SC20H Power stage EL34 bias readings

Vlad58

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Hi guys, My SC20H is not near the power output spec. It puts out may be max a few watts for switch in high and much less than 1W in the low switch position and guitar input connection. It is when both the Master and Pre-amp knobs maxed out. I checked EL34 plate voltage under 300vdc and it looks very low. I suspect some issues with the rectifier or PT. Can someone check Marshall's nominal bias voltages on EL34 of this amp. It would help a lot to track down the problem. Appreciate in advance.
 

PelliX

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First of all, welcome! :welcome:

I could pop mine open on the bench tomorrow after work. What voltages are you measuring after the rectifier and PT? Same issue when hooking into the FX return, I presume?

I'm probably preaching to the choir, but you've tried different EL34's, right?

Perhaps a bit more background would also be handy. Did you buy it like this, did this suddenly happen? How are you measuring the output Wattage you mention? What cab are you using? Does the DI out work as expected (bonus round - just curious).
 

Moony

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I checked EL34 plate voltage under 300vdc and it looks very low

That's pretty normal.
The Studio amps are running around 250-260V with bias at 50-55%.
That's probably also why you won't hear that much changes soundwise when swapping different brands of EL34s.
 

Jon Snell

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Hi guys, My SC20H is not near the power output spec. It puts out may be max a few watts for switch in high and much less than 1W in the low switch position and guitar input connection. It is when both the Master and Pre-amp knobs maxed out. I checked EL34 plate voltage under 300vdc and it looks very low. I suspect some issues with the rectifier or PT. Can someone check Marshall's nominal bias voltages on EL34 of this amp. It would help a lot to track down the problem. Appreciate in advance.
Schematic?
 

PelliX

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That's pretty normal.
The Studio amps are running around 250-260V with bias at 50-55%.
That's probably also why you won't hear that much changes soundwise when swapping different brands of EL34s.

This is what I remember, but for the life of me, I can't find my measurements. Shame on me, this doesn't happen to me often.

Schematic?

You wish. Many do. The schematic is not public nor does it appear to have leaked. The preamp section is pretty much a regular 2203, then there's a couple of relays for the FX loop and the output stage which is cathode biased and voltage regulated as far as High/Low output modes are concerned. It's not switching between pentode and triode.
 

Gene Ballzz

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While not an SC20, a kind member just posted a schematic to the SV20, which I believe to be the same power section, here:


Better save it quick, before the Marshal Document Police show up!

Just Sharin'
Gene
 

Jon Snell

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I found the schematic from Marshall, in my archieves.
Depending on your mains supply voltage:
On low power, the on/off switch position delivers about 135 volts DC to the HT; position CT on the output transformer.
On High power, the on/off switch delivers about 310 volts to CT. If the voltages are completely wrong, check FS1 and FS2 first.
The EL34s are self biasing, because they have a 150R cathode resistor that is shared between them with a 100u 63v electrolytic decoupling them.
I will not post the schematic as I respect copyright laws.
Hope that helps.


Edit; more info added ...
 
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PelliX

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I found the schematic from Marshall, in my archieves.
Depending on your mains supply voltage:
On low power, the on/off switch position delivers about 135 volts DC to the HT; position CT on the output transformer.
On High power, the on/off switch delivers about 310 volts to CT. If the voltages are completely wrong, check FS1 and FS2 first.
The EL34s are self biasing, because they have a 150R cathode resistor that is shared between them with a 100u 63v electrolytic decoupling them.
I will not post the schematic as I respect copyright laws.
Hope that helps.


Edit; more info added ...

Jon, if you have an SC20 schematic in these archives of yours, would you care to share?
 

Jon Snell

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Jon, if you have an SC20 schematic in these archives of yours, would you care to share?
Sorry, I cannot because they are copyright and I will lose my agency rights to documents and components at trade price. That is part of the service agent charter I signed when joining as a service company.
They are only 4 years old and still in production. When they are older, who knows.
 

junk notes

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Sorry, I cannot because they are copyright and I will lose my agency rights to documents and components at trade price. That is part of the service agent charter I signed when joining as a service company.
They are only 4 years old and still in production. When they are older, who knows.
:coffee:
 

PelliX

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Hi guys, My SC20H is not near the power output spec. It puts out may be max a few watts for switch in high and much less than 1W in the low switch position and guitar input connection. It is when both the Master and Pre-amp knobs maxed out. I checked EL34 plate voltage under 300vdc and it looks very low. I suspect some issues with the rectifier or PT. Can someone check Marshall's nominal bias voltages on EL34 of this amp. It would help a lot to track down the problem. Appreciate in advance.

Any chance of an update here and some :photos:?
 

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