- Oct 22, 2020
Exactly. Still, people "ooooh!" and "aaaah!" over the looks. Jim
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.
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!
Those are standard tube sockets. How or if they're connected, I have no idea.
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.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.
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.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...
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.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!
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.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.
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...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:
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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...
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.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.