Switchable output tube craziness turned success.

[elbinster]

Ok, so I've finally gotten around to posting about my version of the GSG. All the PTP wiring was finished up in late sept. and minor fixes/changes were done early August.

Ok, so the gist of this amp was the build the classic 6B4G G*S*G, but also put in the circuitry to run 2A3 and 300B output tubes. All it took was some random soviet switches found on ebay (with absolutely no information as to their specs), and a little schematic modification so I don't have random parts of the circuit trying to kill the output tubes that aren't selected, and lots of crossed (and burned) fingers.

This was my first point-to-point wiring job, and considering I basically have three amps wired in one, I guess it doesn't look too horrible. It's what comes out of the speakers that matters right?

Oh, also, my only other modification was to put an EM84 based VU meter on the front for a little more tube-based showing off to the friends.
Now for the pictures:
Schematic: http://lh5.ggpht.com/_fQar2vXIsLY/THgf3rEa6rI/AAAAAAAACYs/B1jvp9Dr5TI/s800/GSG-multitube.jpg
Mock wiring: http://lh3.ggpht.com/_fQar2vXIsLY/TG2bRpMLLPI/AAAAAAAACX4/dZ1KLwFdDSw/s640/IMG_2040.JPG
Filaments light!: http://lh3.ggpht.com/_fQar2vXIsLY/TG2bRwJmrmI/AAAAAAAACX8/pMvzzxkVNNg/s640/IMG_2048.JPG
I can't believe that star ground works (final wiring): http://lh3.ggpht.com/_fQar2vXIsLY/TG2bUSeUycI/AAAAAAAACYM/02_kgCZdpJU/s640/IMG_2066.JPG
First sonic test (6B4G w/ VU meter): http://lh6.ggpht.com/_fQar2vXIsLY/TG2bVeZLEMI/AAAAAAAACYQ/Z1Q3fBqCF1Y/s800/IMG_2068.JPG
It's final resting place, with 2A3s selected (the 300B sockets are lonely): http://lh5.ggpht.com/_fQar2vXIsLY/THggYDdsFPI/AAAAAAAACY0/YEszdZDV6Aw/s640/IMG_2078.JPG

The only annoyance is the slight hum from the right channel. I've tried several modifications to make it go away, and it's very quiet now, but still there. I can't seem to get the left channel to hum, even if I try, and it doesn't matter what output tube is selected. It sounds to be 60Hz but with no scope, I don't know for sure. Removing the VU circuit doesn't seem to change it either. I'm running out of ideas, but luckily I'm running out of motivation to care, since I can't hear it in my normal couch-sitting spot.

I do have to thank all the people who posted about various GSG mods, and Shannon for coming up with such an awesome sounding yet simple circuit, that I had to build it three times. Oh, btw, who has a set of 300Bs I can borrow to get that portion of the circuit tested (mostly kidding)?


Jesse

[elbinster]

Ok, a few specific questions then.

I've heard that using tube rectification helps prolong the life of the output tubes, but can cause a problem if you turn the amp off and then on again in quick succession. If I want to switch between output tubes, how long should I wait to have the amp cool down? If I'm switching to output tubes that are cold, will I have to wait for the rectifier and VA tube to cool down still?

Also, does anyone know of a good place to get knobs? I can't believe I lucked out and my oven knobs fit the soviet switches, but apparently appliance knobs are the only option I have. Not the most stylish or cheap The shafts are 1/4" D-shaped, and they sit a little recessed in the wood. The switches are hard to throw, so I'll need something bigger than the recessed hole. I'm starting to work up the nerve to whittle a couple of knobs out of wood, as I don't feel like spending $20 a piece of vintage stove knobs.

[DeathRex]

You can get knobs from ebay or mouser. Don't know about the other 2.

[EWBrown]

If the Russian shafts are close enough to 1/4 inch (6.3mm) then you can get some knobs from rat shack, and then break them to get the brass inserts out.

Then carve, whittle, or lathe turn your wooden knobs. THe brass inserts will prevent the wood from splitting, which could be an issue with the hard-to-turn Russian switches.

If you like classic vintage style stove knobs, then the local town/county dump/landfill/recycling center/transfer station is a good source.

Lots of dead/old kitchen appliances end up there, before they get scrapped out fo rthe metal content.

Our local dump masters jealously guard the "scrap metal" pile, and don't generally allow for casual picking, but a few kind words and a gentle application of the "beer wrench" can often loosen up those tough nuts

/ed B