Marshall 2061 - Ceriatone Layout Redplate Help

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geoff 1965
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Re: Marshall 2061 - Ceriatone Layout Redplate Help

Post by geoff 1965 »

is your pin9 voltage still higher than pin7 on the el84's? the 470ohm resistor is your screen resistor so you need to check its wired correctly i.e. your B+ comes through the sag resistor and into the output transformer centre tap then passes through the 470ohm into the screens.
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Re: Marshall 2061 - Ceriatone Layout Redplate Help

Post by l4ent »

geoff 1965 wrote:
Fri 05/22/20 11:44 am
is your pin9 voltage still higher than pin7 on the el84's? the 470ohm resistor is your screen resistor so you need to check its wired correctly i.e. your B+ comes through the sag resistor and into the output transformer centre tap then passes through the 470ohm into the screens.
Screen Shot 2020-05-22 at 1.12.56 PM.png
geoff 1965 wrote:
Fri 05/22/20 11:44 am
your B+ comes through the sag resistor and into the output transformer centre tap then passes through the 470ohm into the screens.
I'm seeing the sag resistor go to standby, standby goes to the 470ohm which is connected to both the OT CT and the end of the 470 ohm screen
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Re: Marshall 2061 - Ceriatone Layout Redplate Help

Post by l4ent »

now im getting

V3 pin 7 371
pin 9 378

V4 7 371
pin 9 378

so yes 9 is still higher
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geoff 1965
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Re: Marshall 2061 - Ceriatone Layout Redplate Help

Post by geoff 1965 »

okay so what voltage do you have going into and the other side of the 470 ohm?
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Re: Marshall 2061 - Ceriatone Layout Redplate Help

Post by l4ent »

381 and 376
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Re: Marshall 2061 - Ceriatone Layout Redplate Help

Post by JMPGuitars »

Add a 1K 3W screen resistor. Resisting the resistor is futile.

That other 470R resistor looks like it decouples the B+ node with the cap next to it. It's not specifically a screen resistor.
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Re: Marshall 2061 - Ceriatone Layout Redplate Help

Post by zaphod_phil »

geoff 1965 wrote:
Fri 05/22/20 11:44 am
your B+ comes through the sag resistor and into the output transformer centre tap then passes through the 470ohm into the screens.
Are you describing an ultralinear configuration?
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Re: Marshall 2061 - Ceriatone Layout Redplate Help

Post by geoff 1965 »

looking at the layout/schematic ZP the only purpose of the large 470R is to drop the B+ going to the screens of both el84's or am i missing something?
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Re: Marshall 2061 - Ceriatone Layout Redplate Help

Post by zaphod_phil »

sorry, I had misunderstood your description. I really hate having to reverse engineer from layout drawings, but I think I've now figured out what you meant. IMO 470 ohms is nowhere near high enough to drop the screen voltage to a safe level. I'm attaching the original Marshall schematic here, which shows a 750 ohm dropper resistor feeding the screens. A step in the right direction, but still much too little in my opinion. Unless screen grids are taken care of, all bets are off as well as the rest of the power stage goes, and please note that screen grids burn out a heck of a lot more easily than anodes (aka "plates"). So I would strongly recommend a 2.2k 5W dropper resistor to get things more where they should be and under control, perhaps also with a 100R blocker resistor connected to each EL84's screen grid pin.

Once the screen grids are at a safe voltage, we can run the EL84s at the 400V approx required for 20W output. I would then also recommend a larger value cathode resistor, say 180 to 220 ohms to keep anode dissipation in check, along with a 1000uF to 2200uF cathode cap.
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Re: Marshall 2061 - Ceriatone Layout Redplate Help

Post by zaphod_phil »

And here for comparison is a schematic of the tremolo version of the 20W lead amp. This one has a shared 100 ohm resistor feeding the two screen grids, after the 750 ohms screen voltage dropper .
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Re: Marshall 2061 - Ceriatone Layout Redplate Help

Post by l4ent »

zaphod_phil wrote:
Sun 05/24/20 5:38 pm
sorry, I had misunderstood your description. I really hate having to reverse engineer from layout drawings, but I think I've now figured out what you meant. IMO 470 ohms is nowhere near high enough to drop the screen voltage to a safe level. I'm attaching the original Marshall schematic here, which shows a 750 ohm dropper resistor feeding the screens. A step in the right direction, but still much too little in my opinion. Unless screen grids are taken care of, all bets are off as well as the rest of the power stage goes, and please note that screen grids burn out a heck of a lot more easily than anodes (aka "plates"). So I would strongly recommend a 2.2k 5W dropper resistor to get things more where they should be and under control, perhaps also with a 100R blocker resistor connected to each EL84's screen grid pin.

Once the screen grids are at a safe voltage, we can run the EL84s at the 400V approx required for 20W output. I would then also recommend a larger value cathode resistor, say 180 to 220 ohms to keep anode dissipation in check, along with a 1000uF to 2200uF cathode cap.
So I upped the bias resistor to 220 ohms. My power tubes no longer redplate, and the math and readings are below.

V1 V2
2. 19.9 16.5
3. 14.98 14.98
7. 374 374
9. 380 380

Voltage across cathode res - 15.01
220 ohm cathode res
EL84
plate to cathode 359


soo that means i'm at 96.7% dissipation per tube / 11.6 watts ?





1) Why would I up the cathode cap? Won't this alter the tone / couldn't I switch between 1000, 2200 and the current 42uf or is there an issue keeping it at 42?

2) That screen dropper remains at 470. Should it be increased and if so is the reason only to drop the voltage? Is this necessary?

3) Screen resistors? Is this the last step here to drop those last few volts / is it really necessary? What am i sacrificng by not having them or is it really just a protection thing incase tubes fail?

I met an old amp guy who says that the whole plate vs cathode can be equal or +- a bit and be fine as long as you bias the tube right. Is there really an issue needing to be corrected? So far I've run the amp for about 3 hours without issue now. No redplating, sounds great with the 220 ohm resistor. Are these next steps really necessary and if so which are crucial as while yes it is running great now i'm not niave I understand more needs to be done but not clear exactly what since just upping the boas resistor solved my redplate issue and it doesn't sound dead or lifeless using such a high cathode resistor at least to my ears (obv not as good as 130% dissipation but)
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Re: Marshall 2061 - Ceriatone Layout Redplate Help

Post by zaphod_phil »

l4ent wrote:
Fri 06/19/20 9:15 am

So I upped the bias resistor to 220 ohms. My power tubes no longer redplate, and the math and readings are below.

V1 V2
2. 19.9 16.5
3. 14.98 14.98
7. 374 374
9. 380 380

Voltage across cathode res - 15.01
220 ohm cathode res
EL84
plate to cathode 359


soo that means i'm at 96.7% dissipation per tube / 11.6 watts ?
That's still too hot and will shorten tube life. You should aim for around 80%, and absolutely no more than 85% dissipation.
Your screen voltage is still too high - 300V is the max for an EL84.
l4ent wrote:
Fri 06/19/20 9:15 am
1) Why would I up the cathode cap? Won't this alter the tone / couldn't I switch between 1000, 2200 and the current 42uf or is there an issue keeping it at 42?

With a 220 ohm cathode resistor and a small cathode cap the cathode bias voltage will spike high on signal peaks, which will bias the tubes too cold and into starvation on those peaks, giving you a ragged distortion tone. The huge cathode cap tames those spikes to give you a better sounding amp, with a tighter bass response too. FYI this mod was actually recommended by one of Marshall's design engineers who used to post on this site, and it also works great on other cathode-biased amps. Many builders have tried this mod and love what it does for their amps. :D
l4ent wrote:
Fri 06/19/20 9:15 am
2) That screen dropper remains at 470. Should it be increased and if so is the reason only to drop the voltage? Is this necessary?
Depends how long you want your power tubes to survive :)
l4ent wrote:
Fri 06/19/20 9:15 am
3) Screen resistors? Is this the last step here to drop those last few volts / is it really necessary? What am i sacrificing by not having them or is it really just a protection thing in case tubes fail?
You're sacrificing tube life time! Yes, it is a protection thing to prevent your tubes from failing in the first place.
l4ent wrote:
Fri 06/19/20 9:15 am
I met an old amp guy who says that the whole plate vs cathode can be equal or +- a bit and be fine as long as you bias the tube right.
I'm not going to tell you my age, but there's folks who might also call me an "old amp guy" - better than being called an old fart. :lol: But he is actually right that high anode voltages aren't so much the issue, provided you keep both screen voltage and anode dissipation in check. So you first need to get your screens at under 300V. Keep in mind that screen grids are a lot more fragile than anodes (aka "plates").
l4ent wrote:
Fri 06/19/20 9:15 am
Is there really an issue needing to be corrected? So far I've run the amp for about 3 hours without issue now. No red-plating, sounds great with the 220 ohm resistor. Are these next steps really necessary and if so which are crucial as while yes it is running great now. i'm not niave I understand more needs to be done but not clear exactly what since just upping the bias resistor solved my redplate issue and it doesn't sound dead or lifeless using such a high cathode resistor at least to my ears (obv not as good as 130% dissipation but)
Yes - your tubes are still biased too hot, even though they're not red-plating. And your screen voltage is still too high. It will be easier to get the amp biased correctly, once the screen voltage is l
below the anodes, like it's meant to be.
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Re: Marshall 2061 - Ceriatone Layout Redplate Help

Post by zaphod_phil »

l4ent wrote:
Wed 05/20/20 3:19 pm

I’m trying to understand why if everything is specd exactly to the layout, why these mods are necessary or If something is off.
A layout from Ceriatone is not authoritative. I am actually the person who introduced Nik at Ceriatone to Marshall amps.Before that they were just making eyelet boards for Fender tweed amps, and I asked him to make me an 18W board. You should go by the Marshall schematic I posted earlier, as well as the EL84 spec sheet - https://frank.pocnet.net/sheets/030/e/EL84.pdf
l4ent wrote:
Wed 05/20/20 3:19 pm
I could use the zener diodes on the center tap to drop b+ Which would be a fun little thing but according to the charts the high b+ isn’t a problem, should I do this?
You shouldn't need to, as high anode voltage is the norm for Marshall 20W amps - just as long as you reduce the screen voltage, as already discussed.
l4ent wrote:
Wed 05/20/20 3:19 pm
it’s that the screen is higher which is causing to redplate
. Yes exactly!
l4ent wrote:
Wed 05/20/20 3:19 pm
understand how adding them will help but concerned about tone as this circuit just sounds so good and they manage to run without em. Do I need to add them?
Only if you don't want red-plating! Any difference in tone will be minimal, except with burnt-out tubes, you won't get no tone! :)
l4ent wrote:
Wed 05/20/20 3:19 pm
I’m also thinking that based on these responses, just dropping the screen voltage by adding these resistors (while yes changing tone a bit) will fix the red plate? Is that the simple solution to start?
I’m not sure where to start here.
Yes, that's exactly where to start. Until then, all bets are off.
l4ent wrote:
Wed 05/20/20 3:19 pm
I see these “solutions” but it strikes me odd it needs to be so different from all of these specs I’m seeing. Shouldn’t it be achievable while being close to the product sold by these companies?
Then you're clearly looking at some wrong specs!

FYI I was brought in a burnt-out Marshall 20W rack-mount power amp for repairs. Guess what, it lacked screen grid resistors, while running its EL84s at around 400V. 8O I replaced the burnt-out parts and added in 1k 3W screen grid resistors, and the amp ran fine, with a very sweet tone!
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Re: Marshall 2061 - Ceriatone Layout Redplate Help

Post by l4ent »

Amp was running fine as in not redplating but I added in two 5W 750 ohm resistors on pin 9, didn’t seem to do much if anything to the voltage. Cathode is still 220ohm. If I drop it were back into redplate territory. Any ideas?
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