Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: smackoj on March 24, 2019, 10:30:40 am

Title: Ampeg Jet Model J-12 with 7868 power tube question
Post by: smackoj on March 24, 2019, 10:30:40 am

I never encountered a 7868 power tube before. This amp came without 6BK11 preamp tubes or power tubes. I saw the price for a pair and thought, "I'll just re-wire for EL84s, I have a bunch lying around. I re-wired only to find out that the 9 pin socket is bigger than an 84 tube's pin design and 84's don't fit. So now I need to re-wire back to run 7868s which I decided to buy rather than change the orig. sockets. It originally had 1k screen resistors but I'm unsure how to wire them back? Pin 2 and pin 6 both connect to Grid 1. I know the factory had a jumper wire from pin 2 to 6 but I don't remember how the grid resistors were connected? The pdf shows the 7868 connections. Thanks amigos   

removed big font tags... sluckey

Title: Re: Ampeg Jet Model J-12 with 7868 power tube question
Post by: shooter on March 24, 2019, 11:27:02 am

here's the only 7868 example I can find

https://el34world.com/charts/Schematics/files/Ampeg/Reverb_rocket_12rt.pdf

Title: Re: Ampeg Jet Model J-12 with 7868 power tube question
Post by: DummyLoad on March 24, 2019, 12:39:59 pm

your amp has 1K control grid (g1) resistors. am fairly certain that ampeg did not use individual screen grid (g2) resistors on 7591/7868 amps that i can find or have seen personally. ALL pins on the 7591 or 7868 are used so you cannot tie feed (PI) end of the 1K grid stoppers to any of the power tube pins. IIRC, the 1K grid stopper are connected from the tag board to 7868 socket. be sure to re-install the g1 jumpers from pin 2 to 6 and the g2 jumper from pin 1 to 7. see note 4 in the attached datasheet.

you should have taken pictures before you made mods.  :icon_biggrin:   hindsight is 20/20...

--pete

Title: Re: Ampeg Jet Model J-12 with 7868 power tube question
Post by: smackoj on March 24, 2019, 01:44:16 pm

Yes senor Load a few pics would have been the best plan. I had no idea there was more than one shape/size of a 9-pin socket? I kept the grid Rs and there were 2 installed from factory.  What does  IIRC  mean plz?

Thanks for the help amigos

Title: Re: Ampeg Jet Model J-12 with 7868 power tube question
Post by: shooter on March 24, 2019, 02:49:42 pm

If I Recall Correctly

Title: Re: Ampeg Jet Model J-12 with 7868 power tube question
Post by: smackoj on March 24, 2019, 09:10:24 pm

"If I Recall Correctly" is a new one on me amigo. I imagined some Resistor/Capacitor designation.  :icon_biggrin: muchas gracias Shooter

Jack D

Title: Re: Ampeg Jet Model J-12 with 7868 power tube question
Post by: shooter on March 25, 2019, 06:59:15 am

 :laugh:
I had to ask also, being a tech guy I was "guessing" down the tech road  :icon_biggrin: