Nasty noise from new amp build ( sound clip included)

[leftu2]

Hey guys!!
I just finished my new build (and 1st ever amp build) of Rob Robinette's 6V6 JCM 800 (head) using a princeton chassis and a Allen TP25 pwr transformer andTP026 output transformer. I have a nasty distorted sound that blending with the note and gets worse when turning up the pre-amp volume. If I back my guitar volume down it will clean up ( with the pre-amp volume around 1-2). I have plenty of volume, every control works. Not buzzing or humming at all at idle (which was what I was really worried about initially...lol).

Here's the link with the audio clip of what it's doing:
https://soundcloud.com/jl2556/micro-jcm-6v6-bad-noise

I have chopp-sticked and continuity checked this thing to death, found a few bad connections and have repaired those (problem is still there). I have readings of all the voltages and will post the schematic, layout, bom and sound clip. I have modded the layout to include a fully bypassed effects loop and bias test points. These came off Rob's site and Rob has looked over my layout to make sure it looked good.
****I have disconnected the effects loop for now as I wanted to make sure there wasn't and issue***

When contacting Rob about this noise  issue, he told me to use shielded wire going to V1 on the 1st tube (grounded only at the jack side) and use a ceramic 1000pf 600v cap across pins 1 & 3 . I had a 1000pf 3kv metalized as I didn't have a ceramic (I hope that's not the issue not using a ceramic). I did both of these with no change.

Per Rob' I used EL34s from a working amp using the 16ohm output ( he explains in the link I'll post). I also used 12AX7 pre-amp tubes from a working amp. Yesterday I put the pwr tubes back in the amp they came out of to make sure they were still o.k., they were. For good measure I swapped the pre-amp tubes 1 at a time to make sure there was not issues there, they were fine.

Voltage readings as follows :
***When I got to work this morning, I realized I forgot to take the B+5 voltage reading at the 22uf 500V cap near the input last night when checking voltages***

Main cap
( JJ 40/20/20/20  500v)

-position "O"  / B+4 is 357V
-position "U"  / B+3 is 403V
-position "X"  / B+2 is 470V
-position "Y"  / B+1 is 471V

Heater Voltage is 6.3V
Plate Voltage is 470V

I don't have a ESR meter to check the caps or a meter that will check diodes ( Gonna try to  get those things this week)

I've re-checked the speaker to another amp to eliminate that as the issue.

I've got all the jacks & pots grounded to the chassis (per the layout).  The grounds are set up on the chassis per Rob's layout.

I converted the power plug to a IEC setup.

Link to Rob's page with description of build (6V6 not the micro). Has his stk layout, bom and schematic and notes.

https://robrobinette.com/RR2104_Master_Volume_Micro.htm

Again, the link with the audio clip of what it's doing:
https://soundcloud.com/jl2556/micro-jcm-6v6-bad-noise

[leftu2]

Here are the instructions for the trannies

[Leevi]

I think you should first localize the problem. Disconnect power amp from the preamp and connect signal (e.g. guitar) directly to the power amp in. Even better if you have a scope. Check the signal after each amplification stage.
/Leevi

[leftu2]

 

I think you should first localize the problem. Disconnect power amp from the preamp and connect signal (e.g. guitar) directly to the power amp in. Even better if you have a scope. Check the signal after each amplification stage.
/Leevi

Checking at the pwr amp section only i assume i pull the wire from pin 1 on v1 and where would i run that to? Would it be pin 5 on the pwr tube closest to the board?
I'm new to amp building (obviously)

[Leevi]

Power amp in is either PIN 2 (most probably) or PIN 7 on the PI tube. There  is a cap connected to that pin. Disconnect the preamp from the cap and connect guitar to that point.
/Leevi

[shooter]

I have a nasty distorted sound

did you swap primary wires on the OT to make sure it's NOT a phasing issue?

[jjasilli]

http://www.geofex.com/ > Tube amp debug page > Unintentional and ugly distortion even when set for clean operation


Also checkout repair flowcharts posted on this Forum.

[brewdude]

I’ve been working on an amp loosely based on a blonde bassman normal channel and had a similar sound if the treble and volume were both maxed. 
I was pulling my hair out trying to find it.  Finally, I tacked a 68pF cap from the plate of V1a to the plate of V1b and it went away.  I presume this indicated I had a high frequency oscillation.  Interesting thing is the snubber didn’t work across either of the plate load resistors individually. 

[leftu2]

I have a nasty distorted sound

did you swap primary wires on the OT to make sure it's NOT a phasing issue?

No, but i dang sure will when i get off work tomorrow. I never knew that could be a issue as there's nothing in robs layout that notes that as a possible issue ( unless I missed it). Before replying to your post, i did a little research on swapping transformer primary leads just to get an idea on the effects of doing so  and saw where someone had a screech at higher volume settings....uh, that would be me....lol. I forgot to mention that when i made this post today,  I kinda forgot about it to be honest as i only turned the volume up far enough to make it screech 2 or 3 times and that was more than a week ago. I assume all that nasty crap is positive feedback being put back through the amp and builds up on itself till it oscillates like a mofo...i hope that's it. 

[shooter]

I assume all that nasty crap is positive feedback being put back through the amp

phasing the OT is a 50/50 crap shoot.  most OT trannies have no phase marks, so leave leads long, swap if screamer.  THEN if it don't fix, dig deeper 

[leftu2]

Man, my post has been read 111 times already this morning, thanks guys for all your suggestions and patience with me being new to amp building .
It'll be a long day at work waiting to get home to try the transformer primary lead swap.
Lol... I've gotten a good lesson in patience with this build for sure. I knew I could follow the layout and put the parts together but could I fix any issues, that's what made me put this build off for a while but as my grandpa always told me, you learn by doing, glad I built it.

[leftu2]

Quick update.....

Lets just say I'm getting closer...lol. Swapping the OT primary leads around solved some of the issue (especially the squeal at 3/4 and up master volume settings).
I can turn the master volume up to about 1/2 and put the preamp gain at 9 o'clock and get a pretty decent clean tone but turning up the pre gain much past that and it goes back to the ugly stuff.

The other thing I found , and I'm sure this is the majority of the issue after researching how voltage dividers work.  In the attenuator section of the preamp I had a 47K resistor instead of the 470K ( the 1st resistor coming off the 2nd coupling cap ). From my understanding, lowering the value of the 1st resistor way too much will increase gain in a bad way and overdrive the overdrive section of the pre amp into "nasty distortion". I'm ordering the correct resistor today and will report back.
Thanks again for the help so far....