Redplate Ceriatone 20w Lead Bass / Marshall 2061

[Dave H]

The original 2061 ran the EL84s way over data sheet spec. too. With nearly 400V B+ and a 125R cathode resistor it's almost guaranteed to red plate. That's how they were able to get 20W out of two cathode biassed EL84s.

There's no easy way to calculate the sag resistor value because it's between the rectifier and first filter cap where the current waveform is a series of narrow pulses. I found the B+ voltage drop to be quite a bit more than 20V when the resistor was between the rec. and first cap. It's not easy to calculate the resistor power dissipation either. I measured it (true RMS) and it was 6W with the amp at 15W output so a 10W resistor should be OK. I'd try a 220R sag resistor to start with then adjust it (if required) to make the plate dissipation 12W or lower.

[Helmholtz]

There's no easy way to calculate the sag resistor value because it's between the rectifier and first filter cap where the current waveform is a series of narrow pulses.

You're absolutely right. I didn't consider the position of the sag resistor and its influence on conduction angle. One could probably use PSUD2 for a better calculation.

[trip]

The original 2061 ran the EL84s way over data sheet spec. too. With nearly 400V B+ and a 125R cathode resistor it's almost guaranteed to red plate. That's how they were able to get 20W out of two cathode biassed EL84s.

There's no easy way to calculate the sag resistor value because it's between the rectifier and first filter cap where the current waveform is a series of narrow pulses. I found the B+ voltage drop to be quite a bit more than 20V when the resistor was between the rec. and first cap. It's not easy to calculate the resistor power dissipation either. I measured it (true RMS) and it was 6W with the amp at 15W output so a 10W resistor should be OK. I'd try a 220R sag resistor to start with then adjust it (if required) to make the plate dissipation 12W or lower.

I see the original specs were high but i can't imagine if i purchased this amp it would automatically redplate? The transformer to my knowledge is specd correct so i'm reallt interested in understanding WHY things aren't matching the layout.

1) I'll definitley try a 220 there, would you recommend moving the resister off the board onto the chassis / using metal resistor? i think 15-20 watt would be good.
2) would there just be another transofrmer i could swap to make everything work? it boggle my mind that it would be so different from what both ceriatone and marshall uses so again i just would love to understand the WHY everything is so high or is it just because the specs WERE for 110 back in the day?

i'm goig to try the sag resistor as a temp solution, but would like to do this as a possibilty sincde it seems pretty simple, any advice? I would need what 2 or 3 of these mounted on the chassis? (as to not affect the sag as much for another option)

[trip]

I see the original specs were high but i can't imagine if i purchased this amp it would automatically redplate? The transformer to my knowledge is specd correct so i'm reallt interested in understanding WHY things aren't matching the layout.

1) I'll definitley try a 220 there, would you recommend moving the resister off the board onto the chassis / using metal resistor? i think 15-20 watt would be good.
2) would there just be another transofrmer i could swap to make everything work? it boggle my mind that it would be so different from what both ceriatone and marshall uses so again i just would love to understand the WHY everything is so high or is it just because the specs WERE for 110 back in the day?

i'm goig to try the sag resistor as a temp solution, but would like to do this as a possibilty sincde it seems pretty simple, any advice? I would need what 2 or 3 of these mounted on the chassis? (as to not affect the sag as much for another option)[ATTACH=CONFIG]58561[/ATTACH]

looking here my voltages are kinga on spec other than pin 9. have to reduce screen voltage, how?

[Helmholtz]

have to reduce screen voltage, how?

THe obvious difference is the higher value (750R instead of 470R ) dropper resistor between the first and second filter cap. But that would only lower your screen voltage by around 4V. I don't think that's enough to avoid the redplating.
In other words the 2022 most likely also had a redplating problem and "ate" tubes as the EL84s were running at roughly 16W plate dissipation according to the voltages in the schematic. Or those original tubes were more forgiving.

[Dave H]

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.

[Helmholtz]

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).

[trip]

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.

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

[trip]

[ATTACH=CONFIG]58564[/ATTACH]

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

[Helmholtz]

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.

[trip]

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?

[trip]

Does this help? Not sure how to get pin 7 / or colts without actually going to pin 7 with the board in place

[The Dude]

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

[g1]

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.

[Helmholtz]

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.