5f11 build no sound

King Fan

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My OT is 50246, which their website shows for a 5E3. They show another for just vibrolux/tremolux and vibroverb.
I have no resistors or pots on hand and would have to order, no problem. Just so I understand, the pot allows us to vary the volts and any extra is shunted to ground through the 33K resistor. Same as the existing 56K sending it to the ground above. I do not understand how it works with only one wire, how do the volts get back or is this just limiting the voltage and the remaining just keeps going? Yep, I am that much of a novice.
Draining the caps, I tried to get a link to work on Robs site on how to, but keep getting an error. Long story short, i have no idea, other than wait for days.

Thanks for the help

Draining caps is easy; the key item on Rob's page is to build a cap snuffer. He gives this link:

https://www.ifixit.com/Guide/Constructing+a+Capacitor+Discharge+Tool/2177

A nice extra is to make one of the wires on yours with an alligator clip on the other end, to clip to the chassis. Then you just clip the gator to the chassis and hold the resistor lead to the positive pole of each filter cap for a few seconds. If you hitch up your meter you can watch it drain and confirm it is drained. Lots of other points could be made -- most hot amps in working order drain caps spontaneously in less than a minute. But measure and drain always works.

As you note, your OT only has one secondary, the yellow = 8ohm, so that's not going to be an issue. I think in supply chain times, we're gonna see lots of substitution.

If you want to replace the 56K bias range resistor with a better value, values as per @jsnwhite are a good idea. I do like a pot, though, and 10K linear pot works nicely. I'm confused my Mojo's diagram. Maybe Jason or somebody with a 5F11 can chime in with a clear layout...

1655078032695.png
 

jsnwhite619

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Lots of other points could be made -- most hot amps in working order drain caps spontaneously in less than a minute. But measure and drain always works.
I've always thought that, but I've recently been messing with my stuff again, and even after 24+ hours unplugged and on the bench, I checked it with my meter to see. The caps still had 16v on them. Better than 400v, but still something - I didn't touch it to find out. :lol:
And yes, alligator clips are great. I bent the resistor lead like a hook, then the clip on the other end, so I'll clip it to the ground bolt I run through on the PT side, hook that resistor end around a capacitor lead, and just leave it that way until I'm done.

. I'm confused my Mojo's diagram. Maybe Jason or somebody with a 5F11 can chime in with a clear layout...
It works is all I can say. I've used it since the first bias pot I added to anything, and it's about as simple as you can get. With the back of the pot and black screwdriver slot facing me (from inside the chassis), turning it left increases the bias level and lowers B+ voltage.
 

King Fan

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It works is all I can say. I've used it since the first bias pot I added to anything, and it's about as simple as you can get. With the back of the pot and black screwdriver slot facing me (from inside the chassis), turning it left increases the bias level and lowers B+ voltage.
Thanks, I shoulda been more clear. The pot part of their drawing is clear -- it's the connections on the board where I got lost. But I see now the brown (bias tap) and yellow (ground?) wires look incomplete because they're supposed to be dashed lines -- they're under the board. Sorry for the distraction. @0Telemetry , I think your intuition was right -- this pot exists to adjust the amount of (negative) bias current that goes to ground. Do you understand what Jason's saying about a better value for the 33K resistor?
 

0Telemetry

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I installed the bias pot and resistor per the MT diagram. No problem, adjusted bias to about -34 volts. This brought me to about 78% efficiency. Plugged in the Tele and no sound, faint squel when I was testing, but nothing. Checked continuity to be sure I did not mess up something when adding pot, all good, diode checks out also. May be a bad idea, but what if I swapped the OT leads back to the original position. Makes me think I should have quit at post 32, if ain't broke don't fix it.
 

D'tar

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If you sorted out the OT phase earlier, there should be no reason to have to revisit a phase issue without any other modifications beyond the bias mod. Is that -34vdc measured at the 6v6 control grid? How about a fresh clear photo of your bias mod.
 

0Telemetry

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Here you go, pics of the mod
 

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andrewRneumann

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The -34 is measured at the positive end of the 24uf cap.

The positive end is connected to ground so should measure 0VDC. Do you mean the negative end of the cap?

If you truly mean the positive end, then I would guess you lost the chassis connection from the ground side and your bias supply is lowering that to -34V. Check for continuity between the positive side of the bias cap and chassis.
 

0Telemetry

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Yep, it is still grounded. I checked all the connections that I thought were impacted by adding the pot.
 

0Telemetry

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Turned off the lights to verify all tubes were glowing. One of the preamps was not. Switched the two and the same one stayed dark. Guess I am in the market for a 12AX7, or should I get a matched pair. And why is it not working.
 

andrewRneumann

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Turned off the lights to verify all tubes were glowing. One of the preamps was not. Switched the two and the same one stayed dark. Guess I am in the market for a 12AX7, or should I get a matched pair. And why is it not working.

That seems like a pretty rare failure. But since it followed the tube, there you go. Curious: what’s the resistance between pins 4 to 9 and pins 5 to 9? (Tube pulled out of circuit.) Pins nice and straight? There are two heater sections, so both have failed.
 

0Telemetry

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I did take a reading with the tube still in, the good tube was .2 Ohms bad one was an OL. My pins 4 and 5 are jumpered together, so only one reading. When I tried do take it out the glass broke, so probably won't get any good readings. Any tricks to getting it out. And is there a a tube manufacture you guys recommend.

This started out like a nice project, but starting to get a little frustrating. Cabinet came out OK though
 

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D'tar

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This started out like a nice project, but starting to get a little frustrating. Cabinet came out OK though
Hey friend, you're at the finish line here! **** happens! Your gonna have a great tube amp that you built yourself! I got a nice 12ax if ya like.... you in usa?
 

andrewRneumann

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I did take a reading with the tube still in, the good tube was .2 Ohms bad one was an OL. My pins 4 and 5 are jumpered together, so only one reading. When I tried do take it out the glass broke, so probably won't get any good readings. Any tricks to getting it out. And is there a a tube manufacture you guys recommend.

This started out like a nice project, but starting to get a little frustrating. Cabinet came out OK though

Bonus! You get to take apart a tube and see what a heater, cathode, grid and plate of a 12AX7 look like. It’s pretty useful to see how it actually looks in there. Be careful with the glass. Maybe a small plastic or wooden wedge gently prying up the base of the tube.

Just like a light bulb—if you lose the vacuum the filaments melt. Most likely the tube cracked before you tried to remove it.
 

0Telemetry

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To all of you that helped, thanks. The new preamp tube did the trick, sounds great.

I am thinking about building a Princeton Reverb next summer; so don't go anywhere, I am sure I will need some troubleshooting advice. Finally, time to replace the one I bought new about 50 years ago. Like a fool I sold it in the mid 70's.
 

0Telemetry

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Oops, spoke too soon. So the B+ is about 380 and the new bias is -33.7. close to spec and much better than the -41, but now I noticed the 6V6's are red plating.
 




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