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gm70 filiment transformers

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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
 
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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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....
 
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
 
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!
 
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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!
 
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
 
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.
 
design around your center value and add 10% to that for line dropout. last thing you want is supply ripple injected straight into your heaters?

CCs can be made whit LM338K excelent units for the price. for better regulation add a sizable cap 1000uF across the output, Dont forget to include a reverse bias diode.
It needs around 4 volt for dropout voltage so whit your 28vdc supply your fine upto about -5% line


face it your going to be dissipating around 20w on a line high. just design around that. regardless of what CCS your using

v4lve
 
Thanks for the advice guys
I've gone to the dark side and ordered some coleman boards. hadn't realised how reasonable they are:)

Re heat, my chassis is made of 2' long sections of 1\8" alloy chanal. with only the heat of 2 cheap bridge packages for the filiment and 2 load resisters for the mu follower driver stage it was getting hotter than i'd like, so i may fit a fan and some extra heatsinking for the filiment boards. I was hoping to get them into the filiment ps box but the pdf says i think 8-10" from the filiment.

have a new ht transformer on its way too due to trying to save 5 mins work taking the rear panel off to drill a hole. drilled straight into the 400w trafo. what a prat!

enzo
 
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I recommend short wiring from Regulator-to-ValveSocket only to avoid pickup from electromagnetic fields.
The Coleman Regulator (from V4) is completely stable, regardless of wiring length. It was tested with 3 metres of cheap twin cable, all scrunched up, without problems.

IF you screen the cable, with braid & foil, high quality cable of low capacitance, you can mount it further away. I have not checked the effect on the sound of doing it though.

The GM70 regulator dissipates 12W (25V in) to 21W (28V in) into the transistors, so a moderately heavy chassis will deal with this unaided.

The point on the chassis near Q5 should be kept below 70 deg C for preference. "warm starts" will easily be accommodated.

Changing to MBR1045 rectifiers will mean very little heat to deal with here.

I trust the improved sound of the regulators, compared to LT1084 heating, will take you from the Dark Side to the Light!
 
I have completed my installation of Rod's regulators in my GM70 amp. 4 of the regulators supply the VT25 drivers and of course the GM70 tubes. Front end is D3A in triode mode.

they work like magic. i have done comparison between CRC smoothing and then Rod's regulators. More air, much quieter and darker background which the CRC cannot provide.

the improvements are by no means subtle. the music has a fresh breathe of life, emotions and nuance. reverting to the CRC PSU, i experience a downsize soundstage and a less lively presentation.

An externally hosted image should be here but it was not working when we last tested it.
 
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This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.