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True Confessions

173.184.77.152
Posted on November 19, 2016 at 11:36:43
Jim Dowdy
Manufacturer

Posts: 1518
Location: Atlanta, GA
Joined: July 22, 2000
 Recently completed a SE EML30A>GM70 amp, utilizing a 4-tube bridge of 866s for rectification.
Things sounded good: B+ and Bias measured perfectly...but about 20 hours in, I began losing rectifiers. I even tried replacing a rectifier transformer...same result.
After about two days of exasperation (and minimal sleep), it *finally* dawned on me that 866s operate on 2.5 volts, NOT 5.0 volts!
New transformers (+ 866s) are on order.

I hope no one else has done anything so silly
:-)

 

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RE: True Confessions, posted on November 19, 2016 at 11:39:54
vinnie2
Audiophile

Posts: 4481
Location: North Carolina
Joined: September 28, 2013
 No way, we never make silly mistakes like that. : ) Mine are a lot worse!

 

RE: True Confessions, posted on November 19, 2016 at 12:40:09
Triode_Kingdom
Audiophile

Posts: 10044
Location: Central Texas
Joined: September 24, 2006
 Sorry to hear you lost the tubes. I hate it when something like that happens. How do you like the GM70s so far?




 

RE: True Confessions, posted on November 19, 2016 at 12:52:57
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
 Jim,

No Hg vapor tubes around here, as toxic vapor and I don't get along. If you insist on Hg vapor tubes, make sure you have a container of powdered sulfur on hand, to deal with the inevitable broken bottle.

The following remarks are FWIW. To eliminate doubled forward drop and be "safe", I'd use a hybrid bridge made from 2X 6CJ3s and Schottky diodes.

BTW, don't feel alone in having a "Cranio-Rectal Syndrome" attack. Everybody, starting with me, suffers from occasional bouts of the "disease".


Eli D.

 

I love the GM70!, posted on November 19, 2016 at 12:53:34
Jim Dowdy
Manufacturer

Posts: 1518
Location: Atlanta, GA
Joined: July 22, 2000
 All-around, it's my favorite output tube.

 

RE: I love the GM70!, posted on November 19, 2016 at 13:18:40
used-hifi
Audiophile

Posts: 1100
Location: Surprise AZ
Joined: March 18, 2003
 youth is waisted on the young, crap happens sometimes


gm70 SE or PSE? as in parafeed

 

the circuit is parafeed (nt), posted on November 19, 2016 at 13:21:33
Jim Dowdy
Manufacturer

Posts: 1518
Location: Atlanta, GA
Joined: July 22, 2000
 nt

 

RE: the circuit is parafeed (nt), posted on November 19, 2016 at 13:22:59
used-hifi
Audiophile

Posts: 1100
Location: Surprise AZ
Joined: March 18, 2003
 yeah kool me too.

maybe tell us little more about the amp if you will?

have you measured it yet?

 

RE: True Confessions, posted on November 19, 2016 at 13:54:56
glenz10183@aol.com
Distributor or Rep

Posts: 84
Location: New York
Joined: September 27, 2003
 I think most of us have had a mulligan or two within this space, Jim. I feel your frustration though. Before and finally realizing after.

 

RE: True Confessions, posted on November 19, 2016 at 15:25:02
Jim Dowdy
Manufacturer

Posts: 1518
Location: Atlanta, GA
Joined: July 22, 2000
 Thanks, George: it's been at least 40 years since I've felt quite this stupid
;-)

 

RE: the circuit is parafeed (nt), posted on November 19, 2016 at 15:26:29
Jim Dowdy
Manufacturer

Posts: 1518
Location: Atlanta, GA
Joined: July 22, 2000
 Let me get the rectifier issue squared away and I'll provide more info.

 

CR inversion, posted on November 19, 2016 at 15:28:56
Jim Dowdy
Manufacturer

Posts: 1518
Location: Atlanta, GA
Joined: July 22, 2000
 Ha - thanks, Eli!

While we're at it: did you ever make progress on that (Class B) PP 46 amp you were talking about?

 

The problem was..., posted on November 19, 2016 at 17:44:15
Jim Dowdy
Manufacturer

Posts: 1518
Location: Atlanta, GA
Joined: July 22, 2000
 that I had it firmly cemented in my mind that 866s operate on 5.0 volt filaments...therefore, I continued to miss the obvious!
:-)

 

RE: CR inversion, posted on November 19, 2016 at 18:52:59
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
 I'm slated to retire (finally!) in 2017. Bench upgrades 1st, then the B2 #46 amp. Some stuff for the project is already on hand. A "big ticket" item, the O/P "iron", will be my next purchase.

Thanks for your interest.


Eli D.

 

I've had the pleasure of hearing Mr. Dowdy's GM70, posted on November 19, 2016 at 19:00:14
1973shovel
Audiophile

Posts: 10117
Location: Greenville SC
Joined: February 25, 2007






Sublime.

Thanks for lugging the three chassis affair to past Horn Fests, Jim.

If I ever win the lottery (the one I don't even buy tickets for) your phone will ring.

 

perhaps because each 866 draws 5 amps? am building with 816's right now!... sublime bass~nT, posted on November 19, 2016 at 23:55:11
Cleantimestream
Audiophile

Posts: 7550
Location: Kentucky
Joined: June 30, 2005
 ~!
The Mind has No Firewall~ U.S. Army War College.

 

I once wired the 2a3 with 5v, it sounded very good......for about 10 hrs (nt), posted on November 20, 2016 at 08:59:44
bouncy ball
Audiophile

Posts: 1221
Location: British Columbia
Joined: July 26, 2003
 .

 

Me too, but no problem..., posted on November 20, 2016 at 09:19:29
SteveBrown
Audiophile

Posts: 2454
Location: Portland, OR
Joined: November 14, 2002
 I was using the JJ 2A3-40 and a DC supply. It loaded the supply down to about 3.5 v but ran just fine. Of course once I discovered it I fixed it, but tubes seem no worse for the wear.

 

RE: I think everyone should share a brain fart experience..., posted on November 20, 2016 at 11:27:00
DAK
Audiophile

Posts: 2712
Location: PACIFIC
Joined: August 8, 2010
 .. since it could help us with trouble shooting later on. We all get a little cocky and start winging it and going on our merry way. And 95% of the time it is all good. But, then that time comes when the synaptic connections were scrambled from previous let's call it "chemical experiments" be they intentional or not.
One experience that sticks out was a problem after building a 2 chassis amp with power supply on 1 chassis and the amp on the other. The ground bus is made on the PS and continued to the output chassis. On all my builds i make the output and input jacks on a separate ground which i connect to the amp chassis. So, on first power up i had very weak sound. Measurements should no problems and since this was somewhat of a different build as i was now using fixed bias instead of cathode bias, i thought maybe that was the issue. So, I tried a different bias scheme. No difference. Perhaps the driver tube? played with different bias on the driver, no difference. Finally, the solder fumes cleared in my head and i thought, Duh!my amp chassis is not grounded to the PS. So i put the jacks on the ground bus and bingo! that was that!

 

I once wired the 300B with 2.5v, it sounded very good..., posted on November 20, 2016 at 20:35:24
Paul Joppa
Industry Professional

Posts: 7295
Location: Seattle, WA
Joined: April 23, 2001
 Long ago I made an amp that dissipated 18 watts and used either 2A3 or 300B. When I accidentally switched to 2.5v with an early Sovtek 300B, it came to about 3.0 volts, but sounded darn good.

I suppose the tube would have an early death from the reduced space charge, but I didn't run it that way for more than an hour or two.

 

LOL...Thoriated Tungsten 866's, posted on November 21, 2016 at 04:40:56
PakProtector
Audiophile

Posts: 12363
Joined: May 14, 2002
 Have not done that particular one yet. But there have been oversights made during construction on at least one occasion...LOL
cheers,
Douglas

Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.

 

Doh!, posted on November 21, 2016 at 11:25:28
Ed Sawyer
Audiophile

Posts: 2595
Joined: November 3, 2000
 Well, at least it wasn't the EML30a getting fried! Those are a lot more dear than 866s. (which if you need any I am sure I have some spares).

 

AS do I...., posted on November 21, 2016 at 11:40:26
Ivan303
Audiophile

Posts: 48887
Location: Cadiere d'azur FRANCE - Santa Fe, NM
Joined: February 26, 2001
 As well as about a TON of iron that would make a good Parafeed 45 driving a Parafeed GM-70. ;-)






First they came for the dumb-asses
And I did not speak out
Because I was not a dumb-ass

 

Heater Supply?, posted on November 22, 2016 at 09:12:28
Triode_Kingdom
Audiophile

Posts: 10044
Location: Central Texas
Joined: September 24, 2006
 I've stayed away from these due to the unusual filament requirements. Maybe one of the inexpensive 24V switchers would be a good candidate for this. Do you use AC or DC on the filament?


 

RE: perhaps because each 866 draws 5 amps? am building with 816's right now!... sublime bass~nT, posted on November 22, 2016 at 11:06:34
PakProtector
Audiophile

Posts: 12363
Joined: May 14, 2002
 LOL...go for the big bottles...816's are pre-amp rectifiers. For size, maybe the 866AX with its skinnier bottle?

My 6CB6A amps run a pair of 866's each. Love the flash when the 6NO45 closes on the plate trans primary.
cheers,
Douglas

Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.

 

RE: I've had the pleasure of hearing Mr. Dowdy's GM70, posted on November 22, 2016 at 14:34:16
Jim Dowdy
Manufacturer

Posts: 1518
Location: Atlanta, GA
Joined: July 22, 2000
 Thanks for the kind words!

 

RE: Heater Supply?, posted on November 22, 2016 at 14:36:24
Jim Dowdy
Manufacturer

Posts: 1518
Location: Atlanta, GA
Joined: July 22, 2000
 Filaments are DC.

For years now, I've been threatening to use tungar rectifiers to accomplish this
;-)

 

816, posted on November 22, 2016 at 15:01:34
Cleantimestream
Audiophile

Posts: 7550
Location: Kentucky
Joined: June 30, 2005
 Normally I would Doug, but aesthetics rules here and the 866A towers over the city of choke and Xtransformers ...; 866 with 811... oh yes.

Besides, you would agree 250 ma. is plenty for a SE 45 amp,

LED bias

for

5842 input


The Mind has No Firewall~ U.S. Army War College.

 

RE: Heater Supply?, posted on November 22, 2016 at 20:59:54
Triode_Kingdom
Audiophile

Posts: 10044
Location: Central Texas
Joined: September 24, 2006
 THAT will scare the neighbors!

 

RE: 816, posted on November 23, 2016 at 03:36:50
PakProtector
Audiophile

Posts: 12363
Joined: May 14, 2002
 OK...a pair of 816's is adequate for a SE 45 amp...:) Whilst you're considering these rectifiers, the 866JR is shorter and w/o the plate connection. More like a 6550 bottle( the Tongue-Sore coke bottle that is ).
cheers,
Douglas

Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.

 

on the drawing board, posted on November 25, 2016 at 11:06:35
Jim Dowdy
Manufacturer

Posts: 1518
Location: Atlanta, GA
Joined: July 22, 2000
 Is a PSE GM70 IT-driven by 46...

 

What current going through the choke on the PSE GM-70?, posted on November 25, 2016 at 20:08:17
Ivan303
Audiophile

Posts: 48887
Location: Cadiere d'azur FRANCE - Santa Fe, NM
Joined: February 26, 2001
 Thinking the Plate Chokes I ordered from Pieter was about 120 ma MAX as it was WAY over-spec'ed for a 45 driving an 845 much like the amp Josh made for Ed Sawyer. Parafeed OT is 5K, if memory serves.

Would have worked well with Eimac 340TL as well, and that's a bit of current with all four sections cooking at once.

HUGE iron with LOTS of Henrys as only Pieter can wind, if you have the $$ and time.

As luck would have it, work intervene, then back to SF and no shop to work in so all that iron is just holding the house to the ground. :-(




First they came for the dumb-asses
And I did not speak out
Because I was not a dumb-ass

 

choke , posted on November 26, 2016 at 06:08:47
Jim Dowdy
Manufacturer

Posts: 1518
Location: Atlanta, GA
Joined: July 22, 2000
 Power supply choke for the four GM70s will carry approx 430 mA.

Parallel SE GM70s will not be parafed - each OPT will be 6k, and designed for about 215 mA.

 

6K, for paralled GM-70?, posted on November 26, 2016 at 14:59:20
Ivan303
Audiophile

Posts: 48887
Location: Cadiere d'azur FRANCE - Santa Fe, NM
Joined: February 26, 2001
 Wondering now i if you feel that GM-70 is more a 211 than an 845, as far as plate resistance goes?

You use carbon or copper plate GM-70s?




First they came for the dumb-asses
And I did not speak out
Because I was not a dumb-ass

 

6K OPT for paralleled SE GM-70, posted on November 27, 2016 at 07:14:23
Jim Dowdy
Manufacturer

Posts: 1518
Location: Atlanta, GA
Joined: July 22, 2000
 Andrea Ciuffoli estimates the R(p) of the GM70 to be 2k3, and I have no reason to question that figure.

I prefer the copper-plate version of the GM70.

 

RE: 6K OPT for paralleled SE GM-70, posted on November 27, 2016 at 08:09:32
TomWh
Audiophile

Posts: 764
Location: Tucson Az
Joined: August 7, 2003
 Hi Jim

If I remember correctly you played a lot with 845's. Can not remember if you used any transformer split push pull.

Well would love to hear the sound differences between 845 vs gm 70. And if you have any information on parallel se vs interstage push pull sound wise would love to hear it.

Thanks Tom

 

RE: 6K OPT for paralleled SE GM-70, posted on November 28, 2016 at 07:09:18
used-hifi
Audiophile

Posts: 1100
Location: Surprise AZ
Joined: March 18, 2003
 good gm70 tubes that have normal current capabilities will have Rp 1.6k

if you have lower current capabilities (measured) then the Rp will be much higher.

I confirmed this with my Conrad vladimir!

Lawrence

 

845, posted on November 28, 2016 at 08:42:11
Jim Dowdy
Manufacturer

Posts: 1518
Location: Atlanta, GA
Joined: July 22, 2000
 I have built a couple of SE 845 amplifiers - neither of them utilized ITs.

 

You're very welcome, Jim, posted on November 28, 2016 at 13:43:20
1973shovel
Audiophile

Posts: 10117
Location: Greenville SC
Joined: February 25, 2007
 Sorry about the late reply.

 

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