Parallel-Push-Pull KT66 organ amp conversion ???

[jon_010101]

I have a pair of gigantic Baldwin organ amps that I want to convert for audio use. I have them running now (new caps), but they use a paraphase 12AX7 to drive four 6L6GCs and, consequently, sound somewhat lousy. My thoughts are as follows:

1) rewire from tetrode to triode, use KT66s vs. 6L6GC
2) dump the 12AX7 for a 6N1P or 6DJ8, running either Dynaco style gain+cathodyne, LTP, or differential amp with input transformer to split the phase.
3) significantly reduce feedback to compensate for lost gain

So... my question is: which one-tube phase splitter/driver configuration should yield the best sound for driving a quad of KT66s? Any tubes besides the 6N1P that might work better?

Also, the power transformer is the size of a cantaloupe... should it care about the extra KT66 filament current draw over the long term? Short term, I've tried it, it stayed cool

Just looking for some suggestions based on others' experiences.

Best regards,
-jon

 

[Eli Duttman]

Jon,

7591s are usually OK combined with 6L6 O/P "iron". The 7591 is easy to drive, sounds GOOD, and has a low heater current draw.

A differential phase splitter made from a 12AT7 provides all the gain needed when a CDP is the source. No preamp is needed.

 

[poobah]

Depending on the organ, the frequency response of your OPT may be lacking in the high-end. Organs only create the frequencies they were designed to... sometimes 6-7 kHz. The designers were keen to take advantage of this and avoid excess interleaving the OPT's.

My point being, you might want to run the response on your OPT's before you commit money to pricey tubes etc...

😉

 

[jon_010101]

7591s are usually OK combined with 6L6 O/P "iron". The 7591 is easy to drive, sounds GOOD, and has a low heater current draw.

A differential phase splitter made from a 12AT7 provides all the gain needed when a CDP is the source. No preamp is needed.

Interesting... 7591... I didn't even consider this option, but it would definitely solve any heater concerns, and would be easier to drive. Hmmmmm....

Depending on the organ, the frequency response of your OPT may be lacking in the high-end. Organs only create the frequencies they were designed to... sometimes 6-7 kHz. The designers were keen to take advantage of this and avoid excess interleaving the OPT's.

My point being, you might want to run the response on your OPT's before you commit money to pricey tubes etc...

This is a great point... The amps are Baldwin and pretty huge. Listening now, they don't seem to lack extension but are a bit springy-sounding and don't image quite right. Not so much bad as "weird" sounding. A bit phasey, perhaps. I'm hoping this is just from the lousy phase-splitter mucking things up. Even still, the power supply is so impressive (cantaloupe-sized transformer + monster choke + PIOs) that I can justify buying new OPTs for them if needed.

Thanks for the replies!