Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: AZJimC on November 28, 2013, 08:43:17 pm
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Howdy,
I have reworked and modified my little 18w amp, and have it nearly to my liking. Ive done the basic 18w circuit, plus reverb, and then went all wanky and added a gain stage, which has exposed a bit more hum than before.
In checking the heater wiring, I learned that there are two 6.3v windings on the tranny, and since It has diode rectification, someone previously wired it with one winding to the pilot lamp, and then grounded one lead and ran this to the Power tubes, and the Phase Inverter. The other 6.3v has a center tap grounded, and that winding is wired to all preamp tube heaters. Neither set is loaded much at all, as they both measure a bit over 7v.
One question is, Exactly how twisted should the heater wires be? These are somewhat lazily twisted, maybe a twist or two every three inches. I'm assuming this would be the source of the 60hz humm i hear at high gain. It does appear that this hum originates with the first stage, and while it is not very loud, it does grow if I increase gain on these early stages. I'm prepared to rewire all the preamp with a very twisted wire set, and likely add the power tubes to this circuit in hopes of bringing the voltage down a bit. I'll then abandon the 6v winding intended for the rectifier tube.
Does this sound like a good approach? Should I leave the power tubes wired to the 1 leg grounded circuit, or would this likely be inducing a bit of hum as well?
Thanks Y'all,
Jim
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Do heater winding like any Fender Blackface amp . Do you what I mean ? There no amp more quiet like a a good Fender BF .
In you case , use only one heater winding from the Power Transformer , with a center tap . Twisted winding , more twist is better , one or two by inches . But there are so many other thing to do to quiet an amp .
Send picture of the amp wiring .
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It'll take me a bit to do photo's, and I'm not sure how helpful they will be, as this is P2P wiring, and a bit rats nesty around the tubes. The only noise that reaches annoying levels is 60hz, so I figured I need to sort out the heater circuit first, and then see where I'm at.
Tube layout is:
V1 - 12ax7, A-input, B-post tone stack gain
V2 - 6av6 -Reverb Recovery
V3 - 6av6 -Trem Oscillator (Shakes V1a cathode)
V4 - 12ax7 -Parallel Reverb Send
V5 - 12ax7, A-Gain B-PI (not positive which sort, but it looks like single triode PI)
V6 -V7
I'll get some pics soon as practical