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Old 11th August 2012, 08:31 PM   #1
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Default gm70 filiment transformers

Hi all. I guess this topic has been done to death....

I'm trying to get an order out for the transformers for ccs regulated 20v/3a gm70 heaters using a ring of two using mje15030 power transisters.

i've been using 18v 100va into 22mf and a 2 x 6.8mh cmc. this works ok, gives the right voltage but i'd like a slower heat up and higher resistance supply.
i'm playing about on psud2 to find a secondary voltage that minus ripple voltage gives me 3-4 volts for thr ccs.
i'm getting 28v, this would be a lot of heat to dissipate. am i doing something wrong? people seem to recommend secondary voltages = regulated dc required.

enzo
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Old 11th August 2012, 08:53 PM   #2
MelB is offline MelB  Canada
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Here is what I am using. Works out to be incredibly efficient for a linear supply. I am going to try some schottky diodes to run a bit more voltage drop through the LT1083. However the snubber diodes I am currently using are really nice sounding and have reduced the transformer humm over the regular diodes I first had in there by quite a bit. I guess you get what you pay for.
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Old 11th August 2012, 10:39 PM   #3
djn is offline djn  United States
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Look at the Rod Coleman shy fully supply boards. He is in England too.
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Old 11th August 2012, 11:19 PM   #4
kevinkr is offline kevinkr  United States
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Rod's boards incorporate a discrete gyrator and CCS - seem to work well and don't hurt the sound. I've been using a set for almost a year in my GM70 amps.
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Old 12th August 2012, 12:19 AM   #5
AJT is offline AJT  Philippines
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Quote:
Originally Posted by enzoastro View Post
Hi all. I guess this topic has been done to death....

I'm trying to get an order out for the transformers for ccs regulated 20v/3a gm70 heaters using a ring of two using mje15030 power transisters.

i've been using 18v 100va into 22mf and a 2 x 6.8mh cmc. this works ok, gives the right voltage but i'd like a slower heat up and higher resistance supply.
i'm playing about on psud2 to find a secondary voltage that minus ripple voltage gives me 3-4 volts for thr ccs.
i'm getting 28v, this would be a lot of heat to dissipate. am i doing something wrong? people seem to recommend secondary voltages = regulated dc required.

enzo
depending on the size of your traffo, you may find that that 28volts dc raw is not that high....i suggest you build the circuit and test it with a load that will draw 3amps at 20 volts a 6.8ohm dummy load can be used to find out how that 28 volts will drop under load....
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Old 12th August 2012, 09:20 AM   #6
mogliaa is offline mogliaa  United Kingdom
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Hi Enzo,
What is the target raw DC voltage you're aiming at? If you are not familiarised with filament supplies and regulators, I think that given a transmitting tube is a challenge itself, Id go for Rod Coleman's board. They are superb and Rod will give also good examples and raw power supply design guidelines as well.

The best transformers for filament supplies in my view is JMS (Peterborough based). They will wind your transformer to exactly the secondary voltage you need for no extra charge. The quality is excellent.
I have just received my filament transformers for 26/4p1L, 46 and 4-65a stages of my SE amp. I can upload some pictures if you like.

For raw PSU design, use PSUD2: unbeatable. You can check your ripple current there to see how many C1 in parallel you will need based on the max ripple current that the chosen capacitor can withstand. Also you can play around with CLC or CRC filtering stages to see how will respond to dynamics and final output ripple given load.

Hope this helps
Ale
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Old 12th August 2012, 10:40 AM   #7
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Thanks very much for the replys!

I've heard nothing but good things about the colman boards, but i'm a bit of a miser ;-) and would like the finished amp to be as diy as possible. I've read pretty much all of the 'new filiment supply' thread and found it really helpful.
A 24v 100va transformer seems to do the job with a lt1083 and 0.42r sense resistor, but when i put an adjustable voltage reg on the supply to share some of the voltage drop heat and reduce ripple i only get the desired voltage with the v reg passing 27+v and if i drop that to ~23v the filiment drops to ~12v. same thing happens with the ring of two circuit. i'm presuming this is due to ripple ?
The mje15030 is a to220 package and

24v x 1.4 =33.6 -~2v diode drop and 1.25 for sense resistor =30.25 which means the to220 package has 30 odd watts a channel to lose.
Not suprisingly i've chosen to put the filiment supply into a seperate chassis!
help!

enzo
p.s great website Mogliaa!

Last edited by enzoastro; 12th August 2012 at 10:55 AM.
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Old 12th August 2012, 01:00 PM   #8
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Thank you for all the recommendations, folks.

Actually, the MJE150xx will give fairly bad performance at 3A, because it needs quite a few volts Vce to achieve reasonable Hfe. This is intrinsic to the transistor, and can't be helped. The required ripple rejection will be hard/impossible to meet, also, because of the amount of base current needed.

Also, with the GM70, you risk violating the SOA of the CCS transistor at cold, or worse at a warm-start, where the filament resistance is much lower, and the transistor sees most of the supply voltage, and all the the current - simultaneously.

The latest version of the GM-70 Regulators buffers both sides of the filament, gives carefully developed frequency-compensation, temperature-compensation of the filament current, and still only needs 4.5V of headroom. Not to mention the best possible sound, especially compared to LT108x regulators. And even a miser would struggle to complain of the cost of these DIY self-assembly kits!
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Old 12th August 2012, 01:24 PM   #9
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Thanks for your helpful reply.
As i think is obvious my knowlege of semiconductors is limited to morgan jones book, and very little of that sticks!

The LT1083 i used actually seemed to handle the cold start ok, as long as it had a decent heatsink it never shut off.

Perhaps i should go with the flow and ask Rod Coleman for some boards, just don't think i'd learn anything that way. I'm not in a hurry to get the amplifier in to its 'finished' format, its been ten years since i started building it, and it started off as a pp sv811-10a! happy for it to be a long and winding road:-)

enzo
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Old 12th August 2012, 10:25 PM   #10
djn is offline djn  United States
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I've been poking away at a GM70 SET for three years now. I am already to wire it up, but life gets in the way. I have learned SOOO much in the process thanks to the good folks here and on AudioKarma. I learn best by talking things out and trial and error so books never done me no good.

Enjoy the road as long as it takes you.
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