I have this to repair and the rust on the chassis is an issue. I can get my brother who is a panel beater to get rid of the rust but I’m wondering how to treat it so it doesn’t rust again for a very long time.
Of course it has unusual values in the volume and tone controls. The volume is 1M Lin pot, the treble is 1M log pot, the bass is 2.2M log pot, the speed control is 1M Lin, and the intensity is 220K Lin.
It’s hard locating pots for the amp. Any thoughts about the repair?
A most unusual repair.
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A most unusual repair.
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Yours Sincerely
Mark Abbott
Mark Abbott
Re: A most unusual repair.
Only one preamp tube? Is there more gain somewhere not shown?
Don't understand what's troubling you about finding replacement pots.
Don't understand what's troubling you about finding replacement pots.
I build and repair tube amps. http://amps.monkeymatic.com
Re: A most unusual repair.
The output valves are 6GW8’s which are a 12AX7 and an EL-84 in one envelope.
1M log is plentiful, as is 220K to 250K Lin, I can find 1M Lin but it’s smaller, and is pcb mount. 2.2M log pot is a tricky one to find.
1M log is plentiful, as is 220K to 250K Lin, I can find 1M Lin but it’s smaller, and is pcb mount. 2.2M log pot is a tricky one to find.
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Yours Sincerely
Mark Abbott
Mark Abbott
Re: A most unusual repair.
Gotta take everything off and remove the rust, then seal it. There are oils meant for this purpose, not sure what is available where you are. You could also spray paint the thing. Maybe a clear coat enamel on the front panel if it's just screen printed and doesn't look too rusty already. Rethinking the ground scheme and only have one chassis ground is probably wise.
Pretty sure alpha still makes 2M pots, but you might be able to take the existing pots apart and clean them.
More pictures.
I will post a circuit up later for reference.
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Yours Sincerely
Mark Abbott
Mark Abbott
Re: A most unusual repair.
The jacks are quite corroded and I doubt the pots have faired any better. I will look for a 2M Alpha pot.maxkracht wrote: ↑Thu Apr 10, 2025 2:07 amGotta take everything off and remove the rust, then seal it. There are oils meant for this purpose, not sure what is available where you are. You could also spray paint the thing. Maybe a clear coat enamel on the front panel if it's just screen printed and doesn't look too rusty already. Rethinking the ground scheme and only have one chassis ground is probably wise.
Pretty sure alpha still makes 2M pots, but you might be able to take the existing pots apart and clean them.
I had thought of using the stuff they seal PCB’s with, though there are probably better options. Not sure about painting it yet.
There is rust on the transformers laminations, I’m not sure how to deal with this. I could remove the rust and seal it with the pcb sealer.
Yours Sincerely
Mark Abbott
Mark Abbott
Re: A most unusual repair.
Usually the rust doesn't creep in between the transformer laminations, It should be pretty tight. I wouldn't worry about that too much, unless they are behaving strangely or heating up. If you end up taking the end bells off of the transformers, be sure to replace any insulation on the screws so you don't accidentally create a shorted turn.
I've worked on a few similarly rusty amps that were mostly fine, just bad connections to chassis and not nice to look at. Usually budget doesn't allow pulling everything off and stripping, so I just redo the grounding and make sure everything is tight. It is just kicking the can down the road for a few more years, but sheet steel shouldn't rust all the way through too quickly if you take it inside.
I've worked on a few similarly rusty amps that were mostly fine, just bad connections to chassis and not nice to look at. Usually budget doesn't allow pulling everything off and stripping, so I just redo the grounding and make sure everything is tight. It is just kicking the can down the road for a few more years, but sheet steel shouldn't rust all the way through too quickly if you take it inside.
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Re: A most unusual repair.
No. Its a high-mu triode and a pentode in one envelope. Its like half of a 12AX7 and some other pentode that is dissimilar from an EL84.
I build effects and amps and do custom builds to order. Boom.
Site: http://ballseffects.com/
Site: http://ballseffects.com/
Re: A most unusual repair.
I build and repair tube amps. http://amps.monkeymatic.com
Re: A most unusual repair.
Paint the chassis with Phosphoric acid to stop the rust.
Trade names vary but Ospho was one back in the day.
Trade names vary but Ospho was one back in the day.
Tube junkie that aspires to become a tri-state bidirectional buss driver.
Re: A most unusual repair.
The 2m alphas in my stash are linear, just checked Mouser and that appears to be all they have. You could just fake a log pot with a tapering resistor.
The next hurdle.
I have gotten the amp to work and make a sound, the tremolo is pitiful but the LDR is exposed to the light which will affect things.
The amp is quite bright sounding but not without some charm, the 2.2M bass pot is shot, it’s like a switch at the end of its travel. The treble pot works fine, and the volume is okay, though I’m not overly fond of a 1M lin volume control. The volume control has a 270pF bright cap across it, which would account for the bright sound of the amp. I had thought of putting that on a pull pot.
https://ampgarage.com/forum/viewtopic.php?f=6&t=37976
The tone controls return triode stage cathode circuitry has me scratching my head. The stage works and it sounds good, but how is the thing working?
The other thing is, is that I suspect this stage doesn’t have a lot of gain and the linear 1M pot gives the amp the impression that it’s louder than it really is.
I think this amp is good for 8 to 10 watts at best.
The amp is quite bright sounding but not without some charm, the 2.2M bass pot is shot, it’s like a switch at the end of its travel. The treble pot works fine, and the volume is okay, though I’m not overly fond of a 1M lin volume control. The volume control has a 270pF bright cap across it, which would account for the bright sound of the amp. I had thought of putting that on a pull pot.
https://ampgarage.com/forum/viewtopic.php?f=6&t=37976
The tone controls return triode stage cathode circuitry has me scratching my head. The stage works and it sounds good, but how is the thing working?
The other thing is, is that I suspect this stage doesn’t have a lot of gain and the linear 1M pot gives the amp the impression that it’s louder than it really is.
I think this amp is good for 8 to 10 watts at best.
Yours Sincerely
Mark Abbott
Mark Abbott
Re: A most unusual repair.
You're probably right, but still worth taking the cover off and checking. Sometimes extra crud builds up wherever the wiper was left in storage and just spraying with contact cleaner won't get rid of it. Also likely that the track is just missing, but only takes a minute to check.
Re: A most unusual repair.
For the chassis, remove the rust by whatever means you believe will be effective. This could be done with sandpaper, sand blasting, a rust remover solution or treatment, or electrolysis. When it is free of rust, find an industrial place that does galvanizing. Plead with them to dip your pitifully small chassis along with whatever industrial level things they are dipping for commercial customers. The "hot dip" is a zinc coating. If you can't do that, I guess you are left with asking an automotive body shop if they can spray a color next to a car they are fixing, or use a rattle can. Here is one company I know about. https://www.southerngalvanizing.com/ The problem is getting them to accept your nuisance job, basically, for free. If they had to charge you for it, it would be cheaper to fabricate a new chassis.