Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: hiegdk on May 22, 2024, 12:34:56 pm
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During a recent build (Matamp GT120 clone) I encountered a soldering error that resulted in what I thought was a really cool sound - though definitely a fault. I've tried to highlight it in the attached screenshot (and the full schematic is there for reference too).
Essentially I failed to tie the two halves of the PI together. The result was that the amp was way down on output power and but it also sounded really cool - kind of a high gain stoner-rock / metal kind of sound that I really liked, just quite quiet compared to how blisteringly loud that amp is when working properly. I then fixed the issue and the amp is great now but I was curious if someone smarter than me could explain what that wiring error would have been doing, and what the knock-on effects of it would be for an amp. Could that mistake be somehow worked into a a proper design - as I said, it sounded pretty good!
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If there was no connection in the place you indicated, the amp wouldn’t work. (Edit upon further inspection, I note it’s supposed to be a DC coupled cathodyne inverter, but without that wire, there is no connection.
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I agree. No connection as you indicated would have resulted in NO sound.
The schematic is drawn wrong and You need a coupling cap there. As to what caused your happy sound, this isn’t clear from the information you provided.
However, the schematic is correct. It's called a direct coupled cathodyne. No cap is required. Sunn used this circuit in most all of their old tube amps. I own two such direct coupled cathodynes. They work just fine...
https://sluckeyamps.com/sunn/sunn_sceptre_1971.pdf
https://sluckeyamps.com/rocky/rocky.pdf
edit... You're too fast Tubeswell. :icon_biggrin:
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I agree. No connection as you indicated would have resulted in NO sound.
The schematic is drawn wrong and You need a coupling cap there. As to what caused your happy sound, this isn’t clear from the information you provided.
However, the schematic is correct. It's called a direct coupled cathodyne. No cap is required. Sunn used this circuit in most all of their old tube amps. I own two such direct coupled cathodynes. They work just fine...
https://sluckeyamps.com/sunn/sunn_sceptre_1971.pdf (https://sluckeyamps.com/sunn/sunn_sceptre_1971.pdf)
https://sluckeyamps.com/rocky/rocky.pdf (https://sluckeyamps.com/rocky/rocky.pdf)
edit... You're too fast Tubeswell. :icon_biggrin:
Yes i realised that after my initial post. I had difficulty squinting at the schematic initially.
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Ah, interesting. So the error wasn't that that connection was entirely missing, it was that the solder connection to pin 7 was bad as I recall - I think the insulator was in the joint a bit. I assumed it was not connected at all, but maybe it was somehow making a partial or poorly conducting connection? What would the effect of adding a resistance between pin 1 and pin 7 in that circuit be?
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I suppose a high resistance solder joint at pin 7 could cause the symptoms you describe. I probably would not call it a cool sound.
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Cold solder joints can have diode-like behavior, though it's very unpredictable and typically only manifests in hi-Z circuits.
BTW, Hi everyone.