A1A Finished...slight problems (Pics Added)

[kcguitar]

I just finished a wreck based off of the A1A schematic and used Paul Ruby's TW Iron from Toneslut...or Moose? I built it into Weber's ModRod Chassis, which is pretty small, but everything fit...just barely. The only mod that I installed was the ppimv rich mod that I found here on the forums. Anyways when I play harder and dig into the strings...at any volume, (even at clean settings) it seems like I'm getting some blocking distortion somewhere in the signal and the volume fades out just a bit, making the bass response very loose and saggy as well. The only cure is throwing in a 12AU7 in the PI. I tried it in V1 and V2 without any luck, so I'm thinking that the PI or poweramp section is where the culprit is. I already tried raising the tail resistor voltage divider to a 470/22k combo instead of the 470/10k without any luck. Any help? Also everybody talks about how this amp is very bright...but my build sounds way darker than my Marshall JMP with the bright cap cut. The good news is that I get no oscillations and noise is very low. This may be due to the very short runs to the tubes due to my layout and the design of the chassis (the tubes are right up against the front of the amp in between the eyelet board and the pots). Kevin.

[Allynmey]

KC, I would have started with a stock TW design and made incremental changes. Change it back to stock to see if the problem persists and then go on from there.
Allynmey

[kcguitar]

The problem was definitely still there stock. I added the ppimv rich mod later.

[dobbhill]

Can you post pics? The layout with the tubes between the pots and the board may be the problem....Maybe some coupling of grid and plate signals where they cross between the board and the pots....Just thinking as I type. I learned the hard way that the layout is an integral part of the amp...In my experience just as important as the parts selection is the parts placement.
D

[kcguitar]

Sure thing. Here's some pics. Mind you this thing sounds amazing with the 12AU7, but I'd like to have it work properly as stock with a 12AX7. Again with the ppimv rich mod, it sounded exactly the same way without the mod and additionally, I read that the master volume is out of the circuit when it's wide open. I already tried "chopsticking" it and moving some wires around but nothing changed at all. I'll try to get some clips up with the 12AX7, and the 12AU7.

[kcguitar]

Anybody? I tried messing with all sorts of values for the tail resistor...using a pot to go all the way from 10k to 50k...it didn't really help that much at all though. It's hard to record what is occurring though in detail. The closest thing that I can think of that it feels/sounds like to me is a noise gate that is set to cut off too much of the sound, and making the sound fuzzy when the gate opens or closes. I have went through almost all of my preamp and power tubes and it is definitely in the amp.

[Allynmey]

KC, let's start with a FULL voltage chart. We will need that to figure out where the problem may be. Also, take a full chassis shot at High resolution so I can look at the wiring.

Allyn

[joe6v6]

Have you checked any of your voltages ? Use the A1b layout found here
https://tubeamparchive.com/viewtopic.php?t=1408 and write down the voltages you have in relation to the ones on this schematic, see if anything is drasticly diffrent. since you mention that your amp isnt bright at all I suspect you may have a wrong value resistor somewhere. Cheack the voltages & let us know.

JOE

[Fischerman]

What are the plate resistors for the PI? The Express schemo calls for a 91k for the input triode and 100k for the second triode but in mine...I had to reduce the 91k to 82k (like many/most other amps). Doing that plus the 22k tail resistor helped quite a bit...but mine still doesn't sound right IMO...plus it doesn't sing like it's supposed to.

It should be pretty bright...the fact that it isn't is strange. I would triple check evrything...and then check it again. Too many times I've looked at something 3x and thought it was right and then finally noticed an error.

[kcguitar]

Ok here goes. With 123V from the wall, I'm reading

B+1 408V
B+2 396V
B+3 312V
B+4 293V
B+5 278V

and I'm biased at -30V. I did not take any liberties at all with the A1a schematic except for the ppimv rich mod, which I removed and noticed no difference. I do have the standard 100k and 82k for the PI plate resistors, as well as a 10k tail. As I stated earlier, raising the tail to 22k did not help. Also I cannot get any higher resolution pics as that is as high as my camera will go. Clicking on the pics opens them up and they are very clear on my computer.

[Jackie Treehorn]

What's going on with the coupling to the 3rd stage? Do you have the 150k grid to ground resistor in place?

Can you check that the 1 meg resistors in the phase inverter are 1 meg and not 100k, 10k etc. Those dale resistors are sometimes easy to mix up.

I like a 1k grid resistor on v2, myself, to eliminate some blocking, but that's probably not the drastic problem you're describing.

[kcguitar]

Yep I have the 150k grounded. It's a weird layout due to the preamp tubes being on the same side as the pots, but the circuit is exact (I hope). I basically flipped the ceriatone layout and ran the wires that would normally be at the front edge of the eyelet board to the middle, so that they would be as far away from the preamp tubes as possible, since otherwise they would be near criss-crossing with eachother. Therefore I ran all preamp grounds together in the middle and ran a single ground, along with the pot wires, along the edge of the chassis away from the tube sockets. I double, and triple checked all components, and yep those PI resistors are 1 megs. I also did try hooking up a pot instead of that 150k grid to ground resistor and playing with it and yep...no change.