Need some help on a 5F1, please.

[AmpGal]

Hi, all. I'm back to working on the 2 amps I set aside in Feb due to major frustration and a few guitars & pedal builds my husband asked for help with. However, for Christmas, all he wanted was for me to get these working, so here we go.

I've taken readings today and I'll post them for your help, but I also want to learn to do this myself. The internet has just too much crap out there... anyone have a good resource for me to read & learn? I want to learn two things, actually... how to look at the readings and see a problem & where to look to fix it... and how cap & resistor values affect tone in amps & pedals.

The first amp is a 5F1 Champ. Voltages are as follows. I think I did it correctly... took readings on the 5Y3, then put that tube in and took readings on the 6V6, put that tube in & took readings on the 12AX7. Also, it looks from reading as though the process for taking VAC readings on the filaments may be different than the rest of the VDCs, where the neg wire of the voltmeter is clipped to the chassis? I don't put both hands anywhere near an amp that's plugged in, so am I really supposed to touch pins 2 & 7 at the same time???

From other builders' posted readings, it looks like something's wrong at the 6V6, but I don't know enough to figure out where to look. I'm concerned I may have a grounding issue somewhere, but I can't figure out where it might be. When you insert the tubes, she is quiet at first then she lets out a horrendous squeal regardless of whether a guitar is plugged into her.

5Y3
1 = 0.13 VDC
2 = 3.66 VDC
3 = 0.30 mVDC
4 = 346 VAC
5 = -0.5 mVDC
6 = 346 VDC
7 = 0.5 mVDC
8 = 3.66 VDC

6V6
1 = 0
2 = 3.47 VAC
3 = 459 VDC
4 = 458 VDC
5 = 0.5 mVDC
6 = 8 - 16 mVDC (constantly changing)
7 = 3.47 VAC
8 = 0

12AX7
1 = 334 VDC
2 = 0.5 VDC
3 = 0
4 = 3.42 VAC
5 = 3.42 VAC
6 = 333 VDC
7 = 0
8 = 0.1 mVDC
9 = 3.42 VAC

Thanks in advance for your help!
Gina

[Topper]

You might have a feedback problem. The 5F1 is supposed to have negative feedback. If you have the output transformer primaries backwards it actually then creates runaway positive feedback and will squeal with a high pitched sound. (Sometimes the color-coding of the OT wires isn't what they say it is supposed to be.)

If that is the problem, simply reverse the OT primaries and that will fix it.

[AmpGal]

Thanks, Topper. The OT instructions did say to reverse the red/blue hookups, which I did. Any danger in putting them back to check it out?

[tubeswell]

Hi, all. I'm back to working on the 2 amps I set aside in Feb due to major frustration and a few guitars & pedal builds my husband asked for help with. However, for Christmas, all he wanted was for me to get these working, so here we go.

So neither amp works at all? Or the lights come on but no sound?, Or there is sound but there is other unwanted noise? Some other clues about the problem will aid diagnosing the problem(s).

The first amp is a 5F1 Champ. Voltages are as follows. I think I did it correctly... took readings on the 5Y3, then put that tube in and took readings on the 6V6, put that tube in & took readings on the 12AX7. Also, it looks from reading as though the process for taking VAC readings on the filaments may be different than the rest of the VDCs, where the neg wire of the voltmeter is clipped to the chassis? I don't put both hands anywhere near an amp that's plugged in, so am I really supposed to touch pins 2 & 7 at the same time???

Its always safer to use meter leads with clips (rather than probes). That way you can attach/unclip the clips with the amp off and switch the amp on to get the reading without putting your hand anywhere inside the chassis when the power is on.

For the (5Y3) rectifier tube you want to know what AC volts are on the rectifier's anodes (Pins 4 and 6), so to get a reading off one pin, you set your V-meter to high VAC and put the red clip on one pin and the black clip somewhere on the chassis. (Alternatively to get a high VAC reading between both pins you could put your black clip on the other pin). Make sure that the clips that are clipped to the socket pins aren't shorting to anything else before you switch the amp on.

But for the B+ reading that has to be with the V-meter set to high VDC (and you measure between Pin 8 of the rectifier socket and the chassis/ground - or alternatively between the +ve end of the reservoir filter cap and the chassis ground)

From other builders' posted readings, it looks like something's wrong at the 6V6, but I don't know enough to figure out where to look. I'm concerned I may have a grounding issue somewhere, but I can't figure out where it might be. When you insert the tubes, she is quiet at first then she lets out a horrendous squeal regardless of whether a guitar is plugged into her.

That squeal could be:

1) positive feedback from the feedback loop (instead of the desired negative feedback). Temporarily disconnecting the feedback loop from the OT secondary to the pre-amp cathode will soon confirm if it is or not. If it is, then swapping the OT primaries around should cure it. (i.e. taking the OT primary winding wire that's attached to the +ve end of the reservoir filter cap and connecting it to pin 3 of the 6V6 socket instead, and at the same time taking the other wire from the end of the OT primary winding that is going to pin 3 of the 6V6 socket and connecting it to the +ve end of the reservoir filter cap instead). Most SE OT primary wires are colour-coded red and blue, but the factory doesn't always get these in the 'right' way around.

2) a microphonic pre-amp tube. In which case, swapping the 12AX7 for a better tube should fix it.

5Y3
1 = 0.13 VDC
2 = 3.66 VDC
3 = 0.30 mVDC
4 = 346 VAC
5 = -0.5 mVDC
6 = 346 VDC
7 = 0.5 mVDC
8 = 3.66 VDC

Take VAC readings from pins 4 and 6 respectively to ground, as suggested earlier. Your VAC meter will need to be set to high (hundreds of volts) VAC for this reading.

Take a VAC reading between pins 2 and 8 to confirm that the rectifier heater is putting out ~5VAC. Your VAC meter can be set to tens of volts AC for this reading.

Take a VDC reading from Pin 8 to ground to confirm that you have high VDC for the B+. Your meter will need to be set to high voltage (hundreds of volts) VDC for this reading.

Be careful not to kill yourself when taking these readings.

6V6
1 = 0
2 = 3.47 VAC
3 = 459 VDC
4 = 458 VDC
5 = 0.5 mVDC
6 = 8 - 16 mVDC (constantly changing)
7 = 3.47 VAC
8 = 0

For the 6V6, you want to know the idle voltages (i.e. the VDC with no signal conditions) for Pin 3 (the plate), Pin 4 (the screen), Pin 5 (the control grid), and Pin 8 (the cathode) respectively to the chassis ground in each case. So use your V-meter setting to VDC. Pins 3 and 4 will each need to be set to high voltage DC. Again, be careful not to kill yourself, nor let the meter clips short to one of the other pins/or to the chassis when taking these readings. You should not get any VDC voltage on Pin 5.

For Pins 2 and 7, which are the heater filament for the 6V6, you should set the Vmeter to 10s of volts VAC and measure between these two pins. You should get about 6.3VAC

12AX7
1 = 334 VDC
2 = 0.5 VDC
3 = 0
4 = 3.42 VAC
5 = 3.42 VAC
6 = 333 VDC
7 = 0
8 = 0.1 mVDC
9 = 3.42 VAC

For the pre-amp tube, you want to know the VDC voltages for Pins 1 and 6 (the plates of each triode), Pins 3 and 8 (the cathodes of each triode) and pins 2 and 7 (the grids of each triode), in each case, from the respective pin in question to the chassis ground. For the readings on Pins 1 and 6, the VDC needs to be set to high-voltage VDC. But for pins 3 and 8, its better to have the Vmeter set to 10s of volts VDC. Same for Pins 2 and 7. You should not get any VDC voltage on pins 2 or 7.

Pins 4-5 and 9 are for the pre-amp tube heater filaments, and your Vmeter should be set to 10s of volts VAC, and measure between pin 9, to pins 4&5. (Pins 4&5 should be tied-(i.e.: soldered) together). You should get about 6.3VAC (as you should have done with the 6V6, because these are respectively wired in parallel to the 6V6's pins 2 and 7, as well as to either side of the pilot lamp).


A word on safety.

Parts of the circuit of the amp (like the plates, screens and B+) are designed to be at a high potential voltage. These points in the circuit are where there is a relatively large deficit of electrons when power is applied. You complete a circuit by connecting a pathway for electrons to 'replenish' that potential deficit from the earth/ground, and when you do so the electrons will rush from the earth/ground towards that high voltage point. The bigger the deficit of electrons at that high-voltage point, the more electrons that will flow towards it when the circuit is connected. High voltages (where there is a high deficit of electrons) attract lots and lots of electrons, which have a lot of energy associated with them. If you accidentally happen to make your body a part of that circuit between the ground and that high voltage point in question, then the electrons will use your body to get to that high voltage point, and in doing so will char the parts of your body (be it blood vessels or muscles or whatever) tissue in the process, which is almost certain to kill you. So you need to take precautions to ensure that you do not make your body a part of the live circuit between any of the high potential voltage points in the circuit, and the earth/ground. Hence what I said about using (well-insulated) clips on your meter leads (instead of probes, which can slip more easily), and attaching and un-clipping them with the power switched off.

Furthermore, the filter capacitors are designed such that they 'store charge', and some 'stored charge' can be present for a while even after the amp is switched off and unplugged. So you should get in the habit of draining the charge in the filter caps, by temporarily connecting a ~10k resistor between the +ve end of the filter caps and their -ve end after you have switched the amp off and unplugged it from the wall. Another way of doing this is to clip your meter leads between one of the pre-amp tube's plates on one lead, and the chassis ground on the other lead, and reading the meter as the DC charge drains off (after the amp is switched off). Yet another way is to permanently solder a 220k 2W 'bleeder' resistor between the +ve end of the reservoir filter cap and the -ve end of that same cap, which allows the filter caps to quickly discharge when you switch the amp off.

You should get in the habit of also unplugging the mains cord from the wall socket each time you switch the amp off to connect and un-clip the meter leads, as an added precaution.

You should also work on a well-insulated floor at a work bench that is well insulated and uncluttered with stuff that can cause accidental slippages or unwanted ground-to-high voltage connections. (Concrete is not a good insulator - in fact is is highly conductive, so don't work on a bare concrete floor.)

Don't work on your amp late at night when you are tired, or when you are otherwise inebriated in any way.

Also you do not want to ruin the circuit (or the circuit's components) by inadvertently making a short/electrical-contact between parts of the circuit that are not designed to be shorted/contacted together. Hence what I said about being careful how you clip your leads to the socket pins.

[AmpGal]

So neither amp works at all? Or the lights come on but no sound?, Or there is sound but there is other unwanted noise? Some other clues about the problem will aid diagnosing the problem(s).

I try to work on one thing at a time or I start confusing what problems go with what amp, especially when the hubby has me doing two or three of the same amp at a time. This one is just a ear-splitting squeal that makes any further evaluation impossible.

For the (5Y3) rectifier tube you want to know what AC volts are on the rectifier's anodes (Pins 4 and 6), so to get a reading off one pin, you set your V-meter to high VAC and put the red clip on one pin and the black clip somewhere on the chassis. (Alternatively to get a high VAC reading between both pins you could put your black clip on the other pin). Make sure that the clips that are clipped to the socket pins aren't shorting to anything else before you switch the amp on.

Good deal. That means my current readings are correct (pin 6 was a typo; it was taken in VAC, not VDC)... as long as my handy "does everything" meter's "auto" setting takes "high" readings, which it appears to. I will do the "between" readings tomorrow, as I normally go to bed between 9 & 9:30 & my brain is already winding down for the night.

But for the B+ reading that has to be with the V-meter set to high VDC (and you measure between Pin 8 of the rectifier socket and the chassis/ground - or alternatively between the +ve end of the reservoir filter cap and the chassis ground)

I read at pin 8 with the black clipped to the chassis, like the others. I do, however, drain at the + end of the filter cap ever time I turn off & unplug the amp.

That squeal could be:

1) positive feedback from the feedback loop (instead of the desired negative feedback). Temporarily disconnecting the feedback loop from the OT secondary to the pre-amp cathode will soon confirm if it is or not. If it is, then swapping the OT primaries around should cure it. (i.e. taking the OT primary winding wire that's attached to the +ve end of the reservoir filter cap and connecting it to pin 3 of the 6V6 socket instead, and at the same time taking the other wire from the end of the OT primary winding that is going to pin 3 of the 6V6 socket and connecting it to the +ve end of the reservoir filter cap instead). Most SE OT primary wires are colour-coded red and blue, but the factory doesn't always get these in the 'right' way around.

2) a microphonic pre-amp tube. In which case, swapping the 12AX7 for a better tube should fix it.

I tried my tube set in a known good 5F1, so I know they're in good working order. I'll do #1 tomorrow, too.

Take VAC readings from pins 4 and 6 respectively to ground, as suggested earlier. Your VAC meter will need to be set to high (hundreds of volts) VAC for this reading.

Take a VAC reading between pins 2 and 8 to confirm that the rectifier heater is putting out ~5VAC. Your VAC meter can be set to tens of volts AC for this reading.

Will do tomorrow.

Take a VDC reading from Pin 8 to ground to confirm that you have high VDC for the B+. Your meter will need to be set to high voltage (hundreds of volts) VDC for this reading.

That is how I took this reading. What would cause it to be only 3.66 VDC?

Be careful not to kill yourself when taking these readings.

I promise. Well, at least so far so good. I have faith & a healthy respect for electrons.

For the 6V6, you want to know the idle voltages (i.e. the VDC with no signal conditions) for Pin 3 (the plate), Pin 4 (the screen), Pin 5 (the control grid), and Pin 8 (the cathode) respectively to the chassis ground in each case. So use your V-meter setting to VDC. Pins 3 and 4 will each need to be set to high voltage DC. Again, be careful not to kill yourself, nor let the meter clips short to one of the other pins/or to the chassis when taking these readings. You should not get any VDC voltage on Pin 5.

Not sure what you mean by "no signal conditions?" Without the rectifier tube in, the 6V6 readings were:

Plate = 3.5 VDC
Screen = 3.54 VDC
Grid = fell each reading from 5mV to 0
Cathode = 0

Any idea why the grid would fall without the 5y3 in, but have .5 mV with it?
Or why the cathode would be at 0 without the 5y3 & fall from 3mV to 0 with it?

For Pins 2 and 7, which are the heater filament for the 6V6, you should set the Vmeter to 10s of volts VAC and measure between these two pins. You should get about 6.3VAC

Will do tomorrow.

For the pre-amp tube, you want to know the VDC voltages for Pins 1 and 6 (the plates of each triode), Pins 3 and 8 (the cathodes of each triode) and pins 2 and 7 (the grids of each triode), in each case, from the respective pin in question to the chassis ground. For the readings on Pins 1 and 6, the VDC needs to be set to high-voltage VDC. But for pins 3 and 8, its better to have the Vmeter set to 10s of volts VDC. Same for Pins 2 and 7. You should not get any VDC voltage on pins 2 or 7.

The plates are 333 & 334 VDC, 3.33 & 3.38 VDC without the rectifier.
I'll check the wiring again, as it looks like I may have accidentally wired pins 7 & 8 backwards & missed it before. Neither they or the grids have any VDC until the rectifier goes in, at which point the readings I gave originally are what I get.

Pins 4-5 and 9 are for the pre-amp tube heater filaments, and your Vmeter should be set to 10s of volts VAC, and measure between pin 9, to pins 4&5. (Pins 4&5 should be tied-(i.e.: soldered) together). You should get about 6.3VAC (as you should have done with the 6V6, because these are respectively wired in parallel to the 6V6's pins 2 and 7, as well as to either side of the pilot lamp).

My filament points are between 3.41 & 3.49 at the tubes. I'll check the pilot end tomorrow, too. Any particular reason mine could be reading roughly half of what it should?

A word on safety.

Parts of the circuit of the amp (like the plates, screens and B+) are designed to be at a high potential voltage. These points in the circuit are where there is a relatively large deficit of electrons when power is applied. You complete a circuit by connecting a pathway for electrons to 'replenish' that potential deficit from the earth/ground, and when you do so the electrons will rush from the earth/ground towards that high voltage point. The bigger the deficit of electrons at that high-voltage point, the more electrons that will flow towards it when the circuit is connected. High voltages (where there is a high deficit of electrons) attract lots and lots of electrons, which have a lot of energy associated with them. If you accidentally happen to make your body a part of that circuit between the ground and that high voltage point in question, then the electrons will use your body to get to that high voltage point, and in doing so will char the parts of your body (be it blood vessels or muscles or whatever) tissue in the process, which is almost certain to kill you. So you need to take precautions to ensure that you do not make your body a part of the live circuit between any of the high potential voltage points in the circuit, and the earth/ground. Hence what I said about using (well-insulated) clips on your meter leads (instead of probes, which can slip more easily), and attaching and un-clipping them with the power switched off.

Furthermore, the filter capacitors are designed such that they 'store charge', and some 'stored charge' can be present for a while even after the amp is switched off and unplugged. So you should get in the habit of draining the charge in the filter caps, by temporarily connecting a ~10k resistor between the +ve end of the filter caps and their -ve end after you have switched the amp off and unplugged it from the wall. Another way of doing this is to clip your meter leads between one of the pre-amp tube's plates on one lead, and the chassis ground on the other lead, and reading the meter as the DC charge drains off (after the amp is switched off). Yet another way is to permanently solder a 220k 2W 'bleeder' resistor between the +ve end of the reservoir filter cap and the -ve end of that same cap, which allows the filter caps to quickly discharge when you switch the amp off.

You should get in the habit of also unplugging the mains cord from the wall socket each time you switch the amp off to connect and un-clip the meter leads, as an added precaution.

You should also work on a well-insulated floor at a work bench that is well insulated and uncluttered with stuff that can cause accidental slippages or unwanted ground-to-high voltage connections. (Concrete is not a good insulator - in fact is is highly conductive, so don't work on a bare concrete floor.)

Don't work on your amp late at night when you are tired, or when you are otherwise inebriated in any way.

Also you do not want to ruin the circuit (or the circuit's components) by inadvertently making a short/electrical-contact between parts of the circuit that are not designed to be shorted/contacted together. Hence what I said about being careful how you clip your leads to the socket pins.

I understand the risks & take all necessary precautions and have been sober for 3 years now. Thanks for the reply! I'll post further readings tomorrow sometime. For now, I'm off to read the 2nd edition of the Audio Cyclopedia by Howard Tremaine so I can learn to do more than just solder & follow schematics.

[tubeswell]

That is how I took this reading. What would cause it to be only 3.66 VDC?

The +ve end of the reservoir filter cap should be wired to Pin 8 of the rectifier tube socket. If you have a low voltage there, either your rectifier tube is shorting or your filter cap has somewhat shorted, or you have a short somewhere, (or you have put the wrong value bleeder resistor in?). With the amp switched off, check the B+ to the chassis ground with your R meter. It should not read as a steady low DC resistance.

Not sure what you mean by "no signal conditions?"

Quiescent. i.e.: Switched on but idling, without any signal inputted into the amp.

Without the rectifier tube in, the 6V6 readings were:

Plate = 3.5 VDC
Screen = 3.54 VDC
Grid = fell each reading from 5mV to 0
Cathode = 0

You won't get any voltage on the plates (or screens, or cathode) without the rectifier tube plugged in. The rectifier takes the AC voltage from the PT secondary High Tension winding and turns it into positive pulses of current that charge the reservoir filter cap to a more-or-less continuous steady high voltage DC (which you should be able to measure at pin 8 of the rectifier tube). This is also connected, via the OT primary winding, to the plate of the 6V6. If the amp has high voltages on eth plates (and screens) with the tubes plugged in, and you still aren't getting a signal, check that you haven't miswired the Ot secondary to a short, or that you haven't omitted any other wires/connections in the signal path between the input jacks and the speaker.

My filament points are between 3.41 & 3.49 at the tubes. I'll check the pilot end tomorrow, too. Any particular reason mine could be reading roughly half of what it should?:

You are only measuring one side of the heater winding-to-ground at a time?. If you measure from one side of the heater winding to the other, you should get around 6.3VAC (with all the tubes plugged in. A PT puts out higher voltages without a load)