Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: moosetone on April 30, 2024, 05:33:43 pm

Title: Troubleshooting 6G15 Build
Post by: moosetone on April 30, 2024, 05:33:43 pm
I am still a rank amateur, but I have so far successfully built three different amps, and rebuilt three vintage ones, so I'm learning my way around. Right now, I am building a 6G15, and this problem has me stumped.

After first-time startup procedure, the output is working (I can hear the springs when tap on the reverb tank). No guitar sound on either the wet or dry side, or when the foot switch is off.

The first issue I've found is that B+2 is way low:

B+ 299.4V
B+1 291V
B+2 84.8V (250V expected)

I've rechecked the layout components and connections, verified the caps are good, verified the dropping resistor is 10K, and tried a second 12AX7. The schematic is below, with the node reading incorrectly circled. I'm also attaching some gut shots.

Any suggestions as to where to look next?
Thanks,
Dan
Title: Re: Troubleshooting 6G15 Build
Post by: AlNewman on April 30, 2024, 06:28:01 pm
Did you try measuring voltages with the 7025 tube removed?

How about values of the 2.2M resistors?
Title: Re: Troubleshooting 6G15 Build
Post by: moosetone on April 30, 2024, 08:02:30 pm
Quote from: AlNewman on April 30, 2024, 06:28:01 pm
Did you try measuring voltages with the 7025 tube removed?

How about values of the 2.2M resistors?

I didn't. So I shall do so tomorrow. Thanks!
Dan
Title: Re: Troubleshooting 6G15 Build
Post by: sluckey on April 30, 2024, 08:52:03 pm
Quote from: AlNewman on April 30, 2024, 06:28:01 pm
How about values of the 2.2M resistors?
Bingo! A 2.2M resistor would be red-red-black-yellow. Your resistors are NOT 2.2M (well at least the one I can see). They are much smaller and that's why the B+2 is so low. See pic...
Title: Re: Troubleshooting 6G15 Build
Post by: AlNewman on April 30, 2024, 10:07:38 pm
Yeah, so you're running 20mA through your 10k resistor, and there's only a few paths to ground.

Let's say you had mistaken your two 2.2M for 2.2K. 

At the bottom of your 10k resistor you are at 84.8V.

84.8/4400 ohms = 19 mA.

Minus whatever the tubes are conducting, looks about right.
Title: Re: Troubleshooting 6G15 Build
Post by: moosetone on May 01, 2024, 09:40:51 am
Sure enough, that's what I did. Doh! Sadly, I have no 2.2M resistors on hand, and just place an order yesterday with Mouser. I miss the days when Radio Shack was just around the corner...
Title: Re: Troubleshooting 6G15 Build
Post by: sluckey on May 01, 2024, 01:01:40 pm
Two 1M resistors would work just as well. The exact value is not critical, but they should both be equal value. Probably any value between 1M and 10M would work.

Title: Re: Troubleshooting 6G15 Build
Post by: moosetone on May 04, 2024, 02:33:50 pm
And with the correct resistors installed, were dripping with reverb. Thanks for the assist!

Dan