Any tips/tricks for my first vintage tube amp repair?

[BoomTexan]

I've got a Selmer Treble n Bass MKIII in the shop today. It came from France and it arrived with a cracked headshell, so I currently have it at a friend who loves woodworking and tube amps (so this is like a dream for him), but when it gets back, I'm gonna start repairing it.

The seller sold it untested as-is, he claims that he got it from a guy in England, played it a couple times, and its been sitting in his basement for 10 years unplayed. I'm gonna check all the capacitors and carbon comp resistors, transformer leads to make sure they aren't dead, and root around for broken solder joints.

Any tips? I'd like some advice on which types of caps I can leave in there, what to do about the can caps, and what to do with the power filtering section. I'm not used to working with diodes (most of my experience is with small tube rectifier amps).

Any help is appreciated! I'm hoping to post a repair video to YouTube when I'm done, and if I have enough $ left over I'll get myself an attenuator and demo this thing cranked and edge of breakup.

 

[Jon Snell]

After the usual physical inspection for lumpy components etc and safety, (Earthing etc);
Use your variac to power it on ... Very Slowly! ... then you will see where any issues are.
After a while check the bias voltage on the EL34s.
If you need new electrolytic capacitors, fit the same type and specification that you remove except for the blue capacitor, the physical size does not metter in that case, just the values etc. There are lots of dual capacitors available from many suppliers.
These amplifiers were built to last.

 

[Lowerleftcoast]

As a father would say, "What are your intentions with my daughter."

Are you making this a museum piece?
Are you turning it for a quick profit?
Are you making it a gigging amp?

Three cap cans are gonna set you back some $. If it is not a museum piece, I would look for alternatives to that. (?Install some tag strips and fit with modern filter caps?)

Rust never sleeps. Clean the jacks, sockets, pots, etc. (The jacks do not appear to have switches. IOW, I doubt any of the jacks will need to be replaced.)

EDIT: I see the echo jacks do have switches. Hopefully they can be cleaned. If the switch contacts are bad, replacement will be in order.

The diodes have impressive datasheet specs. They are from an age where diode technology was improving. My gut tells me these still have plenty of service life left. I would leave them alone if there are no obvious problems.

 

[Paul G.]

All electrolytic caps should be replaced -- as stated before, it may be quite a bit less expensive to use appropriate value single caps (mounted securely) in place of multi cap cans. Clean all pots and jacks. Check bias. Then evaluate what's what before doing anything else.

 

[Phrygian77]

Those Phillips 'mustard' caps can and will leak DC current. Testing for that isn't exactly simple.

 

[AntonyB]

Did you say France?
Voltage is 220V there.

You may need a step up transformer or a different power transformer to match the ~120V to the expected ~220V

Hopefully you are comfortable with high voltages, voltmeter, solder iron and basic electronic concepts.

In general my maintenance steps (collected from Skip Simmon's podcast, plus some of my own):
Even BEFORE POWERING ANYTHING UP:
- check the fuses (plural, you might have more than 1 fuse): correct rating, continuity check with an ohm meter
- check for loose solder joint from speaker all the way back to input jacks (including the wall power section, from hot/neutral through the fuse holder to the power transformer
- check tube sockets and retense the pin holders if needed (wobbly tubes is one thing, but are the tubes firmly held by the pin holders?)
- check electrolytics (power filters section) with an ESR for value (they could still be bad if they leak DC, but you can't tell just yet since your amp is off... keep it off until you finish those few steps)
- check coupling caps with an ESR as well for value

At that point you have done the basic safety and health things.
If any electrolytics is off value (like more/less than 50%), unsolder/clip, and replace with expected value/rating... don't even attempt at powering the thing up without them addressed.

Then use a variac yes to power things up slowly, while using your voltmeter to check things are critical locations (power transformer output, rectifier output, electolytic filters, plates)
Skip likes to use a variac with amp meter... so if things gets out of control, like too much current is drawn, then you can see it right away before things blow up (fuse is here for a reason!).

If this looks logical and simple to you, then report back on your findings.
If this is scary, then bring the amp to a local qualified tech (that is perfectly fine man, we are all on our journey to learn).

 

[BoomTexan]

Did you say France?
Voltage is 220V there.

You may need a step up transformer or a different power transformer to match the ~120V to the expected ~220V

Hopefully you are comfortable with high voltages, voltmeter, solder iron and basic electronic concepts.

In general my maintenance steps (collected from Skip Simmon's podcast, plus some of my own):
Even BEFORE POWERING ANYTHING UP:
- check the fuses (plural, you might have more than 1 fuse): correct rating, continuity check with an ohm meter
- check for loose solder joint from speaker all the way back to input jacks (including the wall power section, from hot/neutral through the fuse holder to the power transformer
- check tube sockets and retense the pin holders if needed (wobbly tubes is one thing, but are the tubes firmly held by the pin holders?)
- check electrolytics (power filters section) with an ESR for value (they could still be bad if they leak DC, but you can't tell just yet since your amp is off... keep it off until you finish those few steps)
- check coupling caps with an ESR as well for value

At that point you have done the basic safety and health things.
If any electrolytics is off value (like more/less than 50%), unsolder/clip, and replace with expected value/rating... don't even attempt at powering the thing up without them addressed.

Then use a variac yes to power things up slowly, while using your voltmeter to check things are critical locations (power transformer output, rectifier output, electolytic filters, plates)
Skip likes to use a variac with amp meter... so if things gets out of control, like too much current is drawn, then you can see it right away before things blow up (fuse is here for a reason!).

If this looks logical and simple to you, then report back on your findings.
If this is scary, then bring the amp to a local qualified tech (that is perfectly fine man, we are all on our journey to learn).

Well, first off, the Selmer Treble n Bass was really unique because it was one of the first multi-voltage switching amps out there. Theres a switch at the top of the transformer to change winding ratios for 250, 240, 200, 125, and 115 volts of input, so that won't be an issue.

I already have a variac and despite it being a crappy Chinese one I got for $45, it works reasonably well and I use it all the time to reform caps and bring stuff up to voltage, so I'm familiar with that process.

I actually don't have an ESR meter at the moment, which is why I was asking for advice as to which caps go bad often. I don't want to spend money unless I need to, and while the ESR meter will pay for itself in the long run, I just don't have the money at the moment. I'm just gonna replace all caps that have a bad reputation or have serious amounts of voltage going through them, i.e. the can caps.

Those Phillips 'mustard' caps can and will leak DC current. Testing for that isn't exactly simple.

Thanks so much for the advice, I'll buy a fresh set of those asap.

As a father would say, "What are your intentions with my daughter."

Are you making this a museum piece?
Are you turning it for a quick profit?
Are you making it a gigging amp?

Three cap cans are gonna set you back some $. If it is not a museum piece, I would look for alternatives to that. (?Install some tag strips and fit with modern filter caps?)

Rust never sleeps. Clean the jacks, sockets, pots, etc. (The jacks do not appear to have switches. IOW, I doubt any of the jacks will need to be replaced.)

EDIT: I see the echo jacks do have switches. Hopefully they can be cleaned. If the switch contacts are bad, replacement will be in order.

The diodes have impressive datasheet specs. They are from an age where diode technology was improving. My gut tells me these still have plenty of service life left. I would leave them alone if there are no obvious problems.

I'm not planning on doing it for a quick profit. I'm gonna try to get it as clean and nice as possible as a showcase project for my fledgling repair business, along with my other Selmer.

Honestly, the can caps aren't too bad of a price. I could get a set of 6 F&T 32uF caps for about $40, or 3 JJ 32+32uF 500V can caps for 50. I'd rather not have the extra holes in the chassis where someone could stick a finger in while reaching for a tube and kill themselves. Plus, if semi-originality costs $10 more and doesn't impair function, I'll stick with it.

I think I'm gonna replace those echo jacks in that case. Didn't know that those could be a problem, thanks for alerting me. Thanks for the heads up about the diodes as well, I'm trying to keep every original part that I possibly can without possibility of major failure. My multimeter has a diode life function, so I'll give those a quick test and hope they're all good. As usual, if one goes bad, swap in a 1N4007, right?

 

[Phrygian77]

Thanks so much for the advice, I'll buy a fresh set of those asap.

Listen, it was simply a warning. You need a good meter with a high input impedance, and you need to lift the output side of the coupling cap out of the circuit, mainly to keep any grid current from the following stage from affecting the test. I suggest you read up on it.

 

[radiocaster]

I actually don't have an ESR meter at the moment, which is why I was asking for advice as to which caps go bad often. I don't want to spend money unless I need to, and while the ESR meter will pay for itself in the long run, I just don't have the money at the moment. I'm just gonna replace all caps that have a bad reputation or have serious amounts of voltage going through them, i.e. the can caps.

Check the capacitance. First drain the ones that don't have a resistor in parallel. Remove all tubes before doing this.

 

[BoomTexan]

Well, after considering cost of the can caps and then looking at the cheapest (non-crappy-chinese) ESR meter on Amazon, I think its a better idea to just get the darn meter. I guess its one of those deals where the earlier you get it the more money you save on cap replacements, so no reason not to. I'll update y'all as soon as the amp gets back from my buddy on Tuesday and I'm able to get inside it for a good long time.

 

[radiocaster]

Well, after considering cost of the can caps and then looking at the cheapest (non-crappy-chinese) ESR meter on Amazon, I think its a better idea to just get the darn meter. I guess its one of those deals where the earlier you get it the more money you save on cap replacements, so no reason not to. I'll update y'all as soon as the amp gets back from my buddy on Tuesday and I'm able to get inside it for a good long time.

Keep in mind bad caps often pass the ESR test.

 

[schmee]

One thing: Get some flux. Remelting that old solder can be near impossible at times and certainly is without overheating stuff! A tiny bit of flux and ouila!

 

[Phrygian77]

Keep in mind bad caps often pass the ESR test.

I just posted about this recently. It depends on the construction of the caps. Modern caps usually fail with a high ESR.

Edit: by modern I mean most electrolytics made in the last 50ish years.

 

[Lowerleftcoast]

My multimeter has a diode life function, so I'll give those a quick test and hope they're all good. As usual, if one goes bad, swap in a 1N4007, right?

If the diodes need replacing, I would use two of the 4007 type in series per side. Some builders prefer the faster switching UF4007 to the 1N4007. So far, my ears can't hear the difference. You can hunt down diodes with higher amperage ratings if you are dead set against *in series* diodes.

I wouldn't change out the yellow caps unless they are giving trouble. ESR meter or lift one leg and clip in a good cap for a test? Keep it simple.

Just change the cap cans and the other e-caps. Perhaps the cans can be gutted and stuffed with new caps. It might be messy but satisfying. Check out youtube for tutorials.

 

[radiocaster]

I just posted about this recently. It depends on the construction of the caps. Modern caps usually fail with a high ESR.

Edit: by modern I mean most electrolytics made in the last 50ish years.

I recapped 2 amps from the 90s. Maybe one or two caps failed ESR, about 12 or 15 caps had about half capacitance. You often don't need to desolder anything to check capacitance either.

The only electrolytics that were good were the ones at the last stage from the power supply, i.e. the ones at the first audio stage, V1 or whatnot. I changed those also, but it seems if they get less juice, they fail less.

 

[BoomTexan]

I recapped 2 amps from the 90s. Maybe one or two caps failed ESR, about 12 or 15 caps had about half capacitance. You often don't need to desolder anything to check capacitance either.

The only electrolytics that were good were the ones at the last stage from the power supply, i.e. the ones at the first audio stage, V1 or whatnot. I changed those also, but it seems if they get less juice, they fail less.

That's what I've heard. That's why black beauty and bumblebee caps are so valuable in guitars but are immediately changed in tube amps. They fail commonly in amps because of high voltage, but as guitars operate at such minimal current and voltage, they just about never go bad.

Same for tone pot caps or preamp caps on amps. Almost never any reason to change them, because there's very little chance of failure (unless they're paper in oil).

 

[Phrygian77]

Maybe one or two caps failed ESR, about 12 or 15 caps had about half capacitance.

What amps are you working on that, first, had that many electrolytic caps, and second, nearly all of them bad? This sounds fishy to me.

That's what I've heard. That's why black beauty and bumblebee caps are so valuable in guitars but are immediately changed in tube amps. They fail commonly in amps because of high voltage, but as guitars operate at such minimal current and voltage, they just about never go bad.

Those are not electrolytic caps.

 

[kbold]

Electrolytics: replace all
Rectifier diodes: replace
Pot's: Deoxit
Safety: try not to die

 

[BoomTexan]

Electrolytics: replace all
Rectifier diodes: replace
Pot's: Deoxit
Safety: try not to die

Hey, I mean, I'll try, but no promises.

Man, I don't know how anything on this godforsaken planet would work without Deoxit and WD40.

 

[BoomTexan]

What amps are you working on that, first, had that many electrolytic caps, and second, nearly all of them bad? This sounds fishy to me.



Those are not electrolytic caps.

I've never personally even seen a Gibson EH150, but I did watch a Fazio Electric video where she replaced several black beauty caps in the amp. They're not electrolytic, but they are paper in oil, and have a far worse reputation than electrolytic. The caps are worth $25 easy for guitars because tone chasers are idiots, but they aren't trusted in the slightest to work as like coupling or DC voltage blocking caps in a tube amp because they fail so frequently.

I mean, I had to replace no less than 7 failed electrolytic capacitors on my Hot Rod Deluxe. Its not impossible that so many could fail on, say, a Marshall JCM900 head.