Unusual (at least for me! lol) single-ended GM-70 amp

[Zonula03]

Greeting fellow members,

This is my first post, my principal interest is (for now) is comparing GM-70 amp schematic, the "contemplative state" before considering building, anyways, what do you guys think of this circuit regarding these aspects :

1 - input impedance?
2 - the use of a output transformer as an input (yes you can warn me about not to go this way, ahah)
3 - is the simplicity of the circuit worth exploring?
4 - the possibility of using a higher plate voltage
5 - the cost (of course I would look for cheaper trans than Tango)

Here is the link to the original page : http://www.ononet.jp/~micky/gm70s.html

Here are the published characteristics

Happy building and listening!


Olivier

 

[TheGimp]

You need a transformer in the filament line for safety. Also this will have a lot of hum unless you add capacitance (LOTS) after the rectifier, or other filter scheme.

 

[Zonula03]

Ooops I forgot to include this detail, he is using an regulated ps for the filament

Look at the rear of the amp

Thanks for you reply TheGimp ;-)

 

[grataku]

yeah,
i was confused how to get 20v from 100vac and a couple of 47ohms res.

 

[6A3sUMMER]

What signal source is driving the 8 Ohm to 3.5k input transformer?

A solid state power amplifier?
Or,
A tube power amplifier?

Are those signal sources stable when they are loaded by nothing other than an essentially unloaded 8 to 3.5k transformer?

 

[daanve]

1. source impedance must be very low unless you strive for a Sakuma style effect box;
3. depends on what you expect: serious "hifi" amplifier? Not worth exploring IMO;
5. you might consider Lundahl LL9202 (6k5 primary impedance - 85mA - 57H).
When you build: take care of correct connection of the capacitors in the negative grid supply (not like in the schematic....).

 

[directdriver]

Looks like a booster amp to be driven by a smaller amp, likely a flea powered one.

 

[Zonula03]

Thanks for all your reply, I understand better now, looks like a booster amp like directdriver mentioned, so for me, not worth building

 

[baudouin0]

Interesting polarity of some of the negative rail caps. The negative regulator will not support the current in the wrong direction if GM70 driven into positive grid current.