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He who hesitates is lost........

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Posted on April 22, 2016 at 04:29:55
vinnie2
Audiophile

Posts: 4481
Location: North Carolina
Joined: September 28, 2013
 I knew I should not have let so much time go by between breadboarding the 26/26/45 amp and actually building it. I drew a schem (see "hot time" below), but I am having troubles now I did not have with the breadboard.
In particular, my B+ is way low by about 150 vdc. The psu module has a nice potted surplus tranny of 880vdc ct and 0.165 A, so by my calcs it ought to have enough umpah to run four 26 tubes and two 45's. I had it hooked up to one channel of the breadboarded amp and it worked just fine.
I am sure I have missed something puting the final version together that isn't showing in the schem. What kind of things would cause the B+ to be too low if the tranny has enough horse power?

PS
I put a larger voltage rated cap in place of the one that blew and a different resistor, but that right channel 45 still has me worried as possibly being the source of the problem, or at the very least a hint as to what is wrong. I first changed out the pot and tried another lower voltage cap and it blew again. Why is it just that cap that is having problems? Is it possible the problem is still there but the higher voltage rating on the new cap is hiding it temporarily?
I am about ready to strip it down and start over and change a few things I don't like anyway, but thought I would see if you guys have any ideas first.

 

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RE: He who hesitates is lost........, posted on April 22, 2016 at 12:37:44
Chip647
Audiophile

Posts: 2653
Location: The South
Joined: December 24, 2012
 If you draw out the schematic, you can find your problem in just a few minutes. You can calculate what the voltages should be, then actually measure them in the circuit. You can easily find which section is drawing too much current. When you build, always test the power supply first with a load to make sure you have that correct before you go and build the rest of the amp.

Measure the load your power amp is being asked to deliver. Measure across the choke or the first resistor to calculate. If you have nothing easy to measure the full load over, insert a 1 ohm 10 watt resistor in your power supply before the first load. You really need to measure what is going on.

 

RE: He who hesitates is lost........, posted on April 22, 2016 at 18:18:36
vinnie2
Audiophile

Posts: 4481
Location: North Carolina
Joined: September 28, 2013
 I designed the power supply with psud2 and had everything tuned up pretty well.
I have the schem drawn up already ("hot time" below in earlier thread), but I have not calculated the voltages. Guess I will give that a try. But I keep coming back to the at same question of why always just the right channel 45?

 

Why always the right channel?, posted on April 22, 2016 at 20:33:52
Chip647
Audiophile

Posts: 2653
Location: The South
Joined: December 24, 2012
 Because something is wrong. You could have a bad tube or a bad part (shorted resistor or a failed coupling cap) Measurements will pin point your problem.

 

RE: Why always the right channel?, posted on April 23, 2016 at 04:31:48
vinnie2
Audiophile

Posts: 4481
Location: North Carolina
Joined: September 28, 2013
 Can I do the voltage checks at less than full B+? I am a little leary of giving it full throttle for as long as it will take to do all the measuremnents. Only had it on for a minute or two so far.

 

If you have a VARIAC, posted on April 23, 2016 at 12:37:55
Chip647
Audiophile

Posts: 2653
Location: The South
Joined: December 24, 2012
 you can ramp up. Measure the overall B+ current and stop the ramp up when you reach the 100mA or so you are shooting for then stop. If this is 65 volts AC so be it. Then you can measure around and find the bad guy causing the high current drain.

 

RE: If you have a VARIAC, posted on April 23, 2016 at 14:22:33
vinnie2
Audiophile

Posts: 4481
Location: North Carolina
Joined: September 28, 2013
 I do have a variac. Where should I be measuring the 100mA?

 

RE: If you have a VARIAC, posted on April 23, 2016 at 20:19:10
Chip647
Audiophile

Posts: 2653
Location: The South
Joined: December 24, 2012
 Early on in the power supply. If you have a CLC or CRC filter as the first section, measure the voltage drop between the two capacitors. Take the voltage and divide it by the resistance of the resistor or the choke DCR. You can monitor this as you ramp up the voltage. When you get to 100mA, stop increasing the voltage and start measuring the various voltage drops in the amp, Plate resistors, cathode resistors, B+ dropping resistors. You will find a voltage drop measurement that is higher than the others or higher than it should be.

 

RE: If you have a VARIAC, posted on April 24, 2016 at 04:42:13
vinnie2
Audiophile

Posts: 4481
Location: North Carolina
Joined: September 28, 2013
 Ok, will do. Will report back when I get some numbers.
Thanks!

 

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