Burning Resistor

[pittsburgh]

  I'm modifying a tape deck to make a tube pre amp. I've run into a heat issue at R57. When turned on the resistor heated up beyond normal operating levels and began smoking. I checked the resistance level and noted a level around 330 Ohms. The recommendation for that resistor is 270 Ohms. I put in place another resistor rated at 270 Ohms/1%/1W  and had the same result. Lots of heat, and the exterior paint of the resistor started to turn black. 

  Could other resistors around the tube be a factor? The resistor level at R58 is 37K/ v. the 33K schematic, the level at R59 is 440 Ohms v. the 390 per the schematic.

  I have had not had the same issue with R23 which is the right channel of the pre amp. I changed out the stock resistor on that side at the same time that I replaced R57 on the left channel.

  I have modified the output of T3 and T2 I've taken the yellow lead and put 10 Ohm 3W resistors in series and send them to new 1/4" female Jacks that have an earth ground. Could this modification possibly be causing the failure at R57?

  I've put a PDF of the schematic on my idisk. If you have ideas and would like to help this link should automatically download the file to your computer. http://files.me.com/natesbasement/5w2n0v

  Thanks for the help.

[Mark Burnley]

...taken the yellow lead and put 10 Ohm 3W resistors in series ...

Hi,

We talked about this in another thread?

The problem here is that if the secondary load resistor is in series with the output wire, then when you have a high-impedance load connected to the output jack, the load on the output transformer winding is 10R + <input impedance of load- presume ~10k> When nothing is connected, the load is infinity-ohms!

You need to wire the 10R resistor across the secondary (i.e. in parallel with the windings) -across yellow and black, and then take your line-out from either side of this resistor.

It's possible you have damaged the valve and/or the output transformer from running it unloaded.

Think about a Fender guitar amp- the output jacks would rather short out the signal than leave it unloaded to protect the output stage!

Valve power output stages would rather see low-ohms than infinity-ohms as a load. A high or infinite load impedance causes voltage stress on the output valve and transformer primary winding.

Meter out the transformer primaries on bothe sides.

Mark

[PRR]

Mis-loading the tube won't burn the cathode resistor.

The tube grid is floating, C30 leaks, or B+ is WAY higher than 300V (but R23 is not smoking).

[Mark Burnley]

No,

But just wondering if your tube was damaged?? Also damaged s/c primary.

Worth checking- and also see if you have disturbed any component legs- very easy to short out when you push them to one side (and forget to move them back) when soldering!

Mark

[pittsburgh]

  I tried putting the 10 Ohm 3W resistor across the OT secondary with the same result. R57 is still running very hot.
  Have I damaged the GT9 Tube? How can I test for that?

[Mark Burnley]

What voltage have you got across the hot resistor?

Measure the voltage from 0V to the cathode and then anode. We can see what current you've got flowing, and what the "spread" of voltage is across the transformer, tube and R. Is your HT rail as per schematic?
 
Mark

[pittsburgh]

R57 Has values of approximately 57/44 MvDC Anode/Cathode. This is very different from R23 which has values of about 2/0 MvDC.

I have removed the fan from the unit. The leads are still connected to the power supply but they don't terminate to another device. My meter shows 120VAC for that pair.  

[pittsburgh]

  Could disconnection of the fan from this machine be causing the increased amount of voltage to flow across the output of R53?

  What about blowing out the primary or secondary windings of the output transformer? Should I attempt to replace the transformer?

  I'm confused about how to keep a load on the output of the transformer. If I have a cable connected to the output jack that's not connected to anything will that work to keep some kind of a load so that the amount of resistance is not infinity?

[Insomniaclown]

    I'm confused about how to keep a load on the output of the transformer. If I have a cable connected to the output jack that's not connected to anything will that work to keep some kind of a load so that the amount of resistance is not infinity?

I don't know about the other questions, but I do know a cable on its own will not provide the required load. You could strap a resistor on the end of the cable to provide the load, or just plug it into another piece of gear. Its just like a speaker output on a guitar amp. Without the speaker giving that 4/8/16ohm load, the amp will fry itself in due time.

As for strapping a resistor on the end of a cable, I've never done it, but a quick search in google should provide lots of easy options for creating a dummy load.

[lassoharp]

  Could disconnection of the fan from this machine be causing the increased amount of voltage to flow across the output of R53?

  What about blowing out the primary or secondary windings of the output transformer? Should I attempt to replace the transformer?

  I'm confused about how to keep a load on the output of the transformer. If I have a cable connected to the output jack that's not connected to anything will that work to keep some kind of a load so that the amount of resistance is not infinity?

The 10ohm 3W resistor across the secondary is the load

[pittsburgh]

  Ok, thanks. I currently have the 3W 10 Ohm resistor across the black and yellow connections coming from T3. I'm having the same result. Is it possible that I've burned up the transformer?

[bdubya]

"Meter out the transformer primaries on bothe sides.

Mark "



Don't know if you caught this last chunk that Mark suggested.  You'd be checking for continuity, to ensure that one of the coils hasn't opened up.  should help narrow it down.

[pittsburgh]

What am I looking for when I meter out on both sides?

[pittsburgh]

I haven't had any luck testing across both sides of the transformer. I tried audible continuity and resistance while powered off. No results there, which could be an answer but I'm not sure. What exactly should I be looking for, and should I be metering while plugged in?

  If I've blown the transformer, what should I get? The service manual only has a serial number, no explanation of what the transformer is doing outside of the block diagram.