Fender TRRI bias issue. Help!

[Enzo]

Did we determine if a power tube had failed? That many times takes out a heater fuse.

[Chuck H]

Steelwitch indicated that each tube was behaving under idle current. But I think that's all we know.

[Enzo]

With the present tubes, but did the 10A fuse blow while they were in use, or was it blown from before? Had a previous power tube failed then?

[Steelwitch]

With the present tubes, but did the 10A fuse blow while they were in use, or was it blown from before? Had a previous power tube failed then?

The fuse did not blow while the old output tubes were in idle. And yes, the output tubes were stable, but the bias range was way too high to be safe. -49 to -52 v

[Steelwitch]

With the present tubes, but did the 10A fuse blow while they were in use, or was it blown from before? Had a previous power tube failed then?

As far as I know, or at least with continuity tests, none of the tubes were shorted. They are the stock tubes from 2015, so I insisted that the client let me put in new JJ's. Either way, these output tubes are probably tired as heck, but not shorted.

[nevetslab]

Can I throw out a question to someone who might know? Why would these 220uf caps be reading over capacitance? It doesn’t make sense to me unless it was a manufacturing error. I tested them in circuit which could explain why they’re testing so high, but typically I get a ballpark reading if it’s within 10% of its rating.

You'd need to measure capacitance with the caps removed from the circuit, or at least one of the leads. I can't recall the last time I've seen an electrolytic cap read greater than the nominal rating. The Tolerance spec'd allows the mfgr to use less foil in the construction and still meet the spec, hence less than the specified value, while within the tolerance spec.

[Leo_Gnardo]

Why would these 220uf caps be reading over capacitance?

What nevets said ^^^ also leaky caps often read above their ratings on some capacitance meters. If I see 20% or more over, I don't think "hey! extra capacitance for free" instead I think "leaky = replace." Time was, way back when 40-50 or so years ago, electrolytic cap specs often read -20%/+80%. Not so much in recent times. Better manufacturing techniques usually put caps in plus/minus 20 or even 10%.

[Steelwitch]

Update: this amp got a recap— open resistors replaced. Bias is still very high. Meter is reading 0485v on the plate at the lowest bias setting

[Steelwitch]

I’m starting to wonder if my bias probe is not giving me the correct readings.. it’s giving me the plate voltage but not in mV like it should

[Helmholtz]

I’m starting to wonder if my bias probe is not giving me the correct readings.. it’s giving me the plate voltage but not in mV like it should

Bias probes insert a 1 Ohm resistor between cathode and ground and allow measuring the voltage across this resistor. The only explanation for several hundred volts at the bias probe is a blown/open resistor. You may verify with your Ohm meter.

[Enzo]

Plate voltage is never in millivolts, unless it is absent. The cathode current in in milliamps, usually read as millivolts across a 1 ohm cathode resistor. Plate voltage will typically be several hundred volts.

[Steelwitch]

Wait! Yeah I’m a huge dingus— just got them biased at 43.6 mA. I think we’re in the clear, I’m gonna put the output through my scope and give it the final check

[nevetslab]

Wait! Yeah I’m a huge dingus— just got them biased at 43.6 mA. I think we’re in the clear, I’m gonna put the output through my scope and give it the final check

43.6mA plate current is high....I'd adjust the bias control to drop them down to, say, 35mA, and verify all four power tubes to be in that range.

I don't know who's bias probe you're using...whether you're using your DMM with the bias probe, or if it's a dedicated bias probe meter with probes....bias would be in mADC, plate voltage would be in +VDC, if there's a grid reading, that would be the applied bias voltage, in -VDC. Normally, the bias current is read with your meter on 200mVDC range. If it reads Plate voltage, it's scaled, and probably also read on the same 200mVDC range, as would bias voltage.