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Thread: Redplate Ceriatone 20w Lead Bass / Marshall 2061

  1. #36
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    The Marshall circuit has a 750R screen supply resistor (the one between the two 50u caps). Your circuit looks like it has 470R. Changing it to 750R would lower the screen voltage a little but probably not enough. I'd try 1k5 or 2k2.

    Edit: Helmholtz posted as I was typing.

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    Here's an internal shot of what the OG from the manufacturer should look like.
    What do you mean with OG?

    Well I’m getting 121 out of the wall or from hot to ground on the iec but from the iec hot to Star ground point I get 108
    Does that mean that there is a voltage difference of 13V between 2 chassis ground points? This would mean bad contacts and a safety issue.
    (Regarding amp voltages only the voltage across the PT primary matters.)


    Please do resolder the horrible lug connections on the IEC receptacle (also a safety issue).

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    Quote Originally Posted by Dave H View Post
    The Marshall circuit has a 750R screen supply resistor (the one between the two 50u caps). Your circuit looks like it has 470R. Changing it to 750R would lower the screen voltage a little but probably not enough. I'd try 1k5 or 2k2.

    Edit: Helmholtz posted as I was typing.

    Click image for larger version. 

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    the marshall 2022 was for reference, this wuld technically be a 2061, ah so not increase the 22 ohm sag resistor but it's the one after it , so how much do i have to up the value here will drop the screen voltage by about 10 volts or so, betwen 1k5 and 2k2 we hope, i think i understand because that resistor obv is the one feeding pin 9 so it should drop the volts there

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    Quote Originally Posted by trip View Post
    Click image for larger version. 

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    the marshall 2022 was for reference, this wuld technically be a 2061, ah so not increase the 22 ohm sag resistor but it's the one after it , so how much do i have to up the value here will drop the screen voltage by about 10 volts or so, betwen 1k5 and 2k2 we hope, i think i understand because that resistor obv is the one feeding pin 9 so it should drop the volts there
    i'm really wondering if i ordered srtraight kit from ceriatone how that wouldn't red plate following the same schematic and using same tranformer

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    i'm really wondering if i ordered srtraight kit from ceriatone how that wouldn't red plate following the same schematic and using same tranformer
    For a given tube type redplating solely depends on tube dissipation. So with same cathode current and voltages all amps/circuits will make the EL84s redplate when plate dissipation exceeds around 14W.

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    Quote Originally Posted by Helmholtz View Post
    For a given tube type redplating solely depends on tube dissipation. So with same cathode current and voltages all amps/circuits will make the EL84s redplate when plate dissipation exceeds around 14W.
    I’m being told conflicting things on different forums. So ur saying the cathode resistor definitely meeds to be swapped or what is the solution?

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    Does this help? Not sure how to get pin 7 / or colts without actually going to pin 7 with the board in place Click image for larger version. 

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  8. #43
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    Try a 220 ohm sag resistor as suggested and see what B+ voltage and idle current is. Tweak from there if necessary.

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    Quote Originally Posted by trip View Post
    i'm really wondering if i ordered srtraight kit from ceriatone how that wouldn't red plate following the same schematic and using same tranformer
    Maybe it would. Or maybe this particular transformer is running at the high end of the allowed tolerance. You did mention the heater is at the high end of tolerance. Does the Ceriatone version of the PT have the same specs?
    I would think that most people would adjust the cathode resistor to get idle dissipation within reason and not worry about the plate voltage. I've seen plenty of circuits running EL84 plates that high or higher. Not saying it's good, but it's not that uncommon.

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    So ur saying the cathode resistor definitely meeds to be swapped or what is the solution?
    To avoid the redplating tube dissipation must be reduced. This can be achieved by either lowering B+ (e.g. by increasing the sag resistor) or by lowering cathode current (increasing cathode resistor and/or lowering screen voltage) - or a combination of both.

    A cathode resistor of 220R or more might cause unwanted crossover distortion.

    I think the solution suggested by Dave H (220R sag resistor and 150R cathode resistor) looks like a reasonable compromise, though still on the hot side.. I guess he verified that this tames the redplating.

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  11. #46
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    There was another similar build (2061) with similar issue. His PT was spec'd at 320-0-320 and yet he had pretty much the same B+ (387V). So that seems a bit odd if yours is supposed to be spec'd lower.
    https://music-electronics-forum.com/...ad.php?t=14380

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    Quote Originally Posted by g1 View Post
    There was another similar build (2061) with similar issue. His PT was spec'd at 320-0-320 and yet he had pretty much the same B+ (387V). So that seems a bit odd if yours is supposed to be spec'd lower.
    https://music-electronics-forum.com/...ad.php?t=14380
    I read the thread and think it wasn't quite clear what the loaded PT secondary voltage was.
    And it's not clear if the 2x290Vrms quoted here is PT spec or measured value.
    Exact values will depend on transformer DCR, sag resistor and actual idle current.

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    Last edited by Helmholtz; 05-21-2020 at 09:40 PM.
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