Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: okies on January 07, 2018, 06:01:10 pm
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Hi, I'm new to the forum, just purchased some parts from the store and seen the forum and am hoping someone can lend some assistance.
I have an old field coil style Harmony amp, tubes are 1-6sl7, 1-6c5, 1-6v6 and a 5y3 rectifier tube. When I got it the filter caps were replaced as were several other caps that were leaky, also a few resistors that had drifted. I used the amp for 6 months or so, just off and on for an hour or so at a time. The other day it just wouldnt power up so I checked the fuse and it had blown. I opened it up and checked closely for wire shorts, I removed all the tubes and powered it up, still no luck and now the PT is smoking. I am going to assume at this point the PT is toast and need a new one. My question is how do I determine what PT to buy? There are small 5 watt Valco style PTs available from several sources but I'm not sure if they would work.
Any help would be appreciated.
Steve
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how do I determine what PT to buy?
start with your tubes filament current, add them up for that rating
do you have a schematic?, do you have DC voltage readings when it worked?
If not figure the 6V6 n 5Y3 will probably operate your B+ to something 250-300vdc, and the 6V6 operating SE you can get a ballpark current from the data sheet. try and figure out what the cathode R is for the 6V6 also.
the BIG thing is why did you PT fry?, bad tube, bad R........you don't want to fry another before you get that one figured out.
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Hey thanks for taking the time, I'll do what you said.
Steve
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> 1-6sl7, 1-6c5, 1-6v6 and a 5y3
That's a Fender Champ, done different (two actual inputs). You can't read the schematic I found, but squinting shows nothing strange.
Uses the PT from a 1935-1950 Big Radio.
Today you buy a "Champ PT".
The power output will be less than a Champ on the same PT, because of field coil drop. That is how these things were, until we had affordable permanent magnets. If the Champ PT gives you a half-Watt less than the original, that's better then zero Watts, and this was never a Jericho amp. But the raw voltages are probably very close, because of the limits of electrolytic caps.
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Here's a better schematic...