#1
Hey,

I just got a quick questions. I have a Kustom 36' Coupe and it says in the owners manual that it has both fixed and self biasing on the power tubes. Does this mean that I have to bias it when I change them?

This is what it says in the manual
The amp uses a mixed-bias system which is a combination of fixed and self-bias techniques to give the user the best of both worlds. The result is that the amp will control its own bias to a certain point, but without the typical reduction in power associated with normal self-biased amp designs. The ’36 Coupe uses two 6L6s in a “push-pull” amplifier configuration.


This is the first tube amp that I have ever owned so it's a learning experience

Thanks!
Guitars:
PRS Custom 24
Gibson Les Paul 60's Tribute
85' MIJ Strat
97' Snakepit Les Paul
LP Traditional 1960 Zebra
MIJ Tele
MIA Strat

Amps:
Silver Jubilee 2525
Peavey Ultra 112
Jet City JCA50H
66' Bassman
Pink Paisley Princeton RV
74' Vibro Champ
#2
Yes you do, the fixed and self bias is there to combine the fast response and non-compressive effect of fixed bias, but the compression and "softness" of distortion of self-bias.
#3
Alright. This may be a bit of a noob question but how hard is it to bias a tube amp? Is this something that I can do myself or am I better of taking it to a tech?

I like to work on my own equipment whenever possible but if there is a risk of really messing my amp up I'd rather not take the chance.

Thanks!
Guitars:
PRS Custom 24
Gibson Les Paul 60's Tribute
85' MIJ Strat
97' Snakepit Les Paul
LP Traditional 1960 Zebra
MIJ Tele
MIA Strat

Amps:
Silver Jubilee 2525
Peavey Ultra 112
Jet City JCA50H
66' Bassman
Pink Paisley Princeton RV
74' Vibro Champ
#4
If you are uncomfortable working with the high (and very dangerous) voltages within a tube amp, then take it to a tech. If you do mess up, you should be able to 1, find out very quickly, and 2, hopefully work fast enough to shut the amp off to fix it. The voltage across the cathode resistors allows you to find the current through them (and through the tubes) by E=IR.

http://www.aikenamps.com/SafetyTips.html
http://www.drifteramps.com/safety.html

http://www.lynx.bc.ca/~jc/bias.html
http://duncanamps.com/technical/lvbias.html
http://www.geofex.com/tubeampfaq/tubefaq.htm#bias
#5
Honestly Jared... I'm not entirely sure if he does need to do this or not...

I'm emailing Kustom about this because there doesn't seem to be any consensus if you have to adjust it or not. I have a feeling that since it is cathode biased you don't... but the odd "fixed cathode bias" is weird.

I'll post again as soon as they reply to my email.
#7
Quote by XgamerGt04
Honestly Jared... I'm not entirely sure if he does need to do this or not...

I'm emailing Kustom about this because there doesn't seem to be any consensus if you have to adjust it or not. I have a feeling that since it is cathode biased you don't... but the odd "fixed cathode bias" is weird.

I'll post again as soon as they reply to my email.


See thats what confused me. Let me know what they have to say.

Also, last year I replaced all of the tubes in this amp with new groover's. I just did a swap out without biasing because I was under the impression that this was a self-bias amp. My amp sounded even better after I had swapped the tubes. Could I have done any damage to my amp by doing this?

BTW blandguitar, thanks for the reading material. I just started picking my way through it and I've already learned a lot!
Guitars:
PRS Custom 24
Gibson Les Paul 60's Tribute
85' MIJ Strat
97' Snakepit Les Paul
LP Traditional 1960 Zebra
MIJ Tele
MIA Strat

Amps:
Silver Jubilee 2525
Peavey Ultra 112
Jet City JCA50H
66' Bassman
Pink Paisley Princeton RV
74' Vibro Champ
#8
Quote by Mark Sharp Kustom Amps
Matthew as long as you are putting in 6L6 power tubes, any brand, and 12AX7 preamp tube just drop them in. no bias is needed.


Looks like the answer is no. I'm guessing that the fixed bias supply is very low voltage and all of the real work is done by the cathode bias system.
#9
You're welcome, there are some good tube threads (and the electronics thread) floating around if you do a search if you're wanting to learn more. Aiken Amps, Geofex, ESP (Elliot Sound Products), and the Duncan tube amps sites all have some good info as well.

Interesting Matt, you reckon the best of both worlds is a bit of a gimmick about it? I guess it depends on what you want out of the power amp, too though.
#10
Quote by XgamerGt04
Looks like the answer is no. I'm guessing that the fixed bias supply is very low voltage and all of the real work is done by the cathode bias system.


Sweet! Although I was looking forward to doing a little fiddling with my amp I'm definitely going to keep reading up on this in case my next amp isn't self-bias.

Thanks for all the help guys!
Guitars:
PRS Custom 24
Gibson Les Paul 60's Tribute
85' MIJ Strat
97' Snakepit Les Paul
LP Traditional 1960 Zebra
MIJ Tele
MIA Strat

Amps:
Silver Jubilee 2525
Peavey Ultra 112
Jet City JCA50H
66' Bassman
Pink Paisley Princeton RV
74' Vibro Champ
#11
Quote by blandguitar
You're welcome, there are some good tube threads (and the electronics thread) floating around if you do a search if you're wanting to learn more. Aiken Amps, Geofex, ESP (Elliot Sound Products), and the Duncan tube amps sites all have some good info as well.

Interesting Matt, you reckon the best of both worlds is a bit of a gimmick about it? I guess it depends on what you want out of the power amp, too though.


Most likely it is a gimmick, but it also could be somewhat of an interesting concept. You would have to do some interesting calculations because the negative voltage applied to the grid and the cathode voltage would have to be taken into account to determine the bias point. I'm guessing more of the voltage comes from the cathode bias, and some of the voltage comes from the fix bias. That way you don't really have to care too much about the grid bias voltage and can just drop in and play.
#12
"Cathode bias often lends a natural compression or 'squishiness' to the sound, due to the increase in bias voltage when one valve enters Class B conditions, though the larger the bypass capacitor, the less will be this effect. A small capacitor (less than 100uF say) also increases non-linear distortion, which may be significant in hifi. Using a very large capacitor (greater than 470uF say), or using no capacitor at all, reduces this effect.
Fixed bias on the other hand, remains the same at all times. This allows maximum output power to be developed, and the reduced compression gives faithful transient response, or a stiff or 'barking' overdriven sound.
Furthermore, there is no reason why we cannot use a little fixed bias and cathode bias simultaneously to achieve the desired mix of compression and 'bark'. "
http://www.freewebs.com/valvewizard1/pp.html

Merlin touches upon the topic there, I'd say using 75% fixed, and 25% self bias would allow for some variation, but depending on the mixture could lead to some neat results, makes me wish I had a house and a lab.