Redplate Ceriatone 20w Lead Bass / Marshall 2061

[Helmholtz]

I don't want to be just guessing i figured there'd be a math formula to figure this out

Probably not impossible but I don't know of a simple formula that allows to calculate the quantitative influence of screen voltage on cathode current with cathode degeneration and out-of-spec tube operation..
One might try a graphical construction using the transfer chart with a 2 step extrapolation for plate and screen voltage, but results won't be very precise.

i need to go back to like 150 ohm for the bias resistor but increase the sag resitor from 30ohm to something like 220ohm ? is'nt this going to affect the tone a lot, why would it be sooo off spec from the layout

I would leave the 165R (not much different from 150R regarding cathode current) resistor for now and use a sag resistor large enough to reduce B+ and screen voltages to values below 360V.
EL84 datasheets limit both plate and screen voltages to 300V max. I am not in favor of exceeding these limits by more than 15% to 20%.
And I don't think that tubes need to be abused/tortured to sound good.

There may be other opinions/experiences.

[trip]

Thank you. Any other thoughts kn the subject?
Won’t upling the sag resistor change tone, so how can we do this with zener diodes or otherwise?

I’m so confused because the layout and parts used and transformer all “should” be operating right.

Before I run to the electric store I want to make sure I get the right parts to get this baby running right I feel like I’m so close

Probably not impossible but I don't know of a simple formula that allows to calculate the quantitative influence of screen voltage on cathode current with cathode degeneration and out-of-spec tube operation..
One might try a graphical construction using the transfer chart with a 2 step extrapolation for plate and screen voltage, but results won't be very precise.



I would leave the 165R (not much different from 150R regarding cathode current) resistor for now and use a sag resistor large enough to reduce B+ and screen voltages to values below 360V.
EL84 datasheets limit both plate and screen voltages to 300V max. I am not in favor of exceeding these limits by more than 15% to 20%.
And I don't think that tubes need to be abused/tortured to sound good.

There may be other opinions/experiences.

[Helmholtz]

Won’t upling the sag resistor change tone, so how can we do this with zener diodes or otherwise?

I mentioned the zener diodes because they won't increase sag.
But you might actually like the extra sag with the resistor because it gives a nice compression effect which helps sustain.
Generally (hot) cathode biased amps tend to have less sag than fixed biased amps, because there is less variation of PS current between idle and full power.

Both resistor and zener will require adequate heat sinking.

A third option would be a power scaling circuit. This would allow for adjusting voltage but otherwise has no benefits over a zener.

[The Dude]

And remember, when figuring plate voltage in a cathode biased amp, you need to subtract the cathode voltage from plate voltage measured to ground. The tube rating is plate to cathode. The tube has no idea if the cathode is grounded or not.

[trip]

And remember, when figuring plate voltage in a cathode biased amp, you need to subtract the cathode voltage from plate voltage measured to ground. The tube rating is plate to cathode. The tube has no idea if the cathode is grounded or not.

I was using the plate to cathode Voltage Method to make that measurement per how to bias an amp on rob robinette website.

I’m just really confused why the specs are so out of wack given it’s following the layout almost exactly. I hear everyone saying drop the plate voltage / screen voltage and all of these options but it’s information overload. Kinda hoping for a do x to get y kind of answer because it seems like I’m only out of spec where it matters for a few things so I’m not sure what my next step is here i see a lot of “possible options” but knowing that it seems almost everyone has this issue with this circuit yet it is a circuit they sell and make money on there has to be a way to make this work with this transformer. “Making a zener” okay but I’m looking for more you need x ohms @ y Watts because without having the math explained to me like a baby learning algebra I literally have no idea and it’s going to result in my guessing which components to switch until it works

[Helmholtz]

You have a total power tube current of 14.23V/165 Ohm = 0.086A or 86mA. Adding 4mA for the small tubes gives a total PS current of 90mA. A 220R sag resistor will drop 220R x 0.09A = 19.8V. So B+ will be about 20V lower.

[The Dude]

Sorry for the silly question, but are the mains correctly wired to the 120V tap and not the 100V tap of the PT?

[Justin Thomas]

Also, have you tried new/different tubes? A lotbof times a tube that has redplated in the past will continue to do so be ause the elements msy have shifted slightly, making it not behave according to original spec.

Did you buy this new & DIY, or from someone else?

Justin

[Helmholtz]

Seems the Marshall 2061 has similar high B+:
https://music-electronics-forum.com/...l=1#post123121

[trip]

You have a total power tube current of 14.23V/165 Ohm = 0.086A or 86mA. Adding 4mA for the small tubes gives a total PS current of 90mA. A 220R sag resistor will drop 220R x 0.09A = 19.8V. So B+ will be about 20V lower.

Here's an internal shot of what the OG from the manufacturer should look like. They use just the 30 ohm. I stared with 22 and am at around 34 right now so that should be fine but i will definitely try upping to 220. This math is VERY helpful so thank, does it affect anything else other than b+? If we lower b+ by 20v, that should get me out of redplate zone?

Sorry for the silly question, but are the mains correctly wired to the 120V tap and not the 100V tap of the PT?

What's the best & safest way to check this? Entirely possible, can't rule out anything.

Also, have you tried new/different tubes? A lotbof times a tube that has redplated in the past will continue to do so be ause the elements msy have shifted slightly, making it not behave according to original spec.

Did you buy this new & DIY, or from someone else?

Justin

i baked the sovteks it came on cause redplate without realizing, tried em again and got some smoke so no good, got new pair of grove tube "7". DOn't immediately start to plate but deifnitley do, started making mods and adjusting circuit to fix after realizing it wasn't the tubes. It was completed from someone else but i have followed every component and circuit and it all seems to line up according to the specified layout, if i built it myself with these parts here it would be identical, but i find it odd that the "official layout" of ceriatone's model has differences to the "official gut shot" provided from ceriatone.

a shot before i upped the sag and bias resistors

A photo of an og marshall if it helps

[The Dude]

......What's the best & safest way to check this? Entirely possible, can't rule out anything.
[ATTACH=CONFIG]58550[/ATTACH]

It's hard to tell for sure, but that looks wrong from what I can see assuming the transformer is like the diagram in your first post. The mains go to the first and third wire in the diagram and don't appear that way in the picture. The picture looks like the first wire is tied off instead of connected. Also, the soldering on the IEC connector looks poor and possibly wrong. It's hard to tell since we can't see the whole IEC connector in the picture. It almost looks like neutral of the connector is grounded, but I can't tell for sure. One wire looks barely soldered at all. Is there a wiring diagram on the transformer itself, or can you give us the number on the transformer so we can check the datasheet? You definitely need to double check the mains input wiring. I hope the soldering in the rest of the amp isn't as bad as it is on the IEC connector!

[trip]

[trip]

It's hard to tell for sure, but that looks wrong from what I can see assuming the transformer is like the diagram in your first post. The mains go to the first and third wire in the diagram and don't appear that way in the picture. The picture looks like the first wire is tied off instead of connected. Also, the soldering on the IEC connector looks poor and possibly wrong. It's hard to tell since we can't see the whole IEC connector in the picture. It almost looks like neutral of the connector is grounded, but I can't tell for sure. One wire looks barely soldered at all. Is there a wiring diagram on the transformer itself, or can you give us the number on the transformer so we can check the datasheet? You definitely need to double check the mains input wiring. I hope the soldering in the rest of the amp isn't as bad as it is on the IEC connector!

transformer is pt-290-50 ht-6179-50.

yes the first of the 3 black wires is taped off looking from this angle wires 2 and 3 are used (or 1 and 2 depending on your orientation)



If it was wrong, wouldnt things blow up? i'm surprised it be working if they are hooked up wrong. definitley want to know how to test and check if it's right but it turns and sounds fine other than well the redplating

marshall schematic

[The Dude]

OK. I believe that's a Heyboer PT. Here's the diagram.

https://www.mojotone.com/transformer...ISH-18W-PT.pdf

So, the transformer is wired across a 120V tap. There is not a 100V primary tap, so we're good there. Can you upload a full picture that includes the entire IEC connector?

Edit: Oh, never mind. I zoomed the picture and just saw the white wire on the IEC connector. It looks right except for poor solder. My apologies for the rabbit hole.

[trip]

not sure why I need to drop the voltage so much when this is the right transformer?

My only options are A- upling sag resistor to 220
B) making some zener diodes to lower b+

?

Nothing else could be awry causing this red plate

I mean based on the bias calc I’m 120% so upling the cathode would not be good solution because the b+ is already so high and it would make it sound like poo, right ?