Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: BigE on June 04, 2011, 12:27:58 pm
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Hi guys. Got a head scratcher and am looking for some suggestions....
On the bench is a SF Pro Reverb 45W Master Volume. It just doesn't have any power and it sounds lifeless. You can dime out it out and it doesn't get near as loud as what it should be. This happens with both channels.
All the parts are stock including the power caps. Voltages on all the tubes are good. Voltages on all the stages of the power caps are good. Bias current is great. Swapped tubes with known good tubes and still no power.
Any ideas/suggestions?
schematic is here...
http://www.webphix.com/schematic%20heaven/www.schematicheaven.com/fenderamps/cbs_45w_mstrvol_pullsw_super-pro-bmstr_rev.pdf (http://www.webphix.com/schematic%20heaven/www.schematicheaven.com/fenderamps/cbs_45w_mstrvol_pullsw_super-pro-bmstr_rev.pdf)
thanks to all in advance!
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Well, if the bypass cap on the cathode of V2B was dried out that would reduce the gain of both channels.
The output transformer could have a shorted turn or something.
You have checked the bias individually on both output tubes?
You may need to use an oscilloscope and do some signal tracing.
Good luck.
Nathan
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Do the pop test on each grid. You might find a stage which pops a lot quieter. Then check the coupling cap there to see if it is leaky. A leaky cap can affect signal strength by affecting the bias of the following stage
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replaced that cathode cap on vb2 and no luck.
Bias on both power tubes are identical.
One thing I did find when doing the pop test was that pin7 on V6B pops lower and kind of with a muffled *thud* sound than the crisp *pop* like the others. So I'm wondering if it's that .1 coupling cap there. (and of course that's the 1 cap I don't have at the moment....
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Lift (unsolder) the far end of the cap (the end that goes to the 1k5 grid stoper and 220k grid return) and test for dcv.
Should be 0vdc.
Brad :icon_biggrin: