GM-70 design approach

[dmac23]

I have been brainstorming a gm 70 design but have been getting stumped in a few areas. This is my first attempt at a high voltage design so I am unsure about a few things regarding the power supply design.

Filter chokes that have a dcwv of 1kV+ seem hard to come by. How do some people go about obtaining these chokes? Is there a way to use a lower test choke and decrease the potential that it sees?

I am planning on making it a SE parallel design using the hammond 1642SE 5K 75W OT. I am thinking of using a 6sn7 type as an input tube followed by an 807 to drive the gm-70’s. I have yet to start drawing any schematics on paper, but I wanted to run this past a few of you and see if anything really stands out as a large problem with this design approach. After looking at the 807s datasheet, do believe it will drive the gm 70 well. Is this a correct assumption? Thanks!

[kevinkr]

I placed the chokes in the CLCLC plate supply to the GM70 on the negative side of the supply in my amps. See old ARRL Radio Amateur Hand Books for useful high voltage supply design details. Since the only the ripple voltage and DC IR drop appear across the choke there is not much stress on the insulation and standard good quality chokes can be used.

I would strongly suggest you check out Electra-Print for your OPTs, they will not be unreasonably expensive and they are quite good from my direct experience. (An extremely expensive GM70 high end amp uses Electra-Print OPTs) I had jack design a pair with 7K primary rated for 150mA and 40W, with a single copper 8 ohm secondary.. In class A1 at 1kV/120mA I am getting a little over 20W, driver stage is an IT coupled triode connected D3A running at 20mA.. (Note: 25W in A1 with 5K primary is possible, I went for higher DF and lower distortion)

This amplifier replaced a 300B SE amplifier I designed 13yrs ago, the goal was to achieve at least 5dB greater output than that amplifier, with considerably better linearity at my usual listening levels. I choose to go with a two stage amplifier because I am finding in my old age that fewer stages sound better. This amp clobbers the old one in terms of resolution and dynamic behavior with my high efficiency 3 way speaker system. (Onken/Ultraflex based)

[cotdt]

Hi kevin, how do you get enough gain with only 2 stages? How much voltage can the D3a swing?

[kevinkr]

The D3A in triode has a measured mu of 80 and can swing absolutely obscene amounts of voltage as long as the driven grid current is not too many mA.. I need about 180Vpp swing to drive the GM70 to full output in A1.. Open circuit swing is in excess of 300Vpp at a couple of % thd on a 200V rail.. (Varies somewhat from sample to sample)

The D3A is coupled to the output stage with a 1:1 IT, gain is more than sufficient for the target output power. The driver stage was not intended to furnish appreciable grid current but is capable of furnishing at least 4mA so some very limited operation into A2 is possible. Given an speaker efficiency of over 100dB and >20W per channel it gets loud.

[cotdt]

Are you using D3A as pentode or triode?

Nvm: you said triode... but have you tried pentode?

[kevinkr]

Originally Posted by cotdt

Are you using D3A as pentode or triode?

Nvm: you said triode... but have you tried pentode?

Pentode is not a good mix with IT, and I am looking for the low source impedance I get with triode connection and no feedback local or otherwise. I prefer the distortion spectrum in triode..

I do have an SE amp with 6J7 pentode driver into a 6V6, (triode or tetrode mode) it is not one of my favorite amps.

[widowmaker]

Originally Posted by dmac23

... After looking at the 807s datasheet, do believe it will drive the gm 70 well. Is this a correct assumption? Thanks!

Believe me, one of best drivers for GM-70 is 807 (in triode connection).
Just try to find russian analog G-807 (Г-807) from ebay for example.
For input tube you can use russian 6N1P-EV (6Н1П-ЕВ).
Very good double triode, long life military production, works very well at anode current about 1mA.
Ua about 6mA for 807 is good enough.
(now somebody will start with comments "aaah, at 6mA linearity is not good, blah blah blah...")

An advice for chokes...
Just buy such chinese winding machine (again ebay) and use reel and core from old trafos,
you just have to find the proper size.
I'm using the same chinese winding sh*t even to wind output transformers for GM-70

[dmac23]

Thanks everyone for the advice, I will look at some old radio books and get a clearer image on the HV supplies.

I looked up the russian 807 on ebay and could not believe the low price on these! How does the sound compare to Sylvania or RCA?

Also no one commented on the thought of running them in parallel to get more power, are there any complications that may occur by doing this?

[widowmaker]

Originally Posted by dmac23

Thanks everyone for the advice, I will look at some old radio books and get a clearer image on the HV supplies.

I looked up the russian 807 on ebay and could not believe the low price on these! How does the sound compare to Sylvania or RCA?

Also no one commented on the thought of running them in parallel to get more power, are there any complications that may occur by doing this?

First what to parallel? Driver or output tube? It's not very clear...

Second... about russian military production.
One of most important advantages is very high vacuum achieved in their
military production (especially in 60's-70's) due to very high military requirements
(don't forget, in US all of military production came from private companies and low production costs was the most important thing there).
In USSR the most important thing was quality of production, not costs.
All military factories was owned by their government (communist party).
Current russian production is not as good as old one.
A very good example is Sovtek production of 2A3 and 300B (also known as Electro Harmonix).

[dmac23]

Very sorry! I was really tired last night and obviously I did not clarify. I was referring to running the GM-70 outputs in parallel.

Very interesting what you said about the Russian production....I have experience with some of the old USSR teflon caps and you sure are right about them not cutting corners

Looks like I will have to order some of these USSR 807s. I have noticed that there are brown and black base versions. Are there any differences between these? Maybe production years?