Blues Jr has a neg voltage turning off V1A.

[Luthin You]

This is a brand new Blues jr , making no sound . All voltages are correct but V1A is turned off , -0.5V on pin 2 ! Its only on the tube side of the grid stopper R7. There should be no negative volts anywhere near it ! Has anyone seen this before ? Where is neg half volt coming from ? Thanks

[Enzo]

Electrons.

Swap that tube for one of the others. Does the same voltage exist?

The amp makes no sound. Did you test the speaker and speaker wires? Are they plugged into the correct jack?

[Jon Snell]

The grid will try to go negative if the cathode is held at ground potential or the valve if faulty.
If you discrlosed the actual model number or Mk1, Mk2, Mk3 etc, we may have more information to hand as they are all slightly different.
If you mean R7, I would think R7 is open circuit.

[vintagekiki]

This is a brand new Blues jr , making no sound .

Is hear reverb noise when reverb pot is set to 10?

[Leo_Gnardo]

Has anyone seen this before ?

Yes but never in a Blues Junior.

Where is neg half volt coming from ?

IF the cathode was shorted to ground, you would develop a small negative voltage on the grid. Some old preamps are run this way, all of them have capacitors between input jack and control grid. Such is not the case with Blues Jr. Check resistance between cathode and ground, maybe you'll find it's zero or nearly zero ohms. Then find the reason for this short circuit, maybe a bypass cap went short. I would never expect that to happen, especially in a new amp, but who knows. It's worth a look.

[Enzo]

And you didn't report the cathode voltage. Tube bias is the difference between grid and cathode, not simply the grid voltage. Does touching your probe to pin 7 make any hum or noise come out? Can't assume that half a volt is silencing the whole machine.

[J M Fahey]

As mentioned above, grids capture some electrons and that makes them negative unless they are leaked to ground, hence the NEED for a "ground discharge/reference" resistor, often known as "grid leak" for obvious reasons.

Usually a very low value voltage, so as mentioned some resistor in the grid to ground path is higher value or open.
Or poor connections, broken tracks or solder, stretched socket connections, etc.

But even -0.5V on grid will not fully cutoff that triode (what´s plate voltage by the way?) I bet you have some other problem and this is just the first "weird" thing you measured.

Personally I´m quite the "functionalist" guy, first try to make stuff work (even if block by block) and once isolated, find why, so I suggest you inject signal and do some tracing to find where/how it stops.

Scope is your Friend [tm], of course.

[Helmholtz]

. All voltages are correct..

This I doubt.
If the grid voltage is negative by 0.5V, the cathode voltage will be off.
The most likely explanation is an open grid (as indicated by Jon Snell).

[Luthin You]

Im sorry my post was unclear , V1A is cut off it has 265V on both ends of plate resistor and 0V on cathode . The amp was quiet , but did have hum in the speaker . It seems to be REV A dated 21mar 01 on the schematic . The -0.5V is not always present on V1A G1 and today it went much higher as I turned up the volume pot with oscillator into i/p . All resistors measure correctly as well as their connections to ground or HT or tube pins . I have tried different tubes and the owner fitted a whole new set before it came to me . When i returned V3 to its socket , the amp had a tone like a hum which the scope shows starting in PI (V3). its not a sine wave but a fairly straight line with 2 dips in neg direction then straight again and 2 dips again . So the stage after the volume is not conducting ( heaters ok) there is signal on its grid but nothing on cath or plate . And then V3 has a strange signal that Im calling an oscillation , with no oscillator going into the i/p . It is truly strange , I added extra filter cap just incase but no change . This is all while Ive had 2 weeks of migraines from covid vax (I think). Thanks so much for your help guys .

[Jazz P Bass]

" V1A is cut off it has 265V on both ends of plate resistor and 0V on cathode "
For whatever reason (bad tube, loose solder joint, faulty resistor} the tube is not conducting.

[Luthin You]

Jazz P Bass , thats correct I cant get that stage to turn on , or work out why its off , while checking around the bypass Cap for V1B fell out of the board , so the soldering is not great , but that stage was working ok.

[Jon Snell]

Being brand new, why are you even looking at it ... warranty?
If the soldering is that bad, Fender need to know the model and serial number to trace back the building of it, just incase there is an issue with a flow soldering machine.

[Luthin You]

To John Snell , all I know is the customer said it was new , I don't know the details yet . Thanks for your help , the negative grid voltage varies from 0.4 up to1.0v as volume is turned up . With oscillator in input , so it does behave like grid bias . But the grid resistor and grid stopper measure perfect same as Rk and Rp it just will not turn on .

[vintagekiki]

... V1A is cut off it has 265V on both ends of plate resistor and 0V on cathode ...

https://www.thetubestore.com/lib/thetubestore/schematics/Fender/Fender-Blues-Junior-Schematic.pdf
If on both sides R3 (100k) has voltage Uz = 265V, and voltage on R4 (1.5k) is 0V, first check that C3 (47u) accidentally is not in short-circuited.
If R4 and C3 are correct, resolder all pins on tube socket V1 there may be somewhere cold solder.
If that doesn't help, replace tube socket V1 most likely is defective (pin7 or pin8 is interrupted)

1) Correction
If on both sides R10 (100k) has voltage Uz = 265V, and voltage on R11 (1.5k) is 0V, first check that C4 (22u) accidentally is not in short-circuited.
If R11 and C4 are correct, resolder all pins on tube socket V1 there may be somewhere cold solder.
If that doesn't help, replace tube socket V1 most likely is defective (pin2 or pin3 is interrupted)

[Helmholtz]

I think it is this version (I would have expected the OP to post the correct schematic):
https://www.thetubestore.com/lib/the...atic-Rev-D.pdf

R7 would be the grid stopper to V1A. If it's open or not connected, V1A will have an open grid and will not conduct, because the open grid charges negative.
(Also look for breaks between PW1A and PW1B.)

Now when the OP connects a voltmeter to the grid, a negative grid leak current will flow through the meter's input resistance and cause a negative voltage drop/reading.
So the meter acts as a large value grid leak resistor.

To test for open grid, measure resistance from grid pin to ground.