Anyone used C3g to drive GM70?

[Ryssen]

Havent yet decided what tube to use to drive my GM70SE.I have loocket at the D3a
813 / GM70 amplifier with Bartolucci transformers
But I think the sensitivity is a little high 1vRMS for full audio signal.
My preamp/DAC gives about 3,5vp so I was thinking in those ranges.
The I thought of the C3g it has 40 in gain,but can it drive GM70 fully?
What would the input sensitivity be with C3g?
On GM70 I will use 950v/100mA -90v.
Write your thoughts.

 

[revintage]

Shouldn´t be a problem with D3a as it only gives you a marging of just over 3dB. You will probably need that.

 

[Pyka]

When connected in triode,D3a gain=80 .Use 1:1.5 interstage transformer.Total gain of D3a+interstage transformer >100.

 

[revintage]

Hey Ryssen,

Forgot to say sensitivity would be close to half ie. 2Vrms. So with 2,5V from your DAC it wouldn´t be much margin.

Also stick to 1:1 as this will probably give a sonically better result. You could then also go bifilar. Then check with Dave Slage for great irons!

 

[kevinkr]

Working on my own GM70 design with triode connected D3A driver via Lundahl 1:1 IT. I have the impression that my design may be marginal in terms of both drive and bandwidth (on the low end).. My custom Edcor power transformers arrived a couple of weeks ago, and when this stupid, brutally hot summer weather goes away I will start on the PSU chassis (the amps themselves are finished) and report back on the results.

The recommendation for Slagle iron is a good one. (I know Dave personally, and am familiar with the quality of his products - good stuff. )

Worst case I will get some other ITs made with higher primary inductance, mine have ~30H and I think it is not going to be enough.. Running at just below 20mA...

I'm also thinking about experimenting with the D3A in pentode connection with RC coupling. The current set up is not capable of real A2 anyway so I would loose little IMHO..

 

[revintage]

What OPT do you use?

Slagles stuff is great, I have used quite a few I have custom-ordered(but they all are). About pentode D3a I remember Michael Koster have found 10k/20mA/150V/150V to be linear.

 

[kevinkr]

What OPT do you use?

Slagles stuff is great, I have used quite a few I have custom-ordered(but they all are).

I have a pair of custom 7K:8 150mA, 40W transformers Jack Elliano at Electra-Print made for me last year. The previous OPTs Jack made for me for a headphone amp worked so much better than I expected that I went back for more..

 

[kevinkr]

Interesting comment on the operating point for pentode connected D3A, I was actually thinking of more or less those exact conditions with stiffly regulated screen of course.

 

[revintage]

Funny you mentioned Jack. Bought two pairs of custom transformers from him a while ago....

 

[Ryssen]

Forgot to say sensitivity would be close to half ie. 2Vrms. So with 2,5V from your DAC it wouldn´t be much margin.

Also stick to 1:1 as this will probably give a sonically better result.

I will go for the D3a then.And I will try capacistors first,maybe later when I can afford it, IT trafo,

 

[gaborbela]

Hello
I just purchased some D3a tubes in case the 3 stage I planed to build I do not like..
I purchased because a year a go it was $7/PC on Ebey now some people ask even $35 to.
I read the D3a drive very well the GM70 in both mode with interstage transformer or coupling caps.
Let us know your result! Thank you. In case you have a good schematic I would be interested..
Greetings Gabor

 

[Ryssen]

Thank you. In case you have a good schematic I would be interested..

No schema,other than I plan to use ledbias and a CCS on the D3a.

 

[gaborbela]

Hello Ryssen

Anything would help! But not urgent at all, when you done yours. Are you planing to use inter stage Xformer?
Specially at the amplifier stage. PS schema I have several..
How I wrote I just purchased 6PC D3a tubes these week, today I purchased 3PC 12GN7A Sylvania tubes.
Just to have these tubes until still available and not to pricy.
What type of OTP you use??

Greetings gabor

 

[revintage]

And I will try capacistors first,maybe later when I can afford it, IT trafo

Hey,
Why not go for the CCS/CVVS PCBs we are eagerly waiting for. You could then try both Low-, and Hi-Z(D3a 2k) and CVVS drive.

 

[djn]

I am planning on driving my GM70 with the 12HL7. I also have Rod's boards for the GM70 fillie heaters.

 

[gaborbela]

Hello

Can someone post a picture from the PC board..
You know the best solution is the P to P soldering. But if the board help certainly I can buy a pair.
I have to see what goes to the board.
djn thanks for your website, I will check it out
Thank to all of you
Look like these GM70 tube getting more & more popular among the DIY-ers..

Greetings Gabor

 

[revintage]

Can someone post a picture from the PC board..

Why not go to Rods thread about it. If it isn´t there, it´s probably because Rod doesn´t want to show the layout. Or ask Rod!

Have the boards for 813. The layout is, if I remember it right, shown in the manual.

So buy a pair, they are extremely cheap!

 

[gaborbela]

Hello

I do not write to see the PC board to make a copy. I want to know what is going on , for what tube was design etc.
I think a sketch would be better in these case..
Who is Rod, where I found his site?
I understand it is extremely cheap , my question can I use it.
I already purchased some parts in wain, what I would like to avoid in the future. I live on disability support in one word on very little money.

Greetings gabor

 

[Ryssen]

Hey,
Why not go for the CCS/CVVS PCBs we are eagerly waiting for. You could then try both Low-, and Hi-Z(D3a 2k) and CVVS drive.

If I know how to use it,yes I can try it.
Do I adjust the CCS as a regular CCS?

But how do I adjust P101/P102?
Is this suposted to be lika a SRPP?

 

[revintage]

Do I adjust the CCS as a regular CCS?

Do you mean:

"Do I adjust the VCCS as a regular CCS?"

In that case, no.