Golden Tube Audio - early 300b mono issue - any experience?

[jaaronb]

Hey folks - I know this is a longshot...

I’m in possession of a pair of what appear to be early production Golden Tube Audio 300b monoblocks. Does anyone have any experience with these?

Here’s the issue: one of them has been repaired/modded, and the other hasn’t. A new choke has been added (yet the original appears to still be in the circuit), among other things.

Trying to figure out how to get these to a good place.

Pics of what I believe to be the stock circuit first....

 

[jaaronb]

Mono #2 with mods or repairs:

 

[nerdorama]

That "choke" you show in the pics of the second unit is a 5v filament transformer. May the original 5v filament supply for the 300b failed so this was added.
John

 

[liteagilis]

That "choke" you show in the pics of the second unit is a 5v filament transformer. May the original 5v filament supply for the 300b failed so this was added.
John

That checks out. Wasn’t golden tube known for having highly suspect parts and reliability issues??

 

[heyraz]

Wasn’t golden tube known for having highly suspect parts and reliability issues??

I think they had design issues, the iron was supposed to be pretty good. For example, I think the SE-40 heater supplies for their 6SN7's were somewhere around 8VDC and the Bridge Rectifier was one of the weak links.

There was a user group over on Yahoo, I don't think it's currently active. Kegger was a member and a guy named Loc Vu was the GTA Guru.
Documentation is nonexistent, the only schematic's I ever saw were derived by users. Sonic Craft had repair kits and parts.

Good luck.

 

[steve...]

Loc Vu moved the golden tube audio forum to https://groups.io/g/golden-tube

The forum is very active and if you join and post there you will get assistance with your GTA 300B monoblocks.

I own 2 regular GTA SE-40s and 2 Special Edition SE-40s. The transformers in all of the GTA amps are excellent. The problems experienced in the early models were related to running the amps near maximum voltages for the power tubes with a design intended for 110-115 volt line voltages. Later models were provisioned for higher voltages.

The voltage gain tube filaments were powered with rectified DC and used dropping resistors to bring the voltages down. The variation in line voltages across the country caused some of these to put near 8 volts on the filaments. I had to increase the size of the dropping resistors in 2 of my amps to get them below 7 volts. The later 2 amps did not require this but my line voltage is only 117 volts. This is a common problem in old tube equipment and I use a variac most of the time. In the case of the GTA amps using rectified DC for the filaments exacerbates problem.

The plate voltages on the output tubes in the SE-40s run close to 500 volts. The output tubes use grid bias. The increased line voltages can easily exceed the specs of the tubes. It is very important to monitor the B+ and adjust the bias to keep the power draws within specs. I use 6L6GC equivalents or 7581s in my amps. I have had to replace some resistors and caps because of the high voltages.

The schematic for the stereo GTA 300Bs at the groups.io website is low resolution and the component values, etc. are unreadable. I am not aware of GTA selling 300B monoblocks. You may have a pair of custom amplifiers that used the GTA 300B transformers. I have a pair of GTA 300B transformers and intend to to do something similar when I retire and have more time.

Steve

 

[steve...]

Would you post a photo of the plate on top of the power transformer so that the text is readable?

Thanks

Steve