Bias Balance "AND" Bias Adjust

[cjl1106]

Hello -

I have read the two topics that are posted on this website regarding Converting the Bias Balance to a Bias Adjust.
I would like to have both.  Does anyone know if that is that possible.  Have you done it.

I have a Fender Rivera-era 83 Concert.
"The Gear Page" forum has over 62 pages dedicated to the Fender Concert and about a dozen pages with regards to the topic of Bias.  However, while a lot of "theory" is thrown around out there - and I have emailed many for clarification of their posts - no one seems to be able to tell me "for sure" if they got both the Bias Balance AND the Bias Adjust working properly.  Plus there seems to be a 50/50 split (in theory) on where to introduce/connect the Bias Adjust Pot into the circuit.

Thanks,
Chuck

[sluckey]

...Converting the Bias Balance to a Bias Adjust... Does anyone know if that is that possible.  Have you done it.

Yes, it's possible. No, I haven't done it. You simply make the bias voltage that feeds the balance pot adjustable.

However, if you're gonna have two bias pots to twiddle, you may consider just installing a separate bias pot for each tube. Much more useful IMO. Kinda like this...

                 http://home.comcast.net/~seluckey/amps/sunn/sunn_sceptre_1971.pdf

[bluesbear]

Sluckey is right. It is easier to just have a seperate bias for each tube. A few years ago, I bought a pair of "matched" RCA 6L6GC's. They were FAR from matched so I did the dual bias trick. It worked fine. I soon got a real matched set and went back to a single pot. Those RCA's still test new (and still don't match). They work great in single ended amps.
Dave

[cjl1106]

Once the Bias in milliamps is set - if I used two Bias Adjust pots - I would set each 6L6 to the same setting, Correct?
What's the difference then, if I set the milliamps through a single Bias Adjust Pot and then use the Balance Pot to bring them to the same setting?  Isn't that achieving the same results - with a lot less work ??

Since there has been much discussion and apparent disagreement on whether to use the insertion of the Bias Pot using the 33K tap (from the Balance Pot) or replacing the 3.9K - I am hoping to hear from somone who has done the installation of the Bias Pot and found that the Balance Pot continues to work.

Thank you

[sluckey]

I am hoping to hear from somone who has done the installation of the Bias Pot

OK. That rules me out. Maybe some of the more experienced guys can chime in.

[bluesbear]

"Isn't that achieving the same results - with a lot less work ??"

I did it with 2 bias pots because it seemed LESS less work to me. I guess it's all perception. I suppose it doesn't really matter; it works either way.
Dave

[cjl1106]

Since there has been much discussion and apparent disagreement on whether to use the insertion of the Bias Pot using the 33K tap (from the Balance Pot) or replacing the 3.9K - I am hoping to hear from somone who has done the installation of the Bias Pot and found that the Balance Pot continues to work.

Thank you

[sluckey]

Since there has been much discussion and apparent disagreement on whether to use the insertion of the Bias Pot using the 33K tap (from the Balance Pot) or replacing the 3.9K - I am hoping to hear from somone who has done the installation of the Bias Pot and found that the Balance Pot continues to work.

You may not find anyone that has 'actually' done this mod. Mostly, people want to just replace the bias balance with the older style bias adjust circuit. The fact that there's been "much discussion and apparent disagreement" simply means that there are more than one way to accomplish your mod. However, it's such a simple mod that most anyone with a decent understanding of electronics could successfully do it. It's not always necessary that a thing has been done before to prove that it can be done.
 

[HotBluePlates]

Once the Bias in milliamps is set - if I used two Bias Adjust pots - I would set each 6L6 to the same setting, Correct?
What's the difference then, if I set the milliamps through a single Bias Adjust Pot and then use the Balance Pot to bring them to the same setting?  Isn't that achieving the same results - with a lot less work ??

I think LooseChange might have done both methods before. Which is easier is in the mind of the beholder.

If you got close with a raw bias adjust, you would tweak the bias balance until the current through each tube lands at the same value. That value will very likely not be the same as what either tube began with (after the raw bias adjust).

The reason folks say it is easier with a pair of bias adjusts is that if you had selected a specific target current value, it is faster to land both tubes on that target; you adjust 1 pot per tube until both are on your target. If you use a bias adjust/bias balance, and whated a specific target current, you have to tweak the adjust, then balance, then adjust again (and maybe balance again). So the other way is (possibly) fewer steps.

[Merlin]

Since there has been much discussion and apparent disagreement on whether to use the insertion of the Bias Pot using the 33K tap (from the Balance Pot) or replacing the 3.9K - I am hoping to hear from somone who has done the installation of the Bias Pot and found that the Balance Pot continues to work.

The most straightforward way (IMO) is to replace the 3.9k resistor with a 10k pot, wired as a variable resistor. (Or use a larger value if you need more range). The balance pot will continue to work perfectly well. Quick change, no messing

[LooseChange]

I think LooseChange might have done both methods before. Which is easier is in the mind of the beholder.

My ears were ringing... Here is everything you ever wanted to do to the power amp...

[jjasilli]

Maybe checkout this thread:  http://www.el34world.com/Forum/index.php?topic=8863.msg80057#msg80057

[cjl1106]

"However, it's such a simple mod that most anyone with a decent understanding of electronics could successfully do it. It's not always necessary that a thing has been done before to prove that it can be done."

SO simple that no one can produce a drawaing or diagram. ??
SO simple that no one has done it. ?
There have been numerous othes on this and other forums seekng the information I'm asking for - but amongst all these Experts no one has ever done it ?

And I don't have to have anyone agree with my question - to make my question Valid!  Those are just the Bullshit Link referers! - hey I dunna know da answer -  but go here and go here and go here !  Think I read sumthin about dat - but I don't knowz da answer!
Bullshit !
  ALL Bullshit !

Now you can ban me because the level of experiece an knowledge her is BULLSHIT.

[sluckey]

Where you been so long?

SO simple that no one can produce a drawaing or diagram. ??
SO simple that no one has done it. ?
There have been numerous othes on this and other forums seekng the information I'm asking for - but amongst all these Experts no one has ever done it ?

Merlin offered a solution. Most everyone else in this thread, including me, thought the dual bias adjust is simpler and better than the balance/adjust circuit. So we offered easy alternatives that accomplish exactly the same results. There are commercial amps out there that do have exactly what you want but I'm not gonna look'em up for you. And even though I could, I'm not inclined to modify a Fender bias balance circuit diagram just for you.

Where were you two weeks ago when I was in such a pissy mood? I probably could have been more helpful then.   

[stingray_65]

I may be a bit out of line here, but here goes.

In order to stand on the shoulders of giants, one has to do the standing.

The gentlemen that have offered their assistance are truly knowledgeable and sincere in their suggestions.

Personally I respect each and every one their opinions.

If you choose not to actually try to modify your bias circuit then don't!

You'll be passing up the opportunity to be the guy who actually has done this!

You'll also miss out on the thrill and excitement of sharing your results with these fine fellows and all the other forums you've looked at so far.

I sincerely hope you take a minute to reconsider your resolute and try out a few of the suggested mods.

My.02 take it or leave it

Ray 

[phsyconoodler]

Sluckey said:"However, if you're gonna have two bias pots to twiddle, you may consider just installing a separate bias pot for each tube. Much more useful IMO."

  This covers what you need to accomplish.Anything else is wasting space and energy to come to terms with.
This makes the most sense of any method of bias adjustment.

[HotBluePlates]

SO simple that no one can produce a drawaing or diagram. ??
SO simple that no one has done it. ?

Sluckey produced a diagram; it was posted in the first reply. That schematic showed independant bias adjusts for each tube. That approach has been written about in Kevin O'Connor's books, right along side the alternate approach of using a bias adjust pot and a bias balance pot.

Merlin provided an answer; it involved replacing 1 resistor with 1 pot. No diagram needed.

Loosechange provided a diagram (showing more than you wanted), and has tried more than one way of balancing the bias of multiple tubes.

There have been numerous othes on this and other forums seekng the information I'm asking for - but amongst all these Experts no one has ever done it ?

Wow.

I haven't done it (though I know it will work) because I don't sweat biasing the way some folks do. As long as the tube isn't redplating, I'm fine.

And I don't have to have anyone agree with my question - to make my question Valid!

I don't think anyone said you did not have a valid question.

I think in the end you asked a specific question of how to add a bias adjust to an existing bias balance, looking for a specific answer (replace the 33k resistor or the 3.9k resistor).

Most folks said they preferred twin bias adjusts over a adjust/balance approach. How to use that method was provided, but also assumed you know enough to incorporate that in the Rivera Concert. Merlin, however, did give an exact method to use your approach (adjust+balance) in your amp.

Asked and answered.

[LooseChange]

Hey, I've done it a bunch of times with perfect success. What's the big deal!?
I've settled down with the same opinion as HBP.

[Geezer]

There have been numerous othes on this and other forums seekng the information I'm asking for - but amongst all these Experts no one has ever done it ?

Wow.

I haven't done it (though I know it will work) because I don't sweat biasing the way some folks do. As long as the tube isn't redplating, I'm fine.

Hey, I've done it a bunch of times with perfect success. What's the big deal!?
I've settled down with the same opinion as HBP.

Right on! 

[bluesbear]

If you can't figure out from the very good descriptions how to do this, most would feel it safer FOR YOU to take the amp to most any decent tech. It may be an easy mod but amps are dangerous inside. No one here actually is trying to impede your quest but no one wants to cause your death. If that is offensive... well, I'm not sorry.
Dave

[hotrod]

I think LooseChange might have done both methods before. Which is easier is in the mind of the beholder.

My ears were ringing... Here is everything you ever wanted to do to the power amp...

Hi guys, new to this forum, I just found this as I'm investigating indivdual bias for a pair of push-pull (as in a Fender Bassman 5F6a for example), and I found this diagram ("Everything") really interesting, where would I find the whole file / schematic as this is obviously only a snippet?

Many thanks!
Stephan

[HotBluePlates]

That is a piece of LooseChange's drawing of the London Power Standard amp, found in The Ultimate Tone Volume 3.


I'm not sure if LooseChange still frequents the board, or has posted the complete redrawing on here before.

[sluckey]

Here's a dual bias circuit I actually have in a Sunn amp. It should be very similar to the 'everything' circuit except it doesn't have a switch for cathode/fixed bias.

[hotrod]

Excellent, thanks! The Sunn schematic enlightens it nicely!
Cheers

[Jack_Hester]

The Guild Thunderstar Bass has a couple 3K pots to balance the Bias of the power tubes.  However, they can run out of range, even on 'matched' tubes, when trying to get the overall Bias at a suitable level.  So, I modified mine to have a Bias level pot.  Another circuit to consider. 

Jack

[pompeiisneaks]

side note, why are all of sluckey's links dead now :(

[sluckey]

Comcast shut down all personal websites.

[pompeiisneaks]

Oh crap, do you have that data somewhere?  I don't mind giving you a dir on my web server to host them if not too big or they don't get hit too often... Do you think more than the forum tends to look at that data?

~Phil

[shortfuse]

I am astonished at the response to the guys that have helped me a lot and I respect more than words can say on this site.  WOW

LC, Geezer, Stingray dont see may posts from you guys these day but glad your still lurking around

Sluckey is the site down indefinitely?

[sluckey]

The amp stuff is only ≈100MB at present. Each project is organized in it's own sub directory, all located under a main directory. I never had any way to determine traffic to the site but I suspect that Hoffman's forum and the Amp Garage generated most of the traffic.

I've considered Go Daddy. Their introductory 'hook' price is attractive. But since this is just a hobby for me, I'm still undecided about paying for a website. If I decide to put the amp stuff back online I'll need about a gig of space with easy FTP access. I occasionally temporarily post big, private files for friends.

The amp site is down indefinitely. If I find a solution and put the stuff back online, I'll post a message on this forum and provide a link.

[p2pAmps]

The amp stuff is only ≈100MB at present. Each project is organized in it's own sub directory, all located under a main directory. I never had any way to determine traffic to the site but I suspect that Hoffman's forum and the Amp Garage generated most of the traffic.

I've considered Go Daddy. Their introductory 'hook' price is attractive. But since this is just a hobby for me, I'm still undecided about paying for a website. If I decide to put the amp stuff back online I'll need about a gig of space with easy FTP access. I occasionally temporarily post big, private files for friends.

The amp site is down indefinitely. If I find a solution and put the stuff back online, I'll post a message on this forum and provide a link.

I bet if you talk nice to Doug and blow in his ear he could create virtual directory for you stuff Steve.  kinda like  http://www.hoffmanamps.com/sluckey

[pompeiisneaks]

If you were thinking of hosting somewhere, you can't beat Digital Ocean, they go as low as 5$ a month for a 20Gb storage and a decent server, and for 10$ it is even better. 

Check them out.

https://www.digitalocean.com/pricing/