Fender Twin/separate power supply questions

[Diverted]

Hi all,

Sorry this is a little bit long but please bear with me.

A few months ago I wanted to see if I could build a small, relatively light single channel/no tremolo version of a Fender Twin AB763, with all its power. I built it on a Princeton chassis with a very light single 12" neodymium 200W speaker). I was able to squeeze everything in and it all works but it is noisy; I wasn't happy with it and so I've decided to try again and could use help.
Because of the small chassis I didn't have room on top to put the filter caps in a doghouse, and I had to put the output transformer all the way to the right and snake the wires a good distance on top of the chassis to make room for a speaker. The four 6L6s barely fit next to the power transformer .. overall, it was noisy and I couldn't get the hissy, white noisy floor to go away, despite trying a bunch of different electrolytic layouts. Plus, with all the transformers on the chassis, it was seriously top heavy.

So I'm going to try again and could use some advice.

My idea now is to put the power transformer on a small chassis at lower right of the cabinet. The power switch will be on that chassis but everything else (faraday shield, HV secondary and center tap, and heater wires) are going to go to an umbillical on a six-pin octal socket. I'm planning to snake that from the right side of the lower chassis to the top left of the upper chassis, where I'm hoping to put the choke and output transformer. I'm going to use a doghouse on top of the chassis this time and I'm going to layout the four 6L6s in a square pattern, as opposed to the four in a line pattern, to keep everything together.

There are six wires in the 18g unshielded umbillical I have:
1. One connects ground of power supply to ground on top chassis.
2. Two heater wires
3. HV-0-HV

I want to put the two big transformers on opposite sides of the chassis for balance; speaker would go roughly in the middle.
 
I have a few questions though:

1. Will snaking that AC from one side to another cause issues? I can't find shielded six conductor cable in a heavy enough gauge to supply the heaters.
I was going to run the umbilical straight up from power transformer at right and tuck it right underneath the faceplate, then head to the other side from there, to keep it as far away from the reverb tank as possible.
2. The reverb tank will have to be a short 9" spring to fit next to the power transformer on the bottom. Should I move the reverb jacks to keep them away from the power transformer? And if so, should I run shielded wire to the jacks inside the chassis, since they won't be in their standard position at back right?

Thanks for hanging in there on this long post. So many questions and unknowns. I'm committed to building a quiet amp and could use any advice on keeping it quiet as I can get. Thanks!

[sluckey]

I would mount the PT on the same side as the OT. And mount the reverb tank so the input jack (8Ω) is closer to the PT than the hi-z output jack.

[Diverted]

I was hoping you wouldn’t say that😂😂
Would you recommend running the umbilical inside a length of copper tube or braid etc., for shielding?  Thanks Sluckey.

[sluckey]

Won't need any shielding if you do it as I suggested. But you really need to protect those wires! 1/2" PVC conduit would be sufficient. EMT would be fine. Copper is fine too but a bit overkill. 1/2" PEX or even garden hose will provide a bit of flexibility. Just looking around my shop and throwing out ideas. 
 

[Diverted]

The umbilical has a super thick coating. I’m going to fasten it to the chassis along its length. The

So, if I put power transformer at bottom left you wouldn’t recommend putting output transformer on opposite side of chassis? What If I shielded it?
Just trying to make it less awkward, with all the weight on one side. Thanks!

[sluckey]

The idea is to keep the noisy power stuff on the same side and keep the sensitive preamp stuff on the other side. Run the umbilical cord straight up fastening it in the corner of the speaker baffle and the cab side wall. DO NOT RUN ACROSS THE LENGTH OF THE MAIN CHASSIS! Remember, you're rebuilding this to hopefully be quieter than the original.

At least that's how I would do it. I did something very similar back in the '60s with my first version of my Sunn Sonora bass amp. It was a head and the chassis was not big enough to hold the PT. So I mounted the PT in the bottom of a 2x15 speaker cab and ran the power cable to an octal plug on the head. The power cable was hard wired to the PT chassis.

[Diverted]

Okie dokie. All on one side it is. The hell with balance, I just want it quiet.
I think I may ditch the idea of running the output tubes in a square pattern and just lay out top chassis as close to original as I can get. Unless you see a benefit to putting them in a square pattern?

Thank you for the encouragement.l👍

[sluckey]

square pattern?

Either way is fine. But each way would affect the board layout a little bit.

[Diverted]

Actually, think I gotta do square as chassis is not long enough for everything in a straight line without really squeezing. (4x6l6, 4x12ax/t).
Question: what’s standard practice 6l6 spacing? 1 1/2” or so between tubes?

[sluckey]

I like 3" centers.

[Diverted]

I had to put the tubes at the end and the output xfmr pretty much in the same spot it is on the original. 6l6 spacing is about 2.75” and the choke first next to the tubes with an inch or so breathing room next to the tubes.

How about putting the choke on the power supply chassis? I’d have to add two additional wires on the umbilical but....

[sluckey]

Bad idea.

[Diverted]

Thought so. Don’t want to mix the choke dc in with the AC stuff.. It all fits anyway, will probably get warm up there.

[Diverted]

Here’s what I think it’s going to be or close. Spread it out as much as I could while still giving speaker enough room. The output transformer will be butting into one of the frame openings but it looks like it’ll all fit.
Any opinion on it? Thanks!

[sluckey]

I'm reminded of trying to put 60 pounds of shit in a 12 pound bucket.   

[Diverted]

Well, that doesn't sound encouraging 

[sluckey]

I'm guilty of doing it myself. Takes a lot of planning and determination. Be prepared to get a little stinky.   

BTW that was a paraphrase for 60 watt amp in a 12 watt chassis.

[Diverted]

Ha! Yeah, pretty much. Story of my life.


Actually, I think (hope) it's going to be OK. There actually should be plenty of room above the 6l6s for the rectifier and bias supply; enough room that hopefully i'll be able to lay it out cleanly.. The board itself is pretty short as it's a stripped down circuit, and won't come anywhere near that area; it'll end before the other side of the output transformer. And there's only going to be vol/treble/mid/bass/reverb, so the controls will be well away from that area too. I'm pretty hopeful but if its still noisy I'm not worried about it. I'll break it down and try something different. I just love messing around and I learn a tiny bit every time I try something new so it's all good.

Once I start drilling holes and populating stuff I will post some pix. I would love to have some extra eyes on it as I go through it.

[sluckey]

The idea will work. Just get a decent layout and grounding scheme. Take your time and use good build practices.

[Diverted]

The grounding scheme is one of the things I was wondering about, I was going to try to follow the fender scheme which is the first filter grounded over by the power supply, and the other filters grounded on the preamp buss.

With the power transformer bolted to the small chassis at the bottom, would it be wise or unwise to ground the faraday shield and HV center tap on one of the transformer lugs, and tie that into one of the wires on the umbilical... then run that ground up into the upper chassis and ground out the other stuff up there on it? (bias supply filter cap positive, 1 ohm 6L6 Cathode resistors etc.)?
Or, should I not ground on the lower chassis, but run farady and HV center tap up the umbilical, then ground them at the upper chassis? I was going to put the power cord up top, as far toward the front and left as I could go (adjacent to and just left of the rectifier and bias supply)


As for the other ground for the second, third and fourth filter cap, I was going to run a 16g copper ground buss wire off the input jack and tie them and all the circuit board grounds to it.

[sluckey]

Your power cord will connect to the PT chassis. Power cord ground wire must connect to this chassis. Call this earth safety ground. Connect the PT shield wire to this chassis also. Do not connect the HT center tap to this chassis.

You must bond the PT chassis to the main chassis for safety. So, there will be a wire in the cable that connects to the same lug as the power cord ground is connected. The other end of this wire will connect to a ground lug that is very close to the socket for the cable plug. This is your main chassis safety earth ground. Nothing else should connect to this lug.

The HT center tap wire from the PT is a separate wire in the cable. No connection on the PT chassis. The other end of the HT center tap terminates in the plug. The corresponding socket pin will connect to a separate ground lug that is close to the socket. This is your power ground. This is the point that the reservoir cap, power tube cathodes, and bias grounds will connect. Keep all the power circuit on this end of the chassis.

[Diverted]

Perfect. Thanks👍👍

[d95err]

Looking at the original post, it seems your problem was mainly hiss (white noise).

All that stuff with positioning of transformers, AC wiring and grounding should affect mainly hum (50/60 or 100/120 Hz).

Are you sure you’re working to fix the actual problem? Or is it something else causing the hiss?

[Diverted]

Honestly, I don’t know. The hiss (too loud to ignore with volume at 0) was the issue and no matter what I tried I could not get rid of it ... rerouting wires, shielding stuff, swapping out resistors tubes caps etc etc etc. with how cramped it was and how much it limited my options, i finally just threw in the towel. Pls, it was very top heavy and wobbly with all that weight up top so I decided just to try again.

[sluckey]

The hiss (too loud to ignore with volume at 0) was the issue

I missed this and went straight to the separate PT chassis discussion. Moving the PT will not cure hiss. That hiss was a completely separate issue.

[Diverted]

It’s OK. I would have started over again anyway as it really wanted to tip if you even looked at it sideways. This way will be way more stable.

[Diverted]

Made some progress. Would love to hear any criticisms.
I had a six-pin umbilical but ended up ditching that for an eight pin octal. I didn't have a male octal jack so I gutted an old dead tube and used that instead for the plug. shinkwrapped it all, ran it through a heavy duty rubber fuel line and anchored it to the chassis with an electrical box conduit clamp. That should hokd!

Pin 1 and 8: heaters
Pin 2 and 7: High voltage secondary
Pin 3 and 6: Earth ground and high voltage center tap
Pin 4 and 5: Primary AC hot send and return for switch.

The faraday shield will stay in the bottom chassis and isn't going anywhere. I'm wondering if it's OK to hard wire the AC neutral to the power transformer primary?
If not, I will have to rethink this and possibly order a nine-pin octal male and female.

Thanks,

Ted

[sluckey]

Sure. Do you intend to put the power switch and fuse on the main chassis also?

But if you are going to be cramped for space I'd suggest power cord, power switch, and fuse on the lower chassis.

What about bias tap? Or will you tap one of the HT wires?

[Diverted]

I'm going to tap the HV for the bias.

I was thinking about putting the fuse on the bottom chassis. Less clutter up top, fewer wires to deal with on the umbilical and there's plenty of room down there.
But the power switch will go up top, fed by pins 4 and 5 on the umbilical.

Thanks!

Ted

[Diverted]

Hi all,

Got some more done today. I redid the umbilical using 18g teflon wire as I didn’t 100 percent trust the cloth wire I used the first time.

Fired it up for the first time with no tubes obviously just to check power supply and all seems good.

Anyway, I would love to hear any critiques or suggestion on the wire routing.

As far as the main power stuff, pretty much all that’s left to do are the 6l6 cathodes.
Currently the power ground has HVCT, bias resistor, first filter cap and bias capacitor on it.

Nodes B, C and D are grounded on the preamp buss which is tied in to the jacks.

Thank you!

[sluckey]

Looks clean. Node B is for the screens right? If so, that cap should also connect to the power ground. I also like to run a dedicated wire from the speaker jack sleeve terminal (ground) to the power ground.

[Diverted]

Thanks Sluckey.
As for running a jumper to power ground for the ouput jack, so I should just isolate the jack from chassis and then run that ground?


Finished wiring this evening and powered up the amp. All voltages look OK (nothing glaring though V1B plate is about 50v low), but there are issues.
1. Low volume ... Very quiet, bedroom quiet, and when turned up it distorts a good bit.
2. No reverb.
I've gone over the wiring a few times and I'm sure I'll find the issue but nothing yet.

Here's a link to a few more photos than I can post on this site:

https://imgur.com/a/eA1kyxn


Voltages:
Rectifier: 445
B node: 443
C node: 430
D node: 403
Bias voltage -43

V1a
P 264
K 1.9

V1B
P 225
K 1.5

V2
P 436
K 8.2

V3A
P 278
K 1.9

V3B
P 291
K 1.9

V4A
P 242
K 95

V4B
P 230
K 95

V5
P440
G 439
K 46.4ma

V6
P 440
G 439
K 44ma

V7
P 441
G 439
K 49ma

V8
P 441
G 439
K 49ma

[sluckey]

...

[Diverted]

Ha!
Now how did I miss that? 🤔
Wife has me watching a crazy art house movie but I will check soon. Thanks!

[sluckey]

...

[Diverted]

Yeah, I found it 
Forgot to put that one in, thank you.

Amp's up and running. Boy is it loud. Much quieter than the last build too so I am happy I redit it.

At this point the only thing I would like t address is the reverb noise. I know reverbs get noisy as you crank them up but this one is pretty hummy/white-noisy. Would you recommend any shielded wire in the reverb signal path? The only place I used shielded in this amp was in V1 input grid.

Thank you for the help!


Edit: I should note that I ran out of 470k resistors so I used 330k for the reverb attenuation resistor. I suppose I could parallel two 1Ms but the 330k would just make the reverb level bit louder overall, correct?

[sluckey]

Shielded cable to and from the reverb pot is probably a good idea.

Seems to be a lot of finicky, hummy reverbs these days. I wonder if the quality of the tanks is a factor. Most of the hum usually comes from the recovery stage. I would try lowering the gain of the recovery amp. I'd first try a 5751 tube for V3. Second, remove the shared cathode cap from V3. And finally lower the plate resistor of the recover amp (looks like pin 1) to about 47K.

Also get a handful of 12AT7s and rotate through V2, reverb driver.

[Diverted]

Thanks.
Stupid question but shielding has always confused me, as in which end generally should be grounded? The end closer to signal source?

[sluckey]

I would just ground both shields at the pot.

[PRR]

> any shielded wire in the reverb signal path? The only place I used shielded in this amp was in V1 input grid.

Tank output is often _lower_ than guitar input. So very likely it wants shielding.

[Diverted]

Quieted the amp quite a bit, thanks for the suggestions.

1. Moved the screen filter (node b) ground to the main power ground.
2. Ran shielded cable to and from the reverb pot.
3. Put a 5751 in V3.

Tried it without the cap in V3 cathode, but got oscillation so put it back in.

Just as it was suggested, I think I will run the output jack ground to the main power ground. Going to isolate the jack from chassis when I do that.

Thanks for the help!

[Diverted]

Finally got around to putting the amp together for a dry run. Still a few more odds and ends to do on the upper and power supply chassis, and I have to Tolex it. But the end is in sight.
There's a slight bit of white noise at idle which I can live with (would love it to be dead quiet though), and there's no reverb hum. So I'm happy!
Thanks for all the helps and tips along the way.