1975 Princeton Reverb, harsh "blatty" distortion...causes?

[Matthews Guitars]

Amp is a freshly recapped '75 Fender Princeton Reverb. Sounds great when kept clean. But when pushed to overdrive, distortion onset is loud and very harsh and blatty.
Bias cap is new, as is the filter multisection canister cap.

Before I do a deep dive into it, I want to see if someone here has some advice as to common failures that can cause this.

All the dreaded "white caps" are long gone. The tone and coupling caps are all original shiny dark blue Paktrons, except one. I see no evidence of DC leakage thru them, and they are very reliable.

[sluckey]

This may help...

https://robrobinette.com/AA1164_Princet ... ested_Mods

Read the first paragraph under the suggested mods section.

[dragonbat13]

I was going to say the same. Probably blocking distortion in the PI. Got the same problem with mine.

Mine is probably coming apart soon to fix that and a trem issue.

And I want to add a mod to the ground hole. I don't know what, however.

[Stevem]

A mod to the ground hole?

What are you referencing with that name?

To this day that's kind of a back woods way of saying time to move the out house.

[dragonbat13]

The hole for the ground switch I'm not using. I might try a negative feedback switch, or maybe a master volume of some sorts.

[sonofmickel]

I built a Deluxe and it sounds a lot better after lowering the B+ closer to original spec.
Also has some reformed old stock capacitors, but I think the biggest difference was getting the voltages closer to stock.
It required switching my Power Transformers primary to the 125v tap, it was on the 120v tap.

[sonofmickel]

You aver get this fixed?

Solution you came to?

[burchyk]

Recently started looking into this myself.

Tried the PI grid stopper as recommended by Rob Robinette, which did not help. Later added pre-PI master volume (for different reasons) which showed that the unpleasant clipping happens at an earlier stage. Plus my BF Champ has somewhat similar clipping character. And it's got no PI.

From the scope I get a suspicion that maybe it's the centre bias with 1k5 cathode resistors throughout which gives most headroom, but then tends to clip rather symmetrically and abruptly, could be the culprit.

I'll be trying larger cathode resistors when I get time. Preliminary impressions from quickly swapping just one are good.

[B Ingram]

Recently started looking into this myself.

Tried ...

Disconnect the feedback loop, and let us know whether it improves.