crazy voltages troubleshooting 5E3

[Enzo]

What kind of meter are you using?

I suspect your meter is loading the circuit to create a confusing reading. If your circuit is a copy of the original 5E3, then pin 7 is the grid of the phase inverter, and it should not sit at zero volts. Unfortunately I don't believe the 20 volts either. I really wouldn't worry much that the 70v has become 55, but try this. Instead of measuring voltage from pin 7 to ground, instead measure the voltage from pin7 to pin 8. Don't measure each to ground and calculate4, measire directly between the pins. If pin 8 is at 55v, then I bet there will be something like 1.5 volts between pin 7 and 8. You may not be able to measure it to ground directly, but that would mean the voltage at pin 7 was really 53.5.


V1 may have some voltages, but the circuits at V2 are different, so whatever is on V1 doesn;t apply to V2.

"Preamp tubes" are not generic, so don;t expect them all to be the same in a circuit. Just as a thin piece of wire could be an E string on your guitar, or it could be part of a cheese cutter. It is the use of something that determines what it is. SO a 12AX7 tube in V1 has no bearing on a 12AX7 as V2.


In the other hand you asked what they are normally biased to. Well, if you have 55 volts on the cathode and 53 volts on the grid, then that tube is biased vy 2 volts just as surely as one with 2v on the cathode and zero on the grid. Bias is the relationship of grid to cathode, not the relationship of either of those to ground.

[mrtelemon]

I measured bewteen 7 and 8 and got 1.3 VDC...I'm just using a cheap digital meter and wish for doing this type of work that I had a good one.. I have looked on different forums with other guys who have built these same cicuits and they all show 0 nada on pin 7 so maybe other 5E3 kits could be different....I can send you the links for yo to see what others are showing if you like.. Here's the meter I have..

[cgates56]

I measured bewteen 7 and 8 and got 1.3 VDC...I'm just using a cheap digital meter and wish for doing this type of work that I had a good one.. I have looked on different forums with other guys who have built these same cicuits and they all show 0 nada on pin 7 so maybe other 5E3 kits could be different....I can send you the links for yo to see what others are showing if you like.. Here's the meter I have..

can you post the schematic you are working from

[mrtelemon]

[g1]

I'm not sure what you meant by this, "shows a voltage at the grid of the phase inverter half of V2."

I said it this way to avoid saying pin 7. A 12AX7 has 2 independant halves. Side A is pin 1,2,&3. Side B is pin 6,7,&8. Some kits (like yours) have side B wired up as the phase inverter, some kits have side A wired as phase inverter. Depending on how it is wired, you may have a voltage on Pin 7 OR pin 2, which ever is wired as the phase inverter half.

I measured bewteen 7 and 8 and got 1.3 VDC...I'm just using a cheap digital meter and wish for doing this type of work that I had a good one.. I have looked on different forums with other guys who have built these same cicuits and they all show 0 nada on pin 7 so maybe other 5E3 kits could be different....I can send you the links for yo to see what others are showing if you like.. Here's the meter I have..

I hope my explanation above cleared this up for you. Circuits with 0 volts at pin 7 of V2 will have around 1.2V on pin8. Circuits with 40 to 70V on pin 8 will have voltage at pin 7 but it will be hard to measure properly as Enzo explained.
The bottom line is if your plate and cathode voltages are correct, (pins 1&6, 3&8), then your grid voltages must be correct, and you can ignore them.

[Bruce / Mission Amps]

#1... there should not be a 12AX7 in V1... a 12AY7 is called for and you'll then see the voltages drop when you fix that part.
#2... there is always about 25vdc-45vdc on lug 7 of the 12AX7, section B... that voltage is part the triode's biasing circuit.
You should note the voltage at the junction of the 1.5k ohm resistor and the 56K resistor, then look at the 1M resistor... it is not grounded anywhere else so that positive voltage floats up to the grid, lug #7 through the 1meg resistor.... there should be a little difference in voltage between the junction of the 1k5 and the 1Meg with respect to the grid, lug7... but that is where the DC voltage is coming from, if the tube is biasing correctly and or you have it wired right.
#3... There is still something wrong I assume, otherwise you'd say the amp is working.

[cgates56]

that was the reason I asked to see the schematic, if it is wired to fender 5e3 specs there is no way those voltages can be correct unless the is a fault in thw wireing, I have built too and sold too many 5e3, 5f11, etc amps and there is always a voltage on pin 7 of the pi.

[cgates56]

also if he would post several closeup chassis photos we could nail this down

[Enzo]

I checked the Trinity web site for the voltage chart he refers to, but apparently it is in the "resources" section of their forum, which as far as I can tell is only available to members.

[cgates56]

yeah, I got nowhere with that too

I did find in an old post on the forum from Stephen at trinity that they did not post voltages on pin 7 of v2

[mrtelemon]

Hi Bruce: The amp is working and other than what I would call annoying hum it works fine and I'm really happy with it and I do have an AY7 in V1...I did go to the junction you mentioned, and I got 48VDC... it sucks not knowing more than I do with this stuff, but I can tell you that I appreciate the help..

[g1]

#3... There is still something wrong I assume, otherwise you'd say the amp is working.

From what I can tell the amp is working:

and although it sounds decent

I measured bewteen 7 and 8 and got 1.3 VDC..

Sounds about right.

I get the impression we are chasing a misprint on the trinity documentation (0 volts at V2 pin 7).
V2 pin 8 voltage may be a bit low but seems to be in the ballpark. We don't know what the B+ from the 5Y3 is so it's hard to tell.
As Bruce mentioned, V1 should be a 12AY7, not 12AX.

[frus]

What kind of meter are you using?

I suspect your meter is loading the circuit to create a confusing reading. If your circuit is a copy of the original 5E3, then pin 7 is the grid of the phase inverter, and it should not sit at zero volts. Unfortunately I don't believe the 20 volts either.

that's right, the PI is bootstrapped, so it has a much higher input impedance than 1 meg, and it interferes with meter's internal impedance. it's a well known effect. I get some bogus reading on my 5F4 too, depending on the meter. 55V on pin 8 is what most people get, that's fine. also check that the voltage between pin 1 and power rail (across the "other" 56k resistor that connects to pin 1) is the same as the voltage between pin 8 and ground, that means it's working properly

[mrtelemon]

pin 1 and 6 of V2 each have a resistor joining together and I took a measurement from there to Grd and got 96VDC and from pin 1 to that point of both resistors got somthing like 286VDC but showed a HV sign....man it sucks being a dumb ass with this stuff

[frus]

OH I'm dumber still! I meant pin 6, the plate on the same triode, not pin 1 !!!
so you would check the voltage across both 56k resistors in the PI section (pin 6 and pin 8 of V2)

apologies!!