Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: wsscott on February 15, 2021, 08:52:12 am
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My musician friend asked me for input on a problem this amp is having. When it is first turned on after being off for overnight, for example, the pilot light is not illuminated, there's no sound, and there's no glow in the tubes for around 2 minutes or more. Then everything springs to life and its fine. If you turn it off for awhile and then turn it on, you get sound after about 15 secs. I haven't taken the chassis out of the cabinet yet, but I did check the voltages at the filaments and they're reading 6.3-6.6 VAC. There's also voltage at the plates, so once it "turns on" its fine.
My guess is that its a failing Power Transformer. I say this because, based on the schematics I've seen, the Power light is wired directly to the power transformer filament leads with nothing in between to prevent power getting to the Power Light. So it seems it should light up as soon as the switch is flicked. The aux. power jack is getting power since its directly connected to the line plug.
If it is a failing PT, should it be replaced asap to avoid any damage resulting to the amp when it should die, or just wait it out?
Thoughts? Thanks.
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Open it up and examine the solder connections on the pilot light. This is the most likely suspect.
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P.T. is rarely a issue.
Sluckey is right
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ALL the connections from the wall to the PT.
That assumes a "dumb" amp. If it has a microprocessor, gut it and build a dumb amp in the works.
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Thanks. I'll check out the pilot light connections first.
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Well that checked out okay. I couldn't get it to re-create the problem. The voltages going to the pilot light and then onto the filaments is as it should be--3.3v on each leg of the pilot light and 6.6 on the filament pins. The only thing that might be the cause is the V-1 tube socket is not very tight and that tube is the rectifier. Could that be the issue?
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Technically its the V-7 socket and not the V-1. Regardless its the rectifier tube.
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Nothing on the rectifier socket could cause the pilot light to go out. Reflow the solder connections on the lamp assembly.
You also said no tubes were lit. If that includes the rectifier tube then look on the primary of the PT as PRR suggested. Look for a loose connection on anything between the power cord and the PT primary wires, ie, fuseholder, switches, etc. Maybe just reflow every solder connection if you don't see anything obvious.
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When I said it checked out okay I meant that not only did the solder connections look good, and reading good values at the pilot light, when I turned it on after sitting all night and the caps totally drained, the light came on within 1-2 secs of turning it on and the tubes started heating up and I had sound after about 15 secs. But your point is good about the lamp. Why it would start working okay without me doing anything to it beats me. I didn't go around jiggling wires or anything, so I don't know. Thoughts?
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Intermittent problems don't usually heal themselves. Everything I said above still needs to be done. Or you can do nothing and give it back to your friend. Then you can fix it when he brings it back. :l2:
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Good point!
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I just realized that the filaments on the rectifier are heated from the yellow 5v wires, and the rest of the tubes are heated from the green 6v wires. Yesterday when I started it up for the first time none of the tubes immediately started warming up, nor the pilot light. It was about 2 minutes or so. So power wasn't getting to either the rectifier from the 5v wiring or the others from the 6v wiring.
How can I test the wiring to see if there is anything happening on the primary or secondary windings?
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Follow PRR advice, answer #3
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How can I test the wiring to see if there is anything happening on the primary or secondary windings?
You been told a couple times already.
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I know, I know.
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Just do it. Just do it. :icon_biggrin:
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Careful observation and patience are required with an intermittent problem.
Courage, you will find :thumbsup:
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I think its the SPST On/Off switch.
I did as everyone suggested. I first ran continuity testing on the Power line from the plug, into the amp, the unswitched socket, and onto the fuse etc. and finally to the switch. Continuity all the way through.
I then checked the continuity on the switch it self... Off-no continuity....On-intermittent!
I then resoldered all the joints. I don't know if the soldering will help, but I think I'm going to recommend replacing the switch.
Since the pilot light and all of the filaments on the 6.3 volt tubes, and the filament on the 5v rectifier tube do NOT light up when it is experiencing the problem, it seems that power is not getting into the transformer. And power gets to the transformer by the on/off switch. Thus that's why I think that's what needs to be replaced. Why its acting intermittently, I can't imagine. I don't know how this simple switch is constructed. Maybe its just wearing out.
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The switch is a mechanical device. It's very common for them to develop intermittent failure prior to complete failure. Good job!
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So I've got to order the switch (you can't find anything like this in Charlottesville), and after I get it installed I'll let you all know if that solved the problem. Thanks for your help.
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...(you can't find anything like this in Charlottesville)...
Not 72mi down 64, either!
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Doug has them in his store - 6th item down on the page.
https://hoffmanamps.com/MyStore/perlshop.cgi?ACTION=enter&template&thispage=Switches &ORDER_ID=!ORDERID!
{EDIT: links into Hoffman's store with "ORDERID" in them will not work for anybody else. (They are linked to your unique IP number.) Take the "&ORDER_ID=!ORDERID!" off the end. -PRR}
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Success!! The switch came in and I installed it this morning, and everything starts right up as it should, ie. the pilot light and the tubes immediately start to glow! Thanks again.
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Interesting, the old switch felt soft and mushy and silent when you flicked it. The new one is crisp and clicks. So that probably should have been a giveaway. But it’s not my amp so I didn’t know what it should feel like. Anyway, good to resolve it cheaply.