Selmer treble and bass sv

[blindboybenton]

Ive got a selmer treble and bass sv ( silver front type )
I'm trying to learn more about valve amps. ( i know my semi conductors ) The service has been done with all the caps and some resistors changed all for the correct values. Ive just brought the TAD Bias master and found that my pair of JJ's EL34 are running at 55ma each. Now is that too high ?. The fixed bias voltages are correct -32v.

What i would like to do is convert to variable bias. I know alot of people say " if it didnt come with it. Don't change it " But i want to do this a part of the learning about them

can anyone give me an idea on the circuit ?

john

[Steve Conner]

I own one of these, I've modded heck out of it Anyway, here's a schematic:
http://www.prowessamplifiers.com/sch...nbasssv50w.pdf

On the right hand side just below the middle, you'll see a 34k resistor in series with the bias winding and diode. Replace this with a 47k pot wired as a variable resistor. That should get you a reasonable range of adjustment.

[blindboybenton]

Thanks.. ill try that.. what other mods have you done.. Im using it for bass and its not the tone. its the power i want alittle more power . once it all done im selling on. so i can buy something else to play with

john

[pdf64]

55mA is almost certainly too high.
-32V bias voltage may have been ok for the Mullard or Brimar EL34 it was designed around; however there's far more variation amongst the EL34 now available, and generally it needs to be higher (ie more negative).
See the various Aiken biasing papers, as well as other good info Tech Info
If you are looking to sell it on, it will probably be more attractive without any mods (apart from adjustable bias).
The output transformer in these is not as beefy as, say, an equivilant Marshall, and the power supply choke arrangement will cause the high voltage to sag; if you try to push bottom E at high power, it will likely saturate and 'fart'.
Pete

[blindboybenton]

Thanks pete.

I did think that 55ma was too high. the JJ's i have are marked up as 40ma. I want to try the variable bias and see. I have a love for this amp but as a bassist i cant really use it at a gig. just cant get enough out of it. Im using a carlsbro 100watt pa and a Selmer PA100/4 most of the time.
john

[blindboybenton]

Hi pete..

That's what i get farty So there is no hope for it.. better do it up and sell on.. like your band videos.. what are you playing ?

[Enzo]

Without knowing the plate voltage of the tube - and thus the voltage across the tube - knowing 55ma is only half the story.


If you buy matched power tubes, and the tubes or boxes are labelled with some current reading, like 40ma, be aware that that reading is NOT a suggested bias setting. The ONLY thing that number means is that in their matching test set up, at whatever voltages and conditions it puts on the tube, then THAT tube drew 40ma. The next tube tested might draw 43ma, and the one after that 38ma. They sort the tubes by that number. It is only a sorting measure, not an adjustment. They want a set of four tubes, they grab four from the 40ma bin (in your case) and box them.

[blindboybenton]

Many thanks enzo.. I was aware that the set of valves tested and matched by the supplier with there own test equipment. Ive got two sets of matched JJ's and both set are about the same. Can you help on a circuit to make the amp variable bias ?. I will check out the plate voltages tonight, once i finished fixing computer mainboards :-(

[Alex R]

In post #2 Steve explained how to fit variable bias, it is simple enough.

Getting significantly more output power isn't going to be possible, you are limited by the number of output tubes and the OT.

[blindboybenton]

Hi steve

Thanks for the info in that vari bias.. ive found a old 50k pot in the component drawers fitted it and it works. Ill order some new preset pots.
John

[pdf64]

The amp may be a bit more punchy with the bias at a more reasonable level.
But the simplest way to get it louder is with speakers, more of them (as long as the total load impedance matches the amp output) or more sensitive drivers in the existing cab.
John, thanks for the interest, in the earlier clips it's a 2x6L6 amp based on a Weber 6A20HP kit through a Fluxtone speaker, in the more recent ones it's my '63 AC30 with a Blue and Greenback. In both cases I set one channel up for higher gain and flip between them with an AB box. Two Shure mics right up to the grille, guitars are a '78 Pro Deluxe and a 2010 R8 Les Pauls.
Pete

[Gtr_tech]

In post #2 Steve explained how to fit variable bias, it is simple enough.

Getting significantly more output power isn't going to be possible, you are limited by the number of output tubes and the OT.

You'd have to change the pwr xfmr as well as the output....