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Old 23rd May 2004, 03:50 PM   #11
dougv is offline dougv  United States
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Yes my dad is a retired electrcian and we do have various amp/voltage meters. I will pass all the info to him since he understands all the electrical terminolgy.
I remember going to radio shack to test tubes for an old TV about 25 years ago.
I have a digtech rp80 modeling pedal don't know if this would help.

So after i get the tubes tested and find a schematic i'll be back with more info. I pass a couple of tv stores/repair shops on my way to work so i'll start their.
Thanks again.
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Old 23rd May 2004, 03:52 PM   #12
dougv is offline dougv  United States
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I'am done tinkering, I read enough warnings about high voltage dangers with amps. Just gathering info now.
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Old 23rd May 2004, 07:43 PM   #13
adamamp is offline adamamp  Canada
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Quote:
Originally posted by dougv
I'am done tinkering, I read enough warnings about high voltage dangers with amps. Just gathering info now.
Great, I will check with you later. What the above poster & I said about the hot chassis is true. That is why there is no exposed metal parts on the amp stock. Ask your Dad if he has an 1 to 1 isolation transformer. Make sure it can handle the power requirements.
This is the easiest way to insure you do not hurt yourselves!
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Old 23rd May 2004, 10:19 PM   #14
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I hope my remarks prove to be useful.

The 25Z6 is a dual diode rectifier with separate cathodes. It is probably wired as a voltage doubler.

The 25L6 is an 11 W. power "pentode" O/P type. For guitar, think in terms of the sound levels a SE 6BQ5 or 6V6 produces. Don't expect the same tone.

The tube complement suggests either 2 cascaded triode gain blocks or a gain block plus a cathode follower and the "final".

The "standard" recommendation to "automatically" replace ALL electrolytic capacitors applies.

The Carbon composition resistors have probably drifted out of spec. and need replacement. Xicon currently makes Carbon comp. resistors that are available from Mouser (www.mouser.com).

I make the dropping resistor in the series heater string to be 200 Ohms. That resistor is dissipating 18 Watts. It is possible to replace the dropping resistor with a capacitor and save some energy. Let's see; 200 Ohms at 60 Hz. works out to be 13.3 muF. Michael Percy stocks a 13 muF. 250 WVDC Axon part that combined with a 10 Ohm surge limiting resistor should be OK, given today's higher mains voltage.
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Old 24th May 2004, 01:53 AM   #15
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I thought I'd better correct this in case anyone tries it - a series capacitive reactance and resistance (like the tube heaters) DON'T add arithmatically - the effective series value is the square root of the sum of the squares. So actually you'd need closer to 400 Ohms of capacitive reactance to sub for 200 Ohms of resistance. It works out that 7 uF is about the right value for a capacitive voltage dropper for 0.3A series string tubes.

It appears that this box has an INPUT transformer too, so it may be safe to use once it's buttoned up. Otherwise NO WAY! Non isolated guitar amps are too dangerous to even THINK about - your next encounter with a mike stand could be your last!
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Old 24th May 2004, 03:00 AM   #16
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This is no joke! Back in the day, high school electronics class to be exact, one of these unisolated amps almost cost one of my classmates his life. Much like you, he was trying to salvage an amp and when he was working on it in class the instructor made him plug it into an isolation transformer. Well he got the amp working and brought it home. He plugged the amp's 2 prong plug in backwards, putting 120v directly on the chassis. Luckily, he touched it with is right hand, but it stung him good. He was lucky and learned a valuable lesson. If there is no power transformer, do not under any circumstances use it for a Guitar amplifier and for heaven sake if you work on it get an isolation transformer. If you have no electronics experience, sell it to a collector and get something else.
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Old 24th May 2004, 04:38 AM   #17
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Quote:
I thought I'd better correct this in case anyone tries it - a series capacitive reactance and resistance (like the tube heaters) DON'T add arithmatically - the effective series value is the square root of the sum of the squares. So actually you'd need closer to 400 Ohms of capacitive reactance to sub for 200 Ohms of resistance. It works out that 7 uF is about the right value for a capacitive voltage dropper for 0.3A series string tubes.
Right you are Tom; I got LAZY. The standard value near 7 muF. is 6.8 muF. I compute that 129 V. mains are needed with a 6.8 muF. part for 300 mA. If the power company lowers its voltage below 120 V., the heaters will be "starved". Paralleling the 6.8 muF. part with a 0.68 muF. part works out to be nearly perfect. Add a 1 Ohm surge limiting resistor and call it done.
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Old 24th May 2004, 10:44 PM   #18
adamamp is offline adamamp  Canada
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Thanks everyone for your responces! The input trafo I suspect is a coupling device and not for the mains power. Not sure unless we find a schematic. Any ideas on that? I do have a fair bit of knowledge(mostly practical) on tube amps. This is why I asked Doug to see if his father has a isolation trafo. Also, please correct me if I am wrong, but to my knowledge a line powered amp is only hot on the chassis if it is plugged in wrong. I was hoping to explain to Dougs dad that hooking a modern polorized plug to the cord after determining which position is right would be a safe alternitive to the isolation trafo.

Doug real busy at this end. Just signed a super guitar player to provide clips for my new amp line when it is ready for release.
Check with you later. Maybe ask any of the techs you check with for a tube tester if they could help us find a schematic. I will also check all my Sams photo facts and Audio Anthology copies for it, when I have time as there is binders of stuff, but no real index. Another idea(the best maybe) would be to just use the output trafo, 6SL7, chassis and tube sockets but add a 6V6 for output, proper power trafo to make a Champ like clone. If we go this route I could scan a schematic and post it here for feedback. Can your dad read a schematic? That will be very important either way. Most of the sybols he should be familar with. Have you talked to him yet? Will he have a fair bit of time to help us?

Cheers all,

Adam

PS. Sorry for the typos, too much work and not enough sleep makes me a very lazy typer
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Old 25th May 2004, 01:31 AM   #19
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Quote:
The input trafo I suspect is a coupling device and not for the mains power.
I agree. Still, that trafo isolates the amp from whatever is driving it. That's good for both safety and ground loop control.

In addition to the polarized plug, a wiring tweak in the voltage doubler B+ supply can increase safety. A full wave doubler connects 1 leg of the AC being rectified to the center of the filter cap. stack. Make that leg the "hot" side of the AC mains. That way, both the signal ground and B+ in the unit are associated with the "neutral" leg of the AC mains.
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Old 25th May 2004, 07:13 PM   #20
dougv is offline dougv  United States
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Adamamp, my dad will have no problem reading and following the schematic. I only see him on the weekends since he lives over an hour away so there is no hurry. If it ends up being a very involved or risky project the amp will goto ebay, but if it can end up like a champ clone that would be worth it. And again no hurry this would just be a weekend project doing alittle at time.
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