What to build?

[squierstrat]

I have a Precision S-35 PA head in working order with tubes.

https://imgur.com/7yd5gx1

I would like to rebuild as a guitar head using the chassis and transformers. Maybe keep the 7868 tubes for a while? Options under consideration include Princeton Reverb (delete Reverb and Tremelo), Trainwreck Express with EL34s and fixed bias, and Bassman (too loud?).

I am especially interested in what the forum has to say about how the transformers fit those power amp topologies, if/how to use a rectifier tube, and what amps just won't work at the voltage provided.

Thanks.

[AlNewman]

Looks like it might be a decent amp as is.  You could try splitting the 2 halves of the first tube at the input, leave one as a mic input and wiring the other as a fender input stage, with a 34k/1m grid, a 1.5k cathode, and a 100k plate, possibly a bypass cap...see what happens.

[PRR]

Agree. Abuse it as-is for a while. It may play fine.

BTW: Grommes~Precision is still in business.

[bmccowan]

Joining the chorus - modify the circuit slowly/incrementally. Grommes/Precision made/make high quality amps. I have a set of Monoblocs for Hifi stereo and have modded a few PA amps for guitar - good stuff. I have never seen this model. After playing it as-is for a while, as others suggest, next step might be to revise inputs/V1 to cathode bias. Nothing wrong with 6eu7s - pretty much a 6 volt 12Ax7. Gibson used them a lot as preamp tubes.
It would be fun/interesting to retain those power tubes - used more in HiFi I think, but Ampeg used them some?
Your question

and what amps just won't work at the voltage provided

I think it is about the power tubes - 7591 is the same spec as 7868 I think. The 6L6 family will work at those voltage levels, as would EL-34. B+ voltage is too high, as is, for EL-84s or 6973s. No reason to convert to a rectifier tube IMO.

[tubeswell]

imgur is infuriatingly slow today

[PRR]

> why grid leak circuits show up so often

They give the maximum gain IF the input signal is VERY small.

Guitars will usually overwhelm them. They also allow the cathode to be hard-grounded, which may reduce heater leakage hum, or use of electrolytic caps (which used to be more trouble than they are now).

[AmberB]

As for the output tubes, the 7868 and the 7591 have nearly identical specs.  The 7591 is basically the octal version of the 7868, Ampeg used 7591s in a couple of their amps.  They're actually a pretty good guitar amp tube, especially if you're looking for an amp in the 30-35 watt range.  As for using EL-34s in place of the 7868s, The EL-34 requires twice as much heater current as the 7868/7591, so the power transformer heater coil may not be up to the task.
I replaced the 7868s in a Bogan CHB 100 with EL-34s, but the Bogan had 4 of the 7868s and I just used a pair of the EL-34s to stay within the heater amperage ability of the heater winding. 
You "could" put a separate heater transformer on the chassis just for the EL-34s if you wanted to use them.  The output transformer impedance is in the right range for EL-34s, but may get hot when you push the amp hard.  You could use cathode bias on the EL-34s to reduce the power output to a more transformer friendly range for the amp.
Just my 2 cents...

[bmccowan]

The 7591 is basically the octal version of the 7868, Ampeg used 7591s in a couple of their amps.

They also used the 7868 itself, early on. Here's a schematic of one.
One of the Grommes/Precision PAs I have waiting in the wings has a pair of 7591s and 2 6SL7s in the preamp. Very Ampeg like circuit.
Good catch on the EL34's heater requirements.

[squierstrat]

Update: Sorry for duplicate post! Wish I could delete.

I have a good condition Precision S-35 PA head, including tubes.

Search or follow this URL for schematic: https://imgur.com/7yd5gx1

My hope is that the transformers are decent and I can build something with them for both amp-building fun and playing fun.

I think I can figure out a fixed or cathode bias solution for any circuit. And I can use a EZ81 rectifier tube to work around no 5v transformer output.

I don't need or want much output power. Princeton Reverb looks like a good fit for PT voltage. I might also like to use some 1950s EL84s I have from a Lowry Organ donor.

Options for keeping the 7868s, at least while the tubes last?

Please let me know your opinions and thoughts. Roasting is fine too, I suppose.

[bmccowan]

I saw this and thought - another one? what are the chances? But I see that you posted basically the same thing a few days ago and got some ideas. I think it'd be helpful to continue on that previous thread.

[bmccowan]

Update: Sorry for duplicate post! Wish I could delete.

I believe the moderators can marry them if you want.
I thought PRR and AlNewman had good ideas about using as is first.
Assuming you don't want to do that, I can only tell you what I might do:
First I would follow PRR and Al's advice far enough to hear what it sounds like.
Next I would follow that Ampeg schematic I posted. Yes Ampegs are clean, but you can add filth later if you want.
Your amp has 6eu7 preamp tubes - they are close kin to 12Ax7s, so I'd use them and adjust the circuit as needed.
I'd get a happy amp first before considering a rectifier tube.
I'd keep my eye out for some backup 7868s as I think it'd be more fun to make those work than to convert to EL84s or other. My experience with Grommes/Precision is that all the components will be high quality, so keep all you can.

[squierstrat]

General Update: Removed the old-style speaker jacks and replaced with one 1/4" jack. Played it with my strat, Mic 1 input to cheap 12" Celestion. Comparable to my Quilter Microblock, but of course different. I am too inexperienced to describe how, I think. I like it better than the Quilter but I need to learn to control the responsiveness? As expected, there is NO preamp distortion, no matter how far I turn up the Mic 1 volume and turn down the master.

Next up: replace Mic 2 with Fender style input. Add a grounded cord. Check the B+ voltages and caps for DC leakage.

Attached a couple pics of the plate I cut (from an old Lowry organ chassis) to cover the speaker jack holes before drilling one for 1/4" jack.