Replacing filter capacitors in JCM 800 with vertical inputs

[shattered]

I'm preparing to replace the filter capacitors of a Marshall JCM 800 2203 with vertical inputs and I have some questions.

1. With amp off and power cord detached, my plan is to leave a cocodrile wire to connect pin 1 from preamp V1 attached to chassis for the capacitors to drain through the plate resistors (already checked the schematics and it makes sense) wait some minutes and verify with a properly rated voltmeter the capacitors are drained, without forgetting to remove it after the work is done. Should I have another consideration such as leave a speaker connected or having the power switch on (without power cable)? It makes no sense to me at least in this amp, but I want to minimize risks.
2. Is a 5/16" nut driver the correct tool to open the capacitor clamp? If so, it's the same size as the truss rod adjustment tool for Gibson guitars, could this do the trick?
3. For the 10uF 160V bias caps, would a F&T 10uF 350V work fine as a replacement assuming they fit physically? Dunno if I should assume they are better quality than an Illinois TTA 10uF 160V. I've also considered Vishay MKT1813610255 for reliability, but I think I prefer to use the same type of capacitor.

[Stevem]

Hello.

I forget the size of the filter can nuts, but I think there metric, not 5/16” but that maybe close enough to work without stripping the nut.

To drain out the power supply filters have the amp on, the turn off the main power switch while leaving the standby switch on.

This will drain out the power supply high voltage from the filters in 30 seconds.




No need for a speaker to be plugged in also.

These amps have shorting type output jacks so even if the amp is powered up and played the self shorted jack will prevent any harm to the output transformer.


Those two 10uf 160 volt caps are not bypass caps, there the power supply filters for the power output section negative bias voltage.

Theres no need to use caps with a higher voltage rating, in fact the caps will live longer if there not so highly overrated in terms of voltage.


NOTE that since these filters are used in the negative bias section be sure that your new ones need to go back in like the old ones with there positive marked end going to ground.

Before you put the amp back on leave do not put the output tubes back in, turn the power switch on with the standby switch off and then confirm that on pin 5 of each output tube that you read at least a negative 53 volts for starters if your output tubs are 6550 types .

Then your ok to pop the output tubes back in

[shattered]

Hello Steve, thank you for the information, it's much appreciated.

I'll make sure the new caps are installed with the same polarity as the old ones. I'd prefer F&T 10µF 160V but they don't seem to be available in that voltage. Not sure if the Illinois TTA 10µF 160V are good enough, a probably better option would be Vishay Sprague 30D106M160DC5A.

The amp uses EL34 tubes, so it should be between -30V and -55V, right? Is it ok to leave the preamp tubes installed? Those sockets feel not as tight as they should so I'd prefer to leave them is possible.

Is it recommended to check bias and adjust if needed after the recap?

Regarding the Standby, it's permanently "on" and never touched like it didn't exist, so the amp powers on just right after flipping the power switch.

[Roe]

do you mean the 10uf 160v bias caps or cathode bypass caps?

[martin manning]

The amp uses EL34 tubes, so it should be between -30V and -55V, right? Is it possible to leave the preamp tubes installed? Those sockets feel not as tight as they should so I'd prefer to leave them is possible. I suppose there is no need to connect the speaker. I'm still a bit reticent to poke inside the amp while powered on, but I don't want to risk this amp so I'll look into this.

Is it recommended to check bias and adjust if needed after the recap?

I'd be a good idea to check the bias as long as you are in it, or learn how if you don't know already. Bias voltage is somewhat informative, but what you really want to know are the plate voltage and the plate current so you can get the plate dissipation. Do you have a method to measure the current? Is this is an early 80's amp?

[shattered]

do you mean the 10uf 160v bias caps or cathode bypass caps?

Bias caps. The only two 10uF 160V electrolytics in the circuit.

The amp uses EL34 tubes, so it should be between -30V and -55V, right? Is it possible to leave the preamp tubes installed? Those sockets feel not as tight as they should so I'd prefer to leave them is possible. I suppose there is no need to connect the speaker. I'm still a bit reticent to poke inside the amp while powered on, but I don't want to risk this amp so I'll look into this.

Is it recommended to check bias and adjust if needed after the recap?

I'd be a good idea to check the bias as long as you are in it, or learn how if you don't know already. Bias voltage is somewhat informative, but what you really want to know are the plate voltage and the plate current so you can get the plate dissipation. Do you have a method to measure the current? Is this is an early 80's amp?

It's an early 80's amp.

Yes, I use the TAD Bias Master that measures voltage drop through resistor instead of interrupting the circuit to measure current. That gives plate + screen current if I recall correctly, so it errs on the safe side.

For plate voltage I use a Brymen BM789 multimeter, rated 1kV CAT III and 600V CAT IV.

[Stevem]

The always most safe thing to do is to set the bias voltage as high as it will go until your ready to check & set it the idle current.

To me cathode stripping of the output tubes a is real possibility over time if you do use the standby switch since the amp uses SS diodes and not a rectifier tube,the preamp tubes not so much.

[shattered]

So for this amp would you use the Standby switch? I tested the switch and it works fine, I found contradictory information so I'm currently not using it.

Yeah, in other amp I recently biased I turned the pot for the coldest setting, then go upwards until 60%-62.5% or so, doing the math using 25W power consumption (EL34) and the plate voltage. In the JCM800 2203 the coldest setting would be turning the bias pot fully counterclockwise, right?

[martin manning]

If your amp is wired like the schematic below, the bias voltage does not come up until the standby is switched to play. The heaters come on with the main power, so the tubes are up to operating temp when you switch to play, but there is no bias for a short time while the bias supply caps charge. Moving the wire leading to the bias supply from the cold to the hot side of the standby is safer. You can easily check your amp by looking for the negative bias voltage in standby. With the bias supply connected directly to the transformer's HT, you should be able to use the standby. By not using the standby (leaving it set to play) the delay while the cathodes come up to temperature gives the bias supply plenty of time to settle in either way.

[martin manning]

PS the coldest bias setting is where you get the most negative voltage. Check with your meter.

[shattered]

The bias probe I use directly shows the mA instead of the negative voltage.

For other amps I set the pot to the coldest side with the amp disconnected, then power on in standby, wait some minutes for the tubes to heat up and flip the standby, adjusting from cold to hotter setting using a non-conductive screwdriver. In example, it starts showing 20mA, then go up to ~35mA or so. If I start with the standby flipped I can see the mA climbing until it settles.

This amp is, according to the previous owner, biased below 70% so I won't check until I do the recap.

[sluckey]

I highly recommend a full evaluation of the amp ***BEFORE*** you fire up the soldering iron.

[martin manning]

Solid advice from Steve L.

I missed that you said this amp is a 2203. In which case it might look like this. Note there is a separate bias winding, and the bias will come alive with the main power. This schematic may not match your amp, but as mentioned above you can easily check for bias voltage in standby, and dial it to the most negative reading if it is. For which way is the coldest bias setting on the bias pot, from the schematic (and usually the case with Marshall bias circuits), the most negative voltage is produced when the resistance to ground at the junction of the bias feed resistors (2x 150k) is maximum. Here you can just measure the resistance across the outer legs of the bias trimmer and set that to maximum without any power applied.

[nuke]

I'm preparing to replace the filter capacitors of a Marshall JCM 800 2203 with vertical inputs and I have some questions.

1. With amp off and power cord detached, my plan is to leave a cocodrile wire to connect pin 1 from preamp V1 attached to chassis for the capacitors to drain through the plate resistors (already checked the schematics and it makes sense) wait some minutes and verify with a properly rated voltmeter the capacitors are drained, without forgetting to remove it after the work is done. Should I have another consideration such as leave a speaker connected or having the power switch on (without power cable)? It makes no sense to me at least in this amp, but I want to minimize risks.
2. Is a 5/16" nut driver the correct tool to open the capacitor clamp? If so, it's the same size as the truss rod adjustment tool for Gibson guitars, could this do the trick?
3. For the 10uF 160V cathode bypass capacitors, would a F&T 10uF 350V work fine as a replacement assuming they fit physically? Dunno if I should assume they are better quality than an Illinois TTA 10uF 160V. I've also considered Vishay MKT1813610255 for reliability, but I think I prefer to use the same type of capacitor.
4. Are cathode bypass capacitor failure modes dangerous for other components, or when they fail they just make the amp sound bad?

1: That will work. I usually just read the remaining charge with a voltmeter at each filter capacitor and if they're zero or close to zero, just proceed with work. If they are significantly higher, figure out why. Put your clip-lead on the 50uf section of the last capacitor that leads to the preamp plate resistors. If you put it on pin 1 of V1, you'll be there all day waiting for it to bleed down.

One thing to be very safety conscious of is the main filters may retain a significant charge if the HT fuse is blown or removed, while the rest may be reading 0v.

2. I don't know what the hardware size is, but I'd suspect metric. Maybe there's a Harbor Freight or similar tool store nearby? I always like a good excuse to buy another tool.

3. I think the other folks covered this well already, as those are the bias supply filters. I wouldn't bother replacing them with film caps. A decent quality electrolytic would do quite well. You'll pay a big premium for axial leaded parts. It is worth looking to see if a radial electrolytic will fit there with a little fiddling. Amplified Parts has an axial 8uf @ 150v that will work fine there, 105c rated and only $1.05 each.

4. The 2203 preamp (in the above listed schematic) section only has one cathode bypass at V1A, a 0.68uf. It is probably a film cap already, so no need to replace it. There are electrolytics in that value, so it is possible. If it is electrolytic, i'd replace it with a 0.68uf film.

[shattered]

I highly recommend a full evaluation of the amp ***BEFORE*** you fire up the soldering iron.

So it would be a nice idea to remove all the tubes and then measure plate voltage and pin 5 of each power tube? I'm afraid the increased B+ voltage could blow some old capacitor. Will continue reading about how to evaluate an amp, thanks for the suggestion.

I replaced the preamp tubes with good ones and once I heard a very slight "pop" while playing, reseated them and it works fine. I've used it for less than two hours or so. Maybe it's the socket of v3 that could be tighter (since in other amps I have to apply more force to insert or remove the tube) or the electrolytics.

Solid advice from Steve L.

I missed that you said this amp is a 2203. In which case it might look like this. Note there is a separate bias winding, and the bias will come alive with the main power. This schematic may not match your amp, but as mentioned above you can easily check for bias voltage in standby, and dial it to the most negative reading if it is. For which way is the coldest bias setting on the bias pot, from the schematic (and usually the case with Marshall bias circuits), the most negative voltage is produced when the resistance to ground at the junction of the bias feed resistors (2x 150k) is maximum. Here you can just measure the resistance across the outer legs of the bias trimmer and set that to maximum without any power applied.

I take note, that's a good idea specially when installing a new set of tubes. Since the amp is biased and I'll be using the same tubes, wouldn't make more sense to adjust if needed? So I wouldn't need to poke inside while amp is on more than necessary.

By the way, I found that Sprague Atom 10uF 150V are available and from what I read they seem adequate for this circuit.