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VTA ST-70 Review (Stereo Tube Amplifier)

Rate this amplifier:

  • 1. Poor (headless panther)

    Votes: 125 63.8%
  • 2. Not terrible (postman panther)

    Votes: 46 23.5%
  • 3. Fine (happy panther)

    Votes: 16 8.2%
  • 4. Great (golfing panther)

    Votes: 9 4.6%

  • Total voters
    196

DSJR

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As a total aside here, you know what I still love about tube/valve amps? THE LOOKS when they're powered!!!!! My ears now as they are, the inevitable response dips at 4kHz into a typical modern speaker will work badly against me, so eff the sound and whatever 'magic' these old amps put in... I have a pair of fully rebuilt (sensitively by Glenn croft) Quad II's. I must bring them down and play them again for old time's sake. They have a 'glorious(ly coloured)' midrange and soft highs which won't favour the speakers here, but they do look fab when powered in a compact less showy way. The GEC KT66's cost a bomb though now, so I only get these amps down for use very occasionally in the twenty five years I've been custodian of them.
 

captainbeefheart

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As a total aside here, you know what I still love about tube/valve amps? THE LOOKS when they're powered!!!!! My ears now as they are, the inevitable response dips at 4kHz into a typical modern speaker will work badly against me, so eff the sound and whatever 'magic' these old amps put in... I have a pair of fully rebuilt (sensitively by Glenn croft) Quad II's. I must bring them down and play them again for old time's sake. They have a 'glorious(ly coloured)' midrange and soft highs which won't favour the speakers here, but they do look fab when powered in a compact less showy way. The GEC KT66's cost a bomb though now, so I only get these amps down for use very occasionally in the twenty five years I've been custodian of them.

Those are very odd designs which I kinda like, I haven't heard one in years. The front end is two pentodes (ef86) run as a paraphase inverter which is not the best for balance/hifi, I don't even think they give you an AC balance adjustment which would be very nice to improve performance. There is some cathode loading similar to McIntosh unity coupled circuit with the KT66 output stage, it's not 50/50 loading like the McIntosh so there is still some gain in the output stage. It's certainly an odd duck from a design standpoint.

If you don't like coloration there are cleaner tube power amplifiers, it's funny because there have been times where I have made a very clean tube power amp and people said "if I wanted clean I'd just purchase a SS amp".
 

SIY

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Those are very odd designs which I kinda like, I haven't heard one in years. The front end is two pentodes (ef86) run as a paraphase inverter which is not the best for balance/hifi, I don't even think they give you an AC balance adjustment which would be very nice to improve performance. There is some cathode loading similar to McIntosh unity coupled circuit with the KT66 output stage, it's not 50/50 loading like the McIntosh so there is still some gain in the output stage. It's certainly an odd duck from a design standpoint.

If you don't like coloration there are cleaner tube power amplifiers, it's funny because there have been times where I have made a very clean tube power amp and people said "if I wanted clean I'd just purchase a SS amp".
Menno vanderVeen did a good analysis of the Quads in one of his books. The conclusion was that there’s some very clever and functional engineering in them with some very nonobvious details. FWIW, the performance is quite decent and they are not colored amps.
 

DSJR

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Those are very odd designs which I kinda like, I haven't heard one in years. The front end is two pentodes (ef86) run as a paraphase inverter which is not the best for balance/hifi, I don't even think they give you an AC balance adjustment which would be very nice to improve performance. There is some cathode loading similar to McIntosh unity coupled circuit with the KT66 output stage, it's not 50/50 loading like the McIntosh so there is still some gain in the output stage. It's certainly an odd duck from a design standpoint.

If you don't like coloration there are cleaner tube power amplifiers, it's funny because there have been times where I have made a very clean tube power amp and people said "if I wanted clean I'd just purchase a SS amp".
I'm long term 'custodian' of these (long story) and they're precious to me if not my first choice of amplifier :D To be honest, my Quad 303 sounds pretty much exactly the same in a casual sighted comparison and using the same speakers ;)
 

captainbeefheart

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Menno vanderVeen did a good analysis of the Quads in one of his books. The conclusion was that there’s some very clever and functional engineering in them with some very nonobvious details. FWIW, the performance is quite decent and they are not colored amps.

The more I think about it I'd have to agree it's a pretty decent circuit, the fact the pentodes are high impedance means the balance between the each phase inverter isn't as bad as with triodes, more likely the accuracy of the divider network between the two stages will be more of a factor in balance.

It does have global feedback and I like the output stage better being pentode with the cathode loading arrangement vs the typical ultra linear. At least the screens are after the choke, should be good enough. Still an AB circuit so I'd like to see a big fat choke feeding the output stage plate supply with a tube rectifier, or better just use silicon diodes with a decent sized (100-220uF) reservoir cap. I've been messing around with active ripple filters lately with AB amps and so far so good. I tend to just reference the plate supply to a divider and then reference screen supply filter to a diode regulated potential, screens supply feeds the preamp tubes also. Anyway I know it's all they had back then but the tube rectifier in an AB amp isn't ideal to me.

I'm going to have to find analysis by VanderVeen to get some actual real numbers of performance. It is a pretty clever circuit.
 

SIY

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The more I think about it I'd have to agree it's a pretty decent circuit, the fact the pentodes are high impedance means the balance between the each phase inverter isn't as bad as with triodes, more likely the accuracy of the divider network between the two stages will be more of a factor in balance.

It does have global feedback and I like the output stage better being pentode with the cathode loading arrangement vs the typical ultra linear. At least the screens are after the choke, should be good enough. Still an AB circuit so I'd like to see a big fat choke feeding the output stage plate supply with a tube rectifier, or better just use silicon diodes with a decent sized (100-220uF) reservoir cap. I've been messing around with active ripple filters lately with AB amps and so far so good. I tend to just reference the plate supply to a divider and then reference screen supply filter to a diode regulated potential, screens supply feeds the preamp tubes also. Anyway I know it's all they had back then but the tube rectifier in an AB amp isn't ideal to me.

I'm going to have to find analysis by VanderVeen to get some actual real numbers of performance. It is a pretty clever circuit.
I *think* Morgan Jones may have also discussed the circuit in Valve Amplifiers 3rd edition. I’m not at home to look but if you have that book, check my memory.
 

captainbeefheart

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I *think* Morgan Jones may have also discussed the circuit in Valve Amplifiers 3rd edition. I’m not at home to look but if you have that book, check my memory.

Your memory has been checked and verified, "Valve Amplifiers" by Morgan Jones does include the Quad II in the book along with the Williamson, Mullard 5-20.

I most likely just didn't pay much attention to it because of the output transformers, I've built both Williamson and 5-20 amps but never a Quad II because of the output transformer. BUT, I also found Menno's analysis which included very specific information about the output transformer which now I can at least build a simulation with the output transformer parameters and possibly even pass the information along to someone that may feel confident enough to try and wind a pair for me. The Mullard 5-20 and Williamson can get along just fine with off the shelf OPT so it was much easier to experiment and build them. I was always partial to the Williamson circuit, it just does it for me.
 

SIY

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Your memory has been checked and verified,
So it's not Alzheimer's. Whew!

The Williamson and Mullard are to this day the most copied circuits. The "improvements" the copycats make are generally not in a good direction.
 

captainbeefheart

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The Williamson and Mullard are to this day the most copied circuits. The "improvements" the copycats make are generally not in a good direction.

Completely agree, the ST70 sans the input pentode is also a highly cloned circuit, typically a 12AX7 up front in common cathode stage coupled to a split load phase inverter to drive push pull finals. Sadly since they ditched the input pentode they lose a lot of gain that was once turned into feedback so the modern triode input stage clones have worse distortion figures. I get it, pentodes can be noisy so why not use a dual triode cascoded to behave like a pentode? That way there you get a similar open loop gain figure to keep feedback about where it was in the original circuits.

That's my latest issue, I was asked to build a SE amplifier for highly efficient horn speakers and although I was using a 6AU6A as the voltage amp stage there was just too much chance to get a noisy 6AU6A. Even screening for good samples I reduced the noise floor by ditching the pentode voltage amp stage for a dual triode cascoded. My opinion is with low power amps like this SE KT88 build, I am getting a clean 12 watts with distortion just under 1% THD, I am using a 5670 with both triodes in parallel as a cathode follower DC coupled to the KT88 to get higher peak power in A2 operation. The 5670 with gm doubled has a fairly low output impedance at the cathode and seems to be doing an ok job of driving peaks into grid current. A mosfet would work better but he doesn't want SS in his amp and doesn't want to fork out money for interstage transformers. The KT88 is run as a pentode and not UL or triode strapped. I am working out the current feedback from speaker for an adjustable damping control that allows for both +/- current feedback to give different output characteristics. Even though the amp can output power higher than 12 watts that's all I rate it for because it's not really meant to be run in grid current mode for long periods of time, with the efficiency of his horn speakers (105db at 1 watt) he won't be using more than a watt or two average but there is more headroom for dynamic range compared to just A1 operation.
 

Gorgonzola

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Those are very odd designs which I kinda like, I haven't heard one in years. The front end is two pentodes (ef86) run as a paraphase inverter which is not the best for balance/hifi, I don't even think they give you an AC balance adjustment which would be very nice to improve performance. There is some cathode loading similar to McIntosh unity coupled circuit with the KT66 output stage, it's not 50/50 loading like the McIntosh so there is still some gain in the output stage. It's certainly an odd duck from a design standpoint.

If you don't like coloration there are cleaner tube power amplifiers, it's funny because there have been times where I have made a very clean tube power amp and people said "if I wanted clean I'd just purchase a SS amp".
In this day & age it seems to me that tube amps are justified only by their colorations, (principally low-order harmonic distortions).
 

SIY

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Completely agree, the ST70 sans the input pentode is also a highly cloned circuit, typically a 12AX7 up front in common cathode stage coupled to a split load phase inverter to drive push pull finals. Sadly since they ditched the input pentode they lose a lot of gain that was once turned into feedback so the modern triode input stage clones have worse distortion figures. I get it, pentodes can be noisy so why not use a dual triode cascoded to behave like a pentode? That way there you get a similar open loop gain figure to keep feedback about where it was in the original circuits.

That's my latest issue, I was asked to build a SE amplifier for highly efficient horn speakers and although I was using a 6AU6A as the voltage amp stage there was just too much chance to get a noisy 6AU6A. Even screening for good samples I reduced the noise floor by ditching the pentode voltage amp stage for a dual triode cascoded. My opinion is with low power amps like this SE KT88 build, I am getting a clean 12 watts with distortion just under 1% THD, I am using a 5670 with both triodes in parallel as a cathode follower DC coupled to the KT88 to get higher peak power in A2 operation. The 5670 with gm doubled has a fairly low output impedance at the cathode and seems to be doing an ok job of driving peaks into grid current. A mosfet would work better but he doesn't want SS in his amp and doesn't want to fork out money for interstage transformers. The KT88 is run as a pentode and not UL or triode strapped. I am working out the current feedback from speaker for an adjustable damping control that allows for both +/- current feedback to give different output characteristics. Even though the amp can output power higher than 12 watts that's all I rate it for because it's not really meant to be run in grid current mode for long periods of time, with the efficiency of his horn speakers (105db at 1 watt) he won't be using more than a watt or two average but there is more headroom for dynamic range compared to just A1 operation.
Interestingly, Tim deParavicini showed me a cascode ECC88 circuit that got astoundingly high gain, using starved current operation (0.5mA). I didn't believe his performance claims so breadboarded it. And damn, he was right. Gain of 300, at 6V RMS out, H2 = -58dB, no detectable higher order.
 

captainbeefheart

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Interestingly, Tim deParavicini showed me a cascode ECC88 circuit that got astoundingly high gain, using starved current operation (0.5mA). I didn't believe his performance claims so breadboarded it. And damn, he was right. Gain of 300, at 6V RMS out, H2 = -58dB, no detectable higher order.

Yes I was amazed with the performance also and sort of scratched my head saying why is nobody doing this? I too found that lowering the current to a starved point the gain went up, I assumed it was because I lowered the current to use a higher value plate load resistor since when using identical triodes gain is gm*Rp. Yes reduced current lowers gm but the higher load impedance more than makes up for the difference. I too got better distortion (total at output of amp) and lower noise when switching from a true pentode.

I'm still early in development of using cascodes so I'm sure I'll find out sooner or later why it's not so popular in the hifi world.
 

SIY

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I'm still early in development of using cascodes so I'm sure I'll find out sooner or later why it's not so popular in the hifi world.
Often high distortion, always negligible PSR, often complicated heater circuits to avoid abusing the heater-to-cathode max voltage ratings. Can be oscillation prone as well if you use RF triodes.
 

captainbeefheart

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Often high distortion, always negligible PSR, often complicated heater circuits to avoid abusing the heater-to-cathode max voltage ratings. Can be oscillation prone as well if you use RF triodes.

The first two issues are there with pentodes anyway unless you mean it's worse. The heater to cathode max voltages is certainly a concern. The 5670 states 100v max so the bottom triode now sits at 90v which is at the same potential at the top triodes cathode and I haven't had any issues yet but I'm still in build mode and it hasn't been in use for years, I guess we'll find out. Since the amp is SE and I planned on using a pentode in the front end the power supply if very robust with extremely low ripple, tube rectified CLCLC feeds the output stage while there is another decouple RC network feeding the Cascode/Pentode stage.
 

SIY

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Have you tried Morgan Jones's clever LED shunt regulator?
 

DonH56

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4th edition of Valve Amplifiers.
I thought I had one of his books, and probably do somewhere, but thought of picking up the latest version. I see Valve Amplifiers and Building Valve Amplifiers on Amazon. What is the difference (if any)?
 

SIY

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I thought I had one of his books, and probably do somewhere, but thought of picking up the latest version. I see Valve Amplifiers and Building Valve Amplifiers on Amazon. What is the difference (if any)?
Much expanded, a lot more on power supplies and noise, more CCS data and analysis, some novel distortion-cancelling circuits... worth the upgrade IMO. He was nice enough to give me a little hat-tip.
 

DonH56

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Much expanded, a lot more on power supplies and noise, more CCS data and analysis, some novel distortion-cancelling circuits... worth the upgrade IMO. He was nice enough to give me a little hat-tip.
Very cool, thanks!
 
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