#124 Build Questions

[lgehrig4]

I wish I shared photos during the build, but better late than never. If I get my last parts shipment before the weekend I should hopefully have this completed by Sunday. Still trying to decide what to use for the ground bus. I was hoping to use something nicer than just stripping the insulation off the 18g wire that I have. Thanks to all who have assisted so far!

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[lgehrig4]

Going back to the test jack for measuring DC in place of the 1/2 power switch. Before this came up I had the cathodes of V4/V6 wired to ground with no resistor. Should I now replace these wires with a 1 Ohm resistor or was that suggestion only for V5/V7? I assume all tubes but I wanted to make sure.

I only plan on using 2 test jacks (+ one ground). Would it be ok to connect each test jack to cathodes if each neighboring pair?

test jack1 ---> V4/V5 pin 8 ---> 1 Ohm ---> ground lug
test jack2 ---> V6/V7 pin 8 ---> 1 Ohm ---> ground lug

[martin manning]

Yes, you can do that. Remember the mV you read will be equal to mA for both tubes, and you can also put your DMM across the cathode jacks to measure the balance, and you can use that measurement while feeding signal in to adjust the PI balance.

[lgehrig4]

My other thought was to use the 1 ohm resistor for each power tube and only connect the test jacks to V5 and V7. This way I can use the jacks for biasing when I know the tubes match, and occasionally remove the chassis to check V4/V6 to mke sure those tube didn't drift.

[lgehrig4]

I was comparing your (with Tony) layout with the main power suppply board form your eyelet board pdf and have a question about the two 100K resistor pairs.

The layout shows the 12st pair (from right) going to the 20uf/2K2(B+5) and then to the FET preamp. It shows the 2nd pair going to B+4 and also to the 20uf/2K2(B+5).

Since B+4 and B+5 are on my power supply board, I was thinking that this is what I need to do.

From right to left

1st 100K resistor pair
One wire from this to b+5 on the board
One wire from this to the FET preamp

2nd 100k resistor pair
One wire from this to b+5 on the board
One wire from this to the FET preamp

This would eliminate the 20uf/2K2(B+5)

How far off am I? Even if I am correct is there a better way to run these wires from the resistor pairs?

[martin manning]

The main filter board has all the filters on it, and eyelets for B+3, 4, and 5 along the edge nearest the preamp board. All you need to do is run a lead from the B+3 eyelet to the PI trimmer, a lead from the B+4 eyelet to the V2 (OD) plate loads half way down the preamp board, and a lead from the B+5 eyelet to the V1 (Clean) plate loads at the far end. The last connection is a lead from the B+5 eyelet on the preamp board to the FET board, basically an extension of the B+5 lead.

[lgehrig4]

Thanks for all your help so far Martin! I think I have it from here, but I want to backtrack to the the T4 transformer before I clip the unused lead.

This Hammond transformer is 12V C.T. so my understanding is that it puts out 24V end to end.

You mentioned earlier that "Feeding 12VAC into the full wave bridge rectifier on your relay supply board will produce raw DC voltage of around 17V". The regulator can handle 7-35V.

Would it be a better practice for any reason to connected the two outer taps (for 24V) and clip the CT? Based on what you said I'm thinking this would end up being 34V raw DC voltage. I don't know much about v regulars and wasn't sure they work more or less efficiently if fed voltage on the higher end of their limitations.

[martin manning]

This Hammond transformer is 12V C.T. so my understanding is that it puts out 24V end to end.

That description means it is 12V end-to-end with a CT. If you have any doubt, measure it!
See GRN-GRN no-load voltage on the data sheet snip below.

Would it be a better practice for any reason to connected the two outer taps (for 24V) and clip the CT? Based on what you said I'm thinking this would end up being 34V raw DC voltage. I don't know much about v regulars and wasn't sure they work more or less efficiently if fed voltage on the higher end of their limitations.

No (and you have ~12VAC). The higher the input voltage, the more heat the regulator will have to dissipate. It's best to provide just enough input voltage to insure that input - output will always be above the regulator's dropout voltage, which is somewhere around 2-3V for a garden variety LM7812. WIth 17V input you have adequate margin.

[lgehrig4]

This Hammond transformer is 12V C.T. so my understanding is that it puts out 24V end to end.

That description means it is 12V end-to-end with a CT. If you have any doubt, measure it!
See GRN-GRN no-load voltage on the data sheet snip below.

Would it be a better practice for any reason to connected the two outer taps (for 24V) and clip the CT? Based on what you said I'm thinking this would end up being 34V raw DC voltage. I don't know much about v regulars and wasn't sure they work more or less efficiently if fed voltage on the higher end of their limitations.

No (and you have ~12VAC). The higher the input voltage, the more heat the regulator will have to dissipate. It's best to provide just enough input voltage to insure that input - output will always be above the regulator's dropout voltage, which is somewhere around 2-3V for a garden variety LM7812. WIth 17V input you have adequate margin.

Thank you once again! I'm so glad I asked. The datasheet included was a list of all models and their specs. Under Secondary (R.M.S.) it says 12V C.T.

I originally asked the question online and found these first 2 answers. I didn't see the Wiki description until I doublechecked today so I needed to clear it up.

AI
"In an electrical context, "12V C.T." typically means a 12-volt device with a center-tapped winding. This allows for two voltage outputs: 12 volts from the center tap to one end, and 24 volts between the two ends. "

Reddit (https://www.reddit.com/r/electronics/co ... ?rdt=44044)
Center tap. So measuring the voltage from the center tap to one of the 12VAC lines would be 12 Volts AC, and measuring the voltage from one 12VAC line to the other 12VAC line would be 24 Volts AC.

Wiki
"Volts center tapped (VCT) describes the voltage output of a center tapped transformer. For example, a 24 VCT transformer will measure 24 VAC across the outer two taps (winding as a whole), and 12 VAC from each outer tap to the center-tap (half winding)."

[martin manning]

Once again, proof that you shouldn't believe everything that random people say on the internet ;^)

[lgehrig4]

Finally done! Martin, thank you so much for your valuble information!

I looked this over a few times, but before I add power I was wondering if anyone could take a look at the photos and see if you noticed anything that doesn't look right. Also, is there a good thread or document here for the initial power up and voltages to check for? I found good information for trim adjustments, but that is about it.

Tools I have to work with. If others are recommended (ie. scope) let me know
Multimeter
Variac

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[lgehrig4]

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[GAStan]

The main power fuse is wired incorrectly. Power in from IEC connector should go to tip. The way it is wired presents a shock hazard if a person inserts a good fuse while it is plugged in.

[lgehrig4]

The main power fuse is wired incorrectly. Power in from IEC connector should go to tip. The way it is wired presents a shock hazard if a person inserts a good fuse while it is plugged in.

Oh shoot, you're right Thanks for noticing!

I was on auto-pilot towards the end and looking at this layout. It looks like the PT primary goes to the tip. Just followed visually without thinking

Screenshot 2025-04-04 at 5.01.09 PM.png

[GAStan]

I fully understand. I get really excited when getting close to the first power-up! Also following the layout of #124 got me on the rear chassis switches for PAB & OD. The actual #124 and layout have slide switches but I used toggle so they operated upside down. Took me a bit to figure out what I had done but I did correct it.

Good looking build! That was all I could see but I may have missed something, hopefully some of the others here will look too.