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Strange input problem after retube


Erik Hoversten (erikhoversten)
Username: erikhoversten

Registered: 11-2013
Posted on Sunday, November 10, 2013 - 06:44 am:   

Hi Folks!-

I'm new to this forum, and I'm not really sure if I should be posting in the "front panel" section or in the "urgent repairs" section. Apologies in advance for the double post.

I've got an HD 130 (with a tube phase-inverter and no reverb) that has a really strange problem. I've had the amp for 20 years, and it's always been great. A few weeks ago, I had it re-tubed with JJ EL 34s. Ever since, something has gone crazy with the inputs.

In the past, I always used both of the inputs (1 and 2) together on the "normal" channel. I used this setup in order to use an A/B switch to switch between two sets of pedals. It didn't matter if I used input 1, input 2, or both together. The tone and the output volume were always the same on the amp.

But since the tube job, the inputs are behaving differently. Input 1 is much louder and broken up than input 2. I am told that it's "normal" for input 1 to be slightly louder, though I never experienced this difference in the past. The most confusing part about the new input volumes is that if I have signal going to input 1 and I plug a 1/4 inch cable into input 2, the output becomes quieter. On the other hand, if I have a signal going to input 2 and I plug a 1/4 inch cable into input 1, the output becomes louder.

I have heard of Fender Twins, etc. having this sort of behavior by design. But I've had this amp for 20 years, and it's never done this before. The guy who re-tubed my amp claims that he didn't touch the preamp or anything else besides the power tubes.

I contacted Terry Loose (a musicman repair guy who is highly rated on this forum.) He was very helpful, and indicated that what I was describing did not sound like normal musicman behavior. But before I ship my amp all the way across the country to him, I figured I might ask if anyone has any thoughts on what I'm describing. If I had to guess, I'd wonder if what I'm experiencing might be some sort of impedance issue or something???

Ian Azrikan (inertian)
Username: inertian

Registered: 10-2013
Posted on Sunday, November 17, 2013 - 11:48 am:   

Hmmm...did the tech rebias your amp after the tube change? If so, he would have had to go inside the chassis, in which case who knows what could have accidentally been touched?

OTOH, it might just be a simple case of a dirty or bad input jack, have you tried cleaning with DeOxit or similar? Also check the connections and values of the input jack resistor to verify everything is still connected and in spec.

Good luck.
Erik Hoversten (erikhoversten)
Username: erikhoversten

Registered: 11-2013
Posted on Tuesday, January 28, 2014 - 04:13 pm:   

I opened up the chassis to have a look at the input section. I plugged 1/4 inch cables into inputs 1 and 2 of the "normal" channel and hooked them up to my muti-meter. With cables plugged into both inputs, input 1 showed 0 resistance. Input 2 showed a reading of 84. As I understand it, the two inputs should give the same reading if both inputs are being used.

To double-check my methods, I repeated my test on the "bass" channel. With cables plugged into inputs 1 and 2 on the bass channel, I got readings of 83 on both.

So it seems like there is something strange going on with the input section of the "normal" channel. But I don't see anything visibly wrong. Any suggestions on how to proceed? I'm hoping that the solid-state preamp isn't the culprit.

Many thanks,