el84 transformer question

Hi everyone,

While doing some servicing at a local venue, one of the guys was throwing out an old blaupunkt tube radio. Needless to say i rescued it and took it home. Only after looking up the schematic i have a small question regarding the output transformer.

This transformer has an extra tap on it's primary, it looks like it's used as a choke for the preceding stages but i'm not sure. Could anyone shed some light on this? I'm hoping to use it as the basis for a tiny practice amp, like a selmer little giant.

schematic is attached below!

greetings,

Thomas

Hi Thomas,
This tap looks like choke for pre stages.
I want to warn you: keep in mind this device is directly coupled to AC line.
It may cause problem if try to ground it or connect to other devices.

what kind of EL84? telefunken, phillips, amperex ?

keep the tube and the output,

build a safe power supply which means buy a power transformer, those voltage doubler circuits are hard on the caps, i bet they ready to pop and that could be a shocking experience,

use the tap on the output for negative or positive ( for hendrix, dont stop playing or it will squeal like a stuck pig,  or just snip the wire and use resistor feedback, or no feedback,

good call on that power tranny did not spot that. Good thing i already ordered a hammond tranny.

All the tubes are haltron if i recall correctly.

Cj thanks for the suggestions. Will try it.

greetings,

Thomas

> transformer has an extra tap on it's primary, it looks like it's used as a choke for the preceding stages

Yes, but moreover the ripple to the small-stages is anti-phase to the ripple in the big bottle so you get tolerable buzz for less capacitor money.

RCA did this a lot in their cheap radios.

You are going to change the small-stages. The cancellation will be upset. No matter, because you are not trying to shave every penny and you will probably be replacing the main filter caps. Use normal values. For an SE power stage, you might go 47uFd-100r-47uFd to OT. Then a few K to 47uFd for the hi-level small tubes, and 10K-22uFd for the first stage in anything longer than a Champ or Junior.

It's not a voltage-doubler but it IS a "Hot Chassis" as CJ says. I don't quite see how it works (the B- return is not shown), but clearly there is no isolation between B- (and Signal Common) and Power Line. That does not matter for radio, and they slip a Phono input in with capacitors, but that's not good enough for Guitar where signal is weaker and you are *always* holding Signal Ground.

I suspect the 4x6 speaker, while adequate for ballett music over the radio, will not survive LOUD guitar playing at the 4-5 Watts this cutie can deliver. Fortunately that's an available car-speaker size, and the impedance seems to be ~~4 ohms.