Next Bogen Conversion soon - need phase splitter advice

[Turbo7MN]

Hey guys, I just bought a Bogen K15 on eBay, it should be here next week. Stock schematic:

http://www.flickerdown.com/forsale/k-15.pdf

Anyway, I want to do a tweed-style amp. The preamp will be set up identical to what I did with my CHB-10A, and I'd like to run 6V6s instead of the 6L6GBs, although I will see how they sound before I decide. I like massive amounts of power tube distortion in my amps (my CHB's power tube breakup starts at 1/4-1/3 volume) and I can't decide which style phase splitter would be best. I can either keep the paraphase style and replace some components to make it similar to a 5D3 Deluxe, use a cathodyne like the 5E3 (though this may result in some unwanted preamp distortion) or I can go long-tailed-pair and maybe clone one from a Blackface circuit.

What I'm really going for is very little preamp distotion and loads of power tube overdrive. Which phase splitter setup would accomplish this better?

Also, would it be a bad idea to get rid of NFB altogether? This amp is going in the garage attic (new jam space), so it will be cranked 99% of the time.

Thanks in advance!

-Darren

[PRNDL]

The K15 looks like an interesting amp.

You might want to try plugging your guitar directly into one of the mic inputs to see how it sounds. If it breaks up nicely, you could modify the preamp and tone sections and get a very unique amp. (I did that with a Heathkit A7E).

As for NFB, it reduces gain and lowers noise and distortion. I believe it's more for the clean sounds, instead of distortion. A simple solution is to replace the NFB resistor with a pot that can be clicked into an open circuit. Another way is to move the resistor onto another one of the speaker taps.

As for switching from 6V6 to 6L6 ... are you sure that can be done with the same transformer? (I don't know).

I'm actually interested in the 12 volt 12AU7 project of yours. I'm thinking of making a Sopht amp to put into a little SS amp cabinet I got at a yard sale.
Did yours work out well?
What transformer did you use? Was it the Collins/Hammond 119DA?

Thanks!

[Turbo7MN]

I actually finished redesigning the circuit a while back. Plugging into the original circuit (after I added the 1/4" jacks of course) sounded pretty good, it was rather dark sounding though and it did not break up nicely with chords. I used the same preamp design that I did with my CHB-10A (loosely based around the 5D2 Princeton and oher tweed designs) and the driver stage didn't get modified much at all.

I scrapped the 6V6 idea and threw a pair of Tung-Sol 5881s in there and it sounded great. I would have had to correct for the different tube's impedance by a factor of two (this amp has 4/8/16 taps so it's fine) but it runs the 6L6s at a pretty low plate voltage and I'm sure any 6V6s could handle it. I've got kind of a strange "overtone" thing going on with a lot of distortion (and especially when I palm mute the srtings) that I'm 99% sure is in the driver stage, but it doesn't bother me that much.

I just put an SPST switch in the NFB loop and I couldn't be happier. I've familiarized myself with NFB much more over the last several months and I decided that would be the best option. Great cleans and "on the edge" overdrive with it on, but cut it and there's more power stage distortion than I'll ever need available.

I paired it with a homemade 112 cab with a Weber 12A125-O and it sounds great.

Thanks for the input. A little late, but you've got some good points.

The sopht amp turned out alright for what it is. You can't expect a full-on tube amp sound when you're running the tubes on 12VDC but I am happy that I did it. I cathode biased both preamp stages with 100k pots to get the bias voltage exact and it sounds a lot better. You can mess with stuff while the amp is running that you woldn't dare do on a real amp, it's fun to play around with. It's loud enough to drive a reasonably efficient speaker to practice volumes too, though lately I've been having fun running the output straight into my '65 Gibson GA-25RVT .

I used a 70V line matching transformer as an OPT. It's a Speco 10 watt, model number T-7010 I believe. I couldn't get the Hammond transformer so I used that, I would have preferred the Hammond though.

-Darren