5F2a voltage: My Grids Ain't Good

[dig22]

My little 5f2a is working like a charm but for the grid 1 voltages on the 12AX7 and 6v6.

1.2 on pin 2 of the 12AX7 7 between 10 & 30 dcv on pin 5 of the 6v6

I've swapped tubes. I've swapped both coupling .02 caps two times each. I've gone over the wiring, tested continuity and resistors. What am I missing?

Here are the recent voltages with new jj6V6 & 12ax7.

6V6
pin3: 343 Dcv
pin4: 314
pin5: 30
pin8: 17.6

12ax7
pin1: 182
pin2: 1.2
pin3: 1.4
pin6: 166
pin7: 0
pin8: 1.39

B+ from left to right
1: 355
2: 355
3: 312
4: 260

the resistors all seemed fine but for the 22k connected to the 1.5k. It read 1.4 I swapped it out and it and the new one also read 1.4 The one I removed tested at 22 so I guess that's what happens.

Please help! And thanks.

[tubeswell]

Can you upload any pics (gutshots) of your build?

[dig22]

Link to 5f2a pics

It's shocking to see the board so close up. Looks like a dirty house.... Let me know if you need any more images

Dig22 5F2A June 15, 2010
click on any image to enlarge or shrink

of note: there's a broken lug on the 12ax7. It hasn't effected anything but I will swap it out. I've re-soldered many lugs and misc connections. But since I swapped out the vintage oil caps, everything I've done seems to make the grid voltage worse. As of this afternoon, pin 2 on the 12AX7 is up to around 25. Pin 5 on the 6V6 is around 75 Vdc. At least it still sounds amazing. Thanks for your help. -D22

[Enzo]

Would be good to let us know that pin 5 is +30v, since it might seem OK to have -30 there. I had to look to see this was cathode biased.

OK, so we expect zero VDC on pin 5 of the 6V6. Looks like the black wire comes from pin 5, drops through the hole and should connect to the junction of the 220k and the cap, sorta hidden behind the input jack in photo 1. So power off, and check, is the 220k resistor OK or open?

Now power on, you get +30v at pin 5, is it also at that resistor? Now, move your meter probe off the eyelet wher the cap, resistor, and wire meet, and poke it into the fiberboard right next to the eyelet. Measure to se if any voltage exists on that fiberboard. If it does, then there is your problem. If not, we move on. Old Fender fiberboards can get conductive.

Now pin 2 of the 12AX7. Is that +1.2VDC or -1.2VDC. +1.2 is problematic. Even a conductive board can only spread so much voltage, and that point - where the two inpuit resistors join, is pretty far from high voltage. Nonetheless, again, stick yout probe on the board next to the eyelet where the two 68k join teh shielded wire, and se if any stray voltage is on the black fiberboard there. If it is -1.2v, that is no problem.

[MWJB]

Looks like the shielding at V1, 2 could be touching the socket pin. the jacket should be cut back & heatshrink applied. How have you grounded the shield at the 2x68K input jack resistors?

You don't need the ground wire from the input jack ground lug to the brass plate, the jack is grounded via it's mechanical connection, ground the 12AX7 cathode wires directly to the rightmost input jack ground lug (separate wire for each). Likewise the grounded 0.0047uf cap from the tone pot, it is more usual to ground this cap to the pot back (remove any varnish first). Ground the LH vol pot tab to the back of the pot also. I suspect poor grounding. Verify ohms readings at the grounded end of components, with meter black lead clipped to main filter ground, meter should show "default" reading, perhaps a third to half an ohm?

Many of your components look like you have tied them in a knot, then untied them before soldering.

[dig22]

From original post:

"the resistors all seemed fine but for the 22k connected to the 1.5k. It read 1.4 I swapped it out and it and the new one also read 1.4 The one I removed tested at 22."

Is this normal? Or is this a problem?

[MWJB]

If, when removed from the circuit, the 22K resistor measured 1.4K, then yes - this is a problem.

However, if you tried measuring it in situ, you will be getting a false reading due to other parts in parallel to ground (effectively the 22K runs to ground, just like the 1.5K cathode resistor connected to it, 22K & 1.5K in parallel make 1.4K).

[dig22]

Would be good to let us know that pin 5 is +30v, since it might seem OK to have -30 there.

it's +30 and going higher. Between +30 & + 75. this is where it was during the build till I replaced the 470 R. The new one still test fine. I also swapped 6V6 when intermittent crackling noise started with volume all the way down. . I should add the amp strill sounds amazing and is really quiet (without guitar plugged in) till the volume is turned up to the 5 O'clock position. Then it has a bot of a windy sound. But not with guitar plugged in.

OK, so we expect zero VDC on pin 5 of the 6V6. Looks like the black wire comes from pin 5, drops through the hole and should connect to the junction of the 220k and the cap, sorta hidden behind the input jack in photo 1. So power off, and check, is the 220k resistor OK or open?

it's ok, but reads closer to 230k

Now power on, you get +30v at pin 5, is it also at that resistor?

yes.

Now, move your meter probe off the eyelet wher the cap, resistor, and wire meet, and poke it into the fiberboard right next to the eyelet. Measure to se if any voltage exists on that fiberboard. If it does, then there is your problem. If not, we move on. Old Fender fiberboards can get conductive.

The meter flutters then sometimes settles between .3 and .5 DCV There are small amounts of voltage at various spots on the board, ranging from about.3 to .5 DCV.

Now pin 2 of the 12AX7. Is that +1.2VDC or -1.2VDC. +1.2 is problematic. Even a conductive board can only spread so much voltage, and that point - where the two inpuit resistors join, is pretty far from high voltage. Nonetheless, again, stick yout probe on the board next to the eyelet where the two 68k join teh shielded wire, and se if any stray voltage is on the black fiberboard there. If it is -1.2v, that is no problem.

1.2 VDC was the good old days. Now it's around +55 VDC. The same stray voltage between .3 & .5 on the board with a fluttering meter when the probe hits the shiny area around eyelet. I'm guessing that from the heat of the the soldering.

[dig22]

If, when removed from the circuit, the 22K resistor measured 1.4K, then yes - this is a problem.

However, if you tried measuring it in situ, you will be getting a false reading due to other parts in parallel to ground (effectively the 22K runs to ground, just like the 1.5K cathode resistor connected to it, 22K & 1.5K in parallel make 1.4K).

so then it's normal.

[MWJB]

What readings did you get when checking ohmage from grounded eyelets (grounded end of 1.5K resistors at 12AX7 pin 3 & pin 8) to main filter ground point/AC cord ground? There is a discrepancy between cathode voltage & voltage dropped accross the V1a plate resistor.

[dig22]

Looks like the shielding at V1, 2 could be touching the socket pin. the jacket should be cut back & heatshrink applied.

I fixed it. But no change in voltage.

How have you grounded the shield at the 2x68K input jack resistors?

I've followed ted weber's layout.

You don't need the ground wire from the input jack ground lug to the brass plate, the jack is grounded via it's mechanical connection, ground the 12AX7 cathode wires directly to the rightmost input jack ground lug (separate wire for each). Likewise the grounded 0.0047uf cap from the tone pot, it is more usual to ground this cap to the pot back (remove any varnish first). Ground the LH vol pot tab to the back of the pot also. I suspect poor grounding.

I haven't had problems with grounding as pictured. The amp is silent,. And before I started futzing with the original coupling caps for this build - two vintage oil caps - the voltage was less than -1 DCV on pin 2 12ax7 and around 4DCV on pin 5 6V6.

Verify ohms readings at the grounded end of components, with meter black lead clipped to main filter ground, meter should show "default" reading, perhaps a third to half an ohm?

looks good.

Many of your components look like you have tied them in a knot, then untied them before soldering.

no knots. Some look like knots when two connecting wires cross at same point.

[MWJB]

Ted's layout has no specific instruction as to how to ground the shield at the junction of the 2x68K resistors, you appear to have used shielded cable, how/where is the shield grounded?

"I haven't had problems with grounding as pictured. The amp is silent,. And before I started futzing with the original coupling caps for this build - two vintage oil caps - the voltage was less than -1 DCV on pin 2 12ax7 and around 4DCV on pin 5 6V6." If you're not having problems, why the thread? I'm not being funny, but things don't tally. You should have 0vdc at idle at both 12AX7 and 6V6 grids...4dc is not acceptable. Vintage oil caps or not, if the cap is blocking the dc effectively then that's all that is required, I have had several vintage oil caps "leak", oil caps were never fitted to Fenders, or any other popular guitar amps, switching from oil caps has not caused your problem. If you suspect the new caps are at fault then replace them...they cost very little.

Your board looks "scratched" & silvery, any ideas why this might be? There seems to also be considerable solder "splatter" around the tube sockets, it'll only take a few minutes to change these.

"looks good" isn't a reading, I'm looking for a number.

[tubeswell]

it's +30 and going higher. Between +30 & + 75.

Somehow or other you have a DC voltage at pin 5 of the 6V6. I think you need to look for bad wiring or short circuiting. Are you absolutely certain that the black wire is going to the junction of the 220k and the .02uF coupling cap on the back of the main board?

[dig22]

What readings did you get when checking ohmage from grounded eyelets (grounded end of 1.5K resistors at 12AX7 pin 3 & pin 8) to main filter ground point/AC cord ground? There is a discrepancy between cathode voltage & voltage dropped accross the V1a plate resistor.

pin3: .5
pin8: .9

[MWJB]

Well, even if that 0.5R at pin 3 cathode resistor is meter default (check by pressing black & red probes together, deduct reading from that 0.5 to give "actual" reading) you still have a good half an ohm at pin 8 cathode. Remake this ground. Make life easier for yourself, add a buss wire to the pot backs & input jacks, ground to this rather than the brass plate.