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AA864/AB165 preamps

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3/30/1999 9:20 PM

AA864/AB165 preamps

I modded my BFBassman AB165 back to typical blackface power. That made it sound much better. Now a possibility is to convert everything back to AA864. I wonder if doing this might clean up the Normal channel a little because it won't have the extra gain stage (it is summed after the extra gain tube in AA864 whereas in AB165 it is before)?

Thanks in advance for any comments.

3/30/1999 11:38 PM

What I did was change the normal channel to AA864 specs, and then I changed the tone stack of the bass channel to the typical guitar tone stack. It worked out great, now there's a normal channel and a high gain channel.

Did you change the feedback circuit into the phase inverter to blackface specs? this helps out alot. Once that is done you have to swap the wires around going to the speaker jack from the output transformer. Otherwise it may squeal.

TC

3/31/1999 3:45 AM
BWilliamson	What are you running for a driver tube. Try a different tube. If you got a 12AX7 go to a AT7 or AY/U7 to clean it up. Unless you don't find the tone currently usable this is easily reversible. Perhaps some different stronger Power tubes to give yourself some more headroom. bw

3/31/1999 9:57 AM

I am running Fender labelled tubes all around (GE 7025, GE12AT7, I think Phillips 6L6's). I have rewired the phase inverter as per BF and used a 1k feedback resistor rather than 820ohm. I am coupling to it with 500pf.

I think I will rewire the Bass channel as suggested (like normal channel with a bright switch). I still wonder about the differences between the AB165/AA864 types of extra gain stages in the bass channel. The AB165 extra gain stage has a feedback loop with a 470k resistor and the grid is grounded with a 470k. The cathode resistor is bypassed with a 25uf/25v similar to the other preamp stages. However the AA864 doesnt have a cathode resistor bypass cap, doesnt have any feedback loop, and the grid is grounded with 220k bypassed with a 0.001uf. Also the coupling caps to the extra gain stage are 0.01 and 0.1 for the 65 vs. 64 respectively.

I would like to have the Normal channel like any BF which is the AA864 way of summing after the extra gain stage with a 220k. But what can I do with the extra gain stage on the Bass channel to tweak a little. I would like to play a little with this extra gain stage to see what it will do with the sound. Any suggestions on changing cathode resistor/bypass caps or plate load resistors on this extra gain stage? Anyone else out there fool around with this extra gain section in the bass channel?

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