Change bias resistor or add voltage dropping resistor - or both

dan40

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@cottontails1959 ...I know that anytime I have experienced redplating in a tube amp, the first thing that alerts me to an issue is a steadily increasing hum that continues to grow louder as more current is pulled through the tube.

I was also wondering if perhaps the primaries on the new OT were backwards. This happens a lot in new builds or when swapping an OT. There is a 50/50 chance that the wires attached to the sockets will be correct. In most cases, the amp emits a loud squeal when they are backwards but in some cases your amp may simply act strange or possibly redplate. The fix is easy...just unsolder the wires on the sockets and swap them. In your case, if the amp sounds good and the tubes appear to be fine then you most likely have the primary wires correct.

Edit: I forgot to mention that this only applies to amps with an NFB circuit. After looking at your schematic again, your amp does not employ nfb so you have nothing to worry about.
 

cottontails1959

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@Phrygian77 @JamesAM ... voltages with Fender 6V6's ... no red-plating :p

wall = 122.5VAC
B+1 = 357VDC
B+2 = 348
B+3 = 308
6V6 plates = 351
6V6 screens = 348
cathode = 20.9VDC (measured cathode resistor at 216 ohms)
RR calc. dissipation = 15.1W / 108%

With my 15W OT, I'm good to go? Also, a higher ohm k-resistor will flatten somewhat the class A part of the load line, so more voltage swing available?
 

Phrygian77

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@cottontails1959 if it's not red plating, yeah, I think you're fine. However, I'm going to say this again, if your goal was slightly increased power and headroom, you really should get the bias down a bit. You have all the numbers, draw a load line, or use one of the online tools.
 

cottontails1959

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Sorted! After ~99% dissipation with a measured 265 ohm cathode resistor with a 300 ohm resistor I ended up at ...

wall = 123.0VAC
B+1 = 371VDC
B+2 = 361
B+3 = 319
6V6 plates = 364
6V6 screens = 361
cathode = 24.0VDC
RR calc. dissipation = 12.8W / 91%

With a 250K audio taper volume pot, nice bedroom volume at 5, clean and loud at 10. Relaxed looking tubes 😁 Sweet!
 

JamesAM

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Sorted! After ~99% dissipation with a measured 265 ohm cathode resistor with a 300 ohm resistor I ended up at ...

wall = 123.0VAC
B+1 = 371VDC
B+2 = 361
B+3 = 319
6V6 plates = 364
6V6 screens = 361
cathode = 24.0VDC
RR calc. dissipation = 12.8W / 91%

With a 250K audio taper volume pot, nice bedroom volume at 5, clean and loud at 10. Relaxed looking tubes 😁 Sweet!
Congratulations and enjoy your new amp! I bet it sounds awesome.
 




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