pittsburgh

Bad tube or a bad engineer?
« on: January 26, 2010, 05:31:46 PM »
 I worked on my tube pre amp project last night and ran into some major problems. I Installed 1/4" jacks using the ground at J30 and J21, the hot is coming from J19 and J 50 through 10W 10 Ohm resistors. Before installing the jacks I was able to listen to audio without problems. After I installed the 1/4" jacks I heard the left channel start to crackle. The tube began to spark. I powered off and tried swapping tubes, but I got more smoke as a result.
  I've not worked with tubes much. I'm wondering if I have a bad tube, they are at least 40 years old, if I have done something wrong with grounding audio outputs, or if something else has failed.
  Thanks for the help.

  -P


pucho812

Re: Bad tube or a bad engineer?
« Reply #1 on: January 26, 2010, 06:20:22 PM »
DId you have to drill holes for the jacks or was their holes already there? I am thinking that if you drilled the holes you have have vibrated something loose like say a solder joint?
You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is.

pittsburgh

Re: Bad tube or a bad engineer?
« Reply #2 on: January 26, 2010, 06:23:05 PM »
Yea, I drilled into the existing case. Any idea where to start checking? What's the word on wether or not a tube is dead or not?

Mark Burnley

Re: Bad tube or a bad engineer?
« Reply #3 on: January 26, 2010, 06:27:37 PM »
What was it that smoked?

Don't forget that those are power output stages, and want to see a ~10R load at all times- so if you connected the output of the transformer to the jack tip through (in series with) a 10R resistor, then the output stage would see no load (or a much larger load than normal- depending on load impedance) Also, the "ground" side of the output transformer has a different symbol to the ground point at J30/J21 (check the symbol- one appears to be 0V, and the other "chassis ground"?)

If the output stage doesn't see the 10R load across the secondary of the transformer, it will definitely cause problems (don't forget that the anode load of the output tube is the reflected impedance of the load on the secondary)

We need to see-

a) exactly how you've wired your 10R resistors to the secondaries

b) if the J30/J21 ground and the "low" side of the secondaries are connected together.

Edit>Which tape machine is this??

Mark
« Last Edit: January 26, 2010, 08:20:16 PM by Mark Burnley »
O_O tape is life O_O

Re: Bad tube or a bad engineer?
« Reply #4 on: January 26, 2010, 07:57:44 PM »
Assuming your jack is 2 terminal (not TRS) connect hot to tip and 'lo side' to sleeve - in other words leave it floating.

Disconnect from the J30/J21 grounds.  I'm wondering if the connection there is accidentally making contact with something it shouldn't.

Also which tube sparked/smoked?  Could be a clue.

pittsburgh

Re: Bad tube or a bad engineer?
« Reply #5 on: January 26, 2010, 11:56:31 PM »
Mark, I've wired the resistors in series with the OT secondary lead coming out from the transformer that connects to J50 and J19. Instead of connecting at J50 and J19 I've simply taken the lead out of the OT and connected that to the jack. After what you mentioned about the ground, I connected the ground of the connectors to chassis with the same result. I'm confused about the loading of this part of the circuit. Also, I'm using the 10W resistors in place of the 3W that were recommended for the job.

Mark Burnley

Re: Bad tube or a bad engineer?
« Reply #6 on: January 27, 2010, 03:38:09 AM »
Hi,

Have you built this from scratch, or are you modifying the original machine?

Best thing to do is remove the 12Ax7 and the 6T9's and measure all the resistor values with your meter. Also check continuity of o/p transformer windings, and that the + and Gnd rails have continuity from the PSU to all points in the circuit.

Make sure the power is OFF and has been off for a while.

(put your meter in Volts range across PSU caps to check, and drain main filter caps with 10k/2W resistor to make sure)

There is also feedback via a 22k resistor to the cathode of the driver stage- have you kept this connection?

Mark
O_O tape is life O_O

pittsburgh

Re: Bad tube or a bad engineer?
« Reply #7 on: January 27, 2010, 04:12:52 PM »
This is a tape machine from the 60's. It's the 3M Wollensak 1280. I'm attempting to modify it into a stereo tube preamp.

tv

Re: Bad tube or a bad engineer?
« Reply #8 on: January 27, 2010, 04:29:59 PM »
Signal path looks just like a FuzzFace into a single-ended valve Class-A guitar amp.

LOL
If you sprinkle when you tinkle, please be neat and wipe the seat.

pittsburgh

Re: Bad tube or a bad engineer?
« Reply #9 on: January 27, 2010, 05:23:12 PM »
  Contrary to what I said about one of the GT9 Tubes smoking, I believe C33 to be the culprit. Is it possible that my problems have been because of a failed Capacitor?

  You're right about the fuzz face, this device has some very nice sounding distortion.


pittsburgh

Re: Bad tube or a bad engineer?
« Reply #10 on: January 27, 2010, 11:55:03 PM »
  I believe I've found the source of what smokes in the 1280. R57, (270 Ohm 10% 1watt), gets very hot when the unit is turned on. I checked resistance for the component and it was 432 Ohms. That's greater than the +/- 27 ohm tolerance. Could this possibly be the source of my problems?

  R57 is used to send pin 9 of the GT9 tube to ground. If there is too much resistance will that cause the current to back up into the tube and cause sparking?

  I measured the resistor in place on the board. Should I pull the resistor out and measure it by itself for a valid, or more accurate reading?

  Also, what happens to a resistor when it breaks down? Does the amount of resistance go up or down?

Re: Bad tube or a bad engineer?
« Reply #11 on: January 28, 2010, 01:15:18 AM »
Mark, I've wired the resistors in series with the OT secondary lead coming out from the transformer that connects to J50 and J19. Instead of connecting at J50 and J19 I've simply taken the lead out of the OT and connected that to the jack. After what you mentioned about the ground, I connected the ground of the connectors to chassis with the same result. I'm confused about the loading of this part of the circuit. Also, I'm using the 10W resistors in place of the 3W that were recommended for the job.



If when wired this way you connected the output to a high impedance load like the input to a sound card or mixer and then disconnected it (open) you may have gotten 'flyback' voltages in the output transformer that could have shorted some of the windings on the primary.  The ouput tube would then have nearly no load/near short thus very high currents in the plate circuit which could easily fry the cathode resistor.

You can check the OT primary resistance and see if it's gotten super low.

Whatever the case it sounds as if you're getting too much current flow in the plate circuit.

pittsburgh

Re: Bad tube or a bad engineer?
« Reply #12 on: March 14, 2010, 08:41:12 PM »
  I'm looking to replace the OT secondary in this schematic after determining that it has been damaged. Given the schematic at the top of the page, can anyone tell me what type of transformer I should get to replace the damaged one currently in place?

  Thanks for the help.

  -P


 

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