PYE TYPE 16H help needed

[Paul Mathew]

I am busy re-capping the above radio. I have been unable to locate a schematic, probably as it is an export model. Tube line-up attached. I am not sure of the orientation of the 50mFd 12V cap in the pic. I assume the red dot, which is connected to chassis is positive? The main electrolytic common negative is also not connected directly to the chassis, which is also causing me some consternation. I would appreciate any help. Thanks, Paul

[ms660]

Looks like the grid bias arrangement as you've drawn it, if that's the case then the +ve of that single electrolytic capacitor (50uF) should be connected to the chassis and it's -ve and the common -ve connection of the HT reservoir/filter capacitor as drawn looks to be correct.

The Pye 36H uses the same arrangement:

https://www.radiomuseum.org/r/pye_36h.html

Lawrence.

[Paul Mathew]

Great. Thanks for confirming that. The negative of the 50mFd cap does go to the grid of the output tube EL33, which must be a negative bias. Time to switch on!

[ms660]

Yes, it should connect to the grid of the output valve via a high value resistor.

Lawrence.

[RussellSher]

But wouldn't the 50uf cap then be reverse-polatised with its positive terminal to the the main chassis?
How does that work?
Regards
Russell

[ms660]

Originally Posted by RussellSher

But wouldn't the 50uf cap then be reverse-polatised with its positive terminal to the the main chassis?
How does that work?
Regards
Russell

If it's a "back bias" circuit, which it appears to be, then the -ve of the bias supply filter capacitor would connect to the -ve of the bias supply which means its +ve connection would be connected to chassis.

In basic terms with that type of circuit there is no cathode bias resistor in the conventional sense.

Instead, the cathode of the output valve is connected direct to the chassis, a resistor is connected between the chassis and the center tap of the HT supply winding on the mains transformer, that means that all the current flowing through the amplifying valves flows through that resistor thus a potential difference between the center tap end and the chassis end of the resistor will exist which means that the voltage at the center tap end will be -ve WRT the output valves cathode (cathode's connected to chassis)

That -ve voltage is used as a source voltage for the valves grid bias, the capacitor across the resistor therefore must have it's +ve connection connected to chassis for the capacitor to be correctly polarized.

This is a common arrangement in many valved receivers.

Lawrence.

[Station X]

Easily checked by connecting a meter across the cap, red lead to cap's positive lead, and noting whether the meter reads in the correct sense.

FWIW the VE301W I'm working on uses a similar arrangement with a 700R resistor, but no parallel capacitor. The reading with a meter connected to the ungrounded end of the resistor is negative wrt chassis.

[Paul Mathew]

There is indeed no resistor on the EL33 output valve cathode, and grid 1 of this tube is fed from the negative lead of the 50mFd, 12V cap via a 1Meg resistor. The voltage on the negative lead reads -5V wrt chassis. I believe a fixed negative bias produces a more powerful output, as opposed to cathode bias. This seems a simple way of achieving this. My understanding of the 50mFd caps orientation, with its positive lead connected to chassis, means the negative lead has to be at a lower potential than 0 volts, hence a negative voltage wrt to chassis. (not sure if this is sound logic) No pun intended Russell!

[RussellSher]

Ok, good one. Thanks for all replies
Russell