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Old 10th March 2007, 02:34 PM   #1
PRNDL is offline PRNDL  United States
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Default isolation transformer for 50C5 amp

I have a 60's tube amp that connects directly to the AC. I want to add an isolation transformer for user protection, but don't know how much current it would need.

The tube lineup is 12AX7, 12AV6 and a pair of 50C5 output tubes into a small (6 inch) speaker. I believe this would be about 5 watts. It has a solid state rectifier.

Specs for the 50C5 are here
http://frank.pocnet.net/sheets/127/5/50C5.pdf

The Hammond 262E6 looks like it might do.
18VA, sec. 120V, DC ma 87, Fil. (heater) 6.3V @ 1.2A

Is 87 mA plenty of current for this?

Thanks!
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Old 10th March 2007, 02:39 PM   #2
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No,
If you look at the tube specs, you will see the filament takes 150ma so the filament string takes 150ma not to mention what the rest of the circuit will pull when you are using the amp. I would say you would need at least a 1/2 amp (500ma) isolation transformer and that is even at the bear minimum. 1 amp would be better.

Here is a 1.2 amp isolation transformer for $12.00 that would be great for your application.

http://www.surpluscenter.com/item.as...-1150&catname=

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Sal Brisindi
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Old 10th March 2007, 02:50 PM   #3
PRNDL is offline PRNDL  United States
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I'm glad I asked!!

How about this one

Hammond P-T169VS 169 Series Line Isolation Transformer

* 175 VA capacity, 1300 secondary current ma.
* Primary 115 VAC, 60 Hz.
* Secondary 115 VAC
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Old 10th March 2007, 02:58 PM   #4
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The Hammond P-T169VS is rated at approx 1.5 amps so that is more than enough but at $38.00 vs. $12.00 from the surplus link I mentioned you can buy 3 for all your isolation needs... :-)

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Sal Brisindi
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Old 11th March 2007, 01:07 AM   #5
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tubesandmore has a hammond thats only 35va and perfect for your amp. Costs 20 bucks. I am building a Harmony h303A clone right now, and I ordered a 120vct filament transformer. Its a little big fro the amp thats for sure...
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Old 11th March 2007, 03:08 AM   #6
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Any time I fix an old line operated guitar amp or even an AA5 radio, I add an isolation transformer. For most applications I use the Triad N-68X. It is rated at 50VA which works out to be 435 mA. Mouser has these for $11.20. This is enough to run your 50C5 amp.

I used one in an Electrolab guitar amp powered by a similar tube lineup (12AU6, 12AV6, 50C5, 35W4). I mounted the transformer in the bottom of the box, below the speaker. The line cord goes directly to the transformer, and the transformer goes to the amp. In this case the transformer is always powered up. This has not been an issue.

They have a 115 / 230 volt dual primary and a single 115 volt secondary. I have been known to hook them up backwards making a 115 to 230 volt transformer. With a SS bridge I get about 280 to 300 volts under load. Add a $7 filament transformer from Mouser and you have enough juice to run a stereo P-P amplifier using 6AQ5's or 6V6's or even EL84's for under $20.
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Old 11th March 2007, 03:21 PM   #7
PRNDL is offline PRNDL  United States
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Thanks for the great info!
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Old 12th March 2007, 04:43 AM   #8
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Tubelab:

I have seen some guitar amplifiers that use a 120v transformer into a full wave voltage doubler, and the result is 280vdc under load, with enough current to run at least a SE el84/6v6 or the likes.

Right now my problem with the isolation transformer is getting the voltage back down to proper operating levels... Hooked up my amp last night and the plates started glowing! I was using FWB ss rectification after an apparent 132vac out of the x-former. Seemed to be getting 180vdc UNDER LOAD. I replaced the bridge with a single 1n4007 ( like a 35w4 would be) and under load I got 140vdc, which is good for a 50c5. Ya know with enough filtering I dont seem to pick out any audible hum from that amp, and I'm only using a 47uf cap on the output of the 1n4001!
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Old 17th June 2013, 03:55 PM   #9
slor is offline slor  United States
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Thanks for the tips! I ended up purchasing a Triad N-68x for a 12AV6/50C5/35W4 project but it runs quite hot. I had intended to install another in a 2 x 50C5 stereo but I'm guessing I'll need one rated a bit higher....
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Old 17th June 2013, 04:54 PM   #10
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A N-68X is probably all you need for the 2X 50C5 amp, if the B+ is SS rectified. What is the tube complement?

Take a look at this thread, where reverse connecting a N-68X was discussed. Reverse connecting a N-68X allows the B+ rectification circuitry and the series heater string to be energized from different sources and that provides considerable benefits.

BTW, if the 1st amp you did employed 1/2 wave rectification, "standing" DC was present. "Standing" DC can make transformers noisy and run hot.
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