First fire-up voltage problem [SOLVED]

[benoit]

Just finished the build in the wee hours of this morning. Nothing popped and all the smoke is still inside all the parts. Plugging in a guitar I get some pretty nice cleans but almost no dirt from the OD channel.

I'm pretty sure the main reason for this is v1a. about 105V at the plate.

The rest are as follows:

V1b: 213

V2a: 173
V2b: 186

V3a: 286
v3b: 289

I was a little concerned about the power tubes. 455 at the plates, which I would guess is fine (bias at 40ma for now on the 6L6s) but 456 at the screens. Given that the screen supply is taken after the choke and the plates before (yes, I double checked that it really is this way in my amp), shouldn't the screens be at a slightly lower voltage?

I'll be doing some reading later today when I have more computer time but if anyone has any suggestions I'm all ears.


Ben

[heisthl]

If it's true and it is wired correctly and the screens are indeed higher, you have just solved our energy crisis!

[dobbhill]

Check your V1 B+ (plates) and make sure they are wired correctly, and your V1 cathode resistor values.
Just a humble suggestion......
D

[Bob-I]

Post the cathode voltages also. I'll bet there's something off on V1a, such as a bad cathode cap or bad ground connection. Close up pics would help also.

[benoit]

Vk for v1a was like .9 or something. v1b was 2.something.

[Bob-I]

Vk for v1a was like .9 or something. v1b was 2.something.

Once time I reversed the Rk's on V1 and got voltages like you listed. Double check. They should both be aroiund 1.7 1.8.

[benoit]

Vk for v1a was like .9 or something. v1b was 2.something.

Once time I reversed the Rk's on V1 and got voltages like you listed. Double check. They should both be aroiund 1.7 1.8.

I did have the cathode resistors switched, but fixing that didn't completely fix the problem. V1a Vk is now 1.2 and V1b Vk is 1.8. Small victory but there's still something wrong.

I've swapped V1 and V2 tubes just to make sure it wasn't a tube problem.

Let's say the V1a cathode bypass cap is a problem. How would I check that without a capacitance meter? I don't have any replacements in that exact value, though I do have some 50v 1uf caps that might do. How critical is that value?

On low volume (1ish on the master) I get some promising tones but above there, even with a shield in place covering the chassis there's a lot of noise. I'm hoping this clears up when the voltage issue is resolved. I have some time now so I'm going to dig into old posts and see if I can find any solutions.

Ben

[benoit]

pics

[benoit]

more pics

[Bob-I]

I did have the cathode resistors switched, but fixing that didn't completely fix the problem. V1a Vk is now 1.2 and V1b Vk is 1.8. Small victory but there's still something wrong.

Are the plate voltages better now?

Let's say the V1a cathode bypass cap is a problem. How would I check that without a capacitance meter? I don't have any replacements in that exact value, though I do have some 50v 1uf caps that might do. How critical is that value?

You could simply disconnect the cap and see if the voltages are correct. Based on the cathode voltages you're showing now I'd say that's not the problem.

On low volume (1ish on the master) I get some promising tones but above there, even with a shield in place covering the chassis there's a lot of noise. I'm hoping this clears up when the voltage issue is resolved. I have some time now so I'm going to dig into old posts and see if I can find any solutions.

Ben

Voltages off doesn't usually cause noise issue like you're describing.

I'd start by grounding the grids of the various preamp tubes to see where the noise is being generated.

You can also ground out the PI input and see if there's still noise. If there is, the problem must be in the output section, maybe a bad ground on the power supply.

If there's no noise with the PI grounded, go to V2b and ground the grid, then V2a, V1b, V1a. This will at least narrow down your troubleshooting.

Nothing jumps out at me in the pics.

[benoit]

Plate voltage is still no good. Is the way I did the fet simulator resistor correct? From the last B+ node a 150k resistor to ground?

[Bob-I]

Plate voltage is still no good. Is the way I did the fet simulator resistor correct? From the last B+ node a 150k resistor to ground?

The 150K resistor is done right.

If you're still only getting 100V on V1a plate, you need to look at that before anything else. Measure the voltage with the tube out, double check the plate load resistor's value. Double check the connections to the tube socket. Make sure there's no hairs from any wires on the tubes sockets shorting.

[dobbhill]

You might try measuring the B+ on both ends of the plate resistor, then using the measured value of the plate resistor, calculate the voltage drop and dissipation of that stage.
Is it the tube, the resistors/caps, or the wire?
Just trying to help..
D

[Structo]

I can't see where your buss wire is grounded but typically it should be grounded at the input ground. That could be a noise source if not done correctly.

[benoit]

I can't see where your buss wire is grounded but typically it should be grounded at the input ground. That could be a noise source if not done correctly.

It is, but I definitely appreciate all the tips I can get.

You might try measuring the B+ on both ends of the plate resistor, then using the measured value of the plate resistor, calculate the voltage drop and dissipation of that stage.
Is it the tube, the resistors/caps, or the wire?
Just trying to help..
D

The tube doesn't seem to be the problem. I've tried the tubes from the PI and OD stage in v1 and nothing changes more than a few volts. The voltages with those tubes in v2 and 3 are just fine. I need to double check what the plate load resistors measure. Don't think it's caps. Wire is good.
B+ on the positive side of the plate loads is about 330 volts IIRC. I guess I'm not 100% sure about the calculations you're talking about - both how to do them and what they tell me. Could you explain? Thanks for the help so far.