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Symptoms of bad coupling caps?


 
10/25/1998 5:57 PM
Jonathan Krogh
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Symptoms of bad coupling caps?
I am doing a traynor YBA1 conversion to a JTM45, i was informed by a YBA1 owner that i should be wary of the .01mf coupling caps on the plate of the phase inverter, as these give problems in these amps, mine did look a little scorched, but i didnt replace them.  
i have redone all the electrolytics, changed some resistor values, but in an adjustable bias etc.  
Yesterday i was playing the amp and suddenly some loudish hum kicked in, i noticed one of the 7027's started glowing red, so i shut it down.  
today, i swapped in 6L6wgb's, rebiased, and played it awhile.  
Then i was suddenly possesed to check the tubes for microphonics,  
on tapping the 6L6's, loud popping and crackling started, then that high hum and plate glowing like before.  
So im beat as to the cause of this,  
would it likely be those caps are the problem?  
thanks
 
10/25/1998 6:28 PM
Kevin Timm
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Sounds like you are loosing bias voltage. Could be several things including the tubes, wiring, bias supply.
 
10/25/1998 8:48 PM
Bruce
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Me too Jon,  
If tapping around the circuit causes this,  
I think you have a bad solder joint or component in the bias supply somewhere and you are loosing the negative voltage.  
By the way, did'nt your Traynor come with 630v Sprague Orange drop caps in that spot?  
 
Bruce
 
10/25/1998 9:07 PM
Jonathan Krogh
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ive just been over the whole bias rig,  
even replaced the pot,  
i clipped my meter leads in on the problem socket and fired up the amp.  
started out idling the 6L6wgb at about 25ma,  
went up slowly to about 30 within 2 minutes, then i knocked on the board, then it ran up quickly to over 50 then i shut off.  
tried this all again with a pair of svetlana 6550c, same thing.  
No it didnt have orange drops, some chessy looking ceramic colored looking stuff, ive been warned about them.  
all resistors check out fine,  
i dont have replacement caps, but have ordered  
but would you think the caps are the prob here/  
thanks
 
10/25/1998 10:32 PM
Bruce
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First of all let's assume you don't have some wierd parasitic oscillation going that can trigger with vibration.  
You didn't say where you clipped the DVM leads to on the socket...  
I've seen coupling caps leak quite few volts to the final tubes and not really kill them. Change them anyhow if they're leaking.  
But a few hundred milivolts cap leakage on the preamp tube will flip out the next preamp stage. A few volts on the power tube grids will not blow them up. Change them anyhow.  
If you hook your DVM (set on the DC +500v scale) and measured the negative bias on the tube's socket while your playing or tapping on the amp and you'll see what the bias voltage is doing if the tube starts to draw excessive current.  
This is why we have two or three DVMs on the bench all the time Jon.  
You can monitor the current, B+ and the grid voltage all at the same time.  
If the grid voltage goes from -35/-45vdc to +300vdc for a few seconds while you are tapping around the board then you know you have a bad coupling cap on that one.  
You can even do this without the tubes plugged in.  
If the bias voltage jumps to zero then the bias supply is probably shorting to ground somewhere or is comming disconnected somewhere.  
The thing is, if the caps are leaking they are leaking positive B+ from the previous stage to a grid that is supposed to be -35v to -55v and it would take a real bad cap to leak enough to turn the tube on like that at idle. But, yes, I have seen that happen too.  
And as a matter of fact I had a BRAND NEW 1998 Marshall at the shop two weeks ago with this exact problem.  
As far as cheap caps go ...the coupling caps from the driver stage have to hold back the B+ AND handle the AC swing from the previous stage too. It's the sum and the differences of those two voltages that the cap is subjected to.  
If the B+ is 375v and the cap is rated at 400v then the AC signal added to that will cause the caps to be overvoltage at high drive levels.  
Too much of that woould sure be tough on them and eventually they would start leaking like a stuck pig.  
If the caps in question are cheap then you know they are going to fail.  
I still think you might have a bad solder joint, solder blob under the board that is shorting the negative bias voltage to ground, or a wire that is coming undone or shorted when oyu tap it or play loud etc etc...  
something like that is what I'd be looking for.  
Change the caps anyhow if you're worried about them.  
 
I know that was wordy!  
Keep us posted and the fix will come up.  
 
Bruce
 
10/26/1998 4:55 AM
Richie
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Johnathan:I would do the things Bruce suggested,I would also look for a bad solder conection,or something on or under the board.Check the tube socket wiring.You might be able to push around on the board to see if you can find a specific place that causes it,or what part of the board the problem is.Fenders get this way sometimes,or stray solder gets under the board,and when you push around it will crap out. [Richie]
 
10/26/1998 10:44 AM
Mark
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Jon, Make sure your transformers are screwed down tight. I had a similar problem with bias going all over and after hours of trying all sorts of stuff I found an extremely loose OT which when tightened made the problem disappear. Mark
 
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