Electrolytic Cap Advice

[BoogieMKIIA]

I've been wanting to play out my Marshall 50W but it has quite a buzz. The person that worked on it over 20 years ago said the replacement caps could be reformed in circuit by playing a loop for some hours, didn't work. I pulled the back cover to swap out tubes and discovered the original Daly caps were still there! Been so long I didn't recall.

Question - replace or try to reform out of circuit?

The tops are flat but haven't yet looked under the chassis. From what I've read I should just replace. 

Related question - what to do with rusted transformer laminations? I wouldn't think what I have would cause hum, want to stop the process and clean it up a bit.

[Cboss]

Those are the most beautiful capacitors I've seen! Here is a link to replacement kits, I think this would do it..F&T, Sprague are great

https://www.amprepairparts.com/marshallcaps.htm

[JGale]

Yes. F&T.

[Cboss]

Those old ones are so cool and iconic, what you could do is leave them in place, Hollow them out and put the new ones inside as they will be smaller, I think this is done often with vintage caps

[BoogieMKIIA]

I have seen used Daly caps on eBay for a good price. I could remove, test and see if they could be reformed. Hate to destroy a “historic” part.

Also hope my diagnosis is correct. 
 

One of the caps is under the PCB, will need to be careful to get to it. I don’t want to unwire to move it enough. If anyone has done this, please proved tips.

[BoogieMKIIA]

Have ordered replacement caps and also some resistors to attempt reforming prior to replacing. Instructions are on the Metropolis forum and seems simple enough to try. I pulled the chassis and the caps look OK from the bottom and too, but can’t tell for sure from appearance.

This is one of my retirement projects. Next up is rebuilding a Gibson Gibsonette. 

[Jimbilly]

I feel I should add for those of you reading this and hesitant to do any of your own work: get a Champ or similar circuit, and start looking at the schematic, map it out, test some resistors, ask a lot of questions. I'm been learning on a Silvertone 1331 (similar to a tweed champ) I recently bought, and I'm gaining some confidence. The simple circuit makes it possible, and it's less of a mystery the more I look at it.  

[BoogieMKIIA]

1 hour ago, Jimbilly said:

I feel I should add for those of you reading this and hesitant to do any of your own work: get a Champ or similar circuit, and start looking at the schematic, map it out, test some resistors, ask a lot of questions. I'm been learning on a Silvertone 1331 (similar to a tweed champ) I recently bought, and I'm gaining some confidence. The simple circuit makes it possible, and it's less of a mystery the more I look at it.  

This “should” be easy for me since I’m an EE, but I didn’t design or build tube amps. All the practical and good technician knowledge comes from doing and asking those that know. I do know enough hopefully to not get killed as I get there. The Gibsonette is very simple, agree something like that is a good place to start. I modded my Marshall in college days to add a master volume per a Marshall schematic. The circuit diagram is simple, mapping it to the physical amp not as much. 

Schematic attached if anyone is interested. The hum stays with V1 pulled, goes away with either V2 or V3 pulled. This suggests the filter caps on the output section may be ok. I could end up changing all the electrolytic filter caps and still have hum. A reason to Ask The Experts.
 

 

jmp_lead_50w_1987.pdf

[Dutchman]

On 10/29/2024 at 2:03 PM, BoogieMKIIA said:

hum stays with V1 pulled, goes away with either V2 or V3 pulled. This suggests the filter caps on the output section may be ok

Change them all while your in there. Also check resistor values. I've found better grounding on older Marshals really helps with all noises. You can test with extension clips while it's running if you feel comfortable doing that. Also some of the boards on Marshals start conducting DC. So check the board to see if it's hot.

[BoogieMKIIA]

I’m going to try to reform the caps first, just to see if I can/how bad they are. The replacements arrived Friday so I can change them as needed. Got the three dual can 50uF and two 150 uF for the bias caps. The bias caps are not original, but 20+ years old.

I am looking for photos of stock amps for examples of lead dress and grounding. Not sure how far off the wiring is from stock, likely not much. The amp was in a basement for awhile and there is some oxidation or funk visible. May get some high purity IPA (not beer) to wipe off the board. May find some screws that need tightening.

Also checking out the Marshall Amp Forum for advice and learning.

[velorush]

A great resource is the Psionic Audio YouTube account. 

https://youtube.com/@psionicaudio?si=lrUmHc6eXVdtAPzZ

 

Great videos of all kinds of amps including loads of all stripes of Marshalls.

[BoogieMKIIA]

1 hour ago, velorush said:

A great resource is the Psionic Audio YouTube account. 

https://youtube.com/@psionicaudio?si=lrUmHc6eXVdtAPzZ

 

Great videos of all kinds of amps including loads of all stripes of Marshalls.

Thanks. I stumbled on a few of those, lots of good info. Stuff on noise and grounding will be helpful. 

I'm reforming the caps according to a thread on the Metropoulos Forum. Seems to be working as I'm down to 8 volts across a 100K resistor between the rectifier and caps. Instructions say you may get to 5V eventually if the caps are in great shape, and under 10V should be good enough. 24 to 48 hours is the max time to let them form. I do have a set of new F&Ts should reforming not take care of the hum.

[BoogieMKIIA]

I didn’t check after reforming with the original caps since I didn’t get to 5V across the 100K resistor and one cap looked iffy. Put in the new F&Ts, still have hum! I think there might be a grounding issue and I’m searching for that.  Sounds like 60Hz hum best I can tell.

The ground wire across the pot bodies broke between Vol 1 and Vol 2, have that jumped at the moment. The ground for the pots comes from a wire on the PCBA at the bias caps. I haven’t changed the bias caps as they were changed when it was converted from 6550s to EL34s and are Nichicon. But May replace just to be sure. I will need to lift the PCBA to do so and look at wires and grounding.

Started tapping on the first two preamp tubes and can hear the tapping. Will recheck swapping some tubes again as well, but thinking that is a symptom and not a cause.

[BoogieMKIIA]

I’m going to play this Marshall at a gig this Saturday - with the regular as back up. Hoping it is now quiet enough outside the house, turned up a bit. Will use a THD Hot Plate and inefficient 1x12. Thanks for advice in this thread and by message.

What I found to date

1. Sprayed and worked the input jacks.

2. Moved the grid wire of V2 away from the twisted heater wires

There is nothing else I have found incorrect so far in the circuit per the schematic and parts. Will get some new tubes next. I noticed some odd 12ax7s rolling them in and out looking for noise. Those are all old Boogie 12AX7s

[HSB0531]

Are those caps single value ( 2-lead ), or multi-value/multi-lead?  If they're single value, you can replace them with a modern electrolytic of the same uf. value which are much smaller than the originals.  A similarly sized modern capacitor has a much higher working voltage.  Upping the voltage rating will greatly increase the lifespan of the cap.  Switching from an 85 degree to 105 degree cap will also make a big difference, especially in a tube chassis. And, finding a capacitor with a tighter tolerance in value is even better.

[BoogieMKIIA]

On 12/13/2024 at 8:13 PM, HSB0531 said:

Are those caps single value ( 2-lead ), or multi-value/multi-lead?  If they're single value, you can replace them with a modern electrolytic of the same uf. value which are much smaller than the originals.  A similarly sized modern capacitor has a much higher working voltage.  Upping the voltage rating will greatly increase the lifespan of the cap.  Switching from an 85 degree to 105 degree cap will also make a big difference, especially in a tube chassis. And, finding a capacitor with a tighter tolerance in value is even better.

I replaced the caps early on with F&Ts, made no difference. Dual 50uF/500V cans, just a little shorter than the originals. I didn't correctly identify 60Hz (heater hum) versus 120Hz (rectifier diodes, filter caps).

I played it out Saturday and there was very little hum, certainly acceptable. Played into one 12" Tone Tubby Chicago Blue using a Hot Plate on 8dB attenuation. Volume was 11 o'clock. Sounded great

 

[HSB0531]

On 12/16/2024 at 12:06 PM, BoogieMKIIA said:

I replaced the caps early on with F&Ts, made no difference. Dual 50uF/500V cans, just a little shorter than the originals. I didn't correctly identify 60Hz (heater hum) versus 120Hz (rectifier diodes, filter caps).

I played it out Saturday and there was very little hum, certainly acceptable. Played into one 12" Tone Tubby Chicago Blue using a Hot Plate on 8dB attenuation. Volume was 11 o'clock. Sounded great

 

Good to hear it's better now.