Bias control, a point not to be taken lightly

[Richie]

I think one of the thing was ,Ken used NOS tubes of the same type. So most all of these would be the same. And i'm sure he had them picked out.
Much different than trying to run some of the current made tubes and brands. Or even if NOS they may differ between brands.
As was posted its easy to mod the bias section if you want more bias range.
Alot may depend on the transformers your useing..voltages,and the tubes you might want to use. Some of the old fender schematics call for -34volts for 6V6 at 410 volts,using the RCA 6v6's
The power supply itself could even be modded. I used an old fender showman amp,which had the twin power section and values. I tried the wreck values,and didn't think it sounded as good. the stock fender power values for that amp sounded more lively.
I think its cool to build a stock express..but more fun to see a few changes can be made here and there,and you have a different sounding and recating amp. Plus if you have the stock built amp,you have something to compare it to.

Richie

[Humbucker]

Richie, you hit the spot. It's hard to get just the sound from 20 years ago or something like that. Tubes, caps, resistors - in general they don't make them like they used to and if they do it's going to cost you. It becomes tiring hawking a bit here and another one there. Even after you put the b-ard together following Mama's apple pie recipe tradition you'll find out the sound is not quite you were after.

The resistors: nowadays metal film (foil) gives you +-2 per cent variation in value. In Leo Fender's time carbon mass resistors were like noise generators and yielded something like +- 20 per cent, if I remember correctly. That alone could explain the minute variations in sound and feel of the amp (of the same era, that is, and definitely between the 50's and today). And speakers...I had two Goodmans 30-watters in an Italian copy amp (piggyback Bassman) some 25 years ago. Same brand, same model, same manufacturing line, even same production quota and still the other sounded tighter and more hi-fi.

When younger I wondered a lot about the recorded sound after trying a few of the actual amps they were using. Only one sounded like the record but there was minimal compression and no other effects. Live, the sound was harsh and sometimes a bit grainy. It was the tape studio time - when you could rely an European Marshall sound good.

A guy borrowed my ADA MP-1 and recorded something like one rhytm track with it. Live sound was great but just too sweet recorded. Today, I think I could plug the ADA in and get as aggressive a sound as one could want.

Yes, I'm a sucker for old Ampegs, Supros, even a 45-watt Marshall and of course a great Vibrolux and a Deluxe - but I'm gigging with a Yamaha G12-80A modeling amp. My first finished-cab-still-to-come Express project proves Richie's point: this one is for comparison. Next one I will mod to suit my taste.

BTW, my version of Kelly Xpress (with all the corrections mentioned by UR and others in the bias thread) uses Tung Sol pre's and matched SED Svetlana EL 34's. Yes, it's loud and starts to feedback harmonically by itself when volume goes ovet three o'clock.

I may try something like old Telefunkens or Philips 12AX7 tubes to tame the beast a bit. And build an attenuator. The volume itself kills me even if the sound is probably one of the most aggressive rock sounds I've encountered.

[sepulchre]

I know this was all posted 8 years ago but, being one of those mentioned as "just getting his feet wet", I would like to see the schematics talked about here but not, or no longer posted. The only ones I can see are Paul's. I may use his in the end but as a student (albeit ancient) I'd like to see others' ideas.

Ken

[M Fowler]

Which schematic the Kelly one?

[sepulchre]

Yes, the Kelly one. I might have an old version of it but not sure.

I'd also like a look at the Francesca files. I've heard so much about them but can't seem to locate copy.

Thanks!

[M Fowler]

Everything your looking for is in the Trainwreck files section.

https://tubeamparchive.com/viewtopic.php?t=1408

[sepulchre]

Thanks a million!

[roberto]

Excellent point: I forgot a wire...

I usually swap the latest resistor and trimmer (resistor to raw bias, trimmer to ground), with the bias taken just after the resistor: this way if the trimmer fails you are sure the bias goes safely colder (bias voltage more negative, bias current goes down).

I use similar values for the pot and the resistor (or maybe the pot with a lower value than the resistor), because the higher the trimpot/resistor value the lower the precision of bias regulation (more voltage variation / degrees of rotation of the bias pot).