Bias .

[lasrup]

I have a Victory Sheriff 25 which is 18 months old.
It has a pair of El 86s. The bias should be 22mv each .There is an external bias adjustment.The variance between the two is at least 8 mv.These are new matched tubes .
The amp can take EL84s which I have tried but cannot get any readings at all.
Help !

 

[Pete Farrington]

The variance between the two is at least 8 mv.These are new matched tubes

What operating condition criteria did the vendor use to match them?
Even if they were matched under realistic conditions, maybe they were subjected to rough handling in shipping.
Hence I suggest to contact the vendor.

EL86 -

https://frank.pocnet.net/sheets/129/e/EL86.pdf

https://frank.pocnet.net/sheets/010/e/EL86.pdf (See bizarre single ended push pull suggested circuit on p3!)

The amp can take EL84s which I have tried but cannot get any readings at all.

Do you mean that you're reading 0mV with EL84 fitted?
If so, are you sure they're good, eg do they work ok in another amp?

 

[VintageSG]

Does the variance follow the valve? If so, 8mV in 22mV is more than unmatched.
Does the variance stay with the socket?, service time.

EL86 to EL84? Hmm. Are you certain your EL84 are fully functional? If they were asked to draw the same anode current as an EL86, bad things may have happened very quickly inside them.

You can install just one valve, power up and check for sound then power down with little risk. The risk is a non-zero risk so your choice.

 

[printer2]

What are the plate and screen voltages with the EL86's, if I may ask?

 

[elpico]

EL86 -

https://frank.pocnet.net/sheets/129/e/EL86.pdf

https://frank.pocnet.net/sheets/010/e/EL86.pdf (See bizarre single ended push pull suggested circuit on p3!)

At some point the common name for that changed to single *rail* push pull, in online posts abbreviated as "SRPP" if you're interested in more like that. The hifi guys are way into all the weird stacked circuits. It's totem pole city over at tubecad or diyaudio.

 

[tubedude]

The Series Regulated Push Pull and the Shunt Regulated Push Pull, along with the White follower, Mu Stage and the other variants are big in Hi-Fi, but few of these circuits are used in instrument amps. Weber used one in a design of theirs, the Smokin' Joe i think. Fun circuits to experiment with. Especially if you like elevated heaters

 

[elpico]

Is that what SRPP is meant to stand for? I learned it the wrong way a long time ago then. Sorry for the bad info

 

[lasrup]

Thanks everybody for your advice.