147 amp cathode resistor

[Drawbar Dave]

I have spent a lot of time going through this amp again checking voltages based on the Benton Electronics page which I’ll list;
Cathode resistor (on the side connected to the bypass cap) is 27 vdc
Off the bridge rectifier (with new diodes) is 462 vdc
After the choke is 440 vdc
After OC3 is 325 vdc
After 10k resistor is 271 vdc

While I have the amp running on my bench with the tubes fitted and a 16 ohm load across the output I notice the cathode resistor gets very hot and starts to smell. Secondly I cannot get a dc reading from the other terminal of the cathode resistor hence my lack of juice to pin 4 of the 6550’s. I put my meter across the cathode resistor in situ and get a reading of 148 ohms.

The amp had been sitting for years. I didn’t power it up until I had changed the can and bypass caps, the output tube and OC3 resistors and the diodes in the rectifier. All the tubes are new except the OC3 which is a NOS 1941 tube.

Every component was done one at a time by desoldering one end, connecting the replacement and the doing the same at the other end. I have triple checked that the components I changed are correct.

Thanks
Dave

[David Anderson]

You can't get a DC reading from the other terminal of the cathode resistor because it's connected to ground, same as the other lead of your voltmeter, presumably.

Are you sure you're counting tube pins correctly? If you count in reverse, you'd count 8, 7, 6, 5, so you'd be looking for a voltage on pin 5 instead of pin 4. It's usually a yellow wire on pin 4.

[Drawbar Dave]

Thank you David. Yes I read it backwards. I have the correct voltage on pin 3 and the voltage drop on pin 4 as per George’s page. Any idea what could cause the cathode resistor to overheat?

I haven’t changed the four resistors relating to the 12AU7 although they are close to their values. I don’t have any more 12AU7’s to hand but could borrow a known working 12AX7 from my Binson to try.

[David Anderson]

Cathode resistors get hot. Check any Leslie amp. That's why they're wirewound and mounted in a chassis hole. If they have dust or gunk on them, it will smell.

Are you thinking that the resistors biasing the 12AU7 have something to do with the 6550 cathode resistor? They don't.

[Drawbar Dave]

Thanks again David. I never realised that about the resistor. I asked what they were made of in the past but no one answered.

[Drawbar Dave]

I have located the problem with the amplifier. The plate voltages on the 12AU7. Pin 1 is 60 vdc while pin 6 is 120 vdc. Every resistor in the amplifier has been carefully replaced as have the orange drops and electrolytics. All other voltages check out. It was reading low before and after I replaced the remaining four resistors around the 12AU7.

[muckelroy]

Check 12AU7 socket voltages with 12AU7 tube removed. Has the tube ever been replaced? Check the cathode voltages of the 12AU7.

Note: This is piece-meal troubleshooting. Most answers to your questions can be found by searching both this forum, and countless other resources that talk about how tube amps work. Photos will help greatly. Otherwise, we are just kind of kicking the can along. Saying components were carefully replaced is all fine and well, but why were they replaced to begin with? What was the problem with this amp that prompted the replacement of every resistor and capacitor in the first place?

[Drawbar Dave]

Check 12AU7 socket voltages with 12AU7 tube removed. Has the tube ever been replaced? Check the cathode voltages of the 12AU7.

Note: This is piece-meal troubleshooting. Most answers to your questions can be found by searching both this forum, and countless other resources that talk about how tube amps work. Photos will help greatly. Otherwise, we are just kind of kicking the can along. Saying components were carefully replaced is all fine and well, but why were they replaced to begin with? What was the problem with this amp that prompted the replacement of every resistor and capacitor in the first place?

Thanks. I have outlined why I wanted to go through this amp before powering up in the first post due to unknown history. All I know is that it was in storage for decades. I’ll stick to the search option rather than asking questions.

[David Anderson]

The obvious test is to plug in a known-good 12AU7, but I know you don't have one. If you're going to do this kind of work, you need 12AU7s on hand. No way around that.

In terms of the 4 resistors setting the 12AU7's operating points, there should be 5, including the shared cathode resistor: two 1M grid-leak resistors, 1.2k shared cathode resistor, and two 56k plate resistors. Perhaps you already replaced one?

[Jyvoipabo]

Hellio,

I think one of the 0.1 / 600 capacitors is bad : the one outgoing from pin 6 of the 12AU7

Remove this 12AU7 and the 6550 who may die.
Check Zero volts on pin 5 (Grid) of each socket of the 6550 you removed.

JP

[Drawbar Dave]

Thanks guys. JP I already replaced all the .1 caps but I have just got home from a 6 hour round trip to pick up my BC which came with a load of spares including an AO-29 and a load of tubes so first thing tomorrow I will try another 12AU7 to see what voltages I get.

[Jyvoipabo]

You can try a 12AX7.
It's the same pinout and this tube is not involved in the feedback loop.
But values will change a lot.
Approx : VBias 2V and VAnode 200V near blocking
If it works (VAnode are the same) you will got a higher gain and a terrific sound.

JP

[Drawbar Dave]

Swapping the tube makes no difference. I tried another 12AU7 I got yesterday.

[David Anderson]

Based on what you've told us, I'd say that you either, for some reason, have a positive DC voltage on the audio input (pin 2), or, based on the previous episode of miscounting tube pins, you've made a mistake that's right in front of you, but you're not seeing it. Or maybe there's a positive voltage on pin 2 because you made a mistake when replacing resistors.

Biasing triodes is very simple, and because of that, the list of what could be wrong is pretty short.

[Drawbar Dave]

Thanks for your help David. I’ll get back to it and retrace my steps. I need to compare it to the schematics as I found the previous owner had fitted an aftermarket relay and connected pins 2 and 5 of the 6 pin to each side of the coil so there could be other errors!