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FDP Forum / Amp Mods, Repairs, and Projects / Hot Rod Deluxe III bias from GT to JJ tubes

Bill D.

Clovis, CA USA

I pick...therefore, I play...
Feb 5th, 2018 07:37 PM   Edit   Profile  

After having some power tube rattle in my Groove Tube power tubes of my Hot Rod Deluxe III, I decided to change the power tubes to JJ's. The factory bias before was set at 60mv at Fenders test point. When I put the JJ's in, without readjusting the bias, it went up to 68mv. I backed it back down to 60mv. All is good, but I'm wondering if I should put it back up to 68mv. I like to run an amp as clean as possible and use a dirt box in front. I'm not really interested in amp distortion. I know some people say that 60mv is a very conservative number that Fender uses to prolong to life. Should I bump the bias up a bit even though I want the amp as clean as possible?

(This message was last edited by Bill D. at 11:30 PM, Feb 5th, 2018)

Roger Ball
Contributing Member


Just one fret off the right note.
Feb 5th, 2018 09:41 PM   Edit   Profile  

If it sounds good to you at 60mV then I suggest you leave it there, especially if you want "clean as possible". Increasing the bias won't improve that but will just run the tubes a little hotter for no reason.

I use a clean boost pedal to provide a hotter signal to the amp input. That way I can get a nice clean sound with the guitar volume down but can get a nice creamy overdrive with the guitar volume up. I also use a separate overdrive box if I need something dirty.

Hope that helps.

Bill D.

Clovis, CA USA

I pick...therefore, I play...
Feb 13th, 2018 10:09 PM   Edit   Profile  

Well, I decided to bump the bias pot to up the voltage from 60mv to 68mv. Just that little bit increased the volume very slightly. I could tell because the hiss increased when I turned the bias pot up. We'll see how it works this weekend. I have to say this this amp does hiss more than any of my older Hot Rod Deville ever did. But that amp is sold because it weighed 60lbs.

(This message was last edited by Bill D. at 12:11 AM, Feb 14th, 2018)



Feb 14th, 2018 03:09 AM   Edit   Profile  

Yes, hotting up the bias moves the power tubes' idle point to a higher gain point on their transfer curve.
If you like that higher gain, and are happy to compromise tube life, perhaps power output (and maybe amp reliability), you could go higher still, past 80mV; the 70% guideline would have it even as high as 100mV!
So if you like the tone at 68mV and you still got big cleans, then you're probably fine.



Why do I keep fixing things that work?
Feb 14th, 2018 10:13 AM   Edit   Profile  

I ran my JJ's in my Hot Rod Deluxe @ 67 and the amp sounded best there. It came alive and really brought out the fullness from the EQ and everything else just felt more responsive. At 74, it sounded glorious but was concerned about longevity of the tubes and transformer and never had a problem but dialed it back to where it still sounds good with less hiss and worry.
I think the JJ tube site stated to set the bias at 72-74 milivolts, but go to his website to confirm as it has been several years since I'd been to it!



Feb 18th, 2018 01:11 AM   Edit   Profile  


Je possède un ampli hot rod deluxe II depuis quelques années. J'ai dû replacer les lampes de puissance d'origine dernièrement.
J'ai donc installé des 5881 Sovtek.
Il se trouve que je rencontre un problème pour le réglage du bias. La valeur mesurée sur le point de test ne dépasse pas 63/64 mV avec le potard à fond.
Savez-vous me donner quelques informations sur cette panne ?

Moderator Emeritus
(with many stars)

NW Pennsylvania

Feb 19th, 2018 03:14 PM   Edit   Profile  


I have a deluxe hot rod amp II in recent years. I had to replace the original power lamps lately.
So I installed 5881 Sovtek.

I happen to have a problem with the bias adjustment. The value measured on the test point does not exceed 63/64 mV with the knob all the way down.

Do you know how to give me some information about this failure?

Thank you

Bill D.

Clovis, CA USA

I pick...therefore, I play...
Feb 20th, 2018 12:28 AM   Edit   Profile  

My HRDx will go down to 48mv, and I've had it up to 118mv before I backed it back down, and it wasn't even all the way up. BTW, I gigged with the bias set at 68mv last weekend, and it did sound better than when set at 60mv. I was actually surprised that I could even hear the difference. Not thin and wimpy. Much fuller and rounder sounding.



Feb 20th, 2018 02:34 AM   Edit   Profile  

"The value measured on the test point does not exceed 63/64 mV with the knob all the way down.

Do you know how to give me some information about this failure?"

That's probably not a failure.
Your tubes are probably just at one end of the performance spectrum. It shows why having adjustable bias is useful.

Does the amp sound good?
ie can you get a big loud clean sound with plenty of bass and punch?

To check that both tubes are good, and that they are a reasonable match to each other, if you take out one of the power tubes, what is the mV reading?
Replace that tube and remove the other, what is the mV reading?

(This message was last edited by pdf64 at 04:44 AM, Feb 20th, 2018)

FDP Forum / Amp Mods, Repairs, and Projects / Hot Rod Deluxe III bias from GT to JJ tubes

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