Tubenit's TOS Build question re: cathode current

[j_bruce]

So I have been working on a TOS build and I am down to one connection left to go: the NFB

In Tubenit's schematic (https://el34world.com/Forum/index.php?action=dlattach;topic=9636.0;attach=15949) runs from a 4.7k resistor to to pin 7 of V4. That is simple enough, but my OT has multiple leads for 4, 8 and 16 ohm speaker loads. I am choosing to use both 4 and 8 ohm jacks so that I can be flexible on speaker selections.

So how do I connect the NFB and the 4.7K?

One thought I had was to use a switch to switch the 4.7k between the two jacks, beyond that how can I do this?

Thanks for the help,
Jerry

[PRR]

The plan shows connected to the "8" tap. Do the same.

You do NOT need (or want) to connect to both taps. The signal is 99.9% the same except for voltage.

[j_bruce]

Thanks, will do.
Jerry

[j_bruce]

So I fired this up this morning and it is a no go.

I put a 1khz signal in and I get a 1 khz signal all the way down the signal path until I hit Pin 3 on the output tubes. I also have no cathode current on both tubes.

What should I be looking at?

pin 3 = 432v
pin4 = 429v
pin 8 - 63v
pin 5 has a clean 1 kh signal coming in and the PPIMV is working fine giving a wide range of signal strength.

 https://el34world.com/Forum/index.php?action=dlattach;topic=9636.0;attach=15949

I would much prefer to find these things myself but I cannot find anything mis-wired or out of spec.

thanks,
Jerry

[sluckey]

pin 8 - 63v

Recheck pin 8 voltage. Also, turn the power off and measure resistance from pin 8 of each output tube to chassis. Also measure resistance from pin 5 of each output tube to chassis.

[j_bruce]

Steve,

Thanks for the note,

both pin 8 readings are 61.9v

pin 8 resistance is 2.2k on both, seems like this should be around 420 ohms?

pin 5 resistance is 1.9k on both tubes, this did not make sense given the 10k right there so I checked the 10k and they measured 1650 ohms. So I checked all my 10 k and they all measured (+-) the same, so there is that. I will have to get some correct values and get them in place. This also makes me suspicious of the other resistors as I had bought a big pack of many values. Hmmm.

I think I will always check values and not rely on the color bands in the future.

Thanks,
Jerry

[sluckey]

Pin 8 should be 430Ω. Pin 5 should be 10K when the MV is on zero and should be about 200K when the MV is turned up to max.

Post a hi-rez pic of the grid and cathode resistors.

Might be a good idea to check all resistors in the amp with your meter.

EDIT... The 1650Ω grid stoppers is not a show stopper, but the 2200Ω cathode resistor is. BTW, that cathode resistor needs to be about 10 watts!

[tubenit]

Please follow Sluckey's excellent advice.   Pin 8 should be 430R/10w

I always measure the resistor nowadays prior to placing it in the amp. Color bands give you a good starting place but values need to be verified.

With respect, Tubenit

[j_bruce]

Just got back home and will follow up in a while. The cathode resistors are 10 watt, the grid are 1/2 watt.

The cathode is a 120 ohm in series with a 300 ohm as that is what I could order to get 420, I also bought a 400 ohm so I could make a variety of combos to dial it in if needed. But that is so far off I am thinking there must be something else wrong in that part of the circuit. I will examine it closely, and grab a couple pictures.

The pin 5 does go up near 200k with the MV knob, so that is working.

I was thinking the cathode resistor being that high would limit the cathode current to much and result in a very "cold" bias thus no sound.

I think checking all resistors is definitely on the list for tonight.

Thanks,
Jerry

[sluckey]

The cathode is a 120 ohm in series with a 300 ohm as that is what I could order to get 420, I also bought a 400 ohm so I could make a variety of combos to dial it in if needed. But that is so far off I am thinking there must be something else wrong in that part of the circuit. I will examine it closely, and grab a couple pictures.

I was thinking the cathode resistor being that high would limit the cathode current to much and result in a very "cold" bias thus no sound.

Well, there's not much in the cathode circuit. Should only take a few minutes to find why the resistance reads 2200Ω. Put your black probe on chassis. Put the red probe on cathode and note reading Should still be 2200Ω. Now move the red probe to the junction of the 120Ω and 300Ω. What reading do you get? Should tell you which resistor is not right. When I say resistor, I'm also including  any 1Ω resistors, all wires, and solder joints involved in the path between pins 8 and chassis.

Another quick test would be to jumper pins 8 of both tubes together then use some gator clip test leads to connect that 400Ω resistor between pins 8 and chassis.

[j_bruce]

Yes, not much to it and I found it easily as you thought I should: bad solder joint on the connection to the ground tab. Touched that up and she is rockin' pretty solid!

I need to go back and address the 10k resistors on the grid, and check all the remaining resistors for correct value.

As is this amp has more tone with my LP than I deserve given my talent. I will need to bring home a harp mic and test it out on that. I think the clean side with the fat control turned way up should be the right neighborhood.