It is currently Mon Mar 16, 2020 3:36 pm

All times are UTC - 7 hours



Post new topic Reply to topic  [ 22 posts ]  Go to page 1, 2  Next
Go to page 1, 2  Next
Author Message
Post subject: 100W Bias control
Posted: Tue Feb 11, 2014 7:01 pm
Offline
Hobbyist
Hobbyist

Joined: Mon Feb 10, 2014 6:59 pm
Posts: 67
One of the mods I want to do on this amp is to make the bias control a true bias, not a fixed or "balance" control. The only type I've found so far though is a balance control with a 4-wire pot like the one that's in mine. What's the best way to configure this so I get an even bias among all four power tubes?

_________________
--Jim


Top
Profile
Fender Play Winter Sale 2020
Post subject: Re: 100W Bias control
Posted: Tue Feb 11, 2014 8:51 pm
Offline
Rock Icon
Rock Icon
User avatar

Joined: Wed Mar 31, 2010 12:48 am
Posts: 26417
Location: Tombstone Territory
If you're referring to your Bassman 100, the closest 4 x 6L6GC configuration with traditional bias-supply adjustment that Leo ever used was the AB763 revision as seen on the Showman, Dual Showman, and Twin Reverb layouts. You'll need a closely-matched quartet of output bottles to achieve optimum symmetry plus a balanced 12AT7 in your phase-inverter stage.

Arjay

_________________
"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


Top
Profile
Post subject: Re: 100W Bias control
Posted: Tue Feb 11, 2014 8:58 pm
Offline
Hobbyist
Hobbyist

Joined: Mon Feb 10, 2014 6:59 pm
Posts: 67
Yes, that's the amp. Thanks. I wonder if it would benefit from having dual bias controls to set each pair separately? I can't see any other way of getting them all spot-on.

_________________
--Jim


Top
Profile
Post subject: Re: 100W Bias control
Posted: Tue Feb 11, 2014 9:47 pm
Offline
Rock Star
Rock Star
User avatar

Joined: Sat Jul 03, 2010 2:18 pm
Posts: 6544
Yeah, I've seen a really pretty neat job of adding another bias pot next to the OEM one in a SF Twin Reverb. If I remember correctly, he put another small breadboard next to the main one. And ran the bias supply resistors and separate grid-to-ground resistors for that push-pull pair of 6L6GC's.

Gotta look in my vast compendium of photos, for the shot of that mod.

:lol:


Top
Profile
Post subject: Re: 100W Bias control
Posted: Tue Feb 11, 2014 11:33 pm
Offline
Professional Musician
Professional Musician

Joined: Fri Jun 26, 2009 6:53 pm
Posts: 2252
Location: Harvard IL
For sure, a standard set-up adjust control, such as the Twin Reverb, only addresses the entire 4-tube bank. I do recall a neat individual trim pot set-up, to further tailor each tube's bias. Lost my amp repair records, but think I picked that up off the Amp Garage forum. Thinkin' Stratele had an idea along these lines ? Art

_________________
None of Us are free, if One of Us is chained !


Top
Profile
Post subject: Re: 100W Bias control
Posted: Wed Feb 12, 2014 8:35 am
Offline
Rock Icon
Rock Icon
User avatar

Joined: Fri Feb 05, 2010 4:31 am
Posts: 14049
Location: Province de Québec, Canada
I'll keep the balance pot and put a bias pot on a terminal strip with few lugs .

The one I use ;

http://www.tubesandmore.com/products/P-0501H


Top
Profile
Post subject: Re: 100W Bias control
Posted: Wed Feb 12, 2014 9:23 am
Offline
Rock Icon
Rock Icon
User avatar

Joined: Wed Mar 31, 2010 12:48 am
Posts: 26417
Location: Tombstone Territory
stratele52 wrote:
I'll keep the balance pot and put a bias pot on a terminal strip with few lugs .

The one I use ;

http://www.tubesandmore.com/products/P-0501H


+1

This might be the most logical and efficient arrangement of all.

Arjay

_________________
"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


Top
Profile
Post subject: Re: 100W Bias control
Posted: Wed Feb 12, 2014 9:56 pm
Offline
Hobbyist
Hobbyist

Joined: Mon Feb 10, 2014 6:59 pm
Posts: 67
Here's what I came up with so far. It just has a balance bias control right now:

Image

_________________
--Jim


Top
Profile
Post subject: Re: 100W Bias control
Posted: Wed Feb 12, 2014 10:01 pm
Offline
Rock Icon
Rock Icon
User avatar

Joined: Wed Mar 31, 2010 12:48 am
Posts: 26417
Location: Tombstone Territory
The theory of operation seems sound.

Arjay

_________________
"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


Top
Profile
Post subject: Re: 100W Bias control
Posted: Thu Feb 13, 2014 7:49 am
Offline
Hobbyist
Hobbyist

Joined: Mon Feb 10, 2014 6:59 pm
Posts: 67
So the 12AX7 is operating simply as a gain booster in this setup, correct?

_________________
--Jim


Top
Profile
Post subject: Re: 100W Bias control
Posted: Thu Feb 13, 2014 8:14 am
Offline
Rock Star
Rock Star
User avatar

Joined: Sat Jul 03, 2010 2:18 pm
Posts: 6544
jimolson1963 wrote:
So the 12AX7 is operating simply as a gain booster in this setup, correct?


Half of the 12AX7 is a gain, the second half is a cathode follower. Prolly less than unity buffer stage to match impedance to the phase inverter. Also note the 100k-ohm local feedback between the grid and anode of the cathode follower stage.


Top
Profile
Post subject: Re: 100W Bias control
Posted: Thu Feb 13, 2014 9:57 am
Offline
Hobbyist
Hobbyist

Joined: Mon Feb 10, 2014 6:59 pm
Posts: 67
The cathode follower helps add current flow back into the circuit taken up by the tone stack, right? Sort of like increasing the diameter of a garden hose so the volume of water is increased, but not the actual water pressure.

_________________
--Jim


Top
Profile
Post subject: Re: 100W Bias control
Posted: Thu Feb 13, 2014 4:18 pm
Offline
Rock Star
Rock Star
User avatar

Joined: Sat Jul 03, 2010 2:18 pm
Posts: 6544
Jim,

In not most applications. You get no gain (voltage or current) in this usage. Esp with the internal, local feedback constraint. You get prolly close to unity gain, but an impedance shift, no loss.


Top
Profile
Post subject: Re: 100W Bias control
Posted: Thu Feb 13, 2014 5:08 pm
Offline
Hobbyist
Hobbyist

Joined: Mon Feb 10, 2014 6:59 pm
Posts: 67
Well that's ok, I'm using a modified Twin pre-amp section to get the tone anyway.

I read some very interesting information about the phase inverting tube as it relates to how the amp breaks up at higher volumes. It seems to me to get that crunch from the output tubes they need to be driven harder than the pre-amp or PI tubes anyway. The guy said most people load up the pre-amp which pushes the PI tube into distortion instead of the power tubes which makes a lot of sense. Whether that's right or not I honestly don't know but it sure seems plausible.

The problem with doing that with this amp is the four output tubes. To get them to break up results in a ton of volume which I don't need, so that puts things in a different light altogether. I have considered only wiring two sockets in a push-pull sequence like a 50w head just so I'm not making my ears bleed to get the sound I want. That also leaves two sockets that could be adapted for reverb use.

I have all kinds of ideas when I work on these things, just tell me if I'm all wet or not when I mention them. :)

_________________
--Jim


Top
Profile
Post subject: Re: 100W Bias control
Posted: Thu Feb 13, 2014 5:26 pm
Offline
Rock Icon
Rock Icon
User avatar

Joined: Fri Feb 05, 2010 4:31 am
Posts: 14049
Location: Province de Québec, Canada
A master volume ?


Top
Profile
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 22 posts ]  Go to page 1, 2  Next
Go to page 1, 2  Next

All times are UTC - 7 hours

Fender Play Winter Sale 2020

Who is online

Users browsing this forum: No registered users and 1 guest


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum

Search for:
Jump to: