Re: UPDATE & ideas ??? snarlin' Pro-Reverb

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9/1/1999 9:04 PM

jeff

Re: UPDATE & ideas ??? snarlin' Pro-Reverb hey all,     i replaced the preamp tube socket, the volume pot, and the power tubes but that was not the problem.     I can 'originate' the problem:     (Loud pulsating feedback, LOW frequency only, and     the power tubes actually pulsate - like in a brown-     face Princton when the tremelo is on)         ONLY by putting the Volume on 7 or more AND the bass control on 5 or more and then hitting the low E string heavy.         I suspect that the "circuit card" has become conductive     but only goes crazy with a really strong and bassy signal     pumped thru it. Any ideas would be much appreciated.

 

9/1/1999 11:20 PM
Carlo	Jeff, if you haven't already done this, try testing the output tubes the same way you tested the preamp tubes ...gently tapping and/or swapping for a known good pair. Also, check for loose connections throughout the bias supply (from the cap and diode to pin 5 of the output tubes).

 

9/3/1999 5:26 AM
jeff	thanks for the ideas Carlo, now that I think about     what u said, this weird problem does cause the     power tubes to behave just like they were having     their bias modulated (like in my brown-face Princeton)     which visually and aurally pulsate and glow blue!     darnest thing is this only occurs when the amp is really     cranked (volume on 7 and bass tone pot >5 .

 

9/3/1999 4:23 PM

Tim C.

Jeff or others,     I am fighting a brown 1961 Bassman head that cuts out at the settings you described, or close to. It only does it in the bass channel. I am really curious. Does you Pro act up in both channels or just one. The Bassman will just shut down to almost no output as you strum a power cord. It only does it when the bass is turned up and the volume is set at about 7 or 8. Has anyone else ever experienced this phenomenon?     Any help is greatly appreciated.     Tim C.

 

9/3/1999 4:44 PM
Gus	check or change the coupling caps

 

9/3/1999 4:56 PM

Doc

There may be "grid blocking" in one of the preamp stages, or possibly the power amp stage. You can find it on a scope, but not every tinkerer has one of those useful troubleshooting tools.         If you're using this amp for guitar, you don't need the extra low bass response which the coupling circuits can pass. For greater stability and resistance against signal blocking on the power stage, the standard 0.1uf coupling caps can be downsized. Also, the bias feed resistors to the 6L6 grids can be made smaller (check the tubes' bias after this mod, because applied grid bias voltage could be different).         Sometimes it's a matter of resoldering all the component connections to remedy a problem such as this. Also, make sure all the plate to grid signal coupling caps are healthy, and the grid stoppers (1.5k?) & screen series resistors (470R?) are good.         You may have already gone through these suggested checks. If so, you may need to bring in the big guns and signal trace each stage with a scope until you find out exactly where the signal block is occurring.

 

9/5/1999 12:55 AM

TubeHead

thanks all you for the ideas:         Gus - coupling caps are new (hell I have replaced     nearly every component in this amp) they are .1     716P sprague's.         Doc- I have a good dual trace scope so you really     have my attention now - this "blocking" you mention-     could you tell me what should I look for? - I have noticed that this amp clips at volume 5 (clipping at the .1 -what I call the 'final coupling caps' that feed the power tubes.         Tim C - thanks for reminding me of one the basic rules     of troubleshooting- stage i/o testing .. I realize     that i have stupidly not tried the faulty setting-     Volume 7 and up, bass >5 on the norm channel - I will     and I'll let you know.         ALso I noticed today the power trannie has rust     on the outside and at some time someone globbed     some black paint on one side to cover or inhibit     this rust .. I have read that rust-ing trannies can     start to act "crazy" because of spurious eddy currents.     Could this cause my 'snarling' problem?

 

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