Powering up old amps

Beyond Black

Well-known member
Later this week, me and my buddy are going to check out two JMP half stacks that are for sale, a 78 and a 79.
They’ve been in storage for at least the last 20 years. How exactly do you go about powering them up slowly with a Variac? I’ve got one and we’re gonna bring it along.
 
Open the amps, visually inspect. Check for burned components.
Check the filter caps with an ESR meter.
Test all the tubes on a tube tester. (or bring known good set with) Try to make sure the output tubes are balanced.
With tubes out, power on w/ variac at 25V. Dial it up 20V every 20 minutes, watching for trouble.
If you reach full power without a blown fuse or trouble, power down and install tubes and connect a speaker (or dummy) load.
Use LBCL (light bulb current limiter) with an incandescent bulb rated about 150watts.
Start up the variac at about 90V. Stay there for a few minutes - watch for smoke, popping noises.
Range all knobs, listening for weird sounds.
Put a signal thru, and see what it sounds like.
 
Open the amps, visually inspect. Check for burned components.
Check the filter caps with an ESR meter.
Test all the tubes on a tube tester. (or bring known good set with) Try to make sure the output tubes are balanced.
With tubes out, power on w/ variac at 25V. Dial it up 20V every 20 minutes, watching for trouble.
If you reach full power without a blown fuse or trouble, power down and install tubes and connect a speaker (or dummy) load.
Use LBCL (light bulb current limiter) with an incandescent bulb rated about 150watts.
Start up the variac at about 90V. Stay there for a few minutes - watch for smoke, popping noises.
Range all knobs, listening for weird sounds.
Put a signal thru, and see what it sounds like.
I pretty much follow the same protocol but I start lower than 25ACV overall it takes me a few hours sometimes, I can't remember what intervals I used. I think it was15-30 minute intervals so PLX's will work just fine. I normally stop at about 90ACV so I don't blast the lower voltage caps on the B+ voltage rail. The current limiter is also a good idea but I have never used it, just variac method and it has always worked well.

The filter caps should reform just fine. There is a more in depth current limiting setup but you have to solder some resistors in and some owner isn't going to let you do that to their amps. So the one PLX mentioned would be the best and fastest for the situation. Don't rush it either do the full intervals for as long as it takes. I usually write all the intervals down a pad of paper and check off the time intervals until I'm done so I'm not just guessing.
 
Open the amps, visually inspect. Check for burned components.
Check the filter caps with an ESR meter.
Test all the tubes on a tube tester. (or bring known good set with) Try to make sure the output tubes are balanced.
With tubes out, power on w/ variac at 25V. Dial it up 20V every 20 minutes, watching for trouble.
If you reach full power without a blown fuse or trouble, power down and install tubes and connect a speaker (or dummy) load.
Use LBCL (light bulb current limiter) with an incandescent bulb rated about 150watts.
Start up the variac at about 90V. Stay there for a few minutes - watch for smoke, popping noises.
Range all knobs, listening for weird sounds.
Put a signal thru, and see what it sounds like.

Caps have to be removed for an ESR measurement so that’s not correct. You’re not going to want to remove all of them one at a time to do that. You cannot do ESR in circuit just like you can’t measure resistance in circuit without one node open.

Light bulb limiters limit current but also drop voltage on the secondary side of the PT, causing current to increase which can stress the PT to the limit of the rms current of the bulb. You can use them for initial short circuit protection when power tubes are installed, but never long term and you’re definitely not going to see full secondary operating voltage with a light bulb limiter. They have their purpose but the correct answer is get a clip on current meter and after testing with a light bulb limiter, just monitor the current on the primary of a variac using the clip on meter and limiter disconnected with the power tubes pulled during initial turn on. Validate the capacitors can take an unloaded full charge and also validate the bias circuit is working correctly with no power tubes installed using a multi meter. Only when those are validated should you install the power tubes. I’d make sure the bias is turned to the most negative voltage possible before reinstalling the power tubes.

I’d also not stick your head over the upside down chassis. Those old Daly caps vent from the bottom which is facing upward towards your face when the chassis is upside down.
 
Damn, sound like there’s a bit to it and it takes some time.
I’m not sure that’s all possible in this situation.
 
Damn, sound like there’s a bit to it and it takes some time.
I’m not sure that’s all possible in this situation.
In that sort of situation, I'd let the seller power the amps on - any way he chooses and see what happens.

Or, if the price is right just buy them for parts.

ymmv
 
Damn, sound like there’s a bit to it and it takes some time.
I’m not sure that’s all possible in this situation.
I think inspecting the caps, then deciding on the condition before attempting to power up is still taking too much of a chance.
I’ll donate my time + 3 F&T caps if I get a free lap dance at a nice Chicago family fun bar/strip joint.
Just sayin.
😬
 
If it helps your decision, any amp I haven’t powered on or can’t power on with good faith I ask for the parts cost or sell it ahead of time at a discounted price at parts cost. If the seller is trying to cash out of in working order vintage prices then he needs to flick the switch or lower the price. You can’t have your cake and eat it too.
 
Damn, sound like there’s a bit to it and it takes some time.
I’m not sure that’s all possible in this situation.
If you and the owner cross your fingers and flip the switches and the amp burns up you likely won't want it anyway would be my guess.

Maybe have him take the amps to a repair technician and have them both brought up on a variac at the tech's shop if he wants a decent price. Otherwise I would not power them on unless he is willing to sell for a large discount if you cannot verify functionable status. The amps could have blown output transformers for all you know.
 
There's a lot of estate sales in my area. This situation comes up a lot..

Someone who doesn't know jack about tube amps is suddenly put in the position of selling one.

"When was the last time this amp was turned on ?"

"Oh, Bob hasn't played in over 30 years, but I'm sure it still works !" -
reaches for power switch. 😖
 
Thankfully I have never had an electrolytic cap pop on me, but I hear when it does happen is sure gets your attention.

When I get in vintage amps, I don't even flip them on. Electrolytic Caps get replaced, tubes get checked, etc.

I did have some vintage Twin reverbs come through that I powered up with a 300Watt light bulb limiter on a variac, they all passed signal amazing.
 
Thankfully I have never had an electrolytic cap pop on me, but I hear when it does happen is sure gets your attention.

When I get in vintage amps, I don't even flip them on. Electrolytic Caps get replaced, tubes get checked, etc.

I did have some vintage Twin reverbs come through that I powered up with a 300Watt light bulb limiter on a variac, they all passed signal amazing.
Those LCR filter caps seems to stay good forever.

I had a pair in an '89 Laney 100w that were still good in 2022.
 
Those LCR filter caps seems to stay good forever.

I had a pair in an '89 Laney 100w that were still good in 2022.
They do, they seem to last much longer than the typical ~20years that electrolytic caps should last. I still replaced them in my Silver jubilee fairly recently.

Since we are talking late 70s Marshalls its likely that the LCRs are still in pretty good shape. I would still plan to change them as Preventative Maintenance, but for the powering up discussion its something to note.
 
Those LCR filter caps seems to stay good forever.

I had a pair in an '89 Laney 100w that were still good in 2022.
Pfff, that’s nothin. My 72 Trem has its original Dalys. Amp sounds killer. Previous owner played it regularly until he passed in 2019. Bought it new in 73.
 
Pfff, that’s nothin. My 72 Trem has its original Dalys. Amp sounds killer. Previous owner played it regularly until he passed in 2019. Bought it new in 73.
Thats like my 71 Super Bass. I just recently changed the caps from the original. Nothing wrong with them but I thought it was time.
 
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