AB165 phase inveter quest.

[trem]

In the schem below.......

Why are the plates of the PI connected, via 220k, to the plates of the power tunes.?

Thank You


https://el34world.com/charts/Schemat...b165_schem.pdf

[pdf64]

It creates a negative feedback loop around the output valves.
So it may be intended to linearise the response

[Helmholtz]

Yes, that's local voltage feedback from plate to grid (aka Schade NFB).
It lowers distortion and output impedance, extends the frequency range but also lowers power tube gain.

Also unusual, the global NFB is applied to the PI input, which requires the NFB signal to be out-of-phase.

[trem]

OK.....thank you.
Was this fairly unique or did Fender, and others, use this quite a bit.?

[Justin Thomas]

The AB165 Bassman is the ONLY Fender that ever had it. Having tried it with & without, I couldn't tell a difference, but I can say the wiring layout is very susceptible to oscillation if you move stuff around. As far as the rest of the preamp, I think it's magical & wonderful. I can answer questions when I get off work. I love these amps; I'm cloning one now.

Jusrin

[loudthud]

IIRC the speaker connections to the OT were reversed, the green wire was grounded and the black wire connected to the speakers.

[Helmholtz]

IIRC the speaker connections to the OT were reversed, the green wire was grounded and the black wire connected to the speakers.

Yes, something like that would seem necessary for this global NFB wiring.

[Justin Thomas]

IIRC the speaker connections to the OT were reversed, the green wire was grounded and the black wire connected to the speakers.

Yup. And it stayed that way for every (factory-issue) 2x6L6 Bassman head til 1981. A lot of people miss this when they're trying to mod or restore these amps.

Jusrin

[Chip tele]

hello to all

I see this is a fairly recent thread on AB165. Not trying to hijack it but have a related question. My 1968-ish AB165 has the mod for adjustable fixed bias. It was working okay about 6 months ago when I last played it. When I plugged it in last week, the volume was normal but with some static on certain notes. I took all the tubes out and tested them on my Precision 912. One triode of the 12AT7 phase inverter tested bad, so I replaced it with one that tested good.. When I turned it on next, the static was gone. However, I lost that loud Fender clean so typical of guitar through these amps. In its place was a weak, unpleasantly distorted sound at less than half the normal volume. Plate voltage on pin 3 of both output tubes (Shuguang 6L6GC) is in spec at 450v. Bias voltage at pin 5 of both output tubes is also in spec at -45 to -50v. Pin 1 of the 12AT7 phase inverter (plate triode 2) is in spec at 210v. However, pin 6 of the 12AT7 (plate triode 1) is way high at 405v. Even with the bias and plate voltages in spec as above, the plate dissipation on both output tubes is only 14-16 milliamps.

Do ya'll have any suggestions as to what could be causing the high voltage on the first triode of the phase inverter? Is that errant voltage on pin 6 of the phase inverter related to the low plate dissipation on the output tubes?

Thanks in advance for your suggestions!

[Chip tele]

p.s. Pin 7 on the phase inverter has some heavy corrosion. The rest of the connections look okay.

[Chuck H]

It would be best to start a new thread for a different amp with it's own issue.

That said... There's a good chance the new 12at7 you installed isn't conducting on one triode. That would explain the high voltage. It could be the tube, solder joints or pin connections in the socket.

Not related to power tube dissipation as far as I can figure.

Please start a new thread. You can reference what you've already posted here if you think it helps.

[Chip tele]

here's the new thread--thanks! https://music-electronics-forum.com/...ultiple-issues