a bias procedure question

[pbman1953]

I recently talked to someone about setting the bias on my bass amps.


I use a Weber Bias rite to get me in the ball park. I've always written down the 60-70% dissipation wattage of the tube I'm working on. Then set the tubes to that range.
I'm learning , due to my procedure, that I'm setting for idle current. I talked to someone that sets his amps to idle at 90%.


Is that ok?


Have I been missing out all this time on a better sounding amp?

[sluckey]

no no

[shooter]

depends, are you talking apples and grapefruit, or trying for apples to apples?


CLASS A (typically referred to as self-biased, or cathode biased) can Idle at 100% since they will run cooler (bias) the harder you drive them.
CLASS A/B (typically Push-pull, fixed bias) needs to be biased cool since the harder you drive them the hotter the (bias) becomes.


don't quote me for your final, it's more complex than that but that gets you started down the road of reading "what is fixed bias compared to self-bias"
enjoy

[mresistor]

I've found that bass amps seem to work just fine in the 50-60% max dissipation range for a fixed bias amp. 

[pbman1953]

So here's the reason I ask-


I bought a new set of Tung Sol 7581A tubes which is supposed to work fine in the Super twin. While checking the bias, the 2 - watt resistors off pin 8 to ground smoked. This has never happened to me. I tested them and thy are dead but  can't read the last ring to tell the value. They are Brown- Black- ?, 


The tubes were in range  settings wise. Can anyone help with the value and reason why those would go?

[sluckey]

There are no resistors from pin 8 to ground unless you put them there. Back up and read your original thread on this amp. Could be very enlightening.

[pbman1953]

You're right, I'm bused. I think the 100k was in there testing but I forgot to remove it. All pin 8's are back to ground


Here are the idle current values i have so far. Please keep in mind that I did install a bias supply pot before the balance pot






Tube value in watts
1- 32
2- 27.2
3- 26.8
4- 32
5- 32
6- 32


What's to be done with 2 & 3 to help them blend better

[dragonbat13]

swap tube  2 and 4.  Only other thing would be individual bias for each tube.  Or bias circuit, bias resistors after the adjustment, to balance the two out of match tubes.

I would just swap 2 and 4, the other things would be a big job.

[pbman1953]

latest-

no hum after using the balance pot

1-27
2-27
3-27
4-26
5-26
6-31
 
1,2,3 are one side
4,5,6- are the other side



Should I go back to the seller and get another one to cover for #6?

[HotBluePlates]

I bought a new set of Tung Sol 7581A tubes which is supposed to work fine in the Super twin. ...

Here are the idle current values ...
Tube value in watts
1- 32
2- 27.2
3- 26.8
4- 32
5- 32
6- 32

latest-
...
1-27
2-27
3-27
4-26
5-26
6-31
...

Why so high on the plate dissipation?  The modern Tung Sol data sheet claims this is a 30 watt tube, and that Super Twin oughta be biased cooler rather than hotter.

Should I go back to the seller and get another one to cover for #6?

Do you think it will help?

Some sellers claim "matched set" but then you find their matching wasn't so great.  And they will probably need you to send the entire set back for a replacement, because it's not very likely they can send you 1 tube that matches up with your others (without them checking all together).

[pbman1953]

Overall, they were pretty close except one. They test at 400 volts but my average is 488. I could go back in and bring them all to 400 volts to find the cathode current at 400, but I'd rather not. I told then that I need 488 volts at 55ma. Right now the hot one is reading 488-490 volts @ 64ma

[sluckey]

Hot x 6. Those tubes will not last very long.

[pbman1953]

The plan now is to send back 2. One to show the measurement I need and the hot one. This way I can level them all off.


Wouldn't that be better?

[sluckey]

The plan now is to send back 2. One to show the measurement I need and the hot one. This way I can level them all off.


Wouldn't that be better?

Of course. Now don't forget to brush your teeth. 

[pdf64]

… I need 488 volts at 55ma. …

What is the rationale for idling so hot? eg all 6L6 have the same nominal characteristics, so why idle hotter than the type (GC) the amp was originally designed for?
The current TungSol brand doesn’t list 7581A on its website.
Design max ratings require derating to accommodate variance in operating conditions eg mains voltage variations.

[sluckey]

http://frank.yueksel.org/sheets/127/7/7581A.pdf

[pbman1953]

https://shop.ehx.com/item/7581a%20tungsolpl/tubes-vacuum-tung-sol/








The website for its distributer

[pdf64]

Apologies, I naively checked the https://tungsol.com/ site, which doesn’t list them.
The 7581A valve info on the ehx site doesn’t note the rating system being used.
https://shop.ehx.com/catalog/tubes-specs/7581A%20TS.pdf

[pbman1953]

No Worries!

[HotBluePlates]

http://frank.yueksel.org/sheets/127/7/7581A.pdf

Historical American Tung Sol data means nothing for this Russian tube.

https://shop.ehx.com/item/7581a%20tungsolpl/tubes-vacuum-tung-sol/

Apologies, I naively checked the https://tungsol.com/ site, which doesn’t list them.

I, too, went straight to Tung Sol's website which has ratings for their tubes, but not the 7581A (only the 30w 7581).  My mistake then.

[tubeswell]

https://shop.ehx.com/item/7581a%20tungsolpl/tubes-vacuum-tung-sol/ The website for its distributer

I highly doubt those tubes would handle 35W plate dissipation. I'd treat them as 25W tubes and bias accordingly. YMMV