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Voltage drop

Crusher

Well-Known Member
May 12, 2007
841
388
73
Louisville, KY
A friend is bringing over a sb220 amp. The problem with it is that when amp is keyed, the plate volts drop to almost zero. I am thinking the cap bank probably needs replaced. Anyone ever see this happen in an amp before?
 

I'm waiting for guy to bring it over. Last I was told, it had no plate Voltage at all. I will have to check rectifiers and caps. See what I find when it gets here. Have another amplifier, it is a Drake L4B with oversized plate transformer. The V doubler circuit was recently rebuilt. New diodes and caps. Now, the amp won't key more than 400W and V drops way down. Guy said there was a spark near or around filament transformer. Almost sounds like a tube is shorted or something on this one.
 
Most likely bad diode rectifier circuit on the L4B.

The SB220 either HV PS or look at the cathode circuit, Heathkit uses a zener for cathode bias and this could be faulty. Just a guess:mellow:

I have a Pride DX 1000, pair of 4-cx250b in it.

Lightning strike job. Really messed up the circuit boards and relay circuits.

Feel like I am doing surgery instead of repair so much circuit board rework to be done.

Would really like to find a source for new boards for this old amp. Any one have any sources?
 
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Nomad radios or Chris Nomad may have something. I know he sells some stuff. He even had a website or something @ 1 point. He is a Guru when it comes to the prides. I will try to find link or something later and see if I can find.
 
Excessive voltage drop under normal load is almost always an open cap or diode in the HV supply. If more than 50% of the voltage is lost under load, it's probably an open cap. I recently had the same problem in an NCL-2000 that one of the caps opened up in. I hadn't even considered this as the problem because 99% of the time this makes the cap short due to the larger voltage drop and take out the HV fuse. To my surprise this was one of the rare times the cap actually showed a virtual open connection and would not support any current. The diodes would be the next thing to check.

The Drake sounds like something may have been damaged in the output section from excessive power. See what the plate current is when it's making 400 watts. Low output and larger than normal voltage drop usually means high current is being drawn as a result of mistuning the tank. That could be the result of a part failing in this area like the plate blocking cap. It's also possible the bias has shorted and loaded the supply way down with idling current. The larger power transformer will bring out the next weak link in the chain.
 
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I recall nomad made some upgraded hv boards I will search his sight.

thanks
 
L4B info

Drake L4B

CRUSHER: Excellent info here...if the L4B has had the B+ increased...
The bottom plate removed from the power supply look at the large wire wound resisters near the main terminal strip...most likely smoked or cracked (look carefully they can look fine...but they have opened up)
Also if the bias was not upgraded as in this article...those big 50k (I think) puppies are shot!:blush:

One check as Shockwave noted the blocking cap(s) near the fan motor...
Also while looking there check the glass tube at the base of the plate choke...those 5 little GREMLINS inside the tube can cause some really crazy S**T to happen....:blushing:
Jeff at Harbach's has got all those goodies...
GL
All the Best
Gary
 
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I just did a new V doubler with 6A diodes. All the caps are new as well. A month old. Yeah its very possible one of the 50k resistors are shot. 1 is no longer black, but gray in color like it got really hot. I will probably just change blocking caps to be safe. Its been sitting here for a few weeks. I'm currently building a 4 tube GI6B. When I finish it, then I can start on drake. I know the guy that runs this drake is a heavy hitter. Last year he went thru 3 sets of tubes. Why he runs it at a 1500W carrier is beyond me. I had to install a fan in power supply to keep it cool. Also he smokes, so everytime he brings it over I have to clean everything before I can work on it. Nasty.
 
I know the guy that runs this drake is a heavy hitter. Last year he went thru 3 sets of tubes. Why he runs it at a 1500W carrier is beyond me. I had to install a fan in power supply to keep it cool. Also he smokes, so everytime he brings it over I have to clean everything before I can work on it. Nasty.

Job security:whistle:
 
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Finally got a chance to look at Drake a bit more, I'd say the 2 50k 50W resistors are toast. Haven't got much further than that, but I see Jeff has a pair pretty cheap on his website. I need to look at article on L4B. Maybe change up bias and just use CT on filament instead of using those drop down resistors for bias. Need to see what they recommend.
 
I know the guy that runs this drake is a heavy hitter. Last year he went thru 3 sets of tubes. Why he runs it at a 1500W carrier is beyond me. I had to install a fan in power supply to keep it cool. Also he smokes, so everytime he brings it over I have to clean everything before I can work on it. Nasty.


Because he is an idiot and has no idea about carrier versus pep ratings. Just a theory I have mind you. :)
 

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