GM70 Amplifier

[Paul Barker]

[Paul Barker]

Ony found two more 56’s making 3 total, one st but 3 more 6J5’s so just enough.

The opt’s are so heavy. 10k not 7k so thats good, wont be as much power but probs better sound.

Now I need to wonder how to combine everything in packages I can lift. Oh boy!

[pre65]

Now I need to wonder how to combine everything in packages I can lift. Oh boy!

Been there and got the T shirt.

Watching with interest Paul.

[Paul Barker]

Well the size and weight of the output transformers have put a spanner in the works. I cant see any more iron going in, except ac indirect heaters, all positive voltages will have to come from the 650v supply. The minus supply will be the only other and as its no more than - 100v not too challenging.

All this in mind, two monoblock signal sections, but the main B+ will arrive at one monoblock, and shared from there to the other monoblock. The decoupling down for each stage probably duplicates on each monoblock.

[pre65]

It suited me to have the big OPT for 833a as a separate part (on it's own base), but I've not seen anyone else do it that way.

With "boat-anchors" bad backs are almost de rigueur.

[Paul Barker]

Im only 66 but I decided not the lift down the 833 as used by me and Nick. I took all the components off little by little, and actually just carrying each output transformer was a challenge. Oh waily waily effing waily. I might have another look at a small amp. There isnt even room at home.

[pre65]

I managed my GK71 amp in 3 parts for each monoblock.

The main amp in 2 sections, both in MDF as a top and bottom and the OPT separate. So 6 pieces in total.

Each part on it's own was liftable and the top/bottom were securely fixed together.

[Paul Barker]

Back to calculations.

I have a 330-0-330 transformer, I want 500vdc for 6j5 so suitable for cap input.

Operate 6j5 250v 10mA -7.3v 730ohm rk bypassed rp 25k next stage CF so use 500k gain 15.38 which should swing 220 peak to peak AC valve is at max 2.5 watts

Former stage 56 ccs load gain almost mu 13.18 means 1.07ac peak to peak sensitivity so neither voltage amps will limit output.

Next step see if sufficient heater windings on the 330-0-330. Back in a minute.

Well, The transformer is pretty close. Might need to unwind a few turns on a small torrid for the 56 , on this transformer the 6v3 2.5 a winding would heat the 6j5 el84’s get heated by the 5 A winding on this transformers. 2 hex Fred diodes 1200v for ht






So to recap. All bases covered with a small torroid unwound for 56 heater.
HT for EL84’s decoupled from the 500v supply, maybe shut regulated. I yet have to calculate the EL 84 operating point. The direct coupled level will be max -65v min -55v in that window is fine as the OpT’s can cope with more current if voltage doesnt sit at 80mA.

Not much more to work out. Have everything I need near enough.

Breadboard it obvs.

[Paul Barker]

This circuit diagram for a 56 stage looks interesting for a play, but not in this project. Maybe use it in a 71a amp with no volume control, just the input transformer. Forget that the second circuit is a 57. I dont have those.

[Nick]

Trying to workout what that tx would have started life for with that high voltage tap. TV or scope is what comes to mind

[Paul Barker]

I guess.

So the breadboard is an Ideal Icos combustion chamber cover plate fairly thick galvanised steel. In a post office type box plastic.

Structural battens to maintain rigidity. The chokes on top will be connected underneath el84 load. All valve bases cut through boiler plate. In jacks and speaker connectors also. 650 B+ High voltage radio ham connectors I usually use. It has most of weight itll get and I can lift it. Project is doable.

[Paul Barker]

Ill need to find two USA 5 pin bases for 56

[Paul Barker]

Ordered

[Mike H]

Trying to workout what that tx would have started life for with that high voltage tap. TV or scope is what comes to mind

Dribble-drool.

Also what did 'Calib.' do?

[Paul Barker]

Trying to workout what that tx would have started life for with that high voltage tap. TV or scope is what comes to mind

Dribble-drool.

Also what did 'Calib.' do?

g to h Calib. Where you connect youre probes to check calibration? Who dares touch any of the unknown connectors. Likethe 6.3v which is at the level of the crt so must be the crt heater supply. Other end of crt supply at centre tap of 660v ac. So many ways things could go wrong. Leave any terminals of that nature open for sure. No room for error.

Ive just realised the amp of the 660vct winding being 140mA means its rated at 70mA if I bridge rectify end to end. I could go choke input, but I’ll have to do some adding up first of current demands. You know I like choke input. By the way I read enough literature on transformer winding I know you shouldnt bridge rectify end to end of centre tipped winding without under rating it because when its valve full wave rectified only half the winding at any one time is passing current. So the “A” which is the surface area of the core is sized for half the current of the and to end ht. But after adding up the unused windings such as the crt winding and some filament voltage capacities we can borrow core area and employ it in our bridge. Or just do no maths and go for it. The extra 100v would allow me to add to the 6J5 anode load for incrementally more gain and swing the loadline slightly better trajectory to slightly lesten 2nd harmonic at extreme voltage.

I’m very glad I bought any of such transformers probably 20 years ago. You would struggle to find them today.