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Old 10th June 2003, 02:00 AM   #1
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Location: Oregon, USA
Default Starting on my first amp, have some questions

Well, that was a pretty obvious thread title The amp I'm building is the JE Labs/Angela SE 2A3 with the SRPP 6SL7 driver and "my own" PS using a 5AR4. Questions:

* What do you think of this layout?

Click the image to open in full size.
Click the image to open in full size.

I had no idea the transformers would be that big, or I would have ordered a 17"x14" chassis (that's a 17"x10"). I was thinking about a layout that's symmetrical around a center line with the PS in the middle, but since I can't put the PS transformer, rectifier tube and PS choke in a straight line without making everything almost touch, I think that option is out (I could still leave the rectifier offset from the line of the transformer/choke, but that won't look very nice). With this layout, I'd put the power switch on the left front, and IEC connector and fuse on the left back. Haven't decided how to do the binding posts and RCAs yet, they'll be somewhere on the rear right. Maybe RCAs on the inside and binding posts on the outside. The filament transformers will be under the hood, probably on the left wall or between the PS transformer and choke.

The other layout option would be to move the 6SL7s closer to each other, and then put my hum balance pots on the outsides of the 6SL7s, in front of the 2A3s. That would let me adjust them without having to reach around hot tubes, but would that bring heater currents too close to the 6SL7s?

* Transformer polarity/phase - how important is it to have the 2 filament transformers in phase? Is there any way to tell other than hooking them up and measuring the voltage difference between leads? For the PS choke, does it matter which way I connect it?

* 125ESE wiring diagram - mine didn't come with any, but I'm assuming that I'll be able to find one online somewhere, probably at the Hammond site.

* Transformer grounding - this is important, right? The PS transformer and choke seem to have paint on the outside casing, so I'm not sure if just using bolts and toothed metal washers will make electrical contact. Do I need to scrape the paint off near one of the bolt holes?

* Power up test sequence - does this make sense?

- Power up with no tubes. Make sure secondary voltages are as expected, i.e. the secondaries have been wired correctly.

- Power off, insert all tubes except rectifier, power up. Verify that all filaments light up, filament voltages look good, and HT secondary voltages look good.

- Power off, remove all tubes, insert only rectifier tube, power up. Verify rectifier filaments heat up, and B+ looks correct at output and driver tube plate pin sockets. Voltages will be high because no current is being drawn (this is the step I'm not sure about... is this a good idea?)

- Finally, power up with all tubes inserted.

Thanks a lot in advance, and I'm sure this will be the first post of many.

Saurav
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Old 10th June 2003, 02:31 AM   #2
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I'm not so cool with all the Trafos so close to the tubes, but.....i think you can pull it off. At least the 6SL7's arent sitting on TOP of any of them.

Good luck to you, Saurav,

-Maz
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Old 10th June 2003, 02:38 AM   #3
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Well, I could get another enclosure and make that the PS and use this one only for the audio circuitry. I'd rather do it right the first time than have to re-do everything because it's too noisy.
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Old 10th June 2003, 02:43 AM   #4
PassFan is offline PassFan  United States
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If you have any doubt about having enough room, order what you need now. You can probably return the chassis before you punch holes in it. Keep us informed
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Old 10th June 2003, 03:04 AM   #5
Colt45 is offline Colt45  Serbia
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looks like plenty of room to me.
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Old 10th June 2003, 03:16 AM   #6
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your layout looks good to me....all your wiring will be short which is important...I dont feel that the transformers will affect the tubes....and on another question you asked the transformers dont need to be grounded...just the chassis


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Old 10th June 2003, 03:29 AM   #7
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OK, if I move the transformers right to the edge of the chassis, I can just about fit the rectifier between the PS choke and transformer. That gives me around 3/4" clearance on both sides of the paper box for the rectifier tube, so it'll be a hair more for the actual tube.

Click the image to open in full size.

Click the image to open in full size.

Is that better? I could also rotate all transformers and have the 125ESEs along the back edge. That would put them a little closer to the PS trafo, but give a little more room for the 2A3s.

I'm sorry about the flash reflection off the chassis, the protective plastic layers are still on.

And this is the largest aluminum chassis that Angela carries, I double checked that. So I could exchange it for a bigger steel chassis (which would be harder to drill and Morgan Jones recommends against using steel), otherwise returning it will be a little difficult, since the first thing I did was to tear up my invoice and throw it away (it had my credit card number on it).
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Old 10th June 2003, 03:37 AM   #8
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You should be fine. A trick for determining the best alignment of the power and output iron is to power up the power transformer and hook a volt meter to the input side of the output iron and measure mV AC. Your looking for the lowest number you can achieve by turning them in different configurations. This is help to minimize induced hum.

Here is a heavely modified Dynaco Mark 3, and you thought yours was close.
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Old 10th June 2003, 03:39 AM   #9
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Quote:
Here is heavely modified Dynaco Mark 3, and you thought yours was close.
Well, yes, but that was probably designed by someone with a little more experience than me

I'll try out the voltmeter solution, though it would make me nervous to have all the secondaries unconnected. I guess I could wire-nut each one individually.
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Old 10th June 2003, 01:36 PM   #10
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Default How bout a breadboard

I don't have the experience of a lot of folks here but how bout a breadboard instead of committing it to final form. It is a nice way to experiment with placement, wire length ectc...and how it effects things. I've been messing around with this stuff for a year and while I got it off the drawing board I have not gotten off the breadboard to date. It makes those almost inevitable mistakes easier to handle, allows you to focus on sonics, and makes tweaks and changes less traumatic too. At least, I think, for a beginner.

Cheers
Craig Ryder
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