Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: EKDENTON on May 02, 2016, 10:27:39 pm
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On this bias circut, the PT I am using has a bias tap (Doug's 041316). If I use the bias tap do i eliminate the 220k resistor and the diode? Or do I just use one side of the HT just as in the schematic? The information on the bias tap doesn't give any voltages. Iv'e never used a PT yet that had the bias tap. That's handy.
I guess it would still need the diode for the negative voltage ... but the resistor?
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No, the diode is absolutely required. You still need a completely independent rectifier (I call it "the backwards diode") to produce the negative voltage you need; a voltage below ground with respect to the main rectifier = the series strings with the common cathodes (bars)
With your bias tap, you are feeding the input to your "backwards" half wave rectifier with 60-80 VAC instead of the big HVAC used to make the amps B+.
You probably want to cut the value of the 220K by ~~80%, though. Consult a Super Reverb or equivalent, 20K - 50K is about right. I like having that resistor there, even though you're likely using the same 1N4007 1000 PIV diode as the main rectifier, meaning the diode is 5x overrated, there's a piece of surge protection you get with such a resistor.
Some folks here, myself included, would suggest placing the stby switch an the other side of the first cap, especially if you have (as shown) a bleeder resistor.
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Okay thanks, i will check those schematics. Yes....after I clicked the post button I remembered that the diode is needed for the negative bias voltage.
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Some folks here, myself included, would suggest placing the stby switch an the other side of the first cap, especially if you have (as shown) a bleeder resistor.
What will moving the standby past the first cap do?
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That schematic clip looks like the Mark Huss Plexi 6V6. If you are following that schematic, I'd recommend to tape up the bias tap wire and follow the schematic.
If you just have to use the bias tap, then change the 220K resistor to 470Ω. You may need to juggle the 470Ω value (sometimes as high as 1K) to give a suitable bias voltage range for 6V6s.
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yes it is. Okay I will just stick with the schematic.
Eleventeen suggested moving the standby switch. I checked the super reverb schem. It has the standby after the first cap. What is the reason for doing that, is it safer wired that way?
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The idea is that with power on but the stby sw open, when you close the stby switch you get a big surge current charging all the caps. That does not especially matter, it would be the same if you flipped both switches at the same time which I reckon most people do anyway. It's likely to cause a big POP that could be a speaker killer especially if the heaters are lit up, and, if it's your habit to "let the tubes warm up" when you flip from stdy to ON after some brief time, that's what will happen. The tubes would be all set to go with nothing on the plates, then you slam them with a big power surge.
With the stby switch after the first filter cap, that pop would be reduced. That's the long and short of it.
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I see what your saying. For whatever reason the last few I wired haven't made any noise at all when you let the amp heaters warm up then turn off the standby switch to power up.
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yes it is. Okay I will just stick with the schematic.
Eleventeen suggested moving the standby switch. I checked the super reverb schem. It has the standby after the first cap. What is the reason for doing that, is it safer wired that way?
Here is Merlin's essay (and theories) on standby switches:
http://www.valvewizard.co.uk/standby.html (http://www.valvewizard.co.uk/standby.html)
I was about to type something snarky about not really knowing why Leo Fender did it a particular way. However, just experiencing a 5AR4 rectifier tube fail shorted in an amp wired with standby before the reservoir cap makes me think I should reconsider...
Cheers,
Chip
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One could definitely see such a thing happening w/a tube rectifier much more than SS diodes, and if the issue with these Russian 5AR4's is a breakdown of heater to cathode insulation, such a surge could certainly produce such a failure by punching through. Good thought.
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breakdown of heater to cathode insulation
The cathode is a separate element but it is electrically connected to the heater inside the 5AR4 tube.
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Right, I misspoke, meant cathode to plate. Inside a 5AR4 or 5V4 the cathode is MUCH closer to the plate than in a 5U4/5Y3.
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Moving the standby past the first cap will isolate the B and C supply caps from the bleeder resistor if the standby switch is in the open position. Which it should be when the amp is turned off. This means A supply cap is discharged and the B and C supply caps will stay charged. Is this correct?
I think the consensus on the standby is that it is not necessary but only a convenience on the type amps that we are building, and some builders here say they have been using amps for some time without a standby with no harmful effects to their amp, so it may stand to reason that whatever inrush of current there may be into a few caps may not damage the amp.
The tubes would be all set to go with nothing on the plates, then you slam them with a big power surge. With the stby switch after the first filter cap, that pop would be reduced. That's the long and short of it.
The plates might be hurt by a large voltage spike but they shouldn't be harmed by having more current than needed, they just draw whatever current they need to operate right? Is this correct?
Just trying to figure out if it is better to keep it as the schematic shows so that the bleeder resistor will drain all the caps instead of the A supply when the standby is in open position.
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One thing you might consider. SS vs tube rectifier.
If you are using a tube rectifier and no standby switch, at startup with cold tubes, the rectifier slowly brings up the voltage as it warms up.
You can watch this happen with most tube rectifiers. It usually takes like 3-5 seconds to get up to operating voltage.
Once the rectifier is warmed up and turned off and back on this happens much quicker, but still kind of ramps up.
SS is sending full strength rectified DC right away.
Maybe something to consider? :dontknow:
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Actually i am using a tube rectifier for this amp. I thought I would try it and see if i like it. I have the diodes on the board so if I don't like it i can always switch to ss.