Replacing a tube in a Fender Pro Jr IV

**El Fundo** · 03-26-2019, 09:06 PM

I have one 5751 but there are two preamp tubes in the amp. Which socket should I put the 5751 in? Or would I need two replacement tubes in order to make a difference?

**dlew919** · 03-26-2019, 09:08 PM

The general rule is if you need to replace one tube you replace all of them.

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**El Fundo** · 03-26-2019, 09:09 PM

I’m just trying to warm up the sound a bit. The Groove Tubes it came with are fine.

**lawson-stone** · 03-26-2019, 09:09 PM

Originally Posted by dlew919

The general rule is if you need to replace one tube you replace all of them.

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I don't think that applies to Preamp tubes. Lots of people swap out V1, putting a 5751 or a 12AT7 to tame the gain a little. You don't swap about all the others in that case. Power tubes yes.

**dlew919** · 03-26-2019, 09:10 PM

Originally Posted by lawson-stone

I don't think that applies to Preamp tubes. Lots of people swap out V1, putting a 5751 or a 12AT7 to tame the gain a little. You don't swap about all the others in that case. Power tubes yes.

Thanks for that.

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**El Fundo** · 03-26-2019, 09:12 PM

Maybe you’re right, but I did the same thing with my DRRI and PRRI and there was a difference in gain and tone. Also, I really didn’t expect there to be two preamp tubes in such a simple amp.

**El Fundo** · 03-26-2019, 09:13 PM

Oh, I just realized you were responding to someone else’s post, Lawson.

**vernon** · 03-26-2019, 09:18 PM

I think a speaker upgrade might “warm up the tone” more than preamp tube. If memory serves, I think I put a Weber AlNiCo 10” in my Pro Jr and it improved the warmth of tone.

**El Fundo** · 03-26-2019, 09:29 PM

I’ve thought about that. Which Weber Alnico 10” did you get? There are several choices, some of which are very expensive.

**lawson-stone** · 03-26-2019, 09:31 PM

Originally Posted by El Fundo

Maybe you’re right, but I did the same thing with my DRRI and PRRI and there was a difference in gain and tone. Also, I really didn’t expect there to be two preamp tubes in such a simple amp.

They aren't just preamp tubes. That amp has (I think) 2 EL84 power tubes, which likely means it's a "push pull" circuit which means you have to have a Phase Inverter tube. So I'm guessing you have a 12AX7 preamp tube, another 12AX7 as a Phase Inverter, then the two EL84 power tubes.

V1 can be replaced by a number of 12AX7 compatible replacements. The 5751 is a popular choice, I have one in my 15 Watt Monoprice that has, like yours, 2 EL84 Power tubes and 2 12AX7's.

**El Fundo** · 03-26-2019, 09:34 PM

Ah, thanks Lawson. I really just wanted to know why there were two 12ax7s. I know zilch about electronics!

**neatomic** · 03-26-2019, 09:47 PM

you want to put that 5751 tube you have in the very first tube spot...thats the first preamp/gain stage of the amp...2nd 12ax7 may handle additional boost circuit and/or phase inverter...(haven't looked at schematic)...its got a ss rectifier..and two el84 power tubes...

change out the power tubes one day..the 2 el84's...for some jj el84's...you'll love them..esp with a celestion 12" speaker..and not expensive!!

cheers

**Longways to Go** · 03-26-2019, 10:12 PM

El Fundo,

the V1 position -- which is where you want to put the 5751 is the first tube on one side of the amp. It is easy to identify because it is the first small tube on one end --- opposite side from the bigger power tubes. They are all in a row -- v1, v2, v3, v4 . . .
v1 & v2 are the little guys, v3 & v4 are the bigger power tubes.

Go for it !!!

**lawson-stone** · 03-26-2019, 10:16 PM

I think you'll like that 5751 in the first preamp spot. It's a sweet tube.

**Greentone** · 03-27-2019, 10:03 AM

V1 is the preamp tube. V2 is the phase inverter, just as Lawson explained. You can go with a preamp tube that has a lower total gain factor for a more tame sound. 12ax7 gain = 100; 12at7 gain = 70; 12ay7 = 40. If you substitute V2--the phase inverter--you are altering not the preamp gain, but the signal that is feeding the power amp tubes. This is a dicier proposition. People do it, but I prefer to leave the 12ax7 in place there.

**vernon** · 03-27-2019, 10:14 AM

Regarding Weber AlNiCo 10” - I’m pretty sure I got model 10A125 30watt... solid improvement over stock.

**D.G.** · 03-27-2019, 03:12 PM

In a Pro Jr. I like to put a MUCH lower gain tube into V2 -- NOT V1 -- like a 12au7. This will give you a LOT more room on the volume dial before the onset of the overdrive.

**leroydobbs** · 03-27-2019, 03:57 PM

Originally Posted by El Fundo

I’ve thought about that. Which Weber Alnico 10” did you get? There are several choices, some of which are very expensive.

The Weber “signature” AlNiCo 10’s (emi made for Weber) sound great very old school Jensen and they are inexpensive.
Don’t handle much power but your pro jr is fine.
Put the 5751 in the 1st preamp spot (farthest from the EL84s).

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**lawson-stone** · 03-27-2019, 06:16 PM

Originally Posted by D.G.

In a Pro Jr. I like to put a MUCH lower gain tube into V2 -- NOT V1 -- like a 12au7. This will give you a LOT more room on the volume dial before the onset of the overdrive.

How can that be? V2 is just a Phase Inverter tube? V1 is the primary boost feature of the preamp section.

**neatomic** · 03-27-2019, 06:30 PM

12ax7s are dual triodes..tubes that can do two things at once...for ex-half can go to reverb driver and 1/2 can go to an extra gain stage...and there are different style phase inverters...without the schematic to the particular amp, you don't know what the 2nd tube is really doing...and pro jr IV amp with a solid state rectifier...many modern tube amps are awash with ss op amps..the tubes are not as key to the overall tone

on some modern amps tube changing finesse is not as obvious as with other (esp. vintage) amps

cheers

**El Fundo** · 03-27-2019, 07:14 PM

Originally Posted by neatomic

change out the power tubes one day..the 2 el84's...for some jj el84's...you'll love them..esp with a celestion 12" speaker

I found some JJs locally and will be picking them up soon. The Celestion you mentioned: are talking about the Alnico Gold? And also, do you mean replacing the 10” with a 12” speaker?

**neatomic** · 03-27-2019, 07:22 PM

great..the jj el-84's will make a marked difference!!...ok sorry, the fender pro iv i was referencing had a celestion 12"//probably one of fenders many limited editions...if you have a 10" speaker..then, yes of course, the celestion alnico gold 10 is one of the best speakers ever...but pricey!!!

depends how far you want to go with your mods!!!...jj power tubes first! gold in the future!!! haha

i have a '65 vox amp, six feet away from me, with 2 jj el84's and a celestion gold 10"..so im not lying!!

cheers

**Gitfiddler** · 03-27-2019, 07:29 PM

Does the Pro Jr. IV have a 1/4" speaker out? If so, simply disconnect the speaker and connect it to an ext. cab of your choice. (Be sure to match ohms!)

I did that with my GA5, Vibro Champ, Champ II and Super Champ amps. They sound 'all grown up' connected to an ext. cab.

**asedas** · 03-28-2019, 09:59 AM

I have Celestion Gold 12" in my PRRI and it made HUUUUGE difference. I recommend this mod. Also the Sovtek 12ax7LDS tube is one of the best new and quite cheap. You can also look for some vintage preamp tube like Philips or RFT.