Bassman AB165

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Mant
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Bassman AB165

Post by Mant »

I have a Bassman AB165 and was staring at the schematics and comparing to the AA864.
Two questions, is there a sonic difference between doing the voltage divider verses negative feedback on the extra gainstage in the bass channel?
And how about the normal channel mixing in post that gainstage (AA864) vs pre that stage(AB165)?

Thanks
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martin manning
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Re: Bassman AB165

Post by martin manning »

In a word, yes. This write-up has been out on the web for a while, and it has some commentary on the differences and possible mods: http://acruhl.freeshell.org/mga/main/AA ... AB165.html
An odd thing with the AB165 is the second stage of the Normal channel takes its power from the 4th supply node, which puts some DC on the grid circuit of the mixer stage. Relocating it to the 5th node will reduce noise.
mauiboy81
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Re: Bassman AB165

Post by mauiboy81 »

Sorry to revive an old thread but I saw Martins post and was curious. Why does the recovery stage of the normal channel being supplied by the fourth power supply node put dc on the grid network of the mixer stage?

I just picked up a ‘68 ab165 and am thinking about having some fun with it.
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Re: Bassman AB165

Post by Stevem »

But in doing that the plate voltage of that normal channel second stage would be some 60 volts less and on top of that you would then have 3 gain stages feed off of one PS node which in and of itself is not a good thing!
I think it would be best to feed it off of a added parallel PS node fed by a 1to 2k resistor!
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mauiboy81
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Re: Bassman AB165

Post by mauiboy81 »

Moving it on the AB165 would put the first two stages of the normal and bass channels on the last node after the 27k. The mixer stage would be the only stage fed by the fourth supply node. On the earlier AA864 all the preamp stages are fed by the last node after the 4k7 which would be three stages for bass and two for normal.

I replaced all the filter caps last night and went ahead and moved the second normal stage to feed off the same node as the first stage. I haven’t had a chance to turn it up but it sounds good at low volume. It’s still noisy but all the carbon comps have drifted way out of spec. All the 100k plate resistors are reading in the 120k-130k range.
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martin manning
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Re: Bassman AB165

Post by martin manning »

mauiboy81 wrote: Wed May 16, 2018 2:53 pm...went ahead and moved the second normal stage to feed off the same node as the first stage. I haven’t had a chance to turn it up but it sounds good at low volume.
I've seen this done before, and it should indeed reduce the noise floor. In the original arrangement there is DC on some resistors where there really shouldn't be.
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martin manning
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Re: Bassman AB165

Post by martin manning »

Sorry, just saw your question above... Different DC voltages and the lack of blocking caps around the local FB and mixing resistors is the reason. Different tubes will move those voltages around too.
mauiboy81
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Re: Bassman AB165

Post by mauiboy81 »

Speaking of the local feedback what is the calculation to determine the amount of feedback?
If I add a cap in series with the 470k to block dc what size would it need to be to not change the frequency response? Then maybe I can add a pot as a variable resistor in series to dial out some feedback.

As far as moving stage two of the normal channel to the last node I don’t mind the extra voltage drop. I’m considering temporarily swapping the places of the 4k7 and 27k to see how it sounds with lower voltage on the mixer as well. Kinda sorta blondish voltages. Maybe try dumble 2k2/22k/2k2 dropping string. Maybe try out the blonde pi in it. Who knows? I need to change a bunch of drifting resistors as it is. The beauty is I can easily take it all back to stock if I want.
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martin manning
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Re: Bassman AB165

Post by martin manning »

The best way to clean things up is to add blocking caps in series with the 220k mixing resistors, say 47n. Not the easiest thing to work into the layout, unfortunately. Feedback calculations are pretty involved, but the gain of the mixer stage is very roughly Rf/Ri, in this case 470k/220k or ~2. Reduction of the NFB (increasing the 470k) might result in overdriving the PI.
Mant
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Re: Bassman AB165

Post by Mant »

I tried removing the 470k feedback resistor for shits and giggles to see what would happen.
Waaaaay too much gain.
Squeally beast.
Couldn’t you do a split plate load Feeding into this stage dropping the gain that way, rather then the local NFB?
Is there any advantage to one over the other?
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