Homebrew 5e3 voltage leak?

[cubby01]

I've been trying to trace down a problem with my 5e3 build. The original problem I was running down was a lot of background noise, like white noise when the volume is anything other than zero. In doing so I find I'm not getting the DC voltages I'd expect at the first filter cap (A). I've found that with 5y3 rectifier and no tubes in I get about 370 vDC. If I put in the preamp and PI it stays pretty close to same value. When I put in the 6v6 tubes I get a reading of only about 252 vDC. This drop isn't normal is it? If I measure as I switch it on with all tubes in, measured at first filter cap (A) it runs up quickly to 370v then after a few seconds it starts dropping and settles down around 252v. When I turn it off voltage drops to near zero within a second or two.

If this is consistent with some kind of leakage how do I go about pinning it down. I don't think it's bad filter caps. They seem to hold a charge as long as the 6v6s are not installed. I've been through the wiring a dozen times, pulling leads to check resistors, resolder, swaping parts, etc.

It is based on Weber 5e3 layout and schematic which is true to original Fender Deluxe. The chassis and board are diy. Components are from weber. Xformers and tubes are vintage and test appropriate to this build.

Thanks,

[Chuck H]

It sounds like your power tubes are drawing too much current. If that is the case it is most likely a bad tube or a mistake in the power tube cathode circuit.

Chuck

[cubby01]

It sounds like your power tubes are drawing too much current. If that is the case it is most likely a bad tube or a mistake in the power tube cathode circuit.

Chuck

I've swapped out various brand tubes in all positions. No changes. When you say "power tube cathode circuit" are you talking about the 250 ohm resistor and 25uf cap in this schematic/layout?
http://www.ampwares.com/schematics/deluxe_5e3.pdf

I have pulled the 5w resistor and verified that it is 250 ohm. I don't know how to verify the little electrolytic but I could swap it when I get a replacement part to see if that makes any difference.

Any other thoughts?

I was having an issue with the sound cutting out for a second and then come back in. This was when volumes were above 3ish and I just chord thrashing. I replaced the OPT and it doesn't cut out anymore but I still get a lot of background noise and the dc voltages is still acting as indicated in the initial post.

[Chuck H]

The most likely reason the voltage would drop only with the power tubes in place is that the power tubes are drawing extreme current. In fact it's the only reason I can think of.

You can simply remove the cathode bypass cap. The amp doesn't need it to operate. But it does improve gain. So, if you remove that cap and the amp behaves, it's the cap. If it's not the cap then I suspect a mistake in the power amp wiring somewhere.

I was having an issue with the sound cutting out for a second and then come back in. This was when volumes were above 3ish and I just chord thrashing. I replaced the OPT and it doesn't cut out anymore but I still get a lot of background noise and the dc voltages is still acting as indicated in the initial post.

The cutting out is probably blocking distortion. The power tubes are drawing so much current that they are trying to draw current from the grids. This builds a charge in the coupling caps that throws off the bias and sends the power tubes into cutoff until they can recover proper bias. This is part of the same condition as above and also indicative of the power tubes drawing way too much current. Since you've tried other tubes and tested the cathode resistor I would suspect the bypass cap or a mistake in the wiring.

What is the tested value of your bias resistors? Test the ground point for those resistors.

With the amp at idle, what is the voltage on top of the cathode resistor?

Chuck

[cubby01]

You can simply remove the cathode bypass cap. The amp doesn't need it to operate. But it does improve gain. So, if you remove that cap and the amp behaves, it's the cap. If it's not the cap then I suspect a mistake in the power amp wiring somewhere.

Lifted the 25uf - No change.

The cutting out is probably blocking distortion. The power tubes are drawing so much current that they are trying to draw current from the grids. This builds a charge in the coupling caps that throws off the bias and sends the power tubes into cutoff until they can recover proper bias. This is part of the same condition as above and also indicative of the power tubes drawing way too much current. Since you've tried other tubes and tested the cathode resistor I would suspect the bypass cap or a mistake in the wiring.

What is the tested value of your bias resistors? Test the ground point for those resistors.

Bias resistors are marked 220K and both test as 218K, (within 1% should be ok right?)

With the amp at idle, what is the voltage on top of the cathode resistor?

It is 17vDC - same at pin 8 (cathode) of both 6v6.

For what it's worth at idle the voltages at each filter cap are now showing as 314, 277, and 209 vDC. This is a little better than yesterday and the only thing I changed since then was the OPT and swapping in some Magnavox branded 6v6's. The initial voltage climbs a bit higher too then drops to these levels.

Also at idle, I'm only getting 100mv (.1vDC) and 70mv (.07vDC) on pin 5 and 6 (grid) of each 6v6 and I was told in another thread I should have a value of at least 1vDC.

I don't doubt I could have a mistake in the wiring or bad solder joint but I just haven't found it yet. I'm just about to the point of disassembling and reassembling on a different board rechecking values along the way. It's a PITA but that route has worked for me in the past.

[Chuck H]

Now that all looks correct based on your unloaded voltages posted earlier. I don't know that the grid of a 6V6 needs to have a volt on it. But I do know that at this time your other voltages look correct. How does the amp sound?

One or both of your power tubes was probably bad. So save that OT you took out for another project. Unless the old tubes smoked it at some point it's probably fine.

Chuck

[cubby01]

How does the amp sound? Chuck

I've still got the same noise floor issue I started with. Noise comes in at any volume other than both pots at zero. It's like white noise static you get between radio stations. Much louder than any acceptable hiss. These are all MF resistors, no CC in this build. I swapped the tone pot with one of the volume pots and the quality of the noise changed but didn't go away. One of the thee pots is new the other are vintage pulls. I plan to replace the older ones to see if that's helps. Other than this background noise the guitar sounds fine. Gets loud real quick and is crunchy about where it's supposed to be I guess.

Thanks for your help.

[Chuck H]

Hiss is generally caused by gain or a noisy tube. Do both channels operate as they should now other than the extra noise?

If so, try pulling tubes. Starting with the preamp tube. If the noise stops then it's coming from your preamp. Please explain the noise in some detail and how it responds to the volume and tone controls.

Chuck