New Express build with Low voltage ???'s

[Bruciep07]

I just finished my first Express build today, my voltages are much lower than expected. I've got 120V coming in, exactly 600V across my sec, but all B+'s are low...
B+1-379
B+2-329
B+3-245
B+4-228
B+5-215
Bias is -18 to -30
They seemed excessively low to me but may be I'm wrong. The lowest bias voltage I can get is -30V, at -23V I'm up to 53mA across the OT which puts me at 17.4W. Do the voltages seem low to anyone else? Could it be a bad diode? Though a rectifier diode wouldn't make my bias voltages higher, would it?
Oh, almost forgot to mention, for the few minutes I got to play it so far, this thing is an absolute monster! It seems so much louder than my JCM 800. Absolutely zero hum dimed which is what I was most afraid of. Though I am a little disappointed with the pots I used, I got the CTS pots with the brass shaft, each one sounds scratchy... Oh well...

[Structo]

What is your wall voltage?

[Bruciep07]

What is your wall voltage?

Sorry, when I said I got 120V coming in, I meant I have exactly 120V at the outlet, should have been more specific. I'm running all JJ's right now, EL34's and 12ax7's. Also, I have to go pull the tubes and double check, but I think my B+ without any tubes was less than 385, I didn't write it down

[Structo]

If the pots sound scratchy that can be an indication that you are getting DC voltage on the pot.

If it is just the first stage volume pot or your guitar pot, sometimes putting a small value cap from the intput to ground will cure that.

[jjman]

I'm guessing a bad filter cap or semi-short on the B2 node. Or the 1k dropper is bad?

That node is dropping 50volts from B1. It "should" drop only 15volts. This implies too much current thru the 1k, or it's not a true 1k.

The other nodes look to be dropping the expected amount.

[Bruciep07]

If the pots sound scratchy that can be an indication that you are getting DC voltage on the pot.

If it is just the first stage volume pot or your guitar pot, sometimes putting a small value cap from the intput to ground will cure that.

The guitar pots sound clean, the amp's volume pot is probably the most noisy, TMB isn't very bad but I can still hear it a little...

[Bruciep07]

I'm guessing a bad filter cap or semi-short on the B2 node. Or the 1k dropper is bad?

That node is dropping 50volts from B1. It "should" drop only 15volts. This implies too much current thru the 1k, or it's not a true 1k.

The other nodes look to be dropping the expected amount.

I'm gonna go measure the 1K again right now, hopefully it's that simple... I checked and re-checked for shorts, checked all my solder joints, all seems ok. Does 379V seem low to you being I have 600V before rectification?

[Bruciep07]

1K measures 996ohms, so it seems there may be a short somewhere after the 1K but before B+2, is that a good assumption? How can I check filter caps for internal shorts? Also, I measured DC Voltage in between the first and second rectifier diodes and ground on both sides and one side was about 15V less than the other, is that normal? The lower V side is the side with bias tap coming off of it it...

[jjman]

1K measures 996ohms, so it seems there may be a short somewhere after the 1K but before B+2, is that a good assumption? How can I check filter caps for internal shorts? Also, I measured DC Voltage in between the first and second rectifier diodes and ground on both sides and one side was about 15V less than the other, is that normal? The lower V side is the side with bias tap coming off of it it...

The Express voltage chart (downloaded from this site) says that the secondaries should be 585vac. Since you have 600vac you should not have such low voltages.

I'm not sure if measuring the DC between diodes is relevant since it's not ~smooth~ DC at those points. It's pulsating DC until you reach the filter(s.) My understanding is that diodes are usually either good, blown, or shorted. They are so cheap it would make sense to change them. Even Radio Shack might have them.

I would change the 40u filter stage on B2 since I don't know how to test filters for (semi)shorts. I don't even know if they can partially short. I do know that Radio Shack does NOT stock caps in ratings/voltages for such an application.

[Bruciep07]

1K measures 996ohms, so it seems there may be a short somewhere after the 1K but before B+2, is that a good assumption? How can I check filter caps for internal shorts? Also, I measured DC Voltage in between the first and second rectifier diodes and ground on both sides and one side was about 15V less than the other, is that normal? The lower V side is the side with bias tap coming off of it it...

The Express voltage chart (downloaded from this site) says that the secondaries should be 585vac. Since you have 600vac you should not have such low voltages.

I'm not sure if measuring the DC between diodes is relevant since it's not ~smooth~ DC at those points. It's pulsating DC until you reach the filter(s.) My understanding is that diodes are usually either good, blown, or shorted. They are so cheap it would make sense to change them. Even Radio Shack might have them.

I would change the 40u filter stage on B2 since I don't know how to test filters for (semi)shorts. I don't even know if they can partially short. I do know that Radio Shack does NOT stock caps in ratings/voltages for such an application.

That's the voltage chart I've been going off of, I've heard of voltages being +/- 10% so 379V B+1 isn't too far off, but the fact that I started out with 600V and have less that 400V after the rectifier seems too far off, plus the large voltage drop across the 25W 1K is, like you said, way too much. I am gonna pull the tubes, swap out some diodes first and see if that brings up my B+1, and then disconnect the single 40uf cap and see if the voltage drop changes across the 1K, does that make sense? I would think if there was a short in that cap, I would see a large drop in current across the 1K if the cap is shorting...

[RB]

Your are not starting with 600 volts. Your tansformer is a 300-0-300 CT and you are rectifing each 300 volt section twice per cycle (that's where 120hz ripple comes from). In theory with no load the 300 volts should charge up the filter cap to 1.414 times the 300 volts which equals 424 volts. If you take out all your tubes and measure your fist filter cap with power on you should get something close to that. The voltage you are measuring is lower because of loading of the power supply. You can measure the voltage drop across the 1k 25 watt and calculate the current at idle. Add up all your tubes expected current draw and see if its close to your actual. If there is a big differance then you got a short or possibly a leaky filter cap or some such thing.

Regards
Randy

[jjman]

50 volts dropped thru a 1000ohm resistor is 50ma. At idle that seems way too much to me. I would guess ~5ma for each screen and ~1ma for each preamp/PI triode. ~15ma~ Also the "expected" amount based on the excel drop of 15volts.

[Bruciep07]

Your are not starting with 600 volts. Your tansformer is a 300-0-300 CT and you are rectifing each 300 volt section twice per cycle (that's where 120hz ripple comes from). In theory with no load the 300 volts should charge up the filter cap to 1.414 times the 300 volts which equals 424 volts. If you take out all your tubes and measure your fist filter cap with power on you should get something close to that. The voltage you are measuring is lower because of loading of the power supply. You can measure the voltage drop across the 1k 25 watt and calculate the current at idle. Add up all your tubes expected current draw and see if its close to your actual. If there is a big differance then you got a short or possibly a leaky filter cap or some such thing.

Regards
Randy

my B+ without any tubes was less than 385, I didn't write it down

Thanks Randy and JJ,
weather you look at it as 300V between the center tap and each secondary, or 600V between each secondary, my transformer is providing the correct voltage. I swapped out the diodes in the rectifier and it brought my B+ unloaded up to around 398V. With my bias set max negative (around -36V) and all tubes installed I'm at around 385V which is closer to expected. My B+2 is much better at 369V but the rest is still a lot lower than expected. And I'm not sure why I suddenly went from a 50V drop to 16V drop from B+1 to B+2, all I changed were the rectifier diodes. With all the tubes pulled there is no current across the 25W 1k, so in my head that kind of eliminates all filter cap shorts with the exception of the first 2 which are before the 1K resistor, sound right? I've tried moving tubes around to see if I see a weird jump in voltage but it didn't seem to make a difference. My next step is to remove the first 2 filter caps from the circuit and replace them with some temporaries and see if makes a difference. I'm also gonna track B+3-5 wiring and see if there is something screwy somewhere that I missed the first 3 times

Thanks for all the replies guys!

[Bruciep07]

The saga continues... I was adjusting the bias, had a 1ohm resistor from cathode to ground, and a fluke DMM across the resistor monitoring current (voltage but you know what I mean), with another DMM measuring plate voltage. I started out at like 6Watts, I slowly dialed down the bias and monitored plateV/cathodeI, at about 45mA, it started humming like crazy, sounded like someone put an SM57 in their mouth and just started humming. The bias was at about -29V, the Plate voltage went from slowly dropping by about a volt at a time, sort of in-line with the bias voltage as I was adjusting it, but then suddenly dropped like 50V and that's when it started screaming at me. I shut it down right away, clocked the bias all the way back again, and turned it back on again, checked my bias voltage, -36, B+ 389V, flipped the stand by switch, gave it 5 seconds, hummed like a mad man again running at under 6Watts. I starting to think I may have a problem with the OT. I'm gonna pull all the tubes again, disconnect the OT from B+1 and measure my Voltage again with nothing connected. What kind of current draw should I get with no tubes installed with just the OT getting power? With a 25V-30V drop across the OT with nothing else installed, is it safe to say something isn't right or is that in the ballpark? Any other ideas? I know it could very well be a bad solder joint somewhere, as the current increases the shitty solder joint doesn't make a good enough connection to pass the increasing current, but I've gone over everything so many times, and I'm decent at soldering, I kind of doubt that's what it is, but it obviously can't be ruled out. I'm kind of at a loss right now... At least this is a good learning experience, and that's what this is all about, isn't it?

[jjman]

I’m not clear on your statement “With a 25V-30V drop across the OT with nothing else installed, is it safe to say something isn't right.” Did you actually take a reading “across” the OT with no tubes installed? What do you mean by “across.” From one primary lead to the other? From the CT to one primary lead?

If you have no tubes installed, I think there should probably be no voltage difference between any of those 3 nodes since there is no current (that should be) flowing from the CT to either primary lead.