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Old 27th February 2009, 07:58 AM   #1
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Default Bogen AP35 amp problem

I have an ancient Bogen AP-35 i acquired and one of the output tubes was fried. It uses 7408's 2 in push pull. I replaced them all with 6V6GT's and the sound now is much softer volume wise and has much less power. Also from about 3/4 to full volume, the sound becomes VERY distorted. Could this be a biasing problem? If so the unit has no bias adjustments that I can see. Should i insert a resistor and a pot in between the 2 tubes for bias that way? If so how would i find what values to use. Also the filaments for the 12ax7's in the phono stage do not work. I think this is somehow connected as the power supply that feeds the cathodes of the power tubes are also used to power the filaments. Very weird setup with this. Also in the schematic, the 2 12ax7's that aren't in the phono section are actuall 6eu7's in the unit. Not sure why.

Also is there any ways of improving the sound?

Any ideas are welcome. Thanks.
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Old 27th February 2009, 09:58 AM   #2
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Sorry, forgot to post the schematic. Probably important huh?
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Old 27th February 2009, 11:29 AM   #3
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C43, the capacitor between the 6V6 cathodes and ground, may be bad.
It certainly is a strange circuit.
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Old 27th February 2009, 12:47 PM   #4
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Another thing.
Since the output tubes derive their operating bias through the filaments of the 12AX7 tubes, a short within one of the output tubes which causes the tubes to draw more current is likely to burn out the filaments of the 12AX7's and possibly cause the electrolytic capacitor to fail.

Not a good design.
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Old 28th February 2009, 05:44 AM   #5
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Well, I took a closer look at it's guts and realized whoever had it before me changed the resistors in the power supply and had many wrong values. Also the electrolytic capacitor in the bias circuit was done (probably original) so I replaced all the resistors in the power supply, replaced the bias cap and put a fresh pair of 12ax7's in the phono section and BLAMO! SUCCESS!

Thanks Frank, I appreciate the knowledge.
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Old 28th February 2009, 02:32 PM   #6
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Congrats!
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Old 15th August 2011, 03:45 PM   #7
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I have one of these -- would it make sense to disconnect the cathodes of the 7408's from the 12AX7's and install a cathode resistor (bypassed) for each pair of power tubes ? start at 250 ohm 10 watt ? thanks
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Old 15th August 2011, 04:21 PM   #8
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And I would obviously have to connect v1 and v2 to the filament supply -- would the pt handle the extra .3 amps ?
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Old 15th August 2011, 04:31 PM   #9
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I would pull V1 and V2 and use a proper cathode resistor for the op tubes. You probably don't need the phono section. Try using the "tape out" RCA jacks to insert your CD player inputs to bypass all those old switches. Ultimately, you could probably connect the RCA jacks directly to the grids of V4. That will do away with the tone controls and provide a much cleaner signal.

Last edited by Palustris; 15th August 2011 at 04:33 PM.
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Old 15th August 2011, 04:48 PM   #10
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cool I'll give it a go -- I don't need the phono stage anyway cause my 59 euro turntable has a preamp built into it anyway.... 2 separate resistors would be best, correct ? One for each p/p pair ? What value would you start with ? Thanks for your input
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