Tube issues in 1975 SFPR

[Jared Purdy]

Greetings,

A friend recently loaned me his 1975 SFPR. We plugged it in on Friday to check it out and so that him and my neighbour could hear my ne LP Traditional , and before I even had the guitar cable plugged in the amp was making this horrid ear piercing feeback like sound, that had a bit of reverbish tone to it.

We unplugged it, tried in a different place, same thing. I was thinking that it might be one of the tubes. He loaned it to me several months ago and when we plugged it in at that time we both remarked how bad it sounded (flat, muddy, no chimmy tone).

He's not too electrically or technically inclined with amps, and this amp has basically sat in his house unused since about 1980. When I got it, I was probably the first person to play it in years. At the time (back in the summer) I replaced the power tubes with a fresh set of GT's, and that alone breathed a tremendous amount of life into it. I had it for a few months and then gave it back to him in the fall. He had not used it once since getting it back from me.

Someone suggested the reverb driver might be at fault. I'm assuming that the reverb driver is the 12AT7? Well, I pulled it, and the problem is solved, well sort of, I just don't have any reverb.

So I have two questions related to this:
1. Can I use a 12AX7 in place of the 12AT7?
2. V1 on the tube chart inside the cab (and I verified this by looking at the tube) has the tube marked as a "7025".

I was under the impression that the PR amps had 3 12AX7's. Is 7025 just another name for a 12AX7?

I'm asking as I replaced the old 12AX7's in V3 & V4 with new GT's (I know not the best tubes in the world but they are new and on my shelf!), and I want to replace the tube in V1 as well, but with the right tube of course.

Thanks for your time.

[MWJB]

You can temporarily sub a 12AX7 for the 12AT7, it might not sound too great?

7025 is really now a defunct designation for a military spec, low noise 12AX7...any 12AX7 that sounds good and is not microphonic will work fine.

GT didn't make tubes, they sold the same tubes that everyone else sells, they have just been through GT's screening & grading process.

[Jazz P Bass]

The first tube, V1, from Fender was always a 7025, which is a 12AX7 with low microphonics.
I have never heard the difference. Put any 12AX7 in there & see what is what.
The reverb tube, V2, should be a 12AT7.
Yeah a 12AX7 will work but it will drive it harder.
Not much, but harder.
As an aside, I have seen 12AT7 tubes that put out as much output voltage as a 12AX7.
(marked incorrectly?)
So unless you have a test setup to measure the tubes, you will have to rely on your ears.

[Jared Purdy]

Thanks for those replies. I did a little experimenting. I have on hand a couple of JJ's, 1 Mullard, and a few GT's (pulled them from a PRRI that I use to have).

Currently, there are two GT's in V3 & V4, two GT 6V6's, the original rectifier, and the original 7025 in V1. I tried a JJ in V2 and it worked, but there is a audible hum coming from the amp, not super loud, but it's there. I then tried a GT, it was better, but the hum was still there. I then tried the Mullard, and it is the quietest of the bunch. I also tried a the Mullard, the GT and JJ in V1 with any one of those in V2 and the hum is worse.

I'm going to get a proper 12AT7, hopefully today, and see if that doesn't take out the hum. When there is nothing in V2, there is no hum at all. I suspect the amp need to be looked at by a qualified tech, and I'm lucky to have one of the best in the country (John Fletcher) about five blocks from me.

As an aside, does anyone know what speaker was used in the mid 70's era SFPR's? The is a blue label on the magnet that says "Fender Musical Instruments Special Design Speaker Made In USA"

[Jared Purdy]

I picked up a GT 12AT7 today and that solved some of the problem. It is defiantely a brighter tube in V2 and the reverb definately sounds better.

Right now I have the original 7025 in V1 (not sure what make it is), V2, V3 & V4 have GT's and there are GT 6V6's in the power section. When I tried putting a GT in V1, the hum is much more noticeable with my LP Traditional plugged in than with my Strat Deluxe. Same if I use the Mullard or the JJ. I tried a GE 7025/12AX7, one of the original tubes from the amp, and it is not better than the GT, JJ or the Mullard in V1. Any thoughts what would cause this, and why it would be worse with the LP Traditional than the Strat Deluxe?? Regards

[pbmw]

If that little Princeton Reverb has sat unused since the early 80's, I'd certainly get it to a tech for a recap. Also, I'd strongly recomend having a grounded plug put on and disconecting the polatity switch just for safety.
I have a 76. Outstanding little amp.

[Chuck H]

+++ on the recap and the AC cord upgrade. Other than that, it's been my experience that most vintage reverb model Fenders hum. And the more you turn up the reverb, the worse it gets. I would think it's a ground scheme problem right from the factory. Reverb tanks and their associated grounding schemes are finicky. I've never opened up a SF reverb amp to examine the ground scheme but I have noticed the hum. Implementig reverbs that don't hum in my own builds usually involves an isolated input circuit. Fender didn't use tanks with isolated inputs but I know from experience that you can do it yourself with a Dremel and some cardboard.

[pbmw]

I've looked at the grounding in my SFPR, I'm fortunate to have a VERY competent amp tech near me. He was kind enough to explain what needs to happen for good grounding and did it to mine. I get a little 60 Hz when I turn up the gain on my compresser, but if I run it in a reasonable way...all sounds well.
I play a LP with P90's through it every now and then and in the single side...it can get noisy. But if I turn up the juice on my A5 Pup'd LP, it works well. I use the reverb every now nad then also. I'm lucky to have an SFPR with a reverb that actually works. Many are somewhat flakey. But you can go a long way towards cureing the Hum and noise thing with proper grounding.