Gretsch Airline amp bias questions

[jpcar]

I'm trying to figure out the bias of this singled ended tube amp.

The amp power supply has been re-capped with the values shown below the originals on the schematic. And it has a 6L6 replacing what should be a 6V6. There's a 10uF bypass cap on V5's cathode to ground in parallel with the 330ohm resistor. I don't see any other changes to the schematic.

I changed back to a 6V6 and checked the bias. (Using the voltage drop and resistance of the cathode resistor). I’m getting idle current of 62mA, with plate voltage at 375v. This seems pretty hot! tedweber.com Bias Calculator says: 6V6 in Class A with plate voltage 375v. 90% dissipation is 28.8mA

I checked the bias with a 6L6 in place out of curiosity. Idle current of 73mA with plate voltage 365v. Weber calculator says 90% dissipation is 46.8mA.

The capacitors tested ok but I replaced them anyway. I went back to stock values. I used a 47uF instead of the 20uF x2. Now, there’s a (not huge but reasonable) 120Hz hum. Reconnecting the 330uF capacitor eliminates the hum. Is this higher value capacitor safe and a good solution? Is this common practice?

Upping the cathode resistor is bringing the bias current down but not anything like enough in get it close to the weber calcuation.
Any thoughts?

[Jon Snell]

My thoughts are many;
Valves vary by manufacturing technique and will differ with anode current readings, calculations are not for valves, they are precise measurements that valves do not adhere to because valves vary so much.
Is the anode glowing? ... No means it has not reached the danger point, yes means too late.
The absolute maximum value filter capacitor is 20u, 47u is pushing it and 330u will destroy the cathode. Always read the datasheet!
Valve dissipation is current multiplied by voltage. That is 100%. So you need cathode current, using simple Ohms Law I=V/R (of the cathode resistor voltage/resistor value) multiplied by the anode voltage multiplied by how hot you want it, say 70%.
Don't forget to look up the data sheet and find the current value of the screen gid, to remove that from your calculations.
http://www.r-type.org/exhib/aai0014.htm
http://www.r-type.org/exhib/aai0136.htm

[mozz]

Yes, go back to stock values on the filters. What kind of 5Y3 are you using? What is your 6.3vac reading?

[Helmholtz]

Agree with the above. Do not use higher than 20µf reservoir caps with a 5Y3. Max. total reservoir capacitance is 32µF.
Main problem is excessive voltages - especially screen supply. But even original schematic voltages indicate a plate dissipation of 16W or 114% of the 14W limit
Cathode current should not be above 40mA.
You may try to increase the dropper resistance between B1 and B2 to maybe 10k/5W.to lower screen voltage to around 300V.
This will also lower hum because of improved screen supply filtering.
Clean screen supply is much more important than clean plate supply.

[jpcar]

Thank you all!

Do not use higher than 20µf reservoir caps with a 5Y3. Max. total reservoir capacitance is 32µF.

So there should only be a single 20uF cap for the B1 supply and not 2 in parallel as the schematic suggests. I'll try that.

What kind of 5Y3 are you using? What is your 6.3vac reading?

The rectifier tube is quite old and is not labeled. It looks like it's probably a 5Y3 judging by it's size and look, but I'm not totally sure. 6.3vac reading is fine.

Is the anode glowing? ... No means it has not reached the danger point, yes means too late.

If you're talking about the rectifier tube, then yes I believe so. It looks brighter than I'm used to seeing. I replaced it with a new JJ 5Y3, and the voltages dropped a bit.
B+ at 360v, B2 and 6V6 plate at 345v, and the cathode resistor voltage drop has lowered to 21v (from 24v). This is better already!

You may try to increase the dropper resistance between B1 and B2 to maybe 10k/5W.to lower screen voltage to around 300V.

I will try this next!

[mozz]

At one point, the JJ 5y3 they were selling were putting out too many volts. Hopefully that is now fixed. A asked about the 6.3 filament voltages because if they are too high that means the transformer was probably meant for 110-117 volts and a bucking transformer is your best cure to drop everything.

[g1]

At one point, the JJ 5y3 they were selling were putting out too many volts.

I think it was the Sovtek that had that problem?

[mozz]

Yes you are right. Old age/too much lead intake/ not even 60 yet.