Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: scstill on June 12, 2021, 08:41:00 am
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I just finished this champ build and have a frustrating problem with very low volume.
I did some checking and research on this and other forums, but have not determined the issue.
This champ circuit has the following mods: a single input, a MV (my other champ builds are quite loud) and an OT with 4/8/16ohm switched secondary. Point to point wired.
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All the voltages noted on the schematic
Checked and double checked the connections and resistor values - all seem ok
Checked the jacks - all seemed ok
Changed out tubes - no change
Bypassed the MV - no change
Bypassed the spkr switch - no change
Tried a different OT - no change
Appreciate any suggestions. Thanks, Steve
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Voltages are normal. I suspect a wiring error, or bad shielded cable.
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You have your nfb resistor crossed out, just wondering why?
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Voltages are normal. I suspect a wiring error, or bad shielded cable.
I replaced all of the shielded wire with non shielded wire
Double checked the connections.
Still extremely low volume.
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Check input jack connection ; a bad wiring may send sound to ground.
Try it with no shorting contact to ground , only + to resistor's tube input and negative to ground.
What I read, you check everything ...Something you do wrong ?
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also try "hot wiring" the speaker, no switches, jacks spade lugs.........
just secondary to speaker
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DPDT switch wiring across the OT secondary looks unusual
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You have your nfb resistor crossed out, just wondering why?
From RobRob....
"The 5F1 amplifier uses negative feedback (NFB) to reduce distortion, increase headroom, decrease damping factor and improve stability but a drawback is it also reduces overall amplifier gain. Negative feedback works by taking the speaker output voltage and feeding it back into the amp's signal stream before the driver or phase inverter circuit. A feedback resistor reduces the voltage to a suitable level before it joins the amp's signal stream. It's negative feedback because the signal is out of phase so when it's injected into the amp's signal stream it reduces the amp's signal voltage."
I wanted to try without NFB but its easy to add back in...
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You have your nfb resistor crossed out, just wondering why?
I wanted to try without it but its easy to add back in...
You can take it out, but you can’t just jumper the NFB resistor with a wire, or it will dump full NFB to the preamp (I’m assuming you didn’t jumper it)
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Did I say I checked everything?
I meant I checked everything except the input resistor (grid leak) on the input jack. :smiley:
I had 1kohm installed. When I replaced with 1Mohm I got the true champ volume
I discovered as I was setting up Latole's suggested input jack test.
The OT switch on the secondary I have done before with good success. Allows any speaker cab to be run from the amp. The Hammond 1760C provides 8k to 3.2/8/16.
did not jumper the NFB. Plan to try it with (22k) and without (open) NFB to see what sounds best. Maybe will add a switch.
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Great ! I'm happy for you.
Most of the time, when building a amp, if the amp do not work, it is 90% a builder mistake