Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: mresistor on June 15, 2016, 11:31:35 am
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Hi folks - I have a Kay 703c sitting here, in operating condition. As it has no power cord with ground and no fuse and so it got me looking here. I found the previous thread that was started by punkykatt.
http://el34world.com/Forum/index.php?topic=19072.msg196599#msg19659 (http://el34world.com/Forum/index.php?topic=19072.msg196599#msg19659)
I understand by reading through the above thread that the best way to make this amp safe is to add an isolation transformer. Pete posted up a pretty good schematic for a safety mod. Has anyone implemented that mod? Would a Hammond 185c 230 work? It's 25VA instead of the 43VA. The owner of the amp wants the iso trx mounted inside, which is only possible if the big e-lytics are moved to clear space around the power cord intrance. I personally would mount it below , hanging from the chassis.
http://www.alliedelec.com/m/d/d4e478caa3db7f42469004951f483748.pdf (http://www.alliedelec.com/m/d/d4e478caa3db7f42469004951f483748.pdf)
I PM'd punkykatt and asked about the results he came up with on his Kay 703c, but so far no answer.
Also on the internet is a link to a an instructable that shows how to mod a similar amp with a Triad N-68X iso trx. But he says without adding a full wave bridge and rectifying the full wave the N-68X got hot.
http://www.instructables.com/id/Isolation-transformer-upgrade-for-old-guitar-amps/?ALLSTEPS (http://www.instructables.com/id/Isolation-transformer-upgrade-for-old-guitar-amps/?ALLSTEPS)
What do you think about what this guy says about the amps half wave tube rectifier and adding an iso trx?
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> It's 27VA instead of the 43VA.
Power draw is 18W in heaters and 12W for plate DC. This is over rating.
> the N-68X got hot.
We don't know if he screwed something up, or if the transformer is really marginal.
Small transformers *do* tend to run hot, by the economics of iron+copper. How hot is hot? With modern materials, a tx can run too hot to hold for many hundreds of hours without failure. How many hours will he play (or idle) his Kay?
I do not know the Kay. I know that my Kent is a big cheap empty box with tons of space for a transformer in the bottom of the speaker space.
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Thanks PRR - so since 43 * .7 is 30 watts the Hammond 185D230 is best.
When I was reading that instructable I had the same questions come to mind. Did he screw up, and how hot is his version of too hot?
I really don't know how much he plays the amp or for how long. Best to plan for worst case scenario I guess.
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I am still puzzled by the instructables guy inferring that the reason for the iso trx getting hot was the fact that the rectifier was only using half wave. So it's only a half wave rectifier, why would that make the iso trx get hot?
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mresistor, Yes, I did use the N-68X isolation transformer and it did not get hot.
Punky
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...inferring that the reason for the iso trx getting hot was the fact that the rectifier was only using half wave...
Got nothing to do with it. Did he actually say that? Or did you interpret it that way?
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Thanks punkykatt good to know.
Steve here is the heading of the section: Step 8: Fixing the half-wave rectifier problem
and down a little ways he states:
is that bad? Well, yes. As the name implies, half-wave rectification only uses one half of the AC waveform, and blocks the other half. Power transformers are really designed to be symmetrically loaded. The flux field collapses as one peak falls, and the transformer expects an equal load--and an equal amount of magnetic force from the complementary peak. Without a load on half the cycle, the collapse of the field causes the transformer core to become saturated much more quickly than normal. That puts a "standing" DC voltage on the transformer. The N-68X, being a small transformer, isn't designed to handle this.
When I first installed it, I tried to use the N-68X with the circuit, as-is. But it immediately became obvious that the transformer became too hot, considering a current draw less than 30 watts.
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Wow! Never heard that before. I would have thought a 1/2 wave rectifier would be LESS load/stress.
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Half-wave certainly is not "less" load.
The idea that the iron saturates is somewhat plausible.
We want the RMS current in the copper winding. Duncan PSD is not set to do this directly. I faked it with an "R-C" first section, very-small R, and monitored that current before the cap.
I get 20% more RMS current for the half-wave connection, while getting 5% less output current (due to lower delivered voltage).
Worth noting that the AC RMS current is two to three times the DC current.
Considering that 1/2 to 2/3 the total power demand is AC heaters, I would not expect ~~20% more RMS current to be a "Wow" difference.
If he has also gone to DC heat, yes you gonna need a bigger transformer.
BTW, all these amps can be re-rigged (and re-tubed) for 12V heat from a small light switcher supply. That leaves only the ~~12W of plate feed to get from a transformer. (Isolated 110V DC supplies are not common items, though isolated 48VDC units can be stacked.)
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PRR - If I were to use the N-68X like the guy did in the instructable, and used a FW bridge rectifier to "fix the half-wave issue" would a 2 amp epoxy bridge do the job? I have a bag of old VARO VS644 epoxy bridge rectifiers, I'm thinking the 2 amp rating is ok but what I am concerned about is the PIV rating. This one should be around 500-600 PIV. I can't find info on this exact bridge rect. but have found info on similar from same manufacturer. They are compact and in this circuit I wouldn't think they would be generating much heat (see pg 17)
https://archive.org/details/VaroSemiconductorSiliconRectifiersAndBridges1981-82 (https://archive.org/details/VaroSemiconductorSiliconRectifiersAndBridges1981-82)
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I think the PIV will be OK - 500/1.4 = 353V - which is more than suitable for this transformer and amp.