Fender Pro Junior gtr amp schematic deletions--perspective/opinions?

[soapfoot]

I've used a Fender Pro Junior (with a better speaker) for many years, but got tired of reflowing solder joints on the PCB-mount tube sockets.

I decided to rebuild the amp using the Hoffman Amps turret board, but noticed in Hoffman's schematic that there are some parts deletions.

The most obvious is the Zobel network on the OT primary (R26 and C11).

The 47pF snubber cap (C10) also seems clear enough in function. Obviously if it was deleted there are unlikely to be stability issues, but maybe it's smoothing out top end a bit, too?

I also notice that the out-of-phase output of the LTP phase inverter has been switched to a more typical configuration (R17 and R18 have been replaced with a single 82k). I'd love to understand why the 9k1 and 91k in series were originally employed. Does this manage gain staging somehow, or does it perhaps mitigate the impact of the above-mentioned 47pF snubber?

If I like the sound of the amp as-is, should I err on the side of building the amp as originally designed, or would the deleted components be unlikely to make much difference?

One other thing--the output transformer was replaced with a larger Mercury Magnetics unit many years ago; I'm not sure how that changes the calculus re: the stock Zobel network across the primary.

Would greatly appreciate any input/insight!

PS: the bias supply has obviously also been reworked/improved to make it adjustable. That's clear to me, and I'll be going with the improved bias supply.

Original schematic (changes circled in red):




Revised schematic:

 

[CJ]

One time. I stuck one of those caps across the pi plates and it made things worse,

The 91k/9.1k thing is to get a slightly better match between output signals, it was probably borrowed from some boutique builder and does absolutely nothing as the tube sections are never matched anyway, you would be hard pressed to notice any difference between that and the normal 82k. And also the output xfmr cancels any signal mismatch in the pwr
tubes. Also, some people say that a mismatched phase inverter sounds better but that is probably bs.

 

[Rusan]

....does absolutely nothing as the tube sections are never matched anyway, you would be hard pressed to notice any difference between that and the normal 82k. And also the output xfmr cancels any signal mismatch in the pwr tubes.

Thank you, CJ! I can't count the times I've been flamed by Aspen Pittman disciples for saying the same things.

 

[soapfoot]

Thanks so much!

What do you think about deleting the Zobel network on the OT primary?

 

[Gus]

A fun read that might be of some help

https://robrobinette.com/Voicing_an_Amp.htmI had not seen the Dumble schematic before

 

[Rusan]

One could speculate that the output stage had some oscillation issues resulting from something to do with the OEM trafo and the enclosed NFB loop, perhaps even exacerbated by a less than optimal PCB layout and wiring scheme and some rather high transconductance power tubes.

Or, maybe they just wanted a more constant impedance loading across the finals...

You did state the OPT had been replaced with a larger Mercury Magnetics, and you're moving the circuit to a turret board. If the finals are unstable sans Zobel in the stock state, it may well be stable after you're finshed your conversion (provided that it was an issue in the first place).

If it were me, I'd try it with and without the RC shunt, while doing a frequency sweep from 80Hz to at least 50kHz with a scope connected across the secondary (terminated into a proper load, of course). If there's no oscillation, then I'd listen to it both ways and see which I preferred.

If it oscillates without the RC shunt and you have to keep it, I think the stock values would be fine. The values should be impedance-scaled to the circuit, so as long as the replacement transformer has the same winding ratio as the OEM unit, it should be okay. Crank 'er up and play a while or input some pink noise, and if the OPT's not getting warm from ultrasonic heating, then you're good to go.

Perhaps CJ will chime in on this with a more definitive answer or at least tell me I'm a complete idiot, as he's forgotten more about guitar amps than I'll ever know.

 

[CJ]

I have never seen a zobel on a guitar output xfmr before so they must have had a real bad problem. The primary leads can cause problems, the Peavey 5150 has a very bad problem with the leads being right next to the preamp tubes, use some chopsticks to move the PRI leads around to see if you can get it to oscillate without the zobel. Of no oscillation then you probably can ditch that thing. There is so much energy in the output xfmr , plenty of current to overwhelm any capacitance which is why you never see copper shields used, which makese think that a zobel would be useless, unless there was some HF ringing that made an ugly sq wave, if you have a signal generator and scope, look at the output at 1k, 10k, and 100k sq wave with and without zobel.
Any stray capacitance in the windings Is easily charged up by all that power.

Since there is not much gain in the phase inverter, it has the least effect on tone out of any tube in the amp, however, there is a Mullard military tube that sounds great as a sub for 12AX or 12AT7, trying to recall the number, CV something,

I met Aspen at the AES show, really nice guy, kind of big which might have caused some health problems.

 

[pucho812]

Aspen was pretty smart at times and other times talked of positive electrons.
may he r.I.p.

 

[Rusan]

there is a Mullard military tube that sounds great as a sub for 12AX or 12AT7, trying to recall the number, CV something,

That would be a Mullard CV4024, the British military 12AT7. I used to have a pair, sold 'em in an amplifier. They're indeed great tubes.

 

[soapfoot]

I don't think Mr. Pittman succumbed to health problems. I believe he had a bad auto accident

 

[pucho812]

I don't think Mr. Pittman succumbed to health problems. I believe he had a bad auto accident

correct. I don't know exactly how it happened but he was pulling a trailer with a van and going to pickup a vintage car to take to sale. Somehow he had an accident on a canyon road. It was a single vehicle accident and the van managed to be on top of the trailer. No one knows how it happen. Aspen was found later and brought to the hospital. He passed while in the hospital after 24 hours.
He was a good friend in the L.A. audio community.

 

[soapfoot]

I have never seen a zobel on a guitar output xfmr before so they must have had a real bad problem. The primary leads can cause problems, the Peavey 5150 has a very bad problem with the leads being right next to the preamp tubes, use some chopsticks to move the PRI leads around to see if you can get it to oscillate without the zobel. Of no oscillation then you probably can ditch that thing. There is so much energy in the output xfmr , plenty of current to overwhelm any capacitance which is why you never see copper shields used, which makese think that a zobel would be useless, unless there was some HF ringing that made an ugly sq wave, if you have a signal generator and scope, look at the output at 1k, 10k, and 100k sq wave with and without zobel.
Any stray capacitance in the windings Is easily charged up by all that power.

Revisiting this after awhile...

I got this amp sounding really good, except that when I was overdriving it decently hard, something that could only be described as an ugly-sounding, almost-unstable distortion happening on the fronts of notes that were attacked fairly-hard.

It didn't sound like blocking distortion... more like something that was almost wanting to oscillate. I didn't remember this behavior from the original PCB layout of this amp, so I thought of this missing Zobel.

I never bothered to scope it, but in a game of simple "spot the difference" I added the Zobel network back in on the OT primary (2200pF 630V polystyrene and 270R metal film) and just like that... unpleasant quasi-ringing behavior gone.

Since I didn't put it on the scope I can't verify for 100% certain, but it seemed pretty obvious to me that this was addressing some kind of real situation. It's working well and sounding great with the Zobel network in place, so I'll leave it in

 

[CJ]

You could do a Seymour Duncan convertible mod. That is put in an edge connector and build different preamp modules these get swapped out when you need anew sound,

 

[soapfoot]

Interesting idea—but I’ve got a few amps, and this one does a unique thing, so I’ll probably just leave it

 

[CJ]

this Crate dirty channel sounds awesome>

 

[murrayatuptown]

Looking at the Amp Books LTP calculator, Long Tailed Pair Calculator
There is quite a bit of gain in the 12AX7 LTP (ignoring NFB), like 25 dB or so. 5751 drops it a decent amount, and 12AU7 drops a lot more, even with different cathode & tail resistors...so much so it doesn't look like a good 'incremental' experiment.

Murray