• 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.

Best GM70 version suited for audio??

Status
Not open for further replies.
Jump to Latest
#21
Hi Gustavo, the 2A3 has a U (gain) of 4 or so and the 300B is not far behind that. And the GM70 has a U of 5, so 4x5=20. That is not much gain. The 12HL7 I am using has a U of about 100 so that is 100x5=500. From all I've read and talked to folks, a gain of 20 will not work. The put that to the real world, I divide my gain of 500 by 33 (into my output transformers (this is just an approximation) and get a watt per channel of 15wpc. Using your gain of 20, it would be .6 WPC. Of course, I don't know what I am doing....just learning myself. Maybe some one else can jump in. Cheers.
 
Last edited:
#22
djn said:
Hi Gustavo, the 2A3 has a U (gain) of 4 or so and the 300B is not far behind that. And the GM70 has a U of 5, so 4x5=20. That is not much gain. The 12HL7 I am using has a U of about 100 so that is 100x5=500. From all I've read and talked to folks, a gain of 20 will not work. The put that to the real world, I divide my gain of 500 by 33 (into my output transformers (this is just an approximation) and get a watt per channel of 15wpc. Using your gain of 20, it would be .6 WPC. Of course, I don't know what I am doing....just learning myself. Maybe some one else can jump in. Cheers.
Click to expand...
Hello DJN,
I assume you are running a sole GM70 drive by a 12HL7, so seems this 12HL7 Pentode is a powerful tube, and yet you have only 15W.
Certainly your GM70 tube will last a century, this is very good however.
Do it is possoble to get 25W output with this 12HL7??
I see the GlowMaster SEP 70W per channel at: SACThailand
they use a ''NOS 6N1P dual triode to input stage'' and a ''NOS 6BL7 large size medium mu twin triode in special anode choke loaded circuit can meet such demand of GM-70'' in the driver stage.
A poster in other Forum said the GlowMaster price is around 5Kusd, IF this is true it is a low price indeed.
I wonder what is an ''CHOKE LOADED CIRCUIT'' after all?? I see this terminology everyday...
Regards, Gustavo
 
Last edited:
#23
From what I know, choke loaded is when there is a choke between the B+ and plate instead of a resistor. I am still a little confused on this point, but there is a difference between what gain does and what higher B+ does. I could get more wpc (like 25wpc) if my B+ was 1200v. The output of that tube is 125 watts so you take 1/5 of that for a hifi amp as max WPC, but that is at max B+.....in this case 1200v. I am running a B+ of 840v so it reduces from there. Now here is were the confusion sets in: The SWAG forumula on gain x gain / by the ratio of the OTs work out well. GO figure.
 
#24
djn said:
From what I know, choke loaded is when there is a choke between the B+ and plate instead of a resistor. I am still a little confused on this point, but there is a difference between what gain does and what higher B+ does. I could get more wpc (like 25wpc) if my B+ was 1200v. The output of that tube is 125 watts so you take 1/5 of that for a hifi amp as max WPC, but that is at max B+.....in this case 1200v. I am running a B+ of 840v so it reduces from there. Now here is were the confusion sets in: The SWAG forumula on gain x gain / by the ratio of the OTs work out well. GO figure.
Click to expand...
Do your amp use a Choke??
Wonder IF physically this part ''CHOKE'' is a transformer or an Voltage Double Circuit?
 
#25
I have two chokes in my Power supply. But none in the signal path. Here is a pix of my power supply and a pic of the signal path schematic I am using. I still have to mess with the voltages a bit but it is basically there.

Post a couple pix of the choke you are talking about. It is most likely a two legged choke as I've never heard of a choke and PT in the same can. Cheers.
 

Attachments

  • Final with 6D22S.JPG
    Final with 6D22S.JPG
    235.1 KB · Views: 580
  • New Image.JPG
    New Image.JPG
    232.8 KB · Views: 571
#27
Hi Gustavo, hi all,

I'm on holiday, not able to access internet very often.

RC coupling is resistor-capacitor coupling, where you have a resistor from B+ to the plate of the input tube, then a capacitor from that junction going to the next stage. If you change the resistor into a special 'plate choke' with very high inductance, then you get a 'choke-loaded' design which can have higher gain.

I don't know much about the circlotron, but I don't think it's the easiest topology for a first tube project. SE with cathode bias is by far the easiest stage to build -- it is automatically stable without any adjustments or fiddling.

If you use high-efficiency speakers, one GM-70 will give you enough power to shake the walls! My best advice I can give is, to keep it simple, but finish the project with success, rather than having a very complicated project which never gets finished (I've ended up in this situation too often! 🙂 )

Best regards,
Kenneth
 
Status
Not open for further replies.
Top Bottom