Mojo 5E3 bias (heat?) issues...

[Mick Bailey]

You're measuring the voltage and your meter resistance (usally 10M) is draining it down, so I would check continuity of the 220k resistors to ground, as these should provide a much lower ground-reference resistance.

[5thumbs]

Hi Mick,
It does the same on the 220k resistors that it does on pin 5, starts high and drains to around .6vdc. Also I changed out all the orange drop coupling caps with equivalent new Mallory 150s which I prefer...no change 😤😪 the plate dissipation is still way off ~18 watts! My friend kind of botched up the 4 input jacks and the 1meg resistors, so I am going to correct and clean up. I pulled the eyelet board and check for any bad wiring underneath and made sure the insulator board covered the top board properly, which it did. I tried a deluxe reverb transformer I had, no luck. What now??.!!

[g1]

Check resistance to ground from each end of those 220K's.

[Mick Bailey]

See #16. And #18

If those resistors aren't connected to ground you'll get excessive current draw and 'farting out' blocking distortion. My money is on you've lost connection to ground.

Those resistors have a much lower value than your meter. If your meter is bleeding the voltage then the resistors can't be connected, or are open. It takes 5 seconds to check them and could save you a whole lot of trouble.

[5thumbs]

Hallelujah!!! My friend didn't install the ground wire on the joined end of the 220k resistors feeding the power tubes! Thanks everyone for the help with that!!!
So now my voltages are:
B+ - 380
Pin 3 - 367
Pin 4 - 331
Cathode - 21
Plate Current .042

So now my math says: (367 - 21) x .042 = 14.5 watts

So should I change the cathode resistor to a 330ohm? seems that would drop the plate current to .032 and give a watt rating of 11 watts, or am I off on my math
again;-)

Thanks for everyone's help!
Tim

[5thumbs]

Went ahead and changed the cathode resistor to a 330ohm 5w. No my readings are:
B+ - 396
Pin 3 - 385
Pin 4 - 355
Cathode - 25
Current .037

Seems to have only dropped it to around 13 watts? Should i keep going larger with the cathode resistor value? Will that make the plate voltage too high?

[The Dude]

Remember that plate voltage ratings on tubes are plate to cathode and not ground. So effectively, you have about 371V across the tube now which keeps you in a reasonably safe zone. There are lots of ways to go here.

1) Lower plate voltage with zener(s) or a choke.
2) Try an even larger cathode resistor.
3) Leave it as is.

If it were mine, I might either try raising the cathode resistor slightly more, or add a choke to drop B+ a bit.

[pdf64]

Is the 5Y3 old production / made in USA etc?
Internet wisdom is that current production put out too high a voltage; that may be across the board, or just Russian made.
Consider getting hold of a functional used one to see if that helps bring the VB+ down; no need for for NOS, though even those are still reasonably cheap.

[Mick Bailey]

I'd stick in a pair of JJ 6V6S tubes and play it. Those tubes will take a lot more punishment than a regular 6V6. They sound pretty good, too.

[5thumbs]

I am using the stock JJ 5y3 and JJ 6V6s that Mojo supplies with their kits. I raised the cathode visitor to a 470ohm as that was the next closest to the 330ohm I had tried above. I played an iPod through it for 2-3 hours at a moderate-low volume. After playing it for that time, I again checked values at idle:
B+ - 403
pin 3 - 395
pin 4 - 368
Cathode - 27.4
current - .027

Seems now that the readings show a little too low dissipation? maybe I need to go back to the 330ohm resistor and call it good.

Tim

[52 Bill]

Seems now that the readings show a little too low dissipation? maybe I need to go back to the 330ohm resistor and call it good.

How does it sound with the lower dissipation? Don't just get caught up in the numbers, use your ears in the decision making process too.

[g1]

Completely agree with 52Bill, there is no such thing as "too little dissipation" unless it sounds bad.
Bias as cold as you can go without making a negative impact on the tone. Your tubes will thank you.

[5thumbs]

I thought it sounded a little flubby (technical term ;-)) especially when I cranked it up past half way.

[The Dude]

Idle current is more in line with what you should have. Have a look at this page.

Weber Bias Calculator

It's quite possible that the "flubby" is not an effect of bias, but something else.

[pdf64]

My experience with cathode bias is that if cathode resistor values are increased much above their vintage value, there will be too much squish (increase in cathode voltage with high signals) results in a thin ready cranked tone.
Far better to reduce the VB+ somehow to control plate dissipation, hence the suggestion of trying a USA made 5Y3.
Too far from class A and it's preferable to move to fixed bias.