Replacing a tube in a Fender Pro Jr IV

**El Fundo** · 03-28-2019, 12:35 PM

Originally Posted by Longways to Go

I don't think there's enough room for the Celestion Gold in the Pro Junior.
Better check for clearance before buying a speaker.

You’re right. I just checked out pics of the Celestion this morning. No way it wil fit. There’s barely enough clearance for the stock speaker.

**asedas** · 03-28-2019, 02:18 PM

There is also Gold 10" Celestion G10 Gold - 10 inch Alnico Guitar Speaker
and probably is the same good - you will notice difference, probably more sweet treble and more equal sound in general.

**Longways to Go** · 03-28-2019, 04:55 PM

Originally Posted by asedas

There is also Gold 10" Celestion G10 Gold - 10 inch Alnico Guitar Speaker
and probably is the same good - you will notice difference, probably more sweet treble and more equal sound in general.

True that this is a great speaker, but the 10" just does not fit in this amp.
I have one sitting around -- it would be in there if that was an option!

PRRI has a little more room to mess around with speakers.

Also, I should add that I am perfectly happy with the stock speaker in my Pro Jr III.

**D.G.** · 03-28-2019, 06:59 PM

Originally Posted by lawson-stone

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

Frankly I don't know why, but it does work. I've owned a few Pro Jr.'s and they all behave the same to the tube swap. I actually keep a 12ax7 in V1 as it's a brighter tone -- which is my preference -- and the 12au7 in v2 then helps control the gain structure and noise floor.

**KirkP** · 03-28-2019, 08:16 PM

Originally Posted by dlew919

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

That’s not a rule. If two output tubes are in push pull it’s best to match them. Otherwise, only replace tubes that are bad.

**leroydobbs** · 03-28-2019, 09:25 PM

Originally Posted by D.G.

Frankly I don't know why, but it does work. I've owned a few Pro Jr.'s and they all behave the same to the tube swap. I actually keep a 12ax7 in V1 as it's a brighter tone -- which is my preference -- and the 12au7 in v2 then helps control the gain structure and noise floor.

The inverter stage is also the driver for the power tubes. Some PI stages produce a net loss from input to output of that stage and some produce gain. The tone control can affect gain at this stage as well. So will the gain factor of the tube in that stage.

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**Alder Statesman** · 03-28-2019, 11:46 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.

A completely different amp, but I have had a 5751 in V1 in my SuperSonic 22 for a couple years. I have a 12AU7 on order that I will be placing in V4. I understand this will reduce the SS22’s notorious idle noise while adding some headroom. The idea is similar DG’s suggestion for the Pro Jr.

This is what I enjoy with tube amps, rolling tubes can lead to some great tones besides being an enjoyable pastime.

**stevo58** · 03-30-2019, 03:15 PM

The phase inverter in a pro jr is a long-tail pair. This means it uses both triodes and has a gain>1.
The 12AX7 is not really a particularly good driver tube (and the phase inverter is a driver function). It is designed as a gain tube. A 12AT7 is a driver tube, and will swing more current than a 12ax7. A 12au7 is also a better driver tube than an AX7.

Changing the phase inverter will most certainly change the tone of the amp. Give it a try and see if you like it. There are many variables, so I won’t make any predictions. Try both an AT and an AU and see what does it for you. I wouldn’t waste a 5751 in the phase inverter stage.

And don’t worry about getting a ‘balanced triode’ unless you are absolutely certain every component in the inverter circuit was selected and individually measured for a perfectly balanced PI. I can assure you that is not the case in a pro jr. Even if it were, you might not like the result.

Steven

**neatomic** · 03-30-2019, 03:58 PM

Originally Posted by El Fundo

You’re right. I just checked out pics of the Celestion this morning. No way it wil fit. There’s barely enough clearance for the stock speaker.

not saying i'd necessarily recommend doing it...but the bell cover can be removed..it's only there for a bit of extra protection... against heat, dust, getting hit etc... jensen sells their alnico speakers with bell covers as an option..extra $$$

here's celestion gold in fender jr III

cheers

**Longways to Go** · 03-30-2019, 05:22 PM

Yeah, I kinda remember seeing that speaker-swap video now that it was brought up.
There's also a guy on a TGP thread who says he tried the Gold and it didn't fit. . . go figure. but hey, it's the internet!

My concern with subbing in that deep of a speaker is that it is so close to the power tubes and dang that amp already runs really really hot !!!! I like the idea of a little more space -- some breathing room. I could also stick a rajun Cajun in there -- I have one sitting around -- but I'm still perfectly satisfied with the stock speaker.

And then there are guys getting pine cabs for 12" speakers built, doing BillM mods, etc. Geeze it's another deep rabbit hole !!! All perfectly cool mind you, but I enjoy that the Pro Junior is cheap & small. Rather than mod the heck out of it, I'd rather sink 2K into a nice handwired Victoria or something . . .

All this talk of the Pro Junior got me looking mine over just to see what was what, as I haven't messed around with it for quite a while and I have been perfectly happy playing it. So my Pro Jr III has the stock speaker and the original power tubes, a NOS Jan Phillips 5751 in V1 and an Electroharmonix 12AY7 in V2. (I must admit I didn't remember the V2 substitution . . . ) I have a pair of Tungsol EL84's that I'll toss in for power tubes when needed. As it stands, the amp is quiet enough for home practice, it's been my go to practice and gigging amp for at least a year now and I'm still perfectly happy with it. I use a reverb pedal, amp tone and volume around 3, and adjust volume from the guitar. It's a simple, small set up with a lovely thick tube tone that I find very easy to use and very pleasing.

The one other "mod" I did which I find very useful is that I removed the metal cage around the tubes so that I can quickly swap out a tube without removing the back cover on the amp. I know the tubes are less protected, but I prefer having the easy access.

**KirkP** · 03-31-2019, 12:27 AM

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.

Originally Posted by lawson-stone

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

When you put a lower gain tube in the PI stage (V2 for the Pro Jr.) I think it’s roughly equivalent to having a master volume in the PI stage and turning it way down. If you turn down a master volume control and want to have the same power coming out of the speaker, you must turn the channel volume and/or the guitar’s volume control up.

I don’t think that buys you any headroom though, because the output tubes still have the same power. In fact, it seems like it might dirty up the signal a bit, since the preamp tubes must be driven harder to get the same power out of the speaker. Of course driving preamp tubes hard isn’t necessarily a bad thing.

By dialing up the preamp volume control you’ve changed the EQ a bit. If there is no bright cap on the volume control, turning the control will tend to increase the treble. If there is a bright cap on the volume control, turning it up should tend to decrease the treble. (That’s because bright caps on volume controls are designed to increase trebles when the control is turned down.)

Feel free to pick this apart. :-)

**lawson-stone** · 03-31-2019, 08:45 AM

Originally Posted by KirkP

When you put a lower gain tube in the PI stage (V2 for the Pro Jr.) I think it’s roughly equivalent to having a master volume in the PI stage and turning it way down. If you turn down a master volume control and want to have the same power coming out of the speaker, you must turn the channel volume and/or the guitar’s volume control up.

I don’t think that buys you any headroom though, because the output tubes still have the same power. In fact, it seems like it might dirty up the signal a bit, since the preamp tubes must be driven harder to get the same power out of the speaker. Of course driving preamp tubes hard isn’t necessarily a bad thing.

By dialing up the preamp volume control you’ve changed the EQ a bit. If there is no bright cap on the volume control, turning the control will tend to increase the treble. If there is a bright cap on the volume control, turning it up should tend to decrease the treble. (That’s because bright caps on volume controls are designed to increase trebles when the control is turned down.)

Feel free to pick this apart. :-)

Actually now that does make sense. The PI is the last relay of the signal to the power tubes, so presumably what happens there would be significant. Never really thought of it before.

**mje** · 03-31-2019, 12:05 PM

The 5751 was designed as a low noise dual triode similar to the 12AX7 in high gain circuits, like oscilloscope amplifiers. The features that make it lower in noise and microphonics also mean it’s more resistant to vibration. It was not meant as a drop-in replacement in an existing device.

As designed, the 5751 has a mu 70 which is closer to that of a 12AT7(mu of 60) than a 12AXT (100) so it’s a popular replacement for a 12AX7 in the kind of applications we’re talking about. However....

today’s newly manufactured tubes are not necessisarily identical in characteristics to the tubes made 50 years ago that have the same labels. Many are or were existing Russian designs that are close enough to work in a particular application and so were given a new label and sold as drop in replacements. Russian 5751s are often relabeled or modified 6N2P tubes.

**Alder Statesman** · 03-31-2019, 01:14 PM

Originally Posted by mje

The 5751 was designed as a low noise dual triode similar to the 12AX7 in high gain circuits, like oscilloscope amplifiers. The features that make it lower in noise and microphonics also mean it’s more resistant to vibration. It was not meant as a drop-in replacement in an existing device.

As designed, the 5751 has a mu 70 which is closer to that of a 12AT7(mu of 60) than a 12AXT (100) so it’s a popular replacement for a 12AX7 in the kind of applications we’re talking about. However....

today’s newly manufactured tubes are not necessisarily identical in characteristics to the tubes made 50 years ago that have the same labels. Many are or were existing Russian designs that are close enough to work in a particular application and so were given a new label and sold as drop in replacements. Russian 5751s are often relabeled or modified 6N2P tubes.

Fortunately there are readily available GE 5751’s that were manufactured in Kentucky through the mid to late 80’s. These JAN tubes will cost more than modern Russian tubes, but will last long enough for my use. Try www.thetubestore.com - Your online source for audio vacuum tubes..

**KirkP** · 03-31-2019, 01:33 PM

Originally Posted by mje

The 5751 was designed as a low noise dual triode similar to the 12AX7 in high gain circuits, like oscilloscope amplifiers. The features that make it lower in noise and microphonics also mean it’s more resistant to vibration. It was not meant as a drop-in replacement in an existing device.

The one 5751 I bought (from a usually reliable source) turned out to be very microphonic!

Is it acceptable to link to a thread from the Harmony Central forum? I pasted a link at the end of this post. #8 has some excellent observations on tube swapping from a circuit design viewpoint. The basic gist of it is that even though a number of dual triode tubes are pin-compatible and will operate when substituted for another in that family, their design parameters may differ in more ways than simply the gain.

12AX7 vs. 12AT7 may be one of the more extreme examples. Fender long-tail pair PIs in the tweed era used 12AX7s. In the blackface era most Fender PIs were redesigned for 12AT7s. The schematics have much different component values. If you swap one tube for another you’ll be operating the PI far from its optimum point and might end up with unexpected results like crossover distortion or clipping.

The 12AX7 5751 swap isn’t a problem because the tube characteristics are pretty similar, but other swaps might not work well. One analogy might be to putting different tires on a performance car. If a car was optimized for one tire type and you swap them for something much different, it might work fine for getting to the grocery store, but you might seriously degrade the high speed performance. Fortunately, tubes are much cheaper than tires and won’t put you in the ditch.

Here are schematics for the Pro Jr PI vs. Deluxe Reverb showing how different those circuits are.

12AY7 or 12AU7 in V1 instead of a 12AX7?- Harmony Central