Recycle an Eico HFT-90 for a preamp

[pchw]

I have a non-working Eico HFT-90 tuner with a good PT. I want to build a preamp using this PT and the tiny 12" x 8" chasis. Candidates are Frank's 12BH7A and 6SN7 schematics. However, I am not sure this PT can provide the power. Here is the PT's specs, according to Sams:
pri: 117V @ 0.3A => 35VA
sec 1: 420VCT @ 40ma
sec 2: 6.3V @ 2.6A

If I use a separate heater transformer and SS rectification, will I be able to get 300V/40ma regulated B+ safely? I am assuming that sec1 can provide more current if sec 2 is not used. Is this a safe assumption? If not, any suggestion on what circuitry to use.

Thanks!!

[Tom Bavis]

Solid state full-wave bridge with choke input could use the full 40 mA rating of the transformer - with cap input you'll have more voltage than you want and about 25 mA available. Measure the winding resistance and we'll see if the rating can be pushed a little...

[pchw]

Hi Tom.

Took some measurements:
- resistence of the high voltage sec winding is 408ohm (not power on).
- Set up a simple 6x4-R-C-R-C-R-C with 2 zener 5378 at the end to form a 200V B+ driving a pair of 12AU7A at 10ma total current draw. The HV secondary was at 460VAC (my wall outlet was at 122V!!), and rectified DC from the 6x4 (pin 7) was 278V.

Attached is Frank's preamp that I want to build. Possibly replace the 12BH7A by 6CG7 or 6SN7 as I have those handy. I am thinking about using 2 x 0D3 in series to the B+ stable at 300V. I estimate this will draw 40ma total, is it close enough? Since I plan to use a separate filament transformer, I am hoping this PT won't be stressed too badly when the HV sec is at the max as there will be nothing drawn from the 2nd secondary.

Thanks

[Tom Bavis]

You'll need about 60 mA RMS to gety 40 mA DC. The transformer secondary will have a dissipation of .06^2 * 408 or 1.5W - reasonable. You'll have to drop almost 300V with a full-wave bridge... 12 Watts. With the existing center-tap rectifier, you can't get 300... or at least not enough to regulate.

You may need two heater windings to stay within heater-cathode ratings.

[pchw]

Thanks, Tom.
300V is quite a bit to drop, maybe using the same topology but replace the 12BH7 by ECC88, that way maybe I can lower the B+ to somewhere at regulated 255+ (OD3 + 0A3) which is doable by CT full wave rectification. What do you think?

[Tom Bavis]

The transformer will run cooler too...