Bias Calculation And B+ 1971 Vibrolux Reverb

[keithb7]

Hi kind folks. I have my 1971 Fender Vibrolux Reverb on the bench today. I installed new B+ caps, bias caps, and cathode caps.
The amp has the bias balance circuit in it.



I have balanced the two 6L6GC tubes using the bias resistor that was in the amp. The resistor in there now 15.49k
Plate voltage measured 415V at 37.5 mA
b+ voltage measures 416V DC or so volts at rectifier pin 8. That seems low to me. Schematic calls for 465V.

According to several bias calculators on the internet I am currently seeing 15.5W plate dissipation or 51.9% on the 6L6GC tubes.
My understanding is, I replace the resistor on the balance pot, center wiper to bring the bias amperage up. However....
Before I do that, maybe I should try and get the B+ voltage up a little? Guessing start with a new rectifier tube? And check everything again?
Tubes are of unknown age. Look like many are original.
I measure 355V AC off the power transformer secondary. Schematic shows 345V. I am getting 122.3 VAC out of my wall.

Unless of course, I leave the B+low and adjust the bias resistor. I am up to trying that. However I am unsure what resistor value I
should go with to get up to 50 or so mA, or 65% plate dissipation on the 6L6GCs.

Thanks for your help. Slow learner here. Hobby-ist. - Keith

[xtian]

You've got a pretty good grip on this, so keep plugging. First, how does the amp sound? Loud enough? Crossover distortion? Decent tone?

How did you measure the 37.5mA tube dissipation?

Certainly, if you have a new rectifier tube, try it to see how the voltages change. Otherwise, don't worry about 415 on the plates. Yes, it's lower than Fender's schematic says, but it's fine for the 6L6s.

Best plan for the bias voltage is to install an additional bias adjustment pot.

Google: add bias adjustment pot to Vibrolux

[nickb]

What is the voltage after the rectifier with the standby switch open? I would expect somewhere around 490V.

Roughly with the switch closed the current is around 80mA. The plate current of the rectifier will be much higher at around 160mA. Looking at the rectifier datasheet, the drop at 160mA will be 40V making the B+ = (335*1.4-40)=457. That's pretty good agreement with the schematic.

If you have a lot less then, at you suggest, try a different rectifier. Another possibility is the bias is a lot higher than you think it is.

The terminology "standby on/off" has always been ambiguous to me.

[g1]

Agree with nick about making sure bias isn't hotter than you think and pulling down B+.
Find out what is causing your low B+ issue, but aside from that, is there a sound problem that makes you want to bias the tubes hotter?
50% sounds like it's probably in the ballpark of the Fender factory settings. If it's not having a major negative impact on the tone, biasing hotter will only serve to reduce tube life.
The popular bias numbers found on the internet do not seem to have much in common with the actual numbers used by many manufacturers of classic tube amps. They sure work in favour of tube sellers though.

[keithb7]

Played around with the bias balance resistor. Dropped it from 15K down to 12K. That did get me up to 50 or so mA to get to 70% dissipation but the amp sounded buzzy and unstable.
I put the 15K back in at I am now at 37.8 mA at 38.1 mA on the two 6L6 tubes. PV at or about 414V to 419V.
According to calculations I am at 52% or so now. Amp sounds very good here.

Voltage after recifier with power and stand by active is 414V. Amp with no sound, (standby switch off?) I measure 502V.

I played around with some spare 6L6 tubes I had, and another 5U4. No b+ voltage improvement. I see some really mixed up current measurements depending on what tube is in there.
I found two, as close as I could get them using balance pot. Will leave it at that for now and go buy a balanced set of new 6L6 tubes and start over. Not having good known tubes
kinda tosses all this measuring to the wind I suppose. Thanks.

[g1]

Just for the heck of it, check for AC voltage on the B+.

[nickb]

I wouldn't worry too much about the matched tubes. if you can balance the idle currents, then that is good enough.

Over 500V with the standby open is a worry. Those first filter cap are rated at 450V. The surge rating is probably something like 475V. On the other hand they have lasted so far....

My estimate for the standby open voltage would have been on the low side, I now realize, as a meter doesn't give an accurate measurement due to distortion of the voltage waveform.

On the other hand, the B+ does still seem lower than expected. With 15K in the bias I expect the currents are in the ballpark and it may just be a case of that's the way the voltage is. The only other thing I can think of is if one side of the rectifier tube wiring was open ( or a bad socket) then the drop would be larger than expected.

If the ripple is high then the B+ will also measure low and I think that is where g1 is headed. With many meters, to measure the AC ripple on the B+, you need to put an 0.01-0.1uF 630V cap in series with the meter to block the DC.

[keithb7]

Ok, new matched Ruby 6L6 tubes just installed. I like to know I am working with something fairly reliable, not a big investment, worth it to me.
Set bias balance. Had 33.5 mA at 434V with 15K bias resistor.
Pulled 15K bias resistor. Installed 10K resistor again.
Each 6L6 is pulling 42 mA at 423 volts. About 59% plate dissipation.

Amp sounds great! Real nice. I assume we had some tired old unstable power tubes? When i got the current up on the old ones they got noisy.
Ok, so I am content with bias and amp tone. Voltages still seem a little low, but amp sounds great. No complaints I guess.

I am not sure how to measure AC ripple on the B+. I don't have cap to put in series with the meter. If there is another way, and someone can
walk me through it, I'll try. Thanks.