Battery eliminator for Beethoven portable SG4

[Daventry 5XX]

Hi All

I'm looking at getting a battery eliminator made for my Beethoven Portable Screen grid 4,but I am unsure of the HT voltages as the original battery is missing and I have been unable to find a Trader sheet or schematic for it.

The nearest that I can find is for a Beethoven TRANSPORTABLE SG 4,this is a different set but does appear to be very similar electronically.

I think that the original valve line up would have been as follows :

V1 : PM12M RF Amplifier
V2 : PM1HL Detector
V3 : PM1HL AF Amplifier
V4 : PM22A Audio Output

My set has a hotch potch of different valves :

V1 : S21
V2 : 210HL
V3 : SS210HL
V4 : SS220P

There is a hand-written label inside the set which gives 2 HT voltages :
108v. and 68v.
Does this sound about right ?

I presume that the 108v(HT1) would be for the anodes on valves 1,2 & 3, and the 68v(HT2) would be for the suppressor grid on V4 .Why 2 different voltages,could it all be made to run off the HT1 voltage ?

It still has the original Grid Bias battery, modified to take 6x1.5v batteries, but again,I am unsure which valves have what bias.(can anyone tell me ?)

The 2v.filament accumulator has been replaced with a cyclon 2v. battery, but as I dont have a charger for it, I will probably get the 2v. supply built into the eliminator.

If anyone can offer any help or advice I would be very grateful.

Alan

[Herald1360]

Radiomuseum suggests the same Mullard lineup as you found except PM2A for V4.

I would expect to find 120V anode supply for PM2A, GB about -4.5 to 6V
PM1HL 108V sounds fine, GB about -1.5 to 3V
PM12M 108V on anode, 68V(ish- you can't get this nominal voltage from a stack of 1.5V cells) screen grid, GB about -4.5V

If your set has a pentode PM22A output, then Anode and screen 120V and GB about -4.5V would suit.

[Daventry 5XX]

Thanks Chris

Thats a great help,but according to the only schematic that I have(which may not be correct)the HT+2 68v. would go to what appears to be a suppressor grid on V4 and the screen grid on V1 gets its voltage from the junction of R2 and R3 .

Is this correct or am I reading it wrong ?

Alan

[Herald1360]

Any chance you could post your circuit?

I was just speculating from data for the valves themselves.

I can't think why a suppressor grid should have 68V on it, usually it's grounded or connected to cathode. Screen grid sounds possible.

[Daventry 5XX]

Hi Chris

The screen grid is connected to cathode but the HT+2 is definitely connected to the suppressor grid.
I will try to post the circuit diagram for you to look at.

Alan

[Mr Moose]

Hello,

The suppressor grid of the PM22A is internally connected to the cathode in the valve so cannot be connected to HT2+.
The drawing of the V4 on the Brodcaster sheet has the grids in the wrong order. The grid nearest the cathode is actually the suppressor grid and is shown correctly as internally connected to the cathode but it should be shown as the top grid. The centre grid on the drawing is the control grid and the top grid connected to HT2+ is the screen grid.

You say that your output valve is an SS220P but this is a triode with a four pin base not a pentode. There is the SS220Pen which is a pentode with a five pin base. How many pins does your output valve have?

Yours, Richard

[Herald1360]

Originally Posted by Dickiebird

Thanks Chris

Thats a great help,but according to the only schematic that I have(which may not be correct)the HT+2 68v. would go to what appears to be a suppressor grid on V4 and the screen grid on V1 gets its voltage from the junction of R2 and R3 .

Is this correct or am I reading it wrong ?

Alan

Hi Alan,

I've now actually looked at the Broadcaster circuit for the Transportable

The rest of the datasheet inter alia quotes HT voltages of 120V and 96V; then voltages on the anode and screen of V4 of 118V and 95V which rather suggest that HT+1 is 120V and HT+2 is 96V.

As you noted, the screen voltage of V1 is resistively derived.

It also quotes two GB voltages of -1.5V and -3V which would logically be GB-1 (V1 and V3) -1.5V and GB-2 (V4) -3V.