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 Post subject: Trutest series string tube stereo amplifier
PostPosted: Mar Mon 02, 2020 8:05 am 
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Joined: Dec Sun 07, 2008 7:05 am
Posts: 4237
Location: Meridian, MS 39307
OK, this is getting pretty close to the bottom of the barrel; but, there was one even lower than this...with a tone switch instead of a tone pot. What we have here is a Trutest (I believe that was a brand that Lafayette used) Japanese-built stereo amplifier from the early-to-mid '60's. It has controls for L volume, R volume, tone, stereo/mono switch, and tuner/ (ceramic) phono switch. The tubes are 12AX7, 35C5 X 2, and 35W4.

It's housed in a metal case and since it uses no power transformer, it's rather dangerous (with the only thing between chassis and one side of the AC line being a .05uf capacitor that is now very leaky). Someone also did a slippy job of changing the filter capacitors, which I've corrected. I've also replaced the AC cord with a polarized line cord and wired the power switch in the hot side of the line. Of course, that safety modification goes out the window if the amplifier is plugged into an incorrectly wired AC outlet. On that note, the safest way to use this is with an isolation transformer. Someone already had tape on one side of the power plug, likely because they'd already gotten the mess knocked out of them and they were marking which way to insert the power cord.

Some of the original disc capacitors looked pretty cooked and a couple were cracked. Even though they tested OK, I still replaced them with film capacitors. The resistors all checked fine and were left alone. The tubes are all Toshiba-branded, except for one RCA replacement.

Before working on it, I briefly tested it with a modern CD player and it works; but, there is bad ground loop hum, regardless of the plug position of the CD player. I've got a few more loose ends to tie up; but, I'll post another picture when I get it finished.


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 Post subject: Re: Trutest series string tube stereo amplifier
PostPosted: Mar Mon 02, 2020 2:45 pm 
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Joined: Nov Wed 07, 2007 12:44 am
Posts: 1414
Location: Kingman, Arizona
I remember those amps and if I remember correctly, they were sold by Lafayette Radio. A friend of mine bought one back in the early 60's. The amp worked, but it isn't exactly high fidelity. I also remember the fireworks when we tried to use two Truetest mono amps for stereo and got the AC polarity reversed. Harry


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 Post subject: Re: Trutest series string tube stereo amplifier
PostPosted: Mar Mon 02, 2020 6:38 pm 
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Joined: Nov Sat 07, 2009 11:37 pm
Posts: 2412
Location: Sayreville, NJ 08872
Trade-marked names Trutest, Argonne, and Lafayette were all house brand names for Wholesale Radio/Radio-Wire Television back in the real "good old days". The were carried over even after the company assumed the name Lafayette Radio Electronics (LRE).

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 Post subject: Re: Trutest series string tube stereo amplifier
PostPosted: Mar Mon 02, 2020 8:28 pm 
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Joined: Jan Tue 07, 2020 1:41 am
Posts: 1244
Location: Fenton, MI 48430
Replace the ceramic audio coupling caps with polypropylene for better sound. The ceramic is the worst sounding and only ok in ham radio sets.


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 Post subject: Re: Trutest series string tube stereo amplifier
PostPosted: Mar Mon 02, 2020 9:59 pm 
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Joined: Dec Sun 07, 2008 7:05 am
Posts: 4237
Location: Meridian, MS 39307
With a record changer plugged in, it works fine. With a tuner or CD player plugged in, I get horrible ground loop hum, no matter the plug position and even if the amp is plugged into an isolation transformer. I just ordered a ground loop isolator and we'll see if that fixes it.

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 Post subject: Re: Trutest series string tube stereo amplifier
PostPosted: Mar Mon 02, 2020 11:38 pm 
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Joined: Dec Sun 07, 2008 7:05 am
Posts: 4237
Location: Meridian, MS 39307
I'm waiting on a ground loop isolator; but, in the meantime, the only way I could get rid of the hum was to jump the capacitor that is between B- and the case/circuit ground (and with the amp plugged into an isolation transformer, for safety). Hopefully, the ground loop isolator will take care of the problem.


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trutest 022.JPG
trutest 022.JPG [ 812.56 KiB | Viewed 535 times ]

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 Post subject: Re: Trutest series string tube stereo amplifier
PostPosted: Mar Mon 02, 2020 11:46 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: 13 Critchley Avenue, PO Box 36, Monteith Ont, P0K 1P0
radiotvnut wrote:
I'm waiting on a ground loop isolator; but, in the meantime, the only way I could get rid of the hum was to jump the capacitor that is between B- and the case/circuit ground (and with the amp plugged into an isolation transformer, for safety). Hopefully, the ground loop isolator will take care of the problem.


Just for hits and giggles, why not try putting the wiring back the way it was designed, then plug it direct into the outlet? It was safe that way for fifty years ..... right up until people started overthinking it. Instead of trying to redesign something that was safer before you changed it, try making sure that your house is wired correctly.


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