'69 Sunn Sceptre questions

[carlsoti]

I've got an old Sunn on the bench for a general health check-up. I haven't dug into it too deeply yet, but it appears to be the early circuit with a can cap and a single additional cap on the B+ before the choke. It's been maintained with a few minor mods and a couple minor issues I plan on fixing. The first filter cap is under-rated, at 500V. This will be replaced with series caps and bleeder resistors to handle the expected voltage. The 3-prong conversion has been done previously with polarity switch taken out of the circuit, but the AC hot to the fuse was put on the "side" conductor of the fuse. That will be corrected, also. The can cap has already been replaced with a CEDist cap from '21, and few other electros have been replaced, as well.

Docs from SLuckey for reference > https://sluckeyamps.com/sunn/sunn_sceptre_1971.pdf

It doesn't appear to be too complicated of a circuit, leaving little room for gain mods, unless I get creative and hack the reverb driver tube circuit or add gain stages. The owner gets his doom-tones from pedals, mostly, so I don't think it's appropriate to get that involved.

I was considering putting the NFB on the polarity switch, but I'm unsure if I need to do anything special due to the UL output section.

Any insights would be greatly appreciated.
TIA

P.S. I see now that the OT might be wired a little differently than the OG schematic. This amp has original markings for 8 and 16 ohm output, but I haven't verified the actual wiring just yet. That may play into how the NFB is wired.

[Stevem]

I have a 71 Sonaro which is the same basic amp as yours other then the effects.

When I first got mine I played around with using it for bass and guitar at fairly loud volumes.

Now I just use it for bass, but if I was going to use it for just guitar I would tell you to audition playing around with smaller value PI output coupling caps .

[carlsoti]

Thanks Stevem. Oddly enough, I met the guy that put this on my bench when he bought a 68 or 69 Sonaro I had found "out in the desert." I really liked that amp, but there's just no flavor when they're down at apartment volume. He's got room to doom, so when he saw this one for sale, he jumped on it. When I asked if he planned a stereo or wet/dry rig with the two, his eyes lit up!

[sluckey]

I don't often see discussions about the Sonaro. It was designed to be a bass amp. Very simple but heavy combo amp. I played bass in a C&W band during '67 and '68. The band owned all the equipment. The bass rig was a Fender Precision Bass and a Sunn Sonaro. I was studying electronics in a trade school at the time and I chose to build the Sonaro as my first project. Here's the real thing and my clone...

https://sluckeyamps.com/misc/sunn_sonora.jpg
https://sluckeyamps.com/misc/1st_build.jpg

Forty years later I had the good luck to pick up a Sceptre on eBay for cheap. I restored it and still have it today. I never could find a Sceptre cab but was able to get a Solarus 2x12 cab. Both are keepers.

https://sluckeyamps.com/sunn/sunn.htm
https://sluckeyamps.com/6v6plexi/P-6V6_09.jpg

Thanks for the memories!

Now back to your Sceptre... It's a powerful, clean amp. I would just fix whatever is broken and not modify at all. Get ya dirt from pedals!

[carlsoti]

Thank you for your input, sluckey. I hold your opinion in high regard, as you've always been very helpful here and in other forums. I'll likely only do what's absolutely necessary with this one. All the prior work on this seems well-done with high quality components. Upon closer inspection, I'm reminded, yet again, that I should not post questions early in the morning based on my memory. The "additional cap" is at the pre-amp end of the power rail, not on the first node, so there's no need for a series cap stack.

The reverb sounds great, as does the tremolo. The only thing that "might" need attention is in regards to the tremolo depth. It seems to "kick on" around 2 or 3 on the knob, though there's no thumping or other unusual sounds associated. While I was futzing late last night, I realized I've got some flakey meter leads, so I scrapped my voltage chart and shut it all down.

[carlsoti]

Follow-up question(s), seeing as this amp seems totally healthy, save for the expected high voltages throughout.

Currently, it's got a modern JJ GZ34 as the rectifier. I'd like to replace it with something with a bit more voltage drop. I've got a NOS Chatham "potato-masher" 5R4 I could drop in, but the datasheet states a max of 4uF for the input capacitor. Is this a place where I can "fudge it" and run that rectifier into the 30uF at the first node without issue seeing as we're not operating near 1kV nor any of the other limits? My gut says there's nothing doomier than using a rectifier spec'd for B-52s, but I'd rather not risk damaging this vintage amp, even if it's not a FVM, Dumble, or Trainwreck.

If the 5R4 can't be used like that, do you think these power transformers are built well enough to support a 5U4 rectifier heater? A 50% increase in heater current seems like a lot for me to risk guessing, as well. FWIW, with the JJ GZ34 and 117VAC input, I had 5.09V on the rectifier heater. All other voltages were marginally higher than some of the older schematics I've seen. With 123VAC on the input, I was tempting fate with 530V coming off the rectifier. The voltage rating on the can cap is 525V.

[sluckey]

Did you see my Sceptre? Raytheon 5R4 connected to a 20µF

[carlsoti]

I'm sure I did see those pics, but I've spent more time looking at the PDF with the schematics and mods you'd done to it. Thanks again!

[TUBEDUDE]

I don't often see discussions about the Sonaro. It was designed to be a bass amp. Very simple but heavy combo amp. I played bass in a C&W band during '67 and '68. The band owned all the equipment. The bass rig was a Fender Precision Bass and a Sunn Sonaro. I was studying electronics in a trade school at the time and I chose to build the Sonaro as my first project. Here's the real thing and my clone...

https://sluckeyamps.com/misc/sunn_sonora.jpg
https://sluckeyamps.com/misc/1st_build.jpg

Forty years later I had the good luck to pick up a Sceptre on eBay for cheap. I restored it and still have it today. I never could find a Sceptre cab but was able to get a Solarus 2x12 cab. Both are keepers.

https://sluckeyamps.com/sunn/sunn.htm
https://sluckeyamps.com/6v6plexi/P-6V6_09.jpg

Thanks for the memories!

Now back to your Sceptre... It's a powerful, clean amp. I would just fix whatever is broken and not modify at all. Get ya dirt from pedals!

Hey Sluckey, what's the deal.with the Strat? It looks like a natural sunburst with the banded edge. Is it a stripped faded Antigua?

[sluckey]

Hey Sluckey, what's the deal.with the Strat? It looks like a natural sunburst with the banded edge. Is it a stripped faded Antigua?

Haha. It used to be red as seen in this pic...



Then I stripped it and an auto body shop painted it yellow. Even later, I painted it antique white.

[TUBEDUDE]

A keeper then. Congrats. This looks like a "find the stoner" picture.