Blatty Express

[danotron]

My Express sound harsh and blatty. The distortion/overdrive just sounds bad....even at lower volumes. Could this be blocking distortion? I don't know what blocking distortion sounds like. My voltage readings are good and I am using Pasific transformers. Is there anything (component changes) that I can try? Am I driving something too hard?

[leadfootdriver]

What model tone caps did you use?

[danotron]

PVC caps..Poly Film/foil...the correct caps in the BOM

[Tillydog]

...The distortion/overdrive just sounds bad....even at lower volumes.

I'll give you what I think is important based on my limited experience with the Express (one amp, but 2 years tweaking):

How are you getting the "lower volumes"? (IMVHO, the Express doesn't suit a master volume if that's where you are).

Is the bias supply exactly as schematic (feed to the 2 x 220k grid leak resistors coming off the 10uF capacitor, not the bias pot wiper)? Have you got the bias dialled in?

Don't overlook lead dress / bad solder joint.

If you can't get anything there, then as a diagnostic, try putting grid stoppers on some/all of the preamp valves (maybe 5k). This will take out some of the highs, but it is only a stepping stone. If this helps the 'blattiness' problem, then remove them one by one to find out which stage(s) is/are causing the problem. You can either work on the stage to eliminate the need for a grid stopper, or refine the value smaller. FWIW, I ended up with a 100R grid stopper on V2 which made a big difference to smoothness. (Aside: I had tweaked & worked on this amp for over a year using a decent 'scope, and had never seen any sign of parasitic oscillations, but they must have been there.)

For completeness, I also have:

18k grid stopper on V1a (I left the 18k at the input because the amp was prone to picking up interference without it, and I couldn't hear any difference with it in place)

820R grid stopper on V1b (per Ingrid);

250k treble pot (I felt there was too much bottom end for me as stock - 6V6s & LP style guitar);

220pF cap between the PI anodes (helped smoothness a little without muffling the highs - the popular choice of 470pF lost too much for me here).
I think mine is about right now, but it took a *lot* of tweaking to get it there. Each mod has been in and out at least twice. YMMV

Then there's the whole "tube rolling" thing...

My 0.02p. Take or leave as you wish

[danotron]

my bias was comming off the wiper for some reason. I fixed that but it was only a little better....I put the 5K's on the grids and there was a large improvement....I will continue to tweak....

thanks for setting me on the right path...

[Tillydog]

thanks for setting me on the right path...

Hey, I never promised it was the *right* path...

I would be interested in your progess keep us posted!

[Kassie]

I ended up with a 100R grid stopper on V2 which made a big difference to smoothness.

Hahaha, so what's it like? Having the ears of a dolphin?

[Tillydog]

Hah! - strangely, dolphin ears not needed, just regular ones. I'm in my 40's, so high frequency hearing has gone, anyway.

I only mention it, because I hadn't ever seen it come up before, and it was a pretty obvious difference grid stopper vs not. I could probably have got away with a few k ohms, but out of curiosity, I tried a 100R, as I had been using them for MOSFET gate snubbers, and it did the trick, so I left it there.

[Kassie]

Hah! - strangely, dolphin ears not needed, just regular ones. I'm in my 40's, so high frequency hearing has gone, anyway.

I only mention it, because I hadn't ever seen it come up before, and it was a pretty obvious difference grid stopper vs not. I could probably have got away with a few k ohms, but out of curiosity, I tried a 100R, as I had been using them for MOSFET gate snubbers, and it did the trick, so I left it there.

Yeah tell me about it.. at 25 I allready lost some high end... that's what stereo headphones and 100 watt guitar amps do for ya.

I mentioned that cuz grid stoppers are supposed to be inaudible, so theory says anyways... maybe even for up to 80K. Let alone 100R. Maybe did you perform it simultanuous with a cap change or something? Or maybe you brought the amp back to functioning stable where it belongs. Anyways.. I have yet to experiment with those grid resistors. I had some mild oscilation the other day when diming all the controls and bright cap.

Here's a link with some info... there is a lot of really good info on that site anyways. http://www.aikenamps.com/InputRes.htm

[Mark]

What output tubes are you using. It might be an idea to sub a set of tubes into the amp first?

[Tillydog]

I mentioned that cuz grid stoppers are supposed to be inaudible, so theory says anyways... maybe even for up to 80K. Let alone 100R.

You're talking about the high frequency roll-off, and by and large I agree (I maintain I can hear 68k vs 51k on a 12AX7, though ).

The other thing they do, and the reason they're needed, is to suppress parasitic oscillations (and ringing, etc.) in the grid wiring, which are even higher in frequency (MHz??), but which can cause odd artifacts in the audio - I think the difference I was hearing was the difference between the presence and absence of these artifacts. I would say it made the amp sound slightly harsh, even when clean.

YMMV

[danotron]

What output tubes are you using. It might be an idea to sub a set of tubes into the amp first?

I am using new Svetlana EL34's......new stock

I just tried some:

JJ 6V6 = sounds bad
Tung-Sol 6V6 = sounds bad
NOS Sylvania EL34 = a little better but still bad
NOS Westinghouse 6CA7 = also a little better but still bad

[danotron]

Well, I changed the peamp a bit. It's a little like a Komet 60 now..but it sounds so good. The bottom end it no longer flabby and the blattiness is gone...still loud but soooooooo gooood!!!

[Miket]

What changes did you end up doing exactly? I'm finding my new expresses distortion to also be a little blatty and fizzy, especially on the trailing end of the notes played. I'm getting kind of a fizzy phaser type sound as the notes sustain.

[Kassie]

I mentioned that cuz grid stoppers are supposed to be inaudible, so theory says anyways... maybe even for up to 80K. Let alone 100R.

You're talking about the high frequency roll-off, and by and large I agree (I maintain I can hear 68k vs 51k on a 12AX7, though ).



YMMV

Ok I just put some gridstoppers.. 15K @ v1.1, 1K @ v1.2 and 100R @ v2.1.
Furthermore I placed the cathode resistors and caps right on the tube socket of v1 instead of on the board. Results, more hearable stability. Indeed go as low as you can and use metal film because the noise floor can increase quite a bit. Especially on v1.2 I put a 1.5K carbon film and the hiss level jumped up big time. a 1k metal film was much more quiet (its 820r per Ingrid, but that doesn't matter at all).

I also put a 4n7 resonance cap in the NFLoop. Which really helps to let the bottom end breathe. I couldn't help but noticing the bass was choking up too much, and started to become flabby and mushy, I would even be bold enough to say that it's almost a design flaw of Ken.

Furthermore I experimented with a 1N mallory cap in parallel with the 0.0022UF (C8, sorta per Ingrid), but I did not like it at all... the amp didn't clean up all that nice anymore and it became way too mushy if the input brightcap was not engaged.. I play a les paul with alnico2 pro's.. which is a dark combi... maybe that mod works very well for bright guitars..

I replaced the 500PF brightcap with a 300PF which I like more.. and keeps the hiss level down as well. 500PF turned my les paul into a Tele, which is quite dreadfull, since I am not communicating with birds.