SUNN OT compatability question

[nateflanigan]

Hi, I'm thinking about building a Sunn 100s bass amp.  A friend has one and it sounds awesome with guitar.  The schematic is pretty straight forward but I'm a little confused by the primary side of the OT.  There's a few more wires than I'm used to, what's the story with the wires going to Pin 4 on the powers tubes?  I have a bassman OT I was hoping to reuse, can this work?

Thanks

[sluckey]

There's a few more wires than I'm used to, what's the story with the wires going to Pin 4 on the powers tubes? 

It's called ultra linear. You can use your bassman OT and wire the screens like a typical amp. You may want to modify the power supply nodes. Notice that the choke is upstream from the plates too? You need a high current choke if you copy the power supply exactly. This is all part of the Sunn clean/loud sound.

You may be interested in this amp. It's the same circuit except the tonestack is voiced for guitar rather than bass...

                  http://www.el34world.com/charts/Schematics/files/sunn/sunn_spectrum_ii.pdf

[Ritchie200]

If the bassman OT does not have UL tap (if aftermarket it may?) you can use it as Sluckey said.  However, as Sluckey said, UL is all part of the 100s sound.  You may be disappointed if you want to copy what you have heard with the original.  UL will make a very significant change in the sound.  But then again, you may really like it without!

Jim

[nateflanigan]

Thank you for clearing that up guys.  The hammond UL trannies are pretty expensive, it looks like edcor has some good options.  I'll probably start with the bassman OT, if nothing else it'll be a good learning experience to hear the difference.  What I like about the sunn is the big clean sound.  It is however incredibly loud and I would likely never turn it up past 2 or 3, maybe with the bassman ot it'll behave more like a normal guitar amp and maybe I'll like that better.  Either way I'll let you know how it goes.

 

[nateflanigan]

I fired up my Sunn build last night, there's a lot of weirdness to sort out.  One thing I really don't understand on the schematic is the "E" node.  Coming off the cathode of the 6an8 (pin 9) between the 680r and 47r resistors there's "point E" which clearly connects to one of the ultra linear tabs on the OT and pin 4 of a power tube.  I just omitted this since I'm using the bassman OT.

I have everything wired up a'la the Sunn schematic up to the grids of the PT's, then pin's 4 of the power tubes each have the 470r resistors which go to the node just off the choke.

What I'm wondering is if I should have that point E connected to pin 4 of the power tube.

Thanks

[FYL]

E is the negative feedback connection. It's taken from the screens (with a 390p cap blocking DC) as well as the 16-ohm tap but you may connect it only to the 16-ohm tap. Check the Dyna MKIII schemo - the Sunn was based on it.



From the Triode Electronics web site.

[sluckey]

What I'm wondering is if I should have that point E connected to pin 4 of the power tube.

No. Point E is the negative feedback loop. Sunn just copied the old Dynaco power amp. They used a standard feedback from the output transformer secondary, but also used a high freq feedback from one of the screens via that 390pF cap. So, just omit the 390pF as you've done already.

[nateflanigan]

Thanks so much guys.  I'll sit down with it once I've finished my coffee and start the usual trouble shooting, I'm sure I'll have more questions.

[FYL]

Sunn just copied the old Dynaco power amp.

They had some kind of OEM deal and used Dyna-sourced iron as well as some boards.

[nateflanigan]

Got it working, had a wrong resistor, all is well now.  It's very very loud and sounds huge even with out the UL OT.  As mentioned before the tone stack is designed for bass so it is a little weird with guitar.  Not that I couldn't dial in some good tones it's just not ideal.  First I think I'll take it to band practice and have our bass player give it a go. 

Two questions...
1) I'd like a little more travel on the volume pot the usable range is roughly between 1 and 3.  If I swapped out the 500k for a 1m would that do the trick?
2) Once I get the tone stack sorted I'd like to add the 1 tube reverb circuit any thoughts on wether that would sit better just before the 6an8 or between it's two halves.

thanks

[FYL]

I'd like a little more travel on the volume pot the usable range is roughly between 1 and 3.  If I swapped out the 500k for a 1m would that do the trick?

Try a 5751 or 12AY7 for V1.

 

[nateflanigan]

Cool I have some 12ay7's around.

[nateflanigan]

That really did the trick, the amp is sounding super nice.  One question though, I have the negative feedback set up "normal" i.e. running off the + terminal of the out put jack.  Is there any reason to use the 16 ohm tap instead?  I suppose I could just try it and see for myself.

I have parts ordered for the spectrum tone stack as well as one tube reverb, once I get that business sorted I'll post some pics.

Thanks again for the help

[nateflanigan]

You may be interested in this amp. It's the same circuit except the tonestack is voiced for guitar rather than bass...

                  http://www.el34world.com/charts/Schematics/files/sunn/sunn_spectrum_ii.pdf

I rewired the tone stack to spectrum specs today, big improvement on an amp I was already digging.  Thanks for posting that sluckey!  I haven't finished the 1 tube reverb circuit yet but I should be able to wrap that up tomorrow, I'm sure I'll have some questions about that.

[DummyLoad]

It's very very loud and sounds huge even with out the UL OT

you're running in tetrode mode - it delivers more power to load than UL with the same B+.

[nateflanigan]

I need to learn more about ultra linear, I understand the wiring now but have no idea what's happening electronically.

[nateflanigan]

Ok, here's a link that I think it explains UL very clearly...

http://www.ampbooks.com/home/amp-technology/ultralinear/

[nateflanigan]

Got the reverb working today, I'm really happy with the sound.  It's different than the usual spring reverb sound, kind of more like a plate reverb, to my ears it has some of that pillowy quality.  Also I switched over to 6L6's, the tonal differences between the JJ kt88's and the sovtek 6l6wxt's I have are pretty minor, but the PT was getting pretty hot with the KT88's.  I gotta bundle up the wiring and generally clean things up but other than that the amp's done.  Oh, and a faceplate, I'm gonna need one of those.

[Ritchie200]

JJ KT88's are basically a 6550.  The only manufacturer making a true to design KT88 (that I have found that doesn't cost $$$$$) is the =C=.  They will sound different (much better to my ears...) compared to the Sovtek, JJ, and EH.  Plus they last infinitely longer.  You said the PT was getting pretty "and" with the KT88's?  Did you mean "hot"!   Isn't that what originally came in this thing?  It should be fine.  So, do you have a use for this extra LOUD amp?!

Jim

[nateflanigan]

"hot" "and" what's the difference?  

I have a pair of =C= 6L6's that I've tried in a few amps, not really my thing, I really prefer the sovteks.  Maybe I'll check out the kt88's though.

The PT is a mojo bassman, I did the math on the heater current and it should be fine, but whatever, I like the 6l6's better anyhow.

Aside from building it just to build it, I wanted a louder amp to use on stage.  I use a deluxe reverb in band practice and it's plenty loud, but I can never hear myself when my band plays shows. 

[Ritchie200]

Aside from building it just to build it, I wanted a louder amp to use on stage.  I use a deluxe reverb in band practice and it's plenty loud, but I can never hear myself when my band plays shows. 

Hah!  Just give the speaker cabinet a nudge to face the drummer if he gets out of hand.  It's great to have the big fist on stage!  Congrats on a nice build!

Jim