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compactron power amp project

nerdorama

nerdorama

AK member
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I have a set of transformers from a Stromberg Carlson integrated. It was the ASR220c I think, a 6V6 PP amp with 7199 VA and PI stages. I have a box of 5HA7 compactrons which are a version of the 7247 dissimilar triode. I think it would be fun to build an all compactron 12 pin power amp on a small chassis box. I have a set of 6JB5's coming to use with them and will use a pair of 6CT3 damper diodes. I think it will be a fun little amp project. If anyone has any experience of a similar project or any other ideas to share please let me know.
Thanks,
John
 
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Compactrons were often re-packaged common tubes. 6JB5 looks a fair bit like a 6V6. Nothing wrong with them, getting the sockets is often the hardest part though.
 
I had a hard time finding the right sockets for the amp I built.
I don't remember which one it was, Novar or Magnoval. I think Novar?
I found a few and snapped them up. These were for the larger 9 pin 6LR8's and a few others I tried though. The difference was pin diameter.
 
The sockets will be the 12 pin cheapies from China. They seem to grip well. Can't say about long term reliability.

@Lavane , I saw a fellow on DIYaudio a couple of years ago who had commissioned a large buy of Chinese Magnoval sockets to be made with the smaller Novar pins. I have a few original Novar sockets but grabbed a dozen or so of the special order sockets just to be sure I had a small supply. I've mainly used them for Novar base damper diodes in power supplies.
 
At the time I had to do a lot of searching just to find the few old stock sockets I found.
I wouldn't mind building another amp if this type with a higher power tube.
5.5wpc is all I get out of the 6LR8 amp.
 
Would love to see a well documented UL PP compaction design/build. I built the 6LU8 and love it, but would like to get just a bit more power.

R
 
RogerA said:
Would love to see a well documented UL PP compaction design/build. I built the 6LU8 and love it, but would like to get just a bit more power.

R
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Pete Millett's Engineers amp uses sweep tubes and if I remember right RF tubes for the drivers. The amp is around 20-25wpc.
 
So many of the compactions are sweep tubes with a low screen voltage limit. Makes it more complicated to do UL.
 
The low screen ratings are more about difference between plate and screen. In straight pentode mode the screen will be at whatever and the plate can actually pull down to very nearly zero volts at full output. With UL that voltage won't be extreme, though it won't be zero. Triode-wired its zero volts and the screen voltage limit really doesn't matter anymore.

Not all tubes do super well in UL mode, and not all want the same tap point so just because you can do it without blowing up the tube doesn't mean its going to perform all that amazingly. Might be better off just running in pentode mode with a screen regulator to drop the voltage within limits. Not many of these things have documented performance so it may be a case of experimenting to figure out what works best.
 
The 6LR8 amp I built can be wired Pentode, UL, and triode strapped.
I thought UL sounded best so left it that way.
 
Sweep tubes are fun to play with but for an ultralinear design you're better off using the tubes it calls for. Lucky for us, 6l6 and 6550 tubes are still available for reasonable prices. Not sweep tube prices, but still workable. At a certain point in this hobby you're going to have to pay extra money for that extra power that you want.
No free lunch here.
I just got finished building some ultralinear 6l6 power amplifiers from scratch, and I can say it is a worthwhile endeavor which will teach you a lot, and you'll end up with some great sounding amplifiers which you can be proud to own,and pass down to several generations of kiddo.
Is your grandkid going to be able or willing to hunt down some obscure black and white TV sweep tube in 60 years, or would it make more sense for them to go to the local music store and buy a pair of 6l6?
 
Double down on a bad hand?
Now you've got the power you wanted but you'll need to source double the number of rare tubes. These haven't been made in 50 years and nobody is going to start.
They're rare now, do you want to try and find matched octets in another few decades? Personally I'd rather just buy 6L6s and move on...
 
nerdorama said:
I have a set of transformers from a Stromberg Carlson integrated. It was the ASR220c I think, a 6V6 PP amp with 7199 VA and PI stages. I have a box of 5HA7 compactrons which are a version of the 7247 dissimilar triode. I think it would be fun to build an all compactron 12 pin power amp on a small chassis box. I have a set of 6JB5's coming to use with them and will use a pair of 6CT3 damper diodes. I think it will be a fun little amp project. If anyone has any experience of a similar project or any other ideas to share please let me know.
Thanks,
John
Click to expand...
I have a 6T9 amp which I built from the Nut's N Volts schematic about ten years ago. It's nice, but my transformer makes too much B+. I have been thinking about redoing it. I recently purchased a pair of 21LU8 tubes as well. I'll probably piece together a SE spud from them.
 
nerdorama said:
I have a set of 6JB5's coming to use with them and will use a pair of 6CT3 damper diodes. I think it will be a fun little amp project.
Click to expand...
It does sound like a fun project. I remember George (tubelab) talking about 6JB5's years ago when Pete created his DCPP amp board. Lot's of experimentation going on back then with different output tubes.

jeff
 
The Stromberg OPT's are quite small but the original amp was a PP 6V6/7408 amp so they should be ok for 10ish watts. Got 1 or 2 other projects ahead of this one but you have to think down the road so there is always a project on tap.
 
I was thinking of building a PP amp at one point using FU-32's.
They're cheap, way cheaper than the western version. But being able to get two of them matched would be mor difficult since each tube has two pentodes inside. Like some sweep tubes, some transmitter tubes can be used to build an audio amp.
 
I built a single ended compactron amp last year. 6JC5, beefier version of the 6JB5, in pentode mode with regulated screen voltage. The driver is a 6BN11, double pentode, triode strapped in a mu-follwer configuration. I threw in a 6CG3 diode for delayed B+. All compactron tubes except for the voltage regulator tubes, 0D3. About 1% THD at 1W, almost all second order. Very fun amp to listen to.1000008120.jpg
 
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