Master volume in cathode biased amp woes

[goldstache]

Wired up a Cathode Biased PP 6L6 amp.  It has gain, tonestack, and a Master Volume. 


I have V1A>TS>Gain>V1B>PI circuit topology.


When I disconnect my feed to V1B's grid (floating noise is there) but I also get guitar signal at the finals??????????


When I hook up a signal generator to the input and leave the preamp disconnected going into V1B's grid I get almost 15vac at lug 3 of my PPIMV??????


Where is this audio signal coming from?  I do have the 6l6's in Ultralinear off my PT. 
PPIMV is wired:
Dual Gang 100KA lug 3's post PI coupling cap.
Lug 2's headed to power tube grid resistors
Lug 1's to same ground as PI's filter cap


Where is this audio being superimposed???? 
Power supply,Cathode bias resistor for 6L6's,  PTCT, OT ground all chassis grounded to one spot and outjack isolated.  Preamp and PI all grounded to buss bar on back of pots.
I am floating the heaters off of the cathode resistor voltage via filament CT. 


Where is this audio coming from???????

[shooter]

I have V1A>TS>Gain>V1B>PI circuit topology.

this statement doesn't indicate PPIMV

I get almost 15vac at lug 3 of my PPIMV??????

this one seems to

have you been following this thread?  is it wired kinda sorta like that?


http://el34world.com/Forum/index.php?topic=21173.0

[goldstache]

My PPIMV is wired right off the PI plate coupling caps. 
With signal gen in the input and feed to V1B grid disconnected (floating)
I have 0 VAC at the phase inverter grids but 15VAC at the plate??????

[sluckey]

A schematic and some good pics of the amp might reveal some clues. Without more info everything is just a WAG.

[shooter]

I have 0 VAC at the phase inverter grids but 15VAC at the plate??????

Sluckeys right, my WAG, If you have no signal on the grid, but amplified signal on the plates, you have something wired wrong, measured wrong.  since you have V1b electrically disconnected what happens when you pull the tube?  15VAC still on PI plate?

[goldstache]

I have the cathode bypass caps for V1A and V1B tied to the same grounding point. I was using a dpdt switch to short them to ground like a boost switch.  I had resistors on the negative leads to stop the pop.  I split the grounds and problem was gone.  The grounds are an inch apart now not shared. 


Strange stuff. 


Sorry, I don't have a schematic written up yet.  I just freaked and posted. 

[HotBluePlates]

Could be any circuit element common to V1a and V1b. Maybe shared power supply component, etc.

Could be crosstalk within the tube.

Agreed we'd have to see a schematic first, and maybe a photo of the actual amp.

[hesamadman]

I would like to see the components connecting V1B to PI. I had the same issue with a cathode bias PP amp. It was in these components. You obviously have a Plate resistor at V1b and hopefully a coupling cap. Do you have any resistor following that CC? What value if so?

[terminalgs]

If you don't have something completely mis-wired, one possible culprit is instrument signal traveling through the power supply (PS) to later stages. This is more common than one might think. nodes in the power supply are typically separated with 10K to 1K resistors, and they feed stages with 47K to 220K plate resistors. big caps to ground are at each node of the PS, hopefully providing a ground for any audible AC frequency.


If a cap isn't functioning (failed cap, bad ground,etc) then the plate R's and those 1K-10Kohm R's only form voltage dividers  for AC signals and let your instrument signal "survive" in that PS area.  signals from consecutive stages might be out of phase, but one is "bigger" than the other, so some signal can survive and sneak down to the plate of the next phase,,,  past a volume control.


Or if the PS isn't well designed, if the amp is assembled from various segments of other circuits, and perhaps  including cathode followers, reverb circuits and extra gain circuits.,, partial signal can "live" in the PS and find its way to other stages.  what is really nefarious is when just some of that stray signal finds its way to another plate but only partial frequencies of the original signal survive..  if its out of phase, or in phase it'll cause tone nightmares... ("this amp has no bottom end!" or "this amp is mud!!")




no matter what, that signal survives past your break in the path.  it is either:

• a mis-wire.
• signal surviving in the PS.
• signal surviving through ground (poor grounding, match sure all grounds are 0ohm to your base ground,, with a quality DMM).
• poor component proximity, or bad circuit boards (like a PCB., seems unlikely here..)

[goldstache]

The power supply serving as a AC ground is what I suspect.  Bad Lead Dress or Grounding Practice!


my grid and plate wires on the PI tube seem to be the culprit.  I was getting all sorts of extra noise and even little pops on the gain and master volume controls usually at max rotation.  Moved V2A's grid wire a bit and V2B's plate wire a bit and the problems ceased.  Should I cinch them down. 


I have 3 ground nodes. 


1.  AC safety ground to chassis by where it enters chassis
2. PTCT, OTGND(speaker jacks isolated), Reservoir cap GND, Cathode bias resistor GND
3.JJ can cap for all other stages ground to buss bar on turret board then fed to a chassis ground on back of pots through 1 connection


This schematic is close to where I started.  Though there are a few changes.  Mainly the reservoir filter not being part of the preamps filter caps ground returns. 


Believe it or not the pic taken is the quietest lead dress I've been able to get with the chopstick.  I thought it might be the OT secondaries on V2B's Plate interacting and they do a bit.  But not in any logical way.  The biggest help was V2A's grid and plate crisscrossing, got rid of a ton of parasitic stuff. 


Hope this helps, its got a lot of gain.  But I dig it.
Bare in mind the V1A,B cathode bypass caps switch in on a DPDT switch (not in schematic)