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The Mighty Bogen MO-200A

Thanks for the info Keg. Much appreciated. I think your experience tends to mirror mine. I like the 6BL8 as well, although it is the lowest rated tube of the bunch. And while Mullard versions do perform well, the American versions I have come into contact with have had dependability problems.

All of these tubes pack a lot of performance into a small bottle, and can run pretty darn hot as a result. I think that is why I gravitate towards the 6AN8 over the 6U8 family. The 6AN8 was developed later than the other tubes, and just seems to be a heartier tube not only on paper, but in service. I know it dampens the tube rolling possibilities, but I would favor max reliability over extended rolling possibilities in this application. All in all, and from a purely subjective standpoint, I have just perceived far fewer problems from 6AN8 tubes than from the 6U8 family of tubes -- when used in more demanding applications. The (American) version of the 6AN8 I have been using for development even employs a carbonized coating on the inside of the envelope to aid in cooling. I don't ever recall seeing a 6U8 family tube constructed with that feature. Do you have any input on this?

Ultimately, I'm just trying to develop the best basic re-design of the Bogen circuit, without going too far off on any particular tangent. Interested folks can always take the new basic model, and tailor it to their own preference.

Your input is always appreciated!

Dave
 
Dave,

I have had a dickens of a time finding quiet 6AN8's myself. Even new ones seem to hum it up like a fiend.

As to Octal ideas. I am forgetting that Octals by and large are dual triode.

But you can really throw abuse at a 6BL7 or 6BX7 and even the 6DN7 (Which Bogen Used For A While).

Frannie
 
Hi Fran -- I have not ultimately ruled that option out either! A 6SN7 could be paralleled for a killer inverter section, and a 6SJ7 could be used for the pentode section. Against that however, the 6SJ7 is notably more microphonic, and ultimately, the 6AN8 does get the job done very nicely.

Heck, for that matter, you could even go with a Mullard style driver as well. I was just trying to keep some assemblance of the "kiss" formula in tack, in keeping with the original Bogen approach -- but I do enjoy all the input and suggestions. Ultimately, as I mentioned earlier, I think the final product should represent something that will be easy for folks to work with (not scare them off), and be economically feasible. Then they can take it from there........

Thoughts have been, and are appreciated!

Dave
 
Fran -- For low hum performance from 6AN8s -- or really any of these small pentode/triode tubes, you need to float the heat supply, referencing it to ground only through a cap as Hafler did. That allows the heater supply to bias itself through the natural heater cathode leakage these tubes all have -- to the lowest point for the quietest operation.

Do you recall if this was the type of installation you were working with with your humming friends (or fiends!)?

Dave
 
Hiya Dave,

Yep I know I am going off the deep end a bit and I figure I am throwing my ideas out for my own personal sense of waste not want not. But the Octal idea hit me when you were discussing cathode current dissipation and some of those Vert Driver Octals were designed to take a beating.

But I can agree that I am going down the over design route.

Frannie
 
Hiya Dave,

Do you recall if this was the type of installation you were working with with your humming friends (or fiends!)?
Click to expand...

Stock Dynaco Mk-3's .. went away in a large part when I switched to Sylvania's and finally went bye bye when I traded the MK-3's away. :)

Frannie
 
Yep, none of these tubes are infallible, that's for sure -- not like so many dual triodes you can count on without batting an eyelash, even when they test dead! I'll keep pondering.......

Thanks for the input.

Dave
 
Hiya,

FYI .. I loved my Mark-3's .. just had something dangled in front of me I could not pass up. I would put Mark-3's back into my rotation any day happily. Just did not want to give the wrong impression with my snarky sounding response.

Anyhow I had to roll a few different 6AN8's to get a set that sounded the same and also were quiet.

The Sylvania Yellow Letter Black Plates were the ticket.

Frannie
 
It may be that the 6AN8 gets the project off the ground, and then variations can always come at a later date.......

Dave
 
Hiya Dave,

Good choice as I know my MO-100 "Project" is hovering precariously down in the basement. Shows what pondering driver tubes and too much misplaced hubris will do.

Yours at least stands a good to excellent chance of lift off.

Frannie
 
I've never been able to get the 6AN8 just right. I know it has to be a good tube due to the many great amps that used them. I've tried playing with parameters with pots and I still can't get it to where I really like it.
Frannie, I have a schematic from a very old Popular Science magazine that runs 6BL7s as the drivers and the outputs. The title of the article was something like "An audio amp from TV tubes" so I guess we aren't doing anything new these days.:D

I'm really looking forward to see how you set this up Dave.
 
Thanks Jay. Keep in mind too that I am retaining R/C coupling between the pentode and triode sections, meaning that once final parameters are determined, they will largely remain a constant from tube to tube, with no interaction between the sections.

Dave
 
What got me scratching my head about this tube was the AA151. Everyone talks about how good the iron is, but mine always sounded "thin". I blamed it on the 6AN8 but I'm really hoping I was wrong (and I bet I was). I built an amp way back when around the 6AN8 and the 151 output iron, and the Blumlien circuit for biasing 6GK6s. I should dig it out and post you a pic. I think it weighed in at 72 pounds. It has dual power supplies with full bridge rectification half tube, and half SS. You would get a giggle. It's the one where I did all the tweeking on the 6AN8 and finally just shelved it. Might be time to resurrect it, after I learn some more about this tube.
 

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What about a tube like the ECC99 as it's in production and has it's positive attributes among users or any other good in production tube.
 
Shadowdog said:
What about a tube like the ECC99 as it's in production and has it's positive attributes among users or any other good in production tube.
Click to expand...

The 12BH7A equivalent? If I needed a bunch of drive, which I do at the moment, I'd think about paralleling a 6BL7/6BX7. Some say you create some distortion when paralleling dual triodes, but the tubes are cheap enough I think you could get a couple with well matched sections which would make the distortion issue, a non-issue.
 
An interesting idea for a driver is a triode connected 6W6. I've always had this in the back of my mind since reading about it as follows from Lynn Olson's site Nutshell HiFi.

http://www.nutshellhifi.com/triode2.html

"I received a tip from John Atwood, the previous Technical Editor at Vacuum Tube Valley, the 6W6 is a "sleeper" tube that makes a great driver, very low distortion, really reasonable price, and readily available. How can I resist at $2.85 apiece from Antique Electronics?

Well, after testing for distortion, beware the Japanese variant of the 6W6, which has more gain and 15 to 20dB more upper-harmonic distortion than the genuine article. The old-stock American and Italian versions were OK in performance, maybe a little worse than a 2A3, but certainly usable. Since these are indirect-heated pentodes (triode-connected for this application), it saves worrying about with the 2.5VAC filament supply and using the hum-cancelling circuit of the Karna driver stage. Conveniently, the 6W6 uses an octal socket with the standard 6L6/EL34/KT88 pinout, so you can substitute any power pentode you like and listen for your favorite flavor of sound. The biasing is also the same as the 45 (in fact the overall specs are similar to a 45 in terms of dissipation and recommended plate current), so the audio circuit can be left at the 45 operating points. Replace the UX4 4-pin socket for the same-size octal socket, connect the screen directly to the plate, provide 2.4 amps of 6.3VAC heater current for the pair of tubes, and off you go."

I've collected a stash of them in both 6W6 and 12W6 versions. They're still cheap as no one seems to talk about them. Some folks don't want to triode connect a pentode or tetrode, but I've done it a number of times and they can sound really nice.

Best,
John
 
On that line of thinking, how about the 6P15P tube, triode-strapped? I've heard they have very nice linear triode curves. Those things are pretty doggone cheap last I bought some. I've run them in place of a 6bq5 in amps that run them conservatively, and they do quite nicely.
 
Thanks for all the interesting ideas. All the tubes mentioned certainly have their attributes and possibilities in the appropriate application, but some hard facts will help to bring some reality to this matter rather quickly.

With 6550 tubes and the current driver topology, a standard 1 volt input sensitivity will require the AF amplifier input stage to provide a gain of about 235, assuming that 17 db of NFB will be maintained as used in the original design. This quickly rules out any triode ever produced on this planet. Even if the original (basically) 2.0 volt input sensitivity is accepted, then the AF stage still has to supply a gain of 118, which no single triode is going to supply with any stability. The only triode that would have a prayer of getting close to this much gain would be a 12AX7 stage employing a CCS plate load element, and positive feedback to further eek out whatever gain hadn't been achieved before instability set in. That is simply not a good row to hoe.

Now if we completely abandon the original basic drive stage topology altogether, then a Mullard configured driver stage becomes the next best bet from a standpoint of simplicity. In this scenario, the AF amplifier stage only needs a working gain of about 35 to maintain a 1 volt input sensitivity with 17 DB of NFB, and assuming a cathode coupled inverter stage gain (to either output) of ~ 6 (typical of say, a 12BH7). But this configuration also needs either a CCS in the long tail of the inverter, or about a stout -80 volt source to supply the inverter's tail resistor if enough current is to flow in this stage to develop the needed drive at each output tube grid across the 47K grid return resistor that 6550 tubes require in fixed bias applications.

Finally, since the unit now uses 2 output tubes per channel, another possibility is to use the two remaining octal sockets in each channel to install a capable Williamson front end, using 6SN7s. This type of front end would require the most tubes of course, and would likely be the most complex. But it would not require any CCSs, could operate solely from the bottom half of the doubler, could still achieve about a 1 volt input sensitivity after feedback, and could also drive the 6550s right out of their sockets.

Against all of this however, the 6AN8 driver DOES do the job very well with a 20% reserve drive capability, and offers the greatest simplicity and economy, with no compromise in performance. If this project is to maintain even one ounce of its MO-100/200 heritage, then its got to be fairly simple, or these things are either going to end up with a lot of rust, or hopelessly lost in the dreaded shuffle that Frannie so aptly warned of.

Also of course are rpampt's thoughts on all of this: After all, it is his amp!

Dave
 
Fair enough, all good points. 6AN8's are certainly not that expensive, so its not like we're looking for some exotic high priced bit of glass here. In a couple minutes of looking around I found many NOS pairs at a lot of places for under $30. With further shopping I bet you could come up with them for even less.
 
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