I keep getting requests about SE amp designs using the 6SF5 driver. Here is what I have come up with for the 6SF5 driving the GM-70. With only one stage you can drive the GM-70 to full power and get 18W out of this lovely transmitting valve:
I don’t think the circuit needs explanation. The driver is described on the 6SF5 blog post. The grid of the GM-70 is properly driven by a source follower which also provides the DC bias using a Rod Coleman regulator (or any other regulator) The output stage relies on the scary 1.1kV HT and also a very good quality 10K OPT
This is not for beginners, so I g
et somehow scared when people ask me how to calculate the loadline of this circuit!
The Rod Coleman regulator is mandatory as no AC heating is viable here. Rod can help you with the LT supply design.
Hope this helps and no one dies in the process of building this amp! You’ve been warned 🙂
You are not limited to the 6SF5 of course, you can use many other great triode (or pentode) drivers here. You just need to have enough headroom and gain to deliver effectively 200-220Vpp. Not many drivers can do this well at low distortion. If you don’t want to pursue the DHT route (e.g. an 801a/VT-25 with a 1:8 SUT) you can look into D3a, E810F as well as many others I have blogged around here:
You can use any of these drivers as shown below. Instead of SiC diode bias if you prefer a good quality resistor (un-bypassed) will work perfectly well in the hybrid mu-follower topology:
Ali
You have 100mA through the 2k2 resistor giving 22W dissipation, but you used a 7W resistor. Am I missing something with Rod Coleman’s DC filament setup?
Hi Anthony. The resistor only gets into operation when the fuse blows – a failure of the bias supply for example. In that case it will take the gm70 to nearly cutoff at about -200v bias and 10mA anode current. I forgot to update the dissipation of that resistor which should be 25W aluminium clad type
I have it in mind to do a build using this Valve, but HT PSU Trafo is the problem here I think & as You say he voltage, have You a suggested PS for this Ale?
HI Alex, in my experience the source follower by mosfet desnaturalize the warm sound of the tubes to SS amp. Im my last amp I use the 2a3 to drive the 211 tube, after listen do not need more experiments is the best by far. about GM70 too I don´t understand why this tube when you have Psvane 211t on 150Eur pair and the sound is very natural, with a gain of 10 the bias is very easy to move -60v.
Hi, well that is not my experience (as well as others). Mosfets when used properly can perform brilliantly as CCS, gyrators and followers.
gM70 is a great piece of engineering and highly reliable as was built to military standards (OTK). They sound great and have plenty of them. When I bought them were far cheaper than 211. Problem with 211 is that is too popular hence the high price of the NOS pieces. The Chinese have been very inconsistent in quality and reliability. Hence I avoid them.
There are better ones in my view like 813 and GK-71, etc
Everyone have their own personal tastes and views., so won’t enter in a debate on that
If you read the post, I’ve got the ask to help with this circuit, which I did
Cheers
Ale
Hi Ale ,I totally agree CCS and gyrator is a big step to increase the sound quality, but about SS followers.. I do not like them. If the GM70 it’s a great piece of engineering I agree with you, but not about the sound again are tastes :-), I have sometimes similar discussion about the Russian capacitors, k40 FT3 .., yes, they are good but not better than Duelund or Jupiter, I think that when you have invested hundreds hours to make your own amplifier or preamp, 200 € / $ more or less is not significant.
About the filaments, I need to try the Rod Collemant regulator, now I use 10k uf 5mH 10k uf with good result
In all cases, I am very greatfull with you for all your contributions. You do an excellent job of forced reading
I also build 211/845/GM70 amplifier. My GM70 used 6B4G/Interstage driving GM70, it sounded great, until the next amp was built, 845/211/Interstage driving GM70, it is just fantastic. GM70 is a great tube, I will take over any 211/845 tubes.
Why 10K OT? Isn’t that a little bit too high? I think 7K is optimal for power and THD is also good.
Driver stage for my GM-70 is 3A5 –> 4P1L –> IT –> GM-70. Over 30W Pout.
Regards
Yes, perhaps 7-8K is better balance. Good thing of GM70 is that can be ran at 800V and sound good. I’d be keen to have 2 stages and avoid 3. However, your 3A5/4P1L is a great choice!
Yes, I’m very happy with 3A5/4P1L combination. I have all the parts to even to higher with B+. I’m aiming at ~1250V 80mA. 🙂
10K load will give lower distortion and lower power. You dont have to stick with 4XRp formula for output transformer.
6SF5 is a very interesting tube, but very rare. I would like to build this amp, but with easy to buy tube. Can you propose a new schematic?
Kind regards, Adalbertus
Already replied same question at DIYaudio thread. Are you Wojtek?
Yes, and after your answer I have bought 6e6p-dr to drive GM-70. But first I have to finish your 300b line stage 🙂 I am curious how Source Follower works? I really like tube distortions and I don’t want to lose it.
Hi Ale,
I am a beginner and I don’t understand how you set GM70 90mA current. Is a M1 ammeter? What is a function of D3/D4 diodes?
Regards, muzzy
Hi Muzzy,
As a beginner, this is definitely not the amp to attempt building. There are simpler (and safer) amps. This amp has extremely high voltage and you shouldn’t look into building this whilst learning on DIY valve audio.
This amp works with fixed bias, Reg1 sets the bias voltage through the source follower IN and OUT. Thus fixing the grid voltage of V2. M1 simply is a current meter and D3 and D4 are reversed diodes to protect the meter from excesive current due to an unexpected voltage increase (i.e. spike) if anything goes wrong.
I recommend you start reading “Building Valve Amplifiers” from Morgan Jones, a sterling book.
Cheers
Ale
Ale, you power the source follower from 0V because you want to avoid A2 operation of the GM-70 valve?
Yes, for simplicity of power supply. Should you want to operate in A2, you need to add A2 headroom plus 20V at least for best performance of the MOSFET follower
I was asking because I use your source follower for my OTL circlotrone with 6c33c and as I know only DHTs work in A2. I am expecting in the mail two sylvania 833a. That will be my next big project, SE in A2 with your gyrators and other goodies you have posted on this incredible blog. Thank you for the great info and insights in this beautiful world of valves!
What is the input sensitivity of this GM70 amplifier?