5f11 build no sound

0Telemetry

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Finished building my first amp, a Mojotone 5F11. No mods or changes, just followed the included diagram and schematic. I have all the tubes in and a speaker connected. The voltage across the heaters were all about 6.9 volts. Rob's site says you should hear a pop when checking heater voltage, no pop. The B1 was 402DC, B2 was 402, and B3 was 310, all higher than Fender shows. I can hear the usual very slight hum, but nothing else. I checked visually and used a continuity tester to verify all connections. I followed Rob's site to check the resistance of the input jacks, all good. Is there any measurable voltage at the output jack? I show 0. Tried the speaker in another amp and it is OK. I even tried touching the end of a cable connected to the input jack, all 3, no buzz or crackle. Really hoped I could figure this out, but it is time for the experts.
 

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bebopbrain

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Excessive B1 suggests little or no idle current on the power tubes.

What is the bias voltage for the power tubes? And old schematic shows it should be negative 31V.

What is the idle current in each power tube? Get this by (carefully) measuring the voltage drop from B+ (output transformer center tap) to each 6V6 plate. Then turn everything off and discharge everything and measure the resistance from B+ to each plate. Or use a current probe.
 

SerpentRuss

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Like bebopbrain stated, high B+ voltage likely indicates no current in the power tubes. Tube cathodes in this design should be directly grounded, perhaps the sockets themselves are mis-wired? Make sure your bias voltage is negative.
 

dan40

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Looking at your pics, your pin 8 connection on the v3 power tube socket looks very cold and in need of more solder. Like the fellas mentioned, with a fixed bias design, pin 8 (cathode) must be grounded to the chassis for proper operation. I would try reflowing both ends of your small ground wire on that socket. There are several other spots that also look like they may need a bit more heat and solder for a solid connection.
 

andrewRneumann

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Just to confirm that the power tubes are not conducting, can you post the voltage measured at B+1 and both power tubes pin 3?

Also please measure the voltage of both power tubes pin 5.

Confirm you can actually see the heaters glowing in the power tubes.
 

0Telemetry

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I should have mentioned that I know very little about what is what. So talk to me like the novice I am. But I am learning. It would be good to tell me DC or AC and pin number on tube sockets.

Yes the jumper is from the tip to the ground tab per the Mojotone diagram.


B+1 seems to oscilate from about 381 to 410, I get the same readings on pins 3 at the 6V6 tubes.

Pin 5 on both power tubes also oscillates from about -35 to -41DC. The oscillating concerns me, shouldn't it be steady.

Bias voltage measured at -40 at the base of the 25uf cap.

I attached a pic of the tubes shot in the dark, to illustrate glow.

I checked the solder and will add some.

Thanks for all the help, you guys are great. Given time I will learn the in and outs.
 

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sds1

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Yes the jumper is from the tip to the ground tab per the Mojotone diagram.
What the diagram probably shows is a connection tip to switch on a switched 12A style jack, which has 3 solder lugs. This is the wrong jack.

What you've done here is sent all signal to ground.

Nothing is ever soldered to the actual tip of the jack.
 

0Telemetry

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Cut the jumper on the output, tuned it on ( with tremelo spped and depth off at pot). Ah progress, from dead to really high pitched squeal, almost deafening. but at least it is noise.
 

0Telemetry

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I am reading through Robs site for troubleshooting, again. He said excess voltage may result from putting a too high a resister. I checked and they are correct.
 

sds1

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Cut the jumper on the output, tuned it on ( with tremelo spped and depth off at pot). Ah progress, from dead to really high pitched squeal, almost deafening. but at least it is noise.
So this can explain the moving voltage readings, normally we measure things at zero-signal condition but this amp has an oscillation so it's expected the voltages would reflect this in some way.

Normally the first go-to for this issue is to swap the OT primaries (blue/brown) from one side of push-pull to the other.

You'd think the primaries would be on the correct sides already according to the Mojotone layout, but IMO should should go ahead and swap them for good measure, see if you get a fix.
 
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King Fan

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It happens again. @D'tar wanders by, immediately spots single wire causing problem, mentions it modestly in thread, continues on to kitchen for glass of milk…. :)

@0Telemetry , welcome. Good job posting your data and pics. FWIW a 'switching' 3-prong Switchcraft 12A jack is safer at the output; can protect your OT if someone forgets to plug in a speaker. Maybe Mojo owes you one, or you can get one on eBay.

Let us know if brown isn’t long enough to swap with blue. Or if you want to find out if that’s the problem before doing the swap, just unsolder the yellow wire (NFB) that goes on to the board from the switch tip For those keeping score at home, here's Mojo's layout:

B3903898-5358-47DB-AE0D-3DFC15939369.jpeg
 
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0Telemetry

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This is crazy, because I have read about swapping the OT leads, but not knowing for sure I was hesitant. Thank heavens for sds1. It is now working and the bias is -31. Still getting higher voltages than I should, under the first filter cap, which think is B1 I have 409 VDC under the second is 409 and the third is 311. Mojotone said to leave it on for about 8 hours prior to playing, is this correct?

And now for complete disclosure. I have been following this forum for well over a year and only joined last January. I waffled for a long time prior to taking on a build. After reading virtually every post in this DIY section, i figured that you guys could save me and sure enough you have. I can't thank everyone enough. Great bunch of folks, thank you.

I will turn 68 on Monday, can't think of a better present to myself.

Please let me know about the voltages.
Carl
 

SerpentRuss

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Well done sir. This is a common theme around here! My voltage is too high!

What does the group think about switching from the black & white PT primary leads to the black/blue & black/green PT primary leads?

View attachment 992679
He could certainly try it for a minute or two. In theory, it will be very, very close, but it could be lower. Couldn't he also put 10 volt Zeners on the center tap? @robrob has that trick on his mods page.
 

dan40

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As the fellas mentioned, it's not uncommon to see higher than expected B+ voltages in many new builds. Some of it may be coming from a higher than average wall voltage but usually it's just down to the PT that was supplied with your kit. The important thing to measure now is the 6.3v heater filaments. If the PT is receiving the correct wall voltage, you should be pretty darn close to 6.3v AC when measuring the filament voltage. Just set your meter to AC and place your meter leads on the solder lugs of the pilot light simultaneously. If you are seeing 6.3v, then your wall voltage is spot on and the PT is operating correctly. If your reading is closer to 7v, your wall voltage is a bit high and that is the reason for your high B+.

High B+ could also be caused by a cold bias on the power tubes. Colder bias helps the tremolo to operate better but it will also create a slightly higher B+ throughout the amp. Did you set the bias when you powered up the amp for the first time?
 

andrewRneumann

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It is now working and the bias is -31. Still getting higher voltages than I should, under the first filter cap, which think is B1 I have 409 VDC under the second is 409 and the third is 311.

If VTADIY.com is to be trusted, your bias should be more like -45V with plate and screen voltages of 409VDC. I'm trying to understand why you aren't getting any voltage drop between the B+1 (OT) and B+2 (screens). There is a 470Ω resistor between the two, so you should be getting some voltage drop due to the current from the screens and the preamp tubes. Maybe your voltmeter is a little imprecise.

You have a non-adjustable fixed bias, so we are fairly limited in what we can do to the bias without changing resistor values around. I'm trying to help you without relying on ordering new resistors, zener diodes, etc.

I would try what I suggested in post #17. This will lower all the voltages in the amp, including the heater voltage and bias voltage (less negative). Make sure to cap the unused transformer leads as they will be live. It may not be the solution, but it's a start. You will have to report back with the voltages including the heater pin voltages measure in AC.

I have no idea why Mojotone would suggest leaving it on for 8 hours before playing. Play it.
 




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