70's Ampeg V4: early breakup

[Jonny toetags]

I've been playing with this amp on and off for a while and here's where I'm at...
The amp is a 70's V4 with what looks like a replacement transformer and a half power switch. (I didn't install either of these, it was bought like that)
I've done the following maintenanceto it:
-proper ground.
-replace out of spec resistors and all electrolytics on the circuit board.
-install bias trimpot
-replace leaking silver Mallory 20/20uF cap on the inside of the chassis with a Sprague 40uF.
-new 6A fuse

The issue I'm having is that the amp is breaking up at just about 3 on either channel. I notice that if I play with the controls, I can get the breakup to be slightly less (for example, the mids pot acts almost like a gain control when the volume is at 3).
The amp is running a quad of TAD 6L6GC's and biased at around 36mA.
The 3 yellow filter caps are still original.
I have read a few posts saying that the V4's from the 70's started breaking up at around 10 oclock on the vol dial, but I've only owned V2's, so I can't compare.
The half power switch doesn't change anything regarding the breakup...only reduces the overall volume by a few decibels.
The TAD's are new...could it be the powertubes?
Is it worth playing with preamp tubes?
(I should note I'm testing with a 2x12 cab with WGS ET90's.)
I'm looking for a little help/guidance. I included a few pics if it helps.

[nsubulysses]

Do you have a scope? I would see what the power output is in watts via a scope by plugging into the input jack and viewing the output, and then again plugging into the ext in/out jack. You should see 100-110W of clean output. If you can see it that output by using the ext in/out jack but not by plugging into the input jack there is a problem in the preamp.

Mid and treble add a lot of volume so the amp will start to distort if it's already at the limits of clean volume.

I would also check cathode voltages of the preamp tubes against the schematic.

[52 Bill]

The amp is a 70's V4 with what looks like a replacement transformer and a half power switch.

You didn't mention which transformer was changed. Which one and with what?

How do the overall power supply voltages compare to the published values?

[g1]

I'm not sure if I understand the difference between "3" and "10 o-clock" on the volume control?
The db switch on the volume control is critical.
You're biased pretty warm compared to factory. At these kind of plate voltages modern production tubes may be compromised.

[1ampman]

I would start looking at the output jacks and output xmer. Start with ohm meter to grd. Then ohmmeter on output..... If you didn't find anything, power up and check voltages... Output xmer grounding and output jacks need to be correct.... Hope this helps.....

[Swamp]

Just in case, Note that the PI is an 12AU7.

Inject a signal on the ext amp jack and see.

This amp have a dam high Plate voltage!