New Build 5F1 DOA - Any help appreciated!

[martin manning]

7 Grid 1 0 - 2.95 as the pot is turned CW

This says the 0.02 cap feeding the pot is leaking DC. Try replacing it.

[wr7r]

At first it was as original - then as I noticed problems, I switched over to floating 6.3 vac heater and ungrounded the other end of the 6.3 vac tap (I had done this on a previous Champ II)...so now is 6.3 volts toeach side of the heaters and on to the power indicator

[wr7r]

aha - ok will do - and let you know....

[Firestorm]

7 Grid 1 0 - 2.95 as the pot is turned CW

This says the 0.02 cap feeding the pot is leaking DC. Try replacing it.

Did you notice that his 6V6 control grid starts out with 12V on it and rises to 18V when everything is warmed up? Not right at all. Something misconnected between the second stage and the output grid. That's more than a leaky cap.

[Firestorm]

Oh.

I can't see your 220K grid load (on the 6V6) in the pic. Make sure that's 220K (not 220R or 2K2). That could account for the grid grabbing electrons on the way by (it's first in line, so to speak).

[martin manning]

Two leaky caps? Looks like maybe the other 0.02 is leaking too. What is the voltage rating on those? What wattage rating is the 6V6 cathode resistor? It should be a 1W at least, better a 2W.

[wr7r]

The .02 are both 630 volt orange drops - I just tried swapping to no avail.

the 6V6 cathode resistor is only 1/2 watt - I will replace that. I did pull up one leg and check the value.

Also checked the value of the 220K and the 100K resistors....I'm running out of things to check...I'm about to take the board out again to makesure there was nothing missed on the back side...

thanks

[ChrisM]

1/2W is way too small for the 6V6 cathode resistor. You need 2W minimum, 5W ideally.

Your power supply resistors also look small. I'd go with 2W there.

[Firestorm]

The .02 are both 630 volt orange drops - I just tried swapping to no avail.

the 6V6 cathode resistor is only 1/2 watt - I will replace that. I did pull up one leg and check the value.

Also checked the value of the 220K and the 100K resistors....I'm running out of things to check...I'm about to take the board out again to makesure there was nothing missed on the back side...

thanks

Damn! I thought that 220K was the culprit. So the question is: why do you see voltage that high on the 6V6 grid? There will be a little, but 18V at operating levels is pretty much what your cathode voltage should be. Tube won't work properly. Maybe recheck that voltage. And then, yes, look under the board for something miswired.

Chris is right: you want a few watts on the 6V6 cathode. Fender used 1W; just barely adequate.

[Structo]

Most likely a wiring error.
We have all been there.

Sometimes the best thing to do is walk away for a day or two and come back fresh.
You will still think about it so you might get a fresh idea.

[Phil_S]

Most likely a wiring error....

This is why I suggested the double verification to the schematic and with a magnifying glass. Either a wiring error or a whisker. You'll find it if you come at it fresh and with a plan.

[martin manning]

All three of the grids should be at zero volts and stay there. Is it possible that the board material is conductive?

[Phil_S]

It is hard to see in the picture, but your speaker jack looks very close to one of the turrets, maybe too close. In 3D, real life, maybe this is nothing.

Also, it looks like wires run under the board. There is nothing wrong about wires under the board. However, after the first time I had a problem with a wire under the board, I learned to avoid doing that because trouble shooting them is too difficult. As far as I'm concerned, any under board wire is a source of trouble until ruled out. No one wants to do it, but if there is nothing else obvious, the board needs to be lifted and wires checked.

[Firestorm]

When Fender ran wires under the board, they were cloth covered. After UL made them switch to plastic-insulated wire, they started to have trouble with component soldering accidentally melting through underboard wires and shorting things. Something to check, maybe.

The presence of DC on the grids is a dead giveaway that something is not right. The grids pick up a little DC in normal operation because they capture a small percentage of the electrons flowing to the plate (that is how grid-leak bias works). But the numbers we're seeing are way above anything normal. Martin may have a point about conductive circuit boards. Where is that board from? I see it's black, which often implies carbon content.

[Structo]

Also, check the speaker cable to make sure it is not shorted or making good contact.

I had a speaker cable go bad one time when I was working on an amp.
I just about tore everything out and thought I had possibly fried the output transformer.
Finally I metered the cable and it was open.