# Supro S6616 Schematic Voltage Question



## Todd MacCulloch (Mar 8, 2018)

I picked up an old PEPCO model 205 amplifier.

Based on the circuit it looks like a clone of the Supro S6616. See schematic PDF 

My amp is noisy, breaks up early and the tremolo doesn’t work so I have some investigating to do. Also there was some _creative_ and messy _repair_ done to the amp that I need to verify.


That aside, my question involves some voltages from the Supro schematic that I don’t understand. 
Here is an annotated screen shot of the schematic








Looking at the last node in the power filtering section we see *130 V *(highlighted as a red square).

That is the same voltage (130 V) in the schematic for *V1-pin_6* and *V2-pin_6 *(green squircles) - ok, that makes sense.

But the schematic also shows* 200 V* for *V1-pin_1* and* 220 V* for *V2-pin_1* (purple squircles).

What is making the plate voltage higher on that side of V1 and V2?

I can see *V2-pin_1* does not have the 270K plate resistor that *V1-pin_1* and *pin_6* and *V2-pin_6* has.

But I don’t understand how the plate voltage can be higher then the *130V* feeding that branch.



Thanks


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## nonreverb (Sep 19, 2006)

If your amp has the grey coloured that say "oil" coupling capacitors, they need to go....all of them.
Probably needs B+ filter caps too.


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## Todd MacCulloch (Mar 8, 2018)

nonreverb said:


> If your amp has the grey coloured that say "oil" coupling capacitors, they need to go....all of them.
> Probably needs B+ filter caps too.


Part of the creative repair was someone disconnected the can-cap and hung a free floating network of radial caps. I plan on tying them down to a terminal strip. I tested the can cap and one of the sections was bad and given the age replacement was a good idea. It looks like they messed up the filtering branch too but that will be easy to address.

most of the other caps are big old ceramic disks.

I’ll post some pics later when I get into changing stuff.


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## cboutilier (Jan 12, 2016)

Mmm. I love old Pepco amps!


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## jb welder (Sep 14, 2010)

Todd MacCulloch said:


> my question involves some voltages from the Supro schematic that I don’t understand.


Lots of errors with those voltages. A bit of ohms law back and forth solving for current and some of the numbers are just not possible.
Whether the supply node voltages or the circuit voltages are more correct is hard to say.


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## Latole (Aug 11, 2020)

"But the schematic also shows 200 V for V1-pin_1 and 220 V for V2-pin_1 (purple squircles).
What is making the plate voltage higher on that side of V1 and V2?.

Answer ; bias resistor value

Your amp issue look not a voltage issue;
You have to sketch a schematic as is first. You can't truss Supro schematic 

Put know good tubes
Tremolo ; most of the time is circuit is well built with no error , the culprit are the four tremolo capacitors ; replace them all.

Noisy ; Power transformer missing, tubes heater bad wiring, amp bad wiring dress
Input jacks ; bad ground or no N.C contact to ground

Breakup early ; check tubes, check 6V6 bias
There is no negative feed back.


I fix many of these amps. I have four of them.
I found some mistake in their circuit.
With some knowledge and few hours of work, they are very good amp

Show good picture of the circuit


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## Latole (Aug 11, 2020)

Pepco 205 ; Picture from Google;


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## Latole (Aug 11, 2020)

" But I don’t understand how the plate voltage can be higher then the 130V feeding that branch."
*____*
Supro schematic is wrong.

You have to determine the voltage of your amp according to its own power supply circuit. Don't rely on assuming a clone. Drawing the schematic is essential


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## cboutilier (Jan 12, 2016)

Latole said:


> Pepco 205 ; Picture from Google;
> 
> View attachment 342145


I have the step-up version with Reverb and Trem.


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## Todd MacCulloch (Mar 8, 2018)

Thanks folks. I figured the schematic voltages were off, but I wanted to check in case there is something I was missing before I changed anything.

I found Merlin’s book to have a great explanation of the power section.

i have drawn my own schematic using scheme-it and I did a layout using diylc. That’s why I am pretty sure it’s full-on clone of the Supro - pretty much every resistor and capacitor value is a match.

i’ll make a separate post when I start the repairs.


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## cboutilier (Jan 12, 2016)

Todd MacCulloch said:


> Thanks folks. I figured the schematic voltages were off, but I wanted to check in case there is something I was missing before I changed anything.
> 
> I found Merlin’s book to have a great explanation of the power section.
> 
> ...


I wouldn't doubt it. Have you also compared it to a Vibrochamp?

My Model 730 is a near identical copy of a Princeton Reverb.


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## Latole (Aug 11, 2020)

Amp noisy may have many issues.

Few easy to fix:
1-add artificial center tap to heater with two 100 ohms resistors
2-replace 2 input jacks.
3-add a ground with a 3 prongs power cord
4- Olds plate resistors may come noisy with time ( hissss)


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## jb welder (Sep 14, 2010)

The heater winding has no center-tap to ground, but one side is grounded like old champs and some other tweed stuff.
If you want to add the artificial center-tap you *must *first disconnect the heaters from ground*.*


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## Latole (Aug 11, 2020)

jb welder said:


> The heater winding has no center-tap to ground, but one side is grounded like old champs and some other tweed stuff.
> If you want to add the artificial center-tap you *must *first disconnect the heaters from ground*.*



You are 100 % right, I forgot to tell. 
Wishing to give more explanation later if Tod Mac C don't know what is a artificial center tap

I do that on 2 amps and amps become more quiet.


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