6G14-A Showman coming out of retirement

[tdvt]

It has been a while since I have been active here, short of actively lurking.
I have a Blonde Showman head, 6g14-A circuit, that is coming out of mothballs & is in need of a general service. It hasn't really been used for about 20 years but, powers on OK (first on the limiter, then straight wall voltage) & basically everything is working, though the output seems a little low for a quad-tube amp. This revision is the 4-5881 version, w/harmonic trem & the typical Fender tone stack, not the earlier version with the James-type tone controls.

https://el34world.com/charts/Schematics/files/Fender/Fender_showman_6g14a.pdf
The amp is fairly original, having only the e-caps (filter caps, by-pass caps, bias caps + one screen grid resistor) replaced by the previous owner more than 25 years ago, along with a 3-wire power cord.
This particular amp is a player, not a collector's example, as the cabinet & grill were both spray-painted probably back in the late 60's or 70's to look more like a blackface version. While I would like to keep it as original as I can, it needs to function reliably, so I will be checking the usual suspects, filter caps first on the list as it is exhibiting some mild hum, more on power-up than when warmed up.
It also has a rather high B+ of about +495V at the first node. Schematic shows +432V at that point. Other voltages seem proportionally correct based on the high B+. Modern wall voltage probably?

Also, while studying the schematic & amp for particulars, I noticed that there are grid stoppers on only two of the output tubes. The 6G8-A Twin of the same era, appears to be identical. I was wondering what the reasoning behind that would be?
Hoping to get it back into working & usable condition with a new speaker cabinet after the electronics are sorted.


I queried about it on the Surfguitar101 forum recently as this seems to be the benchmark surf amp. Who knew?


Thanks, TD

[sluckey]

Nice!

[tdvt]

Replaced all the filter caps & what hum was present earlier is now gone. The last time the caps were replaced the tech who did the work had converted it to the AB763-style 1st node reservoir, with two larger value caps in series (along with the balance resistors), so I kept the same configuration.
Decided to replace all the small electrolytics at the same time & while I was changing out the bias board caps, I added a trim pot to the bias wiring, using guidance from another thread about a 6G13-A Vibrosonic, that has exactly the same bias circuit.    https://el34world.com/Forum/index.php?topic=18761.0

It all works as intended but I do have a question about component ratings. The schematic states 1/2W, which is what was still in there (actually the bias had been tweaked by the addition of a parallel resistor) & 1/2W is what I had on hand. The trim pot I used was one of the 50K Piher trim pots as used on the Hoffman boards.

Safe to assume these are adequate in this application?

The last planned maintenance is to clean/re-tighten all of the output tube sockets & replace the screen grid resistors. This comes back to a question in my first post about this circuit having grid stoppers on just two of the 4 output tubes.

Was there a reason to have done it this way?

This seems to be unique to these earlier quad amps as the later AA/AB763 models have grid-stoppers on all the tubes. An added quirk is that the two grid-stoppers run between the sockets of each tube pair protected by what looks like a piece of soda straw, same for the OT connection between sockets. I had thought someone had added this to this particular amp, but have found pics of other brownface amps with the same feature.

[PRR]

The 1.5k works to suppress push-push RF oscillation in each side-pair. It has about no effect at audio.

There is no added grid-blocking resistance. But the 12AX7 driver tube is already ~~39k so not a strong driver into two grids per side. Also only a crazed man would play a four-6L6 amp at clipping and into grid-blocking!!! (Such crazed men came later...)

Get it working as-new right (new tubes, clean sockets) before you start to think what Leo did wrong.

[tdvt]

Also only a crazed man would play a four-6L6 amp at clipping and into grid-blocking!!! (Such crazed men came later...)

According to Dick Dale, HE WAS that man! And this was "his" amp...
Interesting video interviews out there with DD about his direct involvement with Leo & his part in how the Showman amp came to be (even that he was the "showman" it was named after...at least that is how he tells it)

New power tubes are the last to-do item on the docket, due to the expense.  There are some Sovtek 5881s in there now, though I suspect one is iffy as I thought I caught some flashing out of the corner of my eye one evening. I pulled that one & I am running just a pair of them at the moment to carry on.

Overall, it seems to be running OK, so closing in...