5b3 6v6 current

[kgreene]

I am attempting to service a very beat up old Fender Deluxe 5B3 chassis, and I'm a bit surprised by the idle current thru the 6v6 power tubes, which as measured via my old bias probe, is about 60mA for each tube. The V from cathode to plate is about 350, cathode to ground is 20 with a 250 ohm resistor (actual reading about 270). I'm puzzled by this, because I would expect it to be much lower. Also, the amp has a steady low hum, after filter caps replaced, filament supply checked, etc. Can't find any signal carrying wires that are crowded by AC carriers... Anybody have ideas what gives?

[gingertube]

20 volts across 270R => 74mA or 37mA per tube.
20 Volts is quite a bit lower than I would expect to see.
Is that reading no good or is your bias probe telling you fibs?

With a Vak of 350V that would mean 13 Watts anode + screen dissipation per tube, that is "hot" but not disasterously hot.

With only 20uF on the main filter then power supply ripple will be quite large. The circuit (common cathode resistor) relies on the idle current in the 6V6 s being equal so that the ripple cancels. Any residual hum is likely due to 6V6 idle current mismatch. Short of rewiring to use individual cathode bias resistors and/or relacing the 6V6s with a matched pair there is probably nothing you can do.

Cheers,
Ian

[Stevem]

Well by the RCA tube spec with 285 volts on the plate in fixed bias push pull they should idle at 70 ma.
How much ripple power supply ripple voltage do you have on the plate ?

[martin manning]

A preamp tube could be the source of the hum, but the heater wiring in that amp uses the chassis as one side of the circuit, which is not the best idea. You might consider rewiring the heaters with two wires.

[kgreene]

Can I determine the amount of ripple on the power supply without an oscilloscope? It's true that 20uF is the value of the first (and other power supply caps, but that is how the amp was designed, and I've played thru other similar amps that don't have this much hum.

I'm using an old bias probe that I have from the '80s. It seems to read accurately on other amps. When I use the probe on each individual tube, they are both reading about 59 - 60 mA.

[kgreene]

I thought about re-wiring the heaters. Can I just, detach the ground side and go straight to the filaments, then float each side of the 6.3V heater supply with 100 ohm resistors?

[kgreene]

BTW, thanks for the tips!!

[kgreene]

Also, I did replace the cathode bypass cap, just in case there was a leak.

[martin manning]

I thought about re-wiring the heaters. Can I just, detach the ground side and go straight to the filaments, then float each side of the 6.3V heater supply with 100 ohm resistors?

Yes just run a parallel heater string like the later Fenders. If you already have a grounded heater winding center tap then don't add the 100 ohm resistors.

[kgreene]

Thank you!

[labb]

The layout for the 5B3 has all of the voltages listed. It shows 350 v on the plates of the 6V6's, that would be to ground and 18 volts across the 250 ohm cathode resistor. I would check the pre amp tubes, 6SC7's tend to pick up heater hum. Hard to find 6SC7's these days that are not noisy.

[kgreene]

I'm still a bit mystified about that amount of idle current. Will the 6v6's tolerate that?

[billc]

[quote="kgreene"]I'm still a bit mystified about that amount of idle current. Will the 6v6's tolerate that?[/quote

The numbers don't add up, something is wrong with your test setup....the voltage across the cathode resistor indicates 74 mA total for BOTH tubes, not the 120mA you should see if each tube was really conducting 60mA

[kgreene]

Based on what you are suggesting, I guess it would be a good idea to check the plate current directly in line, and rule out a problem with the bias probe.

[JazzGuitarGimp]

Could an oscillation cause the bias probe to read higher than the DC voltage across the cathode resistor would suggest?