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Parafeed GM70 Prototype Build

72.211.153.103
Posted on December 9, 2016 at 15:04:17
used-hifi
Audiophile

Posts: 1100
Location: Surprise AZ
Joined: March 18, 2003



Hey Guys Finally the rest of the parts I needed to get this off the ground came a few days ago, I got right to it.

Parafeed GM70 being driven by RC couple to 6HV5 with a Mu of 300! plenty of voltage drive and current! Driver has about 6db of headroom A2?!

anyways I have been working out problems including New but bad 6HV5 tubes.

Initial impressions listening through my IPhone on youtoob are Excellent, The 6hv5 TUBE is a dynamic powerhouse, If you value dynamic contrast this tube does it! being plate choke load and using grid chokes keeps all the dynamic Transconductance intact!!

The power supply uses all torriods including Chokes! The chokes (100va)are power transformers that I am experimenting with LOW DCR!!(they work in the neg leg or the positive leg I tried both) this is LSES , Jeffery Medwin/Dennis Fraker ideas not the torriods just the LSES


The output again is a power transformer 100va 230v/6.3v same as the chokes
Issues: hum, some distortion on lower freq stuff even played at low levels

I am tuning the plate chokes with 100k 2watt russian pots

Filiments are being lit via laptop power supply 19v into 20k capacitance




 

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RE: Parafeed GM70 Prototype Build, posted on December 9, 2016 at 15:05:27
used-hifi
Audiophile

Posts: 1100
Location: Surprise AZ
Joined: March 18, 2003





more pics

 

RE: Parafeed GM70 Prototype Build, posted on December 9, 2016 at 15:05:59
used-hifi
Audiophile

Posts: 1100
Location: Surprise AZ
Joined: March 18, 2003



pix

 

RE: Parafeed GM70 Prototype Build, posted on December 9, 2016 at 15:07:01
used-hifi
Audiophile

Posts: 1100
Location: Surprise AZ
Joined: March 18, 2003



pix

 

RE: Parafeed GM70 Prototype Build, posted on December 9, 2016 at 15:07:53
used-hifi
Audiophile

Posts: 1100
Location: Surprise AZ
Joined: March 18, 2003



pix

 

RE: Parafeed GM70 Prototype Build, posted on December 9, 2016 at 15:10:34
used-hifi
Audiophile

Posts: 1100
Location: Surprise AZ
Joined: March 18, 2003



pix

 

RE: Parafeed GM70 Prototype Build, posted on December 9, 2016 at 18:17:37
deafbykhorns
Audiophile

Posts: 1067
Location: Florida
Joined: October 17, 2003
 What kind of voltage and current are you running on the outputs?
Cathode bias?
Are you using the same high voltage for the pentode?
Is the pentode in triode mode or pentode?
Don't you have to run these at 2000+volts to be in the linear region?

 

RE: Parafeed GM70 Prototype Build, posted on December 9, 2016 at 18:26:41
vinnie2
Audiophile

Posts: 4481
Location: North Carolina
Joined: September 28, 2013
 A man after my own heart.... love the bread board!

 

RE: Parafeed GM70 Prototype Build, posted on December 9, 2016 at 18:49:45
used-hifi
Audiophile

Posts: 1100
Location: Surprise AZ
Joined: March 18, 2003
 Hi, The 6hv5 tube is a triode! yeah and with mu 300 huge voltage swing with current :)

catch is yes they need 1kv to come alive

the gm70 is cathode biased 80ma rp voltage is 1200v

for the 6hv5 30ma! 1150 rp volts

listening to this now, I love this amp beautiful with scary dynamics when the music calls for it.

Lawrence

 

RE: Parafeed GM70 Prototype Build, posted on December 9, 2016 at 18:51:26
used-hifi
Audiophile

Posts: 1100
Location: Surprise AZ
Joined: March 18, 2003
 Thanks! it would look better if i 180'd the board as the other side is finished too.

Lawrence

 

RE: Parafeed GM70 Prototype Build, posted on December 9, 2016 at 19:01:13
deafbykhorns
Audiophile

Posts: 1067
Location: Florida
Joined: October 17, 2003
 I thought it was a pentode with that mu!
Great operating point for the gm70, I had mine at 1200v but lowered just under 1100v.
How did you configure the power supply, who's toroidal?
I used spice to model mine ahead of time, not sure if there's a model for that driver....

 

RE: Parafeed GM70 Prototype Build, posted on December 9, 2016 at 19:37:20
used-hifi
Audiophile

Posts: 1100
Location: Surprise AZ
Joined: March 18, 2003
 The power supply transformer is a antek torriod good company BTW right out of New York.

full wave Bridge, 4 schottkey diodes into a single 6c3g Damper diode for very slow ramp up then LcLcRc the Rc is to decouple from main supply to the driver tube, but I am not sure this is necessary and maybe hurting me because these very hi transconductance tubes swings lots of volatge so a good tank of B+ is mandatory and because of the plate choke which IMO kinda does decoupling in itself

not much information on the net about this tube and how to use it.

reading the data sheet gives an idea on what to do since its all an experiment tubes are very forgiving as you prob know

I like cathode bias for many reasons which is why i wanted to experiment with parafeed for the same reasons its very tuneable!

IMO for what I need SE parafeed is best choice I love it!

Lawrence

 

RE: Dowdy Llama inspired?, posted on December 9, 2016 at 21:17:29
deafbykhorns
Audiophile

Posts: 1067
Location: Florida
Joined: October 17, 2003
 Except for the driver and operating voltage, it sounds like a dowdy amp

 

RE: Dowdy Llama inspired?, posted on December 9, 2016 at 21:45:38
used-hifi
Audiophile

Posts: 1100
Location: Surprise AZ
Joined: March 18, 2003
 nah im pretty sure its a Lawrence amp! :)

 

torroids as chokes, posted on December 10, 2016 at 06:15:38
RayP
Audiophile

Posts: 727
Location: Maryland
Joined: June 30, 2005
 Could you describe in more detail the use of torroids as chokes. Do they have an air gap?

Could you also describe the plate chokes and how you are using the pots to 'tune' them?

ray

 

RE: torroids as chokes, posted on December 10, 2016 at 09:40:49
used-hifi
Audiophile

Posts: 1100
Location: Surprise AZ
Joined: March 18, 2003
 Hi Ray, there is no gap in these power transformers used as chokes

I do not recomend you use these as i did but so far after hours of use no failures


The output is the same power transformer as the chokes 230v/6.3v gives a turns ratio 36.5 with an 8ohm speaker the load that the tube sees is 10.658k

Lawrence

 

Update:, posted on December 12, 2016 at 12:09:48
used-hifi
Audiophile

Posts: 1100
Location: Surprise AZ
Joined: March 18, 2003
 I would like to give some more of my thoughts, First if your thinking of building a transmitter type in SE final tube amp, I can heartily say, go for it this amplifier being simple in design with simple power supply LSES is worth trying out

Its sound or lack of its sound for example, this is what is interesting about this amplifier, its very dynamic its very very low distortion in its sound,but I have not measured it yet,its almost too good in a way, but hard to describe until you build one then you will understand what I mean.

I have not hooked up my Large Fulton speakers(in storage) yet only the smaller tempo 3way these being less efficient then the larger fultons I have not clipped the amp playing very loud!



I have looked at the B+ after the plate chokes under very heavy transient (cymbol crash) and cannot see any movement(sag) via my fluke meter
so the power supply torroids and power supply in general are working very well indeed LCLC into the plate choke(final)LCLC and RC into the driver plate choke.

I really like this SE parafeed. I wonder why more have not been built



Lawrence

 

RE: Update:, posted on December 12, 2016 at 13:15:21
vinnie2
Audiophile

Posts: 4481
Location: North Carolina
Joined: September 28, 2013
 Have you posted a schem yet Lawrence?

 

RE: Update:, posted on December 12, 2016 at 13:39:08
used-hifi
Audiophile

Posts: 1100
Location: Surprise AZ
Joined: March 18, 2003
 No there is no schematic, all came from some math and other ideas i saw, if you have something in mind you need to know I would be happy to help and post it,


here is the basic run down how its done.

Main B+ supply is 1250 volts via after the schotkey bridge into damper diode.

I use a choke load on the 6hv5a 30ma cathode bias after the choke its about 1210 volts the choke is 200H@40ma

choke load (parafeed)for the gm70 100H@80ma runs about 1202volts after the choke gm70 is 80ma cathode biased

6hv5a is cap couple so no A2 unless i would do fixed bias which I don't really like but I may still experiment


Nickle grid chokes on the input 6hv5 and the gm70 grids to ground

I hope this helps and if you require specifics email me privately.


Lawrence

 

RE: Update:, posted on December 12, 2016 at 15:32:18
vinnie2
Audiophile

Posts: 4481
Location: North Carolina
Joined: September 28, 2013
 Thanks for the info, but I get the willies whenever the B+ gets that high. I have sworn never to go there. Quite honestly it scares the crap out of me. Please be careful!

 

RE: Update:, posted on December 12, 2016 at 16:18:42
used-hifi
Audiophile

Posts: 1100
Location: Surprise AZ
Joined: March 18, 2003
 Thanks for the warning, but I am very comfortable with these voltages, I have fixed many such higher voltage OTL (beverdge) and or (Acoustat) +-3600v amplifiers 1200 is not that high :)IMO it does not jump out after you


but I will say that I got sting even with little capacitor .022 I had charged up, this little damn thing got me LOL

so Yes everyone please be careful!, this hi voltage stuff is not for everybody.

Happy Holidays Everybody!

Lawrence

 

Update, posted on December 16, 2016 at 20:45:02
used-hifi
Audiophile

Posts: 1100
Location: Surprise AZ
Joined: March 18, 2003
 I have been experimenting with power supply capacitance too much in the first lc doesn't sound good it's quieter but not better, I think I have settled on what I'm going to use.

Next up rod Colman regulators simple to build and quieter then smps and no nasty MHz freqs flying around. Sonically excellent! But let me warn you to many rc filters not good just make it enough experiment!!!

Lastly taking out the oil filed output cap and replaced with one of these very hi current 60 amp makes a big difference! The oil cap was lovely but once you hear this wow!!

I willl not be building SE! Parafeed is it!

Lawrence

P.s the 6hv5a is a fantastic dynamic driver!

 

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