troubleshooting 6CM6 based AB763 2 channel deluxe (amp2)

[Toxophilite]

What would Tube amp building be without the inevitable post build trouble shooting!
So my amp works, there was a couple of arcs on the power tubes as the initial voltages were quite high, even with the 5U4Gb rectifier
I messed around wit the dropping resistors and got it roughly into the ballpark though the Grid 1 voltage is still a little higher than I'd like it  at 420 vdc

It did work right off the bat but with quite quiet and somewhat distorted, messing around with power supply resistors helped get the preamp voltages up to where they should be
However I have a few questions about things that seem quite strange

I have the bias set to -36 vdc (like a 6V6)..but I only measure about 3-4 ma on the cathodes of the 6CM6 


Also my 2nd channel(this is a 2 channel deluxe) is quite kooky
The voltages on the plates of the 12AX7 are about 40-50 volts apart. I tried swapping different tubes, not a big difference

Annnd the volume and treble controls on that channels are very noisy (the pots are new) and the volume control seems to go up..and down as you turn it up
If you turn the treble all the way up it sometimes shuts the channel off

The first channel is working pretty good and it sounding quite nice now that the voltages are closer

[kagliostro]

About the noisy pots .... a defective coupling capacitor ?


K

[Toxophilite]

I guess it could be though they're all brand new
I noticed another curious symptom
The plate voltage of the 2nd channel 12ax7 goes up and down as I adjust the EQ and volume of that channel


(I put the input and EQ on a single 12AX7, rather than inputs on V! and EQ etc on V2)


I've been going over things checking for continuity etc

[sluckey]

I have the bias set to -36 vdc (like a 6V6)..but I only measure about 3-4 ma on the cathodes of the 6CM6 

What voltage readings do you get on all pins for both 6CM6s? Also, what voltages do you have on each power supply filter cap?

The plate voltage of the 2nd channel 12ax7 goes up and down as I adjust the EQ and volume of that channel

I suspect something is not wired correctly.

[Toxophilite]

The readings from both 6CM6s are virtually identical


pin 1 - G2     398 vdc
pin 2 - NC
pin 3- G1     -29(I have the bias currently set to -30 to tame the B+ a bit when it was at -36 the cathode bias was 3.4mv  )
pin 4-heater
pin 5-heater
pin 6 - G1 -   29 (internally connected to pin 3)
pin 7- G3 K   6.5 mv
pin 8- NC
pin 9- P       420


To me these all appear pretty good except for the millivolt/amp part
I'm including the datasheet for 6CM6 which has the pinout , as well a sample of a 6CM6 in circuit from a the radio museum page(I think) I was just using to confirm what I deduced about wiring the 9 pin 6CM6 socket


i've been going over that 2nd channel a bunch, It's identical to the channel right next to it on the board(that works) so I've been comparing and testing continuity and solder joints etc, I will of course continue to do so

[Toxophilite]

Well I was doing more monkeying around
I got nowhere with the 2nd channel despite checking everything
One thing that is telling is that when I unhook the 3 wires leading to the tone pots  I get normal balanced voltage readings at the 12AX7 plates about 190 ( a little high but at least both are the same)
With the EQ hooked up the voltage on the 2nd triode stage for that channel goes up and down with the tone pot and the 1st triode3(input) goes down about 30vdc regardless


My next recourse is to unhook that half of the board(both channels preamp wires) and have a look underneath, though continuity tests indicate that everything is as it should be.
Then yank the pots for that channel and test them


Then if all else fails , replace the tube socket which was a replacement of the original which had a cathode pin broken off whilst cleaning it.


It really seems like it's in the preamp though I can't be sure




The 6CM6s seem to be okay except for the cathode bias weirdness
I was adjusting the bias to see how it affect the cathode bias .
With the bias down to -18.3 I got  18.3 mv on one cathode and 12.3mv on the other

[Toxophilite]

Okay
I've isolated the 2nd channel problem ...somewhat


I was bypassing the EQ
I had the 3 wires to the pots detached from the board
and a jumper going from the 250pf cap to the volume pot
same problem, noisy pot, no guitar, voltage on 2nd triode plate going down as volume is adjusted up


So I completely took the preamp out by detaching the wire that connected the 1st triode plate to the junction of the 100k resistor and the 250pf cap
Then I used 1 .022 cap and ran that straight from the 1st triode plate to the volume pot


Voila it works, the pot isn't noisy and the plate voltages are where they should be and not moving...super loud of course, no tone stack insertion loss, but working


So my problem is on the board somewhere in the tone stack components ( 100k resis, 250pf cap, .047 cap, .1 cap)   They are hooked up identically to the tone stack of the 1st channel next to it and continuity tests show they should be working okay but I'm going to yank them and have a better look


I did pull the board and the underside looked good

[kagliostro]

Have you verified if the caps of the TS that has the trouble are all OK ?


K

[Toxophilite]

THey measure okay and are all brand new, however I haven't tested them for DC leakage

[kagliostro]

To me it will be a good idea to test it for DC leakage, or to try there other caps


K

[Toxophilite]

10 points to Kagliostra! 


I believe a bad 250pf silver mica cap
As I didn't have another I swapped in 2 x 120pf in parallel


I don't have the whole thing hooked up yet as it's TOO late here, but I jumpered from the other side of the (2 120pf) 240 pf cap to the volume knob and it's working fine whereas it wasn't with the other 250pf cap jumpered from the same point


I'll hook it all tomorrow to make sure


Weird to have a brand new BAD silver mica cap. I measured 140k across it with the ohm meter the other 250pf cap on the board measured open

[Toxophilite]

Okay
Second channel sorted through replacing an apparently defective brand new 250 pf silver mica cap(weird!)


the amp right now is working overall though with significantly less clean headroom than my previous deluxe build


if I wanted an amp to achieve a dirty but sweet blues tone this would be the amp to do it


This could be a result of the 6CM6 tubes that I have maybe being weak(i can't test these ones on my tester..I lost that part of the tester book)
I'm going to try and pick up a couple more pairs tomorrow that will hopefully be tested.


If it's still the same after that I might go ahead and swap in the octal sockets and go with the 6V6 tubes, the devil I know!

[Toxophilite]

Well
My in town supplier of tubes I don't already have has NO 6CM6
So I decided to go back to the devil I know and installed some 6V6 sockets and wired them up
The amp seems to working fine except for a couple things


The 2nd channel is really gainy with a fair amount of hiss, (despite tube swaps to make sure it wasn't a tube)


With a 2 channel fender amp can I ground both the preamps to one ground near the inputs??


(on my amp the inputs are beside each other , this was done to keep the 2nd channel input away from the OPT and the power supply)

[Toxophilite]

I realized why my 2nd channel is so gainy...the EQ isn't working


What would make the EQ not work?
It's the 2 band version with the resistor to ground for the mids
I've gone over the circuit a gazillion times. checked the pots, replaced the leads going from the board to the pots
I haven't tried a hammer yet

[DummyLoad]

I realized why my 2nd channel is so gainy...the EQ isn't working


What would make the EQ not work?
It's the 2 band version with the resistor to ground for the mids
I've gone over the circuit a gazillion times. checked the pots, replaced the leads going from the board to the pots
I haven't tried a hammer yet

take a break form it for a day or so. check wiring; check ground to the mid resistor; check for a wrong value (higher) pot or mid resistor if no wiring mistakes. a 68k in place of a 6.8k makes a big difference; check for a bad (open) pot or resistor.


--pete

[Toxophilite]

"a 68k in place of a 6.8k makes a big difference;"


Well, Clearly this man can see looooonnnggg distances


I was checking things last night and I was okay 33k to ground as it should be(looking at the resistor)
Not registering that it should be a 3.3k resistor


I like more mid scoop so I have a 3.3k resistor instead of the 6.68 to emulate the way I usually set the controls on my super (I'm using a super's tone stack (250, .047, .1)


Swapped the 33k out for a 3.3 K and voila the amp works perfectly, it even got rid of the slight hum which i had begun to suspect was a result of the excessive gain


Huzzah!




And thanks a lot everyone


Time to mount these amps in their matching cabs, pictures of that to follow