MojoTone 5E3 Fender Tweed Deluxe - HUM

[Janglin_Jack]

Thanks Alex for responding. I put in two JJ power tubes and measured 415v on pin 3 of the 6V6 and still get 26v at pin 8 on the 6v6. 26/270=.0962 or 96mA for both tubes....48mA each tube? 415-26=389. 389*.096=37.34 or 18.67 watts per tube at idle?

My first question is how can I fix this? 18.67 is 134% of the 14 watt max for the JJ. I have read about Zener diodes. And maybe a new cathode value. I am considering dumping the Mojotone PT and getting a Classic Tone with the proper vintage specs.

My other question is why does the amp sound so good as is and how are so many others out there running these kits with higher voltages with no issues? Is idle dissipation the only consideration, or can the amp run at this level for years? I dont understand the principles of cathode bias, and very confused about this amp.

These are the kinds of posts that confuse me...amp techs with story's about running the amp hot is ok..don't over think, etc.

I've worked on an amp that ran about 130% dissiptation (cathode bias) for over 15,000 of play time. (I can verify this for those who are skeptical). It finally blew the sovtek el84 that was the power tube. NOW.. I dont think this is the norm, but there are plenty of examples of cathode bias tube amps that run tubes well over 100% and the tubes last for a good length of time. Just rock on!

Mike

[alexradium]

JJs can stand easily almost 500V,no problem,just raise that cathode resistor until you get around 90% dissipation at idle.

[Janglin_Jack]

Alex, i think you suggested adding a 130-180 ohm resistor in series, so we would be looking at 400-450 ohm. I have a 470 ohm resistor, (from a Marshall build?). If i remeber right it is a green resistor and I think it is wire wound. It seems to look like it has a ceramic coating, but i cant find a wattage on it.

Do you think a 470 ohm resistor is a good place to start? If so i will drop that in, otherwise i need to order something.

Mike

[alexradium]

Alex, i think you suggested adding a 130-180 ohm resistor in series, so we would be looking at 400-450 ohm. I have a 470 ohm resistor, (from a Marshall build?). If i remeber right it is a green resistor and I think it is wire wound. It seems to look like it has a ceramic coating, but i cant find a wattage on it.

Do you think a 470 ohm resistor is a good place to start? If so i will drop that in, otherwise i need to order something.

Mike

try it and calculate power dissipated

[Janglin_Jack]

try it and calculate power dissipated

Alex..I figured out it is a Welwyn wire wound resistor. I will drop it in and retest.

Mike

[Janglin_Jack]

OK, well. Dropping in the new cathode resistor took on a whole new life. The chassis was tight and I was worried about melting some surrounding wires. I started moving things out of the way and I got to looking at my home made tag board and decided to replace with a fiberglass board I had laying around. So I got out my parts box and populated the board with all new components including my Welwyn 470 ohm cathode resistor. I finished all the soldering late last night and tested it out today.

Now I am getting about 435v on Pin 3 of the 6V6 and about 407v on Pin 4. The cathode is reading about 32v. So I calculate the following:

32v/470 = .068 or 68mA 435v-32v = 403v 403v*.068 = 27.4 or 13.7 watts idle dissipation per tube. I have the JJ 6V6 in there now so they are 14 watts per tube, so I am at 98%. Seems like I have it dialed in now vs the 18w per tube before.

Will give it a workout at practice and see how it goes. Anything else I should be checking or considering??

BTW..Thanks again Alex!!!!

[alexradium]

good,sounds fine.
If you want to go the extra mile and drop some B+ voltage you can consider the Dana Hall VVR power scaling,it works perfectly in this kind of amps and is very affordable.

[Janglin_Jack]

I might look into that. I suspect, if I do anything else to the amp, it would be to dump the Mojotone transformers and get something more vintage. If the amp performs like it did at practice previously, I will keep the amp as is. But now I am very currious about a vintage spec 5E3 sounds with the lower voltage....sometimes you don't know what you are missing.

Mike