Replacing 6L6s with 5881s - JMF Spectra 60t - No Bias Adj - Mod Suggestions?

[Jecarroll7]

New 5881 tubes are running too hot (55ma) but no bias adjustment.

Have attached the schematic, Any suggestions for a bias mod?

[Jazz P Bass]

Is that 55ma for both tubes?
What B+ voltage do you read?
What bias voltage do you read at the grid?
The Power Section schematic shows a 5K bias pot.

[Jecarroll7]

What I should have asked for first is a better set of schematics.

The bias pot shown on the schematics doesn't exist. May have been a factory mod? Can anyone confirm?

Tubes are showing ~55mA each, tubes are matched.

[Dave Curtis, dB AudioTech]

"Tubes are showing ~55mA each, tubes are matched."

What is the plate voltage (pin 3-output tubes)?

Bias voltage (pin 5-output tubes)?

"Any suggestions for a bias mod? "

Jazz mentioned a bias pot on the schematic; there's your suggestion...

[Gtr_tech]

And don't try to replace 6L6GCs with "re issue" 5881s. I'm not talking about the Sovwreck 6L work alikes that have 6L6WGC/5881 printed on them. Those are 6L6 replacements.

Sovtek 5881 / 6L6WGC Power Vacuum Tube
Sovtek 5881WXT Power Vacuum Tube

A "faithful" 5881 replacement...
Tung-Sol 5881 New Production Power Vacuum Tube

These are not intended to be used in 6L6GC circuits due to the lower plate current/voltage ratings.....30w@500v for a GC vs. 23w@400v 5881.

They *will* replace old metal 6L6s and early 6L6G/GA.

[g1]

And don't try to replace 6L6GCs with "re issue" 5881s. I'm not talking about the Sovwreck 6L work alikes that have 6L6WGC/5881 printed on them. Those are 6L6 replacements.

Thanks for the heads-up, I'm surprised to see the tubedepot suggest they can be used in "any 6L6 tube amp", and comparing them to JAN6L6WGB, which can take high plate voltages.
This is more like it: Tung-Sol 5881
("There are some important things to know about the reissue Tungsol 5881 tube before you decide to use it. The Tungsol 5881 is a 23 watt tube and is rated for a maximum plate voltage of 400Vdc.)

[Jecarroll7]

Okay, I have attached a slightly better schematic and have found the bias pot.

I cannot adjust the bias setting at idle any lower than 45mV (target is ~37mV).

To refresh I am running the new production Tung-Sol 5881s (thanks to Gtr-Tech for the heads up) in a 2x6L6 amp.

All knobs are turned down. I've also tried a couple different 6L6 tube sets looking for a cooler set, no luck.

On the output tubes Pin 3 is 462vDC, and Pin 5 is -44vDC.

The bias pot is all the way counterclockwise. All filter caps are fresh.

I've checked all resistors (in circuit and a few out of circuit to make sure) but haven't found anything way off yet.

Starting to chase my tail, appreciate any help.

[Jazz P Bass]

That '-44Vdc' bias voltage sounds more like a EL34 voltage range.
You could try lowering the 15K resistor on the bias supply.
I get the feeling that you do not have enough B- to start with.
-50Vdc to -55Vdc would be more like it.
You could try measuring the voltage right at the diodes for the 18 volt supply to see what is 'available'.

[Jecarroll7]

Good point Jazz. will check the the 18v supply rec diodes tonight. I believe the 60-T is originally a 2x6L6 amp, but haven't been able to confirm. There are reviews that suggest the 120-T uses 4x6L6.

[Jecarroll7]

Yep, not enough B-, at the diodes or the transformer tap.

If this amp came with 6L6s they must have been running them a bit warm. I guess I could do the same,

At 45mA with a 460v plate and 30W dissipation I'm right at 70% for these new production Tung-Sols.

Short of having a new power transformer built, or switching the amp to EL34s any thoughts?

[Jazz P Bass]

Is there a chance that the 18 volt supply is dragging it down?
You could try lifting the diodes to see what is up.

[52 Bill]

If normal troubleshooting does not get the bias voltage high enough, you could either use a doubler circuit to get more voltage or tap off the high tension winding to get there.

At least that doesn't require a new transformer.

[Enzo]

Um... that is C- guys, not B-.


And just for history, the A supply fed the heaters. In the old days, portable tube radios had "ABC batteries" in them, that provided all three supplies.

Battery (vacuum tube) - Wikipedia, the free encyclopedia

[Jazz P Bass]

Agreed.
The correct name is C for the bias supply.
As I really hate typing, I mearly indicated the letter on the schematic.
B.
Then I gave it the minus sign to designate the negative supply.