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FS-100 PF and GM70 and 845 amp DIYers

99.162.1.181

Posted on June 16, 2018 at 08:06:16
Tanker
Audiophile

Posts: 368
Joined: April 17, 2001
I have both FS-100 PF and FS-100 choke from Mike still sitting in the shelf. It's been 10 yrs since I have not touch the soldering iron. I think it's time for me to start my hobby journey again.

1. What are your recommending on operating point for GM70 or 845 and a single stage driver tube for SE? I already have preamp so gain is not a problem. I might interest in driving it into A2 also if A1 does not have enough juice to drive my Merlin speaker. But for now I think pure class A would be fine to drive it.
2. For power supply transformer currently I have 1400 VRMS @300mA. I plan to use full wave bridge rectifier. The plan is using 5R4WGB or 866A with choke input filter. Choke will be installed on the low/return side of the supply for sure. I know I need 3 separate filament tranny for the fullwave bridge. Any recommendation on other rectifier and Damper diode tubes besides 5R4 and 866A? Any problem you see in using 5R4? Any advice on designing such high voltage power supply would greatly appreciated.
3. I have long thought about the capacitors for power supply as well as output cap for the parafeed. 2000 V plus is not easy to find. So the plan is to use 1000V-1100 Volt cap rating connected in series. For output coupling I am able to find 6uF @2000V oil cap. I wanted to use quality cap but not really sure if they rated this high of the voltage. Any brand of output coupling cap do you recommend?
4. The plan is to have two separate mono blocks for left & right. In addition power supply will be separate from the main driver and output tube components. Therefore, the system will have 4 blocks. The reason is that I wanted to build and use this as a universal power supply (including negative bias voltage) for modification later to drive other configuration.
5. Thanks in advance for all the inputs and recommendation. Also understood Mike does not warranty on using his gears on such a high voltage.

 

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RE: FS-100 PF and GM70 and 845 amp DIYers, posted on June 20, 2018 at 13:15:25
Paul Joppa
Industry Professional

Posts: 7007
Location: Seattle, WA
Joined: April 23, 2001
The main issue is, as you noted, the high voltage. My advice is to make sure that everything - even the hookup wire! - must be rated for the voltage to which is it connected. It is, again as noted, very difficult to meet this requirement. Nevertheless, I am unwilling to deviate from it. Safety first!

 

RE: FS-100 PF and GM70 and 845 amp DIYers, posted on June 21, 2018 at 18:24:47
Tanker
Audiophile

Posts: 368
Joined: April 17, 2001
Paul,

Thanks for the advice. Yes I am well aware of the safety issue and the high voltage hook up wire. In reality I have used this same transformer for my 300B power supply which supplies up to 700V using the center tap. Hook up wires can handle 10KV and 16KV. I will definitely be more carefully when playing with 1KV power supply.

 

RE: FS-100 PF and GM70 and 845 amp DIYers, posted on June 25, 2018 at 14:45:32
gluca
Audiophile

Posts: 1102
Location: SomeWhere Islands
Joined: February 17, 2005
Hi there

what mA is your choke rated for? Mine was 100mA and I ran the GM70 at something like 90mA, PSU was full wave 866A and LCLC filter for power tube and full wave 866A LCLC for driver. I used stacked Film/Oil ASC motor run capacitor in the power supply and, if memory serves me right, a NOS 2uF/2kV sangamo paper/oil capacitor at the output. Take care of the filaments, go for a coleman regulator.

866A's are brutal, a damper diode in series might help in slowing down the initial transient.

That amp sounded great and with EIMACs was even better.

 

RE: FS-100 PF and GM70 and 845 amp DIYers, posted on June 25, 2018 at 20:08:07
Tanker
Audiophile

Posts: 368
Joined: April 17, 2001
Gluca,

Thanks for the info. My choke was also rated at 100mA. Do you run your B+ at 1KV @90mA? Do you remember what total capacitance is in your power supply design? and power output you can get out of the GM70 & FS-100PF? Thanks

 

RE: FS-100 PF and GM70 and 845 amp DIYers, posted on June 26, 2018 at 00:46:40
gluca
Audiophile

Posts: 1102
Location: SomeWhere Islands
Joined: February 17, 2005
B+ was somewhere around 900V and I had 100uF as total C in the PSU. I was not shooting at max power as I have very efficient horns, you'd better plan for A2 to have some extra juice but I guess 15W would be possible in A1

 

211 Works Well Also..., posted on July 22, 2018 at 19:15:11
Triode_Kingdom
Audiophile

Posts: 8246
Location: Texas
Joined: September 24, 2006
What does the "PF" stand for? And who said the OPTs aren't guaranteed at 1KV? Did Mike say that? I have a pair of FS-100 OPTs (not PF) residing in my 211 monos, and it's my understanding that's the type of service they were designed for. The PS provides about 1,050V and each output idles at 75-80mA. The driver is a choke loaded 6BL7 follower. Each amplifier outputs about 15W in Class A and can be driven to near the 30W power rating of the transformers in A2. Filaments are powered with ultrasonic heaters operating at roughly 60 kHz. Be warned, it's a major project putting all of this together. I'm still revising after more than 10 years. :)




--------------------------
Buy Chinese. Bury freedom.

 

RE: 211 Works Well Also..., posted on July 24, 2018 at 03:33:34
triode3
Manufacturer

Posts: 263
Location: Midwest
Joined: August 23, 2005
PF is ParaFeed.

I can not recall exactly, but about 15 years ago and Mike told me what the voltage rating on the wire of the transformers is/was as we were having an OPT discussion. Perhaps he will drop in and let us know the maximum voltage. With all of the high voltage projects around here, it must be at least 1KV.

 

RE: 211 Works Well Also..., posted on July 24, 2018 at 17:19:02
Paul Joppa
Industry Professional

Posts: 7007
Location: Seattle, WA
Joined: April 23, 2001
Here's a little background, while we wait for Mike to chime in.

The FS-100PF was a special parafeed version. The core is a slightly shorter stack than the original series feed version. There are some extra tricks to the construction. There was a matching plate choke.

With an 845 at maximum power, it would likely run at 1250v/80mA; with a grid bias of 195v plus choke DC losses, that would be around 1500v of B+ if you used cathode bias.

I remember that at the time Mike was concerned about very high voltages, though I no longer recall exactly how that played out. IIRC, there was some discussion on the forums about whether the traditional hi-pot test was adequate for safety/liability purposes in the more modern world. Here's an interesting white paper on testing, from a maker of the test gear:

http://www.hipot.com/docs/whitepapers/basic-facts.pdf

 

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