[R-390] Modded my R-390A...

[Helmut Usbeck]

Phil,
Glad you liked my audio mod, Roy Morgan likes to send other peoples work
out without giving credits. I havn't had any problems with the zener on
the 6aq5 shorting, but you never know.  Mine's been cooking for a couple
of years now.  In regards to the output impedance using the 5000z
transformenr it matches a 6aq5 triode fine with -15 bias.  The
distortion is a lot less than running as a pentode.  I would also run
-17 to -18 volts instead of -15 as a pentode.  Draws less current and
should sound better.(My original setup before I went triode.)Just
courious if you removed the 2 feedback paths in the original circuit. 
There are other changes than just changing the output tube.  Just
courious if you did them.

www.geocities.com/husbeck for the complete mod.

Regards,
Helm.  WB2ADT

Philip Atchley wrote:
> 
> Hello all.
> Today I did a couple things that I've been wantietenplg to do to the R-390A.tch
> years).   Yes, I put a 220 Ohm 25 Watt (I had the 25W) resistor in the power
> supply to "compensate" for the diodes smaller Voltage drop.
> 
> Secondly, I wanted to change the "Local" audio amplifier to a circuit that
> would provide more drive to my speaker (which is not overly efficient)..
> Sometime back Roy Morgan had sent me an email converting the local audio to
> a 6AQ5 which was wired as a Triode.  This same "mod" changed several othervolts 
> items on the audio deck for better biasing of the 12AU7 tubes, removal of
> feedback etc.  ALL being aimed towards lower distortion and wider bandwidth.
> This was GOOD as I had a small audio output transformer scrounged from a set
> that used a 6AQ5 as the output section.  This transformer had BOTH an 8 Ohm
> and a 500 Ohm tap on the secondary.  I chose to use the 8 Ohm and ignor the
> 500 Ohm winding (I have the "line audio" output if I need 600 Ohms.  For
> this reason, rather than wire the 6AQ5 as a Triode I opted to use it as a  
> Pentode so it'd match the transformer well.  And rather than use the 15 Volt
> Zener for Cathode bias of the 6AQ5 (as the mod sheet showed) I chose to use
> a 390 Ohm 2 Watt wirewound resistor.  I DON't like Zeners in cathode biased
> circuits as they 'usually' fail in the SHORTED mode and this would zero bias
> the output tube, drawing excessive current and possibly (likely) doing other
> damage!!!
> 
> I removed the original "Local Audio" output transformer and the new
> transformer fit and mounted well in the original space, though only with one
> mounting leg screwed down.  A terminal strip was mounted below chassis to
> use as tie lugs for the leads that connected to the original transformer and
> the new transformer.
> 
> THIS MOD IS 100% REVERSABLE AS I CEKPT THE TRANSFORMER ETC AND NO HOLES WERE
> DRILLED!
> 
> The results??  Fidelity is much improved and the audio drive is more than
> sufficient for the speaker I'm using.  YES I know this is a communications
> receiver but that doesn't mean it has to have lousy audio 8^)
> 
> 73 de Phil  KO6BB
> 
> Loving home provided for wayward Boatanchor Receivers.
> LF Modified R-390A '67 EAC  W/Heath HD-11 Q-Multiplier.
> Hallicrafters  SX-71 ('53) & 5R10A ('53).
> Homebrew 'Hybrid'  Longwave CW receiver.
> Realistic  DX-160 & SP-150 ('78).
> Merced, Central California,
> 37.18N  120.29W  CM97sh.
> 
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