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91A debugging problems....

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Posted on April 18, 2015 at 11:38:44
vinnie2
Audiophile

Posts: 4481
Location: North Carolina
Joined: September 28, 2013
I finally got around to building my 91A clone, and I have been going through the psu and checking voltages. Filament voltages all look good, but when I tried bringing up the B+ nothing seems to be getting past the 5ar4 recitier. First thing I did was test the 5ar4 and it tested strong, so then I dropped in another 5ar4 just to be sure and still nothing.
I have AC voltage on the 5ar4 plates, but no B+ at the cathode. I have checked the schem a couple of times and everything seems ok, but obviously it's not. I had this happen once before but i can't remember what the cause was. I have a 50k bleeder resistor in the psu circuit, but it's acting like there is no load or something. I would appreciate any hints you folks might have as to what is likely to cause this.
Thanks

 

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RE: 91A debugging problems...., posted on April 18, 2015 at 11:43:00
Alpha Al
Industry Professional

Posts: 2958
Location: N. Carolina
Joined: February 16, 2004
Contributor
  Since: December 3, 2015
 Sounds to me like your HV center-tap is not grounded, keeping in in "standby" mode.(it's usually a red & yellow wire)

 

RE: 91A debugging problems...., posted on April 18, 2015 at 12:32:03
vinnie2
Audiophile

Posts: 4481
Location: North Carolina
Joined: September 28, 2013
 Nope. I just checked again and it is grounded and I have ac voltage at the plates of the 5ar4. Just no dc at the cathode.

 

RE: 91A debugging problems...., posted on April 18, 2015 at 13:03:02
REL54
Audiophile

Posts: 1944
Location: Virginia Coast
Joined: January 3, 2003
 Since this is a new build it possible that maybe perhaps the wires from the transformer are not connected to the correct tabs on the tube's socket. Maybe it is something that easy. Who knows...... Just a thought. At any rate, when you find out what it is be sure to let us know since we would then be able to [attempt] making the same error in the future. :-)

Then again, may its not a build out error but some mechanical problem.

roN

 

RE: 91A debugging problems...., posted on April 18, 2015 at 15:42:30
vinnie2
Audiophile

Posts: 4481
Location: North Carolina
Joined: September 28, 2013
 One of these days I will get it through my head permanently that pin-outs are numbered from the key looking at the BOTTOM of the socket! Went back to check the wiring and said "wait a minute!"

I will change the wiring around tomorrow and I think that will help a great deal. : )

Thanks for the reminder.

 

update, posted on April 18, 2015 at 16:44:58
vinnie2
Audiophile

Posts: 4481
Location: North Carolina
Joined: September 28, 2013
 Well it appears I have more than one thing wrong. I corrected the wiring on the 5AR4 and I still have the same problem, no b+ showing at the cathode.
Anyone have any other ideas? What else would cause this type of problem?

 

RE: update, posted on April 18, 2015 at 17:36:23
Chip647
Audiophile

Posts: 2653
Location: The South
Joined: December 24, 2012

Pins 2 and 8 should have 5 volts of AC across them.
Pins 4 and 6 should have +400 volts AC across them.
Pin 8 should have DC if there is a cap connected to it.

 

RE: update, posted on April 18, 2015 at 18:29:32
vinnie2
Audiophile

Posts: 4481
Location: North Carolina
Joined: September 28, 2013
 I am getting way more than 400 vac across 4 and 6. It's like there is no place for it to go. There is an C-LC-LC-R filter connected to 8, but I get not dc at that pin.

 

What is your AC across 2 and 8?, posted on April 18, 2015 at 18:50:54
Chip647
Audiophile

Posts: 2653
Location: The South
Joined: December 24, 2012

 

RE: What is your AC across 2 and 8?, posted on April 19, 2015 at 02:56:27
vinnie2
Audiophile

Posts: 4481
Location: North Carolina
Joined: September 28, 2013
 I never got up to full voltage because the ac on 4 & 6 was 600vac and climbing at about 2/3 throttle on the variac, so I shut it down. I checked 2 & 8 once when 4 & 6 were between 300 to 400vac and it was about 3.5 to 4 vac. It seemed to be about proportional. I will check it again this afternoon when I get back from taking my wife to the airport. She is going to visit the grand kids in Alaska for 2 weeks, so it looks like I will have a bit of time on my hands. Hoping to clear this up and finish up the amp and do the tweaking while she is gone.

PS I had a couple extra minutes so made a couple of measurements. With the variac at the 60 volt mark on the dial I got 475 vac on 4 & 6 and 2.8 vac on 2 & 8. Seems proportional, yes?
Also, I checked and I have continuity from the #8 pin to the ground pin on the ac plug. I assume that means the caps are grounded properly and voltage should be able to flow from the 5ar4.

 

RE: What is your AC across 2 and 8?, posted on April 19, 2015 at 06:28:48
SteveBrown
Audiophile

Posts: 2454
Location: Portland, OR
Joined: November 14, 2002
 Yikes! Sounds like you've shorted pin 8 to ground! Make sure your cap isn't shorted and make sure you didn't wire the choke from cap to ground - don't power up again till you find that... :-)

 

update 2, posted on April 19, 2015 at 07:43:53
vinnie2
Audiophile

Posts: 4481
Location: North Carolina
Joined: September 28, 2013
 Ok, found another problem; I had the center tap on the 5v winding for the 5ar4 grounded and it should not have been. Took that off and I am now getting B+.
I am using a hammond 278x power tranny rated at 400-0-400, so if I put a probe on each plate of the 5ar4 should I read close to 800 vac or 400 vac?
I stopped before I got to full power, but I was at 600+ vac with a probe on each 5ar4 plate with a B+ of 330 vdc. It looks like if I were at 800 vac on the plates I would be close to the 400+ vdc I am supposed to have for B+.

 

RE: update 2, posted on April 19, 2015 at 10:47:25
Tre'
Industry Professional

Posts: 17305
Location: So. Cal.
Joined: February 9, 2002
 I'm glad you found that before damaging your power transformer.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: update 2, posted on April 19, 2015 at 12:01:52
vinnie2
Audiophile

Posts: 4481
Location: North Carolina
Joined: September 28, 2013
 So what should I be reading with a probe clipped to each plate of the 5AR4, 800 vac?

 

RE: update 2, posted on April 19, 2015 at 12:30:24
Tre'
Industry Professional

Posts: 17305
Location: So. Cal.
Joined: February 9, 2002
 Yes.


If the high voltage winding on your power transformer is 400-0-400 then measuring from plate to plate on the 5ar4 should show 800vac under load.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: update 2, posted on April 19, 2015 at 12:55:31
vinnie2
Audiophile

Posts: 4481
Location: North Carolina
Joined: September 28, 2013
 Great, because that is what I have, and that gives me 425 for B+ which is exactly what I want. Now if I can just get that *&^% left channel to put music out the other end. Got some more debugging to do, but at least the psu is good to go.
Thanks

 

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