Perplexing Hum in Plexi 50

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roadhouse
Junior tube assistant
Posts: 1
(4/10/04 7:37 am)
 Perplexing Hum in Plexi 50
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 I just built Dougs Plexi 50. All new parts. Sounds great. I also incorporated Dougs Western Electric Reverb circuit into the design. It works well also.

I have a hum in the key of B I can't get rid of that does not change with volume level. And there is mild distortion at low volumes where I expect clean.

When checking voltages I noticed the hum got louder when testing pin 5 and 6 on one of the power tubes. When I checked the resistance on the 5.6k resistor across pin 5 and 6 it tests out at 6.2k. The other tests out at 5.6k. Could this be the culprit? Could it be the tube? I also am getting higher pitch static.
I have checked all grounding and tried to follow Dougs grounding layout. I've checked and rechecked all resistor values and traced all wiring. As a matter of fact I completely rewired the unit because I got a different chassis for it. The hum was there before and is still there now.
Here are my voltages:

9 pin pre amp tubes
(Valve 2 is the reverb circuit)
Pin        Valve 1        Valve 2        Valve 3        Valve 4
1        66        435         125         32        
2        0        0        0        9.4        
3        3.09        6.12        .8        32        
4        AC        AC        AC        AC        
5        AC        AC        AC        AC        
6        40        83        217        168
7        0        0        124        10.25
8        1.17        1.09        130        32        
9        AC        AC        AC        AC        

8 pin power tubes
Pin        Valve        Valve
1        .027        .027
2        AC        AC
3        448        448
4        442        445
5        -.033        -.032
6        -.033        -.032
7        AC        AC
8        .027        .027

B+ 280

The transformers are bought from Doug. The MPT50 and JMP50.

 
Tiny Daddy
I will work on all amps
Posts: 608
(4/10/04 8:58 am)
 Re: Perplexing Hum in Plexi 50
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 Where's the bias on the power tubes? Pin 5 should be measuring negative something, maybe -30V? Also, add a 1-ohm resistor on the cathode of each output tube (pin 1 to ground) so you can measure tube current.
 
roadhouse
Posts: 2
(4/12/04 4:37 am)
 Perplexing Hum in Plexi 50
--------------------------------------------------------------------------------
 The bias is at -32ma. I have tried moving through the entire bias range. The hum goes away when biased cold.

I found the source of the high pitch static and put a mini coax on the reverb mix pot to the 250p cap(see reverb schematic) but didn't do anything for the hum.

It is not a loud hum but just enough to concern me that something's not right.

Last night I nixed the master and went back to to just a single volume. I think this may have taken care of the mild distortion at lower volumes. I like the sound without the master much better.
 
Lucid Alice
Senior tube assistant
Posts: 194
(4/13/04 9:18 pm)
 Re: Perplexing Hum in Plexi 50
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 A thought on the master volume. You could use a pot with a switch. Wire the switch to disconnect the MV circuit. That way you could still have the MV when you wanted it.
 
roadhouse
Posts: 3
(4/14/04 8:20 am)
 Perplexing Hum in Plexi 50
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 Thanks for the advice on the switch. Thought about that and will reconsider.

I found the source of the 'mild distortion'. Dougs layout for the reverb circuit mod shows 2 - 22uf caps for the filters. Replaced those with 50's as shown in the Plexi circuit.

I just throw the parts together and have not advanced to the level of circuit design but are the 22uf caps a mistake?

In any case this amp sounds absolutely wonderful now. Provides me with some long lost inspiration for playing.

So far I have built the bassman amp, the revibe and now the plexi. Sold my Cybertwin. There is no comparison to a PTP tube amp.
 
 
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