Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: ajeffcote on June 09, 2013, 01:46:39 pm
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I recently acquired a Mojo kit tweed twin low power built a year or so back. The amp sounds pretty good, but I think there is room for improvement.
First, there is a ringing when I play notes on the B string. Not on all notes, but not on particular notes either. If I reduce the pick attack enough the ringing is not heard. Turning the treble pot down also reduces the ringing.Tried swapping all the tubes and it's still there. Is this just the way it should sound?
The tone stack is pretty strange. The bass pot turned all the way down and up to about 3 results in a big loss of volume. Then around 9 to full on the lows are reduced. From 3 to 9 there is no change that I can hear. And the amp is quite treble laden. I find myself dialing in humbuckers at 3-4 on the treble pot.
I don't have much experience with this stuff, just a couple of blackface style, hand held builds. But I do have plenty of time on my hands.
With 2 channels, I'm thinking of eliminating 1 and using the extra tube for a bias mod trem circuit. I would also like to voice the amp a little differently. I love the cleans from the Hot Rod Deville 2-12. Can I get ther from what I have without a total revamp?
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The vague way you describe the ringing sounds like it could be oscillation from bad wiring, or some other badly laid out or soldered aspect. Hard to tell without more information.
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Describing sound is not one of my strong points. If you've tapped around on the chassis close to a bad tube and heard a ringing, tinking sound, that's in the ballpark. When I strike a note that rings, it is not overwhelming but more of a nuisance. The ringing is not sustained, rather it's short lived. So far it is happening only with the B string. Tried different guitars, still the same. I'll get some pics tomorrow. The amp is very quiet when idling.
A few of non standard issues. An extra main filter cap was added, and the pre-amp is all 12AX7s, no AY7s. Both the main filters are installed before the standby switch (so the rectifier charges them more slowly), a pot was installed for bias adjustment. Leads were shortened where-ever possible.
Got to find my camera!