Hi folks. Thought I was finished with my little summer project (an ax84 4-4-0). It is built and sounds pretty good. But... the honeymoon is over a bit and I think I'm hearing something going on that prolly should not be there.
The tail end of long notes seem to have a swirl/swoosh "thing" going on. You can hear it as the main note decays. It may be there the whole time but perhaps fades/swirls in as the main note dies. It kinda sounds like it may not be a frequency related to the main note. Gain/distortion make it worse or more noticeable.
Here is a little sound clip I made. The amp is set at noon on all controls. First two notes are with the guitar full up. The next two notes I back off the guitar a bit. It is then mostly clean but there is a bit of this "effect" left. Final two notes the guitar is backed off even more and is fully clean and you can't hear it. Bottom plate used and does it with multiple guitars.
http://home.comcast.net/~mike.romberg/swirl.mp3
The 4-4-0 is pretty much exactly an express from the input through the phase inverter. Except that it does not have the 47pf cap on the PI. I'm gonna try putting that in today. I also used an edcor OT with a 10k primary and am using a speaker load of twice the rating to change the reflected impedance to 20k. I thought this was OK. But I'm not 100% sure.
Pictures of my build are here:
http://home.comcast.net/~mike.romberg/a ... index.html
In particular the guts:
http://home.comcast.net/~mike.romberg/a ... 0.JPG.html
I may chopstick the treble wire and the NFB wires off the presence pots a bit and see if that does anything.
Does this "sound" seem familiar to anyone? Anything else I should check? Anyone spot something in my layout?
Thanks,
Mike
Swirl/Shoosh in 4-4-0
Moderators: pompeiisneaks, Colossal
It was tubes!
After adding the PI cap, readjusting wires and re-flowing joints, nothing. The ugly swirl/phasor type thing was still there. And then I stumbled across the fix. It was tubes the whole time!
I've always been a bit skeptical about the various discussions on the relative virtues of different tubes. In the other amps I have, tubes can make a minor difference. And it is very minor. But not with this thing. I'm now all in on the notion that the express is very finicky about tubes.
I don't have a massive amount of tubes to play with. But I did have enough to get it sounding right. To start with, it really preferred a chinese 12ax7 in V1. Everything else would not do. It even did not like my old Mullard there
. V2 did not seem as big of a factor, so it got one of those new things with the TungSol label.
Now the phase inverter is a bit interesting. The 4-4-0 has a 12ay7 in there. From what I read it is because some folks ran into blocking distortion or something with a bigger gain tube. I thought I'd plug in a 12ax7 and see how bad it was. Result... Pure awesome.
Putting a 12ax7 into V3 made an even bigger difference than the V1 change. The swirl/swoosh thing is gone. The distortion is much smoother and of course the gain is up.
I'll put her into the awsome cabinet ampmike built for her in a bit and post a final picture or two in a bit.
Mike
I've always been a bit skeptical about the various discussions on the relative virtues of different tubes. In the other amps I have, tubes can make a minor difference. And it is very minor. But not with this thing. I'm now all in on the notion that the express is very finicky about tubes.
I don't have a massive amount of tubes to play with. But I did have enough to get it sounding right. To start with, it really preferred a chinese 12ax7 in V1. Everything else would not do. It even did not like my old Mullard there
. V2 did not seem as big of a factor, so it got one of those new things with the TungSol label.Now the phase inverter is a bit interesting. The 4-4-0 has a 12ay7 in there. From what I read it is because some folks ran into blocking distortion or something with a bigger gain tube. I thought I'd plug in a 12ax7 and see how bad it was. Result... Pure awesome.
Putting a 12ax7 into V3 made an even bigger difference than the V1 change. The swirl/swoosh thing is gone. The distortion is much smoother and of course the gain is up.
I'll put her into the awsome cabinet ampmike built for her in a bit and post a final picture or two in a bit.
Mike
Re: Swirl/Shoosh in 4-4-0
on the express as your note decays your power supply noise will increase with no master volume. I would look at Supply caps. Solen are big but work nice as supply caps ad a choke as a pi filter. The express is an extremely sensitive circuit, no grid stoppers, or any attenuation everything matters. Try to localize the problem to a particular section. I built the ax84 l2l It is my favorite gainy amp and easy on the ears it was not easy though. Grounding is very important as is many other factors. Reward - Killer Amp.