Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Twosteps on March 25, 2019, 02:30:41 am
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Hi, I think I need to hear some suggestions from you guys once again. Yesterday I mess with my amp a bit to change the on/off switch to a dpst standby but after I finished the problem occurred. The amp loss a lot of volume and sounded very fuzzy and sitar like. I have no idea what could cause this as the amp was absolutely fine before I replaced the switch. Here's what I've noticed that may or may not have something to do with the symptom.
- B+ was 480v, that's high, last I remember it was 420v and I was worrying already so I changed the power tubes to JJ's to withstand that. (power from the wall is swinging between 230v - 240v).
- First I thought the first filter cap was blown because when I tapped it, it made an audible noise. So I checked & replaced the filter caps and actually they are fine.
- One of the power tubes has purple spot glow which I didn't noticed before so I tried swap another set in, same result..
- Checked the power resistor and it was fine too.
I think that was way out of the way to check what happened since all I did was a couple of soldering jobs around the switch. I'd really appreciate any comments/suggestions.
Oh, it the same amp I always messing with - 5E9A
Thanks very much.
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The first thing to suspect should be the last thing you did to the amp. Maybe the switch is not wired correctly. Or maybe something else inadvertently happened while you were changing the switch.
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"Sitar - 420"? Are you sure this is an amp issue?
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@Sluckey - that what I thought so I reverse the process and change back to how it was including the old switch but no luck, anyway, I'll stay away from it today to clear my head and will tackle it again tmr.
@bmc, I honestly don't know what's the problem so far, last I checked I really don't think anything's broken. The issue is the loss of volume & a fuzzy sitar like sound.
I mentioned the voltage just in case.
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Sorry, I was just being a wiseguy. Sitar music and 420 are associated with a certain dry refreshment.
But, joking aside - I have experienced that problem and it has always been a sketchy or forgotten solder joint. Something could connect fine for a while, but get jarred loose when you work on the amp. If you have a hard time finding the problem, a simple signal generator and tracer can be made from a smart phone or pad with a tone generating ap and a small practice amp. That setup has saved me a lot of time chasing signal problems.
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With the work done, a sensitive wire might have been moved and set the amp oscillating.
Have you fitted 1 ohm power tube cathode resistors?
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Don’t know the history of the amp you are messing with. Is it an original example? Custom build? My question is if custom is there a effects loop?
Jim
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@bmc haha, sorry I was slow yesterday & didn't get the joke :laugh: but yeah, I'm sensing that it might be something stupid I've done, either that or cold joint. I'll see today.
@pdf no, I haven't
@ritchie it's my build, been using these parts & trannies for many projects, but really love this circuit and want to stop here. So it could be that some part might fail due to that but I doubt it though, every time I rebuild I also rechecked the parts if they're ok and replace the ones that's needed. Oh, and there's no effect loop, just straight forward 5E9A.
Thanks for chiming in guys, I'll checked it today and report. :worthy1:
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You changed a power switch to a standby switch?? Is this switch in the primary (mains) side, or in the b+ line? If the former, it can't have anything to do with your problem. Anyway, you ruled it out.
Bad solder joint is always a possibility. I had low volume sitar-like sounds from faulty coupling caps, though it came with an irregular tumbling thundery noise. Had it twice now with faulty silver mica caps from the same batch! they only had very little dc leakage so not that easy to spot. Thought i'd mention it because symptom appears similar.
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Found it! Since me and everyone here suspecting more or less the same thing some stupid physical mistakes or some bad joints, I went looking for it.
Considering the voltage that came out of the rectifier tube has gone up 60v higher, I don't know what the right technical term but I've been suspecting there might be some sort of constipation(please excuse my limited knowledge haha) somewhere in the power section. PDF's comments reminded me that the cathode resistor that I use has leads that are really difficult to solder. I couldn't tell from looking at it but when I wiggled it, voila!, it was actually barely touch the eyelet. I know it was a silly incident but again I've learnt something, so thanks very much guys!
BTW, I really couldn't recommend this circuit enough, right between 5E3 and 5G9 - to me a 5E3 is too much of a cowboy and 5G9 is too close to brown/blackface amps. This one has it just right especially if you're not a pedal user.
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Hey, it happens to everyone, so not a stupid mistake IMO. Just the other day I traced an error and found that a tube socket eyelet with two connections had one good solder joint and one crappy one. All part of the process. And you just put the 5E9A on my list.