Is img turned off?

[RB]

I just took some voltage measurements on the amp. A couple things that caught my attention were;
1)The DC voltage on the grid of the last triode. This is because the grid load resistor on this stage is connected to the cathode of the previous stage.
2)Neither stage have bypassed cathodes.

I am not an engineer so all of this could be way off but it appears that we are dc biasing the last stage. That bias voltage also has an out of phase single floating on top of it giving us some NFB. I can’t explain it precisely but I believe that the dc bias, the NFB and the lack of bypass caps allow the last stage to track and interact with signal conditions/levels (under a certain range of conditions) and smooth things out something similar to a compressor. And I say this based on the fact that this is how the amp sounds and feels when you play it. When set up for medium-high gain it acts like it has a compressor sustainer type pedal in front of it.

At lower gain levels it gives you a great clean tone typical of a well designed SE amp . If it is fully cranked it also gives up the goods but there is a range of gain settings just above medium gain and just under fully cranked that this amp just delivers singing sustain even with the master turned down.

I have built a number of SE amps with multiple gains stages and could never achieve this middle ground of sweet singing sustain that this amp produces. Over the years I have parted-out all my SE projects (except one) because they never really satisfied me like this little amp. The one that survived over time is a 5F2 style amp with big output iron, higher current rated power transformer and a KT66. It’s a kick ass champ on steroids but it is basically a one trick pony and conversation piece.

David Allen came up with a great design with this SE amp. I am having so much fun with it I am thinking about rebuilding it with reverb.


RB

[bcmatt]

Are you going to rebuild it with reverb?

I just popped a 12au7 (from an old Leslie amp) into the first preamp tube slot tonight before practice and used this amp.
I wanted mostly cleans, and this gave me more clean throughout the span of the volume knob but it also seemed to smooth things for me as well. As it went into overdrive, it seemed smoother and creamier as well (maybe like a tweed deluxe). Ya, I was really loving the semi-clean tones on this tonight with my strat. I'll leave in the 12au7 for a while and this amp will get a bit more frequent use in the near future.

[RB]

Hello Matt

I am trying to decide how to add reverb to this amp. I really don’t have any space on the front for the reverb control even if I sandwiched another board and tube into the chassis. I am thinking about a small outboard chassis like you would see on tweed style amp reverb retrofits.

I have also played around with the preamp tubes. I am currently using a pair of NOS GE 5751 (about 70% gain of a 12AX7) and a KT66. This is producing a really thick creamy sustain perfect for the type of blues I like to play. I had good results with a pair of 12AY7s for thick clean tone also. All in all this is a very versatile little amp clean to mean. I also have used Mullard 12AX7s with a Siemans EL34 with great results for Marshall style tones.

RB

[bcmatt]

I could see now how reverb would be a really great addition to this amp. I haven't yet developed the guts to ever try adding reverb to any build. Now that I see that it's not so hard to find reverb transformers, I plan to some day. I'll want to hear lots of details on how it goes if you do it. I agree that space would be an issue. I would think that an outboard separate reverb unit may be more useful and practical, like what fender makes for around $500. I guess this would require an effects loop too. Do you have quite a bit of experience adding reverb to amps before?

So, would you say that you are using the KT66 most often?
My default has just been an EH 6V6GT. I briefly tried a JJ EL34 and an old GE 6L6 (from the old Leslie amp as well). I will have to try them again with these newer cleaner sounds I am trying this amp for now. I'm seeing this amp through different eyes now. Before, it was just to get grungy plexi sounds alone in my bedroom. Now, I'm using it for the clean but thick bluesy tones like you mention, and finding myself using it in actual band situations more than before.

[azatplayer]

Well do a search and you find some gold.
Ive been looking for a worthy SE EL34 build for ages. came across this read.
Maybe some more gain than i like, but im sure it would be tweakable.
Really been looking for this rocket preamp in a SE big bottle design, and following mcracers progress. I have ordered a deluxe PT and have a 10 watt SE 5-7K OT on hand. All the bits in my drawers for the rest of it.
Was this or the derailment, whcih is kinda cool too.
Anyone got some gold to share in a EL34 SE layout?

[Structo]

Are you going to rebuild it with reverb?

I just popped a 12au7 (from an old Leslie amp) into the first preamp tube slot tonight before practice and used this amp.
I wanted mostly cleans, and this gave me more clean throughout the span of the volume knob but it also seemed to smooth things for me as well. As it went into overdrive, it seemed smoother and creamier as well (maybe like a tweed deluxe). Ya, I was really loving the semi-clean tones on this tonight with my strat. I'll leave in the 12au7 for a while and this amp will get a bit more frequent use in the near future.

I'm not surprised it cleaned it up.

The 12AU7 has a gain factor of only 17-20.
Compared to a 12AX7 with 100.
The only problem with the AU7 is that it draws a bunch of current so it may tax your PT if it is soft.
Try a 12AT7 if you have one around.

[azatplayer]

Well i built it.
This amp screams! Man its got gain and some more gain. But cleans up pretty well.
Im just giving it some thru a 6V6 AND 5Y3, 5751 IN V1 and old ge in V2.
VERY sweet tones with my tele.
Gotta work out the voltages tho. Im using a deluxe PT and getting about 370v at te plate on a EL34. around 26V on the cathode using a 410 R resistor. Im using a 50uf bypass cap which i think i will change out to a 25uf, has a little loose bottom and loads of it. The schem uses a 25uf, but i figured the EL34 could do with some more.
6L6 is very sweet too.
Back to playing

[Structo]

I believe I read somewhere that 25uf bypass will pass all the frequencies a guitar can generate.
Over at the 18 watt forum guys are using 500uf on up and they say that most of them like it better than the stock 100uf.

[azatplayer]

well the class act was fun for about five minutes. So continued the never ending saga of my lookingh for a worthy SE big bottle amp.

Then, i decided to give a champ a go, i had tried a tweed some time ago, as i had an extra pre hole in this build i went with a blackface vibrachamp.

Had the deluxe PT so ive ended up with, 5U4G rec into a KT66 at 400V on the plates, feeding the standard schem with a few little changes in the front end. I used a .68uf cap first cathode to try and get rid of the loose bottom, worked but made it a little bright, reduced the 100k slope to 50k, helped, then dropped a .001cap across the cathode and first plate on V1, perfect!

Took it down my mates guitar store this morning after the last mods, put it into an orange quad, and yeah baby! This is the tone ive been looking for.
Very much like my rocket, big and clean with that same jangly breakup, and LOUD!

The tube trem works very sweetly, using a 12AY7, felt it softened it some.
The trem is fed off the screen voltage on the KT which is a fait bit higher than a standard VChamp, the AY7 seemed to be the right tone, still not fully sure how the trem circuit works, that is the relationship of plates to cathodes, only just built it.

So, my task is now to get it as compact as i can and fit it into a head shell.

Love it!