5f11 build no sound

Snfoilhat

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Hi there, you're getting help from an excellent group of people and I can't add much to that. Trying to follow the thread, there have been a few places where it was hard to interpret, so maybe there's some miscommunication masking a technical issue and the fix.
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.
This has a contradiction in it that stands out. If the original 5F11 ran < 340VDC plate voltage and -31VDC bias, that's well enough, but it doesn't make sense to have 380VDC B+ in a modern repro and aim for -31VDC bias. One part of the spec can't be dissociated from another because they're not independent.

The other thing was also about bias voltage -- in a perfectly working circuit, there are a number of different places you could measure the bias voltage. But since this build is not yet perfect, it seems to me that all the bias voltage measurements should be taken from the control grids of the power tubes to chassis. Pin 5 is the place where the bias matters and the chassis is your ground reference in several different places in the amp. The 5F11 has all the tremolo intensity circuitry connected to the bias supply to make the bias-vary trem system work and maybe you haven't shaken all that down yet. Good luck!
 

NTC

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Whatever you do, you need the bias voltage to be more negative. Do you know how to measure bias current and calculate plate dissipation?
 

0Telemetry

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After reading Rob's red plate trouble shooting, I lowered the bias voltage as low as I could get it with the pot, about 32.8. Still red plating and humming. Started back from scratch checking voltages and bias. The numbers all seemed OK, until I checked the resistance through the OT at the tube closest to the rectifier. Would somebody please tell me which is V1 and which is V2? It was a whopping 314.4 while the other was 175.7. This yielded a plate dissipation of 113%. I imagine this is the problem, but what is causing the higher number.
 

King Fan

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Tubes are numbered from the input end as V1 etc. You say your numbers are ohms, “resistance?“ Amp off, plate to center tap? Are you reading this Rob page with his example?

 

andrewRneumann

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After reading Rob's red plate trouble shooting, I lowered the bias voltage as low as I could get it with the pot, about 32.8. Still red plating and humming. Started back from scratch checking voltages and bias. The numbers all seemed OK, until I checked the resistance through the OT at the tube closest to the rectifier. Would somebody please tell me which is V1 and which is V2? It was a whopping 314.4 while the other was 175.7. This yielded a plate dissipation of 113%. I imagine this is the problem, but what is causing the higher number.

By "lower" do you mean less negative? We want more negative to cool off the current. Instead of -32.8V you may need -35 or -40V for cooler operation.
 

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Yes, I followed it verbatim. He mentions that you can measure the voltage drop directly, which I did also. The values are off by about a half volt from the subtract the pin 3 from B+.

LIfe was good until I added the adjustable bias pot. I am going to bypass the pot and just try a couple of different value resistors to see what happens.

Alas, now I know where to get the V1, thanks
 

King Fan

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I know this is frustrating. Before you abandon the pot, which should be the perfect answer to a blind resistor hunt, you might review Andrew's question about bias current *range*. And it might help to remind us your present bias resistor/pot values and layout, and the voltages you get at different points along the bias circuit. Finally, you might share the steps and numbers you get when you calculate bias. We could help you more if we know where you were, and speaking from my past troubleshooting frustrations, the review might even clarify things for you.
 

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Removed the bias pot, and set the 56K resistor back in place using jumpers. No impact at all, still red plating and humming.
 

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By "lower" do you mean less negative? We want more negative to cool off the current. Instead of -32.8V you may need -35 or -40V for cooler operation.
Interesting that I was getting -41 prior to the adjustable bias mod and all was good. But listening to the other posts and having a wiring diagram from Fender that said -31 I installed the pot to get it lower. Too bad I can't hit a reset button and go back about 2 weeks.
 

andrewRneumann

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Interesting that I was getting -41 prior to the adjustable bias mod and all was good. But listening to the other posts and having a wiring diagram from Fender that said -31 I installed the pot to get it lower. Too bad I can't hit a reset button and go back about 2 weeks.

Ha… it’s just somewhere in between. I just wanted to make sure you were going the correct way with your adjustment. So you can’t get any lower (more negative) with the pot you have? You can’t get -35V e.g.?
 

0Telemetry

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The pot gave me bias measurements of -32.8 up to about -38.5. It was a 10K linear pot with a 33K resistor to ground, installed per Mojotone drawing. It red plating at any setting. While it was in and showing -32.8 I had a B+ of 384.3 DC. pin 3 on V1 was 376.6, pin 3 on V2 was 377.4, this provided a voltage drop of 7.7 and 6.9 respectively. I also measured the voltage drop across the plate pin and center tap and got a drop on V1 of 8.3 volts and V2 of 7.25, as noted earlier about a half volt difference. After pulling the pot and reinstalling the 56K resistor the bias was once again -41 but still red plating. I checked all the connections for continuity and all good. In reviewing my numbers from a couple of weeks ago, i noticed that the OT resistance was not that much different than this time. could be due to not warming up the amp too long this time since it was redplating. Sooner or later I will win and get thing going.
 

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0Telemetry

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Now wouldn't that be real newby error mistaking redplate. I do not remember seeing the middle glow, just top and bottom. The middle red glow I am seeing may not be a plate, it appears to be showing through a small square window. If you can tell me for sure it is heater glow, firstly I will go out in the yard and do a few cartwheels and secondly never show my face around this forum again, it will be more red than the heater glow. lol
 

King Fan

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That makes sense. Don't blush -- it happens to a lot of us. Yes, there are often little windows that reveal the heaters.

My new amp Zen: Take deep calm breaths and wait for D'tar to figure it out. :)

Can we go back to %MPD? You can't leave the forum now -- you have us all in suspense.
 

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good point D'tar. It only took a few minutes to reinstall the pot. Still would like to see the bias a -31, best now is -32.7. It would be easy enough to change out the 33K for maybe a 28K that I have. Darn this stuff is addicting isn't it. some would say i am not only committed, but certifiable.
Thanks again to all of you.
 

King Fan

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Stand back, I'm gonna beat the poor horse now that it's not dead. I completely ignore bias current if my amp runs at 50-70% MPD. If it runs too hot, I need more negative voltage, but yours is no longer redplating, and your quest for 31 is not useful -- those bias voltages on old schematics aren't targets like the other voltages. The actual target is whatever bias voltage it takes to get 50-70% MPD, which depends on variables like the individual tube and the B+ / plate voltage. Can you pretend you never heard of that 31? :)

Also can you confirm your amp-off resistance in ohms from the center tap to the power tube plates? You mentioned it a couple times but I was confused. Also, I know you know where the plate pins are, but I find the whole description of measuring at the OT CT gets confusing. Is yours wired from the Mojo layout? In case this helps...

5F11 bias copy.jpeg

Oh, and those green circles? If your amp is wired like this, and you replace those gray jumpers with 1% 1Ω resistors (½W is fine) we can make measuring bias a 10-second operation. Let me know if you're interested...
 

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The amp is wired exactly per the Mojotone drawing, except the two OT leads were switched and the ground on the speaker jack was removed. OK, last time I am measuring, B+379.0 DC, V1 at pin 3 is 372.7, V2 at pin 3 is 372.8. Ohms between CT and pin 3 on V1 336 on V2 ohms are 304. Giving a voltage drop of 6.3 and 7.0, milliamps are 18.8 and 23. Plugging into Rob's calculator shows % plate dissipation of 48 and 59. These number were taken after playing for an hour, so the amp was good and warm. These are a little cold so I will crank up the pot and what the heck measure again, tomorrow.
 

King Fan

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Now we're talking!

In case I confused you mentioning V1, I put your output tube numbers on the diagram. V1 is the first, input end, tube, so these are V3 and V4.

And if you're at nearly 50-60% MPD, you're almost home. :) A little mismatch is OK, increases the nice harmonics, though 10-11% may be a bit much. Either way, check the trem as you bias -- too hot suppresses it. And bias by ear, with and without trem, then remeasure bias and make sure you're somewhere in the 50-70% zone.
 




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