Winter is coming... Time to start GM70 SE parafeed amp - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 21st September 2013, 07:54 AM   #1
diyAudio Member
 
yagoolar's Avatar
 
Join Date: Oct 2004
Location: Mazowieckie, Jozefoslaw
Send a message via ICQ to yagoolar Send a message via Skype™ to yagoolar
Lightbulb Winter is coming... Time to start GM70 SE parafeed amp

I decided to make GM70 SE amp several years ago. I have been reading about such projects, drawing schematics, collecting tubes, chokes, caps, cables and looking for design inspiritation since then Every end of the summer I said to myself: "Now is the time to start", but I quit then..
Last week I spoke to my colleague and he asked if I could make an amp for him. "Yes, sure, why not. Winter is coming, rains start pouring down so I will have more time to spend".
Am I afraid? A bit. I feel respect for HV because 500V is the highest voltage used in an amp so far.

Project specification (assumptions, limitations, requirements will be written next).
  Reply With Quote
Old 21st September 2013, 07:01 PM   #2
diyAudio Member
 
Palustris's Avatar
 
Join Date: Aug 2006
Location: Cape Cod
I have had a 1kV 845 amp on the drawing board for almost a decade with most of the parts bought. Am I afraid? A bit....
  Reply With Quote
Old 21st September 2013, 07:11 PM   #3
kevinkr is offline kevinkr  United States
diyAudio Moderator
 
kevinkr's Avatar
 
Join Date: Sep 2004
Location: Boston, Massachusetts
Blog Entries: 6
When I was building my GM70 amps I went out on a limb and tried a number of things like IT drive and a very odd single stage driver built around a triode connected D3A. The two amplifier chassis sat 90% complete for a year while I waffled about whether I should pour more money into this ill-advised project.

Fear of high voltages, and the potential disappointment of a design that failed to deliver were part of the issue.

I had originally planned on complex tube based regulated supplies and then decided to go with a very simple solid state bridge rectified CLCLC based HV supply. Bias and filament supplies followed. I completed the supplies in a matter of a week or so once I decided to proceed. (Had to wait for some custom power transformers from Edcor.)

When I finished the build out, did the preliminary checks, and fired the amps up for the first time I could have kicked myself in the head. They were clearly better than anything I had listened to up to that point, and a few minor tweaks were all that were required to address some minor issues.

They are now the only amps in the listening room (rest sold) and I certainly have no regrets having built them, and they have proved extremely reliable and unfinicky as well.

Get going guys!
__________________
"Extraordinary claims require extraordinary evidence." - Carl Sagan
  Reply With Quote
Old 21st September 2013, 08:29 PM   #4
diyAudio Member
 
Palustris's Avatar
 
Join Date: Aug 2006
Location: Cape Cod
Thanks Kevin!

I will get to it after I finish another couple of projects that I have started. I realize now that 845 tubes are out of my budget and that the GM70 will work perfectly well. Below is the chassis and power supply I will use with the Magnequest FS 100 output transformers. Although I will have to rewire it and update the PS caps, the Ballantyne engineers have done much of the critical power supply design and safety engineering already. A Front Panel Express plate (or plates) will cover the original top.
Attached Images
File Type: jpg Ballantyne_amp2.jpg (131.9 KB, 1079 views)
  Reply With Quote
Old 21st September 2013, 08:54 PM   #5
kevinkr is offline kevinkr  United States
diyAudio Moderator
 
kevinkr's Avatar
 
Join Date: Sep 2004
Location: Boston, Massachusetts
Blog Entries: 6
The GM70 may well prove to be more reliable, I have no idea what current 845 production quality is like. So far I have only succeeded in wearing them out, no catastrophic behavior.

I will say I weed out the ones with bubbles in the glass. I may just be being ridiculous, but I do not know anything about glass and its integrity considerations. I figure 125W of plate dissipation and 60W filaments means a lot of stress. At one point I measured the glass temp and as I recall it was a pretty balmy 175C. (!) So I figure a breach through a bubble is possible and catastrophic. I've seen at least two tubes out of the 16 or so I have purchased so far with this issue. I have both copper plates and graphites, and lately I think I have come to prefer the graphite version overall. Never seen a bubble in the envelope of a copper plate GM70 but I only have 3 pairs.
__________________
"Extraordinary claims require extraordinary evidence." - Carl Sagan
  Reply With Quote
Old 6th October 2013, 08:55 PM   #6
diyAudio Member
 
yagoolar's Avatar
 
Join Date: Oct 2004
Location: Mazowieckie, Jozefoslaw
Send a message via ICQ to yagoolar Send a message via Skype™ to yagoolar
Project specs (1st approach):
1. Technical
* single ended parallel feed output stage GM70
* 2 stages
* Si devices allowed e.g. as CCS's or buffers
* should drive a pair of ELACs FS 207.2 JET in 70 m2 room
* monoblocs with a remote volume knob
2. Visual
* clean - just a flat surface and one GM70 tube, caps & xformers hidden
* light and thin
* modern material used, e.g. Ceramic glass
* minimal - look at Stixx's designs which inspire me
3. Functional
* 3 sources: CD, tuner and a flac player
* built-in display (vol/source) lit while operational, finger touch or proximity activated
* standby timer
  Reply With Quote
Old 9th October 2013, 07:36 PM   #7
diyAudio Member
 
Join Date: Feb 2010
Quote:
Originally Posted by kevinkr View Post
The GM70 may well prove to be more reliable, I have no idea what current 845 production quality is like...
I've built SET's off of 2A3, GU-50 (triode-strapped), 6C33C, and GM-70 (among others), and I'll tell you this: GM-70 have been among the most reliable tubes I've ever used. I was originally going to make a dual-socketed GM-70 amp capable of running 845's as well, but after playing with the Shuguang 845's, I decided not to bother. Not that the 845's were bad, but the GM-70's trounced 'em in terms of linearity, sturdiness, and cost. Can't say enough about 'em.
Quote:
Originally Posted by kevinkr View Post
...At one point I measured the glass temp and as I recall it was a pretty balmy 175C. (!)
Don't worry. I've had a couple 6C33C's that I was suspicious of, but they held up (and are holding up) just fine...and GM-70's run cool compared to 6C33C's (God, those things are hot).
  Reply With Quote
Old 9th October 2013, 08:30 PM   #8
kevinkr is offline kevinkr  United States
diyAudio Moderator
 
kevinkr's Avatar
 
Join Date: Sep 2004
Location: Boston, Massachusetts
Blog Entries: 6
Overall I am very happy with my decision to build mono blocks based on the GM70, it has proven quite reliable, and provides excellent performance and consistency. My second set of tubes are not far now from the 1K hour mark and have not been a cause for concern.

My concern about envelope temperature only relates to the few I have with inclusions (air bubbles). I suspect the glass is thick enough, but....
__________________
"Extraordinary claims require extraordinary evidence." - Carl Sagan
  Reply With Quote
Old 9th October 2013, 11:53 PM   #9
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
Yes, when you start going much about 500V, many things change- ask yourself why sweep tubes either use plate caps or have isolated plate pins. Arcing and corona become serious issues as any ham who has built high power linears will tell you.

The wrist strap will not help you- that's useful for static, not anything with current.
__________________
You might be screaming "No, no, no" and all they hear is "Who wants cake?" Let me tell you something: They all do. They all want cake.- Wilford Brimley
  Reply With Quote
Old 10th October 2013, 12:49 AM   #10
kevinkr is offline kevinkr  United States
diyAudio Moderator
 
kevinkr's Avatar
 
Join Date: Sep 2004
Location: Boston, Massachusetts
Blog Entries: 6
I have had a few issues with corona, have not had arcing issues.

NOS GM70 I have purchased usually rustle and crackle around the base when first installed. Quite disconcerting the first time you encounter this. A few hours of use usually solves this, sometimes (rarely) a good scrub with alcohol and a toothbrush helps.

All wiring in my amps carrying a kV or more is double insulated. (Note at full power the output transformer primary sees 1kVdc and as much 1kVpp in class A1 operation, and was designed for these operating voltages - well for A2 actually.)

Note that I have NEVER worked on these amps with 1kV applied to the output stage. Covers on and all measurements done externally. An accident is so permanent and I am not willing to give myself the benefit of the doubt.

When I was younger I worked on ham gear with some pretty high voltages (not this high) and did not worry about it.

Other designs utilized raw supplies of up to 700V, even there I saw quite a difference as compared to what I see at 1kV.
__________________
"Extraordinary claims require extraordinary evidence." - Carl Sagan
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
FINALLY I can start building my GM70 djn Tubes / Valves 47 5th November 2013 06:01 AM
GM70 SE Amplifier gabrielbecheanu Tubes / Valves 35 18th July 2011 04:01 AM
Well, it is time to start thinking about winter projects. GM70 djn Tubes / Valves 14 17th August 2010 08:49 AM
GM70/GM70 parafeed SET? louischia Tubes / Valves 10 4th December 2008 03:12 PM
Soft start & Dead time for TL494 ifrythings Power Supplies 9 18th May 2008 08:31 PM


New To Site? Need Help?

All times are GMT. The time now is 10:27 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2