The previously stated warnings are valid. However, something decent may be possible.
PP 6BM8s are good for approx. 8 WPC. Triode wired 6V6s will yield about the same power. Run the trioded 6V6s at lowish B+ and relatively high plate current. That will reduce the internal resistance and fit better with the O/P "iron".
PP 6BM8s are good for approx. 8 WPC. Triode wired 6V6s will yield about the same power. Run the trioded 6V6s at lowish B+ and relatively high plate current. That will reduce the internal resistance and fit better with the O/P "iron".
6LU8 could be subbed in for 6BM8 with minor mods, or for just outputs only:
6AV5, 6GF5, 6EZ5, 6HE5/6JB5/6JC5, 6FW5, 12/6GE5, 12/6JN6
Check heater power available. 6V6 tubes are only rated for 40 mA DC plate current, versus 50 mA DC for the 6BM8. The subs above range from 75 mA up to 175 mA and can easily handle the 5.6K OT. 6EZ5, 6HE5/6JB5/6JC5 are closest to a supersized 6V6. 6EZ5 is pin compatible with 6V6.
6LR8 is another one, same as 6LU8 with triode driver and big pentode. 6KY8 is another possibility, with triode driver and big pentode. 6KY8 is on the $1 list at ESRC.
6MF8 has a 12AT7 like triode driver section with a 6EZ5 pentode section. On the $1 list at ESRC,
6AV5, 6GF5, 6EZ5, 6HE5/6JB5/6JC5, 6FW5, 12/6GE5, 12/6JN6
Check heater power available. 6V6 tubes are only rated for 40 mA DC plate current, versus 50 mA DC for the 6BM8. The subs above range from 75 mA up to 175 mA and can easily handle the 5.6K OT. 6EZ5, 6HE5/6JB5/6JC5 are closest to a supersized 6V6. 6EZ5 is pin compatible with 6V6.
6LR8 is another one, same as 6LU8 with triode driver and big pentode. 6KY8 is another possibility, with triode driver and big pentode. 6KY8 is on the $1 list at ESRC.
6MF8 has a 12AT7 like triode driver section with a 6EZ5 pentode section. On the $1 list at ESRC,
First, try the amp with the 6BM8s, but connect your 6 or 8 Ohm speakers on the 4 Ohm tap (I hope you have a 4 Ohm tap). Does it sound better?
Now try the following, using 6V6 tube and the 5.6k transformer, it may work very good, but it may have one tradeoff:
Rewire for 6V6 tubes, including re-biasing (hopefully individual self bias resistors with individual bypass caps). Then reconnect your loudspeakers: If you have a 6 Ohm speaker, put it on the 4 Ohm tap. 5.6k x 6/4 = 8.4k primary If you have an 8 Ohm speaker, put it on the 4 Ohm tap. 5.6k x 8/4 = 11.2k primary
reminder: A typical 6 Ohm loudspeaker may have a minimum impedance of 5 or 4 Ohms. And an 8 Ohm loudspeaker may have a minimum of impedance of 4 or 3 Ohms. That applies, no matter what output transformer you use.
Now for the possible tradeoff: If the inductance of the 5.6k primary is too low, there will be a fall off of the lowest bass frequencies. Suppose when using the 5.6k transformer, that its inductance gives you a bass response of -3dB at 20Hz, using an 8 Ohm load on the 8 Ohm tap (-1dB @ 40Hz). With an 8 Ohm load on the 4 Ohm tap, it will be - 3dB at 40 Hz (-1 dB @ 80Hz). The 6 Ohm load on the 4 Ohm tap will be -3dB at 30 Hz (-1dB @ 60Hz). You may not miss much bass, especially if your speakers are not flat to 50, 40, 30, or 20 Hz.
Try those 6V6 tubes, the 5.6k transformers and put the speaker on the 4 Ohm tap, you just may like the sound of that.
How about a schematic?
Now try the following, using 6V6 tube and the 5.6k transformer, it may work very good, but it may have one tradeoff:
Rewire for 6V6 tubes, including re-biasing (hopefully individual self bias resistors with individual bypass caps). Then reconnect your loudspeakers: If you have a 6 Ohm speaker, put it on the 4 Ohm tap. 5.6k x 6/4 = 8.4k primary If you have an 8 Ohm speaker, put it on the 4 Ohm tap. 5.6k x 8/4 = 11.2k primary
reminder: A typical 6 Ohm loudspeaker may have a minimum impedance of 5 or 4 Ohms. And an 8 Ohm loudspeaker may have a minimum of impedance of 4 or 3 Ohms. That applies, no matter what output transformer you use.
Now for the possible tradeoff: If the inductance of the 5.6k primary is too low, there will be a fall off of the lowest bass frequencies. Suppose when using the 5.6k transformer, that its inductance gives you a bass response of -3dB at 20Hz, using an 8 Ohm load on the 8 Ohm tap (-1dB @ 40Hz). With an 8 Ohm load on the 4 Ohm tap, it will be - 3dB at 40 Hz (-1 dB @ 80Hz). The 6 Ohm load on the 4 Ohm tap will be -3dB at 30 Hz (-1dB @ 60Hz). You may not miss much bass, especially if your speakers are not flat to 50, 40, 30, or 20 Hz.
Try those 6V6 tubes, the 5.6k transformers and put the speaker on the 4 Ohm tap, you just may like the sound of that.
How about a schematic?
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These are from originally 60's HIFI amp ( I think 8 ohm) so I'd say they would be good for 12W. I all ready have stereo 6BM8 with similar OTs, I just don't like the sound of the tube, kind of sounds like EL84 with less early distortion. I don't mind them for a guitar amp but not for HiFi. I thought there might be something I could do, maybe regarding plate voltage to match them closer to 6V6. Someone here mentioned running parallel 6V6s to these OTs. I actually had these OTs in parallel 6V6 amp I've build many years ego, they're still in the amp 
But compared to my single ended 6V6 amps I have, it's just way too inefficient for the little more power it puts out. It sounds nice and fat but classic PP would be much cleaner at higher levels.
But compared to my single ended 6V6 amps I have, it's just way too inefficient for the little more power it puts out. It sounds nice and fat but classic PP would be much cleaner at higher levels.
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Shunt Schade N Fdbk works well at reducing distortion and may well fix the 6BM8 sound.
However, the 6BM8 pentode has badly rounded knee curves, meant for reducing 2nd harmonic in SE use (causing increased 3rd H). Most any of the tubes mentioned above have more squared up plate curve knees, and will work well in P-P. ie, May be even better to fix the problem at the source.
The 6EZ5, 6HE5/6JB5/6JC5, 6MF8 tubes are like 2nd generation 6V6 tubes with the screen current kinks cleaned up. And they have twice the current capability of the 6V6 tube. (handling 5.6K easily) 6EZ5 is pin compatible with 6V6, so you can try either tube easily. 6MF8 is on the ESRC $1 list and includes a Mu 58 12AT7 like triode as well for driver use.
6LU8 is another popular Pentode + Triode driver tube,
However, the 6BM8 pentode has badly rounded knee curves, meant for reducing 2nd harmonic in SE use (causing increased 3rd H). Most any of the tubes mentioned above have more squared up plate curve knees, and will work well in P-P. ie, May be even better to fix the problem at the source.
The 6EZ5, 6HE5/6JB5/6JC5, 6MF8 tubes are like 2nd generation 6V6 tubes with the screen current kinks cleaned up. And they have twice the current capability of the 6V6 tube. (handling 5.6K easily) 6EZ5 is pin compatible with 6V6, so you can try either tube easily. 6MF8 is on the ESRC $1 list and includes a Mu 58 12AT7 like triode as well for driver use.
6LU8 is another popular Pentode + Triode driver tube,
No, different tubes have different characteristics. Personally much prefer Octal tubes like 6V6, KT66 etc for HFI. Tubes like EL84, 6BM8 were mostly used in cheap HIFI amps and there is a reason for it, cheaper to make. If you put side by side EL84 and 6V6 HIFI amp and listen, you'll hear the huge difference in sound production.
Some people say that all tubes can be made sound the same depending on circuit design, if that was the case than there would be only few power tubes to choose from. Why would people pay 100's of Dollars for vintage KT66 for their amp project if they just could use new EL34 and tweak the circuit to the right sound.
Some people say that all tubes can be made sound the same depending on circuit design, if that was the case than there would be only few power tubes to choose from. Why would people pay 100's of Dollars for vintage KT66 for their amp project if they just could use new EL34 and tweak the circuit to the right sound.
Thanks for that. I've never tried any of those but think will be worth an experiment. I've been building with what I have in stock for years, 6V6, KT66, 6GW8, 6BM8, EL84, EL34. I've build amps with all of those, both guitar and HIFI. That's all I had to work with so far so I can't speak for other tubes.
They got smaller because that is the way of the world. It's the same reason the 6CG7 exists. The EL84 needs less drive than a 6V6. The 6AQ5 is a 6V6 (electrically) in a B7A package. The Soviet 6P1P is a 6V6 in a B9A package. Both work in a 6V6 circuit with different sockets. The Soviet 6P6S is also a 6V6 equivalent and since it's octal, it will plug and play. Since I prefer to run triode connection, I find 6P1P is great since it's linear and cheap, that and 6P43P.
I suspect people buy expensive NOS KT66 tubes because they are just stupid/unaware of cheaper alternatives, or they are audiophools. Just my opinion, I could be wrong.
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The ARRL Handbook has an interesting 6BQ6 PP output stage using 4 K Ohm load. The screen voltage is regulated w/ a VR150 from G2 to cathode. The low G2 volts the tube uses leaves a litle more swing for the plate and it will still do lots of current. The 5.6K load would be light on them. BTW 12BQ6s are very cheap.
6AN8-6BQ6 modulator
The EL84 can be an excellent performing tube. I've never played with a 6BM8, but I have no doubt that it can have excellent performance. It really depends on the circuit where it is used. I think that a lot of the (misinformation?) that floats around about different tubes being "magical" is that certain people have substituted them into amplifiers that aren't designed for them, leading to distortion that they perceive as "magical". Had they run a distortion test, they may have felt rather differently.
Most well-designed push-pull amplifiers sound similar, because they all perform well with low distortion and good frequency response. When people run tubes in a single-ended configuration with no negative feedback, all bets are off. You've got high output impedance in many cases, a gapped transformer with DC flowing through it, and no distortion cancellation. That gapped transformer with DC flowing through it is a big part of the "Neve sound", which generally isn't considered to be low-distortion.
With that engineering rant over, I personally think the 12BQ6 is a great choice for that iron, much better than a 6V6. Higher primary transformer impedance will yield slightly lower power, but likely lower distortion and a better damping factor.
Most well-designed push-pull amplifiers sound similar, because they all perform well with low distortion and good frequency response. When people run tubes in a single-ended configuration with no negative feedback, all bets are off. You've got high output impedance in many cases, a gapped transformer with DC flowing through it, and no distortion cancellation. That gapped transformer with DC flowing through it is a big part of the "Neve sound", which generally isn't considered to be low-distortion.
With that engineering rant over, I personally think the 12BQ6 is a great choice for that iron, much better than a 6V6. Higher primary transformer impedance will yield slightly lower power, but likely lower distortion and a better damping factor.
Whatever impedance is optimum for a particular amplifier, most loudspeakers are not optimum impedance over their complete frequency range.
EL84 with 12 Watt plates does not equal KT66 with 25 Watt plates.
Oh . . . and I like Both EL84 and KT66.
And I have enjoyed listening 6V6 tubes too.
Etc.
Before the internet, how many people had easy access to Russian tube types?
EL84 with 12 Watt plates does not equal KT66 with 25 Watt plates.
Oh . . . and I like Both EL84 and KT66.
And I have enjoyed listening 6V6 tubes too.
Etc.
Before the internet, how many people had easy access to Russian tube types?
Interesting design... pretty classic topology from what I can see, but it looks like a more modern take on it... do I see LED biasing on the input Pentode?
I've been playing with a design in LTspice right now that might be of interest, it should be usable with a lot of different output tubes. I'll try to post the design when I have time. It's low-gain, designed to use my balanced microphone preamplifier module (reconfigured for closer to 15dB of gain) as the input.
For a small amp, you might be able to get away with driving the output tubes directly from this module, particularly if you pushed the power rails up to +/- 24 volts. It is, however, solid-state. You would have a hybrid amp... not everyone's cup of tea.
Design thread can be found here:
Solid-State Balanced Microphone Preamplifier
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