Layout Advice Needed for Project Amp

For those building and repairing amps on their own. Learna and share ideas.

Re: Layout Advice Needed for Project Amp

Postby Unit_1 on Sat Jun 13, 2015 10:04 pm

dnpqkc wrote:Ok thanks! I'm found a radio shack instrument cable I'm going to use. I have a few more questions, but I can only remember two of them at the moment:

1) My power transformer has two available secondary windings - 540v and 680v.

http://www.tedweber.com/media/import/w025130sch.JPG

My plan was to borrow the output circuit of Weber's plan for the Fender deluxe style power section, which appears to use the 680v winding.

http://www.tedweber.com/media/kits/5e3_schem.jpg

Is this a bit much for the 6v6s, especially given the fact that I'm using 1n4007s for rectification rather than a tube rectifier?

My other thought was to use the Deluxe Reverb style, which is fixed bias but with the lower-voltage winding.

http://www.tedweber.com/media/kits/6a20_schem.jpg

This is weird because I read that it was the deluxe reverb that pushed the 6v6s beyond their limit. :?

I kinda favor a cathode-biased approach for the simplicity.


2) Do you like to run hook up wires to the main board from underneath or on top, and why? Top seems easier to trouble shoot, and bottom seems better for a more polished appearance.


I used to put the wires underneath, but now favor above - if you're using eyelets the wires are kind of a pain on top, but I only use turrets now and it's really super easy.

It's much easier trouble shooting and verifying, and if there is a mistake OH so much easier to fix than removing every soldier and lifting the board, which I've had to do...

WHY did I have to do it

Well, funny you would ask :mrgreen:

It was a fender build, with eyelets, underboard wires and a bottom insulated sheet instead of stand off's (which I also use now instead of the fender sandwich). When I tightened the bolts, it shoved the bottom solder of the powertube cathode ground into the b+ supply wire and bingo, the 380+ volts was being directed to ground across a 300 ohm five watt resistor.

Smoking hot that resistor got in just seconds, and it was *REALLY SMOKIN'*

So a perfect storm of gotchas, the sandwich insulation, the under board wire, cloth wire which penetrates easily and another reason I use teflon wire now instead.

Had to remove ALL the wires, pull the board, move the wire over half an inch, use silicone to glue it in place, and then put it all back together.

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Re: Layout Advice Needed for Project Amp

Postby dnpqkc on Sun Jun 14, 2015 4:59 pm

Thanks for the advice! Does anyone know what 47 pf capacitor between the phase inverter plates does?

http://www.tedweber.com/media/kits/6m45p_schem.jpg
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Re: Layout Advice Needed for Project Amp

Postby PeterS on Sun Jun 14, 2015 9:03 pm

Reduces the gain at very high frequencies, possibly to prevent oscillation.
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Re: Layout Advice Needed for Project Amp

Postby dnpqkc on Thu Jul 02, 2015 11:21 pm

I have the board assembled, and now I am backtracking to get the power supply done. Here is a schematic I have drawn up for the power supply:

https://drive.google.com/file/d/0BxAS2o ... sp=sharing

I'm not sure what resistors I should use because (1) I don't know what the voltage coming out of the rectifier is going to be, and (2) I don't really know what voltages I should be shooting for at each stage. Any guidance would be greatly appreciated.

One idea I had was to use the values the original Plexi uses, which are 22k, 10k, and 10k for R1, R2, and R3, respectively, power up and check the voltages, and then adjust from there.
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Re: Layout Advice Needed for Project Amp

Postby PeterS on Fri Jul 03, 2015 2:03 pm

use the values the original Plexi uses, which are 22k, 10k, and 10k for R1, R2, and R3, respectively, power up and check the voltages


That's a good approach. You can also guesstimate the voltage drops by looking at the current drawn by each preamp stage.
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Re: Layout Advice Needed for Project Amp

Postby dnpqkc on Fri Jul 03, 2015 9:34 pm

Thanks again, man. I really appreciate your ongoing advice.

I notice that the 5E3 (which design I am using for the output section) does not have 470 ohm resistor preceding the screen grids, as most other designs do, including the Plexi, the Fender Deluxe Reverb, Princeton Reverb, etc. I'm thinking this is because the 5E3 uses a pretty high-value resistor (4.7k) in the power supply to feed the screen grids. Designs that include the 470 ohm screen grid resistors either preceded by a choke or lower-value resistor, for example the 1k in the Princeton Reverb.

Since I am using a choke, I think I'm going to go ahead and include the 470 ohm screen grid resistors. Thoughts?
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Re: Layout Advice Needed for Project Amp

Postby PeterS on Sat Jul 04, 2015 1:24 pm

Yes I would add the screen resistor. The trade off is, lower values = more power, higher values = longer tube life. If you used a high value like 10k you'd notice a few watts missing, but the effect of 470 you'll never notice.
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Re: Layout Advice Needed for Project Amp

Postby dnpqkc on Sat Jul 11, 2015 8:32 pm

I powered up for the first time today. I get 341 volts from the rectifier with the standby off. With the standby on, I get:

1) 485v at power supply stages A and B
2) 473 at C
3) ~470 to 472 at D and E

The heaters check in at 6.84V.

https://drive.google.com/file/d/0BxAS2o ... sp=sharing

What I have left:

1) Solder the control grid and cathode wires to the output-tube sockets
2) Solder a few ground connections to the bus bar
3) Wire up the input jacks

:excited:
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Re: Layout Advice Needed for Project Amp

Postby dnpqkc on Sun Jul 12, 2015 9:20 pm

Got out the brand new power tubes I ordered 6 months ago, and one of them looked wierd - no silverish appearance at the top. Installed all the tubes and sure enough, that one doesn't glow at all. I'm still getting over 400 volts at A and B on the power supply. The preamp sections are down in the 300s.
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Re: Layout Advice Needed for Project Amp

Postby dnpqkc on Sun Jul 12, 2015 9:48 pm

The voltages for the power tubes will go down when there are two working tubes in, correct?
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Re: Layout Advice Needed for Project Amp

Postby dnpqkc on Mon Jul 13, 2015 3:18 am

I think I may have fried my output transformer. There was a short on the secondary between the common lead and the 16 ohm tap. I didn't have it on long, but there is continuity between the various secondary taps and between them and the common lead. I don't know what kind of readings I'm supposed to get on an OT, but that doesn't seem good. Is it fried?
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Re: Layout Advice Needed for Project Amp

Postby dnpqkc on Mon Jul 13, 2015 3:28 am

Well, I just tested the continuity between the speaker leads on my AC30 and got continuity as well, so maybe false alarm.
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Re: Layout Advice Needed for Project Amp

Postby PeterS on Mon Jul 13, 2015 2:57 pm

The secondary should give you low readings --- less than an ohm usually. The primary should be somewhere in the ballpark of about 50 ohms to a few hundred ohms per side, and the sides should be roughly similar.
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Re: Layout Advice Needed for Project Amp

Postby dnpqkc on Tue Jul 14, 2015 1:00 am

Ok, just threw a new set of 6v6s, powered up, and... Oscillation! I suppose the first thing I'll try is reversing the OT primary leads. Other than that, are there any likely causes?
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Re: Layout Advice Needed for Project Amp

Postby dnpqkc on Tue Jul 14, 2015 1:53 am

I did a little bit to isolate the problem:

1) Removed V1 - still oscillates
2) Removed V2 - still oscillates
3) Removed V3 (the PI) - no oscillation

Thoughts? I haven't tried reversing the OT primary leads yet.
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