Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: BlackCrowe604 on February 15, 2021, 09:27:29 pm

Title: Stumped - Marshall JMP 2203; Tube not conducting current
Post by: BlackCrowe604 on February 15, 2021, 09:27:29 pm

Hi All,

My JMP is making me scratch my head. Hoping someone may be able to provide some direction on this one. It has an additional preamp tube added, and a master volume. (4x) Preamp tubes & (4x) EL34s (V4 - V7).

I was running 4 bias probes, and V7 had 0Ma current. I proceeded to check voltages at tube sockets.

Initial Voltage Check @ Tube Sockets:

      1.   2.    3.     4.      5.     6.     7.   8.
V7. 0.   H.   499.  0.      -41.  NC.   H.   0
V6. 0.   H.   500.  500.  -41.  NC.   H.   0
V5. 0.   H.   500.  500.  -41.  NC.   H.   0
V4. 0.   H.   499.  500.  -41.  NC.   H.   0

So, from initial check, screen grid stuck out. Checked socket, it was missing, I replaced with 1Kohm.

After replacing 1Kohm screen grid, and measuring 1Kohm between pin 4&6, re-checked voltages, results below:

     1.   2.    3.     4.      5.     6.     7.   8.
V7. 0.   H.   493.  493   -41.  NC.   H.   0
V6. 0.   H.   494.  493.  -41.  NC.   H.   0
V5. 0.   H.   494.  493.  -40.  NC.   H.   0
V4. 0.   H.   494.  493.  -41.  NC.   H.   0

Pin 4 now has voltage. Awesome! However.....

I replaced tubes, let them run for a bit, then noticed through bias probes that yet again, V7 is not conducting current. 0mA.
I rechecked new resistor and measures 1K ohm.

Is there something I am missing here, or perhaps not considering? What else should I check?

Any help would be much appreciated.

I hope all is well despite the crazy times we are living in lately.

Cheers!

Title: Re: Stumped - Marshall JMP 2203; Tube not conducting current
Post by: sluckey on February 15, 2021, 09:52:56 pm

Screen resistor should be 1K, not 100K.

Title: Re: Stumped - Marshall JMP 2203; Tube not conducting current
Post by: BlackCrowe604 on February 15, 2021, 10:35:49 pm

Good catch, corrected.
Sorry, typo in post. Screen grids are all 1K. Replacement screen grid is also 1K.

Title: Re: Stumped - Marshall JMP 2203; Tube not conducting current
Post by: sluckey on February 15, 2021, 10:53:42 pm

Try a different bias probe with V7.

Do you measure zero ohms between V7 pins 1/8 and ground?

Title: Re: Stumped - Marshall JMP 2203; Tube not conducting current
Post by: BlackCrowe604 on February 15, 2021, 11:24:30 pm

V7 Pin #1 to ground = 0.22ohms
V7 Pin #8 to ground = 0.22ohms

new bias probe showing 19mA thru the tube!! thanks!!

will try the rest of the probes individually to confirm if one of the probes is causing errors.