need help

[HiGain]

Well, it don't work... I have a problem with my two rock kit.

I fired it up with the 100W light bulb current limiter trick. At first the light came on fairly strong, but not full on. And I had no heater voltage. This was with the standy switch in standby mode, so that only the bias circuit and two filter caps were in circuit.

So, at the suggestion of Bill at two rock (nice guy), I disconnected the high voltage secondary leads.

Viola. I now have heater voltage... And, interestingly, I now hear the hum of the PT, which I did not hear before when I first fired it up. Bill felt this indicated a partial short somewhere in my power supply.

Next. I measure the high voltage secondary leads: 505VAC. Looks good.

Next I check my rectifier diodes. Each one has a drop of around .55 volts and infinity in the other direction. Seems good.

Next, I hook the secondary leads only to the rectifier diodes, with nothing connected after the diodes. Voltage is only around 200 volts!!!!

First question: Where is my voltage? Shouldn't it be nearly double that?

Just for kicks, I measure the resistance from the secondary leads to ground: 15 ohms...

Second question: does this seem low?

Any suggestions?

Thank you.

Jake

[LeftyStrat]

I always go though Paul Ruby's startup guide when first firing up and amp:

http://www.paulrubyamplification.com/in ... rstPowerUp

This should help you find any wiring mistakes.

Sounds like you may have a short somewhere.

[Elcabong]

Next, I hook the secondary leads only to the rectifier diodes, with nothing connected after the diodes. Voltage is only around 200 volts!!!!

First question: Where is my voltage? Shouldn't it be nearly double that?

Just for kicks, I measure the resistance from the secondary leads to ground: 15 ohms...

Second question: does this seem low?

Jake

First, how are you measuring it? Using DC on your meter? Without any filtering, the average DC is .636 x the peak voltage.

Second, 15 ohms indicates something is wrong. Ckeck your wiring. Good thing you used the light bulb; usually, it will come on bright as the capacitors are charging and then it will fade down to the idle current.

[tubedogsmith]

Eliminate the goofy stuff first. Make sure your meter is set for A/C when measuring A/C and D/C when measuring D/C. Make sure you have a good battery in your meter. Check all your grounds.

[HiGain]

Thanks for the responses.

I checked out Ruby's site. Good information, which I used.

I am measuring with a DMM. When reading Ruby's site, I learned that the voltage after the rectifier needs a load to charge up. So, after carefully checking everything, I fired the amp up again and got 350 volts right after the diodes.

Here's where I'm puzzled.

First, I NEVER found an error. I desoldered the 6.3VAC secondaries and resoldered them, and desoldered some power supply wires and resoldered them. The only thing I can think of is that the heater secondaries were shorted somehow. All other wiring was clean and well spaced.

Second, when I initially turned on the amp, the 100W lightbulb was fairly bright (indicating a strong amount of current draw), and I heard NO transformer hum. Now, after the amp seems to be working correctly, the light bulb remains dim, and I hear the right amount of transformer hum. I have no idea what this means.

Last, I used the Kelly 90 bias circuit. With the 20K pot maxed out, I only get down to -8V. So, I guess I will need to add some restance from the pot to ground. But, I think this might be a byproduct the lower than normal transformer voltage.

If anyone could share some light on this oddness, please do.

Thanks again,
Jake

[Robert]

Last, I used the Kelly 90 bias circuit. With the 20K pot maxed out, I only get down to -8V. So, I guess I will need to add some restance from the pot to ground. But, I think this might be a byproduct the lower than normal transformer voltage.

That sounds like deja vu to me. I had mistakenly used the wrong resistor value on the power tube screen resistors (pin 4). It should be 1K, I miss read in a hurry and used 100K. Lesson learned, double or triple check all your values. Trust the Kelly schematic. You do not need to start adding in resistors. You have made a mistake somewhere. That is the same thing I did until I found my mistake too. I too used the Kelly powersection for my Ruby. My problem was diagnosed off of a voltage chart. Post yours and I will compare them to my build.

Next. I measure the high voltage secondary leads: 505VAC. Looks good.

That doesn't sound good to me. My PT Hi volt secondary is around 600v.

[HiGain]

That doesn't sound good to me. My PT Hi volt secondary is around 600v.

OK, first problem fixed. I was using the lower voltage secondaries. I switched to the yellow wires, and I now have the following voltages:

This is with standby switch in operation and all tubes EXCEPT power tubes installed.

PT secondary: 571VAC (almost exactly the same as my other two rock)
B+1 386
B+2 382
B+3 305
B+4 288
B+5 274

Here's the problem. My bias voltage is still maxed out at -8VDC. I checked very quickly with my 100W light bulb current limiter to see if I would here hiss with the power tubes, and I did. However, all my voltages dropped, which suggests, the power tubes were drawing a lot of current, right?

My bias circuit is: from one lead of HV secondary > 10K R > 110K R >10uF cap to ground > 15K R > 10uF cap to ground > 47K R > wiper of 20K pot, with one end to bias and the other end to ground.

Thanks for the help,
Jake

[LeftyStrat]

There's supposed to be a diode in there. Also, my copy of the kelly bias circuit is:

220k -> diode -> 10uf (positive to ground) -> 15k -> 10uf (positive to ground) -> 47k -> 20k pot

[HiGain]

There's supposed to be a diode in there. Also, my copy of the kelly bias circuit is:

220k -> diode -> 10uf (positive to ground) -> 15k -> 10uf (positive to ground) -> 47k -> 20k po

Good catch... Yes, I have the diode in there, cathode toward the PT. I used two 110K resistors instead of one 220K resistor.

I really can't find a problem... I've looked over the bias circuit again and again. The only thing I can think of, is that the PT VAC on the kelly 90 schematic is around 585, while I have around 570. Maybe the lower voltage is the difference.

I'm going to take some time away, and go back with some fresh eyes. I'm also going to check the 10uF caps for leakage.

Jake

[doctord02]

I've heard several people comment that the bias circuit doesnt really work as drawn - I know that was the case with mine; I could never get better than -11v with the stock circuit. Here's what I did - changed the 220k to 100k, eliminated the 47k and substituted a 50k multi turn Bourns wirewound pot for the 20k pot.

Works like a champ now and I can get around -50v if I need it. I keep it at -30v, just like KF suggested.

The Kelly90 schem and the A1a schem have the same bias circuit if I recall correctly... I wonder why that seems to be consistently wrong...

[HiGain]

Got it.

Thanks for the input on the bias supply. I reduced the 220K to 110K and added a 27k resistor to ground from the pot. I now have a working wreck!

It sounds great. No problems at all. It's more smooth than I expected. Very aggressive, but also very smooth, easy on the ears distortion, with plenty of top end. Interesting. Also, it's more dark sounding than my el84 two rock wreck clone.

I have a question about gain, which I am going to make in a separate post.

Thanks everyone for the great feedback!!

Jake