Help me understand this old Gibson phase inverter

[frankeg]

http://www.ampwares.com/schematics/gibson/ga40.pdf

I found two undocumented component changes in this phase inverter. The first was a .01uf cap in place of the .005uf cap which I understand as a high pass filter and the .005uf cap was removing too much bass. The 2nd change was a 10k resistor in place of the 7.5k in the feedback loop. Not sure on this? Less feedback?. The whole thing looks like a version of a paraphase inverter but I'm not sure. Just trying to understand it. fg

[Chuck H]

It's a paraphase. The .005 cap is simply rolling off HF. The .01 cap will roll off at a lower knee frequency. So the .01 actually reduces lower frequencies more than the .005. There is no feedback loop or local feedback. Notice that the cathode is not elevated from ground at the junction of that 7.5k resistor. The 7.5k is simply the shunt end of a voltage divider feeding the grid of the other triode. I guess 10k made for a preferred balance between the outputs.

[dkevin]

I have seen at least 2 versions of the GA-40 (using 2 ea. 5879's for preamp) schematics. There are some subtle but undeniable differences in components used and design. I suppose if I had to guess it had something to do with the shift in cabinet design from the early buffalo grain/speckled bird covering to the mid-late 50's tweed/buffalo grain covering to the narrow panel all tweed late 50's early 60 version. There was a the schematic you referenced as well as one posted by Greg Cutshaw and there may well be more.

[Rhodesplyr]

Like Chuck said, it's a garden-variety paraphase inverter, very common in 50s amps. It takes advantage of the phase reversal in the top half of the inverter, between input and output, by feeding that inverted signal, divided down by resistors, to drive the lower half of the inverter/driver tube.

I have seen at least 2 versions of the GA-40 (using 2 ea. 5879's for preamp) schematics. There are some subtle but undeniable differences in components used and design.

Gibson changed designs constantly. It was like a nervous habit for them. If you ever find a Gibson amp that precisely matches the published schematic, THAT will be a find.

I have this exact Gibson amp in the shop at the moment. Sad part is that it lived near the beach for several years, and there is rust coming up through the paint on the control panel.

[frankeg]

The one I have is a 1960 narrow panel tweed with components on both sides of the eyelet board. It matches the schematic I posted previously except for those 2 component values and the two sides of the 12AX7 were reversed. Besides replacing the electrolytics, most of the caps and several resistors are out of spec. Since it no longer works as "stock", I'm thinking about replacing all the caps/resistors and mounting everything on the front. I'll put the big PS caps on small terminal strips near the rectifier. Thanks for the replies!