Has anybody used this PCB for a preamp? The price is right, and Im just looking for something to go with my 6B4G SET. It looks like a relatively simple build, but Im wondering if the circuit is tried and true? I have a boatload of 6SL7s, so Im wondering what Id have to change in order to use them instead of the 6SN7? It looks like a pretty good PCB. I know some tube purists arent crazy about PCBs, but Im pretty new at this kind of stuff, so Im going with kit-type stuff for now until I understand this stuff more.
No.
Tried and true, but questionable. The SRPP was originally intended as an active pull-up/active pull-down circuit for driving T-lines. As such, it does this very well. However, due to the impossibility of a "P-Channel" VT, the SRPP goes out of balance for loads other than that for which it was designed.
For most audio use, it's a weird-looking circuit that became trendy. You'd be better off using an active plate load. Some folks swear by 'em, but then there is a huge personal preference factor at work when it comes to audio.
For this preamp, 6SL7s probably won't do: the type has way more gain than the 6SN7, and way less current sourcing capability. You probably don't want either.
I'm building a modified version of the Borbely 6C33C 15W SE design and it has as it's first stage what Mr Borbely describes as "series connected" 6SN7, but what looks to me to be a 6SN7 SRPP design. I'm considering replacing this stage with something else to drive the input of the following 6J5 stage. I don't completely understand how the SRPP stage functions but if it is indeed a Push Pull design I'd rather not have it as the front end for a single ended amplifier. The whole point (I thought) in building a single ended design was to avoid the pitfalls of P-P, so adding this stage contradicts that? I dunno. Any suggestions on an alternative single ended stage to replace the SRPP? This first stage really only needs to develop about 9V P-P to drive the 6J5 stage.
It is an SRPP, and one of those cases where it's operating into a nearly open circuit. That will keep it in balance since there is just the one current path: through both triodes. Another case of "this looks trendy, so let's use it". You'd do better by using another 6J5 (or two sections of a 6SN7) and giving the first stage an active plate load. That will get you more gain and better linearity than an SRPP. Seeing that it operates into a second small signal stage with a 1MEG grid resistor, a Lo-Z output isn't all that important anyway.
It is definitely a push-pull: a symmetrical (if properly loaded) active pull-up/active pull-down circuit. That was the whole point of developing it in the first place.
Ok, I took a first stab at replacing the srpp with something really simple. Have a look and tell me what you think, if you have a second.
An externally hosted image should be here but it was not working when we last tested it.
You can also think of the top SRPP tube as being a simple drop-in replacement for the anode resistor, 1 triode + 1 small resistor replaces the big anode resistor. It can (with equal cathode resistors and tubes) force a mid-HT balance however - that being a good thing or not depends upon your desired anode voltage at idle.
IMO the srpp always sounds better than it really should - very dynamic - no idea why, but there you are. Maybe it's the 'freer' voltage swing - or just the simplicity. It is however my preferred stage - you can also convert it to a mu follower with ease.
Thanks to both of you. I'm going to try it as drawn here, adding the RC network per Jeb D' s suggestion and see how it performs. The other advantage to what I have here is I may be able to leave out the 100V bias for the filaments. While the SRPP may have crossed the cathode/heater threshold, the simpler stage I've replaced it with should only see about 2 volts difference between the 6.3v heater and the cathode Bias voltage. I may end up needing enough open loop gain for some NFB, I'm just going to wait and see how the amp performs. I have a small room and 12x12 and 93db sensitivity speakers, so I may not need to push it to it's 15 watt potential. If I can get a good result without the NFB then I may not need as much open loop gain. Also, my output transformers are 600 ohm eastern audio which don't come with much documentation. They are labeled as being 150VA so I think they should be fine, but I don't want to go pushing them too hard either. I guess my expectation is that I will never need the full performance from this amplifier and if I can make it sound good with 10-12 watts I think I'll be satisfied. (for now anyway) I am a little concerned about the performance with my speakers, they are a multiway design with two 8" woofers on the bottom and, two 5.25" drivers up top and a 1" tweeter. Ive never popped them open but I'm sure theres a nasty crossover in there. Probably not ideally suited to single ended, but again, we'll just see how it sounds. Perhaps this will be the incentive to build a nice pair of full range speakers and use the multiways for home theatre. Sorry for rambling, just thinking out loud here. 
You could soup them up by mounting them externally! I did this with a floor-stander (with a handy plinth), using screws (brass I think) to get the connections out. It sounded hugely better than before - but I upgraded the crossover at the same time so I can't be sure how much effect it had.
Probably not ideally suited to single ended, but again, we'll just see how it sounds. Perhaps this will be the incentive to build a nice pair of full range speakers and use the multiways for home theatre. Sorry for rambling, just thinking out loud here.
I drive 2-ways with my SE amp, I found by adding rather a lot of feedback over the driver-output tube pair I got a very well balanced sound out of it - far better than the cost of the amp would suggest. No other feedback is used - this method of presenting a low impedance drive to the OP transformer is an idea described by Kuei Yang Wang (here), and it really works remarkably well. Switching to the 4ohm tap appears to make it sound even better (but a bit quieter) - so there seems to be something in this approach of muscling the primary and lettering the secondary and speaker just get on with it
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