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Post subject: 6G3 Deluxe Brownface Bias Mod Issue
Posted: Tue Oct 30, 2012 8:51 am
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I have a 6G3 that has been recapped by another tech, and while that all seems fine, he/she has modified the small board that sets the bias voltage etc.

Standard values seen in the schematic are:

100K to the diode from TR1. 22K across a 25uf/50v cap to set the voltage.

The tech who modded this has done the following.

Change the 100K to 1K (although this looks like an earlier change)
Change the 22K to 1K & Added a 50K trimmer in series with it
Change the cap to 100uf

When I measure the output stage now I get a B+ OF 380v at the plates when I set -26v bias. Tube current is running a little cold at 26ma. If I start to increase the trimmer to compensate for the plate current, the B+ climbs far to high, and the -26 starts heading a long way north of where it should be.

I can't fathom why the tech used these values? I would have though simple dropping the 22k to 10K and adding a 20k trimmer would be sufficient. Also why did he go to 100uf?

Any thoughts appreciated.

He also changed the 6V6s for Electroharmonix, which are rated higher than NOS tubes, according to the datasheet.

I did try to add a photo but the forum does not seem to allow use of the img and url tags?


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Post subject: Re: 6G3 Deluxe Brownface Bias Mod Issue
Posted: Tue Oct 30, 2012 9:09 am
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I believe the major cause of your issue was exchanging that series 100K resistor for a 1K. I'd set that back to stock or at most, nothing higher than 22K.

The 100ufd bias cap seems excessive -- 40ufd should be sufficient for the purpose.

The 50K trimmer pot in series with a 1K fixed-value resistor is a relatively common mod and a perfectly acceptable method to provide a sufficiently-variable range for biasing a variety of tube brands.

Arjay

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Post subject: Re: 6G3 Deluxe Brownface Bias Mod Issue
Posted: Tue Oct 30, 2012 9:21 am
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Retroverbial wrote:
I believe the major cause of your issue was exchanging that series 100K resistor for a 1K. I'd set that back to stock or at most, nothing higher than 22K.

Arjay


I have no idea why that was done, but it's being swapped for the correct value.

Thanks for the reply.

N


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Post subject: Re: 6G3 Deluxe Brownface Bias Mod Issue
Posted: Tue Oct 30, 2012 9:39 am
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Schemo:

http://www.webphix.com/schematic%20heav ... _schem.pdf

First off, for clarity sake, this isn't B+. Let's call it C-. It is the negative voltage of the BIAS POWER SUPPLY. Used to set the idle bias in this push-pull amp.

1.) 100mfd/100VDC(?) cap will work. She went to this value, because that's what she had in her stock bin. You can get away with a range of 25mfd/50VDC to 250mfd/250VDC here. Higher voltage rating, for same mfd = better heat tolerance. Size is a constraint.

2.) Going from stock 100K-ohm to 1K-ohm. Are you sure about this? 1-kohm = (brown-black-red). This would not drop the negative voltage to a range of -25 to 30VDC.

3.) And that's why the C- rail to ground resistor HAD to be changed from 22k-ohms to 1-kohms. This isn't good as it makes the C- rail carry a lot more current for the voltage needed. Much of which is being lost & dumped into ground. BIAS SUPPLIES need only mA's of current to operate.

4.) 50K-ohm pot is also a bit high for a BIAS POWER Supply trim pot. You should use a 10-kohm one. It'll give you a bit better range of control. You can do this by subbing the OEM 22-kohm (C- to ground) resistor with a 15-kohm one. Then, put the 10-kohm pot in series BEFORE this set resistor.

5.) At 380VDC anode voltage, you prolly get better tone with around 30mA idle current (cathode-to-ground) on the 6V6GTs. This would be around 11 watts idle dissipation per 6V6GT.


Good luck with that1


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Post subject: Re: 6G3 Deluxe Brownface Bias Mod Issue
Posted: Tue Oct 30, 2012 10:07 am
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I agree with EVERYTHING you said.

B+ refers to the increase I see at the plate when trying to get -26v. The schematic calls for 365v

Yes I am sure about the 1K. I've been doing this a while :P Brown Black Red and Gold tolerance. Looks like an old 2w resistor, so maybe it was a previous repair that the last tech didn't spot, or care about...

I have photos but I cannot get this damned forum to allow me to add either an url or img

I even tried to paste a link to my dropbox to a pdf, but that won't work either.

I even had to remove your quoted post to allow this reply, so maybe it's because I am not allowed to post anything other than plain text being a new forum member? I did write to the admins, but no reply for the last 2 days...


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Post subject: Re: 6G3 Deluxe Brownface Bias Mod Issue
Posted: Tue Oct 30, 2012 10:19 am
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Didn't mean to question your skills. Just did the C- thing as a convention.

You using IE9 or some MS derivative, as a browser? You may need to diddle with the TOOLS, NET SETTINGS -- to get photos to paste correctly.

Or just use Firefox & save all the hassles.

I use IE9 & can paste from Photobucket, no problems (after some setting adjustments).

1k-ohm dropping resistor brings the voltage into a DR AB763 range. But, the blackface DR runs a LOT more anode plate voltage. So it needs more negative BIAS SUPPLY voltage.

BTW... if you are looking for a slot-shaft 10-kohm pot:

http://www.tubesandmore.com/products/R-V10KL-BP-FND


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Post subject: Re: 6G3 Deluxe Brownface Bias Mod Issue
Posted: Tue Oct 30, 2012 10:32 am
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BMW2002Ti wrote:
1k-ohm dropping resistor brings the voltage into a DR AB763 range. But, the blackface DR runs a LOT more anode plate voltage. So it needs more negative BIAS SUPPLY voltage.


+1

A fifty-volt difference (365VDC for the 6G3 vs 415VDC for the AB763).

Quite substantial, actually.

Arjay

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Post subject: Re: 6G3 Deluxe Brownface Bias Mod Issue
Posted: Tue Oct 30, 2012 2:28 pm
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BMW2002Ti wrote:
Didn't mean to question your skills. Just did the C- thing as a convention.

You using IE9 or some MS derivative, as a browser? You may need to diddle with the TOOLS, NET SETTINGS -- to get photos to paste correctly.

Or just use Firefox & save all the hassles.

I use IE9 & can paste from Photobucket, no problems (after some setting adjustments).

1k-ohm dropping resistor brings the voltage into a DR AB763 range. But, the blackface DR runs a LOT more anode plate voltage. So it needs more negative BIAS SUPPLY voltage.

BTW... if you are looking for a slot-shaft 10-kohm pot:

http://www.tubesandmore.com/products/R-V10KL-BP-FND


No Windows garbage here :-) Only MAC. Tested both Safari and FF.

However, I found why! If your url has an % in it, which is often used in html when you have a space between words in a filename, such as 'this picture.jpg' instead of thispicture.jpg, then the Fender forum barfs. I know spaces are evil in filenames!

Google Picasa automatically adds % symbols to your filenames as you generate links if there are spaces in the originally uploaded photo. Most forums tolerate this, but not Fender :)

Interestingly I did find if I substitute the % for a big X, then Fender parses it fine. Bizarre!

No I know why, I will post the extra info below in another post to this thread.

This is the error the forum gives.

Image


Last edited by nodric on Tue Oct 30, 2012 2:30 pm, edited 1 time in total.

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Post subject: Re: 6G3 Deluxe Brownface Bias Mod Issue
Posted: Tue Oct 30, 2012 2:29 pm
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Here is the schematic I am working from (ok Layout), and the actual board as modded by another tech/s.

Image

Image


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Post subject: Re: 6G3 Deluxe Brownface Bias Mod Issue
Posted: Tue Oct 30, 2012 2:39 pm
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I'd set it back to 6G3 standards. Replace the OEM 22k-ohm with a set 15-20k-ohm resistor and a GOOD (read: not all-plastic junk) pot. Bias is important. Not a place to save money.

If you don't want to spring for a "normal-sized" pot --- then just juggle the C- to ground resistor (OEM 22 k-ohm) until to get into the correct idle bias range for your 6V6GTs. It'll be a lot more reliable than having that funky plastic thing in the circuit.

IMHO. Good luck with your amp. It sure is worth the effort to do things correctly. The one reason the darn amp has lasted so long, in the first place.


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Post subject: Re: 6G3 Deluxe Brownface Bias Mod Issue
Posted: Tue Oct 30, 2012 2:51 pm
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BMW2002Ti wrote:

IMHO. Good luck with your amp. It sure is worth the effort to do things correctly. The one reason the darn amp has lasted so long, in the first place.


I agree!

Too many 'techs' take liberties (I'm being polite) when messing around in amps. Why can't they take pride in doing it right, and trying to use the right values of parts :)

Oh I'm a tech too, but Fender is not my expertise, hence why I asked about what I found before I started to pull it out. Who knows it may have been a cool mod...

Since a full size metal pot is a couple of bucks, I'm sure I can spring for it, since I have restored this amp for a client :)


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Post subject: Re: 6G3 Deluxe Brownface Bias Mod Issue
Posted: Tue Oct 30, 2012 3:01 pm
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I hear you! Cool thing about a "real" pot, you can tap a hole (think it's 3/8-inch) and make the thing adjustable, without pulling the chassis. Plus, it looks a whole lot better. You prolly already knew that! :lol:

You do Marshalls? I can't tell you how many JTM-45 and JCM800 went across the bench, when I first started. Vintage Fenders are laid out better and last longer per fix. At the moment, the market is better now for the vintage Fender stuff, than almost any other brand.

Image


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Post subject: Re: 6G3 Deluxe Brownface Bias Mod Issue
Posted: Wed Oct 31, 2012 8:31 am
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BMW2002Ti wrote:
At the moment, the market is better now for the vintage Fender stuff, than almost any other brand.


Yes I do any amp :) I do like Fender amps though, and there are a lot about at the moment. This client has 10, and so far he has bought me two to work on :) Hope he brings the rest!


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Post subject: Re: 6G3 Deluxe Brownface Bias Mod Issue
Posted: Wed Oct 31, 2012 10:22 am
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nodric wrote:
This client has 10, and so far he has bought me two to work on :) Hope he brings the rest!


We'd like to see pics of them as well!

8)

Arjay

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