7868 project ideas

[mark enger]

I have a conn organ amp with a quad of 7868 and a a couple 12ax7 was wanting too do a convesion to something any ideas, what about ampeg? what model = a good rock amp Ive never even played an ampeg guitar amp, I do have a rocket board i havent used maybe a 4 7868 rocket or a 5e3? any idea what ot id need for 4 7868? the conn runs 2 ots so id need 1 bigger one for a quad thanks mark

[rdjones]

I have a conn organ amp with a quad of 7868 and a a couple 12ax7 was wanting too do a convesion to something any ideas, what about ampeg? what model = a good rock amp Ive never even played an ampeg guitar amp, I do have a rocket board i havent used maybe a 4 7868 rocket or a 5e3? any idea what ot id need for 4 7868? the conn runs 2 ots so id need 1 bigger one for a quad thanks mark

Those are powerful little tubes:
http://www.nj7p.org/Common/Tube/SQL/Tub ... ?Type=7868

3k-3.4k @ 100W ?

rd

[ampgeek]

The "typical" Ampeg plan is set up for VERY clean, low distortion tone. I played a V-4 for 25 years and didn't know anything else about any other amp! It took pedals really, really well...so....getting a decent punchy to modern tone wasn't too difficult.

Looks like 6.6K OT for a pair or 3.3K for a quad with 450V on the plates.

http://www.duncanamps.com/tdsl/show.php?des=7868

I have never built or played a Rocket. It does use quite a different tube, in many ways, than the 7868. However, I know that many here have built Rockets into 6L6/5881 type tubes (and others I am sure).

So....have a go at it. That is just the reason why you harbor the same addiction that we all do!! You are sure to discover something new along the way.

Good luck,
Dave O.

[martin manning]

I have a conn organ amp with a quad of 7868 and a a couple 12ax7 was wanting too do a convesion to something any ideas, what about ampeg? what model = a good rock amp Ive never even played an ampeg guitar amp, I do have a rocket board i havent used maybe a 4 7868 rocket or a 5e3? any idea what ot id need for 4 7868? the conn runs 2 ots so id need 1 bigger one for a quad thanks mark

If this amp is working now, what plate voltage do you have? If it's not working, what's the PT secondary AC voltage? This is important for the OT selection.

[VacuumVoodoo]

7868 is a 7591 in a Magnoval bottle & base.
OT selection is very simple: any OT presenting Raa=6.6k to the tubes in PP will do, At 450V B+ you will get close to 50W, at 300V around 25W.
7868/7591 tubes are perfect for power scaled amps as their optimal Raa load is the same 6.6k over full range of B+ voltages.

[martin manning]

If you want to make it that simple it is ;^) I see that the recommended load is constant on the data sheet, but at the higher voltages the screen is spec'd 50-100V below the plate. If the plate and screen voltages are held close to the same at 400-450V, the load line drops well below the knee. At 400V, I might use a 2k5 load (for a quad). At 450V, it probably should stay at 3k3 to avoid exceeding rated Pa.

[passfan]

If its the chassis I'm thinking of ( open frame OTs over and under on the chassis ) the OTs are too small for the higher wattage. Keep it small or do away with them. I had one of these and was tempted to do as you but decided against it. My PT had been run hard ( being set up as a doubler ) and had varnish leaking out of it ( a problem with a number of Conn circuits ) I wouldn't hesitate to use the 7868 tube and it would be welcome to see someone do a rocket with them

[passfan]

Duplicate

[martin manning]

For anybody thinking of starting fresh with 7868's, the originals are Novar, not Magnoval, which have larger pins on the same centers. Current production EH 7868 are Magnoval. These can be forced into a Novar socket, but then Novar-based tube pins will no longer be gripped tightly.

[VacuumVoodoo]

If you want to make it that simple it is ;^) I see that the recommended load is constant on the data sheet, but at the higher voltages the screen is spec'd 50-100V below the plate. If the plate and screen voltages are held close to the same at 400-450V, the load line drops well below the knee. At 400V, I might use a 2k5 load (for a quad). At 450V, it probably should stay at 3k3 to avoid exceeding rated Pa.

It's simple enough for R&R BTW it's ok to work with load line dropping below or rising above the knee. People are already doing it when they swap power tubes for a different type.
You are correct about Novar vs. Magnoval. Chinese ceramic sockets called Magnoval are in fact Novars and are barely usable. Ex Soviet ones are ok, they have spring loaded contact forks that can be loosened to accept current production 7686 and tightened back to work reliably with NOS tubes. Novar pins are 40mils (1mm), Magnoval pins are 47mil (1.2mm). Of course it's not recommended to do this loosen/tighten operation too often.

[rdjones]

For anybody thinking of starting fresh with 7868's, the originals are Novar, not Magnoval, which have larger pins on the same centers. Current production EH 7868 are Magnoval. These can be forced into a Novar socket, but then Novar-based tube pins will no longer be gripped tightly.

I found some more information about the 2 very similar socket types at

http://www.junkbox.com/electronics/Comp ... ndex.shtml

About halfway down the page is "9-Pin Compactrons": Novars and Magnovals.
Among other info is:
"If you plug a Magnoval tube into a Novar socket, the larger pins will damage the pin contacts on the socket. You can wreck the socket of a late-era tube tester by trying to test a Magnoval in the Novar socket."

Tubedepot sells what they describe as a "9 Pin Novar / Magnoval Socket" without specifying which it actually is. Caution advised.

RedDog Steve

[renshen1957]

7868 is a 7591 in a Magnoval bottle & base.
OT selection is very simple: any OT presenting Raa=6.6k to the tubes in PP will do, At 450V B+ you will get close to 50W, at 300V around 25W.
7868/7591 tubes are perfect for power scaled amps as their optimal Raa load is the same 6.6k over full range of B+ voltages.

Hi Aleksander,

Thank you for the power scale insight.

New Sensor is now manufactures the tube.

Best regards,

Steve