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oldschooltube 8th August 2012 04:38 PM

Power Transformer Help
I am looking to upgrade the power transformer in my S-5 Electronics K16ls kit amp but I have done away with the ss rectifier and replaced it with 2 paralled 6BY5GA tubes. The amp uses 8x 6005 tubes in ppp and 2x 5670 tubes. The info I got from S-5 about the stock power transformer is 120v primary, 180v secondary (actually measures around 200VAC) @ 250mA, 6v secondary @ 5.5A. I would like to find a power transformer that has slightly higher HV to account for the voltage drop across the rectifier tubes but I have had no luck finding something that will supply 225-230V @ 250-300mA. I don't want to go any higher on the HV and cook the 6005s. Any ideas would be greatly appreciated.

kevinkr 8th August 2012 05:28 PM

Ask Edcor to make you a power transformer, possibly they already have one in their catalog to suit, and if that is the case you will not have to pay the $40 NRE cost for a custom design.. :D They're quite obliging, and this is what I do for most of my designs.

Osvaldo de Banfield 8th August 2012 05:49 PM

An alternative solution is to use a higher voltage transformer's secondary, and go to choke input filter. This last, also extend the rectifier´s life.

oldschooltube 8th August 2012 06:15 PM

Thanks Kevin. I will see what Edcor can come up with. Taking the tube compliment of this amp into account what do you think my target B+ should be after rectification and filtering? 6005s have a Max plate voltage of 250 so I was going for about 210-220V. Sound right??

TheGimp 8th August 2012 08:38 PM

GE Data sheet gives a recommended operating point at 250V plate and G2, -15V G1, 70mA for a pair of tubes AB1 PP.

If the kit runs cathode bias, you can subtract the magnitude of the G1 voltage from the plate voltage.

You also drop a few volts across the OPT, so a B+ of 265-275 is reasonable.

hollowstatefan 9th August 2012 04:11 PM

You've probably taken into account, but you'll need to supply heater current for the 6BY5GAs, too, each 1.5A @ 6.3 V.

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