New to me SF Pro Reverb

[Smokebreak]

I picked up a '79/80 Pro Reverb yesterday locally off CL for $425. Kinda unbelievable. Sat on the Austin listing for two weeks, even with a "make me an offer" plea. Worked into the deal was that I fix a Blues Deluxe, that does the random channel switching thing. Fellow techs will know that takes well under an hour, and $1 worth of parts.

This is one of the 70W Pros w, the UL OT. I'm sure that's why it sat there. So much BS out there about these amps. From what I've read(honestly am just starting to dive into what UL even is, for the first time), these aren't even truly UL amps, more just designed to decrease screen dissipation . They also have the master volume, and the tube matching adjust, as opposed to the bias adjust.

I had marked up the schematic for the amp for all the things I was going to do to get it closer to BF specs, before I even got it. Seems like conventional wisdom has always been "tube matching sucks", "yank the MV".
Being a good student of the amp gods, I resisted the urge to immediately take the thing apart upon receipt. I played it for a while, as it was given to me, and it sounds fantastic, even with an odd combo of a red stickered made in China eminence and the dreaded celestion 70-80. So I pulled the chassis to see what I was working with.

The only mods were a doghouse cap job, done nicely with F&Ts, and a pair of JJ 6L6s. Inside of the chassis was 100% stock, barring a replaced 2W 470r screen resistor. I'm not even sure I've ever seen one of those before! Very cute sandblock.
All original preamp tubes
Plates are 515V, screens just under that. The rest of the voltages, from the PI on down, are really close to BF Pro specs...which is is relatively high for Fenders, but still "classic" nonethless.

So I sat there wondering what I reeealllly even need to do to the amp.
It sounds great, but good speakers will make it sound even better.
The MV is actually entirely useful in such a loud, punchy amp...certainly for stage use. Even turning it down from 10 to 8 can help find a volume sweet spot, which can get be difficult on the fly, depending on the gig.
The tube matching pot was also useful. I twisted it till both tubes were at 32mA. That bias point works for me at the moment, and the only reason I can see modifying that circuit would be to increase the range of -V needed, and I certainly wouldn't need to convert it to a traditional bias pot to do that.
The only issue with the stock arrangement is the the grid feeds are something like 68K, instead of the usual 220k.

So my only complaint is that the amp maybe sounds just a bit too clean, punchy....sterile if you will, but there need not be a wholesale change to things.
The first filters are 220uf series, and that's a bunch for a 50W amp. I'm considering knocking those down to 100uf, and calling it a day.
I'm curious what the argument would be for ultimately changing the bias feeds to 220K.
At this point the only things I've done is install 5W 470r on the screens, and replaced the bypass caps in the trem section. I've also changed the 820r or whatever it is in the cathode of the reverb driver to the traditonal 2.2k/25u. Forget to mention the reverb sounded a little off. Oh the trem wasn't working, but that was just bc a cap lead was broken.
Also the PI is the 47K plates, 330K grids, Isn't this a better driver arrangement?
There are a couple other little things, but they are fading away as I sit here and play through the amp.


I've attached a much more readable schematic for a redrawn Twin UL 135W, but the Pro is the same with just the 2 tube output.

[martin manning]

...these aren't even truly UL amps, more just designed to decrease screen dissipation . They also have the master volume, and the tube matching adjust, as opposed to the bias adjust.

Nice score. BF-like preamp and clean power amp with MV? What's not to like? Smaller reservoir caps might be a good thing. I'm not sure what the screen taps are positioned, but it could be something close to "Ultralinear." Generically, that can be called "distributed load."

The tube matching pot was also useful. I twisted it till both tubes were at 32mA. That bias point works for me at the moment, and the only reason I can see modifying that circuit would be to increase the range of -V needed, and I certainly wouldn't need to convert it to a traditional bias pot to do that. The only issue with the stock arrangement is the the grid feeds are something like 68K, instead of the usual 220k.

Sounds like you are good for now with the bias balance control, but I would rather add a little trim pot to the bias circuit and keep the balance control. Best of both worlds. I'm not sure what they were thinking with the 47k and 68k grid resistors. I'd be tempted to go with the traditional 220k.

[Smokebreak]

Thanks Martin, and good to hear from you.
From what I understand the screens are at 10 or 12% so still in Pentode, just basically high plates and screens.
I'll hang a trimmer off the balance pot so it's there when I need it. I'm temped to do to 220K too, I figure I'll only really hear the difference at loud gigs, maybe?

As far as speakers, I'm not sure I've ever had to deal with a 2X12, for personal use. Was always a Vibrolux and Super guy when I was young, then went to 1X12 the past decade. I absolutely LOVE the Emi Allesandro in a Fender 1X12, but after talking to CE last week, seems Emi guitar production speakers on the whole is up in the air. Any intel on this? I'm totally open to speaker recommendations, and still kinda surprised what's in there isn't awful

[Ten Over]

Here is a schematic for your amp.

[pdf64]

They’re great amps!
I guess that the design engineer’s remit was loud and clean, and it does that, albeit that stock speakers tended to be a weak point.
The low value of the output valve grid leaks may be to reduce the gain and max available signal swing from the LTP, and to reduce the charging time constants in the grid circuit.
So one change creates a 3 pronged approach to mitigate for bias shift / blocking distortion.
Hence increasing them to 220k will increase (power amp) gain a dB or 2, but make the amp more liable to farting out when pushed hard.

Note that the speaker jacks are arranged in series and include impedance switching.

[Ten Over]

The speaker jacks are connected in parallel.

If you plug into the Speaker jack only, then it connects your load to the 4 Ohm winding.

If you plug into the Ext.Speaker jack only, then it connects your load to the 2 Ohm tap.

If you plug into both jacks, then it connects to the 2 Ohm tap and connects your two loads in parallel.

[pdf64]

Oh, yeah, sorry, I assumed it would be the same as the TR 135.

[SoulFetish]

The output tubes matching circuit is awesome. You can really dial these in to idle super quiet.
I be usually just install an adjustable bias control designed to work within the existing OTM circuit.
Here is the schematic of what it looks like

01BF802D-6401-4370-B47D-B5FA8EBCC65A.jpeg

[martin manning]

The output tubes matching circuit is awesome. You can really dial these in to idle super quiet.
I be usually just install an adjustable bias control designed to work within the existing OTM circuit.
Here is the schematic of what it looks like

That appears to be a schematic for a later version of the 70W Pro Reverb that has both bias level and balance pots. There are a couple of ways to add a bias level to the one Smokebreak has, and it probably comes down to where is the easiest place to hang a trimmer given the layout.

I'm temped to do to 220K too, I figure I'll only really hear the difference at loud gigs, maybe?

You would increase gain, and the likelihood of blocking distortion due to the larger time constant as Pete says. I'm going to conclude that the miss-matched grid resistors (47k on the inverting side and and 68k on the non-inverting side) are an alternate way to balance the PI outputs. Note that the plate loads are equal at 47k. There will be a small difference in the frequency response at the low end doing it that way, but that's probably not going to be noticeable.

[Ten Over]

That appears to be a schematic for a later version of the 70W Pro Reverb that has both bias level and balance pots. There are a couple of ways to add a bias level to the one Smokebreak has, and it probably comes down to where is the easiest place to hang a trimmer given the layout.

That schematic has been altered. Fender never made a 70W Pro Reverb with both bias level and balance pots.

If he never uses the Line Out/Recording jack, he could remove that jack and install a bias level pot in that hole. It is a little bit tight right there, so it may be necessary to use a miniature 16mm pot.

[SoulFetish]

The output tubes matching circuit is awesome. You can really dial these in to idle super quiet.
I be usually just install an adjustable bias control designed to work within the existing OTM circuit.
Here is the schematic of what it looks like

That appears to be a schematic for a later version of the 70W Pro Reverb that has both bias level and balance pots. There are a couple of ways to add a bias level to the one Smokebreak has, and it probably comes down to where is the easiest place to hang a trimmer given the layout.

I'm temped to do to 220K too, I figure I'll only really hear the difference at loud gigs, maybe?

You would increase gain, and the likelihood of blocking distortion due to the larger time constant as Pete says. I'm going to conclude that the miss-matched grid resistors (47k on the inverting side and and 68k on the non-inverting side) are an alternate way to balance the PI outputs. Note that the plate loads are equal at 47k. There will be a small difference in the frequency response at the low end doing it that way, but that's probably not going to be noticeable.

I altered the schematic. I should make a note on it, and date it so that it doesn't go into circulation and cause confusion.

[SoulFetish]

I didn't want to alter the rear panel on the Pro I worked on recently. First, it was for a customer who is strictly a musician with no interest in how a bias even works. That's okay, it's what he pays me for. In this case, I didn't want to put such a critical control where he has easy access and could get confused. Plus, for this amp, I thought the more elegant solution was to install the bias control in the traditional Fender location on the chassis. There's was enough estate there, so I center punched the install location, drilled out the mounting hole, and wired it inline with the stock circuit. I think repurposing unused, or obsolete real estate on either the front or back panel is totally fine. I just thought this was the way to go for this job. I happened to use a multiturn pot, because I had a surplus. But here is the finished job.

SF70 Pro Reverb Bias control.jpg

SF70 Pro Reverb Bias control external.jpg

[martin manning]

I altered the schematic. I should make a note on it, and date it so that it doesn't go into circulation and cause confusion.

Absolutely, and especially since you made such a good job of making the added bias level pot match the original graphics and lettering!