1979 Super Reverb - Distortion on Vibrato Channel

[Axtman]

The Normal Channel sounds fine, but if I turn up the Vibrato channel past 3 it goes into distortion. I have confirmed on an oscilloscope that the Vibrato Channel goes into clipping early.

I have checked tubes, swapped tubes, checked voltages, checked resistors, changed electrolytics capacitors (they needed it anyway), checked coupling and tone stack caps, etc. I am running out of options. Any ideas would be appreciated.

cbs_70w_mstrvol_pullsw_super-pro-bmstr_rev.pdf

[Helmholtz]

... checked voltages

As you checked voltages, are all Vibrato channel plate and cathode voltages within spec? If not, please post values.

What's the first point from input, where you can see the clipping?

[vintagekiki]

At which tubes occurs first clipping?
V2a (pin1), V2b (pin6) or V4b (pin6)
1)
The Vibrato channel (3¹/₂mV) has a extra gain stage (V4b) compared to the Normal channel (9mV).

[g1]

Make sure the pull boost circuit isn't 'on' due to miswiring or wrong type switch installed.

[Chuck H]

It might also be worth noting that the reverb/vibrato channel in the BF designs really DOES have more gain than the "normal" channel (there's an extra gain stage for mixing that favors the effects channel). One reason players prefer that channel even if they don't use reverb or vibrato.

[pdf64]

Are you sure there are no mods? eg cathode bypass on V3 may, with a low master volume setting, cause this.

[vintagekiki]

V3 (12AT7) is reverb send (drive) tube.

[pdf64]

V3 (12AT7) is reverb send (drive) tube.

Yes, when its cathode is bypassed, its g-k is typically the earliest clipping point in a Fender reverb channel preamp.
Perhaps due to the asymmetry and frequency selective nature of the clipping, it sounds awful. But without a master volume, it is masked by the deep overdrive at the power valve g1-k clipping points.
But perhaps it’s just a coincidence that when SF models got a master vol, the V3 cathode bypass was removed

[Helmholtz]

The behaviour could be completely normal, as Chuck indicated (depending on amount of distortion).
All my BF amps exhibit some distortion with a humbucker guitar, starting just above 3 on vib channel. And vol pot tapers may vary.

[vintagekiki]

At which tubes occurs first clipping?
V2a (pin1), V2b (pin6) or V4b (pin6)
1)
The Vibrato channel (3¹/₂mV) has a extra gain stage (V4b) compared to the Normal channel (9mV).

[pdf64]

The clipping I’m referring to will be apparent at the V2b anode pin6 waveform, ie the positive peaks will be clipped, when the signal becomes sufficiently large enough for them to pull the instantaneous V3g-k >=0.

[vintagekiki]

The clipping I’m referring to will be apparent at the V2b anode pin6 waveform, ie the positive peaks will be clipped, when the signal becomes sufficiently large enough for them to pull the instantaneous V3g-k >=0.

But ...

V3 (12AT7) is reverb send (drive) tube.

1)

[pdf64]

Oh dear, I don’t seem to be explaining things well.
Are you familiar with the concept of ‘grid clipping’?
Apologies that might sound condescending, I don’t mean it to be taken that way, I just want to make sure we’re all on the same page.
Aiken covers it a bit in the paragraphs after the ‘Reasons’ heading, he describes it as ‘diode clamping’ https://www.aikenamps.com/index.php/...ing-distortion
Whatever, the result is that waveforms larger than about 10Vpeak at V2b anode are distorted, due to the V3 grids clipping or clamping the positive going wave at that level. Assuming that the V3 cathode is about 10VDC.
Clearly the signal at V3 anodes will also be clipped.

[vintagekiki]

https://www.youtube.com/watch?v=vLjz9G9n1Ts

[g1]

waveforms larger than about 10Vpeak at V2b anode are distorted, due to the V3 grids clipping or clamping the positive going wave at that level. Assuming that the V3 cathode is about 10VDC.
Clearly the signal at V3 anodes will also be clipped.

Pulling V3 would be a quick test to see if it's grid is causing clipping at V2 plate?