Scratch built 6G15 has issue

Fender Amp Discussion

Moderators: pompeiisneaks, Colossal

Blues Hound
Posts: 121
Joined: Sun Apr 09, 2006 1:58 am

Scratch built 6G15 has issue

Post by Blues Hound »

Built a 6G15 Reverb head, but getting too high of voltage at V1 pin 1, should be 160V, getting 245.6V and getting 0.0V at Pin 3 V1 which should be 1.2V

B+3 is at 249.6V, cannot figure out why I am only getting 4V drop across the 100K resistor that feeds V1 pin 1.

B+1 = 291.2V
B+2 = 288.2v
B+3 = 249.6V

V1 = 12AX7
Pin 1 = 245.1VDC (160V)
Pin 2 = goes to RCA Output jack and footswitch
Pin 3 = 0.0V (1.2V)
Pin 6 = 249.6V
Pin 7 = 111.9V
Pin 8 = 122.7V

V2 = 12AT7
Pin 1 = 104.7V
Pin 2 = Input Jack
Pin 3 = 1.8V
Pin 6 = 113.4V
Pin 7 = Dwell Pot Center tab
Pin 8 = 2V

V3 = 6K6
Pin 4 = 289.2V
Pin 8 = 21.3V

All the other voltages look within reason, just V1 Pin 1 and pin 3.

Changed the resistor just in case it was bad...still high voltage at Pin 1.

Any help would be appreciated.

Will post some pictures tomorrow.

Thanks,
Steve
User avatar
pompeiisneaks
Site Admin
Posts: 4222
Joined: Sat Jan 14, 2017 4:36 pm
Location: Washington State, USA
Contact:

Re: Scratch built 6G15 has issue

Post by pompeiisneaks »

Sounds like that tube may not be conducting. Make sure the heater is getting a good 6.3 V on pins 4/5 and 9. Is the tube glowing at all? (see how you also have 0v at the cathode pin 3? another sign it's not conducting) Either that or you may have a bad tube?

~Phil
tUber Nerd!
sluckey
Posts: 3062
Joined: Sun Jul 22, 2007 7:48 pm
Location: Mobile, AL
Contact:

Re: Scratch built 6G15 has issue

Post by sluckey »

In addition to what Phil said... Measure resistance from pin 3 to chassis (meter lead directly on pin 3 on the socket). Should be 1.5KΩ. If not, check the value of that resistor and make sure the other end of that resistor is connected to chassis.
Blues Hound
Posts: 121
Joined: Sun Apr 09, 2006 1:58 am

Re: Scratch built 6G15 has issue

Post by Blues Hound »

Checked the AC to the heaters, V1 pins 4/5 and pin 9 have 6.8VAC to them...put in a brand new EH 12AX7 and the heaters are not glowing, put in a GE 12AX7 and only one side was glowing.

Checked to see if something was shorted, but cannot find anything shorted.

Steve
Blues Hound
Posts: 121
Joined: Sun Apr 09, 2006 1:58 am

Re: Scratch built 6G15 has issue

Post by Blues Hound »

Measured from V1 Pin 3 to the top of the 1.5K resistor, getting 1.5K, measured from Pin 1 to the top of the 100K resistor, getting 99K...

Took the nine pin socket out, tested all the pins from bottom to top, all ring out good with no shorts between any pins...

Reinstalled the socket, put all the leads back, rechecked all connections, everything checks out okay, put in 12AX7 and still the same result...

Having something to drink and not sure what to do next...
User avatar
martin manning
Posts: 13080
Joined: Sun Jul 06, 2008 12:43 am
Location: 39°06' N 84°30' W

Re: Scratch built 6G15 has issue

Post by martin manning »

The work you have done so far doesn't tell you if the socket is making good contact with the tube pins. Measure voltage across the plate resistor, or both ends to ground. That will tell you if there is current flowing. It can be difficult to see the heater glowing in some 12AX7 tubes.
sluckey
Posts: 3062
Joined: Sun Jul 22, 2007 7:48 pm
Location: Mobile, AL
Contact:

Re: Scratch built 6G15 has issue

Post by sluckey »

Measured from V1 Pin 3 to the top of the 1.5K resistor,
That doesn't tell you if the resistor is properly connected to ground. Measure resistasnce directly from socket pin 3 to CHASSIS.
User avatar
martin manning
Posts: 13080
Joined: Sun Jul 06, 2008 12:43 am
Location: 39°06' N 84°30' W

Re: Scratch built 6G15 has issue

Post by martin manning »

Blues Hound wrote: Sat Jan 06, 2018 6:22 pmMeasured from V1 Pin 3 to the top of the 1.5K resistor, getting 1.5K...
To clarify, the "top" of this resistor would be the end connected to the cathode, at pin 3.
sluckey
Posts: 3062
Joined: Sun Jul 22, 2007 7:48 pm
Location: Mobile, AL
Contact:

Re: Scratch built 6G15 has issue

Post by sluckey »

Unless you are looking at the board on the Fender layout. Then the top of the resistor is connected to chassis.
Blues Hound
Posts: 121
Joined: Sun Apr 09, 2006 1:58 am

Re: Scratch built 6G15 has issue

Post by Blues Hound »

Martin,

Pulled the tube in V1 and measured AC to the heaters, V1 pin 9 to 5 = 6.9V, pin 4 to 9 3.8V.

Looked at the pin opening, seems a little bigger than the others, took a bodkin and closed up the opening.

That seems to have fixed the problem...ordering a new socket...

Thanks everyone for the help...

Steve
User avatar
martin manning
Posts: 13080
Joined: Sun Jul 06, 2008 12:43 am
Location: 39°06' N 84°30' W

Re: Scratch built 6G15 has issue

Post by martin manning »

sluckey wrote: Sat Jan 06, 2018 7:07 pm Unless you are looking at the board on the Fender layout. Then the top of the resistor is connected to chassis.
True enough Steve, but I think the inference is that the "top" is the end expected to have higher voltage, similar to the convention of calling the input (CW) lug of a pot the top, and the grounded lug the "bottom," as it would typically appear on a schematic. ...Or maybe you just forgot the ;^)


Glad you got it sorted out Blues Hound Steve!
sluckey
Posts: 3062
Joined: Sun Jul 22, 2007 7:48 pm
Location: Mobile, AL
Contact:

Re: Scratch built 6G15 has issue

Post by sluckey »

martin manning wrote: Sat Jan 06, 2018 6:55 pm
Blues Hound wrote: Sat Jan 06, 2018 6:22 pmMeasured from V1 Pin 3 to the top of the 1.5K resistor, getting 1.5K...
To clarify, the "top" of this resistor would be the end connected to the cathode, at pin 3.
So he has a 1.5KΩ resistive wire between pin 3 and the resistor? :mrgreen:
Blues Hound
Posts: 121
Joined: Sun Apr 09, 2006 1:58 am

Re: Scratch built 6G15 has issue

Post by Blues Hound »

Quick update, changed out the tube socket and all the voltages are good, now...there seems to be an issue with the mix and tone pots.

If I turn up the tone above 1, I am getting a nasty hiss/sizzle and if the mix and tone are both turned up to 4 or above, the hiss goes to the point of a high pitched shrill.

I have tried new tubes for both the 12AX7 and 12AT7 as well as a new MOD reverb pan, still there.

Thanks,
Steve
User avatar
martin manning
Posts: 13080
Joined: Sun Jul 06, 2008 12:43 am
Location: 39°06' N 84°30' W

Re: Scratch built 6G15 has issue

Post by martin manning »

This is with nothing plugged in? The effect of the tone control seems to say the source of the noise is in the wet side.

If you have a scope, and the noise occurs with nothing plugged in, you can just probe along the two signal paths at the input grids and after the coupling caps to see if this is a parasitic oscillation. Pictures of your unit would help with spotting potential lead dress issues.
Blues Hound
Posts: 121
Joined: Sun Apr 09, 2006 1:58 am

Re: Scratch built 6G15 has issue

Post by Blues Hound »

Martin,

Yes, with no guitar plugged in.

Here are some pictures of the chassis.
IMG_0381.JPG
IMG_0382.JPG
IMG_0384.JPG
Put 800hz to the input jack and traced through the .5 uf cap into Pin 7 V1, then from Pin 8 V1 through the .1 uf cap out to the 250k Mix pot, signal was good all the way through and over to the output jack.

Signal was good at all three lugs on the 205k Mix pot, but on the other side of the 250pf cap from the Mix pot to the Tone pot, the signale in not good.

I am seeing the same bad signal at Pin 1, 2 and 3 of V1.

Thanks,
Steve
You do not have the required permissions to view the files attached to this post.
Post Reply