Rebuilding an old Ampeg Rocket into a Marshall 18 watt clone

[olddawg]

I have an old 1961 Ampeg Rocket. It is in pretty rough shape so it is not really valuable (I actually rescued it from a dumpster) or collectable. It has 6v6 outputs and the circuit more closely resembles at R12 Jet in the Joe Piazza Schematics. I fixed it but have never really liked the way it sounds. What I want to do is gut it and build a single channel 18 watt Marshall clone with just one volume control, one tone control, and a tremolo speed control.

First, will I have a problem with the B+ changing from 6L6s to EL 84s? I can always use the Zener trick to lower it. Second will the original OT be usable for EL84s? And third, does anyone know of a schematic/layout diagram for such a beast? People usually use the none tremolo channel for clones and have a more complex tone stack.

Any and all comments or suggestions are appreciated. I will be replacing all of the tube sockets with new ceramics and doing a complete rebuild except for the cabinet, iron, and chassis.

[olddawg]

http://www.webphix.com/schematic%20h...s/j12b_jet.pdf

[olddawg]

Seems like I need a Matchless Spitfire with no MV and a tremolo.

[olddawg]

So.. Is the typical operating B+ voltage for a PP 6V6 amp enough for an EL84 PP circuit for 18-20 watts?

[Justin Thomas]

Olddawg,

I'd think it's fine, maybe a tiny bit steep. I've got a Spitfire running EL-84s at about 355V. I'm using JJs w. a 5Y3. What's the B+ in your Rocket? Ampeg tended toward conservative, so I doubt they're doing like Fenders and running @ 420V... Not sure about the OT. Could you do it with the 6V6s?

I'd be happy to check my books for voltages when I get home, but I believe Ampeg quit using 6V6s before they started putting voltages on schematics. Can you check the B+ in the Rocket? Only Ampeg I owned with 6V6s was before I knew what B+ was, otherwise I'da checked.

Justin

[olddawg]

Yeah. I haven't measured the B+ yet. I've been using the amp as an extension cabinet. It does have 450v filter caps. I should have asked if anyone has converted a 6V6 amp to El84s and do 6V6 amps typically have a lower or higher B+ than an EL84 amp?

[olddawg]

I guess nobody is going to bite ? I should have asked if the same OT is usable on both 6v6 and EL84 pp amps and is the typical 6v6 B+ compatible with EL84s. I see that some amps (the Rebel and such) use both. I'm just trying to decide if I should bother.

[Dave H]

I guess nobody is going to bite ? I should have asked if the same OT is usable on both 6v6 and EL84 pp amps and is the typical 6v6 B+ compatible with EL84s. I see that some amps (the Rebel and such) use both. I'm just trying to decide if I should bother.

I think the OT will be fine for EL84s. It was for cathode biased 6V6s so it is probably about 8k. If the B+ is 355V as stated it will be OK too. Use a 150R cathode resistor like the 18W Lite circuit if the B+ is on the high side.

[J M Fahey]

*Or* you might split the problem in 2 , optimizing each half: keep the 6V6 power stage , I'd prefer a happy 6V6 than a tortured EL84 any day of the week, and build any Marshall (or any other brand) preamp you like.

In fact, you may "Marshallize" your 6V6 stage by using "Marshall 18" NFB, phase splitter, etc, but of course +V voltages, biasing, screen resistors and transformer suitable to 6V6.
Best of both worlds.

[olddawg]

*Or* you might split the problem in 2 , optimizing each half: keep the 6V6 power stage , I'd prefer a happy 6V6 than a tortured EL84 any day of the week, and build any Marshall (or any other brand) preamp you like.

In fact, you may "Marshallize" your 6V6 stage by using "Marshall 18" NFB, phase splitter, etc, but of course +V voltages, biasing, screen resistors and transformer suitable to 6V6.
Best of both worlds.

That's a good suggestion. I have seen 18 watt clones with 6v6 outputs on the 18 watt.com site. It's just that I use my 18 watt head a lot. I was going to build one into this cabinet on this chassis to have a sleeper and less crap to carry around. If I did what you suggest I could retube the Ampeg first and see what it sounds like fresh. I never have liked it, but have never tuned it up because I have other amps. I almost bought a Bugera V22. Then decided to mess with this.

[cyclone]

You should be fine with that OT, you might test it but 6.6k is typical for primary impedance for 6v6's, so it should be near that to work with el84's, they run 7-8k typically but 6600 would work fine. The el84 hammond reverb amps OT's I convert are 6900. You'll be fine with b+ up to 400 volts, 20 or so ma at the cathode with JJ's or slightly higher current and voltage with el84m's (6p14p-ev) Tubes will last 6 months or so, couple hours a day. Hey, my shop is next to a graveyard, I get to crank it up with no complaints from the neighbors!!

One thing, if you open that thing up and fine any pink Cornell Dubilier molded caps, replace every one, even tone control, tremolo etc. I rebuilt a 64 r12r recently, a couple of funky resistors and every one (11 I think) of those pink bastards were bad, tested fine with a multimeter but the Sprague showed them to be leakier than a screen door. All the Wima signal caps were fine, even the ancient original filter can electrolytics tested fine. I've heard the green CD molded capsare more robust, better more deadly PCB's inside or something.

All that aside, I concur that using the original output section and possibly re-arranging the gain structure and tone stack a bit would be the best choice. You should fix it first, you might find you like it in working shape. Tremolo and reverb on that r12r were absolutely intoxicating. I'm trying to buy it from the guy, he's a douchy metalhead with money problems

The amp was heavily modified, from the factory! It had been converted from octal preamp/output tubes to compactrons and 7868's. There were at least three ampeg tube location stickers on top of each other in the cab, and three schematics on the back panel glued on top of each other. It was owned from new by the guys uncle, an accordion player in NYC. More to follow in a separate thread, I took a lot of pics, it was a fun one

[olddawg]

You should be fine with that OT, you might test it but 6.6k is typical for primary impedance for 6v6's, so it should be near that to work with el84's, they run 7-8k typically but 6600 would work fine. The el84 hammond reverb amps OT's I convert are 6900. You'll be fine with b+ up to 400 volts, 20 or so ma at the cathode with JJ's or slightly higher current and voltage with el84m's (6p14p-ev) Tubes will last 6 months or so, couple hours a day. Hey, my shop is next to a graveyard, I get to crank it up with no complaints from the neighbors!!

One thing, if you open that thing up and fine any pink Cornell Dubilier molded caps, replace every one, even tone control, tremolo etc. I rebuilt a 64 r12r recently, a couple of funky resistors and every one (11 I think) of those pink bastards were bad, tested fine with a multimeter but the Sprague showed them to be leakier than a screen door. All the Wima signal caps were fine, even the ancient original filter can electrolytics tested fine. I've heard the green CD molded capsare more robust, better more deadly PCB's inside or something.

All that aside, I concur that using the original output section and possibly re-arranging the gain structure and tone stack a bit would be the best choice. You should fix it first, you might find you like it in working shape. Tremolo and reverb on that r12r were absolutely intoxicating. I'm trying to buy it from the guy, he's a douchy metalhead with money problems

The amp was heavily modified, from the factory! It had been converted from octal preamp/output tubes to compactrons and 7868's. There were at least three ampeg tube location stickers on top of each other in the cab, and three schematics on the back panel glued on top of each other. It was owned from new by the guys uncle, an accordion player in NYC. More to follow in a separate thread, I took a lot of pics, it was a fun one

This one is bone stock except for the Multicap and the speaker. I have the speaker but it is blown. It might have the original tubes. I just replaced the cap and cleaned it up. It works, even the term, but I don't like the way it breaks up. Maybe if I replaced the tubes, did a total recap, and speced the resistors it might sound better. The amp was free.

[olddawg]

I shelved this project (Again!). I'm using it to drive my talk-box on stage. Works great for that as is.