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Hammond PT question about 5 vct.

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Posted on August 4, 2015 at 20:32:33
DAK
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 In my latest 6L6 RH SE amp, I have a Hammond power trans 278x, and I am having no B+ after hooking up all the wires. I hooked up yellow x 2 to the 5v rectifier solder pins 2&8 if I am not mistaken. The red x 2 to rectifier socket pins 4&6. Green is 6.3v, and black x 2 is AC. There is CT green/black, red/black and yellow/black. I have all the CT grounded at the star ground. I tested the PT before hand and got good readings. I remember in the Scott's that the 5vct derives the B+ is that how I should do this? Thank you for clearing that up. cheers, Dak

 

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RE: Hammond PT question about 5 vct., posted on August 4, 2015 at 21:20:53
Tre'
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Location: So. Cal.
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 "I remember in the Scott's that the 5vct derives the B+ is that how I should do this?"

Yes, you can do that or take the B+ off one side but don't ground the CT of that filament winding whatever you do.

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

 

RE: Scott 296 has 5vct, posted on August 4, 2015 at 21:55:48
DAK
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 In that model, the B+ is derived from the 5v CT. Is there any advantage to doing it that way. Or should I just tape off the CT and take the B+ off of pin8. I have always wondered why the Scott296 did that. I checked a bunch of schematics and that is the only amp that did that. Cheers, Dak

 

RE: Scott 296 has 5vct, posted on August 4, 2015 at 23:04:02
Paul Joppa
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Location: Seattle, WA
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 Taking B+ off the filament winding CT removes the 5v of 60Hz hum, leaving only the rectified ripple at 120Hz and harmonics. Since the 120Hz is IIRC some 60% of the output DC voltage, that small 60Hz component has little practical effect. It's just the principle of the thing.

 

RE: , posted on August 5, 2015 at 01:21:13
DAK
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Posts: 2712
Location: PACIFIC
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 So, the B+ value derived from the 5VCT will be no different than the value taken from P in 8 of the rectifier socket?
I did a complete rebuild of a Scott 296 and it is a very nice sounding amplifier. I like the sound a lot more than almost any other PP amp. cheers, Dak

 

Hardly any difference, however..., posted on August 5, 2015 at 05:22:50
Chip647
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Posts: 2652
Location: The South
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 Taking the B+ off the center tap is a nice clean way to do it from a wiring perspective.
If you are dealing with directly heated rectifiers (5U4, 5Y3, etc.) center tap is the way to go in my book. If you are dealing with indirect (5AR4, etc.), pin 8 is the way to go.

 

RE: Hardly any difference, however..., posted on August 5, 2015 at 09:03:59
DAK
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Posts: 2712
Location: PACIFIC
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 Hi chip, can you explain your statement in more detail? tyvm, Dak
If you are dealing with directly heated rectifiers (5U4, 5Y3, etc.) center tap is the way to go in my book. If you are dealing with indirect (5AR4, etc.), pin 8 is the way to go.
t

 

So, did your power transformer survive? (nt), posted on August 5, 2015 at 10:42:48
Tre'
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 .
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: So, did your power transformer survive? (nt), posted on August 5, 2015 at 12:01:23
DAK
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 Yeah, no problem. I was just doing initial testing at 10vac on the variac, with a SS tube rectifier replacement. Now, I have over 50vdc with no load, @ 10vac input. I am using a LCLC filter and it looks like I need some bigger inductors? Maybe a dropping resistor at the end? What about a LCLCLC? once I get the rest of the circuit installed I will know for sure. Thank you for the help. regards, Dak

 

RE: Hardly any difference, however..., posted on August 5, 2015 at 12:17:15
Chip647
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Posts: 2652
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The cathode of an indirectly heated rectifier hangs off the Pin 8. If you take the current off of pin 2 you put the B+ current back through the heater which will add stress to the heater.

 

RE: So, did your power transformer survive? (nt), posted on August 5, 2015 at 12:48:35
Tre'
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Posts: 17305
Location: So. Cal.
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 "I was just doing initial testing at 10vac on the variac"

Smart move!

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

 

RE: Thank you for that tidbit!, posted on August 5, 2015 at 13:00:02
DAK
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 Ahhhh. I knew there was a good reason why all the schematic have it done that way so, I have always copied running the B+ off of pin8 on an octal rectifier, but never really understood why that was done. Thank you for enlightening me. cheers, Dak

 

RE: So, did your power transformer survive? (nt), posted on August 5, 2015 at 13:11:45
DAK
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Posts: 2712
Location: PACIFIC
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 I always start with the power supply when making a new amp. It is the heart of the amp and really needs to be sorted before adding on the other circuitry. So, i choose the power trans, find a suitable location on the chassis. Add on the fuse, on/off switch, and IEC socket. Find suitable location around the power trans for the inductors and caps, then, put your tube circuitry as far away from there as possible. Thank you for the helpful tips! regards, Dak

 

That is how I did it..., posted on August 5, 2015 at 23:09:57
91derlust
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Posts: 1101
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 on my first amp build just because I read it in some old build instructions somewhere. I run DH rectifiers, so it seems I lucked out!

Cheers,
91.

"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein

 

RE: Oddly enouogh.., posted on August 6, 2015 at 09:07:17
DAK
Audiophile

Posts: 2712
Location: PACIFIC
Joined: August 8, 2010
 The Scott 296 ran 5AR4 rectifiers. But, the Scott 272 ran 5u4gb x 2. Both of these with the CT 5 volt filament. The 296 came with GE 5ar4 which is a great little tube. cheers, Dak

 

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