Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: jada3909 on August 21, 2023, 01:04:41 pm
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Hello,
I'm building a 5f1 'champ' amplifier using the hoffman layout and turret board. I did the dreaded smoke test a few days back and got no smoke! Unfortunately, I got no sound also, except some hum. I started reading other posts looking for stats on a healthy 5f1 and started doing my own measurements. First resistance measurements:
V1 (12AX7) Resistance
Pin2 - 33.7K
Pin3 - 1.48K
Pin7 - 1.4
Pin8 - 1.4K
V2 (6V6)
Pin5 - 220.7K
Pin8 - 462.....(Note this measurement was a factor of 10 less than what the other forum member got, 4700).
Next I measured voltages...here's where things go sideways.
Rectifier (5U4)
Pin2 - 337AC
Pin4 - 337AC
Pin6 - 311.8
Pin8 - 312
V2 (6V6GT)
Pin3 - 311
All other pins on V2 had very small voltage readings, most less than 1 volt!
The same for V2 (12AX7), all very small voltages.
Finally, I measured the voltage at the 470 ohm, 5 Watt resistor to ground and that was a very small value as well.
I read in the forum how some new caps can fail so I swapped out the 16uF cap to no avail.
I bought a pair of 6V6GT tubes on ebay and they both behave the same way...could they both be bad?
Aside from tossing the whole thing in the trash, does anyone have any suggestions on how to proceed?
Thank you!
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I'd suspect something wasn't wired up properly, but I'd suggest posting some good photos of your build so people can see the wiring in detail. In fact, posting such photos and the layout/schematic is what people here expect to see when someone is asking for help troubleshooting.
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Ok, good point! I'll get on that now!
Thank you!
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Aside from tossing the whole thing in the trash, does anyone have any suggestions on how to proceed?
Yes. First off, take some high res photos of your project so that we can see your work and layout; please post these. It really helps. Most often this will be a simple wiring error. In fact, Doug wrote up this page https://el34world.com/Forum/index.php?topic=17701.0 (https://el34world.com/Forum/index.php?topic=17701.0) to help you troubleshoot.
From your post, you say that you used the Hoffman layout. Did you make any changes? That layout is located here: https://el34world.com/Hoffman/files/Hoffman_5F1.pdf (https://el34world.com/Hoffman/files/Hoffman_5F1.pdf)
What model power and output transformers did you use?
As for tubes, well, you never know what you're going to get on ePay. Your issue is probably not a tube problem (but I can't rule that out).
This amp will sound great! Your hiccup is a normal part of the startup process. It does take some time to sort these things out.
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Here are some pics...
My power transformer is a mercury magnetic FTCP-59 and my output transformer is another ebay gem that was identified as a 'vintage' hammond. Model unknown.
Thank you!
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this is ALWAYS a good place to start when things ain't what they should be;
https://el34world.com/Forum/index.php?topic=5.0
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I don't know how you read those shiny blue resistors. The first thing I see is the resistors coming out of the input jacks, they look like 6k8 to me or maybe 6.8r.
Dave
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It's hard to see how your tube sockets are wired up - you may want to double check the wiring on those. Also can't tell about the output jack - looks like maybe one too many wires connected there?
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Can we get a picture of that output transformer? Did you test it?
Dave
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Hello and thank you for looking at my post...
1. The resistors hanging off the input jacks are 68K...It looks kind of messy
2. The output jack is a real pandora's box. The output transformer came out of an ebay amp build that I bought. I didn't like the build quality and tube choices so I went with Doug's
design. I tore up the wiring on the output socket more than once because I wasn't sure what wire from the output transformer to wire to the tip pin of the output jack. I decided
to connect the output transformer to the output jack like the previous builder did. Perhaps that was a mistake...If the output transformer wires were reversed would that cause
the big problems I am having?
3. The output transformer is a real black box. There is no make or model info. It was identified as a 'vintage hammond' on ebay but there is nothing on the bell end, (only have 1 of 2).
4. Maybe I should replace the output transformer. does anyone have a suggestion on the impedance, 5K, 7K, 8K.?
Thank you for your time!
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I wouldn't rule out the output transformer just yet.
I agree, something looks a little fishy with the wiring on the output jack and octal socket.
Maybe try getting another piçture looking down on the sockets and output jack
How many wires off of the transformer? What colour are they? There is (probably) wires coming off of each "side" of the tranny. That should be a good indication of where to start testing.
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Go double check the wiring on the rectifier. You list:
Rectifier (5U4)
Pin2 - 337AC
Pin4 - 337AC
Pin6 - 311.8
Pin8 - 312
But shouldn't Pins 4 and 6 have the same voltage feeding in from the power transformer? Either it's hooked up wrong or you got the pin numbers wrong in the chart.
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Thats a good observation about the rectifier voltages! I will look into that tomorrow! I'll also try to get some better pics.
Thank you all for your time!
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Looks like you've done some fairly good work there.
Surely the problem can be tracked down.
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Follow the voltage from the power supply to V1. So transformer output, rectifier output, then check the voltage at each filter capacitor in order, this should tell you where you are losing your voltage, which seems to be the case. I would suspect an open at power supply dropper resistor, probably the first one because you have voltage at the plate but not the screen, but let's not jump to conclusions.
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Rectifier (5U4)
Pin2 - 337AC
Pin4 - 337AC
Pin6 - 311.8
Pin8 - 312
V2 (6V6GT)
Pin3 - 311
All other pins on V2 had very small voltage readings, most less than 1 volt!
The same for V2 (12AX7), all very small voltages.
See question-mark and arrows in diagram (These should be highish DC voltages - so use the 'hundreds-of-volts' setting on your V-meter)
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I think I got it!! Notice the plus side of the 16uF capacitor in my 'pic5.png', that is supposed to go to pin 8 on the rectifier. I can't wait to see if that fixes this!!!
Thanks to everyone!! Thanks dogburn for getting me to look at that rectifier! Maybe this fixes it,... or maybe not!
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that is supposed to go to pin 8 on the rectifier.
and where did you actually put it?
before you let smoke out, you really should trace EVERYTHING against the schematic AND the layout. It will find 99.9% of installed errors
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He probably forgot to run a wire from the rec to the first filter cap, so it wasn't put anywhere. He's got the OT ran direct from the rec but none of the rest of the power supply is even connected.
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you meant PT, not OT? otherwise smoke will be released :icon_biggrin:
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No. He measured B+ at the plate which means power was run to the OT. The OT was run direct from the rec but no wire was run to the first filter cap. My best guess anyway.