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First Power-up in 55 Years - Need Advice

Kasanay

Kasanay

Active Member
I am making arrangements to acquire a pair of OTL amplifiers that were build in the early '60s. I should have my hands on them within a couple of weeks. These amps have been sitting on a shelf sense (at least) 1986 and maybe much longer than that. In fact, there is a possibility that these have never been used.
I'm looking for advice on steps to take prior to bringing these on line.
 
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First, blow off the dust, since they undoubtably have a nice coat by now. Personally, I'd pull the tubes to inspect the pins and sockets for corrosion, since humidity changes have their way with such connections. Finally, bring them up on a variac, don't just plug them in and fire them up--if you don't have access to a variac, use a 100w light bulb in series in the power source. Make sure you have a load on the outputs (either a dummy load or a pair of junk speakers) when you attempt to bring them up.
 
savatage1973 said:
First, blow off the dust, since they undoubtably have a nice coat by now. Personally, I'd pull the tubes to inspect the pins and sockets for corrosion, since humidity changes have their way with such connections. Finally, bring them up on a variac, don't just plug them in and fire them up--if you don't have access to a variac, use a 100w light bulb in series in the power source. Make sure you have a load on the outputs (either a dummy load or a pair of junk speakers) when you attempt to bring them up.
Click to expand...
Thanks for the reply. I do have a variac. Should I be monitoring any such as DC or AC voltages?
 
Kasanay said:
Thanks for the reply. I do have a variac. Should I be monitoring any such as DC or AC voltages?
Click to expand...

Bring them up slow on the variac. The big thing to monitor as you get them up to full voltage is the bias on the output tubes. If you see any "redplating", power them back down so you don't lose a tube.
 
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Post the schematic of the original build if you have it or at least pics of the units for comment. Some of the early OTLs derived their B+ straight off the AC mains and did not use an isolation transformer. Depending on how they were built they could be hazardous if you aren't aware of that.
 
And do not power them up on speakers! Use appropriate resistors instead. Any failures of an OTL amp,especially a higher powered model,can be terminal(and spectacular!)
for the speakers involved.Verify that they are stable,and check the output for offset before going ''live''.
 
Don't pull the tubes and mess around with them, chances are that you actually destroy them.
Power up ( with variac) and speakers connected, watch voltages and sign of smoke. Stop if
anything smokes or hums, fix the problem and try again.

When at fullpower see if sound passes and sound reasonable.
 
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soundmotor said:
Post the schematic of the original build if you have it or at least pics of the units for comment. Some of the early OTLs derived their B+ straight off the AC mains and did not use an isolation transformer. Depending on how they were built they could be hazardous if you aren't aware of that.
Click to expand...
I have posted a photo (try to not soil yourself) the model that I am negotiating. I do not have any photos of the actual items. These do have isolation transformers. In fact, each amp has a power supply on a separate chassis.
One thing that I am concerned about is the time-delay relay. Apparently, the filaments are given a good amount of warm-up time before the B+ comes on. I thinking that even with a variac, the plates will be hit hard with power when the relay closes. Not sure how to prepare for that.
 
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Kasanay said:
I have posted a photo (try to not soil yourself) the model that I am negotiating. I do not have any photos of the actual items. These do have isolation transformers. In fact, each amp has a power supply on a separate chassis.
One thing that I am concerned about is the time-delay relay. Apparently, the filaments are given a good amount of warm-up time before the B+ comes on. I thinking that even with a variac, the plates will be hit hard with power when the relay closes. Not sure how to prepare for that.
Click to expand...

Oh hell yes! Are those 12B4's? If yes I'd be curious if it follows the classic Futterman layout.
 
I've never heard of that company before but did find the name attached to another custom built, non-OTL system.

Those are wicked cool, will look forward to reading progress updates.
 
soundmotor said:
I've never heard of that company before but did find the name attached to another custom built, non-OTL system.

Those are wicked cool, will look forward to reading progress updates.
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I'd be intrested i reading anything about the company. If you have the link to the other system, I'd love the to check it out.
I already own an MP Engineering system that is missing the power amps. This will make the system complete.

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