Epiphone Electar/Zephyr Back Biasing!

[drewl]

The subject of back Biasing came up recently in a thread about a Silvertone 1474, I believe.

Well, restoring a late 40's Electar/Zephyr I noticed no cathode resistor, pins 1 & 8 of the 6L6's be grounded, so I'm wondering- WTF is going on here?

Sure enough, with the field coil speaker connected and output tubes in, there's negative bias voltage on pin 5's!

I need to draw out a schematic, ones I found aren't the same.
Damn, they made a few versions of this amp. This has tremolo.

The HV center tap is connected to a dual 40uf/450v cap ground, BUT- the two positive sides are used for filtering the screens and a B+ node, not grounded as shown on the Aiken site.
There is a big old adjustable ceramic resistor between the HV center tap and ground, to adjust the bias.

So back Biasing does (did?) Exist in the real world.

[mozz]

Have you looked on the Riders PA schematics manual for the correct one? Do you know if Epiphone was the original maker?

[drewl]

What is the "Riders PA" schematic manual?

From everything be I've found it is a late 40's Epiphone Electar/Zephyr .

[Enzo]

I THINK what he refers to is the series of Rider guides. They were a similar idea as Sams Photofact, but from an earlier time. A service you subscribed to if you ran a radio/tv repair shop.

[drewl]

Fascinating captain.
A dozen or so caps and resistors and this old relic is screaming and warbling almost like new.
I wouldn't know as this was built long before I was born.

Just need to straighten the top control panel and it's dirty grubby wiring and she's good to go.

[mozz]

Read there was 3 or more schematics for this amp over the years.
http://prewaramps.org/schematics.htm

[drewl]

Thanks, found that site.
None of those are close.

[LarBal]

Better late than never! This I my personal Zephyr that I traced out years ago. It has the back biasing. I actually pulled out the schematic I drew to restore it, as I've mucked with it so much over the years, I don't even know what's what with it. I'm actually going to restore it to what I think the 2nd generation of the Zephyrs were...all 6SL7s in the pre, and a cathode resistor on the outputs