Help in determing bias on Blue Jr. Mark 1

[Chuck H]

1R cathode resistors would make this easy peasy, removing opportunity for measurement errors / circuit disturbances etc to mess things up.

Board mounted sockets with ribbon cable would make that harder. If there's room in the closed chassis I suppose you could do it like this. Make sure to use 1% resistors and shrink tube covering any bare leads.

[ArturoQ]

Not exactly sure what you mean by 'transformers HT line' which I think is the same as center tap.
You should be measuring DC volts from CP2 to CP1, then CP2 to CP3. Just to be sure there is nothing funny going on, remove the PI tube V3 while doing those measurements.

Good Day. To preface, I am using an Amprobe AM530 meter. I pulled the V3 Tube and the readings remained basically the same. Measured at V4 Pin 7 @ approx. 700 mVdc and V5 Pin 7 @ 672 mVdc which appears basically the same. I am having a fundamental understanding issue.........

[ArturoQ]

1R cathode resistors would make this easy peasy, removing opportunity for measurement errors / circuit disturbances etc to mess things up.

Good Day to you. Thanks for the reply and advice.The amp works, since I replaced the V1 tube. Waiting on a mullard, reputedly low microphonic tube to replace the one I had old extra. Did throw out the other 12AX7's I had laying about as they tested really bad.. Getting desperate, first to understand my inability and second to actually measure the "plate dissipation" to check on altering R37 or R37 and R22 to cool down the bias. My terminology is not necessarily accurate, apologies.

[ArturoQ]

Board mounted sockets with ribbon cable would make that harder. If there's room in the closed chassis I suppose you could do it like this. Make sure to use 1% resistors and shrink tube covering any bare leads.

Howdy and thanx. Seem to be running out of options. Don't know what I could be messing up consistently in my measurements of this unsuccessful attempt. Looking for clearer minds and viewpoints... thanks again

[g1]

Is it possible the amp is actually idling that cold? It still would be functional. What is DC voltage at each end of R37 ?

[ArturoQ]

Is it possible the amp is actually idling that cold? It still would be functional. What is DC voltage at each end of R37 ?

To further clarify this Fender is an American made and market model for our 120 Vac,

It R37 is -27.05 Vdc and -11 Vdc and for the fun of it, R31 is -11 Vdc and .1 mVdc. Currently out of my depth for an explanation,..

Thanx

[g1]

Are power tube pin9 DC voltages good (around 310V) ?
If so, seems to be some kind of measurement glitch, check your meter at low DC volts like an AA or AAA battery.

[Chuck H]

Board mounted sockets with ribbon cable would make that harder. If there's room in the closed chassis I suppose you could do it like this. Make sure to use 1% resistors and shrink tube covering any bare leads.

Having gotten 4 likes on this post I just want to itterate that soldering resistors to pads like this IS NOT ideal. There is no insertion or wrap to secure the connection and it relies entirely on the solder joint. That said, I HAVE done things like this in personal and friends amps with no problems to date. YMMV. I just thought it was important to note that there absolutely are criteria for "proper" component connections and solder joints WRT reliability. This modification breaks the rules. Proceed as you wish and at your own risk.

[g1]

Having gotten 4 likes on this post I just want to itterate that soldering resistors to pads like this IS NOT ideal. There is no insertion or wrap to secure the connection and it relies entirely on the solder joint.

If nothing else it can be done temporarily to check the idle currents. Cut traces could be jumpered back in a more secure manner than leaving resistors hanging there, though I don't think that would really be an issue, they could be affixed with adhesive.

[ArturoQ]

Are power tube pin9 DC voltages good (around 310V) ?
If so, seems to be some kind of measurement glitch, check your meter at low DC volts like an AA or AAA battery.

Back again, Pin 9 on both El84s shows 326.6 Vdc. Measures correctly on AA battery. CP2 to CP1 read O.O mVdc up to 672 mVdc from off until it was fully on..................

[g1]

If you don't want to cut the traces, your AM530 is quite capable of measuring current, you just have to be very careful, and remember to disconnect the probes at the meter end as soon as you are done (to avoid damage elsewhere if you forget it's set up for current).
You would need to disconnect the CP1 connection and insert the meter in series, then repeat for CP3. Meter set up for DC mA measurement.

[ArturoQ]

If you don't want to cut the traces, your AM530 is quite capable of measuring current, you just have to be very careful, and remember to disconnect the probes at the meter end as soon as you are done (to avoid damage elsewhere if you forget it's set up for current).
You would need to disconnect the CP1 connection and insert the meter in series, then repeat for CP3. Meter set up for DC mA measurement.

Thanks once again for your perseverance. Will try that tomorrow. Since my obvious ignorance of the amp circuitry and operation, is there anywhere I can get to and clip in a 1 Ohm resistor with alligator clips and put it to ground??

[g1]

No, using the 1 ohm resistor method requires it to be placed in series, which means cutting a trace or disconnecting one wire of the ribbon cable, which would be fairly difficult.

[ArturoQ]

No, using the 1 ohm resistor method requires it to be placed in series, which means cutting a trace or disconnecting one wire of the ribbon cable, which would be fairly difficult.

Oh well, can't win them all.. As an aside, I have not been receiving notification of new reply via email. Is there a way to enable this feature?
Good evening to all.

[g1]

I have not been receiving notification of new reply via email. Is there a way to enable this feature?

Sometimes the system just doesn't work. As you are the one that started the topic, you should be automatically subscribed. Double check that your email notifications are turned on. (in 'user settings', under 'notifications' tab)