First fire-up voltage problem [SOLVED]

[mat]

I'm not sure if this is helpful at all but I have had couple times similar problems like You. One time it was bad tube socket. The other time it was cold solder joint on one of the pots (Gary (glassman) helped me on that one). In one picture, if I see right, there seems to be a piece of wire on the pre volume pot to the casing of the pot. It might be a hair also. Hope You find the solution.

[benoit]

Wow mat, good eye It was a hair. I did find that I had the preamp and OD B+ leads reversed, and fixing that cut down drastically on the clean channel noise. Plate voltage on V1a is still around 113V and the cathode is now about 0.96V. V1b is at 180ish on the plates and 1.7 on the cathode. I've replaced the v1a plate resistor with one I know is good.

I broke the bright switch as I was flipping the amp over and this happened before first fire-up so maybe I should fix that but it seems like it shouldn't have much effect on v1a's bum voltages.

A friend who is a local tech recommended I check the 1M to ground at the input and the 22k at the grid of v1a, so I did, with no luck.

I've now tried 3 known good 12ax7s in the amp so I really don't think it's the tube. I guess the thing to do now is check continuity on the socket.

Any other suggestions?

[Bob-I]

I didn't see where you measured either the plate load resistor on V1a or the voltage drop across it. More than once I've installed the incorrect resistor.

[dogears]

Please measure the plate load and cathode resistors in circuit.... Thanks!

Although my money is on the socket.... If there is excessive resistance at the plate contact pin you will have this real low voltage....

Wow mat, good eye It was a hair. I did find that I had the preamp and OD B+ leads reversed, and fixing that cut down drastically on the clean channel noise. Plate voltage on V1a is still around 113V and the cathode is now about 0.96V. V1b is at 180ish on the plates and 1.7 on the cathode. I've replaced the v1a plate resistor with one I know is good.

I broke the bright switch as I was flipping the amp over and this happened before first fire-up so maybe I should fix that but it seems like it shouldn't have much effect on v1a's bum voltages.

A friend who is a local tech recommended I check the 1M to ground at the input and the 22k at the grid of v1a, so I did, with no luck.

I've now tried 3 known good 12ax7s in the amp so I really don't think it's the tube. I guess the thing to do now is check continuity on the socket.

Any other suggestions?

[benoit]

I didn't see where you measured either the plate load resistor on V1a or the voltage drop across it. More than once I've installed the incorrect resistor.

Out of circuit it measures about 220k (217 i think). I just replaced it because the old one didn't read right. In circuit I'm confused because when I set my meter's range to 2000k it reads 917k. Set to 200k it reads 90k Set to 20k it reads 9.something. This is with the amp on. With the amp off it drops steadily down from 900V. Voltage drop across it is 277V. Drop across the cathode resistor is .97V. In circuit it measures 779V. I apologize for not providing all the right measurements but I'm still learning about some of the electronic theory.

Ben

[Bob-I]

I didn't see where you measured either the plate load resistor on V1a or the voltage drop across it. More than once I've installed the incorrect resistor.

Out of circuit it measures about 220k (217 i think). I just replaced it because the old one didn't read right. In circuit I'm confused because when I set my meter's range to 2000k it reads 917k. Set to 200k it reads 90k Set to 20k it reads 9.something. This is with the amp on. With the amp off it drops steadily down from 900V. Voltage drop across it is 277V. Drop across the cathode resistor is .97V. In circuit it measures 779V. I apologize for not providing all the right measurements but I'm still learning about some of the electronic theory.

Ben

I'm thinking something is wrong with your meter. The voltage drop across that resistor should be in the range of 100-120V, not 270.

You should be able to measure the resistance of Rp once the amp is unplugged and it's been off for a few minutes. Your readings are all wacked out. I'd first check the batteries in the meter and measure several resistors. If you don't get a consistant reading, get a new meter.

The theory is fairly simple. V=IR so the voltage across Rp of 277 = I *220,000. That's .0012. Taking the same calculation across the cathode you get .00029. These current measurements should be the current through the tube and they should be roughly equal.

To compare to my amp, I have 112 across Rp and 1.6 across Rk. Both work out to .00005.

Check that meter.

[ic-racer]

I didn't see where you measured either the plate load resistor on V1a or the voltage drop across it. More than once I've installed the incorrect resistor.

Out of circuit it measures about 220k (217 i think). I just replaced it because the old one didn't read right. In circuit I'm confused because when I set my meter's range to 2000k it reads 917k. Set to 200k it reads 90k Set to 20k it reads 9.something. This is with the amp on. With the amp off it drops steadily down from 900V. Voltage drop across it is 277V. Drop across the cathode resistor is .97V. In circuit it measures 779V. I apologize for not providing all the right measurements but I'm still learning about some of the electronic theory.

Ben

I believe they mean to measure the VOLTAGE across these resistors with the amp up and running. If you have this voltage and the resistance, you can calculate the bias current flowing in and out of the tube. Just more ways to see if anything is funny.

[danotron]

Out of circuit it measures about 220k (217 i think). I just replaced it because the old one didn't read right. In circuit I'm confused because when I set my meter's range to 2000k it reads 917k. Set to 200k it reads 90k Set to 20k it reads 9.something. This is with the amp on.
Ben

Ben

Don't measure resistance with the amp on. Your meter may be buggered.

[benoit]

I just tried a fresh 9-volt in the meter and it appears to be messed up. With the amp off for several hours I couldn't get a steady reading on the plate load resistance value in-circuit. It's a cheapo ten dollar craftsman and I've gotten a few years out of it. I'll have to do some research on who's good and relatively inexpensive.

[dobbhill]

I highly suspectV1a plate resistor, cathode resistor, or cathode cap. I suggest removing the cathode cap first and testing voltages without it. Also, check the cathode resisitor one more time.
Maybe the FET simuator should be removed until this is resolved.
D

[benoit]

I did a little more checking. The meter seems to actually be more reliable than I thought. So..

Here's what I know:

Plate resistor is correct - 219k
Cathode resistor is correct 3.31k

Plate voltage hovers around 100V, +- 5V depending on the tube in v1.
Cathode voltage is still about .95V. This has been relatively constant as well despite changes. Changes include measuring without a cathode bypass cap and subbing a few different 1uf caps (the only value I had on hand).

I also checked the v1a socket as best I could by checking for continuity between a short piece of bus wire stuck into each hole in the socket and the corresponding lug on the other side. I don't know if this would be enough to tell if the socket were the problem, as that bus wire wasn't quite as large in diameter as a 12ax7 tube's pin.

I don't see at this point how it could be anything other than the socket, unless it's something not mentioned yet. I'll have to see if anyone has a better meter I can borrow as I'm not sure when I'll be able to afford one myself.

Please keep the ideas coming, and if my logic used to rule out possibilities is flawed let me know and I'll double check things.

The amp sounds pretty tasty already, other than being too noisy on the OD side.

[benoit]

I did a little more checking. The meter seems to actually be more reliable than I thought. So..

Here's what I know:

Plate resistor is correct - 219k
Cathode resistor is correct 3.31k

Plate voltage hovers around 100V, +- 5V depending on the tube in v1.
Cathode voltage is still about .95V. This has been relatively constant as well despite changes. Changes include measuring without a cathode bypass cap and subbing a few different 1uf caps (the only value I had on hand).

I also checked the v1a socket as best I could by checking for continuity between a short piece of bus wire stuck into each hole in the socket and the corresponding lug on the other side. I don't know if this would be enough to tell if the socket were the problem, as that bus wire wasn't quite as large in diameter as a 12ax7 tube's pin.

I don't see at this point how it could be anything other than the socket, unless it's something not mentioned yet. I'll have to see if anyone has a better meter I can borrow as I'm not sure when I'll be able to afford one myself.

Please keep the ideas coming, and if my logic used to rule out possibilities is flawed let me know and I'll double check things.

The amp sounds pretty tasty already, other than being too noisy on the OD side.

[Bob-I]

I'm puzzled. Tube sockets aren't usually this consistant when they're bad. Jiggle the tube and the measurements change.

Still you're drawing a lot of current through V1a and that's not right. I suspect something wrong in the V1 wiring, possibly a little hair crossing terminals or a bad solder joint. I don't see a pic of V1 clearly. This problem could also be under the board where you can't see it.

I wouldn't give up until I figured out why the voltage on V1a plate is so low, and why there's so much drop across Rp. Guarenteed that it's the same thing causing both symptons.

[Bob-I]

BTW, noise is V1 will be worse in OD mode. I'm thinking if you find the source of the voltage problem, the noise will be resolved too.

[danotron]

Just looking at the pictures...

What is the 150k resistor on the Dale plate resistors doing? Looks like it is just going from B+ to ground. Is that correct?

This problem has to be something simple....were just missing it.

Can you post the schematic and layout that you used?