Converted My SE Express into PP w/ 6v6's

[strelok]

So I finally got some time to revisit my SE express and try to figure out where that maddening oscillation was coming from. The only think I could think of was the board layout was causing me problems. So I decided I would try and rearrange it to make it more like original to see if that helped. If it did then I would continue with my plan to convert it to PP with 6v6s as in its current configuration it was just not doing it for me. I think the lack of the LTP was a major contributing factor. Anyways last week I ordered parts and got started on Saturday. Turns out the eyelets I ordered were too small and would be problematic to try and get some of the spots with 3-4 connections in one hole to fit. So I said screw it and built the thing up with terminal strips that we had on hand. The board had been getting on my nerves for a while now with this amp, I like boards just fine on designs were everything's been hammered out but this is a one off kinda prototype-y thing and having to pull the board every time I wanted to get to a jumper or something like that was getting old. In any case here's a few pics documenting what I did:


Amp as it was at the start of the weekend.

Gutted, additional octal socket hole cut out and installed, filaments wired and starting on pre-amp supply.

Terminal Strips installed.

Populated

PS wiring

Pre-Amp Wiring.

Completed Build.

Left some extra slack on the OT secondaries in case I had to reverse them. Got lucky and didn't need to.


Quite pleased with how in came out. I kinda winged it on the layout, just trying to keep to the order components are placed for the pre-amp in Ken's original design. Had to move the cathode circuit of the first gain stage up near the tube socket as I ran out of room. Got it on the test bench, came up on the variac without a hitch, plate voltage is around 375, biased it at 28ma or right around 75%. Plugged it in and it sounds sooooooo much better than it did before. I'm quite happy with it. It still has a bit of that parasitic oscillation sitting on top of certain notes however it sounds so much better than before it was hard for me to care. I realized later that I forgot the cap across the PI plates (and also the bright cap) so I'm hoping that will help. I was quite surprised to see it be completely stable, even with no bottom cover. I thought for sure the first time I fired it up it would be a squealing mess given how mushed together everything is. It does squeal with the guitar volume all the way up and gain cranked but I think that's more due to micro-phonics with regards to the pickups in the cheap shop guitar, I doubt they are potted. Still I'm going to go to the hardware store this week and pick up some sheet metal and make one, that might help with the fizzy oscillations as well. Nonetheless I'm loving the way it sounds, its DAMN loud, I had to run it variac'd down to about 70VAC as it was getting late and I didn't want the neighbors getting pissed. Sure gets nice and chewey and brown down at those voltages, would love to add a VVR but not sure where I'd put the damn thing The only reason I ever built it as a SE amp was I didn't think I could squeeze a PP and associated circuitry in there, glad I was able to prove myself wrong. Will get some sound clips up soon.

[joeboo88]

that looks great, have you seen RJ's post, where he made an attenuator on top of the chassis.... you could always put stuff on top of the chassis like he did
I think it looks cool

https://tubeamparchive.com/viewtopic.php?f=1&t=29919

[strelok]

Thanks! Yeah I did see that, but now that you mention it, it gave me the idea that I might be able to put the VVR board on top of the chassis somewhere and cover it in a housing bent out of a piece of sheet metal. I really like the way the amp sounds at lower voltages, even if its just 10 volts lower on primary side, helps tame the highs a bit.

[shipwreck]

Neat wiring! Did you twist the wires by hand or with a drill chuck? What type and gauge # of wire did you use?

I am wondering, for those that use the drill chuck method, how do you deal with the filament wiring, where one of the wires are longer than the other (visible in the pictures on this page).

[strelok]

The company I work for actually stocks spools of pre-twisted wire, so I just use left over clippings for my own builds. I think the green/white is 20 AWG and the red/blue is 18. I have used the drill method in the past and it works very well but I don't bother these days since I have access to the pre-twisted stuff. With regards to your second question what I did on this build was go from the left most tube socket to the right, I cut the twisted pair to a bit longer than what I need for the longest connection, untwist it and straiten it out as best I can then just cut each one to the length needed. It takes a lot of fiddling and messing about with it and is very time consuming but its worth it when they look nice in the end. Its just a matter of being patient with it and I always think my heaters could look nicer but what I ended up with is the result of how much time and patience I had this past weekend so I'm cool with it haha.

[strelok]

Been spending the last few days tweaking here and there. Trying to tame the highs a bit. First added a 250pf cap across the plates of the PI. Seemed to help a bit. Definitely made a difference in that fizzy oscillation sitting on top of the notes that I've been chasing for a while now. Also changed the PI coupling cap from .022 to .1. That seemed to make the biggest difference. Added some bass and filled the sound out a bit, I figured if I added to the bass there would be less power available at the output for highs to get through and that indeed seemed to make a difference. Also seemed to make the tone stack much more usable, before it didn't really seem to much of anything. I ended up changing the PI plate-to-plate cap to 500pf as I still felt that it could use a bit more roll off on the highs and that seemed to help much to my surprise, I thought going to a value that large would make the amp too dull but It seemed to be just right in this case, fizzy oscillation seems to be totally gone too. Also considering upping the output coupling caps to .1's from .022's but I don't have any more at the moment. Will probably try playing around with longer cables next, the one I've been using is pretty short. Also I'm using a new speaker (Weber 1225) even though I had it broken in I suspect it will probably mellow out more as time goes on.

I measured the output on the scope and was getting 14 watts full clip. I had to do a double take and make sure I had calculated correctly because it just seems so much louder than that. I would have expected more out of a pair of 6v6s initially but I remember the OT I used is really only good for about 15 watts so that makes sense. I misread the box and what I thought was 6600 ohm connection was actually 5600, changed it to 6800, didn't really have any noticeable effect power or otherwise.

[M Fowler]

Nicely done and I dig the Edcor PT.

[strelok]

Thanks! Its way overkill for this application, think the B+ is good for some 250ma, its left over from when this amp was single ended w/ a KT120. I could have done a full sized express with EL34s and a bigger OT but I didn't want to spend the money given that 14W is already way louder than I need. Thing kills my ears even just playing through a 1x12 but it sure sounds nice.