SHFA2 power transformer replacement….

[frankb]

I’m working on a SHFA2 amp with an open power transformer winding for the -35V bias.  I replaced it with a power transformer from a SHFA3.  Before I replaced it I did voltage measurements with the 6973’s removed (because there was no bias on them) but all the other tubes in place and got 400V B+.  After replacing it I only have 330V B+.  The orig. one measures 97 ohms across its B+ winding; the replacement only measures 80 ohms.  I checked part numbers and found they are different.  The SHFA1 & 2 used the same power transformer where the SHFA3, 4, & 5 use another one - different part no.  My question is: the machine that it’s going into will be strictly “home use”.  Probably never even have extension speakers connected to it.  Will the lower B+ cause inadequate volume or bad sound quality ?   Thanks for any response!

- FrankB

[jukenorman]

Hi Frank,
No simple answer I think! First of all 400V is the absolute maximum for a 6973 but what you're measuring will be higher because the 6973s have been removed and not pulling current. So the 330V that you're measuring after transformer replacement will drop with the 6973s in circuit, probably down to just below 300V where it should be up around 365/370V

So yes, you will certainly have reduced volume but whether it's adequate?

The power supplies for SHFA2 and SHFA3 are configured differently, the SHFA2 uses a choke. You could "tweak" the power supply components (you would possibly have to remove the choke and configure that section as the SHFA3) - the AC output of the transformers won't be the same otherwise the DC voltages would match! Incidentally, measuring resistance of the windings doesn't prove much because the voltage is determined by the number of turns on the winding.

In your position, I think that I might have been inclined to have left the original transformer and investigate adding an additional small transformer for the biasing.

Norman.

[frankb]

Thank you very much for your reply.  It never occurred to me to add a small transformer for just the bias.  

FrankB

[frankb]

I just tried jumping out the choke. Wow! Now I have 415V. Yes, I guess I'll reconfigure the filter section as a SHFA3. Thanks for the suggestion!

[armandh]

I found that re wiring the output sockets for 6CW5/EL86 is a great choice, a more rugged and much less expensive tube

it did need a bit more bias so I bridged the selenium rectifier with a silicon diode and added an input cap to the filter chain

[armandh]

Sept 20, 2022 17:13:17 GMT armandh said:

[jukenorman]

Hi Armandh,
You make it all sound so easy! There is a substantial amount of work required changing 6973 to RL86. For a start, the pin outs are totally different so there is quite a bit of rewiring required and that's not the easiest job on Seeburg stereo 6973 amps. Next while the 6973 anode voltage (B+) runs about 370V, for an EL86 it's of the order of 200V - so major design change to the power supply configuration required there! I couldn't comment on the biasing, never having used an EL86.

I would argue that the EL84 with anode voltage of 300V might be a better choice, still more robust than the 6973 but still quite challenging and not for the novice I would say. I have had EL84s replacing 6973s in my AMi Continental amp. for the last thirty years and I am of the opinion that as well as being more robust, the sound is also better!

The EL86 is a little bit cheaper than the EL84 though!

Norman.

[armandh]

it is not as easy as I made it sound but not all that bad as I recall the Fil. were unchanged
and yes the voltage is a bit out of the tube's rating
[6cw5] were often used in vertical output circuits a hard life needing a rugged tube.
the overall results with the raised bias were great, sounded superb. but I suspect that was not the only reason.
my SHFA-1 had bad output XFs. I used a pair of ultra-linier XF I picked up at the Daton ham fest in the early '70s
if it ever need more I have a pair saved from a Dynaco stereo 70. none had any 70v out.