5f11 dropping B+ voltage

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Mike52
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5f11 dropping B+ voltage

Post by Mike52 »

Heyall, long time lurker.

I've got a 5f11 build with a 350-50-0-350 transformer with B+ voltage at 386vdc. I've been struggling to get that B+ down to something that makes more sense for a Vibrolux. This is obviously a fixed bias amp. I tried installing strings of zener diodes between the rectifier and B+1 and didn't see voltage drops consistent with what I was expecting. I was expecting to see a voltage drop of, with say a 12v 5w zener, to get a 85-90% the value of the zener. Any ideas on why this isn't working?

I first tried fixing the zener to the PT CT ground. This obviously didn't work because it's a fixed bias amp.

I'm kinda lost for figuring why zeners between the B+ isn't working.
Stevem
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Re: 5f11 dropping B+ voltage

Post by Stevem »

Fender used a 300/300 PT so why don't you do the same, by the time you get done jerking around with dropping out 40 volts of rectified B+ by means of a large heat sinked resistor , or big wattage reversed biased Zeners like a NTE brand 5275AK you can be up and running faster!

Something like a Hammond 270fx can be had from Mouser electronics for 74 bucks .
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Not screaming like the passengers in his car!😊

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M Fowler
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Re: 5f11 dropping B+ voltage

Post by M Fowler »

And after removing that PT 350-0-350 build a nice single channel Super Reverb or a 50w version of the John Mayor Signature Reverb.

Mark
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xtian
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Re: 5f11 dropping B+ voltage

Post by xtian »

You can drop up to 80 volts by using a MOSFET B+ reducer on the HT CT: http://ampgarage.com/forum/viewtopic.php?t=27035
I build and repair tube amps. http://amps.monkeymatic.com
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Guy77
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Re: 5f11 dropping B+ voltage

Post by Guy77 »

M Fowler wrote: Mon Feb 19, 2018 1:46 pm And after removing that PT 350-0-350 build a nice single channel Super Reverb or a 50w version of the John Mayor Signature Reverb.

Mark
Yes great advise "John Mayer Signature" is a great sounding amp , you will love it Mike52.

Cheers
Guy
Mike52
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Re: 5f11 dropping B+ voltage

Post by Mike52 »

Stevem wrote: Mon Feb 19, 2018 12:59 pm Fender used a 300/300 PT so why don't you do the same,
Yeah, I think that's the answer that I'm arriving at. But I'm new to amp building and I'd like to understand what I'm seeing here. This is the first time I've used zeners and the first time ive tried to reduce B+. Do you have any idea what I'm seeing here?
I used the following:
12v5w 1n5349b
25v5w 1n5360b
43v5w 1n5367b

I experimented with 3watt metal comp resistors and got more predictable results (voltage dropped by the amount expected and resistor got hot quickly as expected). But the zeners didnt behave at all the way I'd read they would. I'd be grateful if you guys could educate me here.
sluckey
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Re: 5f11 dropping B+ voltage

Post by sluckey »

A 15 volt zener will drop 15 volts when connected reversed biased in a "well filtered" DC supply. (Emphasis on well filtered). But if the DC is not well filtered such as the pulsing dc output of the rectifiers then the voltage drop across the zeners will be less predictable. And if you connect a zener so that it is forward biased it will act just as a regular diode and the voltage drop will only be about 0.6v for each zener. Is this what you experienced? You have not been very specific about the zeners you used, or the actual voltage drop you measured, or the actual voltage you have, or the voltage you would like to have.
Mike52
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Re: 5f11 dropping B+ voltage

Post by Mike52 »

Ok. So B+ 386vdc.
3x12v string of zeners brought it to 370vdc. 16vdc drop.
One 12v zener = 382vdc. 4vdc drop.
2x25v zeners = 361vdc. 25vdc drop.
One 25v zener = 377vdc. 9vdc drop.
One 43v zener = 366vdc. 20vdc.
2x43v zener = 343vdc. 43vdc drop.

I'm aiming for B+ just north of 340vdc, and 350vdc would be just perfect. Overall I'd read about using axial zener diodes to reduce B+ voltages and i wanted to try it for the learning experience. No one else has mentioned how this responds to well filtered DC, so thats new to me. So is what I'm seeing here typical of running zeners between B+ and a 5Y3?

I'm asking the question because I couldn't make sense off the outcomes I was getting. I appreciate any explanation from folks way more knowledgeable that me.
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Re: 5f11 dropping B+ voltage

Post by sluckey »

Try adding a 22µF cap directly to pin 8 of the 5Y3. That will add some extra filtering before you hit the zeners and should increase the voltage drop on the zeners.
Mike52
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Re: 5f11 dropping B+ voltage

Post by Mike52 »

sluckey wrote: Mon Feb 19, 2018 9:28 pm Try adding a 22µF cap directly to pin 8 of the 5Y3. That will add some extra filtering before you hit the zeners and should increase the voltage drop on the zeners.
Ok, so 5y3 --> 22uf cap --> zener string --> B+ ?

Also, is the 22uf value important?

Ps: I've got a 22uf 50v electrolytic. And a .22uf 600v polypro cap. Hmm, this plan may have come to a screeching halt.
sluckey
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Re: 5f11 dropping B+ voltage

Post by sluckey »

Connect the cap directly to pin 8 of the 5Y3. Other end to power ground. 22µF was just a safe ballpark value. You can probably safely go up to 40µF with a 5Y3.

Are you using a real 5Y3? I ask because an NOS 5Y3 will have a larger voltage drop than the modern 5Y3s.

Another idea... Replace that 56K resistor across the bias cap with a 50K pot in series with a 22K resistor so you can adjust the bias. Then you can run the 6V6s a bit hotter and this will bring the B+ down.

I personally don't use zeners for this purpose. I believe in using the correct PT to get the desired B+. I also believe that 386V is perfectly OK for any fixed bias 6V6 amp.
Mike52
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Re: 5f11 dropping B+ voltage

Post by Mike52 »

sluckey wrote: Mon Feb 19, 2018 10:18 pm Connect the cap directly to pin 8 of the 5Y3. Other end to power ground. 22µF was just a safe ballpark value. You can probably safely go up to 40µF with a 5Y3.

Are you using a real 5Y3? I ask because an NOS 5Y3 will have a larger voltage drop than the modern 5Y3s.

Another idea... Replace that 56K resistor across the bias cap with a 50K pot in series with a 22K resistor so you can adjust the bias. Then you can run the 6V6s a bit hotter and this will bring the B+ down.

I personally don't use zeners for this purpose. I believe in using the correct PT to get the desired B+. I also believe that 386V is perfectly OK for any fixed bias 6V6 amp.
It's a JJ 5y3.
Mike52
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Re: 5f11 dropping B+ voltage

Post by Mike52 »

sluckey wrote: Mon Feb 19, 2018 10:18 pm
Another idea... Replace that 56K resistor across the bias cap with a 50K pot in series with a 22K resistor so you can adjust the bias. Then you can run the 6V6s a bit hotter and this will bring the B+ down.

I personally don't use zeners for this purpose. I believe in using the correct PT to get the desired B+. I also believe that 386V is perfectly OK for any fixed bias 6V6 amp.
What's the mathematical formula for that bias string combo? I'm using a 10k pot. Using a smaller tail resistor would give the pot a greater voltage swing, right?

I wanted lower B+ voltages just to see what effect it has on the amp characteristics in general. My theory is that it will have a positive effect. So yeah, this was just an experiment. I think I'm swimming upstream here (btw, pin 8 is also the OT Center tap). I don't think zeners to drop B+ is a practical option for this amp.

The perfect solution would be to spec out a PT that's appropriate for this amp and not just take Mojotones recommendation next time. Live and learn.
sluckey
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Re: 5f11 dropping B+ voltage

Post by sluckey »

What's the mathematical formula for that bias string combo? I'm using a 10k pot. Using a smaller tail resistor would give the pot a greater voltage swing, right?
You never mentioned you had a non stock bias circuit. The only math I used was just "off the top of my head" reasoning... The stock circuit used a 56K resistor. By using a 22K resistor and a 50K pot you can go as low as 22K and as high as 72K. This will allow you to vary the bias voltage above and below the original circuit value.
(btw, pin 8 is also the OT Center tap)
Well, you simply move the OT lead to the original 16µF cap. This may be a factor for why you did not get the desired voltage drop to begin with. There must be sufficient current flowing through the zener for predictable results.
Mike52
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Re: 5f11 dropping B+ voltage

Post by Mike52 »

sluckey wrote: Tue Feb 20, 2018 12:08 am
What's the mathematical formula for that bias string combo? I'm using a 10k pot. Using a smaller tail resistor would give the pot a greater voltage swing, right?
You never mentioned you had a non stock bias circuit. The only math I used was just "off the top of my head" reasoning... The stock circuit used a 56K resistor. By using a 22K resistor and a 50K pot you can go as low as 22K and as high as 72K. This will allow you to vary the bias voltage above and below the original circuit value.
The bias circuit I'm describing is the same as what you've mentioned apart from the values. It's also a very common 5f11 mod, mojotone sold it stock with their kits once upon a time.

The stock circuit used a 10k into a selenium rectifier and then to ground through a 56k resistor. The modern equivalent uses a 1n4007 1,000v 1a diode.

My bias circuit copies Mojotones old layout, as a basis to start. I've got a 10k going to a 10k pot + 33k resistor to ground. This doesn't give much sweep of the pot. Playing with the math earlier I found that a 4.7k resistor to ground should give me a lot more adjustment to the pot. But I haven't tried it in practice. Does that sound right?
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