2 Stage GM-70 amp in A2... possible?

[quikie22]

Thinking of building a GM-70 amplifier in Single Ended.

Was wondering if a 2 stage amplifier would be able to achieve A2. Any ideas?

If yes, what topology and what driver tube would be suitable.

 

[cotdt]

Why A2? Just turn up the voltage.

D3A -> GM70

 

[quikie22]

How does turning up the voltage drive it into A2?

 

[cotdt]

That's not what I said. If you want more power, simply increase the voltage in A1. In A2, the load from the GM70 is really low maybe 1k, so you'll need a 300B to drive it, which means 3 stages. Even then, I've never tried it, it might not even work.

 

[quikie22]

Meaning not quite possible to do A2 with 2 stages. ok.

 

[cotdt]

I never said it was not possible. I only said that it was pointless, because you can simply give it more voltage in A1.

You can try a D3A -> MOSFET -> GM70, but it may or may not work.

Why are you so obsessed with A2? To get worse sound quality while gaining nothing?

 

[quikie22]

Thanks for the replies.

As for the assertion that I am obsessed with A2, please refer to my first post. I just wanted to know if it could be done. The answers thus far seem to point to the fact that my original hopes may not be quite possible.

Kind regards.

 

[mwiebe]

There is a thread somewhere here by Andrea Ciuffoli on two-stage 845 SET which will get you partly there. Depending on how much grid current you want to push.
Matt

845 Amplifier with only two stages

 

[Palustris]

"Was wondering if a 2 stage amplifier would be able to achieve A2."

What input sensitivity are you aiming for? If you have a preamp capable of ten volts output, maybe; otherwise probably not. But you need to start by defining how much voltage you need to drive the GM70 into A2 and then work backwards to the input. In other words if you need 150VRMS drive (to pick a random number!), how will you get from your input voltage (that you haven't defined) to 150VRMS in one stage?

 

[CijTele52]

It's very difficult. To me it's almost impossible. Driving GM70 to A2 requires low impedance driver that can make 300Vpp. From my experience, tubes with Rp > 1.5K cannot push power tube's grid positive. I once tried E810F/7788 and found it's having hard time to push 211 to A2. GM70 is even more difficult, you can see the picture.

 

[MrCurwen]

Sure it's possible!

Input stage: 6SL7 with gyrator load.

Then a MOSFET source follower (perhaps loaded with a CCS) driving the grid. Follower sets the grid bias for the GM70. Downside is you'll need a negative supply, but that's not too bad.

 

[Palustris]

That's three stages.

 

[djn]

I am doing a 12GM7A into a GM70. Should work well. I don't know what A1 and A2 really means, but my compression drivers are 113db efficient and the GM70 should put out about 12WPC so that will work just right.

 

[joda]

yes, possible
regards

 

[quikie22]

Interesting circuit concept joda. I guess if I have a high gain preamp, I could skip the first step-up transformer....

 

[JZatopa]

Here is a 2 stage GM70 amplifier that I think is a wonderful design by Greenvalve. I have asked him to elaborate on it but unfortunately he currently isn't able to spend much time discussing it. This amp exemplifies what I think the future of audio needs to start looking like. That is, using every modern technology and technique to get the best sound. We need to stop holding onto tradition for the sake of tradition and start looking at using the best design for the job. If I were to try to undertake such an amp the only thing I might do different would be a powerdrive, ala tubelab, instead of a choke; using a power jfet. Hope it helps you out.

Greenvalves - The Maxium GM70

 

[cotdt]

If you use a preamp, then that is 3 stages.

 

[flatheadmurre]

Here is a 2 stage GM70 amplifier that I think is a wonderful design by Greenvalve. I have asked him to elaborate on it but unfortunately he currently isn't able to spend much time discussing it. This amp exemplifies what I think the future of audio needs to start looking like. That is, using every modern technology and technique to get the best sound. We need to stop holding onto tradition for the sake of tradition and start looking at using the best design for the job. If I were to try to undertake such an amp the only thing I might do different would be a powerdrive, ala tubelab, instead of a choke; using a power jfet. Hope it helps you out.

Greenvalves - The Maxium GM70

With that amount of SMPS i hope they´re high quality.

If not top notch and properly implemented HF noise and transmitting tubes can end up as a gigant oscillator.

That´s perhaps the main reason for using non effiecent EI-core transformers, their good rejection and bad coupling of HF noise.

 

[quikie22]

thanks DJnubz

wow... a high gain transmitter driving another transmitter.... that sounds like an idea....
any other candidates for the driver tube apart from the 826? the 826 curves show it to be less than linear....

 

[Vincent77]

Here is a 2 stage GM70 amplifier that I think is a wonderful design by Greenvalve.

That design produces about 10W in A1... And the stacked SMPSs are a bad idea, IMHO, as the noise also adds up.


As for the JAC music schematic posted by joda above, it's a joke meant to sell a ton of expensive iron to naive people.