I just finished building one channel of my 300b amp. I switched it on I had my DVM monitoring the DC coming out of it. As the tube heated up, the DC increased and then after a few seconds more purple sparks then my DVM showed 0V due to the 6A fuse blowing ..
The DC on the cathode of the GZ34 reaches about 100V before this happens.
The amp has a 300b, SV83 pentode, and GZ34 rectifier.
I also have a 100K, 2W bleeder resistor.
GZ34 datasheet says max input cap is 60uF and I only have 5.6uF so it can't be that..
Any suggestions?
my PS schematic is shown below:
The DC on the cathode of the GZ34 reaches about 100V before this happens.
The amp has a 300b, SV83 pentode, and GZ34 rectifier.
I also have a 100K, 2W bleeder resistor.
GZ34 datasheet says max input cap is 60uF and I only have 5.6uF so it can't be that..
Any suggestions?
my PS schematic is shown below:
Attachments
Well you're most certianly taking far too much current for an amp of this magnitude...
You've got the PSUD simulation all wrong.
I ran the PSUD simulation here and checked current on the transformer, you're drawing 5amps! all of the time!
Voltage across the current sink is 0.012v! this should be 375volts!
You should see 0.50 amps across those GZ34's at maximum especially in real life...
Okay heres your problem, take away L2 & C3.
Replace C2 with 100uF on the 1st choke and leave C1 as is..
Try it like that..
The problem is according to PSUD is that your final voltage comes out as 500+ volts whereas the original transformer voltage was only 375... which is expected.
Try drawing more current to get the voltage down or decrease your transformer ohmage so it supplies less current.
You've got the PSUD simulation all wrong.
I ran the PSUD simulation here and checked current on the transformer, you're drawing 5amps! all of the time!
Voltage across the current sink is 0.012v! this should be 375volts!
You should see 0.50 amps across those GZ34's at maximum especially in real life...
Okay heres your problem, take away L2 & C3.
Replace C2 with 100uF on the 1st choke and leave C1 as is..
Try it like that..
The problem is according to PSUD is that your final voltage comes out as 500+ volts whereas the original transformer voltage was only 375... which is expected.
Try drawing more current to get the voltage down or decrease your transformer ohmage so it supplies less current.
What the hell!
It was telling me that it was morbidly unsafe, now if I re-build the simulation it's perfectly fine!?
Okay here we go, try 50hz against 60hz...
With 60hz the simulation draws far too much current while under 50hz it runs 'fine'
That still doesn't produce the 0v supply I seen earlier tho.
It was telling me that it was morbidly unsafe, now if I re-build the simulation it's perfectly fine!?
Okay here we go, try 50hz against 60hz...
With 60hz the simulation draws far too much current while under 50hz it runs 'fine'
That still doesn't produce the 0v supply I seen earlier tho.
I would seriously doubt your power tranny has a secondary resistance of 0.5 ohms. Disconnect it, and measure the resistance with a multimeter. Use this actual number in your simulation.
However I believe you may have a short somewhere. Double and triple check your HT connections and your capacitors. **With the amp disconnected and discharged**, what is the resistance between B+ and ground?
However I believe you may have a short somewhere. Double and triple check your HT connections and your capacitors. **With the amp disconnected and discharged**, what is the resistance between B+ and ground?
Thanks for the replies but I didn't have access at work today...
I created this simulation again and there were no complaints. The correct secondary resistance on the PT is 120R.
Our electricity here in Canada is at 60Hz.
Resistance from B+ to ground is around 22K.
djmiddelkoop: I'll try what you've suggested..
I created this simulation again and there were no complaints. The correct secondary resistance on the PT is 120R.
Our electricity here in Canada is at 60Hz.
Resistance from B+ to ground is around 22K.
djmiddelkoop: I'll try what you've suggested..
I haven't measured the current draw yet. I'll do that when I get home. But I've tried another 300B and also another GZ34. Same problem occurs.
The 300B heater supply is DC regulated by an LT1085. Does that mean I need to ground the centre tap of the transformer?
I measured no resistance between the grid and cathode pins. So i guess there isn't any short.??
This is my first amp, by the way. It's kind of discouraging...
Thanks for the responses!
The 300B heater supply is DC regulated by an LT1085. Does that mean I need to ground the centre tap of the transformer?
I measured no resistance between the grid and cathode pins. So i guess there isn't any short.??
This is my first amp, by the way. It's kind of discouraging...
Thanks for the responses!
I found the problem.. I had the LT1085 bolted to the case through and it was not properly insulated.. Somehow, the screw was touching the chassis which is also ground.
The voltage was able to climb as high as 250V until the sparks came again.. Then i tried another GZ34 and it worked fine!
Thanks all for your help!
The voltage was able to climb as high as 250V until the sparks came again.. Then i tried another GZ34 and it worked fine!
Thanks all for your help!
fdegrove said:Hi,
Sure sounds like it, doesn't it?
A gallon of diesel may help here....
Cheers,
Well........, I dont actually use diesel in my Mercedes Diesel, .................. I use Veggy oil, as fuel!!
Anyway, to get back to it, Does the 300B show any stress--Glowing abnormally or arcing?--Is the G1 volts OK? Are the PSU caps old or maybe need re-forming
As a quick test replace the GZ with some silicon and series resistors of around 120ohm 10W ww, and test, Then you wont damage your GZ anymore--These are stupidly expensive for what is just a rec!
After you cure the fault go back to the GZ--I used this dodge many times when the customer didnt want to fork out for a new GZ!
Yeah.. too bad diesel has such a stigma around it here in north america. I loved my diesel audi when i was in germany.
From building this amp, I've come to realize how fragile the GZ34 is. I wish I tried SS while I was trying to fix the problem.
Now, I'm just trying to iron the hum problems..
From building this amp, I've come to realize how fragile the GZ34 is. I wish I tried SS while I was trying to fix the problem.
Now, I'm just trying to iron the hum problems..
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