7868 SE amp

[A27]

Just finished, well it needs some more tweeks as far as the ps goes. Anyway 7868 single ended amp. This was a pull from a dilapidated console. Wards Airline. I found the sams photofact from sams no less. I built the amp section only, re-used the opt xfmrs with a tube power supply. A 5BC3 to go with the novar bases of the 7868's. I used a turret board for the main parts. The sound was really low. I of course blamed myself for something to be amiss. No, I built it right. The deal was I went off the sams schematic. Well guess what? Mis-print!! The pin numbers they had for the 7868 were for the EL84. Yes I bought the right schemo too. chassis number from the chassis. Wrong on the schematic. Hmm, no it was not cool to see lightning dance in the 7868 envelope. I would love it if sams would comp me a pair of 7868's but I don't see that happening. Oh well I found the mistake and it works now. With the only other two 7868's I have. Hmm, when schematics lie...I've added some pics of the amp too.

 

[electroking]

I was looking at the 1954 Sam's schematic for a Stromberg-Carlson AM-FM
receiver (with PP 6F6 outputs), and one of the tubes on the schematic was a dual
triode, but labeled 12AU6 (which of course would be a pentode). It turns out
it was a 12AU7. So these things do happen. Good luck with this project!

 

[AlphaEcho35]

That's a realy nice looking amp. You moved everything to a new case, huh?

 

[A27]

Thanks. I don't mind using so-so tubes for the first power up, but then again the build usually works. 7868's are too expensive to be messing around with. The circuit was strange in that it had 250V on the plates of the 12AX7 and then to a 180K resistor going to the cathode that also tied in with the feedback loop. Hmm, what they do that for? This amp is bass heavy too. No bypass cap on the cathode of the 12AX7 either. A 100uF on the cathode of the 7868. I only changed the cap between the 12AX7 going into the 7868 from a .01uF to a .47uF. I built a RCA based 6V6 se amp that is just amazing. It sounds better than this one. Ah well, more tweaks.

 

[jaymanaa]

Sorry about the bad luck there, I've been bitten by schematic errors too. What a cool amp you ended up with though. I'm sitting here trying to imagine what single ended 7868s might sound like. I bet they're pretty sweet, with a good bottom for single ended in general (just guessing).

 

[JBL GUY]

Looks very nice. Good luck with your tweaks.

 

[gearhound]

VERY nice looking build!

Steve

 

[A27]

Thanks all. It's a little bass heavy or seems that way. The highs are there but it lacks sizzle like some other equipment I have. Spent some time on the chassis. Those things are time consuming. I've been trying to come up with an easier chassis build. Time to buy some materials and see how it works out. There just has to be an easier way. The Hammond boxes are thin and pricey. I'm thinkin a parrish magic line square sided cake pan upside down with a wood base might be easy. Then a routed recess for the connections. I'll post all about it. I have a 6SN7 tube line stage that needs some listening tests and a chassis.

 

[mhardy6647]

cool rectifier; never saw one of those before...

 

[A27]

Supposed to be the 5U4 in a novar base. I was always going to do the 6973 or 7868 amps in the back of the RCA manuals. They used the 5BC3 rectifier. Has a "TV tube" look to it.

 

[analog addict]

Just got one in a SE 6BQ5 amp...

It's apparently identical to a 5U4GB. Would need an octal socket rewire....

 

[ab0ez]

You can take a look at the attached schematic and compare operating parameters of the Bogen CH10 7868 SE amp. Even with the smallish OPTS, these can be modified into fairly respectable monoblocks and I suspect your amp can be tweaked for sound improvements also. Nice job, by the way.

Jim, AB0EZ

 

[jaymanaa]

You can take a look at the attached schematic and compare operating parameters of the Bogen CH10 7868 SE amp. Even with the smallish OPTS, these can be modified into fairly respectable monoblocks and I suspect your amp can be tweaked for sound improvements also. Nice job, by the way.

Jim, AB0EZ

That's a nifty front end on that one.

 

[kvflyer]

Nice Job! The 7868 is a close cousin to the 7591 and often goes unnoticed. So, sometimes, you can snag a few for not too much money. And the rectifier is a later follow on to the 5U4 which was around when Moby-Dick was a minnow.

As for schematic errors? Sams was known for errors at times. But they were very handy and could be a lifesaver if you couldn't get your hands on the manufacturer's service data.

 

[gearhound]

Bass heavy, huh?

A lot of DIY builders will INCREASE the size of the coupling caps, to bring up the bass response.
I see no reason why DECREASING the size of the coupling caps, wouldn't have the opposite effect?

Anyone want to chime in one this one?

Steve

 

[AudioSoul]

Great looking little amp. About how much power does something like that put out?

 

[BinaryMike]

It's a little bass heavy or seems that way. The highs are there but it lacks sizzle like some other equipment I have.

Console amps are often designed to 'voice' or tailor overall system frequency response, rather than deliver flat frequency response. If you post or link to the schematic, we might be able to spot the offending circuit features.

 

[Kegger]

Console amps are often designed to 'voice' or tailor overall system frequency response, rather than deliver flat frequency response. If you post or link to the schematic, we might be able to spot the offending circuit features.

Yah what he said..

---

I've got several 7868's and 7591's, but I just haven't brought myself to build
anything with them, I have thought of single ended ultra linear several times
as they should perform well in that environment with there characteristics.
(I've used them in several vintage amps, but never "built" with them as of yet)

 

[Coorsman57]

Found one this am at a swapmeet, cut it in half and rewired as needed. Will dress acordingly tomorrow.

 

[Coorsman57]

3 tube 11 watt about as simple as it gets.