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Twin 135 lacks volume


 
4/12/1998 9:54 PM
Charles
Twin 135 lacks volume
OK, this ones a stumper.  
 
I've got a Twin 135 in the shop... the one with the UL output trans.  
 
It lacks volume on both channels, about equally. I'd say it sounds about as loud as a Champ!  
 
The 6L6's have 505vdc on the plates, about the same on the screens. Amp has new screen grid resistors, new cap in the power supply. There is some slight hiss when each channel is turned up full, which sounds about normal. But when playing... there's no volume!  
 
Checked out the phase inverter caps/resistors and they're ok. Did a test on the grids of the power tubes (touched w/meter lead). I expected a pretty loud pop, but am only getting a fairly low volume one. Tubes are running at about 28ma per tube, no signal.  
 
Hell of it, I tried all new tubes, same thing.  
 
Tested my "test speaker" on another amp, no problem there. But even with the signal generator, it's just not there volume wise.  
 
This one really stumps me. Any help appreciated.  
 
Charles
 
4/13/1998 7:42 AM
Doc

If all the tubes, including preamp, have correct DC operating voltages on all the elements, and the output transformer is good, there is a signal coupling problem between two stages. Could be as simple as an open coupling capacitor.  
 
The easiest way to find the problem is to signal trace, either with a small high gain audio amp & speaker, or with an oscilloscope. Connect an audio tone source at the input jack. Look at the signal at the plate, then the next stage's grid. You've found the problem area if the signal doesn't appear at a tube's plate, or doesn't appear on both sides of a coupling capacitor.  
 
Doc
 
4/13/1998 4:28 PM
Charles

Thanks Doc.. sometimes I need a reality check.
 
4/15/1998 9:20 PM
Charles

For anyone who's interested...  
 
It just so happened that all, yes all, of the preamp tubes that he sent with the chassis were pretty rotten.  
 
Unlikely... but that's the way it goes sometimes.
 
4/16/1998 8:11 AM
Doc

Whenever I get an amp in for repair, I pull each tube and test it for transconductance & shorts on a tube tester just to get an idea where I'm starting from. This doesn't screen out noisy tubes, but will tell you if, for instance, one side of a 12AX7 isn't working, or a power tube is wasted.  
 
I sort of assume that when someone posts a perplexing problem here, that they've made all the obvious checks.  
 
Always check the tubes, and also wiggle the tube slightly while it's installed to check for poor/intermittent socket pin connections.
 
4/18/1998 6:14 PM
Charles

Well, you're right of course!  
 
Usually... if I suspect a bad tube, I'll just  
do a quick swap with the vibro tube, or swap with the other chanel. If that doesn't do it,  
I have a set of "shop tubes" that I'll use to replace the entire set. However, the shop tubes were in another amp and I thought it very unlikely that ALL the tubes would have @#@#$ gain. They did.  
 
Of course overlooking the obvious makes one frustrated since the path towards obscurity draws one more distant from the real solution.  
 
I'll neatly package this experience and put it away in my bio-notebook.  
 
Thanks Doc
 
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