MojoTone 5E3 Fender Tweed Deluxe - HUM

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Re: MojoTone 5E3 Fender Tweed Deluxe - HUM

Post by demonufo » Fri Aug 13, 2010 3:00 pm

Depends on what spec the PT is.
What is the PT btw?

It could be a case of the PT not being able to deliver enough current, which would draw the voltage down, and probably be the cause of the hum.

Read the spec sheet of the PT, and calculate the current draw of all your tubes. Heater currents included.
So I like purple, okay!!!!!!

83.7% of all statistics are made up on the spot!

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Re: MojoTone 5E3 Fender Tweed Deluxe - HUM

Post by Janglin_Jack » Fri Aug 13, 2010 6:19 pm

demonufo wrote:Depends on what spec the PT is.
What is the PT btw?

It could be a case of the PT not being able to deliver enough current, which would draw the voltage down, and probably be the cause of the hum.

Read the spec sheet of the PT, and calculate the current draw of all your tubes. Heater currents included.
This is a Mojotone MOJO756EX 384v-0-384v. I don't know any other specs. While I was searching for something on the web, I found a thread talking about this being too high. I guess the voltage would be around 420v after rectification?

Update.....I read where the B+ should be around 350v not 420v. I used my variac to bring the voltage down to about 350 and the amp is dead quiet. I guess I need to either get a different PT or try a Zener Diode to knock down the voltage.

Mike

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Re: MojoTone 5E3 Fender Tweed Deluxe - HUM

Post by Janglin_Jack » Wed Aug 18, 2010 3:48 am

I put in the 100ohm CT resistors again at the first 6V6 tube, pin 2 to 8 and pin 7 to 8. Amp is now silent!! Working very well. My only concern now, is the plate voltage on the 6V6 tube. Reading about 400-405v on the plate. Isn't the max voltage on a 6V6 350v?? Am I just going to burn through tubes or can the 6V6 deal with these voltages? Again the PT is 384v - 0 - 384v. I have read a NOS 5Y3 recto might reduce the voltages more than say a Sovtek. Should I be concerned with these voltages??

Mike

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Re: MojoTone 5E3 Fender Tweed Deluxe - HUM

Post by elronhoover » Wed Aug 18, 2010 11:03 am

Janglin_Jack wrote:I put in the 100ohm CT resistors again at the first 6V6 tube, pin 2 to 8 and pin 7 to 8. Amp is now silent!! Working very well. My only concern now, is the plate voltage on the 6V6 tube. Reading about 400-405v on the plate. Isn't the max voltage on a 6V6 350v?? Am I just going to burn through tubes or can the 6V6 deal with these voltages? Again the PT is 384v - 0 - 384v. I have read a NOS 5Y3 recto might reduce the voltages more than say a Sovtek. Should I be concerned with these voltages??

Mike
That plate V is kind of high for a 5e3 for sure..what is your wall VAC?

Are you running a sovtek-labelled 5y3? I've read somewhere that those are really a 5v4, and that using a 'real' 5y3 will drop it, maybe 30v or so vs. a sovtek, if I recall correctly. A NOS 5y3 is still pretty cheap ($10-15) and plentiful, and will last for a long time. For this amp, I'd say it's definitely worth getting.

As for 6v6 max plate voltages, this has been readily exceeded in many 6v6 amps. It's important to use a 'better' 6v6 in these cases if you can, and keep the bias in a sane range. I have a '79 deluxe reverb, stock, that puts 438v on the 6v6's @ 120vac. I use NOS GE JAN wafer base 6v6's in it, and have been for over 20 years. I keep it biased right at 70% idle, and I get about 4-5 years out of a pair with moderate usage, 2-3 years with heavy usage, and never had one fail.

Definitely check your bias and adjust your cathode R as necessary to keep it in range (I go for about 90-95% max, diss. @ idle on a 5e3 for bias), and you should be OK w/ that plate V, but again, it's not gonna sound as sweet as a real 5e3 should being that high..

Dave
guess you only get one chance in life to play a song that goes like this.....

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Re: MojoTone 5E3 Fender Tweed Deluxe - HUM

Post by Janglin_Jack » Wed Aug 18, 2010 2:58 pm

120v at the wall. Checked the spec on the EH 6V6 and I guess they can handle 475v. So for now, I guess no issue.

Mike

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Re: MojoTone 5E3 Fender Tweed Deluxe - HUM

Post by Janglin_Jack » Sun Aug 22, 2010 2:01 pm

Janglin_Jack wrote:120v at the wall. Checked the spec on the EH 6V6 and I guess they can handle 475v. So for now, I guess no issue.

Mike
I spoke too soon. My buddy played the amp at high volume and after a few minutes the amp cut off and stopped playing abruptly. He turned it off and then 30sec to 1min, turned it back on and it played normally. It then shut off again after a few minutes.

Is this likely a tube going into cutoff?? I am beginning to hate this 5E3.

Mike

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Re: MojoTone 5E3 Fender Tweed Deluxe - HUM

Post by elronhoover » Sun Aug 22, 2010 2:42 pm

Janglin_Jack wrote:
Janglin_Jack wrote:120v at the wall. Checked the spec on the EH 6V6 and I guess they can handle 475v. So for now, I guess no issue.

Mike
I spoke too soon. My buddy played the amp at high volume and after a few minutes the amp cut off and stopped playing abruptly. He turned it off and then 30sec to 1min, turned it back on and it played normally. It then shut off again after a few minutes.

Is this likely a tube going into cutoff?? I am beginning to hate this 5E3.

Mike
Not sure what you mean by 'cut off and stop playing abruptly', assuming you mean that no sound is produced at the speaker at all for some length of time and the amp remains powered up, no its not tube cutoff.

Since it was repeatable at least twice, and after the amp has been running for a bit, I'd suspect a heat and/or vibration related change like a cold solder joint causing lost continuity. Do you have solid continuity on your 6v6 cathode resistor? A bad solder joint here could shut down the output as you describe if it opens after it heats up or vibrates, as could anywhere up the signal line to the input.

If you can repeat this, pull the back panel and check out whats going on when it happens; check your continuity and voltages from your outputs backwards. If you get it to happen, leave it as such and touch a probe to the grid pin on a 6v6, then v2a, then v1a/b and see if you dont get a pop noise out the speaker at any point. Also try chopsticking thru the board, wiring, and sockets/jacks while running a signal thru the input to see if you can break it anywhere.

Have you checked the bias? If you are running a stock 250r cathode resistor and those plate voltages, I'll bet you are cooking way hot in there..

Dave
guess you only get one chance in life to play a song that goes like this.....

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Re: MojoTone 5E3 Fender Tweed Deluxe - HUM

Post by mbratch » Tue Dec 14, 2010 6:46 pm

Janglin_Jack wrote:I put in the 100ohm CT resistors again at the first 6V6 tube, pin 2 to 8 and pin 7 to 8. Amp is now silent!! Working very well. My only concern now, is the plate voltage on the 6V6 tube. Reading about 400-405v on the plate. Isn't the max voltage on a 6V6 350v?? Am I just going to burn through tubes or can the 6V6 deal with these voltages? Again the PT is 384v - 0 - 384v. I have read a NOS 5Y3 recto might reduce the voltages more than say a Sovtek. Should I be concerned with these voltages??
I have the Mojo kit as well. Great kit, but you do have to know to put the two 100R in there for virtual center tap on the heater wiring. I also found that the EH 12AY7 (is that what you have) introduces some hum. I tried two of them, same thing. I put a NOS 12AY7 and that went away.

Also, yes, the Sovtek 5Y3 runs higher voltage than a NOS 5Y3. It's closer to a 5V4. Also the Mojo power transformer will output higher voltage, especially at modern wall voltages, then vintage spec. So try a NOS 5Y3 and see where that leaves you. A Deluxe Reverb will run 6V6's significantly higher than 400V, so I wouldn't worry about the voltage as much as the plate power dissipation. So do the calculation for that. Power = (V_plate - V_cathode) * (V_cathode/R_cathode) watts. Divide by about .7 and that result should be < 14 watts. That's the rule of thumb I use anyway for fixed bias. I think for cathode bias, though, you wouldn't do the .7 divide. There will be varying opinions on that. :)

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Re: MojoTone 5E3 Fender Tweed Deluxe - HUM

Post by viejo » Sun Mar 06, 2011 7:23 pm

I think the problem is in your filament circuit. I would start by rewiring it with the 100 ohm resistors as an artificial center tap and rewire it so it is physically far away from the grid wires,either up above the tubes or against the chassis tight on the chassis on the opposite side of the tube from the leads coming from the board. It looks to me in the photos that your filament wires are actually touching your preamp grid wires

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Re: MojoTone 5E3 Fender Tweed Deluxe - HUM

Post by Janglin_Jack » Fri May 20, 2011 6:02 pm

Janglin_Jack wrote:
Janglin_Jack wrote:120v at the wall. Checked the spec on the EH 6V6 and I guess they can handle 475v. So for now, I guess no issue.

Mike
I spoke too soon. My buddy played the amp at high volume and after a few minutes the amp cut off and stopped playing abruptly. He turned it off and then 30sec to 1min, turned it back on and it played normally. It then shut off again after a few minutes.

Is this likely a tube going into cutoff?? I am beginning to hate this 5E3.

Mike
I haven't revisited this thread in a while and thanks for the additional suggestions. It turned out there was a solder blob on one of the preamp pins that was grounding. Cleared it away and the amp works wonderfully!!

Thanks,
Mike

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Re: MojoTone 5E3 Fender Tweed Deluxe - HUM

Post by Janglin_Jack » Thu Oct 30, 2014 3:39 am

I got this Tweed from my buddy.. Didn't change a thing. It is just a beast. Raw rocking tones with volume up...cleans up and sounds killer...really nice amp. Kept the mojotone iron...love the amp

Mike

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Re: MojoTone 5E3 Fender Tweed Deluxe - HUM

Post by Janglin_Jack » Mon Nov 03, 2014 1:12 am

Played the amp at practice the other nite, both volumes were up at various levels all night, and used various pedals, etc. After 2 1/2 hours or so, i had a moment with no sound. The band kept playing , so i am not sure if it was totally silent, but i couldn't hear anything out of it. I quickly check the back to see it the tubes were glowing and it appeared they were. I shut it down to prevent any damage. I did a quick check of connections to see if any cable were removed etc.. I didnt notice anything odd, so i powered it back on. I noticed the bright volume was all the way down....must have spun the volume down when I was in a panic. I dont remember doing it, but i must have. Amp worked fine for the remainder of the nite,

Could failing tubes be responsible, and if so which? I am guessing not the powers tubes, more likely preamp or rectifier? Since it was an intermittent failure and temporary, i am not sure what to test. i want to start by replacing some tubes, but should i test and check some other things first? Not sure where to start with this.

Mike

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Re: MojoTone 5E3 Fender Tweed Deluxe - HUM

Post by Janglin_Jack » Wed Nov 05, 2014 1:13 pm

This 5E3 has Mojotone iron - the PT is 384-0-384.

I did some tests in a few spots and my readings were 420v DC on pins 2/8 on the rectifier, 375v AC on pins 4/6 on the rectifier. I get 405v DC at pin 3 of the 6V6 tubes and 378v DC on pin 4. I read about 26v DC on pin 8 of the 6V6.

If I am calculating this in the correct way, the Sovtek 5Y3 is stepping up the voltage from 375v to 420v by a factor of 1.12, (right at the theoretical 1.13 for a 5Y3).

I am using a 270 ohm cathode resistor. I am not sure how to measure bias current with a cathode bias circuit. I will have to read more about that. I found some calculations, but not sure I am doing this right. I got 32mA bias current by calculating 26/405=.064 .064x1000=64mA 64/2=32mA average for each tube???

The 6V6 are the older EH 6V6GT with the black tinted glass. I can't find the specs on them.

Do these numbers look OK? I need to read more about cathode bias and how to do it. I am not sure where I am at with bias. Am I going to burn up these tubes or am I in a safe range?

Mike

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Re: MojoTone 5E3 Fender Tweed Deluxe - HUM

Post by Janglin_Jack » Thu Nov 06, 2014 5:34 am

What happened to all the amp builders? I get this is a Marshall forum...I was here early on. I know the site had some struggles, but did all the amp builders move on? Larry and Roe...SDM anyone? :peace:

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Re: MojoTone 5E3 Fender Tweed Deluxe - HUM

Post by alexradium » Sat Nov 15, 2014 5:36 am

Janglin_Jack wrote:This 5E3 has Mojotone iron - the PT is 384-0-384.

I did some tests in a few spots and my readings were 420v DC on pins 2/8 on the rectifier, 375v AC on pins 4/6 on the rectifier. I get 405v DC at pin 3 of the 6V6 tubes and 378v DC on pin 4. I read about 26v DC on pin 8 of the 6V6.

If I am calculating this in the correct way, the Sovtek 5Y3 is stepping up the voltage from 375v to 420v by a factor of 1.12, (right at the theoretical 1.13 for a 5Y3).

I am using a 270 ohm cathode resistor. I am not sure how to measure bias current with a cathode bias circuit. I will have to read more about that. I found some calculations, but not sure I am doing this right. I got 32mA bias current by calculating 26/405=.064 .064x1000=64mA 64/2=32mA average for each tube???

The 6V6 are the older EH 6V6GT with the black tinted glass. I can't find the specs on them.

Do these numbers look OK? I need to read more about cathode bias and how to do it. I am not sure where I am at with bias. Am I going to burn up these tubes or am I in a safe range?

Mike
first try good known preamp tubes.

your bias current is 26V divided by 270 ohm,that means the power tubes are dissipating 37 watts at idle,the nominal value for 2 6V6s is 24/28w.
Most likely they're gone,(russian tubes can't get those voltages and power)i'd say get a couple of matched JJs,and watch the cathode voltage,if its still at 26V raise the cathode resistor,put a 130/180 ohm in series.

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