Amp SVT-CL Strange Issue, Help Request

[g1]

One thing about the voltages on the schematic, the notes say the voltages in the rectangles are AC signal voltages with specified amount applied to the input.
But you were able to get rid of the negative voltage anyway.
How much ripple are you seeing on the +/- 15V rails?

[Delta362]

Glad you found the culprit in IC2

[nevetslab]

I've been using this one

The parallel diodes across the screen resistors have been deleted per Tech Note published some years ago. I'd have to dig that out if interested. Screen resistors had been changed from 22 ohms to 220 ohms at that same time, as we see in this schematic

[Helmholtz]

The diodes across the screen resistors essentially short the resistors, so render them ineffective in normal operation.
Don't understand the purpose.

[nevetslab]

The diodes across the screen resistors essentially short the resistors, so render them ineffective in normal operation.
Don't understand the purpose.

Nor do I. I've attached the Tech Note where they instruct replacing the 22 ohm screen resistors with the 220 ohm parts, and to remove the parallel 1N4007 diodes across them, but no explanation given for why they were included in the first place.

Technical Information Bulletin (TIB0003, June 2001) -- Power tube PCB modifications.pdf

[g1]

I thought the diodes were some kind of 'fail-safe' for the resistors, showing up around Rev.F of the Magnavox SVT's. Not good if you want the screen R's to have fusing function.

[Helmholtz]

A screen resistor has a twofold function:

1) It improves stabilty by damping RF resonance.
2) It protects the screen by instantly lowering screen voltage when screen current rises.

A parallel diode clamps the voltage across the resistor to maybe 0.7V while having a very low dynamic resistance.
This should prevent either of the beneficial effects of the resistor.

[The Dude]

Yes. It really makes no sense at all. It's almost like the resistor is a "backup" for the diode. But if that's the case, why use a 220 ohm? Must have been free LSD day in the design shop.

[g1]

Sometimes it's about protecting the circuit board from burning. Gingertube talked about this elsewhere regarding the older SVT's. Fender also did this in 'The Twin' when the cathode 1R resistors started burning up circuit boards and cost a lot in warranty claims.
For the old 22R screen resistors, the diode would only take over when screen current hit around 30mA, so it's not completely nullifying the resistor.
https://www.audioasylum.com/forums/t...16/161752.html

[The Dude]

So, instead of burning up a 10 cent screen resistor when the tube shorts/over-conducts, we have a diode take over and allow the tube and possibly the OT to melt. It still doesn't make much sense to me.

[g1]

So, instead of burning up a 10 cent screen resistor when the tube shorts/over-conducts, we have a diode take over and allow the tube and possibly the OT to melt. It still doesn't make much sense to me.

I know. I think it's a carry over from when tubes where cheap (and more robust) and circuit boards were at a premium. Hopefully the fuse blows in time, and if you can get away without labour, that factors in too. But I guess Ampeg realized it didn't make sense anymore, thus the diode removal.
In the Fender case, the warranty labour costs got to be so much that they came up with the 'DNR' lists. Ironically, a lot of the warranty labour costs were doing all the stupid paperwork.

[T9c]

VolcanoHead, Thanks for the info you posted on the fix pertaining to u2 on the pwr bd. That fixed the amp I was laboring with, although I previously ordered c26 (.68uF 250v) I didn't believe they were the cause of the -14vdc on c26.

Rick
T9cRepair