Hi - I was just wondering if anyone had produced a compact PP amp using two tubes. That is a single double-triode to perform pre-amp and phase split and a double power pentode (such as 6DZ7) as a PP driver?
If so, I guess quite a compact stereo PP could be made with just 4 tubes assuming SS rectification.
They say curiosity killed the cat, but I don't have a cat so it's worth the risk...
If so, I guess quite a compact stereo PP could be made with just 4 tubes assuming SS rectification.
They say curiosity killed the cat, but I don't have a cat so it's worth the risk...
From an OTL perspective..
Large output valves typically need some driving. So when you look at a line level to output level, the voltage and current output are too much - if you can narrow down the output speaker to high impedance then you may be able to.
Having one tube ahead of the PP output tubes links the gain as both stages in most tubes have similar gain but when it comes to a simple one valve phase splitter you don't get any real gain - also IIRC cathode side is sensitive to impedance.
It's possible. There's many examples out there. Just power levels may be low. Bung in a output transformer and your laughing - driving 8ohm speakers without complicating things means an output transformer for each channel, or adding SS to the output stage. One other note - you could use solid state as PP cathode followers if you don't mind them helping out with current duties. Broksie does this with his modified Brazilian OTL but the same technique would work for driving a large output transformer from a smaller number of tubes.
I suspect it's possible to do this in two ways - I'm looking at this from a headphone amp perspective (given I'm designing my own);
1. input signal -> input triode (amplification) -> simple phase split triode (no gain) -> output tubes [optional transformer]
2. input signal-> differential amp (Long tail pair uses both triodes) -> output tubes [optional transformer]
I'm more familiar with 2, this has additional noise cancellation benefits but all your amplification is in the differential amp to drive the output tubes voltage swing input required and the output swing. In the case of OTL, an 6AS7 doesn't give you voltage amplification but instead actually gives you attenuation, however at the same time it gives you current into a low impedance.
With a transformer you're solving a lot of those issues but then have to design around the other issues.
So that small amp is going to get bigger - either with more tubes (OTL) thus a larger powersupply or more ironware (transformers).
Large output valves typically need some driving. So when you look at a line level to output level, the voltage and current output are too much - if you can narrow down the output speaker to high impedance then you may be able to.
Having one tube ahead of the PP output tubes links the gain as both stages in most tubes have similar gain but when it comes to a simple one valve phase splitter you don't get any real gain - also IIRC cathode side is sensitive to impedance.
It's possible. There's many examples out there. Just power levels may be low. Bung in a output transformer and your laughing - driving 8ohm speakers without complicating things means an output transformer for each channel, or adding SS to the output stage. One other note - you could use solid state as PP cathode followers if you don't mind them helping out with current duties. Broksie does this with his modified Brazilian OTL but the same technique would work for driving a large output transformer from a smaller number of tubes.
I suspect it's possible to do this in two ways - I'm looking at this from a headphone amp perspective (given I'm designing my own);
1. input signal -> input triode (amplification) -> simple phase split triode (no gain) -> output tubes [optional transformer]
2. input signal-> differential amp (Long tail pair uses both triodes) -> output tubes [optional transformer]
I'm more familiar with 2, this has additional noise cancellation benefits but all your amplification is in the differential amp to drive the output tubes voltage swing input required and the output swing. In the case of OTL, an 6AS7 doesn't give you voltage amplification but instead actually gives you attenuation, however at the same time it gives you current into a low impedance.
With a transformer you're solving a lot of those issues but then have to design around the other issues.
So that small amp is going to get bigger - either with more tubes (OTL) thus a larger powersupply or more ironware (transformers).
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I'd noticed that certain single-ended amps were using a double-triode (ECC83, 6SN7 etc.) wired in parallel and some critics had cited this as a 'waste of a triode'. Since you would get the same gain with using half the ECC83, this was taking me down the 'can I use a single triode as a phase splitter/driver' since the gain bit is already done. That rabbit hole took me down the dual pentode in one bottle to make a PP amp.
For sure I'm not expecting to get the power equivalent of two x EL34, but maybe 2 x 6V6 might be an interesting experiment.
For sure I'm not expecting to get the power equivalent of two x EL34, but maybe 2 x 6V6 might be an interesting experiment.
The ECL86 is a cute package - I've done some searching and found a nifty circuit which was used in the Dansette Mk2 record player: 2 x EL86 to provide a mono 10w PP amplifier using a split-phase driver derived from one triode and the other triode providing pre-amplification..
The Dissident Audio is a good shout too.
Thanks for the quick responses!
Paralleling tubes is not a waste - but induces some issues like doubling gate capacitance for the input signal. This can affect frequency so needs designing in not simply bodging into an existing design.
A single valve is: output = F(signal) + noise distribution.
Paralleled valves: output = (FA(signal)+ FB(signal) + noise_distributionFAB + noise_distributionA + noise_distributionB) /2
If normalised by /2 and tube amplification function F() for A & B are very close resulting in only a small amount of signal deviation noise difference, noise_distributionFAB, then... the main noise distributions average out but the signals reinforce. So your signal-noise ratio goes up.
There will still be some noise added (noise_distributionFAB) at signal levels as the valves are never matched.
If you use a differential amp paralleled, the common noise is cancelled out - however noise which is non-common is remains along with the signal. By paralleling your signal noise (FAB) becomes averaged vs the signal, increasing SNR further.
You could put an high input opamp on the anode and cathode - sample the wave form and cancel it out for the op amp output leaving the noise.. then subtract that noise from the signal by feedback. (you can't subtract from the phases directly). This needs to be done per tube. A differential LTP does this by cancelling through the noise being transmitted out of both cathode and anode for each valve.
TL;DR is that paralleled tubes increase your signal-noise-ratio but add some noise in by doing it.
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If you want a differential two tube PP: EL86 Push Pull Triode Connection Diffrential Amplifier
Even if you don't go LTP, there's some good experience and observations around using EL86 in PP for the output stage that may help you.
Even if you don't go LTP, there's some good experience and observations around using EL86 in PP for the output stage that may help you.
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The 6GW8/ECL86 is scarce. Solve the heater supply issue and the more, if not readily, available PCL86 is a possibility.
The 6BM8/ECL82 remains relatively easy to source and was used fairly recently by ASL in monoblocks that attracted some following. ASL's schematic is provided below.
The 6BM8/ECL82 remains relatively easy to source and was used fairly recently by ASL in monoblocks that attracted some following. ASL's schematic is provided below.
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Soviet 6F3P (6Ф3П), which are 6BM8/ECL82 rough equivalent, is even easier/cheaper to source.
Sounds good too (unlike the 6F5P).
Several DIY two tubes per channel P-P amps have been made using the 6LU8 - 6LR8 - 6MF8 triode/ pentode tubes intended for vertical sweep (frame output) in a TV set. I made a breadboard version of one and managed to squeeze over 30 WPC out of it without undue stress to the tubes.
Several versions have been seen on this forum, but the schematic is similar to the one shown in post #8. You could run it at 300 volts or so of B+ into a 5K CT opt for 20+ WPC, but I ran mine at just over 400 volts on the main B+ and just under 300 volts on the output tube screens with a zener / mosfet filter to drop the B+ to the screens. Power was about 35 WPC into a 6600 ohm OPT. Trying a 5K opt made well over 40 WPC, but brought redness to the plates of the output tubes.
These tubes may not be too common in the UK or Europe, but are still in plentiful supply here.
The 6LR8 is a 9 pin Novar tube similar to a Magnoval but with smaller diameter pins. Sockets may be hard to find in the UK. The 6KY8A is a smaller 9 pin Novar tube with the same pinout as the 6LR8. 20+ WPC can be had from them. The 6GF7 is a dual triode in the same package. All have the same pinout except pin 7 is a NC on the triode, so they can all work in the same amp.
The 6LU8 and 6MF8 are 12 pin Compactron tubes. The 6LU8 is identical to the 6LR8 except for the base, both are stout tubes capable of more than the 14 watt dissipation rating. The 6MF8 has the same pinout as the 6LU8, but carries a 12 watt plate rating and has lower transconductance. You can still get 25 WPC from two of them. There are also smaller sized variants of these tubes with the same pinout, but I don't remember the numbers.
I made the breadboard in true Tubelab style by wiring Novar tube sockets up to an early prototype of the Tubelab SPP board. That board uses a single 12AT7 and 2 X EL84's or 6CW5's per channel. I never made a board for the P-P version, and the pictures for the prototype were lost when I dropped my computer.
I made a SPUD SE amp that fit the Novar tubes. It just used one tube per channel. It started out as a furball of wires connecting a turret board to a SSE board, and evolved to a PC board. The SSE uses 1/2 of a 12AT7 and typical octal output tube per channel. I had the board set up for 6EZ5's at the time. The 6LR8 triode is similar to 1/2 of a 12AT7 and it's pentode is like the 6EZ5. I used 6LR8, 6KY8, and 6GF7 tubes in it.
Several versions have been seen on this forum, but the schematic is similar to the one shown in post #8. You could run it at 300 volts or so of B+ into a 5K CT opt for 20+ WPC, but I ran mine at just over 400 volts on the main B+ and just under 300 volts on the output tube screens with a zener / mosfet filter to drop the B+ to the screens. Power was about 35 WPC into a 6600 ohm OPT. Trying a 5K opt made well over 40 WPC, but brought redness to the plates of the output tubes.
These tubes may not be too common in the UK or Europe, but are still in plentiful supply here.
The 6LR8 is a 9 pin Novar tube similar to a Magnoval but with smaller diameter pins. Sockets may be hard to find in the UK. The 6KY8A is a smaller 9 pin Novar tube with the same pinout as the 6LR8. 20+ WPC can be had from them. The 6GF7 is a dual triode in the same package. All have the same pinout except pin 7 is a NC on the triode, so they can all work in the same amp.
The 6LU8 and 6MF8 are 12 pin Compactron tubes. The 6LU8 is identical to the 6LR8 except for the base, both are stout tubes capable of more than the 14 watt dissipation rating. The 6MF8 has the same pinout as the 6LU8, but carries a 12 watt plate rating and has lower transconductance. You can still get 25 WPC from two of them. There are also smaller sized variants of these tubes with the same pinout, but I don't remember the numbers.
I made the breadboard in true Tubelab style by wiring Novar tube sockets up to an early prototype of the Tubelab SPP board. That board uses a single 12AT7 and 2 X EL84's or 6CW5's per channel. I never made a board for the P-P version, and the pictures for the prototype were lost when I dropped my computer.
I made a SPUD SE amp that fit the Novar tubes. It just used one tube per channel. It started out as a furball of wires connecting a turret board to a SSE board, and evolved to a PC board. The SSE uses 1/2 of a 12AT7 and typical octal output tube per channel. I had the board set up for 6EZ5's at the time. The 6LR8 triode is similar to 1/2 of a 12AT7 and it's pentode is like the 6EZ5. I used 6LR8, 6KY8, and 6GF7 tubes in it.
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It is possible to build a mono push-pull amplifier with a single tube ECLL800 / 6KH8 . It is a compactron-like device with two power pentodes and a special phase inverter triode. This now scarce tube has been mostly used on early stereo German table top tube radios, add-on powered speakers (to add the second channel to a mono record player), and even some guitar practice amplifiers. Power output can reach 9W, but on cost-optimized design 5W is a more likely outcome. Those designs had flaws that made this tube disappear rather quickly from the market. Tube was expensive and life was limited, due to the very high heat that had to be dissipatesd in a cramped space; the output transformer required a high (11k) impedence: it was impossible to get good low frequency response from the tiny transformer that had to fit on those compact designs.
To get full power, 8V are needed at the triode grid: a modern low impedence 2V source can be directly connected with a 1:4 input transformer. I wonder if a similar tube had ever be manufactured in the compactron range.
To get full power, 8V are needed at the triode grid: a modern low impedence 2V source can be directly connected with a 1:4 input transformer. I wonder if a similar tube had ever be manufactured in the compactron range.
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