Sunn Build/Conversion project - Need some advice

[quintviskup]

Hello everyone,
I wanted to take a second to say thank you for even existing haha.
This forum is spectacular.

Now onto the meat of my topic.
I'm a new builder and still learning all of this stuff (again thanks for even existing because this has been a wealth of information)
I recently acquired an old Fender super twin which I had considered rebuilding and cleaning up.... however ...

I am pals with Tubegeek (James) and we started talking, it would be kinda cool to rebuild the super twin but it would be even cooler to transform this beast into a Sunn o))) heheh.
This amp has Ultra linear transformers which I know was a big part of the Sunn sound.

I mostly want to know if I am on the right track here? is this a good idea or would I be better off just restoring the super twin (god I really want to make this into a bad ass Sunn).

Now don't worry about me blowing myself up, I have tubegeek to consult with if I get lost and of the 2 amps I've built so far I have made sure he was present at power up ... and so far both amps work like a damn charm.

So far I have been looking at some different layouts and schematics, I am using the Weber 6S100 and the Torres "Bass Amp" as some starting points. Naturally tubegeek and I would have to work out some finer points when it comes to the power section (mostly because the weber design does not use ultra linear transformers and both designs are for much lower wattage amps by comparison to what the super twin is rated at).

I know there are a few Sunn folks on here so I am hoping you fine folks can chime in and give me some direction (maybe if one of you has a layout closer to the original Sunns i.e. Model T or 200/2000s

I would primarily be using this for bass so I'm not really looking at the reverb or tremolo circuits like in the sceptre.
Just big unadulterated boomy power (then I clearly need to start a doom band)

Thanks again in advance for any information, advice or just conversation about this.

Quint

[eleventeen]

Probably not a bad idea. No doubt the Sunn is much simpler than the Fender. Keep the same number and type of output tubes and power supply and you are probably 70% of the way there. Sluckey is the SunnGod. I am sure he will weigh in.

[quintviskup]

I was sort of waiting for SLucky ... I've stalked a lot of his posts here 
I appreciate your feedback so far too.
I was wondering if I could (or should) use the same tubes (considering I just bought all new tubes for the super twin in case I was going to restore it).
Also just looking at the guts of the Sunns its already a way cleaner and simpler design (which is the charm for me).
This twin has that weird active EQ and the goofy distortion circuit ... ugh wires everywhere

[sluckey]

I'd first be sure the power amp in the Fender works. Then I'd plan to use the Fender power amp and power supply. There's not gonna be much difference in the sound of the Fender versus the Sunn power amp.

Then carefully remove all the Fender preamp, tone, reverb, etc. circuitry. You want to leave V4,V5,V6,V7,V8,V9,V10,V11 and all power supply circuits intact and working.

Then build the simple one tube PTP Sunn 2000 bass preamp circuit and connect it to the Fender IAW the attached schematic.

Maybe get James to look over this plan and discuss it with you and see if he likes the idea. If you should decide to tackle this ambitious project please share with us. We like pictures. Good luck...

EDIT... I've replaced my original conversion drawing with the current simpler drawing. This eliminates the 6AN8 phase inverter and uses the Fender phase inverter. The Fender power amp circuit remains completely original and there should be no NFB issues.

[eleventeen]

Is there any truth to the bit of lore floating around that 6AN8s are hard to get and/or their quality is not what it was once expected to be?


No amp I have uses them but the last piece of junk I got has 3-4 of them.

[sluckey]

I was able to get a sleeve of NOS mil-spec 6AN8s 10 years ago when I restored my Sceptre. They seem to be fine.

[quintviskup]

That there my friends is why I knew this forum was the place to look.
Thank you so much Sluckey.
I will be sure to post lots of photos as I go through this project. In the end hopefully some audio clips too.

I'd first be sure the power amp in the Fender works. Then I'd plan to use the Fender power amp and power supply. There's not gonna be much difference in the sound of the Fender versus the Sunn power amp.

Then carefully remove all the Fender preamp, tone, reverb, etc. circuitry. You want to leave V5,V6,V7,V8,V9,V10,V11 and all power supply circuits intact and working.

Then build the simple PTP Sunn 2000 bass preamp and PI circuit and connect it to the Fender IAW the attached schematic.

Maybe get James to look over this plan and discuss it with you and see if he likes the idea. If you should decide to tackle this ambitious project please share with us. We like pictures. Good luck...

[sluckey]

Please note the new drawing in reply #3.

[quintviskup]

Hi guys,
Ive spent the last few days looking over the schematic and creating a parts list.
Please correct me if I'm wrong but the original design used KT88 for the power section (I think), will this be an issue with the transformer from the Fender Super Twin (ultra linear but the super twin was designed to use 6L6).

Also James and I had a chat about using a solid state rectifier rather than tubes .... a few reasons behind that thought... the solid state should be a little tighter and more responsive (which is nice for bass) and then I don't have to add another transformer to power the rectifier stage (no taps on the transformer Fender used for the super twin) ... and ya I don't want a million pound amplifier only a half million ... 
What are your thoughts on both of these topics?

[PRR]

> original design used KT88

6550. (Granted that 6550 and KT88 are the same except height.)

6550 is very much a "large 6L6GC".

The Fender output stage is well designed and integrated. It is also *already built* for you. It is a solid low-distortion un-harsh-clipping thing. Pretty much same as the Sunn. Fender did their numbers and decided that, at the 160W level, three-pair 6L6 was a dime cheaper than two-pair of 6550. (In part because Fender bought 6L6 by the truckload.)

I would not hesitate to send four-pair 6L6GC to do the work of two-pair 6550. At the 150W level, three-pair 6L6 is IMHO ample. I trust Fender's claim of 160W. And these amps are not known to die a lot.

But yeah: if you like the look and taste of 6550/KT88, rip the six 6L6 and put in two of the tall boys. (Check fit!) Minor re-biasing will dial it in. (A bias-mod on the Fender is good in any case.)

> using a solid state rectifier rather than tubes .

Sluckey showed you SS rectification. JUST like Fender built it. Don't touch a thing.

This is a "dashboard swap". We keeping the same engine. The big UL Fender is a fine engine just the way it is. You wanted a simpler control path. Sluckey nailed it.

Sunn "had" to use bottle rectifiers when the 2000 was new because suitable SS rects were more expensive than double-up GZ34. I'm undecided about glass rects for guitar (depends on the design and how you play/abuse it). For tube PA and bass, glass rect is IMHO "stupid" now that we have a choice. I always hated tube rectifiers. Bulky, hot, wasteful, more wires. Much of my younger work was ripping-out 5U4s to make IMO better amplifiers.

[quintviskup]

Thank you prr.
Pretty much confirmed exactly what me and James were discussing.

[sluckey]

Just a caution... a 6L6 requires .9A of filament current. A KT88 requires 1.6A filament current. If you replace those six 6L6s with six KT88s (they may not physically fit) the power transformer will need to supply an additional 4.2A of filament current. Can the PT do that? I don't know.

Also... If you decide to only use two or four KT88s, they will be expecting a higher speaker load through the OT. Since the OT only has a single 4Ω output tap, you'll need to adjust the speaker impedance at the speaker cab.

[PRR]

Six KT88 sure is hard work for the PT.

Four KT88 can do the work of six 6L6, since a 6550/KT88 is roughly one-and-a-half 6L6.

Impedance relations will be fine. Use the marked numbers.

And the added heater power, 6 Watts, is "small" in the overall scheme of a 300+VA PT.