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Old 30th September 2008, 09:17 AM   #1
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Default GM70/GM70 parafeed SET?

Hi guys!

after reading about Sakuma's 845:845 and the parafeed adoption of il Monstro, I am tempted to build a GM70 driving GM70!
This is the bare schematic that i intend to build. But I have yet to do all the calculations.
Click the image to open in full size.
Basically, it is a parafeed, SET with interstage.
I wonder if it will work...
What do you guys think? Will it sound good?
Feel free to comment and give your suggestions!

- Louis
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Old 30th September 2008, 10:18 AM   #2
Gluca is offline Gluca  Italy
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Yes it will work (well I suppose the design and building will be properly executed).

A friend of mine recently built a GM70 into interstage transformer into parallel GM70's and he likes it. I have not listened to that amp yet but I know I like GM70; on the other hand I believe I would not like an amp with the same driver and output tube.

May I suggest you to use a 10Y/801A as driver? Possibly directly coupled.

Gianluca
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Old 11th October 2008, 02:27 PM   #3
noyan is offline noyan  Turkey
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Default Gm70-gm70

Hi,
I don`t see any reason for using one GM70 tube as a driver. Lots of heat and energy loss for nothing. GM70 is easy tube to drive, does not need big watts. Better if you use a 300B or 2A3 or 6B4G tube instead of GM70 as driver. And you have to use one input tube. As input tube, 6S45P will be good choice i believe. Interstage is very good choice for large sound, parafeed output is difficult, more complex and expensive i think. Input choke and anode choke for input tube are also very important for detailed sound (for upper freq. respond). If i am instead of you, i would do this amp with negative fixed bias and remove the cathode electrolytic capacitors & resistors for more dynamic sound.

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Old 11th October 2008, 06:45 PM   #4
JoshK is offline JoshK  Canada
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the secondary of the interstage makes a great place to use a bias supply for fixed bias. i'd opt for an 801 too probably. i however, like the parafeed idea. higher wattage se opt's are hard to make with widebandwidth so a parafeed opt would have some benefits.
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Old 11th October 2008, 06:58 PM   #5
Gluca is offline Gluca  Italy
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HEY! You need that potentiometer to be low impedance ... so what preamplifier are you going to use to drive it properly?
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Old 11th October 2008, 08:22 PM   #6
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I have heard from others that putting the capacitor on the low side of the output transformer (as pictured) sounds better. However, having the transformer primary directly on the plate will expose it to excessively high voltage. This is assuming you're using 900 volts or more as needed by this tube without positive grid bias.
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Old 12th October 2008, 09:06 AM   #7
noyan is offline noyan  Turkey
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Default GM70-GM70

Quote:
Originally posted by JoshK
the secondary of the interstage makes a great place to use a bias supply for fixed bias. i'd opt for an 801 too probably. i however, like the parafeed idea. higher wattage se opt's are hard to make with widebandwidth so a parafeed opt would have some benefits.
I aggree with you about some benefits. Besides, instead of one SE OPT need to use two (one parafeed+one opt) and It makes the cost higher, more complex, more heavy. Serial capacitor at the output also makes me concern. Quality of it is the another parameter for good sound. I think bi-amping is better solution. A GM70 for bass and lower mids, a 300B for upper mids and highs???

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Old 3rd November 2008, 06:39 AM   #8
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Click the image to open in full size.

Click the image to open in full size.

Attached is the new schematics for the GM70 SET and the PS for it.
I will be using RCA 83 rectifers for the PS.
Power trannies for B+ should be 2 pieces of 400V-0-400V @450mA each.
I will be using a C3g preamp w/ volume control to drive this amp.
What do you guys think about these now?

-Louis
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Old 3rd November 2008, 08:18 AM   #9
noyan is offline noyan  Turkey
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Default Gm70 Parafeed Set

Hi,
My comments are below:

-What is the reason of using a GM70 as driver? Gain will be not enough, too much heat will produce and big power (electricity) you will lost for nothing. A 300B or even 2A3-6B4G would work very fine. GM70 tube is not very hungry to put out 30 clean Watts in SET design. If it is so hungry like you think, then your external preamp can not drive the first tube!!! Your amp will be very heavy. External preamp will not be enough to drive your amplifier. Better if you use an input tube for enough gain and headroom.

-You are using DHT triodes for finest sound quality but beside using auto bias circuits. If i am instead of you, i would go with fixed bias for more dynamic sound. I don`t like to use caps between cathode and ground.

-You are using one source B+ for both tubes. You have also a choke trans between them and it is so good. But, in my opinion better way to provide power B+ from the different sourches for each tubes for no compromise solution.

-You should buy very high quality audio grade and expensive C3-C4 caps.

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Old 4th December 2008, 03:55 PM   #10
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Default Re: Gm70 Parafeed Set

thanks noyan for your suggestions!

Quote:
Originally posted by noyan
What is the reason of using a GM70 as driver?
actually, there is no special reason to use GM70 as driver. i just like the idea!
isn't it why people choose to DIY rather than buying off the shelves?


Quote:
Originally posted by noyan
Your amp will be very heavy. External preamp will not be enough to drive your amplifier.
i am planning to drive the input tube with a step-up trans. hopefully it works...
with the irons onboard, i'll be expecting the two channels to weigh more than 200kilos!


Quote:
Originally posted by noyan
If i am instead of you, i would go with fixed bias for more dynamic sound.
i prefer auto biasing for it's simplicity and i found the sound to be clearer and with more details. i think it is very subjective when it comes to sound... or gals...

Quote:
Originally posted by noyan
in my opinion better way to provide power B+ from the different sourches for each tubes for no compromise solution.
the current PS is already very heavy (and costly too!) i guess i will stick with the choke between B+ for the two stages...
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