Screening plate size, McMichael 808 please.

[boater sam]

The screening plate and insulator is missing from my 808.
It should be covering the base of V3, double diode triode HL41DD & the 2nd IF transformer, secured by 2 screws into hank bushes, which are present.
I would like to remake this part, has anyone an idea of the size and shape please?

The output transformer has lost its guts, just the mounting frame left.
Has anyone a set from which I could work out the lamination stack size to make a replacement? Or even the winding details as well would be brilliant.

Thanks,
Sam.

[Ed_Dinning]

Hi Sam, if you know the output valve and the speaker impedance it is easy to work out the ratio , you could then find a suitable scrap one and use that.
As to the exact type, you will need to find one to work from and assume that you can get the probably imperial pattern lams.

Ed

[Murphyv310]

I wonder if David Boynes has one of these sets? If so he may be able to help with the plate.

Crossed with Ed.

[boater sam]

I've worked out the ratio as 42 to 1, the output valve is AC5PEN, 5200 ohms into a 3 ohm speaker. The one that someone has lashed in is 52 to 1 and too big to fit in the frame.
It needs to be a round 430 ohms resistance on the primary, so a small gauge wire will be required, maybe 48swg.
The mounting frame is very small, I will have to strip it out and measure it if I can find time, I doubt I have a transformer with such small laminations.

[Ed_Dinning]

Hi Sam, 48 SWG may be too small for the current drawn by an AC5 PEN.
The calculations get a bit more involved and will depend on the actual core cross sectional area selected.

Ed

[boater sam]

Hi Ed.

Valve spec is 40mA Ia with 180 ohm cathode resistor, 4.85W. In the 808 it runs a 250 ohm resistor.

I'm guessing a primary of round 2100 turns, secondary 50 turns but the bobbin must have been tiny.

Sam.

[Ed_Dinning]

Hi Sam, find a transformer that will fit, with as big a stack as possible then I can calculate for you.

Ed

[boater sam]

Thanks Ed, I'll try to get into it and measure the frame then find a transformer with lams that will fit this weekend. Providing everything hasn't floated or blown away!
Sam.

[boater sam]

I've found a transformer from a Bush with almost the right size "E" and "I" laminations, its very small at 42mm X 35mm. This is from an AC11 which uses an UL41 output valve. It has an open circuit primary, very common to fail this way.

The stack size needs to be 13mm maximum. To get it into the existing frame I will need to grind nearly a millimetre off the end.
The problem will be getting enough copper into the small window, 21mm X 7mm with the necessary bobbin.
The bobbin will have to have end cheeks as I cannot wind without.

As the quoted anode current is 21.5 mA minimum the primary wire gauge will be problematical.
The ratio works out at 41.6 to 1 taking the ra as 5200 ohms and the speaker at 3 ohms.
I'll let Ed Dinning have the figures and see what he can come up with.

Sam.

[boater sam]

Thanks to Ed Dinning I now have a workable set of details for winding a new transformer. Its going to be a tight fit getting enough copper into the lams. A rainy day job when all the many jobs on the boat are finished.

No progress on the screening plate size but assuming it is not critical I will see what looks right and fits the captive nuts.

Sam.