Bogen MX30A tweaker amp

[glassesnvests]

I've picked up a Bogen MX30A for free from a funeral parlor that was shutting down, and I'm wanting to make it into something useful. I thought I'd pick your guys' brains as for ideas.
Schematic:
http://www.makearadio.com/schematics/images/bogen-mx30a-06.jpg
http://www.makearadio.com/schematics/images/bogen-mx30a-07.jpg


The 7868 datasheet says that they have a 6k6 plate-plate impedance at the voltages, that that should be ripe for 6V6 territory, but the plate voltage is a little high (unless I just stick to JJ's).
Datasheet:
http://www.nj7p.org/Tubes/PDFs/Frank/049-RCA_Cuningham/7868.pdf


Any experience with the 7868's/any cool amps that use them? I'd imagine with them being used in a PA they're fairly prone to clean operation. I could use a nice clean amp. The 7247 in the preamp limiting stage could  be useful, I really dig using those.


I did have the thought that since the PA has so many channels, I could set up a middle of the road power stage and make multiple, differently voiced preamp inputs. Maybe clean, crunch-y, and hi-ish gain channels? Just tossing crazy ideas around. Just wanting to have fun with this one and try out new stuff beyond the Fender/Marshall/Vox clone realm.
Thanks!
GnV

[DummyLoad]

i like the octal version of the 7868: the 7591. replace the sockets with octal sockets and run 7591, or you could reconfigure bias for 6L6GB/5881 as well.

--pete

[Paul1453]

i like the octal version of the 7868: the 7591. replace the sockets with octal sockets and run 7591, or you could reconfigure bias for 6L6GB/5881 as well.

--pete

I've been watching a few of these on Ebay, haven't pulled the trigger yet.

Is there a real need to replace the output tubes if they are still good?

Are there just some simple mods to be made to this like you did on the Bogen K10?
A quick, down and dirty conversion to a guitar amp maybe?

[DummyLoad]

i like the octal version of the 7868: the 7591. replace the sockets with octal sockets and run 7591, or you could reconfigure bias for 6L6GB/5881 as well.

--pete

I've been watching a few of these on Ebay, haven't pulled the trigger yet.

Is there a real need to replace the output tubes if they are still good?

Are there just some simple mods to be made to this like you did on the Bogen K10?
A quick, down and dirty conversion to a guitar amp maybe?

no, keep them if they have life left. only reason i suggested is that usually with surplus PA amps the output tubes are shot or close to it. the 7591 are in production and are more common NOS.


re: simple mods: i couldn't load schematic - keeps timing out. i will try later.


-pete

[PRR]

7868 is more a small 6L6, _NOT_ any sort of 6V6.

It isn't quite as hefty as a 6L6GC, more like 6L6G.

It has higher gain than 6L6, so will need different bias.

If you are sure the 7868 are good, leave the whole power stage alone. Bogen output stages make excellent instrument amps. Play with R67 "62K"(?) at 150K, 330K, to "loosen-up" the power stage for less abrupt breakup and more color at medium levels.

What DL says: 7868 is 7591 in different base. Today decent 7591 are available, 7868 are scarce. Only difference is the socket and wiring.

When large stereo hi-fis were built with 6L6 they were just huge, also not lowest-THD. 7591 is slimmed-down and the added gain means less strain in the driver, lower THD number. But in home hi-fi, even smaller is even better, so the base-less 7868 was developed. In the end, supply exceeded demand, prices fell, Bogen liked that.

[Paul1453]

If you are sure the 7868 are good, leave the whole power stage alone. Bogen output stages make excellent instrument amps.

The little Bogen K10 made me start watching the bigger ones too.  Armed with PRR's high praise of Bogen's output stages, I thought the K10 sounded fantastic, and the eagerly anticipated mods for this DL has promised to post when his Internet is working better.  I may have to take a shot at one of these myself.   

Please let us know how you feel your conversion turns out.  Did it give you all the nice clean headroom you were after?

[silverfox]

I'm using similar PT and OT in a 6L6 build and it sounds great. It came out of this amp.

As for cool amps that use those tubes: Ampeg had a number of designs.

silverfox.

[sluckey]

As for cool amps that use those tubes: Ampeg had a number of designs.

Which one uses a 7868?

[silverfox]

Okay I suppose what I'll find now is that the author that drew the schematic was not correct. This is one of the ones I used as a reference for the reply. Let me know, (I'm quite sure some one will), if I've got misinformation.

silverfox.

[sluckey]

That wasn't a challenge. I know that Ampeg liked hifi tubes. I looked at a lot of '60s Ampeg schematics while gathering info for a 7591 project last month. I didn't recall seeing any 7868 circuits so I was just curious.

THX... Steve

[Willabe]

Huh, they have a 1 pot hooked up to both power tube cathodes (K), 1 K on each opposite end of the pot, with wiper to ground for K bias balance.

I wonder how well that works?   

[silverfox]

Yeah I wasn't so sure after that so I went out to find some confirming information and got this: http://vintageamps.com/plexiboard/viewtopic.php?f=52&t=24185

They only used them for a year. post futher down the page. " Re: Ampegs with 7868 output tubes by Baron Von Machinenmann on Sun Jun 01, 2003 4:07 pm
If any of you closet ampeg guys are out there, I e-mailed the man (jess oliver)  himself.Extremely polite gentleman-his comment was: "Ampeg was told by Sylvania that the 7868 was the new and improved output tube to use. (No cost increase) Naturally we changed, just as we did a few years earlier from 6V6's to 7591's. We were just trying to stay in step with the times. Hope this answers your question,"

Sluckey- "That wasn't a challenge."  I've learned everything I know about tube amps and to a degree radio in the last several years from the various contributions by Forum members with years of experience. Questions like yours and others are welcome as they serve to keep my "conclusions" correct or go fix them; like the recently misfired Hetrodyne principle application... If for no other reason "there be Monsters out there" that love to tear up false contentions.

Regards,

silverfox.

[sluckey]

Huh, they have a 1 pot hooked up to both power tube cathodes (K), 1 K on each opposite end of the pot, with wiper to ground for K bias balance.

I wonder how well that works?   

Where do you see any 1K resistors connected to the cathode balance pot?

[Willabe]

No, K = cathode here. If I meant 100,000 ohm 1000 ohm it would have been 1K, no space. 

....... to both power tube cathodes (K), 1 K on each opposite end of the pot, with wiper to ground for K bias balance.

 

[sluckey]

No, K = cathode here. If I meant 100,000 ohm it would have been 1K, no space. 

Duh, I get it. But I bet you would have written 100K. 

I've seen that cathode balance in several hi-fi circuits. In fact, the Bogen MX30A uses a 30Ω balance pot. I don't have any experience with that balance pot but I would suppose it works fine for small imbalances.

[Willabe]

Duh, I get it. But I bet you would have written 100K. 

D'oh.         

I've seen that cathode balance in several hi-fi circuits. In fact, the Bogen MX30A uses a 30Ω balance pot. I don't have any experience with that balance pot but I would suppose it works fine for small imbalances.

Well maybe someone here will try it in a build and report back?   

[sluckey]

I just remembered that my AO-39 conversion used that balance pot, except that amp was also cathode biased. I don't recall messing with the pot though. At that point I was working on getting a LTP PI tested.

http://sluckeyamps.com/hammond/hammond.htm

[Willabe]

Hmm, interesting.

[HotBluePlates]

I've seen that cathode balance in several hi-fi circuits. In fact, the Bogen MX30A uses a 30Ω balance pot. I don't have any experience with that balance pot but I would suppose it works fine for small imbalances.

I agree. I've used a cathode-bias version (see below) in an amp before, and it balanced the tube currents quite well. Never tried it in a fixed-bias circuit.

Note on the schematic below: The grid reference resistors from the coupling caps to the pot say "100Ω 1w" but that's a typo... They really would be ~100kΩ each, in line with what you'd expect for grid reference resistors.

[PRR]

The SB-12 and the Standel do it different.

Standell has cathodes tied together, and diddles the grid voltages.

SB-12 has cathodes 100 Ohms apart. Centered, each cathode has 50 Ohms in series to ground. If one tube is "too hot" you will dial more R in that side, less in the other side. "Too hot" is also likely "more gain". The added R to balance idle current *tends* to also balance gains.

Point for study: if overall idle current is set very low, Gm will be very low, and another 0-100r in the cathode hardly changes the gain. You want non-tiny idle current to get gain balance. This may be "assured" for "good" amps because you do not want idle current and Gm very-low or it will crossover-distort. But many hi-volt guitar amps push this point. (And may not be so adverse to crossover.)

Standell is taken from Heath and Williamson? Then the ~~140 Ohm common cathode resistance may also be scaled to work with the ~~200r 1/Gm of the cathodes to compensate crossover and Gm-doubling. SB-12 does not try to be clever.