• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

C3g:GM70:GM70 schematic advice

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
attached is my schematic. im not good with my calculations. advice needed for my schematic designs and component values. Thank you!

B1+ = 280V
B2+ = 800V

interstage driving 1st GM70 = 3:1
interstage driving 2nd GM70 = 1:1
 

Attachments

  • full.gif
    full.gif
    8.3 KB · Views: 1,078
Ex-Moderator R.I.P.
Joined 2005
being a 3 stage design, is there any sense in the GM70 driver

GM70 should be easy to drive, thus a 2stage design ought to be possible


maybe first stage and its interstage trafo could easily be replaced by a line driver trafo with gain
and being SE, the GM70 driver intertage could be coupled for gain as well
 
Administrator
Joined 2004
Paid Member
Where people get the idea that the GM70 is particularly easy to drive I'll never know, it's not really - at least if done properly.. I'm not a big fan of the quasi-Sakuma approach and recommend ditching the driver GM70 - I'm not sure what it brings you other than some theoretical distortion canceling. Better I think to make things as linear as possible on a stage by stage basis.

You can use a single stage driver based on such tubes as the D3A (the choice I made in my GM70 project), 7788 or C3G.. Choke, transformer or CCS coupling are all options.

The GM70 requires 3A @ 20V for filament heating, not to mention north of 100W plate dissipation in most cases, and I can't imagine using more than one of these per channel in an SE amp. Depending on operating conditions chosen, 20 - 25W is achievable with a single stage driver and the GM70 running in class A1.
 
Administrator
Joined 2004
Paid Member
Here you go: how GM-70 tubes were used in Russian amplifiers originally.

Hi Wavebourne,
Now that is what I call really complex.. :D Quite beyond anything I have ever contemplated. Wondering what it sounds like, have you heard one of these?

What sort of power output? It seems designed to deliver very high power.

I noticed a phase shift oscillator in there driving a small power amplifier, very odd. Is this a projector amplifier or something similar? Unfortunately not speaking any Russian and the considerable complexity of the schematic make it hard to decipher. There is both a lot of voltage gain, and the driver stages are robust to say the least.
 
Last edited:
Hi Wavebourne,
Now that is what I call really complex.. :D Quite beyond anything I have ever contemplated. Wondering what it sounds like, have you heard one of these?

What sort of power output, several hundred watts?

They were used on stadiums, for example. :D
Can you imagine a bunch of aluminum bucket horns on poles driven by such an amp? How well would they sound? :D

Output power was 600W. As you may see, GM-70 tubes were driven by 6L6 cathode followers that were driven by 6L6 push-pull stage.

I noticed a phase shift oscillator in there driving a small power amplifier, very odd. Is this a projector amplifier or something similar?

The amp had all measurement equipment inside.

Usually they were used in a reserve combination, when one was working, while other was fully serviced and ready for the duty.
 
Last edited:
Administrator
Joined 2004
Paid Member
Wow! :D

Certainly explains why the GM70 graphite plate version at least is so ubiquitous and inexpensive. I assume a lot of public venues and soccer stadiums had at least one of these back in the old USSR.

Super cool.. I bet those pa horns sounded none to good, and the amplifier is a bit big for anyone other than Dick Burwin to fit into his listening room.. Guess we'll never know.. :D

:cheers:
 
Last edited:
So these GM70 amps were for PA use.
Well, even the 300B was produced to serve in theatre amps when I remember well.
For driving the GM70 there are many possibilities of course.
GM70 driver might seem over the top (at least economy wise) but at the same time there is some elegance in this Sakuma style and, when done well, it might sound excellent.
D3a driver...hmmmm a bit weak in my opinion, OK for voltage amplification but otherwise maybe a bit weak to drive the GM70 grid with it's 15mA or so current.
Personally I would choose something like 2A3, 300B or 6L6 to drive the GM70 through a 1:1 interstage transformer; this gives headroom into A2 territory.
With for instance 5687 or 6H30 as voltage amplifier you have a three stage amplifier with around 1 VRMS of input sensitivity.

Pieter
 
That 600W thingy is peanuts :)

look at pg 148 and 149 of this pdf

Wavebourn: can you translate a bit of the text, so we know what is going on in the above amplifier?

It is about TY-5-3 amp. :D

Ok, take your popcorn.


Load in both cases is of choke type since resistive load requires higher anode voltage and generates more heat. Anode voltage of the driver stage is a sum of voltages of anode rectified and negative bias rectifier for output stage. Load of a bias rectifier by driver stage stabilizes bias voltage of an output stage".

The book is a good but sporadic collection of schematics and tube data, but author's comments are quite silly. ;)
 
Administrator
Joined 2004
Paid Member
So these GM70 amps were for PA use.
<snip>
D3a driver...hmmmm a bit weak in my opinion, OK for voltage amplification but otherwise maybe a bit weak to drive the GM70 grid with it's 15mA or so current.
Personally I would choose something like 2A3, 300B or 6L6 to drive the GM70 through a 1:1 interstage transformer; this gives headroom into A2 territory.
With for instance 5687 or 6H30 as voltage amplifier you have a three stage amplifier with around 1 VRMS of input sensitivity.

Pieter

Actually you will find a whole lot of SE amplifiers including many 300B designs with a less robust driver than the D3A.. I'm running them at 20mA, and they will swing >350Vpp into 50K (Choke or 1:1 transformer) which IMO ain't shabby at all. I'm running them very hot obviously to do this. As I am only interested in class A1 operation the available current is more than sufficient and the increased complexity and loss of transparency with the added driver stage is unwarranted in my application. (I've designed a lot of 3 stage amps over the years, and in some ways comparably well designed 2 stage amps have sometimes been slightly superior in resolution.) The amp in question is designed for a target output power of 20W and will be used with my Onken speaker system which is slightly more than 100dBspl/w/m efficient.

Incidentally WE was trying to build theater amps with SOTA performance for the time so the 300B was about as good a power tube as they knew how to make. The GM70 of course was also quite good, but the design goals may have been different, given apparently what it was often used for - that it works as well and sounds as good as it does might not have been quite as deliberate as in the case of the 300B. Wavebourne might have some more (accurate) thoughts on the subject.
 
Last edited:
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.