troubleshoot after recap

[hebaton]

just finished a full recap ( everything electrolytic ) on a 1970 Dual showman reverb.
When I fire it up, I get sound, no noise but one tube immediately starts red plating. I am assuming something about bias ?
There are two electrolytics on the bias circuit but I didn’t have those, so the old ones are still in place. Also, the power tubes I got with the amp are miss matched, looks like two pairs of different type. Not the same bottle shape. The amp worked before I changed the caps, but I wanted to give the thing a fresh start. I also removed 2000pf caps on two power tubes as suggested by fellow board members. Looks like an AA769 version...

What should I look at first ?

[roberto]

Have you set the bias at the coldest point?

Report all voltages and check if they fit in a +-10% or better +-5% with these:
https://schematicheaven.net/fenderamps/ ... _schem.pdf

[sluckey]

There are two electrolytics on the bias circuit... ...Looks like an AA769 version...

AA769 only has one bias cap. Two bias caps would make this an AA270.

[hebaton]

There are two electrolytics on the bias circuit... ...Looks like an AA769 version...

AA769 only has one bias cap. Two bias caps would make this an AA270.

here is a pic of the bias circuit before the recap ( that part did not change )
I swaped tubes and the issue remains with the socket, not the tube...

[hebaton]

I cant seem to find 50uf@70v as requested. I assume I can use 50uf@1000v instead ?

[xtian]

I cant seem to find 50uf@70v as requested. I assume I can use 50uf@1000v instead ?

Use 47uf/63volts or higher. Much easier to find.

[roberto]

I cant seem to find 50uf@70v as requested. I assume I can use 50uf@1000v instead ?

I guess it's a typo and you meant 100V. If so, it's ok!

[Stevem]

The first rule of Thumb to live by before you put output tubes back in, and fire up any fixed bias amp after a recap job is to confirm that you have in your case, bias voltage on pin 5 of over -49 volts on all the 4 output tubes.

[pdf64]

Always use a light bulb limiter for the initial power up.

[hebaton]

I cant seem to find 50uf@70v as requested. I assume I can use 50uf@1000v instead ?

I guess it's a typo and you meant 100V. If so, it's ok!

Yep, absolutely a typo !

[hebaton]

The first rule of Thumb to live by before you put output tubes back in, and fire up any fixed bias amp after a recap job is to confirm that you have in your case, bias voltage on pin 5 of over -49 volts on all the 4 output tubes.

AM I correct to believe I should ajust bias to get around -45V at pin5 on 6L6s with 440 plate voltage ?

[martin manning]

You really need to measure the current. You can do that in several ways: use a bias meter, measure cathode voltage after installing 1 ohm 1% resistors between the cathodes and ground (mA = mV), or use the DC resistance and voltage drop across the OT primary to calculate it. I would shoot for about 65% of the 6L6's 30W max plate dissipation or (0.65 x 30W)/440V = 44 mA.

[hebaton]

I tryed working trough the DC resistance method but unless I was oing something wrong, I measured a voltage drop of 2,8v that meant 74,6ma makes no sense ?

[martin manning]

Carefully remeasure DC resistance from the CT to each plate lead, and then the two voltage drops, and post them. Should be straightforward.

[hebaton]

Just tasted 440 dc at the tip of a finger... will go back to measure... Loooks like 39ohms on one pair and 37,4 on the other pair...