Vibrolux Reverb with 680Ω in Reverb Driver Cathode

[patlaw]

This video talks about a common problem with the 680Ω resistor used in the cathode of the 12AT7 reverb driver in a 1979 Silverface Vibrolux Reverb. While I have several schematics for VRs, I can't find one that does not have the bypassed 2.2KΩ resistor in that position. Can someone point me to that schematic?

https://youtu.be/GgmDeVMgrr8?t=228

[g1]

see pg.2 of attachment

[Helmholtz]

See here: https://www.jazzguitar.be/forum/guit...ux-reverb.html

Posts #17 (and #4).

For some reason this post wasn't processed earlier.

[patlaw]

Thanks, g1 and Helmholtz. I wonder why this schematic is so hard to find. Whoever marked this one up had an original copy.

[Helmholtz]

I actually do have an original factory schematic, but it's full of my pencilled notes, so...
If it matters to you, I might copy it (to save my notes), try to erase the notes, scan and post it here.
Doesn't give a model number, though.

[patlaw]

I would LOVE to have a good copy. So would the Internet! It's actually one of my two personal amps.

[Helmholtz]

Ok, here you are: img003.pdf
Hope, it helps.

BTW, have you seen my two Vibrolux Rev. threads? One is about reverb induced distortion artefacts, the other about slew rate distortion.

[patlaw]

Absolutely awesome! Thanks for taking the time. I assume you don't mind if I offer the schematic to some of the schematic sites. Please let me know if you have an objection.

I have not seen your threads, but I will now go look for them. I try to read all of your threads as I always learn something from them. And I've been doing this for 53 years.

[Helmholtz]

I assume you don't mind if I offer the schematic to some of the schematic sites.

Sure, no problem. Didn't know this was rare and sought after.
Original must be from my shop days in the early 80s, when this was just a contemporary Fender amp I serviced.

I try to read all of your threads as I always learn something from them. And I've been doing this for 53 years.

Thanks, but I've been here for 4 years only .

Here you are:

https://music-electronics-forum.com/...ibrolux-reverb

https://music-electronics-forum.com/...ibrolux-reverb

[patlaw]

Thanks, but I've been here for 4 years only .

Ok, let me try that again. "And I've been doing repairs for 53 years."

Thanks.

[Helmholtz]

Referring to that Psionic video, I can't think of a reason that would cause the 680R cathode resistor to get hot (except if V3 is bad).
At idle the voltage across it is 6V, resulting in only 53mW dissipation.
Even if cathode voltage rises to maybe 10V with large input, dissipation would be only 147mW.
But maybe cathode voltage can get even higher with transducer saturation? Should be measured.

CC resistors tend to increase (never decrease except if totally carbonized) in value with age, but as cathode resistor value increases, tube current lowers.
E.g. with a 2.2k cathode resistor cathode voltage is around 9V, resulting in only 37mW dissipation.
This shows that resistor dissipation actually drops with increasing resistance.

Unfortunately the video doesn't provide an explanation nor lets us know the actual resistance value.
Always hate not knowing why.

In any case and in order to not change reverb signature the straight forward cure seems to be a 680R cathode resistor with a higher (say 1W) power rating.

[patlaw]

Referring to that Psionic video, I can't think of a reason that would cause the 680R cathode resistor to get hot (except if V3 is bad).
In any case and in order to not change reverb signature the straight forward cure seems to be a 680R cathode resistor with a higher (say 1W) power rating.

I thought I understood him to say that the 680Ω resistor causes the tube to run too hot and die sooner. I haven't done any of the calculations. In any case, until this schematic, all cathode resistors I've seen in this circuit were 2.2KΩ bypassed.

[Helmholtz]

I thought I understood him to say that the 680Ω resistor causes the tube to run too hot and die sooner.

Yes, that might be a better reason to increase the resistor value.
Total tube dissipation at idle is 3.7W, but likely increases at large signal.
Tube limit is 5W.

Nevertheless it was the the resistor - not the tube - which caused the smoke and I don't understand why.

[g1]

Nevertheless it was the the resistor - not the tube - which caused the smoke and I don't understand why.

At 6m45 seconds he mentioned that the 12AT7 was bad. And that he replaced both the tube and the resistor. I don't understand why he did not clearly state that a bad tube would have been responsible for the burning resistor. He sort of made it seem that the resistor burnt because of it's value and the tube was coincidental. At least that was my impression.

[patlaw]

At 6m45 seconds he mentioned that the 12AT7 was bad. And that he replaced both the tube and the resistor. I don't understand why he did not clearly state that a bad tube would have been responsible for the burning resistor. He sort of made it seem that the resistor burnt because of it's value and the tube was coincidental. At least that was my impression.

And that may be totally true. However, the impression I got from some of his comments is that the "problem" with the 680Ω resistor is well known. Since the versions of this amp before and after this version had the 2.2K resistor, I'd be inclined to "update" the circuit if I were working on one. As I mentioned earlier, a big part of my interest is I have a Blackface Silverface Vibrolux Rever from about 1981 that probably has this arrangement since it has the pull boost knob. I've never been into the amp. I thought it had failed on me this past spring, but it turned out to be a loose speaker connection.