I have been working on converting an old Bogen CHB35A into a guitar amp recently. I have finished cascading the preamp stages for more gain and distortion as well as replacing the original tone stack with a Marshall-type one. However, the output tubes (the amplifier uses two 7868s) seem a bit weak, and for a poor high school student like myself, $50 a pair is a bit pricy. The amp would be treated somewhat roughly, and I do not want to have to worry about replacing output tubes constantly at this price. My question is this: are there any cheaper tubes out there that can be used in place of the 7868 without significant circuit modifications? I would not mind installing new sockets for a tube type with a different base, but I would like to be able to use the original OPT.
Thanks in advance,
Matthew
Thanks in advance,
Matthew
7591A is the same tube but different base and pinout. The JJ brand are $14 each at:
www.tubesandmore.com
This will probably be about as cheap as available new.
Electrically, the 7591A is the exact same tube as the 7868. So is the 6GM5, IIRC.
Daniel
www.tubesandmore.com
This will probably be about as cheap as available new.
Electrically, the 7591A is the exact same tube as the 7868. So is the 6GM5, IIRC.
Daniel
6GT5 is the same pinout, but has a much lower screen voltage limit. A screen dropping resistor and capacitor would do it. But they're not a WHOLE lot cheaper than 7868s... 12GT5 and 17GT5 are DIRT cheap. Add a 12V transformer (in series with the 6.3V for 17GT5s) and you're in business. The 6DQ6 and 6GW6 are electrically the same, but need octal sockets and plate caps (maybe not enough height)... 6FW5 (uprated 6AV5) is another possibility. Come to think of it - this amp uses a doubler for the power supply. Get the lower screen voltage from the half-voltage point on the doubler.
6AV5 / 6FW5 is similar with no top cap... and it's a tube Bogen liked to use as well.
You'll want a variable bias or screen supply if you're using any of the alternate tubes... I'd put a pot in the existing bias supply, in place of the fixed resistor to ground. You may need to adjust some other resistors to get more bias voltage, though that will depend on hte screen voltage that you end up with. You could also vary the screen supply for biasing...
You'll want a variable bias or screen supply if you're using any of the alternate tubes... I'd put a pot in the existing bias supply, in place of the fixed resistor to ground. You may need to adjust some other resistors to get more bias voltage, though that will depend on hte screen voltage that you end up with. You could also vary the screen supply for biasing...
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