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How much current capability to replace a blown power trans?

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Posted on October 27, 2014 at 11:43:25
DAK
Audiophile

Posts: 2712
Location: PACIFIC
Joined: August 8, 2010
 I have a simple tube preamp with 2 6sn7 tubes with a blown PT. I want to replace it but am not sure about the B+ current capacity needed for the new pt. What values in the tube's technical data do you need to determine the PT current demand? TIA, cheers, Dak

 

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RE: How much current capability to replace a blown power trans?, posted on October 27, 2014 at 11:59:25
Alex Kitic
Audiophile

Posts: 182
Location: Serbia
Joined: June 25, 2014
 For the heaters, it's rather easy. Assuming that the preamp uses 2x 6SN7 and the heaters are AC, the 6.3V winding needs to be capable of at least 1.2A (2x 6SN7 heaters in parallel).

The same is true of the winding for the rectifier: if the rectifier is meant to be 5Y3GT, than the winding needs to supply 5V 2A.

As for the HT windig, which is probably CT - it depends on the voltage current it is required (meant) to supply, and the type of power supply.

Usually 1x 6SN7 (both triodes) per channel requires between 6 and 15mA, thus both channels would require between 12 and 30mA. If the HT secondary is CT for full wave rectification, and the power supply is cap first (like C-L-C), the AC current of the HT secondary needs to match the DC current of the B+.

You can add some current capability to the HT secondary, but it is not strictly necessary. Most probably it would end up being capable of 50mA and not 30mA because of standard wire thickness... and some would jump in ecstasy at the "oversized" power transformer.

If we reassume, 6.3V 1.2A (would be probably 1.5A in practice) = 9.45VA
and 5V 2A = 10VA, plus let's say 300V-0-300V 50mA = 30VA, which in the end sums to 50VA. That's most probably a fair measure for your preamp ;)
******

http://rh-amps.blogspot.com/

 

no data on the pt tags? (nt), posted on October 27, 2014 at 12:13:38
vinnie2
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Posts: 4481
Location: North Carolina
Joined: September 28, 2013
 nt

 

RE: How much current capability to replace a blown power trans?, posted on October 27, 2014 at 12:37:24
Tre'
Industry Professional

Posts: 17305
Location: So. Cal.
Joined: February 9, 2002
 Is this preamp DIY or store bought?

If store bought, who's the maker and what's the model number?

If DIY, do you have a schematic?

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: How much current capability to replace a blown power trans?, posted on October 27, 2014 at 14:29:17
DAK
Audiophile

Posts: 2712
Location: PACIFIC
Joined: August 8, 2010
 The preamp is an AE3. I have no info on this preamp at all, nor have I seen any schematics. I bought it with the blown PT and I definitely need to replace it. I will not use a replacement from Audio Electronics as that one costs too much especially since I have a handful of preamp power trans and I am just trying to pick one that will be the best fit. The AE3 is SS rectified but since all my power trans have a rectifier tap I will be retrofitting one. The chassis even has the mounting hole for one.
regards, Dak

 

RE: How much current capability to replace a blown power trans?, posted on October 27, 2014 at 14:32:37
DAK
Audiophile

Posts: 2712
Location: PACIFIC
Joined: August 8, 2010
 Thank you very much for that info. I have several to choose from that would be perfect for the job. have a great day! Dak

 

RE: How much current capability to replace a blown power trans?, posted on October 27, 2014 at 14:47:17
Tre'
Industry Professional

Posts: 17305
Location: So. Cal.
Joined: February 9, 2002
 The heater requirement for the 6sn7 is listed on the data sheet as is the heater requirement for whatever rectifier tube you plan to use.

The question is "how much B+ current does the circuit draw and at what voltage?"

It's anybody's guess what that circuit looks like.

If you trace the circuit and draw it we would still have to guess at the B+ power supply voltage.

Tre'


Have Fun and Enjoy the Music
"Still Working the Problem"

 

Isn't it DC coupled?, posted on October 27, 2014 at 21:07:48
David Smith
 I would think it might need more than 600V CT, no?

Dave

 

RE: How much current capability to replace a blown power trans?, posted on October 28, 2014 at 08:35:37
DAK
Audiophile

Posts: 2712
Location: PACIFIC
Joined: August 8, 2010
 Hi Tre, a friend has one of these and I can ask him to make some voltage measurements. What would I need to know besides the anode voltage of the 6sn7 ? regards, Dak

 

RE: Isn't it DC coupled?, posted on October 28, 2014 at 08:39:16
DAK
Audiophile

Posts: 2712
Location: PACIFIC
Joined: August 8, 2010
 Hi Dave, you mean between the first triode and the second section? I would need to check that and let you know. regards, dak

 

RE: How much current capability to replace a blown power trans?, posted on October 28, 2014 at 08:52:50
Tre'
Industry Professional

Posts: 17305
Location: So. Cal.
Joined: February 9, 2002
 Get the cathode voltages for all the 6sn7 cathodes.

Using ohms law you can then calculate the total current draw from the B+ supply assuming there isn't any shunt current (like a bleeder resistor or voltage regulator, etc.)

Yeah, If you know the B+ at the output of the filter and the total current draw you can get a power transformer that will make the thing work as designed.

If there is a voltage drop resistor in the power supply that carries all the current you could have him give the voltage measurements on either side of that.

I wish you had a schematic. I can't find one on line.

Tre'


Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: How much current capability to replace a blown power trans?, posted on October 28, 2014 at 11:09:34
DAK
Audiophile

Posts: 2712
Location: PACIFIC
Joined: August 8, 2010
 Hey Tre, yeah, I don't think Cary (AES), gives out any of its schematics as I tried contacting them about this before. I will post the voltages when I get them from my buddy. cheers, Dak

 

RE: How much current capability to replace a blown power trans?, posted on October 29, 2014 at 14:43:13
sony6060
Audiophile

Posts: 1465
Location: USA
Joined: August 8, 2014
 80ma secondary with capacitor input & 50-60ma with choke input.

 

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