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

[5thumbs]

Hi gang,
I have a friend's 5e3 Mojotone build that I am trying to troubleshoot. It was farting out then cutting out after about 20 minutes of playing at around 2/3 full up and getting super hot - you can't touch the chassis above the power tubes/rectifier and you can't lay your hand on the power transformer.
The voltages with the 5y3 Rectifier:
pin 3 - 305
pin 4 - 221
cathode - 36
B+ at first cap - 323
Bias - .072

So that looked cold to me coming in around 9 watts per tube. The 5w 270ohm ceramic resistor was discolored and the lettering was faded nearly off (heat?). I installed 2-500ohm 10W parallel resistors in it's place along with a new 22uf 50v cap. This stopped some of the cathode resistor drift from the heat, and it doesn't cut out completely, but still gets dirty and distorted after 20 minutes of play. Anybody ran into this problem? Do you think the Mojo PT is overheating and changing voltages? are the amp voltages too low to begin with?

[g1]

So that looked cold to me coming in around 9 watts per tube.

Can you show your math for this? I get around 19watts per tube.

[Tone Meister]

With those voltages it seems like you might have something drawing excess current. You want the plate voltage to be right around 350V so your plate voltage is a little low. The stock cathode resistor would be 250R, of course. Yesterday, I messed with a Tyler 5e3 that was very distorted and hummed badly. The power tubes were under-biased so the tubes were drawing excess current and the amp sounded distorted and mushy. I installed a 300R and an old stock 5Y3 and the plate voltages ended up right at 349V and the JJ 6V6s were biased for about 12 watts.

I fixed the hum by separating the preamp and power amp grounds, which were tied together by the builder. Made all the difference with the hum.

Which 5Y3 rectifier do you have? Do you have another set of power tubes to try?

[The Dude]

I'm calculating over 66mA per tube. IMO, way too much idle current. As g1 says, about 19W dissipation per tube. I would think around 24mA would be more in acceptable range.

On a side note , 5thumbs, I went right by Longmont, CO last weekend!

[g1]

The high idle dissipation is also probably why the plate voltage is low.

[Tone Meister]

The high idle dissipation is also probably why the plate voltage is low.

Yeah, I must have been writing my reply when G1 posted. I know that when I dropped the dissipation by about 4 watts in the Tyler 5e3, it made all the difference in that amp.

[5thumbs]

okay, I divided the result by 2 for a push/pull amp, duh! My math: (305 - 36) x .07 = 18.83 watts. How do I lower the the idle plate dissipation? Use a higher cathode resistor?

[The Dude]

Yes, bump the cathode resistor value. Using I=V/R, you can calculate the current through the cathode resistor. Then divide by 2 to approximate idle current for each tube. I say "approximate" because we don't know for certain that each tube is passing the same amount of current. Assuming reasonably matched tubes, it gets you close enough.

[Mick Bailey]

Just a thought - did you check the board isn't leaking DC and throwing the bias off?

[5thumbs]

Hi Mick,
How do I check for that?

[pdf64]

Check the static voltage on the power tube control grids, pin5, should be 0Vdc.

[5thumbs]

Pin 5 on the 6V6's reads 2-3VDC when I first put my meter on it but quickly goes down to around .6VDC as I hold the meter probe on the pin and settles there. What do i do to find the cause of the leaking DC?

[pdf64]

Suggest that V2 is removed and you re-check, in case the amp is oscillating.

[5thumbs]

I pulled V2 and am getting the same thing. In fact when i first touch pin 5 with the meter it starts around 20vdc and slowly drops to around .6vdc on both tubes. Once it has "drained" it seems when I test it again it starts around 1-2VDC and goes down as if it is building up voltage slowly?

[g1]

Maybe disconnect the coupling caps to see if it's them or the board.