Eagle Supre 6sn7 V1 cascoded w/MOSFET CF

[surfsup]

Well here she is. Definitely a clean amp. She is loud at 400V on B+, a little lighter at 325 on lower secondaries. Ignore the tube recto socket that has been removed. The amp doesn't really break up much at all maxxed out. The cleans sound awesome though, I have to admit I really like it though I wanted it to break up a little. So I still have the 1k5 on V1 Rk, changing that should help I'm thinking. Just as a recap, to get a little touch sensitivity, I can:

Lower the Rk (or raise it I guess)
Lower the voltage at the plate to give it less headroom

Anything else? I thought the second CF with a 56k Rk would compress it some.

[img1048]http://chicagocadcam.com/ChrisHahn/supre/6sn7_cc_1.jpg[/img]

[Structo]

I don't mean to be overly critical, but your layout on the board is very congested.
You will be getting a lot of cross talk and interference like that.

What are the green wires going from the board to the power tube?

So this is the amp that is frying a resistor?

[surfsup]

Not frying a resistor anymore. Yea she's not too pretty, needs to be cleaned up a bit. The extra cap ground lead is long, was testing where to ground it. The two green leads are the powertube cathode and nfb lead and i figured that would be obvious enough to draw commentary. I received the board with the turrets already mounted so i am working within a fixed envelope.

I've modded the amp quite a bit too, and actually think i'm making quite good use of the original layout and space.

[RJ Guitars]

That is a very unique build, did you create a schematic?

Do I have this sorted out correctly that it is a 6SN7 output tube and the amp doesn't break up? That's something you're not not gonna see every day.

Thanks for posting, lots to be learned from this idea.

rj

[soma_hero]

I think it's a 6SN7 in cascode, with a mosfet source follower and then a 12ax7 gain stage with another cathode follower going into an EL34

[surfsup]

Here is the schematic as it currently stands. The changes I want to make in an effort to get it to break up earlier:

1- move the MV to after the CF instead of the tonestack to try and get the second tube's GS to overdrive
2- add a gridstop to the power tube
3- see what a 56/82/100k will do on the cascode plate
4- try and decrease the Rk on the cascode

[surfsup]

Okay I moved the MV to between the 12ax7 and EL34. Much better. Now its more a guitar amp. The amp has a slight metallic quality to it. Very hard to describe. I literally breathe on the strings and they start going making eerie metal sounds. Amp sounds good. I have the bypass cap on the 12ax7 GS on a switch to cut it out. Its somewhat clean, then hitting the switch I get more drive for sure. Before there wasn't much of a difference at all with that. Just a little louder.

I do notice now there isn't much control with the bass pot, it could be tweaked for more response. Anyway, updated the schemo, though the PT grid stop is not actually in the amp (yet) and no grid leak for V2 GS - I figured the tone stack would be the grid leak.

[ChrisM]

If the TS is after the CF your going to want to scale the stack for the different input impedance caused by the CF. Look at a Marshall TS.

[RJ Guitars]

ok now I get the 6SN7 function. I dunno why I thought you were using it for an output tube... maybe that was another conversation we had.

You are really setup to try a 6SL7 for that first stage if you want more drive. I have several old stock tubes here but just today I got notice of a package at the post office that I believe is a dozen Russian 6SL7ish tubes. You gotta love a tube that costs $2 and represents the transition of cold war era military technology into sonic satisfaction.

[surfsup]

I got to play this about two hours so far today fairly cranked with just one retired couple in the building right now. I am digging the amp at this point. Very touch sensitive. Holding the pick lightly and strumming gets me nice clean chords. Firm grip and jamming the strings gets me a nicely distorted chord with pleasing tone. Same with single strings or double stops, etc. The MV move to before the PA seems like it was what it needed so the signal is preserved as much as possible through the TS into the 12ax7 GS.

Modding today, I paralleled a 1k2 with the 1k5 on the CC input stage Rk. I think that's a bit too dirty as even single strings have a little distortion. I am trying to decide whether to leave it 1k5 or parallel a 2k7 with it (removing the 1k5 is gonna prob be too much trouble) to get me in the 1k0 ballpark. They both sound good, 1k5 being really clean with the 12ax7 cap switched out, but just not quite enough dirt I think if I bypass the R. Maybe I will try and adjust the gain on that tube a touch with a different Rk on the GS or put a 47k on the CF Rk instead of the 56k. Still no grid stop on the EL34.

Putting a Strymon Brig in front of it sounds really good. Better than the x10 clone even for clean delays. So overall I'm pretty happy my first attempt to cascode a tube turned out so well and I haven't even had to change one component yet...probably just beginner's luck.

[surfsup]

You are really setup to try a 6SL7 for that first stage if you want more drive.

Maybe on a different amp RJ. I am liking this one right now (except these damned turrets... ). Maybe I'll get one of those from you when I order some stuff.

[tubeswell]

Hi there surfsup, you might also get more drive if you ditch the 1M master and 220k grid leak for the EL34 and put a 250k pot there. (With the 1M and 220k in parallel, you have 180k with the pot dimed). 2CW

[surfsup]

Tubeswell thx for the tip! Im actually going to move that pot over a hole so when i do that i will try removing that and paralleling some different Rs across the 1m to see what that does. I am expecting a scope to show up soon as well so it'll be fun with that new toy moving forward.

[surfsup]

A few updates including schematic...

I cleaned up the wiring as best I could, looks a lot cleaner now. Put in a Sylvania NOS 6SN7. Way way better.

I put a 330k on the 1M MV pot's outer lugs for ~250k total. I also added a 1k grid stopper to the EL34, but I think that is going to come back out because it seemed to lose a little bit of dynamics.

Unfortunately that burned B+ rail resistor scorched my board. I didn't try it yet but I'm hoping some rubbing alcohol with a q-tip will clean some of that up a bit. If anyone has any tips I would appreciate that.

I didn't move the MV pot either, because I might add a presence control to the amp between the treble and grid of the next tube. From what I understand I can do a 22k/3n/25k series to ground here. So I wanted to leave the hole where it was because its in perfect position to do this and the MV has to run over the board to get to the EL34 anyway, so in either spot it wouldn't matter. I made the runs shielded.

The other thing I might do is put the 56k Rk on the tube CF on a switch to go from maybe 100k to either 56k or a value a bit higher. Seems it might be affecting the cleans just a touch. At guitar volume on 7 it is still somewhat clean but not quite. I am thinking the added compression at this stage is preventing it from getting cleaner. Turn it up to 10, she's got some gain for sure.

The NFB NA/5k/10k switch is subtle, I was going to just take it out but decided to leave it in as it isn't making things noisier or anything. If I was doing this amp again from scratch, I wouldn't bother with it.

The GS before this CF has its Ck on a switch to switch it out or in. Very useful. Kind of a HB/SC switch.

All in all there is no hum audible until its at about 8 on the MV. At 10, she's still very quiet. Sounds very nice.

[img877]http://chicagocadcam.com/ChrisHahn/supr ... ual_v3.jpg[/img]

[RJ Guitars]

Surfsup,

Nice personalization documentation on that build. Great to see the voltages on the schematic... something I have not done as well but should be.

I am still waiting on someone to come through with evidence of an audible difference between the two voltage taps. Your circuit is clearly a bit different that the straight forward 5E1 Champ... can you hear anything due to the B+ differential?

rj