Starting my AB763 style build

[hesamadman]

I started putting my AB763 on the breadboard today. I wanted to do it in sections. Do the power supply and filtering then check voltages. Do the power amp, plug a guitar into the input of PI and check for sound (i know it wont be loud). The do the preamp.


Well I have the power supply on. Choke and all filter caps. No tubes in or anything but I checked my voltages. According to sluckeys layout I should have 428vdc on A and B and 350 and 308 in C and D.


I just checked my voltages and I have about 490 on the first two and about 460-480 on the last. Is this a result of there not being any load on the power supply? Or is it from my SS rectifier? I figure the rectifier wouldnt matter since there wouldnt be any sag without any signal (im assuming).

[eleventeen]

If you're 1: using a SS rectifier and 2: have no load, the voltages will be quite high, maybe 60-80 volts high. Perfectly normal and to be expected. ~35 from the diode p/s, 35 from no load. Just like you have.


Keep going. And don't blow up 450 vdc caps with 490 volts if you can avoid it.

[Willabe]

Is this a result of there not being any load on the power supply?

Yes.

Don't leave it on long with it unloaded IF your filter caps aren't rated for at least 500dcv.

[labb]

I just got to ask, why are you bread boarding a proven design.

[hesamadman]

I just got to ask, why are you bread boarding a proven design.

Im only using the power amp. I am doing something completely different with pre and adding a tremolo circuit from something else.

If you're 1: using a SS rectifier and 2: have no load, the voltages will be quite high, maybe 60-80 volts high. Perfectly normal and to be expected. ~35 from the diode p/s, 35 from no load. Just like you have.


Keep going. And don't blow up 450 vdc caps with 490 volts if you can avoid it.

Thanks everyone YIKES!! forgot about the caps! Thanks for that info!

[sluckey]

I'd say your PS is fine. Remember, with no load, those filter caps will sit for hours with 400V on them even with power off. You may consider temporarily putting a 220K 3W bleeder resistor across the first filter cap just to drain the supply when you switch off.

[eleventeen]

Excellent idea, bleeder resistor. If it's a breadboard and you might have this or that other section connected or not and your power supply is in effect an "island", as a safety matter you should throw a bleeder R in there, Sluckey's 220K 3 watt is perfect.

[rosincore]

I agree with eleventeen and sluckey with the bleed resistor. Above all respect that there is 400 V DC sitting there not saying a thing and won't until you contact it, and believe me it will FLOOR you. One hand in the chassis till you drain those things, they bite badly and will leave a mark.

[hesamadman]

OK thanks guys. Good point and an eye opener. Though I drain the caps every time I turn it off, I did learn something here. I thought the 220k 1 watt resistor on Sluckeys schematic (before the choke) is a bleed resistor. And indeed it is, however that wont do any good on the rest of the circuitry with the stand by switch flipped.


Let me ask you guys this also. Like I mentioned before the only adjustable fixed bias amp I have done is the 2204. On this amp I have a desired voltage of -35v for the grid. As I adjust my bias pot this voltage obviously changes. So I get it to -35 volts. Is this all I need to do to adjust the bias OR do I need to take max dissipation of the 6v6 (12 watts?? 70% of that would be 8.4 watts) and divide that by my plate voltage of my 6v6, which is 433volts, and get a total of .019 (19mv)...... measure across the 1 ohm resistor on the 6v6 and chassis?

[sluckey]

Do you actually have a STBY switch on a breadboard? Why would you flip it when you turn the power off?

[hesamadman]

Do you actually have a STBY switch on a breadboard? Why would you flip it when you turn the power off?

Yes. I put a stand by on the board. I know there is absolutely no need. But I always flip the stand by when I turn the power off out of habit. No reason.


I may have answered my own question as far the bias goes. I checked the mv on the 1ohm resister and it was very high. So I adjusted it to the 19mv according to my math.

[sluckey]

But I always flip the stand by when I turn the power off out of habit. No reason.

Why not develop a habit to not flip it so the caps will drain as the tubes fade out?

[hesamadman]

Why not develop a habit to not flip it so the caps will drain as the tubes fade out?

After today thats what Ill do. 


Learning something new everyday.....even if some of it is really really simple.

[Willabe]

On this amp I have a desired voltage of -35v for the grid. As I adjust my bias pot this voltage obviously changes.
.......measure across the 1 ohm resistor on the 6v6 and chassis?

Yes, measure across a 1 ohm R and then do the math.

Trying to bias power tubes by the listed -dcv on the schematic is not going to work most of the time.

Any -bias dcv listed on a schematic will probably only get you in the ball park. On old schematics like Fender issued, at that time they were using RCA tubes and they were from the 'golden age' of production.

Fender could set the bias conservatively at the factory, maybe ~60%(?) and those tubes were probably close enough to get away with it at the time so they could publish that -bias dcv.

But ALL tubes do vary in output so you need a 5 to 10dcv swing up/down centered around that published -dcv to dial in the tubes for dissipation.

[PRR]

> develop a habit to not flip it so the caps will drain as the tubes fade out?

Or disconnect it so flipping makes no flipping difference.