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Amp Stuff => EZ Board postings => Topic started by: Geezer on June 18, 2005, 06:19:29 am
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Hoffman Amplifiers
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> 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
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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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