5f11 build no sound

King Fan

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Am I having a brain cramp, friends, or is a 350-0-350 PT crazy hot for an amp that had a schematic B+ of 340V on 110 wall power? If I'm thinking straight, it seems like we're lucky to have voltages as low as they are here.

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.

@0Telemetry that's refreshingly honest. Do you know how to measure bias on your amp? Are you comfortable measuring voltages? Maybe AC heater voltages (pin-to-pin as noted -- this at least isn't very dangerous) and DC voltages (pin-to-chassis) on all pins 1-3 and 6-8 in the preamp, and pins 3 and 4 on the output tubes? If so, how about measuring rectifier pins 4 and 6 for AC voltage (you can measure these each pin-to-chassis to decrease the load on your meter).

@andrewRneumann , I like your PT primary idea. For our OP's benefit, I'm thinking you're using the bigger voltage gap between these two primaries (130V) to cut down the secondary voltage coming out. 0Telemetry, let us know what you think and how you want to proceed. Oh, and re your B+ and B+1 voltages, let us ask: what meter are you using?
 

0Telemetry

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I am OK with measuring voltages, it is a Fluke. I measured the AC across the pilot light 6.61 and following the green wires at each tube I continued to get 6.61 all the way down . Pin 8 on rectifier 411DC ,Pin 3 on both power tubes 409, pin 5 on both power tubes -39.81 DC. Preamp tubes, Pin1 206 and 212, pin 3, 1.62 and 1.59, pin 6 209.5 and 259.3, pin 8 1.61 and 56.41. The first preamp value is the the tube to far right, second value is for one near center.

I thought to measure the bias you checked the DC just above the diode. Let me know if you need additional readings.
I have no problem with changing wires from PT. I played through it this AM and all sounded good, I tried all 3 inputs and the tremelo, no problems.
I really appreciate all the help.
 

King Fan

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Good stuff. You’re right, bias *current* is pin 5. But effect of that current on power dissipation varies — target in fixed bias is 50-70% of tube's rated max dissipation, which is 14W for the 6V6 — and is what we’re most interested in.

With your Fluke you should be all set to use Rob's 'OT resistance method:'

https://robrobinette.com/How_to_Bias_a_Tube_Amp.htm

And since you have a good meter, can you recheck pin 4 screen voltages on your output tubes? As Andy says, should usually be a little less than pin 3 plate.

Finally, if it sounds good, let's proceed methodically. Bias measurement/calculation first; and smart folks can think about your preamp voltages. For comparison, Fender's values are still good targets.

http://www.prowessamplifiers.com/schematics/fender/Vibrolux_5f11-Schematic.html

At a glance, your high B+ is elevating all your plate voltages; given that hot PT we may want to work to reduce them. But bias before that.
 

0Telemetry

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Started to follow Robs bias procedure. Since mine is wired with the center tap going to pin 8 on the rectifier tube it was easy to just measure the drop. Well not easy it oscillates about a volt, so best guess left side power tube 4.763 vdc and right side about 4.395. Never thought I would best guess to 3 decimal places. Next I measured the resistance between center tap and pin 3, 319.7 Ohms on 1 and 287.4 on tube 2. Did the division and came up with 14.9 and 15.3 milliamps respectively. I then went to his bias calculator and plugged in the values. Looking at the milliamp calcuation it would appear to be 47%, which if I read his description correctly that is lower than Fender put the original amps, 50 to 60%. I also plugged in the plate to cathode voltage of about 380, again it varied several volts. Tremelo is off so I do not know what causes the oscillations on the meter. Nice exercise, but with the voltage varying so much on the meter the numbers could be off by 20% either way. I had the amp on for some time like he said and I have not noticed any red plating or does that only happen with a guitar plugged in and playing.
 

0Telemetry

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I noticed on the PDF showing the Fender voltages it says + or - 20%. That may be for consideration of today's voltages. Given my 123.6 this morning and currently 116. 5, the numbers may not be as bad as they appear. This of course is based on my complete newbie/novice knowledge. Interesting that Mojotone does not list voltages on the layout sheet or the schematic.
 

King Fan

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You're doing good work. Details: Fender's voltages are a perfect target, except you'll need to allow for your high B+. Likewise Fender's +/– 20% is fine, but it means amp to amp or house to house; itt doesn't mean morning to afternoon (or moment to moment -- :)). If your wall voltage varies that much AM to PM, that could account for it. But otherwise no.

Likewise if the trem is definitely off, the voltages shouldn't fluctuate. Hopeful moment: Are you setting trem to zero, or unplugging the footswitch? IIRC setting Depth to zero doesn't actually kill the oscillation.

You're right, based on those measurements, your bias is definitely cold. As @dan40 mentioned above, that can elevate B+ somewhat -- and the amp won't sound its best either. But as @andrewRneumann noticed, you don't have Mojo's adjustable bias mod. (Did they send that pot etc.? Any chance your could add it? It's job is to help you adjust your fixed bias.)

1654979354108.jpeg


Otherwise you can adjust your bias by swapping in different values of 'bias resistor' -- that 56K. Doug Hoffman talks us through an example (for a different amp) -- I've heavily edited to keep it simple: "Example: Your bias is too cold. You have too much negative voltage in the bias circuit. Make the bias range resistor larger in value and try again. Keep adjusting the bias range resistor until you can get bias in the right range...."

My smarter friends may help. FWIW lots of builders here know the 5F11 circuit. I'll put up the Bat Signal for @jsnwhite619....
 

0Telemetry

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No they did not include the bias mod. I watched a video of Uncle Doug setting the bias with resisters, not too bad. I plugged in the trem switch, turned of the trem and then unplugged, also played to verify, no tremelo.
 

0Telemetry

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No, that was prior to the testing, all voltages were taken afterward.

Regarding the Uncle Doug video, he makes everything look simple. Soldering the resistors in would be easy, but getting the right one would be the challenge, at least for me.

I looked at the bias mod, eleminate one resistor and solder in a pot. Looks easy peasy. Perhaps that is the way to go.
 

wangdaning

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No, that was prior to the testing, all voltages were taken afterward.

Regarding the Uncle Doug video, he makes everything look simple. Soldering the resistors in would be easy, but getting the right one would be the challenge, at least for me.

I looked at the bias mod, eleminate one resistor and solder in a pot. Looks easy peasy. Perhaps that is the way to go.
I think if you use Rob's calculator you can manipulate the resistor value to get see where you need to be to get the correct bias.
 

0Telemetry

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Read through an older post by DennisM ( Nov 2020) on biasing a 5F11. My numbers and his are nearly identical, he commented that his sounds great. I agree I played it for a long time last night, very happy. I play mostly clean. With that said, I think I will adhere to the old saying of "if it ain't broke don't fix it".

I will continue to read Rob's site and try to learn more.

Thanks
 

jsnwhite619

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Scrolled through a bit, but a lot to cover coming in this late. What's the current status of things? One question - did you use Mojo's 768 output transformer? Going by your first pics and their diagram, the yellow tap you have connected is for a 4ohm speaker - is this correct & intentional? If you have an 8ohm speaker in there, you're running the tubes at a 4k primary instead of 8k like they should.

I've built several 5f10/5f11 amps, but my first was a Mojo small parts kit. The stock bias resistor value was WAY too high and cold. I thought my meter was broken because it was so far off from what it should have been. Every one I've built used the high voltage instead of a 50v bias tap - not sure what you're working with here - but they all ended up with 10k bias pot and 17k-22k resistor on top of that, so if you want to average that out, the neighborhood of 25k vs the 56k that was supplied with their kit.

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1655044774767.png
 

NTC

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For the fluctuating voltages, the best thing is to plug in the tremolo pedal and switch it to make the fluctuations go away. The jack for the switch should also have been a three terminal jack like the speaker is supposed to have - though the original didn't have that. (Looking at the schematic, the original didn't have a three terminal SPEAKER shorting jack, either. ) EDITED to add the word "speaker. "
 
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King Fan

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Scrolled through a bit, but a lot to cover coming in this late. What's the current status of things?

Yeah, sorry, Jason, you're joining a program already in progress. Good question about the OT. I'll let the OP update and confirm, but also: IIRC he has this PT, and he's getting high B+ (?~410) and apparently cold bias (<50%); he didn't do the Mojo bias adjust mod but he's considering retrofitting it. AFAIK he has some fluctuation on plate voltage but he notes he turned 'off' the trem and unplugged the trem switch. I may have misled him; I like the note from @NTC about leaving it plugged in and using the switch. Then if it continues, we check the switch and jack are working right.

1655047201263.jpeg


And he seems to have a good attitude, learning before doing.... :)
I think I will adhere to the old saying of "if it ain't broke don't fix it".

Good idea. We're a bunch that likes to fix stuff, but slow and steady wins the race. Don't rush to fix the B+, and maybe only do the bias after you read up. Over time, learning to adjust bias (and add bias adjust pots to old amps) is great.

Trying different B+ values is fun and sonically rewarding, but if you can get bias right and output tubes happy, not a must do now, and not quite as easy. Sometimes an NOS 5Y3 will drop more than a new one -- in fact, what brand do you have in there now? -- but your high PT voltage hints that may not do the trick. IME the easiest, most useful way is to build a bucking transformer -- cheap and easy -- Rob has instructions on his 5E3 mods page, and quite a few threads exist here.

Annnd, which OT *do* you have?
 
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jsnwhite619

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Took a while to find the thread, but my original startup voltages and level with a 33k bias resistor on the pot vs paralleling another 33k on there for 16.5k.

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1655058301691.png


 

0Telemetry

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Scrolled through a bit, but a lot to cover coming in this late. What's the current status of things? One question - did you use Mojo's 768 output transformer? Going by your first pics and their diagram, the yellow tap you have connected is for a 4ohm speaker - is this correct & intentional? If you have an 8ohm speaker in there, you're running the tubes at a 4k primary instead of 8k like they should.

I've built several 5f10/5f11 amps, but my first was a Mojo small parts kit. The stock bias resistor value was WAY too high and cold. I thought my meter was broken because it was so far off from what it should have been. Every one I've built used the high voltage instead of a 50v bias tap - not sure what you're working with here - but they all ended up with 10k bias pot and 17k-22k resistor on top of that, so if you want to average that out, the neighborhood of 25k vs the 56k that was supplied with their kit.

View attachment 993252
View attachment 993253
Jason, my OT shows the 8 ohm as the yellow wire, different number than your 68. Odd they don't try to keep the color scheme consistent. Also, are you suggesting I do the bias pot mod with a 25K resistor instead the of the 33K MT shows in the diagram or just replace the existing 56 with a 25?

King Fan, I have the same PT as you show 757EX.

NTC, tried your method and much better, not still but only varying a few 10ths or hundredths.
 

jsnwhite619

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@0Telemetry Out of curiosity, which OT do you have? The pot makes life a LOT easier, and the hole is already right there in the chassis. I just checked my Harvard type amp here and where it's set with the pot right now, it's between 31k-32k of resistance there. So, if you know how to drain the caps to make it safe to work inside now (and be sure it's unplugged from the wall), I would replace the 56k bias resistor with something in the 27k-33k range and try it again. Use your meter to double check the ground connection & continuity of anything and everything you touch with a soldering iron from here on - both sides of every component and be sure that everything still has a good connection each time you heat/change anything. Nothing like having to find new problems that you accidentally created.;)

It's going to be tough working behind the pilot lamp. If you have extra resistors to choose from, it may be easier to tack on another 56k or 68k in parallel with it just to see if it helps in the right direction. Here's a handy calculator. https://www.digikey.com/en/resource...rsion-calculator-parallel-and-series-resistor
 

0Telemetry

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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
 




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