Troubleshooting question . preamp voltages

[amplifiednation]

I have a 100w build I'm trying to finish and the amp is powered up fine, but preamp voltages on two tubes aren't dropping when a 12AX7 is installed.

I have tested cathode voltage and it goes from 0v to 5v when I put the tube in, but the plates are just sitting at 353v... and should be closer to 200v,
cathode should be at 1.3v.

Does this mean the tubes aren't pulling current? Could I have a bad ground at the cathode?

Any help would be awesome. I've built this amp before so I'm pretty familiar with the circuit. The power amp is working perfect and voltages there and on the other 3 preamp tubes are within spec.

Thx
T

[martin manning]

Have you tried another tube, just in case? The plate voltage staying high means that there is no current flowing through the plate resistor, but there is ~4x the expected current flowing through the cathode resistor. I suspect a wiring error.

[amplifiednation]

I just figured it out. 1.5M cathode resistors are not the same as 1.5k cathode resistors.

Whoops!!!

Glad I spent 6+ hours testing and pulling the amp apart. It's better when you can learn something rom troubleshooting that isn't "pay more attention"

Should be making music soon.

[alvarezh]

Good catch there Taylor.

I don't trust myself and most of us have no supervisor to check our work. Same thing happened to me me once, but the missed value was on the plate resistor.

My solution has been to pick and test every component before assembly. During assembly, I do it again each time I am going to install a part. It takes longer but beats a 6 hour trouble shoot and a post at TAG!

Enjoy you amp.

[martin manning]

My solution has been to pick and test every component before assembly. During assembly, I do it again each time I am going to install a part. It takes longer but beats a 6 hour trouble shoot and a post at TAG!

Ya, but then what are the rest of us going to do for fun?

[alvarezh]

Not only that Martin.

When I was 15 years old I would buy Flying Magazine. There was a monthly column labeled "I learned flying from that". It was about stories of pilots getting in trouble and living through it. It definitely helped me stay out of trouble and stretch my life a little longer.

The amount of knowledge I have gained here by reading other builder/repair problems has been enormous.

Cheers.

[amplifiednation]

I measure them too, I guess I just wasnt paying attention when I populated the board. Voltages now spot on! Now on to troubleshoot the signal!

[martin manning]

There was a monthly column labeled "I learned flying from that". It was about stories of pilots getting in trouble and living through it. It definitely helped me stay out of trouble and stretch my life a little longer.

Ha! I read that column too!

[alvarezh]

Ha! trouble shoot the signal?

Sounds to me you need a well earned vacation. These type of mistakes are usually associated with over stressed situations, working too fast or already (brain) tired from doing other chores.

Not good!

All the best.

[amplifiednation]

Ha! trouble shoot the signal?

Sounds to me you need a well earned vacation. These type of mistakes are usually associated with over stressed situations, working too fast or already (brain) tired from doing other chores.

Not good!

All the best.

Too much work for sure! I'm on the couch until the AM. Thanks for the help guys

[Reeltarded]

There was a monthly column labeled "I learned flying from that". It was about stories of pilots getting in trouble and living through it. It definitely helped me stay out of trouble and stretch my life a little longer.

Ha! I read that column too!

Me too! I miss magazines and flying. Aww..

[alvarezh]

Oh yeah Miles? Next time I'm in Georgia I'll make sure to rent a C172. I pay…..you fly it!

Cheers!

[Reeltarded]

No ticket. I haven't logged a minute since 2001.

We'll just steal a plane. We didn't mean any harm... we were just having fun.

[rdjones]

There was a monthly column labeled "I learned flying from that". It was about stories of pilots getting in trouble and living through it. It definitely helped me stay out of trouble and stretch my life a little longer.

Ha! I read that column too!

Me too! I miss magazines and flying. Aww..

Me three.

Dad had a 172 for years before they tightened up the medical requirements on GenAv.

rd

[alvarezh]

No prob, I'll pay for the instructor. You will take the left seat though. Flying VFR is like riding a bike… you never forget. An extra $35.00 is only 10% of a new 12AX7 Tlefnkn in original box.

W'll check out the chicken farms around your area, seen dozens just north of Atlanta. Haven't seen any peach trees though, must be farther North.

All the best.