SET Asylum

Single Ended Triodes (SETs), the ultimate tube lovers dream.

Return to SET Asylum

Message Sort: Post Order or Asylum Reverse Threaded

Low Voltage to Heaters

72.211.68.9
Posted on July 17, 2015 at 17:01:28
mashley
Audiophile

Posts: 355
Location: New Orleans
Joined: July 31, 2010



I have a problem that I need some help troubleshooting?

Replaced the standard KBU4 bridges with schottky's in both my monoblocks, and these are the figures I'm getting>
Tubes in (loaded)
Left amp Right amp
Left Trafo secondary 13.3v a/c Right> 13.1v a/c
Left Bridge in 13.3v a/c Right> 12v a/c
Left Bridge out after resistor 11.03v dc Right> 11.91v dc

Have reduced the resistors in the left amp temporarily so I can listen without damaging 6SN7's to 0.1R (all three.) the figures above represent this. I have taken the temp of all the schottky's with my laser thermometer and all four diodes both sides are the same temp so I don't think any diodes are at fault. I can't test the 4700uf my esr meter doesn't go that high.
Have tested resistors after trafo and bridge and they are correct.

Right amp I can reduce the resistor after the bridge to get a nice 13v for this side. But on the left amp already I'm using 0.1R resistors!



 

Hide full thread outline!
    ...
RE: Low Voltage to Heaters, posted on July 17, 2015 at 19:18:40
Triode_Kingdom
Audiophile

Posts: 10048
Location: Central Texas
Joined: September 24, 2006
 "Have reduced the resistors in the left amp temporarily ... to 0.1R "

I think you need to measure the voltage across each resistor, calculate the current and compare the results to the other channel. That will help in determining whether the low voltage is the result of too much current draw or if it's caused by too high a resistance somewhere.


 

RE: Low Voltage to Heaters, posted on July 18, 2015 at 11:22:10
Caucasian Blackplate
Industry Professional

Posts: 8313
Location: Seattle
Joined: June 18, 2004
 You left off the most important piece of information - which Schottky diodes did you use?


I still think this is a horrible idea. You have a DC supply that's isolated from the signal path and working properly to begin with, so why mess with it?

 

Low Voltage to Heaters, posted on July 18, 2015 at 15:05:12
mashley
Audiophile

Posts: 355
Location: New Orleans
Joined: July 31, 2010
 Can I just lift one of the secondaries and put my mm between and measure total current draw, will this suffice?

 

Low Voltage to Heaters, posted on July 18, 2015 at 16:45:19
mashley
Audiophile

Posts: 355
Location: New Orleans
Joined: July 31, 2010
 "You left off the most important piece of information - which Schottky diodes did you use?
I still think this is a horrible idea. You have a DC supply that's isolated from the signal path and working properly to begin with, so why mess with it?"

I replaced the KBU4 bridge in both amps for a Schottky bridge 4xVishay VT5202 Datasheet - http://www.vishay.com/docs/87700/vt5202-m3.pdf

I'm just trying to squeeze a bit more performance out of these amps, and wanted to try and see how it sounds that's all, it's super simple to go back to original, do you really think the benefits a schottky bridge can offer, when put in this position are just wasted or so minimal as to be a waste of time?

 

RE: Low Voltage to Heaters, posted on July 18, 2015 at 16:54:49
Caucasian Blackplate
Industry Professional

Posts: 8313
Location: Seattle
Joined: June 18, 2004
 You've picked the wrong diode for the job.

I would put the amp back to stock.

 

Low Voltage to Heaters, posted on July 18, 2015 at 17:07:12
mashley
Audiophile

Posts: 355
Location: New Orleans
Joined: July 31, 2010
 "You've picked the wrong diode for the job."

I bloody have? That is so disappointing!
Hmm ok then, I'm so done with this schottky attempt it's just kicked my ass one last time, back up on the bench tomorrow...

 

RE: Low Voltage to Heaters, posted on July 19, 2015 at 08:27:31
Triode_Kingdom
Audiophile

Posts: 10048
Location: Central Texas
Joined: September 24, 2006
 "You've picked the wrong diode for the job."

Why do you say that? Just from a quick look at the specs, I don't see a problem.




 

RE: Low Voltage to Heaters, posted on July 19, 2015 at 10:59:37
Caucasian Blackplate
Industry Professional

Posts: 8313
Location: Seattle
Joined: June 18, 2004
 One could get less forward voltage drop from a diode rated for lower voltages.

There are also a fair number of tubes being heated by the device, as well as the cap input filter, so one must do a little head scratching at how hot those things are going to get at ~2A current. The 502 will pop up about 70C per Watt, so one would expect a temperature rise of more than 100C, which will simply cook the diodes where they sit.

A 1N5820 will exhibit just a hair more than half these losses.

The KBU will deal with the power much more gracefully, as the package will exhibit much lower temperature rise.

 

Low Voltage to Heaters, posted on July 19, 2015 at 19:34:42
mashley
Audiophile

Posts: 355
Location: New Orleans
Joined: July 31, 2010
 CB, I Followed your advice and put the KBU4's back in there. As well as all the 1R resistors on the (low voltage/problem) amp.

The good amp is up to 12.57v after the last resistor so I've gained 0.5v.
The other amp, with the 1R resistors now(3) back in I'm only getting 9.5v?

I measured the current draw, good amp 0.6A, bad amp 0.5A (lifted the wire from the final resistor and used crocodile clips to run through my mm ( hope this was right?)

I had both amps up on the bench and tested every resistor in the linestage and the two caps and they all are correct as well as reading the same both sides, I cant check the 4700uf cap as my esr meter goes out of range.

I need some help here, have no idea what to do next?

(Just fyi am running film caps 100uf in the linestage, was advised this would be fine, removed lytics)I bypassed these with soviet silver mica's.

 

RE: Low Voltage to Heaters, posted on July 19, 2015 at 20:19:31
Triode_Kingdom
Audiophile

Posts: 10048
Location: Central Texas
Joined: September 24, 2006
 "I measured the current draw, good amp 0.6A, bad amp 0.5A (lifted the wire from the final resistor and used crocodile clips to run through my mm"

Are you unable to measure voltages on all the resistors like I suggested earlier? That's the way to get to the bottom of this problem quickly. Really, reinstalling the original diodes was a complete waste of time without making those measurements first. At this point, it appears one of the resistors ahead of the bridge in the bad channel might have increased in value. It's also possible the 4,700u in that channel is failing. Lacking an ESR meter, you could swap the caps and check whether the problem changes channels.

 

Low Voltage to Heaters, posted on July 19, 2015 at 22:28:00
mashley
Audiophile

Posts: 355
Location: New Orleans
Joined: July 31, 2010
 The resistors are all good, brand new mills MRA's tested!
That only leaves the 4700uf cap, I'll drop in another lytic in a few days just to see if the voltage returns to normal, if so I'll order "another" mundorf m-lytic, I didn't suspect this cap "too" much because I thought these m-lytics were supposed to have a very long lifespan/were bulletproof.

Yay, I hope this is it....

Will post results..

 

Low Voltage to Heaters, posted on July 21, 2015 at 11:59:39
mashley
Audiophile

Posts: 355
Location: New Orleans
Joined: July 31, 2010
 It's apparently not the 4700uf Cap causing the low voltage to the heaters, I swapped it out for another 4700uf I had in my stash and still have the same problem.

Here are the currents through each resistor>
Good amp - The two 1R Resistors before diode bridge 0.7A each resistor
1R Resistor after bridge .63A

Bad Amp - The two 1R Resistors before diode bridge 0.6A each
1R Resistor after bridge 0.5a

What next?

 

RE: Low Voltage to Heaters, posted on July 21, 2015 at 12:16:32
danlaudionut
Audiophile

Posts: 5480
Location: Schenectady
Joined: June 6, 2002
 The resistors are good.
The cap is good.
The only things left is either ...
KBU4 bridge
or
Wiring/soldering problem.

DanL



 

Low Voltage to Heaters, posted on July 21, 2015 at 15:27:17
mashley
Audiophile

Posts: 355
Location: New Orleans
Joined: July 31, 2010
 "The resistors are good.
The cap is good.
The only things left is either ...
KBU4 bridge or Wiring/soldering problem.
DanL "


So I need to check the linestage board and the bridge board only,
for cold solder/wiring problem is this correct?
And other than visual how would I use my mm to test the active circuit to detect the fault location?

 

RE: Low Voltage to Heaters, posted on July 21, 2015 at 15:51:40
danlaudionut
Audiophile

Posts: 5480
Location: Schenectady
Joined: June 6, 2002
 Left Trafo secondary 13.3v a/c Right> 13.1v a/c
Left Bridge in 13.3v a/c Right> 12v a/c
Left Bridge out after resistor 11.03v dc Right> 11.91v dc

What I see is -
Slight loading of the secondary of the trafo
(13.3v vs 13.1v)
Heavy voltage drop across bridge
(13.3v vs 12v)

One of the diodes in that bridge is
not working right - I think leaking.
No way to test that with a meter.
Just replace it.

DanL




 

Low Voltage to Heaters, posted on July 23, 2015 at 12:25:33
mashley
Audiophile

Posts: 355
Location: New Orleans
Joined: July 31, 2010
 The problem was with my soldering, had damaged the trace at the through hole. Finally tracked it down to the 4700uf positive side, I soldered a small jumper across to F3, the cap was effectively out of the circuit.
I was up in the linestage board all sunday looking for the problem, the rectifier board looked fine and I had a Russky silver mica bypassing the big cap it was obstructing my view.
Dang that was so frustrating and such a tiny problem!
Voltages the same on both sides now, Yay!!!

Thanks all of you for your comments and input, really helped me stay motivated I was beginning to get dis-heartened, Ya'll kept me pointed me in the right direction don't think I could've done it without your help...

 

FOLLOWUP..., posted on July 24, 2015 at 22:15:48
Triode_Kingdom
Audiophile

Posts: 10048
Location: Central Texas
Joined: September 24, 2006
 The OP has resolved the problem now, but I want to expand on what you've said. I still don't see a problem substituting the Schottky diodes.

"One could get less forward voltage drop from a diode rated for lower voltages."

The Schottky replacements exhibit a voltage drop roughly the same as the diodes in the original bridge. Lower drop diodes could be used, but this filament supply isn't regulated. It would seem beneficial not to install parts that might raise the voltage above that of the original design.

"The 502 will pop up about 70C per Watt, so one would expect a temperature rise of more than 100C, which will simply cook the diodes where they sit."

Unless I'm misinterpreting the modification, each bridge that was replaced is powering two 6SN7s. That's 1.2A. Each diode of the bridge sees less than that (average), but even using the full current for calculations, that's about 0.8W per diode. This will cause the VT5202 junction to rise to about 69C in free air if ambient is 25C. The junction is rated to 175C.

"A 1N5820 will exhibit just a hair more than half these losses."

The 1N5820 will drop 0.475V at it rating of 3A. That's a dissipation of 1.425W. The VT5202 Schottky datasheet shows a dissipation of about 1.95W at that current. That means the 1N5820 will dissipate roughly 73% of the power of the Schottky. That doesn't seem a very significant difference to me, particularly given that A) the bridge is operating well under 3A, so real dissipation differences are very small, and B) the VT5202 Schottkys will exhibit power dissipation virtually identical to the bridge they replaced.

If you think any of this is in error, let me know. I just don't see anything here that justifies telling the OP he selected the wrong diodes.

 

Page processed in 0.020 seconds.

[ Top Top of Page ] [ Contact Us ] [ Support/Wish List ] [ Copyright © 2024, Audio Asylum, all rights reserved ]
[ General ] [ Speakers ] [ Tubes ] [ Vinyl ] [ Digital ] [ Hi-Rez ] [ Video Asylum ] [ Cables ] [ Tweaks/DIY ] [ Music ] [ Films ]