Slew rate distortion in a 1970 Fender Vibrolux reverb.

[Helmholtz]

Sorry, I'm not trying to be difficult and not being very clear. What I was wondering about was when playing with a guitar at typical Fender clean settings.
In particular I'm wondering about how the affect on the sound would correspond to what people complain about Fender SF not sounding as good as blackface.
Is this something that would make someone think 'this amp is broken', or rather think 'people are right, silver face don't sound as good'.

Are you speaking about the slew rate distortion or about oscillation?
In its original state the amp was stable. Removing the 2nF caps caused instability.

[pdf64]

From occasional forum threads, the parasitic oscillation issue occurs when techs / enthusiastic owners buy or otherwise come to mod a silver panel Fender to AB763, and wonder why the amp then sounds awful. See link in post #3.

[g1]

Are you speaking about the slew rate distortion or about oscillation?
In its original state the amp was stable. Removing the 2nF caps caused instability.

Ok, I missed that you had removed the caps and that is what caused the parasitic. So I guess my question would be 'what does the slew rate distortion sound like'. But I guess you can't really compare to the sound with 'no cap induced slew rate distortion' because of the oscillation.

[Chuck H]

Ok, I missed that you had removed the caps and that is what caused the parasitic. So I guess my question would be 'what does the slew rate distortion sound like'. But I guess you can't really compare to the sound with 'no cap induced slew rate distortion' because of the oscillation.

If I read this thread correctly...

Removing the grid to ground caps reduced the slew rate distortion. And removing the ground caps caused the oscillation.

I think it's generally believed that those caps weren't necessary in the BF version because the lead dress was different. I cannot confirm this, but looking at the spaghetti plate wiring in some SF amps I don't doubt it. If it's true then the slew rate distortion present in the SF model would seem to be one of the major tonal differences between the BF and SF models.

So I'm guessing that your inquiry is about the audibility of the slew rate distortion. I've been following because I'm interested as well.

[loudthud]

Were any changes ever made to the 022848 such as the number of interleaved windings ?

[Helmholtz]

Were any changes ever made to the 022848 such as the number of interleaved windings ?

I don't think so.

Degree of interleaving shows in leakage inductance (Ls).
I measured a few originals from the 60s as well as Fender reissues and Ls was always around 90mH, which is a relatively high value (the OT of my BF SR measures only 8mH).

[Helmholtz]

Removing the grid to ground caps reduced the slew rate distortion. And removing the ground caps caused the oscillation.

Correct.

I don't feel able to say how slew rate distortion sounds on its own.
One of the reasons being that I can't play the amp loud enough in my house, so sound tests will have to wait till completion.

I wasn't happy with the sound of this amp. My reference is my '64 BF SR.
Even my brown Vibroverb RI sounded better than this SF Vibrolux Rev.

The amp showed an unpleasant, early distortion with a LP and lacked responsiveness and detail.
First thing I noticed was B+ being low by 50V. Replacing the 5U4 with a GZ34 brought voltages up to more "normal" values.
Then I found the reverb "artefact" (see my other thread).
And last I studied the effect of the 2nF caps and found the slew rate distortion.

Removing the 2nF grid caps did away with the slew rate distortion.
But now I have to use a 300pF cap between the PI plates.
This essentially corresponds to 600pF caps at the power tube grids.
Means that slew rate limiting starts at a 3.3 times higher frequency than before, i.e. above 10kHz.


Slew rate limiting means the amp can't respond fast enough to rapid changes of the signal voltage. This should result in a loss of detail and changed attack.
Literature describes transient IM distortion (TIM) as a related effect.
E.g. higher frequency content of a slew limited signal will be suppressed along the slopes.

[Pedro Vecino]

Does your Vibrolux Silverface have a 0.01uF capacitor at the input of the phase inverter and 100K feed bias resistors?
If so, I would try 0.001uF and 220K since the first set generates greater depth and worse articulation.

[Helmholtz]

Does your Vibrolux Silverface have a 0.01uF capacitor at the input of the phase inverter and 100K feed bias resistors?
If so, I would try 0.001uF and 220K since the first set generates greater depth and worse articulation.

Thanks.

Amp has a 10nF PI coupling cap and 1M bias feed resistors (I think it's version A1270).
I'm not going to change that right now.
MIght look into that if I'm not satisfied with my results.

[Pedro Vecino]

Amp has a 10nF PI coupling cap and 1M bias feed resistors (I think it's version A1270).

I've never seen a Fender with that value on those resistors. Can't it be a mistake?

[Helmholtz]

I've never seen a Fender with that value on those resistors. Can't it be a mistake?

1M PI bias feed resistors are common with many Fender BF and Tweed amps (as well as Marshalls).
Do you mean the power tube grid leaks/feeds instead?

BTW, all grid bias feed resistors also serve as grid leaks.

[Pedro Vecino]

I have read that term several times (feed bias resistors) and I always associated it with the resistors that come from the bias circuit to the grids of the power tubes. Perhaps I have misinterpreted it.
I mean these:

[Helmholtz]

I have read that term several times (feed bias resistors) and I always associated it with the resistors that come from the bias circuit to the grids of the power tubes.

I see, so you mean the resistors at the power tube grids. Your post #23 sounded as if you meant the PI grid resistors. In both cases you might call those resistors grid leaks or grid feeds as they serve both purposes (leak and feed).
Regarding the power tube grid leaks/bias feeds, the amp has 2x100k.
This is in line with the 6L6 tube data which recommends 100k max for fixed bias.
These resistors load the PI, but a 12AT7 with 47k plate resistors can deliver more signal current than a 12AX7 with 100k plate resistors, so it should be ok.
Increasing to 220k would increase power tube drive and lower the LP corner frequency with the 0.1µ coupling caps, but is likely to increase blocking distortion.

I don't want to change too much at the same time, but might change those resistors at a later point.

[g1]

Just to clarify the common terminology.
I'm not saying it's wrong, but I've never seen those PI grid resistors called 'bias feed'. I think the term bias feed is used exclusively for power tubes when fixed bias is used.
With cathode (self) bias, for either preamp or power tubes, they are then referred to as 'grid leaks'.

[Helmholtz]

Just to clarify the common terminology.
I'm not saying it's wrong, but I've never seen those PI grid resistors called 'bias feed'.

As said, all those resistors serve as grid bias reference as well as grid leaks.
The grid leak path is always between grid and cathode, no matter if actual grid voltage is positive, zero or negative and independent of how grid bias is produced.

As I'm not natively familiar with English terms I go by technical meaning rather than tradition.

But in this case it suffices to specify PI or power tubes.