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7868 or 7591 Push-Pull Ulta-Linear Design Help

Harlon

Harlon

Active Member
So here is a window into how my brain works.

Step one: found a Bogen CHB100 on my local Craigslist for cheap.

It had a quad of these. \/ (that's my attempt at a down arrow) These are 7868 power tubes.
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I've had a desire to build an amp in the 20-25 watt range so the hunt began. Looked up the tube data for the 7868 online and found it was good for that. The ps is good for at least 425. So I started looking for hifi schematics using the 7868.
By chance I found this \/ (another down arrow) on one of Andyman's threads here on AK.
image.jpeg

So this leads to the inquiry. I want to build a push pull hifi amp around the power supply and tubes from the Bogen. I understand that the 7868 and 7591 are the same tube with different bases. I like the schematic above because I've wanted to try something ultralinear and here is a schematic I can build to without worrying about guessing on cathode and NFB resistor values. I do this all without a scope.

The inquiry: Does anyone know of any other existing push pull UL 7868 or 7591 amp designs, or just great pentode designs for these tubes.

image.jpeg
 
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Probably a set of these.
What I've found on the tube data sheets all points towards 6.6k. For this project I'm starting a little backwards with the ps and output tubes. The main goal is to get to that 20-25 watt range so as far as outputs I'm open.
 
Most of the 7868/7591 schematics you run into are very similar. Not many seemed to be UL, but you can find examples from Scott, Sherwood and Fisher for straight pentode mode without problems at all. If you don't want fixed bias, Eico ST-40 is a cathode bias design. Most of the rest you'll find are fixed bias.


The trick with feedback is that it depends on the output transformers. Unless you have the precise ones that a particular design is based around, you will probably need to fiddle with it a bit.
 
gadget73 said:
The trick with feedback is that it depends on the output transformers. Unless you have the precise ones that a particular design is based around, you will probably need to fiddle with it a bit.
Click to expand...

Fiddling I can do. Fiddling I might even be good at. One of my favorite things to do is sit with my jumpers and a couple value resistors or coupling caps and put my Oscillo-ears on.
 
Looks like ultralinear is not the way to go with these tubes. I've read here and there that they don't play nice with the 40-43% screen taps available on most modern opt transformers. Not sure all that goes into it, but apparently these have been used with st35 circuits tied into 6.6k transformers. Apparently these can be used with Dave's EFB circuit as well.
 
Eico, Fisher, Sherwood, Scott, and McIntosh (it wouldn't shock me if a missed a few or more here) all made good variants with these. One of there designs might tickle your fancy. Many are similar. Just some ideas.
 
Why are you not using the Bogen circuit? You have the chassis, power transformer, output transformers and tubes.
 
tcdriver said:
Why are you not using the Bogen circuit? You have the chassis, power transformer, output transformers and tubes.
Click to expand...
The answer can be found in the original post of this thread. The Bogen was a means to an end to get a quad of tubes and power supply that can handle 20-25, 25-30 watts in a hifi push pull circuit. Just like I've done with all my amps, I will start out by doing a quick recap to get it up and running so I can get a feel for the original amp. But again this is a means to an end. I have a 6BQ5 and 6V6 amp. Both of which I greatly enjoy, but I'm looking for an amp that will fill more space. Thus I'm looking for an amp to take me into the next output tier. From the 12-17 zone into the 20-25-30 zone.
The Bogen came up super cheap so here I am.
 
I believe the CHB-100 was a mono amp anyway, running the tubes in PPP mode. Won't get you much joy for stereo, and its quite possibly frequency limited besides.
 
gadget73 said:
I believe the CHB-100 was a mono amp anyway, running the tubes in PPP mode. Won't get you much joy for stereo, and its quite possibly frequency limited besides.
Click to expand...
Yeah they are a PPP mono block. I've seen some stuff on people cleaning up the circuit and output connections and using them as a pair of mono blocks. I hope I don't need that kind of power to fill the space I want to fill.
 
Big power can be fun. I'm working on a pair of mono Bogen amps that will do far more than I need. One is built and it clocks in right around 90 watts, depending which of my meters you want to believe. You can still listen to them at lower levels but pipe organs definitely sound better with some wattage behind them.

Bigger problem can be finding a twin. I paid $20 for the first of these amps, the second one cost me $90 I think, and it took me 2 years to come across one.
 
I'd be willing to bet something like the Dyna Z565 output running 7591/7868 in cathode bias would make an excellent amplifier.
(lower percentage screen tap and cathode bias should make a great fit)
 
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