• WANTED: Happy members who like to discuss audio and other topics related to our interest. Desire to learn and share knowledge of science required as is 20 years of participation in forums (not all true). There are daily reviews of audio hardware and expert members to help answer your questions. Click here to have your audio equipment measured for free!

VTA ST-70 Review (Stereo Tube Amplifier)

Rate this amplifier:

  • 1. Poor (headless panther)

    Votes: 123 64.7%
  • 2. Not terrible (postman panther)

    Votes: 45 23.7%
  • 3. Fine (happy panther)

    Votes: 16 8.4%
  • 4. Great (golfing panther)

    Votes: 6 3.2%

  • Total voters
    190

captainbeefheart

Active Member
Joined
Apr 18, 2022
Messages
276
Likes
355
Exactly. Still, people "ooooh!" and "aaaah!" over the looks. ;) Jim

What I find funny is the looks is a direct confirmation that they aren't taking advantage of modern parts and techniques. For example people that love the look of "tube amps" like the Jadis is exactly like they were 70 years ago with the large can capacitors sticking out from the chassis. It is grossly common in the boutique guitar tube amplifier market as well. I don't get it, these modern replicas of the old style capacitors test worse and are like 10x-20x the cost. We have fantastic radial/snap-in style capacitors that are very affordable because that's what the market wants and where the technology gets put into, knowing this is why I choose to use modern capacitors. I'd much rather see a board inside full of high quality modern capacitors rated for 10,000hrs at max ripple current and 105°C vs these huge can caps sticking out the top of the chassis.
 

dfuller

Major Contributor
Joined
Apr 26, 2020
Messages
2,002
Likes
2,762
It is grossly common in the boutique guitar tube amplifier market as well.
Some of the guitar amps straight up don't have space in the chassis, or are built in an old method where a PCB wouldn't work.
 

captainbeefheart

Active Member
Joined
Apr 18, 2022
Messages
276
Likes
355
Some of the guitar amps straight up don't have space in the chassis, or are built in an old method where a PCB wouldn't work.

My experience has been the opposite. I have a universal PCB board that I use to replace power supply capacitors and found plenty of room in chassis to install. If the PCB won't fit or it's a very expensive and rare unit I'll use tag boards to install new capacitors. I have had the exception where yes it's either replace the multisection can or gut it and stuff with new parts. Fenders where the power supply caps are in the doghouse and many use more expensive axial types I either just use axial or i also had two boards made that mount vertically allowing enough room for radial caps to be mounted in there. I just remove the caps and sandwiched fiber boards all as one and give it back to the customer, the new vertical boards have L brackets that mount directly to where the fiber board fasteners are. The entire job is faster than removing the old caps from the fiber boards and installing new ones as all I need to do is mount the new boards and connect the wires running to the circuitry. Marshall 50/100 watt heads like the JCM 800 have tons of room, typically directly above where the large can caps are mounted is where the new board mounts and I just leave the old can caps in to look original. Again this leaves the amp original and the new board can be removed and easily reversed for people that are picky about having exactly original form for collectors concerns, same as the Fender which is why I give the old cap board back to the owner so they always have the option to restore it to it's original condition for collectors purposes. I can't even remember the last guitar amp where I didn't have enough room to do anything modern where my only option was replace the multisection can or gut it and stuff it, typically I find that more with hifi gear like vintage HH Scott integrated amplifiers where there is zero room for an additional board.
 

Arnas

Member
Joined
Nov 9, 2020
Messages
59
Likes
44
Audiophools would describe it as: Having phenomenal sense of depth, details that are not exaggerated, tons of micro and macro details, beautiful timbre and why measurements doesnt matter + as a bonus they gonna say that Thx, Benchmarks, Toppings sucks!
 

captainbeefheart

Active Member
Joined
Apr 18, 2022
Messages
276
Likes
355
Audiophools would describe it as: Having phenomenal sense of depth, details that are not exaggerated, tons of micro and macro details, beautiful timbre and why measurements doesnt matter + as a bonus they gonna say that Thx, Benchmarks, Toppings sucks!

I think it's a disconnect of understanding the data and correlating it to what you can hear.

For example a friend of mine purchased a SS amplifier after having the Crimson 275. We all know how the Crimson 275 tested, he claims the 275 has much better bass and the amplifier had a much fuller sound and so his subjective experience is that he prefers the Crimson 275. But it's not that the SS amp sucks, it's just that's it's accurate. He is now hearing a flat response with low distortion in the bass region where the 275 has a high output impedance which will track a lumpy speaker response especially the resonant frequency in the bass region. The 30% THD at low frequencies is adding content that was not there so of course it's going to sound fuller.

It's ok to prefer the poor performing amplifier but it's not ok to claim it's better than the more accurate amplifier.
 

BruceGA

New Member
Joined
Apr 28, 2022
Messages
1
Likes
1
Those are standard tube sockets. How or if they're connected, I have no idea.

One could look up the manual (original) online. The Dynaco preamp connected to the amp through the multipin sockets for power.

The original ST-70 was a budget amp kit. I have one from the late '60s, put together by a friend. I paid him $50 for it. I used it for 20 plus years with the original tubes, then replaced the front end board with another design. It's on the shelf at the moment, wiating for me to try another front end design. I did replace the P/S transformer, as the original got pretty hot with our TVA power here running right at 125 VAC. I could have used a variac...

It sounded ok. I mostly used it on my JBL 4311s. Never tried on my Klipsch La Scalas, which would have been super easy to power considering their efficiency. My 2A2 tube amps sounded better that the Dynaco, but I'm now using a modified Crown D-45.
 
Last edited:

Fiddlefye

Member
Joined
Jan 11, 2022
Messages
19
Likes
16
I think it's a disconnect of understanding the data and correlating it to what you can hear.

For example a friend of mine purchased a SS amplifier after having the Crimson 275. We all know how the Crimson 275 tested, he claims the 275 has much better bass and the amplifier had a much fuller sound and so his subjective experience is that he prefers the Crimson 275. But it's not that the SS amp sucks, it's just that's it's accurate. He is now hearing a flat response with low distortion in the bass region where the 275 has a high output impedance which will track a lumpy speaker response especially the resonant frequency in the bass region. The 30% THD at low frequencies is adding content that was not there so of course it's going to sound fuller.

It's ok to prefer the poor performing amplifier but it's not ok to claim it's better than the more accurate amplifier.
In point of fact there are plenty of "good" SS amps that have really quite dismal low-end performance, "accurate" or not. I've had quite a few that measured similarly but behaved very differently when hooked to a speaker.
 

captainbeefheart

Active Member
Joined
Apr 18, 2022
Messages
276
Likes
355
In point of fact there are plenty of "good" SS amps that have really quite dismal low-end performance, "accurate" or not. I've had quite a few that measured similarly but behaved very differently when hooked to a speaker.

Yes sadly many companies only do testing on a purely resistive dummy load when they are driving reactive loads in the real world. I too have seen many SS amplifiers have adequate phase margin with a resistive load yet start to destabilize under slightly reactive loads showing overshoot and ringing. Some have gone into full oscillation with too much capacitance.

There are boutique solid state amps that perform similar to tube amps, Pass Labs is one that comes to mind. Instead of a common drain output stage where the output is taken from the source pin, there has been a trend of going with a common source output stage that takes the output from the drain. These will have higher output impedance and are similar to common cathode tube output stages where the output is taken from the plate. Early solid state amps were common emitter which is the same as common source except using BJT. There may be some companies using BJT in this fashion like the old days of common emitter output stages but I typically see Mosfets with these designs.
 
Last edited:

Fiddlefye

Member
Joined
Jan 11, 2022
Messages
19
Likes
16
Yes sadly many companies only do testing on a purely resistive dummy load when they are driving reactive loads in the real world. I too have seen many SS amplifiers have adequate phase margin with a resistive load yet start to destabilize under slightly reactive loads showing overshoot and ringing. Some have gone into full oscillation with too much capacitance.

There are boutique solid state amps that perform similar to tube amps, Pass Labs is one that comes to mind. Instead of a common drain output stage where the output is taken from the source pin, there has been a trend of going with a common source output stage that takes the output from the drain. These will have higher output impedance and are similar to common cathode tube output stages where the output is taken from the plate. Early solid state amps were common emitter which is the same as common source except using BJT. There may be some companies using BJT in this fashion like the old days of common emitter output stages but I typically see Mosfets with these designs.
I'll probably get battered for this but.... my own personal situation is that I run a CJ MV-45 that I've owned for 3.5 decades (currently running KT-77s and at this point upgraded about as far as such things can be). Yeah, it probably wouldn't pass measure around here (looks damned good in basic scope measurements though). It puts out 40-or-so w/ch - more than enough into very high resolution 92db efficiency speakers in 12x12 listening space.

The rub, though - I live in a house with hot water heat and no central air and running a tube power amp in the summer can turn the room into a bit of an Easybake Oven. I usually sub in a SS amp for the hot months and have owned or had pass through countless highly regarded amps of all descriptions. Part of the equation is finding an amp with an input sensitivity similar to the MV-45 so it matches decently with the existing rest of the system. The only one that really managed to fill the shoes to my liking was a beautifully restored Krell KSA-50 and of course it produces just as much heat as a tube amp (or more). The search continues...
 

captainbeefheart

Active Member
Joined
Apr 18, 2022
Messages
276
Likes
355
I'll probably get battered for this but.... my own personal situation is that I run a CJ MV-45 that I've owned for 3.5 decades (currently running KT-77s and at this point upgraded about as far as such things can be). Yeah, it probably wouldn't pass measure around here (looks damned good in basic scope measurements though). It puts out 40-or-so w/ch - more than enough into very high resolution 92db efficiency speakers in 12x12 listening space.

The rub, though - I live in a house with hot water heat and no central air and running a tube power amp in the summer can turn the room into a bit of an Easybake Oven. I usually sub in a SS amp for the hot months and have owned or had pass through countless highly regarded amps of all descriptions. Part of the equation is finding an amp with an input sensitivity similar to the MV-45 so it matches decently with the existing rest of the system. The only one that really managed to fill the shoes to my liking was a beautifully restored Krell KSA-50 and of course it produces just as much heat as a tube amp (or more). The search continues...

What level do you listen to music at? You have fairly efficient speakers and my guess is like many with efficient speakers is you use a lot less power than you think. You might get away with something in the 10 watt range that puts out less heat. I have a fully horn loaded speakers but I am selling them that are 105db at 1 watt and the speakers I am using now are 96db at 1 watt, with the 96db speakers I can get away with a 5 watt amplifier and it can get VERY loud. With the 92db I would think 10 watts would be enough, gets you over 102db and 20 watts 105db.

I'm thinking a 6BQ5 push pull amp with fixed bias, run higher voltage and lower current, you should be able to get 20 watts out easy with much less heat vs the CJ amp.

Va = 350v
Ia = 18mA

Should run cool enough to use in the summer and you are only losing -3db vs your CJ amplifier as far as sound pressure is concerned.
 

Fiddlefye

Member
Joined
Jan 11, 2022
Messages
19
Likes
16
What level do you listen to music at? You have fairly efficient speakers and my guess is like many with efficient speakers is you use a lot less power than you think. You might get away with something in the 10 watt range that puts out less heat. I have a fully horn loaded speakers but I am selling them that are 105db at 1 watt and the speakers I am using now are 96db at 1 watt, with the 96db speakers I can get away with a 5 watt amplifier and it can get VERY loud. With the 92db I would think 10 watts would be enough, gets you over 102db and 20 watts 105db.

I'm thinking a 6BQ5 push pull amp with fixed bias, run higher voltage and lower current, you should be able to get 20 watts out easy with much less heat vs the CJ amp.

Va = 350v
Ia = 18mA

Should run cool enough to use in the summer and you are only losing -3db vs your CJ amplifier as far as sound pressure is concerned.
Interesting idea - a smaller tube amp isn't something I'd contemplated. I don't listen at "concert" levels at most (not sure how many actual "db"). One thing that I am curious - my tech and I did a mod on the MV-45 recently to install test points so i can set the bias lower than the original LED system would have it and the amp does run considerably cooler as a result of the lower settings. Line voltages here just keep climbing and were becoming problematic. The new approach completely eliminated the mechanical noise in the power transformer and the accompanying 60hz hum. Maybe it will end up being tolerable? We shall see. When we've had a cooler than normal summer I've managed in past a few times just fine.

Still, I do get some fun playing with an alternate amp in the system from time to time!
 

Attachments

  • _DSC5766.jpeg
    _DSC5766.jpeg
    164.4 KB · Views: 15

captainbeefheart

Active Member
Joined
Apr 18, 2022
Messages
276
Likes
355
Interesting idea - a smaller tube amp isn't something I'd contemplated. I don't listen at "concert" levels at most (not sure how many actual "db"). One thing that I am curious - my tech and I did a mod on the MV-45 recently to install test points so i can set the bias lower than the original LED system would have it and the amp does run considerably cooler as a result of the lower settings. Line voltages here just keep climbing and were becoming problematic. The new approach completely eliminated the mechanical noise in the power transformer and the accompanying 60hz hum. Maybe it will end up being tolerable? We shall see. When we've had a cooler than normal summer I've managed in past a few times just fine.

Still, I do get some fun playing with an alternate amp in the system from time to time!

Did your tech adjust bias while viewing on a scope and or distortion analyzer? I use both to set bias on customers amplifiers and you would be surprised how low you can set the bias without any crossover distortion. That's the issue with biasing too cold, you may reach a point where the tubes stay "off" too long and where they pass the zero crossing there is a time lag on the "handoff", this is called crossover distortion and can sound quite bad. I often set the bias just above where crossover distortion disappears to account for any line fluctuation error. It would be very helpful for your tech to tell you a specific current reading to not go below. I am guessing you have either 1 ohm or 10 ohm cathode resistors installed? You hook up your volt meter and measure say 60mV with 1 ohm which equates to 60mA or with 10 ohm 600mV is 60mA. Depending on the circuit you may be able to go below 40mA or less without any crossover distortion, any more current is a waste of energy and tube life. To sum this up, it would be ideal if your tech found that say a reading of 35mV (35mA) is the lowest you can set the bias before distortion starts to increase and sound quality will suffer, you really shouldn't be doing it by ear.

As I have mentioned there have been many Class AB push pull amps come in, say similar to your amp with EL34 type tubes that say bias is set to 45mA at 450v, that's 20 watts dissipation with a 25 watt tube, that's 80% max dissipation of plate. I have found you can get as low as 50% max dissipation without seeing crossover distortion. So you could probably get away with setting the tube at around 30mA instead of 45mA without it causing any harm to sound quality.

McIntosh amps use bifilar wound output transformers with loads placed not only in the anode circuit but also the cathode circuit, it's called unity coupled circuit because the output stage has such low gain from the loading it's practically unity gain, typically I measure a gain of 1.2x which is much lower than your average output stage. This tells us the output stage is near 100% feedback. I have seen these amps biased at like 10mA per tube still meet distortion spec, 6L6 is a 30 watt tube so that puts the dissipation at around 15% of maximum. The tubes run very cold and last nearly forever in these amps. They are basically Class B amplifiers because of how cold they are biased yet you see no crossover distortion.

So looking for a McIntosh tube amplifier for summer use could be an option and you won't suffer any power loss, say a MC275 and you are actually increasing your power output capability yet running with a highly reduced amount of heat. These amps are very expensive, I think around $6,000 now but there is a Chinese clone that looks very well built, there is one guy in a group I am in that has one and he loves it and says it's just as nicely built as the McIntosh for 1/4 the price. I get it if you aren't into trusting a Chinese clone company, and $2,000 is still a lot of money.

You don't always have to go with mainstream amp companies, you can always find an amp builder that does it as a hobby to build you something specific to your needs. Make sure they aren't builders that can only copy schematics, you want one that has good engineering chops that can design the amp specifically for your needs. That's what I do, I don't copy schematics and sell the amps as is, I go through a design process with my customers where we make goals for the design specific to their needs and then I design the circuit from the ground up. This often requires me to do an in house call to take some measurements for a real world power requirement for them because that's critical and I don't like to guess what power they will require. I have a very accurate method of measuring power that includes headroom into the equation, it's simple enough for me to walk through customers over the phone so long as they know how to use a volt meter and has a digital source. I take two measurements, one is your average listening power level and the other is we turn the music up to maximum they will ever listen to, this is their max power needed. This method has converted many people that once thought they needed 350 watts of power to realize they only need say 60 watts. With efficient speakers the results are even more drastic, with 92db speakers the loudest power needed is typically around 20 watts. With my 96db speakers my maximum is power needed is only 12 watts and that was VERY Loud, too loud really but I wanted to be safe. My average listening level was much less than 1 watt and most peoples average listening is going to be typically 1 watt or less. Even if you just guess your average listening power is 1 watt, which for you is 92db at 1 meter away, this is often much louder than peoples average listening, typically people listen to around 80db, for 12db of dynamic headroom puts your maximum power at 15 watts. That puts your peaks sound pressure levels of 104db which is close to concert level. So for 92db speakers I'd be comfortable to say you can easily get away with a 15 watt amplifier.
 

captainbeefheart

Active Member
Joined
Apr 18, 2022
Messages
276
Likes
355
I found the schematic for the MV45.

The LM1458 is used as a comparator, the cathode sense resistor for the power tubes is 20 ohms. I can see how since the power transformer is rated for 115v input you not only have higher plate voltage, it's supposed to be around 470v which with higher mains is probably closer to 500v, the problem of having the comparator reference voltage also be higher now since it's just a divider that sets it is going to increase the amount of current where the LED goes from on to off. These two issues compound each other since now the comparator is going to set current higher you also have higher plate voltage. Your tech could have adjusted resistor values for the comparator to set a lower idle bias current instead of adding the tip jacks making you measure bias.

I'm guessing he left the 20 ohm resistors in the cathode and you are probably setting bias to around 800mV or .8v. This is equivalent to 40mA through each tube. This is still quite hot bias at 80% dissipation. For 60% dissipation I'd reduce the bias reading to .6v or 600mV and it should still sound good.
 

Fiddlefye

Member
Joined
Jan 11, 2022
Messages
19
Likes
16
I found the schematic for the MV45.

The LM1458 is used as a comparator, the cathode sense resistor for the power tubes is 20 ohms. I can see how since the power transformer is rated for 115v input you not only have higher plate voltage, it's supposed to be around 470v which with higher mains is probably closer to 500v, the problem of having the comparator reference voltage also be higher now since it's just a divider that sets it is going to increase the amount of current where the LED goes from on to off. These two issues compound each other since now the comparator is going to set current higher you also have higher plate voltage. Your tech could have adjusted resistor values for the comparator to set a lower idle bias current instead of adding the tip jacks making you measure bias.

I'm guessing he left the 20 ohm resistors in the cathode and you are probably setting bias to around 800mV or .8v. This is equivalent to 40mA through each tube. This is still quite hot bias at 80% dissipation. For 60% dissipation I'd reduce the bias reading to .6v or 600mV and it should still sound good.
Well done, Sherlock! That is exactly how it is working out, with bias shifting (over the course of the daily swings). Using the original LED set-up we were getting readings around 1.0 - 1.1v and that was just way too high! I'm running between .8v and .9 so far (what my tech thought sounded best) and have played around a little at lower values, but still early days of experimentation. We tried the possibilities running it on a variac at between 110 and 115v and decided that the circuit ran pretty heavily into Class A even then, so that is part of the original design plan - just not cooking so hot as it does today. We discussed both approaches and in the end decided that using the digital multimeter was probably more accurate in the long run anyway. With the test points in the front it isn't a big deal and I can kinda ignore the flashing LEDs as they now only light up when I'm really cranking things. Glorious sounding thing it is, though - especially since getting every sort of upgrade we could think of done to it.
 
Last edited:

captainbeefheart

Active Member
Joined
Apr 18, 2022
Messages
276
Likes
355
Thank you Watson ;)

It's still a Class AB amp just biased hot, the power tubes will go into cutoff even at 50mA (1v reading). The only way to make a true Class A amp with EL34's is to run the plate voltage well below the 470v, more like 300v and bias current at 80mA, that should keep the tubes in conduction the full cycle. The plate voltage is just far too high to run true Class A.

It's optimal for the tech to give the lowest bias setting that doesn't show crossover distortion but from experience with these amps I'm confident you can easily go down to 30mA (.6v) and still have great sound, that's still 60% dissipation which is about average for Class AB amps.

Keep in mind that I am talking plate current and when you are taking a cathode reading it is the sum of plate current and screen current. CJ doesn't have any screen resistors which is odd, I'd add 100 ohm resistors directly on pin 4 but I believe these have the power tubes directly onto a circuit board so your tech needs to either cut a trace and install the resistors or if the wire from the output transformer is close enough to pin 4 just install it there. If not you can still easily measure screen current via measuring the DC resistance between the output transformer center tap and the screen taps. Once you write down this resistance place one lead on the center tap and the other at pin 4 and measure the voltage drop across this resistance, use ohms law to figure out screen current and deduct it from the total cathode current. Or you can just estimate roughly 3-5mA for the idle screen currents. So if you are measuring .8 or 40mA cathode current you are really at 37mA plate current. This still puts you at 72% dissipation. Not bad and if you like the sound then just leave it alone, it's better than running near 50mA like with the LED which would put you at about 100% max dissipation of an EL34, it gives you really not much of an advantage because you are still going to be running into cutoff, the Class B part of operation. I just haven't found any advantages to running Class AB amps like this really hot. To get a true Class A circuit you'll need to run much higher current, 80-85mA and 250-300v on the plate.

I have found with high voltage EL34 amps, 450-500v and fixed bias that you can go down as far as 50% dissipation without any crossover distortion but it also depends on load and feedback etc...

Instead of a variac, talk to your tech about a bucking transformer, they are a lot less expensive and you can fit it in the chassis. All you need is a filament transformer 6.3v, make sure phase is correct for voltage reduction because if not you will add the 6v to your current mains voltage and be way too high. Your tech should know what to do and how to wire it up. It's what I typically do for vintage amps and modern mains voltages. If there is no room for a bucking transformer I typically run a zener string on the full wave rectifier center tap to reduce B+, in your case with this amp which has a full wave bridge rectifier you would place the zener string between D1 and D3 to ground. The bucking transformer is better because it lowers all the secondary voltages down which is good because too high heater voltage will diminish tube life. If I can't use a bucking transformer and I use the zener string trick and I feel the heater voltage is too high I just add a very small amount of resistance to the heater circuit to drop it down closer to 6.3. I have seen vintage amps with 7v on the heaters which is too high in my book. This amp runs around 4 amps on the 6.3v tap, getting voltage down say .5v only requires a resistor value of .125 ohms. Use a precision wirewound current sense resistor rated for like 5 watts.

Something like this:

 

Fiddlefye

Member
Joined
Jan 11, 2022
Messages
19
Likes
16
Thank you Watson ;)

It's still a Class AB amp just biased hot, the power tubes will go into cutoff even at 50mA (1v reading). The only way to make a true Class A amp with EL34's is to run the plate voltage well below the 470v, more like 300v and bias current at 80mA, that should keep the tubes in conduction the full cycle. The plate voltage is just far too high to run true Class A.

It's optimal for the tech to give the lowest bias setting that doesn't show crossover distortion but from experience with these amps I'm confident you can easily go down to 30mA (.6v) and still have great sound, that's still 60% dissipation which is about average for Class AB amps.

Keep in mind that I am talking plate current and when you are taking a cathode reading it is the sum of plate current and screen current. CJ doesn't have any screen resistors which is odd, I'd add 100 ohm resistors directly on pin 4 but I believe these have the power tubes directly onto a circuit board so your tech needs to either cut a trace and install the resistors or if the wire from the output transformer is close enough to pin 4 just install it there. If not you can still easily measure screen current via measuring the DC resistance between the output transformer center tap and the screen taps. Once you write down this resistance place one lead on the center tap and the other at pin 4 and measure the voltage drop across this resistance, use ohms law to figure out screen current and deduct it from the total cathode current. Or you can just estimate roughly 3-5mA for the idle screen currents. So if you are measuring .8 or 40mA cathode current you are really at 37mA plate current. This still puts you at 72% dissipation. Not bad and if you like the sound then just leave it alone, it's better than running near 50mA like with the LED which would put you at about 100% max dissipation of an EL34, it gives you really not much of an advantage because you are still going to be running into cutoff, the Class B part of operation. I just haven't found any advantages to running Class AB amps like this really hot. To get a true Class A circuit you'll need to run much higher current, 80-85mA and 250-300v on the plate.

I have found with high voltage EL34 amps, 450-500v and fixed bias that you can go down as far as 50% dissipation without any crossover distortion but it also depends on load and feedback etc...

Instead of a variac, talk to your tech about a bucking transformer, they are a lot less expensive and you can fit it in the chassis. All you need is a filament transformer 6.3v, make sure phase is correct for voltage reduction because if not you will add the 6v to your current mains voltage and be way too high. Your tech should know what to do and how to wire it up. It's what I typically do for vintage amps and modern mains voltages. If there is no room for a bucking transformer I typically run a zener string on the full wave rectifier center tap to reduce B+, in your case with this amp which has a full wave bridge rectifier you would place the zener string between D1 and D3 to ground. The bucking transformer is better because it lowers all the secondary voltages down which is good because too high heater voltage will diminish tube life. If I can't use a bucking transformer and I use the zener string trick and I feel the heater voltage is too high I just add a very small amount of resistance to the heater circuit to drop it down closer to 6.3. I have seen vintage amps with 7v on the heaters which is too high in my book. This amp runs around 4 amps on the 6.3v tap, getting voltage down say .5v only requires a resistor value of .125 ohms. Use a precision wirewound current sense resistor rated for like 5 watts.

Something like this:

Ah, the variac - the one I mentioned has been bolted to my tech's bench for the entire two decades plus I've known him. I have pondered a bucking transformer, discussed it with a friend who uses one with the Stereo 70, actually. Maybe at some point in future? All sorts of possibilities with these old dearies. For now I'm just enjoying how wonderful it sounds...
 

captainbeefheart

Active Member
Joined
Apr 18, 2022
Messages
276
Likes
355
Ah, the variac - the one I mentioned has been bolted to my tech's bench for the entire two decades plus I've known him. I have pondered a bucking transformer, discussed it with a friend who uses one with the Stereo 70, actually. Maybe at some point in future? All sorts of possibilities with these old dearies. For now I'm just enjoying how wonderful it sounds...

If you decide to go with the good old bucking transformer route make sure you have enough space under the hood for it. I found the 6.3v filament transformer works well, most voltages around here tend to be higher than 120v, an average of 122-123v. The 6.3v will bring you down to where you want to be. A easy way to find the rating you need is the AC mains fuse amperage rating. I believe the M45 is a 5A fuse, this means you want a 32VA rating. The Hammond 166Q6 would be a good solid choice rated for 6A and will cost you about $30. Another good choice which is cheaper at around $22 is the Triad F-18X. I have seen people under size them where the core ran saturated and caused more problems then helped. You most likely would be okay with a 4A transformer if the 6A is too big, like a Triad F43X which is only like $16. If I can I like to run the bucking transformer conservatively so I prefer to overrate it.
 
Last edited:

Fiddlefye

Member
Joined
Jan 11, 2022
Messages
19
Likes
16
If you decide to go with the good old bucking transformer route make sure you have enough space under the hood for it. I found the 6.3v filament transformer works well, most voltages around here tend to be higher than 120v, an average of 122-123v. The 6.3v will bring you down to where you want to be. A easy way to find the rating you need is the AC mains fuse amperage rating. I believe the M45 is a 5A fuse, this means you want a 32VA rating. The Hammond 166Q6 would be a good solid choice rated for 6A and will cost you about $30. Another good choice which is cheaper at around $22 is the Triad F-18X. I have seen people under size them where the core ran saturated and caused more problems then helped. You most likely would be okay with a 4A transformer if the 6A is too big, like a Triad F43X which is only like $16. If I can I like to run the bucking transformer conservatively so I prefer to overrate it.
Ya know, maybe I'll take a look through my little stash on Hammond transformers and see if there is anything that might serve the purpose. I am afraid that given how tight things are inside the case it would have to go outside somehow most likely.
 

captainbeefheart

Active Member
Joined
Apr 18, 2022
Messages
276
Likes
355
Another thing to mention is the voltage rating of the power supply capacitors. The amp has a solid state rectifier and when powered on the tubes are not conducting so there is no load to pull the voltage down to operating voltage, the caps are rated for 500v and you might be exceeding this limit during power on cycle. I never liked the idea of setting the cap ratings for working voltages and not no load voltages, someone may try and power the amp up without tubes.

Let us know what you find in your transformer stash and if you decide to install one I'd be happy to assist. I figured the CJ amps don't have a ton of real estate so if you don't have room or don't want to mount above chassis I'll help you with the other methods to keep the amp running optimal.
 
Top Bottom