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Post subject: Help identifying this Fender Blues Junior Mod on V4, V5
Posted: Sat Aug 09, 2014 10:24 am
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So I bought a used Fender Blues Junior online to mod it with the Billm mods.
I receive the amp I found it dosen't sound exactly like the other Blues Junior I have tried.
I open the rear panel and found the ouptut tube have been moded.
There is a resistor added on v4 and v5.
I'm not a expert tech so I can't says what this mod is.
I haved check online and haven't seen this one anywhere.
If someone can identify this mod and what it done to the amp it would be really great.
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Post subject: Re: Help identifying this Fender Blues Junior Mod on V4, V5
Posted: Sat Aug 09, 2014 1:57 pm
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Hi Hulrek,

It appears that they have added a resistor in series with each output tube Plate. There are a very few amps that use Plate resistors, usually very low values, like 10 ohms, maybe 100 at the most. I can't see the color code clearly enough on yours to determine the value.

Personally, I would ditch those resistors, bridge the gaps in the traces and add a bias adjustment pot.

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Post subject: Re: Help identifying this Fender Blues Junior Mod on V4, V5
Posted: Sat Aug 09, 2014 2:25 pm
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shimmilou wrote:
Personally, I would ditch those resistors, bridge the gaps in the traces and add a bias adjustment pot.


+1

Much more utility with a variable idle bias and the output tubes may enjoy an extended service life (EL84's run notoriously hot).

Arjay

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Post subject: Re: Help identifying this Fender Blues Junior Mod on V4, V5
Posted: Sat Aug 09, 2014 7:05 pm
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Thanks for your fast reply!

I was thinking it ad something to do with the bias as it have all the symptoms of a Blues Junior Running with cooler tube than usual.

Quote:
Personally, I would ditch those resistors, bridge the gaps in the traces and add a bias adjustment pot.


I was thinking about adding a adjustment pot for the bias (Billm mod)
I will try to return the Amp as I just bought it otherwise I'll do as you says.

Here a close up of the resistor, I think value is 20 Ohms 10%

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Post subject: Re: Help identifying this Fender Blues Junior Mod on V4, V5
Posted: Sun Aug 10, 2014 2:12 am
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Somebody thinking it is a 2 ohms resistor to read bias ?

Worst , thinking it is a 1 ohms :lol:


Last edited by stratele52 on Sun Aug 10, 2014 2:51 am, edited 1 time in total.

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Post subject: Re: Help identifying this Fender Blues Junior Mod on V4, V5
Posted: Sun Aug 10, 2014 2:38 am
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Looks like 2-ohm, 2% tolerance. Plate resistors in this range are basically "fuses." Excess current draw = popped resistor = no voltage to the anode plate. Not sure if this is an issue in the Blues Jr.

Is an issue in amps that use output tubes that can draw a lot of voltage and current. Like 6550's or KT-88's. Esp if the output section has multiple parallel push-pull pairs.

Ampeg uses anode resistor on each 6550 of their SVT amp, for this reason.


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Post subject: Re: Help identifying this Fender Blues Junior Mod on V4, V5
Posted: Sun Aug 10, 2014 2:51 am
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BMW2002Ti wrote:
Ampeg uses anode resistor on each 6550 of their SVT amp, for this reason.


+1

As did Ed Jahns when he designed Fender's monstrous 400PS.

Arjay

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Post subject: Re: Help identifying this Fender Blues Junior Mod on V4, V5
Posted: Sun Aug 10, 2014 10:35 am
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The resistors are not used as fuses. If a fuse is needed......use a fuse. The only resistors that also function as fuses are called "fusible links", and are designed to blow with excess current. There are no amps that use fusible links on output tube Plates or on the screens. Resistors used on amplifier output tube Plates are for balancing and/or to counter instabilities on amps with more than two output tubes.

I am not sure where the misconception about using resistors as fuses came from, but it is yet another internet myth that has absolutely no basis in fact. :wink:

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Post subject: Re: Help identifying this Fender Blues Junior Mod on V4, V5
Posted: Mon Aug 11, 2014 4:03 pm
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Hi, thank again for your answer.
Have to admit I have not understand all the detail since I'm still newbie in tube amp electronic.

I have the choice to return the amp or get 100$ back witch is interesting.
The person who sell it to me is not the first owner and did not know about the mod (maybe?, anyway).

So i was wondering :

Do you think this mod may have damaged other part of the amp?

Do you think I may have been put there to correct a other problem with the amp?
(by problem I meen Defect not bias ajust)

Someone also suggest me it may have been factory done to correct a amp problem but at the look of the solder I doubt it.

What would you do?

Ok maybe stupid question but can I just bypass the resistor with aligator clip to ear the amp without it or do I need to disconect them first?

Thanks


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Post subject: Re: Help identifying this Fender Blues Junior Mod on V4, V5
Posted: Mon Aug 11, 2014 4:27 pm
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Definitely not a factory job. I doubt if it was done as a fix for anything (imaginary problem maybe). Possibly done to read Plate current? Or maybe someone just mucking around. :?

It would be tempting to keep the amp and undo this "mod", for $100. That is a good idea to jumper the resistors and check the sound first.

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Post subject: Re: Help identifying this Fender Blues Junior Mod on V4, V5
Posted: Mon Aug 11, 2014 5:21 pm
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shimmilou wrote:
The resistors are not used as fuses. If a fuse is needed......use a fuse. The only resistors that also function as fuses are called "fusible links", and are designed to blow with excess current. There are no amps that use fusible links on output tube Plates or on the screens. Resistors used on amplifier output tube Plates are for balancing and/or to counter instabilities on amps with more than two output tubes.

I am not sure where the misconception about using resistors as fuses came from, but it is yet another internet myth that has absolutely no basis in fact. :wink:



Thanks, for the reply. Still learning, here. :D


OK... "ghetto" fuse. Or device to control parasitic oscillation. Not sure what the function of these resistors are. But, I've seen them in several high wattage paralleled push-pull stage amps. And apparently, in Ham rigs.

http://music-electronics-forum.com/t22558/

http://music-electronics-forum.com/t23031/


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Post subject: Re: Help identifying this Fender Blues Junior Mod on V4, V5
Posted: Mon Aug 11, 2014 7:41 pm
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IIUC, resistors used on the Plates of output tubes, usually in amps with 4 or more output tubes, are to help balance the load and reduce instabilities. The Ampeg SVT is one of the few well-known amps that I can think of that use them.

I suppose that people see the screen grid resistors blown in their amps when the tubes short and wrongly conclude that the resistors are supposed to blow to somehow protect the amp. To me it seems obvious that a component that has a wide, unpredictable range of when it will blow, and when it does blow sprays molten debris all over the board, charring the board, and sometimes catching fire (CC), is definitely not anyone's choice for protection.

Add the fact that many amp makers use relatively high wattage screen grid resistors and it is clear that they are not meant to blow. Also note that there are more and more amps that use......wait for it.......fuses on the Plates to protect the amp (called HT fuses). Another clue is that when a screen grid resistor blows, the tube Plate is still supplied with high voltage. Not really much protection there. :wink:

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Post subject: Re: Help identifying this Fender Blues Junior Mod on V4, V5
Posted: Tue Aug 12, 2014 1:25 am
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shimmilou wrote:

It would be tempting to keep the amp and undo this "mod", for $100. That is a good idea to jumper the resistors and check the sound first.



+1


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Post subject: Re: Help identifying this Fender Blues Junior Mod on V4, V5
Posted: Tue Aug 12, 2014 1:34 am
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stratele52 wrote:
shimmilou wrote:

It would be tempting to keep the amp and undo this "mod", for $100. That is a good idea to jumper the resistors and check the sound first.



+1


+2

And jumpering is much more prudent than trying to restore the damaged traces on the PCB (messing with those cheapass boards is whistling past the graveyard). If the alligator-clip trick produces the desired result I would merely solder a jumper in place across the leads of those resistors and be done with it.

Arjay

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