Replacing filter capacitors in JCM 800 with vertical inputs

[sluckey]

I'm talking about the speaker jack outputs. I think they should be non-shorting since they are wired in parallel... otherwise at least one of them would short if it has less than two speakers connected.

Only one of the parallel jacks is a shorting type. The other jack is non shorting. Fender uses parallel jacks. The SPKR jack is a shorting type and the EXT SPKR jack is non shorting. Been like this since the '60s. Works very well. You do need to understand how they work and how to use them.

[shattered]

It makes sense if one of them disconnects the other when it has a cable plugged. I have to look into this, thank you.

[sluckey]

So if you connect just one speaker you must use the shorting connector, but that's not specified in the rear panel. It seems dangerous as hell since if you connect to the non-shorting one there will be a short in the other.

It's specified in the owners manual. You must have a speaker connected to the SPKR jack to have any sound. Plugging into the EXT. SPKR jack only works if there is also something plugged into the SPKR jack.

Maybe the non-shorting one disconnects the other when it has a cable plugged?

Impossible

It makes sense if one of them disconnects the other when it has a cable plugged.

Doesn't work that way. Look at the very simple circuit in the attached file. It really is just that simple. Don't overthink.

[shattered]

Then we're talking about different amps. In the rear panel it just says "LOUDSPEAKER OUTPUTS" on top of two female jack connectors. No indication of external speaker.

I also checked the manual (Master Volume And Lead Amplifiers Handbook) and it just says: "Parallel connected jacks for loudspeaker connections. Speaker load must always be connected. If one or both sockets are used, total impedance must be matched to selector and must not be less than 4 ohms." in page 4.

[sluckey]

Then we're talking about different amps.

Aha! Yes! Your amp does not utilize shorting jacks. (Actually, those 4 lug cliff jacks do have switches on them, but they are not wired to use the switches.) Nothing that has been said about shorting jacks applies to your amp. Sorry for adding more confusion.

We can thank Stevem for throwing the shorting jack red herring into the discussion.

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.

[Stevem]

Yes, I fully take the blame!

[shattered]

Before the recap I took these measurements with the DC voltmeter. I used a 100W lightbulb as current limiter during all the process and it was mostly off, just a bit of light from the filament, no tubes installed, no speaker - just a 220 ohm 5W resistor soldered in parallel the output jack for OT protection.

• B+ is 471V
• I measured just the positive leads of the four 50+50uF filter caps since I didn't observe the negative leg of two of them is connected to the positive of the other two. First group measured 232V, the second group 470V, on both + legs.
• The one under the PCB 433V on both positive leads (on each side of the resistor that is directly connected to it).
• The other one at the corner 443V

Regarding bias voltage, first I measured DC at the 10µF bias capacitor leg that connects to 47K and 15K resistors, it was -37V at the current setting. Turned the potentiometer to the "MAX" position indicated in the silkscreen and now voltage is set to -45V (against ground). I verified pin 5 of every power tube socket and I get the same value.

Forgot to say that just after power on, with or without standby, no tubes installed and no speaker connection, I can hear a slight hum from the amp, probably mains frequency. This happened before when I used the amp (with tubes and speaker, before this test) and the sound does not get to the speaker, could this be normal and not neccesarily a problem if the amp works fine? is tightening the nuts of the the PT a delicate or risky procedure?

[martin manning]

Sounds good, but set the bias at the most negative voltage when you power up for the first time with tubes installed. Keep in mind that if you have a bulb limiter in, your voltage measurements will be a bit low. The hum is in the transformer. New caps might reduce it some. Check the mounting bolts and snug them if they are loose. They trend to loosen over time due to magnetostrsiction.

[shattered]

Recapping went fine and the amp is roaring again. Bias caps (Arcolytic brand) were probably leaky since they measure 14µF, and two of the big filter caps were bulging in the bottom.

Thank you all! In addition to having a more reliable amplifier, I learned a few things.

[martin manning]

Well done!