Cathodyne or LTP for a pair of 7868's?

[trevorus]

I am converting an old Bogen CHB35, and I am wondering the benefits of leaving the 6c4 as opposed to changing it out for a 12au7 or some such in LTP. One benefit is the availability of tubes is better, but I suppose I could also run the 12au7 in cathodyne as well, but I would only really be using one side of it, and the other would be unused. As it is, I can changed the input stage to a 12ax7, leaving the other side unused. A side question, would it be advisable to use both gain stages of V1 in this for a single channel, single input amp?

[Robert M. Martinelli]

Hi Trevor,
If your intention is to convert the amp for guitar use, I'd say I'd mod the PI into an LTP.

The original PI (well, actually the whole amp) was intended for P.A. use, so it's good until you stay in its "linear" region, when overdriven, it tends to sound unpleasant, though this effects can be tamed to an extent with an accurate design...all in all, I think it's easier, and more rewarding, to choose the LTP topology, adapting its features (symmetry, gain etc) to your needs. Some kind of attenuation (divider) might prove necessary due to the LTP's higher gain compared to the original PI, though you might want (like) to "squeeze" some "power stage OD" out of the thing, just to hear how it sounds...you can always try different levels of attenuation later on.

JM2CW

Hope this helps

Best regards

Bob

[defaced]

I suppose I could also run the 12au7 in cathodyne as well, but I would only really be using one side of it, and the other would be unused.

Usually a cathodyne is accompanied by a preceding gain stage so the PI has some gain - that could use the other side of the AU7. I think your choice of PI will depend on what you want this amp to do. I like linear power amps so I tend to like the cathodyne setup. This is not the case for everyone though.

[trevorus]

Well, I have been fiddling with it a bit, and changing V1 to a 12ax7, cascading A into B with a volume in between for pre gain, changing the plate load schemes around a bit, beefing up the pre all around makes a HUGE difference. I can still push it a bit too far and get that nasty distortion out of it, but it's creamy and mean as hell before that, and it is LOUD. Next up is to put in a bias pot, and make sure that's dead on, and see where to go from there. Then after all that, I have to clean up all this rats nest I have put together!