Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: dude on July 24, 2025, 05:43:51 pm
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I just installed a Hammond 125 DSE OT in my Vibro Champ, used the 10K primary into an 8 ohm 10" speaker. Amp played fine on lower volume setting but when I turned the volume to 10, not so good. Volume was crackling and distorted, cutting in and out, like an issue. Lower volume much better but digging in started issue again. Could this be related to the location of the OT to the PT? I also changed the 2 tone coupling caps to orange drops, could one be leaking? Want to eliminate a location problem first.
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What OT was in there before, don't most Champs have a 5k primary, would the swap to a 10k primary be a cause?
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Did you mount the OT where the filter cap can was? If so, what did you do in terms of the filter caps?
Did you add that extension for the speaker cable at the same time you changed the OT?
As I understand it, the placement of the OT in relation to the PT is more likely to produce hum than the crackling sounds and other issues you report. So look at the other things you changed.
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Played fine with another 9.2k primary OT but mounted in sane spot as the tiny 5k original OT. 6v6 can run fine on 7 to 10k impedance. But yes the original OT was 5k but the speaker was 3.2 ohms.
The spk extension is just a conversion to a 1/4" jack, played with it. All l did was two changes the OT and two caps in tone circuit. I know lead dress can cause issues but hum like mentioned, no hum. Just when strings are hit hard the crackling and bad breaking up distortion happens.
I'll put the caps back l had before to eliminate that, the only other issue was the Hammond OT has long leads, rather then cut them l twisted them to spots that wound cause no lead dress issues but maybe they are, l seriously doubt it. Usually what one did before is the cause.
Just what causes a bad distortion on any volume only when hitting the strings hard..?
And yes installed three filters where the can was, on standoffs inside the amp, was done yeas ago.
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More likely to do with your wiring or something else you did (or inadvertently did), or something else entirely, rather than being caused by PT/OT EMF interaction
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More likely to do with your wiring or something else you did (or inadvertently did), or something else entirely, rather than being caused by PT/OT EMF interaction
Yes, found the problem, blackface amps used a brass plate behind the volume pots, thin ground wires soldered to the plate. Terrible design, IMO, one of the tin grounds lifted loose was just sitting on top of the solder blob. I should probably take the grounds put them on a buss bar grounded near the input jack.
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If you double the OT impedance you must revise Bias and B+
Using the new 10k / 8ohm Transformer you can connect a 4ohm speaker to the 8ohm intake and obtain a 5k reflected impedance to the Primary
Franco