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Old 5th November 2009, 09:38 PM   #1
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Default Isolation Transformer Question

Hi all! I bought a silvertone record player that had two 50eh5 tubes paired with a 35w4 rectifier. Hot chassis design. So I read up on installing an isolation transformer and bought a hammond 169RS. Thought I hooked it up correctly and .... ... nothing (no power comming from the secondary windings????). I read some more about the combination of a 35w4 rectifier and an isolation transformer and read that "half wave rectification" is bad for the transformer. Might this be my problem? Do I have the leads from the secondary side of the transformer reversed (is that possible, as both were yellow and unlabled)? Sorry for the noob questions, but this is my first time messing with a hot chassis tube design.

Any help would be greatly appreciated. I'll send pics soon.

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Old 5th November 2009, 10:20 PM   #2
SY is offline SY  United States
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First, let me congratulate you on your immediate recognition that an isolation transformer is VITAL.

Second, I would guess that the no-voltage is because of miswiring. Disconnect the secondary leads, hook them to an AC voltmeter, then power up the primary and see what you have. And do this very carefully!
"You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is."
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Old 6th November 2009, 10:18 AM   #3
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half way rectification loads a transformer with a DC component which may drive it into saturation, no transformer likes that........
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Old 6th November 2009, 01:33 PM   #4
TheGimp is offline TheGimp  United States
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A schematic (of hte amp) would be really helpful if you have one.
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Old 6th November 2009, 09:03 PM   #5
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Thanks for the help guys. I did a little more reading and found a helpful page. I bought some epoxy 3 amp diodes and will make a bridge rectifier and install it in line with the 35W4 tube as shown on this link.

Fixing the half-wave rectifier problem

Hopefully this will eliminate all problems. I'll send pic and the results soon!

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Old 10th November 2009, 09:43 PM   #6
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Well that helped. I now have the heaters glowing, but no sound coming out? Hmmph... I've done a little repackaging as you'll see in the photos below. I am trying to make a nice little desk amp for work. I did not change any of the wiring of the original amp, since I have no schematic. I replaced the capacitors, checked the resistors, and crammed it all into a nice little box. I'm not sure what I should be checking next. Any suggestions on the voltages I should read on the 50eh5 tubes? The original power was soldered onto pin #2 of the 35W4 rectifier, then went in series with the power for the turntable, then came back and tied onto pins 4 and 5. I read online that pin 2 on the 35w4 rectifier goes to nothing so I hooked the power from the rectified isolation transformer to pins 4 & 5. Sorry if I'm rambling, but I'm drawing a blank as to why I've got no sound....

Click the image to open in full size.
Click the image to open in full size.

Any thoughts would be greatly appreciated!

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Old 10th November 2009, 11:52 PM   #7
tomchr is offline tomchr  Canada
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Did the amp work before you modified it?

I suggest making sure that you have the correct heater voltages and B+. You can do this (relatively) safely by pulling the tubes and measuring from the top of the tube sockets.

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Old 30th November 2009, 06:23 PM   #8
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Well I worked before I re-boxed the amp, and now after a few nights of scratching my head and checking every solder point, it's making music. Due to the cramped box and a not so pretty layout it's got a bit of a HUMMMmmmm. I tracked the problem down to a grounded input compounded with a cold joint on an output transformer.

That aside, I'm pretty pleased with the outcome. Now onto taking the original 5" drivers and making some funky cabinets for them. I'm thinking of making some half sphere's on a lathe and play'n with hole sizes on the back until I get the sound just right.

More to come.

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Old 30th November 2009, 07:28 PM   #9
dhaen is offline dhaen  Europe
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Maybe my eyes are deceiving me, but it looks as if the negative terminal of the bridge rectifier is connected to the bottom of the volume control.
IF this is the star earthing point then it's ok. If not then your earthing scheme is probably causing the hum.
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