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Old 20th November 2007, 01:48 AM   #1
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Question 7868 substitutes

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
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Old 20th November 2007, 02:25 AM   #2
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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
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Old 20th November 2007, 03:31 AM   #3
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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.
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Old 20th November 2007, 04:14 AM   #4
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Thanks for the help. I think I'll go with the 6DQ6 since I can get them dirt cheap off of Ebay. I don't think the hieght will be a problem, since I am no planning on using the original case anyway.
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Old 20th November 2007, 02:18 PM   #5
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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...
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Old 21st November 2007, 03:29 AM   #6
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Will do. I was planning on modifying the bias supply anyway, but now I actually have incentive to do so.
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Old 21st November 2007, 03:40 AM   #7
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Default Re: 7868 substitutes

Quote:
Originally posted by AmpKiller66
(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. Matthew
seems I had one pair RCA 7868
I'll check latter
$50 a pair
they are much cheaper
but may expensive in shipping
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Old 21st November 2007, 04:19 PM   #8
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Question 7868 substitutes

Hi Ampkiller.

I'm hoping to mod a Masco PA with a cascade preamp like you describe. Could you post a schematic please if possible?

Thanks,

Mike
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