Sunn Model-T/200s Hybrid

[BetterOffShred]

Hey guys, while I'm waiting for some funds to finish my current project, I figured I'd start planning out something for later.   I really love the Model-T by Sunn, and I think it has a great tone and growl like none other, but I want something a little more manageable.   So I thought about finding a Sunn power amp that used 2x6550's and stuffing that behind the Model T preamp/PI.    So I found the Sorado/200s/Sonic II/1-40 schematic and basically copy and pasted some stuff..

So I have almost no experience in amp design (obviously) but I do know a little bit about how they work from building and modding.  I wanted SS rectification so I copied a common SS style, and put in the Merlin style standby switching.   Another concern on this part is that the Transformer Spec'd is the Dynaco original, but if I'm going SS rectification I probably could use a simpler transformer.  The specs on the Dynaco type are : Rated 400-0-400V (800VCT) at 200 ma (actually, will deliver up to 300 ma continuous, albeit at a slightly lower voltage) , 5V at 3A, 6.3VCT at 5A, also has 60V bias tap. Primary 117VAC 60 Hz.
  Thoughts welcome!

The Sorado schematic has a strange NFB loop (strange when all you look at is fenders I guess) so I copied the Model T for that part, with the 16 ohm tap heading to ground through a 22k and a 10k similar to a bassman.   

I also deleted the "Out" jacks as the original Model T has 5 jacks.. I think normal brite and both is sufficient. 

I wanted cathode bias, so I copied a Deluxe style cathode bias, but I'm not sure how to calculate those values yet.   More to learn there, but I think its wired correctly for what it is.

The B+ power taps are within 20V between the T and the Sorado schematics, so I figured go with the Sorado values and maybe end up changing the voltage dropping resistors a bit to get the PI the right voltage.

So anyway, Take a look, I'd love to hear where I've gone grievously wrong in my schematic, as I'm sure there are multiple problems!   But I'd love to discuss it with anyone who's interested.    Thanks for looking!

-Brett

[sluckey]

Looks workable to me.

[BetterOffShred]

   Well that's good news!  Thanks for looking!

One question I have..  On the Sorado, the 6550's screens and plates are hooked right up to the OT, but on the Model T there is 5W resistors between all of these and the transformer, do these exist because of your A node B+ voltage and bringing the plate voltage to desired spec, and they just didn't need to lower it at all on the Sorado based off transformer spec and A node voltage?     I need to read the power amp sections on Merlin's site again!

-Brett

[terminalgs]

One question I have..  On the Sorado, the 6550's screens and plates are hooked right up to the OT, but on the Model T there is 5W resistors between all of these and the transformer, do these exist because of your A node B+ voltage and bringing the plate voltage to desired spec, and they just didn't need to lower it at all on the Sorado based off transformer spec and A node voltage?     I need to read the power amp sections on Merlin's site again!

Those are ultra-linear transformers. On a 2x6550 amp, connect them directly.  I think the 50ohm R's on the plates and the 1.5Kohms on the screens on a Model-T are to buffer the two parallel tubes from one another (but you don't need them on a 2x6550 amp).

[BetterOffShred]

Ah I see,  Well that's another thing for me to read up on.  Thanks!
-Brett

[sluckey]

Sunn used those resistors on all their quad 6550 amps. They did not use them on any of the 2x6550 amps. I think it has something to do with oscillation but not sure. Other big amps used them too.

I would stay with the Dynaco specs for all iron. You can get the whole bundle for under $300 from Triode.

Why cathode bias? Is it just for simplicity? Be aware that idle current will be much higher than in a fixed bias design. I hope that expensive iron is up to the task. I would stay with fixed bias. I have a Sceptre and the OT is bigger than the PT.

You may consider using separate cathode R/C on the preamp tube so you can get more difference between the bright and normal channel. As is, the only difference between the channels is a 470pF cap on one of the mixing resistors. Just something to think about.

[BetterOffShred]

Yes Sluckey, I was looking those channels over and saw the only difference was that little 470pf, haha!  Honestly I'll probably run it in the "Both" mode for the extra gain anyway. So you're saying to maybe split out the RC bias on pins 3 and 8 of those stages to "color" one channel a little more?  That could be fun to play with. 

As for Cathode bias, I just like it better.  It compresses and sounds neat.  But that being said, I was thinking about maybe making it switchable.   I think some of the "Magic" of these amps is the fixed bias, but I certainly can't afford to burn up that Dynaco iron.   Yeah the PT and OT and choke all from Triode, though I'd rather buy from Doug 

Realistically what effect other than slightly higher voltage (I think) will running the SS rect have on this amp?  If I stay with fixed bias I can just use the Sorado schematic and ignore the yellows right? The fixed bias portion is super easy to add back into the schematic.  I'll do that and re-upload it.
Thanks for looking!
-Brett

[BetterOffShred]

Here's a slightly updated version of the schematic with the fixed bias added back in, and the first 12AX7 cathodes split out.   
-Brett

[sluckey]

Realistically what effect other than slightly higher voltage (I think) will running the SS rect have on this amp?  If I stay with fixed bias I can just use the Sorado schematic and ignore the yellows right?

The Sunn 1200 always used SS and just ignored the yellow wires.

[PRR]

A proper cathode bias for 6550 at 480V wants about a 10K or 11K output transformer. Which is not any stock item at this power level (around 40W). You want to be nearer 400V and 6K loading for cathode bias.

The fixed-bias plan is thoroughly vetted by Dyna and Sunn, sure to work, 50W-100W output. This is what these amps did originally, and why they are famous. Leave the cathode bias for "little" amps.

[BetterOffShred]

Alright, that's sound advice.   I appreciate you fellas looking at it! 
-Brett

[PRR]

Link to schematics so we all on same page.
http://el34world.com/charts/Schematics/files/sunn/sunn_sorado.pdf
http://el34world.com/charts/Schematics/files/sunn/sunn_model_t.pdf

> Sorado schematic has a strange NFB loop

Looks utterly conventional to me. It is in fact same-as Fender's AA Champ. The Sorado has more parts but the NFB is the same except one different R and two added Cs for supersonic stability. These values are probably taken from Dyna(?) and somewhat specific to the OT used.

The T's NFB (the whole driver section) is "strange", and clearly adapted from Fender's big late amps with enough value-changes to avoid direct stealing. That's another well-loved plan.

http://el34world.com/charts/Schematics/files/fender/Fender_champ_aa764_schem.pdf

[BetterOffShred]

Fair enough!   I'm still a novice at most of this stuff.   And yes, the T's NFB looks like a bassman to me.  I may put a pot in instead of that 22k, I did that on my bassman and I really like it.   

I think this amp will probably sound pretty good, if you like Model T's. 

-Brett

[BetterOffShred]

Ok so one final change (for now)  I removed the line out jack and associated resistors and went to a straight ahead bassman style NFB loop, and added a A25k pot (maybe too much?) I think I put a 10k in my Bassman, and it gets pretty loose when I turn the pot all the way up, which I like.   Will this be ok as it is drawn in the schematic?

I had also missed the diode after the standby switch, and I changed the Bias supply diode to a 1N4007 since I have them and the 1N2070 is only 400v, 750ma.   And I added one more rectifier diode to each leg of the power supply, because.. Well That's what Leo used.

-Brett

[sluckey]

I had also missed the diode after the standby switch

Where did you see that? Except on your latest drawing?

And I added one more rectifier diode to each leg of the power supply, because.. Well That's what Leo used.

I promise you, Leo would not have wasted money on six diodes if he had 1000V/1A cheap diodes available back then.

[BetterOffShred]

The diode is from Merlin's site, on standby switches.   At the bottom of the page.   

And yes you're probably right!  Cheap!  I like to use quality components as much as I can I suppose!

-Brett

[sluckey]

Quality 1N4007s are cheap. Last I bought were 4 pennies each. But they weren't around back in the '60s. Even the 400V diodes like Leo used were expensive.

[ernie_jr]

if you are using this for bass, you may wnat to change the caps feeding the grids on the 6550's from 0.25 to 0.1. Sunn did this in the builds but did not update the schematics. I have had 5 diff ones and they all had 0.1mfd caps there

[sluckey]

There's a nicely done 2xKT88 Model T clone on ebay with some good pics. May give you some ideas about layout. Looks like some Hammond transformers but I can't tell which ones. Seller would probably tell you if you ask. Anyhow, could be worth a look and steal the pics for future reference.

http://www.ebay.com/itm/Hand-Built-Sunn-Model-T-80-100W-KT88-clone-guitar-Tube-Amplifier-/142210135250?hash=item211c6274d2:g:vPwAAOSwv9hW3OKW

[BetterOffShred]

Yeah I saw that KT88 2 power tube clone, I have a feeling he probably did what my schematic looks like.   

Thanks for the tip Ernie, I play guitar mostly, but I have a couple 7 strings and down tune a lot, so I figured I'd want it to be basically the bass specs.   That's the sound I like out of these anyway!

And Yes Sluckey, we are in a time where the components are cheap and higher quality than ever before, so It's definitely worth getting into this stuff if you enjoy it!

-Brett

[BetterOffShred]

Hey guys, just another little updated schematic with correct parts numbering, and then I drew up a crappy layout for the main board and a little bias board which I uploaded as well.   

525V and 600V caps are hard to come by, but I found MIEC branded caps in 600V rating on Ebay fairly handily, so I'll probably just use all 600V in the power filtering section.   

-Brett