Marshall 2061 - Ceriatone Layout Redplate Help
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Re: Marshall 2061 - Ceriatone Layout Redplate Help
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
geoff 1965 wrote: ↑Fri 05/22/20 11:44 amis 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.
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 screengeoff 1965 wrote: ↑Fri 05/22/20 11:44 amyour 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
now im getting
V3 pin 7 371
pin 9 378
V4 7 371
pin 9 378
so yes 9 is still higher
V3 pin 7 371
pin 9 378
V4 7 371
pin 9 378
so yes 9 is still higher
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Re: Marshall 2061 - Ceriatone Layout Redplate Help
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
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.
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
Are you describing an ultralinear configuration?geoff 1965 wrote: ↑Fri 05/22/20 11:44 amyour 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
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
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.
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
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
So I upped the bias resistor to 220 ohms. My power tubes no longer redplate, and the math and readings are below.zaphod_phil wrote: ↑Sun 05/24/20 5:38 pmsorry, 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.
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
That's still too hot and will shorten tube life. You should aim for around 80%, and absolutely no more than 85% dissipation.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 ?
Your screen voltage is still too high - 300V is the max for an EL84.
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.
Depends how long you want your power tubes to survive
You're sacrificing tube life time! Yes, it is a protection thing to prevent your tubes from failing in the first place.
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. 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").
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 ll4ent wrote: ↑Fri 06/19/20 9:15 amIs 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)
below the anodes, like it's meant to be.
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Re: Marshall 2061 - Ceriatone Layout Redplate Help
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
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.
. Yes exactly!
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!
Yes, that's exactly where to start. Until then, all bets are off.
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. 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
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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Re: Marshall 2061 - Ceriatone Layout Redplate Help
Here's what I did for my clone. I used a Hammond 270DX PT (undersized), coupled with a salvaged 20W Davis 201 OT. My B+ worked out to be 371 VDC. The EL84 anodes are roughly 360 VDC, screens about 10 volts lower. In order to keep the preamp voltages up near the original Marshall schematic, I reduced the value of the screen filter dropping resistor to 470R. I however fit 1K 2W screen resistors to keep the EL84 screen voltage below that of the plates.
I fit separate power tube cathode resistors and capacitors, that work out to a single 150R resistor and roughly 100 uF capacitor.
I've attached the schematic I prepared for this amp and the annotated voltages. Never a hint of redplating and this amp turned out to be a real screamer. Plenty loud enough for my basement workshop (where I play through a 4x10" cab) and no need for an attenuator, but I only crank it for short periods. I love it. I hope this helps.
I fit separate power tube cathode resistors and capacitors, that work out to a single 150R resistor and roughly 100 uF capacitor.
I've attached the schematic I prepared for this amp and the annotated voltages. Never a hint of redplating and this amp turned out to be a real screamer. Plenty loud enough for my basement workshop (where I play through a 4x10" cab) and no need for an attenuator, but I only crank it for short periods. I love it. I hope this helps.
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