[R-390] R 390A Audio Upgrade - Again

[Perry Sandeen]

Yo Bubba Dudes!
There have probably been as many *A* audio upgrades as any other mod. Some are more successful than others but most have limitations as they just do some mods to the existing circuit which for long term listening: just suck. BTDTGTTS! The Achilles heal of all the mods is that they still used the OEM audio output transformer. Although in Chuck Felton's mod he did use some negative feedback.

The best mod (and yes it is not reversible so *purists* can stop reading now) is the one done by Bill Kleronomos SK who did them for a side business until he passed.
What made this the best by far is that it used a Hammond true Hi-Fi P-P output transformer with negative feedback.
That said IMNSHO there were seveal shortcomings to his design.
First he used a Amperex 6360 9 pin dual heptode(?) mounted in place of the last 6AK6 tube used for the local audio output. This tube runs very, very hot.

Secondly he changed the line output stage to a 6AH6 as the 1/2 12AU7 driver for the 6AK6 tube was removed because of the need for a 6DJ6 phase inverter.  The line audio stage and its associated precision resistors are used in the A's calibration.
Thirdly, he left unused OEM audio output transformer in place and mounted the new audio transformer where the squelch circuit would be installed.
I started to improve on this by relocating the under chassis dropping resistors to the top of the chassis and in their place install new Panasonic electrolytics so that the OEM filter cap sockets would be available to use for P-P 6AQ5 audio tubes.  With a little fitness I was able to remont the OEM filter chokes in the space behind the power tanformer.  An alternate plan was to use the Hammond 1.25 Hy chokes from Triode Electronics and up the filter caps to 330 uF each.  This would give the same or better attenuation as the OEM ckt but take far less space. I also planned to do as W. Li did so well was to do a star ground sustem.

I added another 9 pin tube so there would be the same 12AU7 driver so the 6AK6 line driver circuit would remain the same.
Then I screwed up big time.
I felt the 800 narrow filter was useless and cut it out.  That left me with a huge mess of wires that I never got straightened out.  Waiting for *more quality bench time* never happened.
I believe that the improvements I started would combine the best on chassis audio with the best reliability.
I would be delighted to email all of my circuits, notes, pictures and an unpublished experimental article to all who might want to do this mod.
If one leaves in the narrow filter I believe doing the mod is not too difficult.
BTW my audio module had the $#^%&^( MFP.  I found that a simple engraving tool was great for removing it from solder terminals.  Wear a mask and don't inhale it.  it's still hazardous even at 60 years old.
Regards,
Perrier