• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

QRP03 - Tube Buffered Gainclone

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peranders

peranders

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Joined 2002
sjostromaudio.com
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A couple of years back, do you remember the guy *****? He stole quite much money from our members and a few paid for a pcb but never got one. Those of you who remember know that I managed these catastrophal groub buys but the Tube Buffered Gainclone was not dealt with since there were no pcb's done and quite much work to make them good. This fall I have drawn the schematic of my take of the Tube Buffered Gainclone. The inventor Joe Rasmussen at Joe Rasmussen Pages is my inspiration source and a speaking parter.

The schematics is at the sig below. Some resistor values need to be tuned. Anybody with experience about White Cathode Followers?

Notice that my design has lot's of options and I won't explain in detail right now.
 
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gfiandy

gfiandy

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Joined 2003
Hi, just had a look at your sch. Sorry cant help with white cathode followers, however I have had quite a lot of experience with LM3886 and found that for good stability into low loads >= 4R you need some bulk decoupling within about 2 cm of the package. I found you needed at least 22uF, without this it works ok but on large transients you get momentary instability which saps headroom and causes distortion. This would be quit hard to identify without a distortion analyser where you can view the residual. I was using an AudioPrecision.

I recommend adding these as I don't think you will be able to get the 1000uF caps close enough.

Regards,
Andrew
 
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av-trouvaille

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Joined 2003
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When the transformer(s) are excluded from the pcb it would offer more flexibility in choosing the iron that is availvable/preferred, and in the layout of the case at hand. An alternative would be a pcb where the transformer part can be separated (breakable).
Would it be possible to have an option of a full wave CT rectifier (two diodes) or a full wave bridge (four diodes)? Soundwise, I prefer a CT version also in solid state amplifiers.
 
peranders

peranders

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Joined 2002
sjostromaudio.com
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Personally I think it's a good idea to actually have the transformers on the pcb. Then you will eliminate one source of trouble/safety issues.

I don't like breakable pcb's in such areas.

I have halv wave rectification because it's hard to find a transformer with 50 VAC or more. Besides, I only want max 10 mA. CT plus two diodes would give me only 35 volts instead of 70 V.

25% of the pcb layout ready :nod:
 
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