Filtering help on a Dirty Shirley Marshall Style amp...

[cluster]

Hello all,

Disclaimer: I'm a newb at certain aspects of amp design.

So, I'm in the middle of designing my own Marshall style amp based on the spirit of a Dirty Shirley layout but with my own tweets, here and there. Just need some help understanding the power/mains filtering arrangement here. Looking at the attached gut shots, the DS is basically a JCM800 front end preamp (with tweaks) merged with a JTM45 power section. My question is on the 16/16uf mains cap mounted on the side of the chassis. Can anyone comment on how this can is wired? Series or Parallel? What are the 2 resistors wired from each lug to ground and what is the purpose of these resistors? Next, what is that white cap going to ground off the rectifier and what is it's purpose? Also, why isn't there a flying 1k 5w resistor off the 8.2K dropdown resistor going to the 470R screen resistors??



without my edits...

[Chuck H]

The 16/16 can sections aren't in series or parallel. I think one section of that can is the screen grid filter and the other is for the PI node. The resistors are probably one HV rail dropper and one HV bleed resistor. Though I don't think the screen node is a good place for this since it can be decoupled from the main filter with the standby switch, so I could be wrong. There is no flying resistor to between the rail resistor and the individual screen resistors because there shouldn't be. You would still need the filter attached to the screen resistor end. So there does seem to be a dropping resistor I can't locate. The white cap next to the rectifier is probably a HV hash filter of some kind.

[cluster]

thanks for that input. what is weird is that normally their is a 32/32uf in that position (not a 16/16uf). Maybe this one got swapped out for the one on top next to the rectifier?? Usually the one next to the rectifier is the 16/16uf. See attached layout.



OTOH I found this layout on the web that has a 16/16uf on the side like the Dirty Shirley.

The 16/16 can sections aren't in series or parallel. I think one section of that can is the screen grid filter and the other is for the PI node. The resistors are probably one HV rail dropper and one HV bleed resistor. Though I don't think the screen node is a good place for this since it can be decoupled from the main filter with the standby switch, so I could be wrong. There is no flying resistor to between the rail resistor and the individual screen resistors because there shouldn't be. You would still need the filter attached to the screen resistor end. So there does seem to be a dropping resistor I can't locate. The white cap next to the rectifier is probably a HV hash filter of some kind.

[Chuck H]

Yes I've seen that some amps use the 16/16 and others the 32/32. It also seems that some amps DO use that can as the main filter and the one parked near the rectifier as the next HV node.?. I'm sure some Marshall guru expert guy could tell you why with years, the serial numbers and current owners I wouldn't worry about that. I can't see just how things are wired on that filter, but it's possible that the "other" resistor there is the one I couldn't locate. That is, the plate to screen HV rail resistor. Ordinarily you'd see a 5W or 10W part for that purpose and that looks like a 2W or 3W. That amp doesn't use a choke like the Marshall as far as I know.

[cluster]

Found this...

Yes I've seen that some amps use the 16/16 and others the 32/32. I'm sure some Marshall guru expert guy could tell you why with years, the serial numbers and current owners I wouldn't worry about that. I can't see just how things are wired on that filter, but it's possible that the "other" resistor there is the one I couldn't locate. That is, the plate to screen HV rail resistor. Ordinarily you'd see a 5W or 10W part for that purpose and that looks like a 2W or 3W. That amp doesn't use a choke like the Marshall as far as I know.

[cluster]

Found something on the sloclone forum about using that cap for elevated heats. thoughts??

[g1]

Could be.
You need to clarify what you are trying to achieve.
The pics in post #1 are an actual Friedman Dirty Shirley I assume?
You are trying to reverse engineer it?
Are the pics in post #5 the same amp or an exact same revision?

There are thousands of variants of basic Marshall circuits out there, pick 1 you want to ask questions about so that we can discuss the same thing. There is no point in looking at Ceriatone (or any other) layouts unless you know them to be the circuit you want to discuss.
32 vs 16 filter caps would be simple 'flavouring' and you will find examples of both. We should hone in on the 1 particular circuit you are concerned with.

[cluster]

Thanks for your input. I was looking at that Freidman (Dirty Shirley) specifically because of the JCM800 preamp / JTM45 Power section mix. I want to do a similar thing but then I saw that cap and couldn't figure it out. But after some study it does appear to be used (half of it anyways) as a way of elevating the heater voltage to eliminate hum. Can't tell what the resistor values are on that cap. It could be that that design is prone to hum, so elevated heater might be required. Just learning. I've never installed elevated heaters before, so this would be new for me. Just read the lord valve page on it.

Could be.
You need to clarify what you are trying to achieve.
The pics in post #1 are an actual Friedman Dirty Shirley I assume?
You are trying to reverse engineer it?
Are the pics in post #5 the same amp or an exact same revision?

There are thousands of variants of basic Marshall circuits out there, pick 1 you want to ask questions about so that we can discuss the same thing. There is no point in looking at Ceriatone (or any other) layouts unless you know them to be the circuit you want to discuss.
32 vs 16 filter caps would be simple 'flavouring' and you will find examples of both. We should hone in on the 1 particular circuit you are concerned with.

[cluster]

This actually brings up an important point.

1) Is elevating the heaters dangerous since it ties the heater to HV?
2) Is it bad for the PT?
3) Is elevating heaters necessary if proper lead dress is observed??

[g1]

No danger or problem for PT.
It can help when all other hum reduction methods have been exhausted.
In this case it may have been used to deal with the heater-cathode voltage problem in the cathode follower stage. (Which may not be a real problem as nickb has demonstrated).
In the cathode follower stage you have a high cathode voltage. The 12AX7 spec for heater to cathode voltage may be exceeded. Elevating the heater ensures the spec is not exceeded.

[Chuck H]

This^^^^^^^^^^^^^^^^^^^^^^^

Elevating the heaters on the possibility that the cathode to heater differential was causing all those rusky tube failures is something I've built into amps just to be on the safe side. It seems likely Friedman might do it too.

Examining the pics it looks like that can is wired in series with the heater CT elevated at the junction. It's likely some HV node is using that as an 8uf filter cap, but I can't tell which because the wires run under the board. Probably the screens or the PI.

[cluster]

Thanks everyone. Here is another pic. It seems one resistor is 820 ohms and the other is 470K.

This^^^^^^^^^^^^^^^^^^^^^^^

Elevating the heaters on the possibility that the cathode to heater differential was causing all those rusky tube failures is something I've built into amps just to be on the safe side. It seems likely Friedman might do it too.

Examining the pics it looks like that can is wired in series with the heater CT elevated at the junction. It's likely some HV node is using that as an 8uf filter cap, but I can't tell which because the wires run under the board. Probably the screens or the PI.

[g1]

That shows better, I was confused about the cap terminal orientation. Should be 82K I think, not 820R.

Cluster, do these have a choke? If so, maybe the 1K screen dropper is over there somewhere?
The little white cap off the rectifier is sometimes drawn at the standby switch, like on this mojotone drawing.

[cluster]

yes... this amp has a choke.

Chuck, the funny part is that from what I can see, the terminal being used as the 'junction' on the cap is the (-) terminal, and one of the + terminals appears to be grounded.

Cluster, do these have a choke? If so, maybe the 1K screen dropper is over there somewhere?
The little white cap off the rectifier is sometimes drawn at the standby switch, like on this mojotone drawing.

[cluster]

I see Grey/Red/Black (or brown): 82ohm or 820ohm. 82K would be Grey/Red/Orange.