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Futterman OTL3 - R29 gets very hot

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Posted on February 27, 2020 at 13:29:51
therkild
Audiophile

Posts: 4
Location: Jylland
Joined: February 24, 2020
 Hi
First thread from me in this forum.
I'm in the process of restoring a pair of 30 years old OTL 3 clones made locally her in Denmark.
All capacitors, tube sockets, potmeters and more or less all diodes are new.Tubes should be fine they were tested by the electrical engineer (friend) from whom I bought the set.
I'm slowly ramping the first one up by means of a variac with 20 ohm (power resistor) across the speaker terminals, and a 600 ohm PSU connected to the input rca, no input signal applied yet.
When I reach approx 60-70 % of the rated supply voltage (140 volt of 220)
it pulls approx 0,5 amp and the tubes starts glowing.
However, resistor 29 from pin 9 of V10 (3' grid) gets very hot, meaning there's a potential short circuit. Resistor value is 2,2kOhm which should be correct.
Any ideas what to look for?
Kind Regards
Thomas

 

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RE: Futterman OTL3 - R29 gets very hot, posted on February 28, 2020 at 07:36:39
rogerh113
Audiophile

Posts: 508
Location: San Francisco South Bay
Joined: October 20, 2014
 Do you have a schematic you could post?

When I was originally looking at the H3, I found out that the voltage regulation tubes do not turn on until they have a sufficient voltage across them. I was wondering at the time how they, and the neighboring circuitry, would react to being brought up on a variac. That portion of the amp is certainly not 'linear' due to the nature of the regulators.

regards -- Roger

 

RE: Futterman OTL3 - R29 gets very hot, posted on February 28, 2020 at 11:35:25
therkild
Audiophile

Posts: 4
Location: Jylland
Joined: February 24, 2020
 Hi Roger
I found the fault, poor soldering in a connection at one of the tubes.
What I'm struggling with right now is the voltage level across test point one and pin 11 (grid 2) on either V6, V7 & V8. According to the instruction I have there should be around 170 volt, adjusted by R16. I only get half ot it i.e 85 VDC, so think I need to look into the rectifying circuit.

Step learning curve but that's okay, I enjoy learning day by day.

Diagram attached, I have a corresponding version from whomever built it back in the 80'ties.

Regards
Thomas

 

RE: Futterman OTL3 - R29 gets very hot, posted on March 1, 2020 at 06:43:09
rogerh113
Audiophile

Posts: 508
Location: San Francisco South Bay
Joined: October 20, 2014
 Hi,

That link was to all google images for the OTL3. Which one is it that you have, or all they all the same?

Not sure I can help, but I will take a look.

regards -- Roger

 

RE: Futterman OTL3 - R29 gets very hot, posted on March 2, 2020 at 01:32:28
therkild
Audiophile

Posts: 4
Location: Jylland
Joined: February 24, 2020



Hi Roger
Specific diagram attached.

Regards
Thomas

 

RE: Futterman OTL3 - R29 gets very hot, posted on March 2, 2020 at 07:48:18
rogerh113
Audiophile

Posts: 508
Location: San Francisco South Bay
Joined: October 20, 2014
 Might want to check test pin 4 to see if it is correct. Much closer to the rectifier bridge, and if that voltage is OK, it is something further down that line (like V5 and associated circuitry, or even further down that line, pulling it down).

 

I'd be looking at the screen regulator for that circuit., posted on March 3, 2020 at 14:21:25
Ralph
Manufacturer

Posts: 4778
Location: Minnesota
Joined: April 24, 2002
 I would test the tubes and also the range of the potentiometer in the shunt regulator circuit for starters.

 

RE: Futterman OTL3 - R29 gets very hot, posted on April 13, 2020 at 22:42:31
airheadair
Audiophile

Posts: 393
Location: California
Joined: October 18, 2010
 Did you ever get this working? Is this a NYAL OTL3?
I'd love to hear how it turns out.

 

RE: Futterman OTL3 - R29 gets very hot, posted on April 15, 2020 at 03:25:01
therkild
Audiophile

Posts: 4
Location: Jylland
Joined: February 24, 2020
 Hi
No I ended giving up and handled them back to my friend.
Whenever I got one problem solved, the next appeared. Example, during trimming of the voltage levels in one of them, I noticed a significant change in current draw when I inserted my voltage meter probes in the test socket. A modern digital voltmeter in the meg. ohm category shouldn't influence a circuit causing it to draw more amp. Likely a damage in the print. I simply couldn't get to a point where they were stable.

My friend is currently doing a full rebuild, including changing them to PL 519 tubes. He's been designing and constructing tube amps for 30 years so he's far more capable of diagnosing why they behave like they do.
Will be interesting to hear them once he get them up and running.

Regards
Thomas

 

RE: Futterman OTL3 - R29 gets very hot, posted on April 29, 2020 at 14:02:04
rhinohifi714
Audiophile

Posts: 174
Location: oklahoma
Joined: June 13, 2009
 I believe that is a bleed resistor, doesn't do anything til the unit is powered off, then it bleeds all the flash capacitors in the unit.

They do get hot at that time.

no reason to worry though.

 

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