Servicing a vintage amp for gigging reliability

[rpjazzguitar]

I would have some trepidation about having a tech who I don't know very well (because I don't know any) start working on things that aren't broken.

My old Ampeg was built to last. I'd be worried that the tech might make things worse. Maybe that's unreasonable, but it would be worrisome for me.

If I knew how to do the work myself, well, I might still be worried about breaking something.

If I was gigging the Ampeg (I don't), I think I'd carry a spare amp and fix the Ampeg when it broke.

[DawgBone]

I replaced the mdf with 3/4" marine plywood way back when I converted to 115 on the speaker. So I don't trust to luck or patches but also don't fix things that aren't broken. No luck there just inspection, cleaning and observation. That twin runs with fire breathing metal guitarists, double bass drums and has survived full throttle opening sets for bands like Blue Oyster Cult, Robin Trower, Night Ranger etc ...
The price of a full on rebuild after a complete melt down is still better than the full service 10-20 years before it is needed since those years will have passed without the cost and it will be ahead of the replacement component lifespan by the extra time it has lasted.
Obviously we all run with what we are comfortable with, I do use new tubes and off on switches.

Sounds good until you damage a transformer.....

[Cavalier]

Sounds good until you damage a transformer.....

A trifle, I can get a whole SF for parts for$500. Save some spares for the next 40 to 50 year service and I'm way ahead.

[sgosnell]

It would be nice, I suppose, to buy expensive fans and do expensive mods, but I don't use that amp very often, and I have no added cost at all. The fan came from an old computer enclosure taking up space in the attic, and the wall wart was from something I don't even remember. A couple of screws from the parts bucket in the garage, some solder and shrink wrap already on hand, and it's done. The fan is attached to the upper cross bracket on the back of the amp by the two short screws. Cutting holes in the cabinet is not something I have ever even thought about, because it's not necessary. Airflow is good enough for jazz as is.

[bluejaybill]

I recently paid ~ $775 to get my '91 Boogie Mk IVa combo fixed up, with tubes, caps, and whatever else. It sounds amazing now, and I'm happy I did it.

But, I have a 50Watt 'drip edge' Fender Bassman head that needs the most work. A friend of mine who repairs foreclosed on houses found the head in a basement and gave it to me. It had no tubes, looked like it was ridden hard and put away wet, and was clearly modified. The 2nd input on both channels had been changed to pots, and someone who saw a pic of it told me it was likely a 'Marshall mod'. I've been considering getting it overhauled and back up and running for two years, but you can find used examples in the $400-$500 range and it mat cost considerably more to get the work done. If I do get it sorted, I'll ask for before and after pics from my amp guy though.

https://jim777.com/images/amps/bassman_69.jpg

However, the $400-500 Bassman will probably need much of the same work!

Having said that, I'd be careful if the amp was in a damp environment. That can damage the turret boards I believe. Something to look into.

[Phil59]

A good deluxe is pretty sweet but I'm still gonna put my money on you liking the Super better! Please come back and let me know your thoughts when you get the chance to give it a thorough comparison.

I got the blackface DR back from the shop today. It sounds great. It's loud for a DR. It has a period-correct replacement speaker, 1969 Jensen C12N Concert, 50 watts. The previous speaker was a Jensen Special Design C12Q.

BUT the Super knocks it out of the park: rich, full, shimmering sound. CLEAN. I compared them at roughly the same volume, 7 on the DR, 5 on the SR, with tone controls at 5 across the board. My practice space is pretty big.

Strange story. Here is a summary of the work done on the DR:

"Replaced incorrect output tubes (from EL34 to 6V6), removed all modifications (Class A to AB and fixed bias to variable, vibrato/reverb blend). Replaced screen grid resistors, replaced arcing on/off switch, soldered all bad joints, chemically treated controls and sockets. Tested under load and live."

I bought that amp 30-plus years ago. It has been used often but far from constantly, and never at high volumes for an extended period. I've had a few buy offers along the way. The amp guy has serviced it two or three times before. How did these mods go undetected? The previous work the amp guy did was not extensive. Plus, he said that one of his employees probably did the previous work. Is that explanation believable? Now he does all the work himself.

[orri]

3. Cathode Bypass resistors.

They are called cathode bias resistors (or just cathode resistors).

There are also cathode bypass capacitors (but you had already said replace all caps).


I like your point about replacing all caps. I guess the common practise is to replace only the electrolytic ones, but an amp from 75 is now 50 years old.
If it has 10-40 film/ceramic/other caps most of them will probably work for another 50 years but it is likely that one will not. And if the amp is already open, it is relatively easy and inexpensive to just change them all while you're at it. Especially if you intend to reflow all the solder joints anyway.
Perhaps something like changing all electrolytics every 10-20 years is and changing the other ones every 20-40 years is enough.

[DawgBone]

A trifle, I can get a whole SF for parts for$500. Save some spares for the next 40 to 50 year service and I'm way ahead.

I bought my last Twin in working order for $500, lol.

Most guys aren't going to want a parts amp laying around but I can 100% see where this isn't a bad idea.

[nevershouldhavesoldit]

Most guys aren't going to want a parts amp laying around but I can 100% see where this isn't a bad idea.

What spare parts can you get out of a derelict amp that you'd actually want to use? The small components are useless, not rebuildable, and cheap to buy new. The transformers, switches, sockets etc are likely to be on their last legs unless they were replaced just before the amp was retired. And if that's the case, something else sidelined it and you won't know what it was until you try to use a failed part. You're paying a lot of money for a few old parts that you could have gotten new for a lot less. The speakers are also a crap shoot unless they're fresh. And if they're good, most of us would sell them separately from an amp that's out of service before selling it for parts.

I'd rather put the $500 into new spare parts for an amp that I loved and planned to keep using forever.

[Cavalier]

What spare parts can you get out of a derelict amp that you'd actually want to use? The small components are useless, not rebuildable, and cheap to buy new. The transformers, switches, sockets etc are likely to be on their last legs unless they were replaced just before the amp was retired. And if that's the case, something else sidelined it and you won't know what it was until you try to use a failed part. You're paying a lot of money for a few old parts that you could have gotten new for a lot less. The speakers are also a crap shoot unless they're fresh. And if they're good, most of us would sell them separately from an amp that's out of service before selling it for parts.

I'd rather put the $500 into new spare parts for an amp that I loved and planned to keep using forever.

I'm going to disagree. I use those old derelict parts all the time. It's called my amp haha. Works great.

Can you actually get those transformers new? Mine is a SF linear Twin that puts out 135 watts RMS.

The only transformer I ever blew was in a 50s Kay practice amp that was overdriven to death so blowing one is a pretty small scale worry.

[DawgBone]

I'm going to disagree. I use those old derelict parts all the time. It's called my amp haha. Works great.

Can you actually get those transformers new? Mine is a SF linear Twin that puts out 135 watts RMS.

The only transformer I ever blew was in a 50s Kay practice amp that was overdriven to death so blowing one is a pretty small scale worry.

He makes a good point though. You blow a transformer and a new one is $200 from Mercury, that's quite a bit less than a $500 junk Twin. Most of the small parts aren't worth hanging onto. The only "expensive" part I've replaced was on my beater '72 was a choke that had come loose. Was like $60 from Mercury.

[nevershouldhavesoldit]

I'm going to disagree.

I’d have been surprised if you didn’t. You must have a lot more experience than I do.

Can you actually get those transformers new?

Start with Hammond, Pacific Audio, and Mercury Magnetics. There are other sources, but they’re my go-to makers. I have Mercuries in my ‘70 Twin (power & output), my ‘76 Princeton (output), and a 2x10 2x6L6 Pro clone built by a friend and left to me (along with the Twin and Princeton) when he died of a very rare illness.

I rebuilt the Twin & Princeton and have been trying to make the clone work well since I got it - it just sounds terrible. His daughter told me he never got it to sound great either. It has a very odd EQ system with a 6 way “vari-tone” rotary switch that changes the tone stack architecture. At this point, I’m just going to rip out the board and build a BF Vibrolux clone on the chassis using those great trannies. I may even leave out the vibro, since I don’t use it.

If I need a transformer, I check Mercury first. They’re very well made and every bit as robust as the original Fender OEM units. But all 3 above are excellent. For a power trans for your 135W Twin, there’s part # 013830 from TubeAmpDoctor in the UK (I don’t know who makes it) and a similar one made in California by Pacific Audio (both about $250 USD).

[Cavalier]

Helpful gents, thanks on transformer leads. I don't need one but will see if those places are still in business if I ever do. Again a donor might be the ticket if those places have shut down.

I may have a advantage over you guys in that lots of my amp use is for electric violins. Bowed notes don't have the sharp transients of pick attacks so less spikes and sharp vibration.