New build, Express a bit harsh

[Smokebreak]

I finished up an Express build with parts I had on hand, which was an organ chassis and trannies. I used the schematic in the "express documents" section. I'm an inexperienced builder but really wanted to put one together, as I've read so much about them lately.

The amp sounds really good, really touch-sensitive, but it gets a little harsh up top as I dig in.

I'm thinking that the harshness is a result of my PT putting around 440VDC on the plates of 6V6. Does anyone have experience with this? If so, I'll do the zener trick, but am looking for some guidance as to where my plates should be, as I've seen a lot of numbers around. 400V? 375V?
Thanks- J

[Reeltarded]

Seems like 370-395v is a good place and 440 does get more than a little nasty. What tubes are those? JJs 6v6s is supposed to be 'good' to over 450v but I don't think that is where they sound good probably from hearing a bunch of amps on the interwebs.

[Smokebreak]

Ok thanks! That's what I was hoping. I just cranked it for the first time and it indeed gets quite harsh. I have JJs in now, but I started with my cheap "tester tubes" yesterday to get it going. They are Shugs that seem to be able to take beating!
For the Zener trick, I just want to make sure I'm doing this right... Say 3 or 4 14V 5W zeners between PT CT and ground?

[Reeltarded]

Hmm wait for a brain to show up and give a more informed answer. I have never used a zener dump but I think dropping around 10% is a good place to shoot for, if not just a hair more. In my head I see you ditching maybe 4 something watts at the output.

Ok, I better go before I start getting all pseudomathish.

Me? I'm a theoretical arithmetician.

[vibratoking]

I would want to drop that B+ as well. Here is a nice paper on it conveniently posted, as always, by Aiken.

http://www.aikenamps.com/BackBiasing.html

Should provide you with all the insight and math that you need.

[selloutrr]

Deleted

[Smokebreak]

Thanks for that. Very good read. I've read it a few times, but i still don't understand *how* B+ is dropped, in theory.

I see that there is negative voltage developed across the resistor( or zener?) on the PT CT side, but then how does that translate into positive voltage at ground side....which is what I assume I'm doing with the zener "trick" - since voltage is measure in reference to ground, if the ground becomes more positive, it gives the illusion(for lack of a better term) that B+ is lower. Am I on the right track here?

Also, as per the paper, (even though this is a bit off topic for my concerns, I believe)why would one need only -11V or so for a bias supply(I'm assuming to grids). That seems really low to be useful unless you're running like 700V on the plates. Obviously I have something wrong here...

[Smokebreak]

After some more reading, I was way off base. Ground stays at 0 potentential. The neg voltage that occurs at the CT carries over to the B+. I don't know why I made that so difficult...Anyways I dropped 30V w/ three 15V 5W zeners. I'm down to 410V on the plates, and the difference in tone is pretty amazing. Now I see the reverence for these amps. I'll drop another 10 or 15V when I can heat sink it properly...or just get another PT...or just build 5 more variations.

[Bob S]

I'm pleased you got it sorted.
390 - 400 should sound sweet.
Bet you won't stop at 5 variations...

[vibratoking]

Glad you persevered and got to a good place with it. Victory sounds good, don't it?

[Smokebreak]

Yes it does! Well, having spent the last two weeks now modding, fiddling, and annoying my neighbors, I thought I'd compile my experiences here.

-took out the zener drop...amp voltages took a little while to stabilize on startup, with strange volume drops, and I just didn't feel comfortable with them. So, I'm back to the harshness, but it's still not as harsh as my 18w lite that has all the tricks. Perhaps I'm being way to picky. Lesson learned : get the correct PT

-I was getting oscillations with the vol/treble dimed, and after endless rewiring, and destroying the guts cosmetically, I finally realized just how sensitive V1 is to tube choice. I swapped out a bunch, old and new, and was amazed how each 12a_7 produced a different amp. I finally settled on a new Sovtek and bingo, oscillations were gone, and yielded the crunch I liked the best

-I tried the 47K tail/ 1K Rk in PI and couldn't really tell a huge difference from stock 10k/470ohm

- the mod I found most useful was changing the 150K grid leak between stage 2 and 3. 47k in series with 100K pot as a variable resistor yielded a nice sweep that dials back the gain. I found I liked it around 100K. I lost a little of the adorable wildness of the amp, but I think this amp has plenty of that on tap!

-installed LarMar PPIMV that is really great! Of course I lose some of that sweet PA drive at lower settings, but my neighbors are already sending thank you cards. This is my first MV, but I can see using it a lot in the future, as cranked, it seems to effectively take it out of the circuit, and I can still play Dio riffs at 2am

-I ignored the bright caps initially, but found 500p helps humbuckers a lot

Being an old metalhead that plays country and jazz for a living, I basically have no working need for this amp, but I've fallen in love with it a bit. But, now that the honeymoon is drawing to an end, I have suddenly become obsessed with a new gal, the Rocket, more specifically, the _Wreck. I understand the sensitivity here, so I'll approach it here as an AC30 TB w/ 60s filtering, 2xEL84, if that is acceptable here? I mean no disrespect, but I'm sure I'll have some questions, and I'm a little unsure what the status quo is , in terms of discussing the good Dr.'s current production amp. I've seen some builds on this board, but if I need to stop right there just let me know...

[Blackburn]

Good that you got the amp to your liking! Would love to hear a clip if you get to that sometime.

And the Rocket... Man, the Rocket/AC30TB circuit is reeeeeaaal sweet. The Z Wreck is very similar to the AC30TB, and differs from the Rocket in the filtering and B+ string. You seem to have a handle on what's going on with it. My favorite build so far is my AC30TB and I play it often. I think you'll enjoy that amp more because of what you say you do and this one will serve that arena more than the craziness of the Express/Liverpool. Why not just build it with the quad of EL84s? This circuit isn't nearly as loud and even getting a hint of dirt, can be mild and may or may not bother the neighbors, depending on your situation.

David

[Smokebreak]

Thanks David. I'm waiting on the faceplate so I'll get some pics and clips up when I zip it up...pics of the cab,that is, the amp is kinda a mess..which is partly why I wanted to do a simple 2x.
I've only built a handful of amps and most of them have been inside gutted organ amps, which is tight, so I haven't really had a chance to work on proper lead dress, as I've been just trying to get everything to fit(express is inside ao-43).
However, my first proper sized blank chassis is on its way, so I wanted to take the opportunity to stretch out a little and work on that stuff.
The other factor is that before I even started thinking about the rocket build, I ordered some Weber transformers, and a couple of them are 20w deluxe type(540CT,150ma)and a Deluxe OT, so economics and bird in hand is a consideration here too. The Deluxe OT is only 6.6K primary though...should I be looking for something more in the 10K range for 2xEL84?
That said, I am still wrestling with 2x vs 4x, as if it turns out well, I'm sure I'll be doing a 4x at some point.
Oh any thoughts on using Weber xformers in this build is welcome too. I have never used a PT that was more than $60, so I have no point of reference.

[Blackburn]

8K for two EL84s. I actually have a Mercury Magnetics Vox AC15 OT that I'm selling right now, brand new. If you want to look into good, inexpensive trannies, try Edcor and Heyboer. I don't know much about Weber trannies. Edcor PTs are spec'd for a quad of EL84s and are very cost friendly. I have one now that I'm about to use. For an OT with a quad of EL84s, I hear that Triode Electronics' version of the Dynaco A470 is good and isn't that expensive. 4.3K primary. I have two original Dynacos and have used one in my AC30 build. I dig it! If you haven't done too much building, a quad may be somewhat difficult, however, you only get better with practice and I say go for it. Those amps sound wonderful with EL84 quads. Really "big" sound.

[Smokebreak]

Ok thanks. Great info. As tempting as it is, I just can't drop any more cash on iron right now. The Dynaco looks great though. Triode sells a classictone "TW" PT for $100 too, I see. not bad. Hopefully the 6.6K OT I've got coming will do..I guess it'll push the El84s a bit more, at the least. My chassis arrived from dirtydawg this afternoon, so I'm excited! Super quick too...a couple were custom bent, and I only ordered them on Friday.
I'll start a new thread for this build. Thanks again -J