Advice needed...

[TriodeLuvr]

Yes, that's it. If each resistor is 120K 1/2W, it will draw only 1/2 mA from the B+ supply and filaments will be elevated to about +60 V.

Jack

[The4thWatcher13]

OK, hopefully I've taken everything you mentioned into account.
I think, as far as the layout and the socket components are concerned, this may be what "the doctor" (Jack) ordered. I also made use of Josh's layout and noticed that the pre-amp section and the power supply/ power tube sections were grounded to the chassis separately, so I tried to apply that as well. So one ground for V1 & V2, jack and pots, there will be one for the power supply and three prong power inlet, but I'm not sure where the center-tap for the output trans cathode winding. (The PS ground?) The center tap for the heater trans will go to the heater elevation resistors as Jack recommended. The signal path is indicated in Blue. There is only one little signal wiring overlap and that's under the tube at pins 6 & 7. It's the best config I've been able to come up with. The four plate resistors (black) and one resistor (R6) which is part of the phase inverter have been left hanging. I believe the image constitutes all of the tube connections and wiring (not including transformer and power)
Let me know what you think. If certain lines don't connect where they "should" it's probably an imaging glitch. (something I've been trying to eliminate but no luck just yet) Also, I did start out as Jack recommended, on paper. Lot's and lot's of possibilities printed out and scribbled out. (..just an example. Not the working version which was completed on the computer)
Thanks much!
M

[TriodeLuvr]

You're almost done. A few 3- or 4-pin terminal strips will finish it up. I would use strips with no ground lugs, other than maybe in the power supply area. It's easier and more effective to use isolated lugs on strips for grounding in the audio section, then run wires from each of those lugs to the star ground elsewhere. I think this layout will work well for you, looking forward to hearing your report on the sound!

Jack

[The4thWatcher13]

You're almost done. A few 3- or 4-pin terminal strips will finish it up. I would use strips with no ground lugs, other than maybe in the power supply area. It's easier and more effective to use isolated lugs on strips for grounding in the audio section, then run wires from each of those lugs to the star ground elsewhere. I think this layout will work well for you, looking forward to hearing your report on the sound! Jack

Wow, good news! I'm working out the power supply layout today and figuring out where the terminal strips will be needed. I've got some but will have to order the others. I've posted the updated schematic with the heater elevation resistors the the considered grounding scheme. What I was wondering about is the output transformer cathode ground. Can it be connected to the PS ground? As drawn all three transformers are grounded to the same lug but the heater trans CT is "elevated" at the same point.

[TriodeLuvr]

Yes, use the PS ground for the cathode winding CT. Connect the CT resistor to the ground lead of C3-B if possible.

[The4thWatcher13]

Yes, use the PS ground for the cathode winding CT. Connect the CT resistor to the ground lead of C3-B if possible.

Okie dokie, the resistor placement shouldn't be a problem.
A couple quick questions:
1.Should I be concerned about AC and ground wires either crossing over or running parallel to one another? I seem to remember reading about that and I'm searching again now for that info. In either case I'm having a bear of a night trying to figure out how to mount and run wires while considering this. It's actually making me crazy.
2. Should I be concerned about the number of ground connections I've got? I can imagine one wants to keep it to a minimum. I've been referencing Josh's grounding scheme diagram and have been trying to keep it to 3 ground points.
On another note: the tone pot is a stacked AC switch /Variable resistor type, and I was thinking of bypassing the pot mounted AC switch and going with a regular "bat" style switch. That will remove the "mains" AC from the tone pot, pre-amp proximity completely, just in case. Well, I can't figure out what type of terminals I'm gonna need until I figure out this AC, ground, layout situation. What were we talking about earlier? Internal real estate?! There may not be a lot of parts but there ain't a lot of room in there either, Maynard!
The diagram below is where I am right now.... 6 or 7 different power supply layouts later...I'm toasted again. Everything is there except the heater wiring,..and all the lugs,....I think. See ya tomorrow!

[TriodeLuvr]

The work you've done so far looks good to me. Don't wear yourself out, it's easy to over-think these things!

1.Should I be concerned about AC and ground wires either crossing over or running parallel to one another?

All the AC wiring is on one side of the chassis, so I don't see a problem. If you meant signal-carrying wires, lead lengths are short and you're not working with an ultra-high-gain design. Again, I don't see a problem.

2. Should I be concerned about the number of ground connections I've got? I can imagine one wants to keep it to a minimum. I've been referencing Josh's grounding scheme diagram and have been trying to keep it to 3 ground points.

The only concern I have is the chassis ground at the lower left of the CAD drawing. I would remove this and add a ground wire from the terminal strip back to the power supply ground near the top. The only other chassis-connected ground should be the green wire from the AC cord.

On another note: the tone pot is a stacked AC switch /Variable resistor type, and I was thinking of bypassing the pot mounted AC switch and going with a regular "bat" style switch. That will remove the "mains" AC from the tone pot, pre-amp proximity completely, just in case.

I think the separate switch will be more convenient to operate, but OTOH, the tone pot switch is already there. If you want to use the existing switch, route the two AC-carrying wires away from the pot at right angles along the edge of the chassis. They don't need to be twisted because they're not out of phase like filament wires. Just do whatever is necessary to keep them tucked away. It's difficult to know whether the proximity of this switch to the 'AV6 will create hum, but it's easy enough to add the separate switch later and shorten the wires if that's the case.

Remember, part of the process of designing and building a new amp is debugging after it's built. Put it together with that in mind and don't sweat the little stuff. Actually, yours is simpler than many. I think it's going to work great right out of the box!

Jack

[The4thWatcher13]

Thanks Jack, that takes a load off my mind. All points taken.
So I guess as long as all of the AC components are on one side, away from the audio circuits, the way transformer and filter cap grounds are run isn't a problem with regard to proximity to one another. Otherwise I'd have to "commit" myself right this very minute!
OK, time to settle on a config and get lugs.
Thanks again!

[The4thWatcher13]

Well, things are moving along smoothly aside from some issues ordering lugs. The chassis is all but completely drilled and all parts are here. I've been considering putting a pilot lamp in as well. Not sure what kind yet. I've got a "Linrose B1052C5", 125VAC, neon lamp but not sure if using that in the AC primary is a good thing or not.
Other than that I'm just about ready to consider a paint job and start building. I went the drill press step bit route for the holes and the amp building Gods smiled upon me by directing me to a 10" in Ryobi drill press in excellent condition for $50 bucks! Ya gotta love it.

[The4thWatcher13]

Greetings, all is going well so far with the build, delays not withstanding. But I have a quick question: I've posted a zoom in of the schematic and I noticed that the way it's drawn that C7 allows the tone control to have +61V across pins 1 & 2. I'm wondering if it should be like that, or if I should use "C7" to isolate the tone control from the "V2" plate pin 1? I've been looking at various tone controls on old amps, especially Vox, but I don't see one similar. And honestly I'm not really sure how it works, but it did, ..and didn't sound bad either.
Any info would be appreciated. I'll keep looking around also..
Thanks,
M

[TriodeLuvr]

The schematic is correct as-is. DC current is blocked at the pot by C5. Every terminal of the pot has exactly the same DC voltage. The tone circuit works by shunting high frequencies plate-to-plate. That reduces the differential across the plates and therefore also at the 6CU5 grids.

Jack

[The4thWatcher13]

The schematic is correct as-is. DC current is blocked at the pot by C5. Every terminal of the pot has exactly the same DC voltage. The tone circuit works by shunting high frequencies plate-to-plate. That reduces the differential across the plates and therefore also at the 6CU5 grids.
Jack

Wow! I have no idea how that works but I'll take your word for it! Looks like I've got even more homework.
I do my best to understand this stuff but sometimes I just have to go "Wow, ...cool". I'll leave it as it is. I'm hoping to be done wiring it up in the next couple of days, and all is well. But no need hurrying.
Thanks Jack, I hope all is going well with you and yours as well.
M

[The4thWatcher13]

Well, comrades, I knew something was wrong. And I believe it indeed is. That being I believe that when I did my layout drawing I flipped my socket numbers! I think it's that the wiring diagram was drawn from "above" and the socket images are representations of the view from below! Luckily I haven't soldered anything except the output socket and the output transformer.
I won't even go into the different ways I've thought about this.
I'm not even sure if re-doing the wiring diagram will solve this if it can even be done properly. The problem has to do with the socket numbering. In "reality" my pin 1 is where 7 should be and the same with the 9 pin. They're backward! Honestly I'm not sure exactly sure how bad or really bad I've messed up. Just getting my head around what went wrong continues make me dizzy. I'm hoping re-orienting the sockets 180 deg and re-drawing the layout will save me. Wish me luck!

[TriodeLuvr]

Unless you see burned resistors or other parts, it's probably OK. Tubes are tough. Fix the wiring and try again.

Jack

[TriodeLuvr]

Pay no attention to this post.

[TriodeLuvr]

You know, this circuit has an interesting feature I didn't notice before. Do you see where the 470K resistor R8 is connected to the junction of R6 and R7? I saw that this morning and assumed it was an error in your reverse engineering of the amp. In a post I have now deleted, I advised you to ground that end of R8 instead, which is how the grid resistor of the lower output tube would normally be treated. However, I ran the circuit through a SPICE simulator a little later and discovered something unusual.

With the grid resistor grounded, output of the lower triode is much higher than it should be, roughly 10X as much as the upper triode. Changing the ratio of R6/R8 can correct that, but it doesn't explain what's going on. IOW, if the connection to R8 shown in the schematic is an error, why are the two triodes so badly unbalanced when its "corrected"? I'm sure you didn't get the R8 connection AND the values of R6/R7 wrong. I eventually realized that R8 is applying negative feedback at the lower triode. When R8 is connected as shown in the schematic, gain of the lower triode decreases and the PI becomes very well balanced with the values used for R6/R7.

So why didn't the factory simply use a different ratio for R6/R7 and ground the grid resistor as is usually done? As it turns out, this is a very effective technique to compensate for differences in gain within the two sections of the 12AX7. I proved this by temporarily substituting a 12AT7 for the lower triode. Amplification factor of the 12AT7 is 60, whereas the 12AX7 is 100. In a standard grounded cathode circuit, this creates approximately 5 dB difference in gain. That's also what should happen when the lower triode in this PI is changed to a 12AT7. Instead, because of the feedback circuit, the gain of the lower triode only decreased about 3/4 dB. The two sections of a 12AX7 would of course be much better matched than 60/100 even in a worst case scenario. That means this circuit really works well under any normal circumstance at improving PI balance. Between this circuit and the cathode feedback windings on the output transformer, this is (was) a pretty good hi-fi amp.

Anyway, I just thought this was worthy of mention so we both understand the circuit a little better. It could also be important to know in case you decide to unbalance the PI for an experiment at some future date. If you ever decide to remove this feature and ground R8, output of the two triodes can be reasonably well balanced by reducing the value of R6 from 47K to about 7.5K or 8.2K. That will put the two triode outputs within one dB of each other, and you can experiment from there.

Incidentally, I'm sorry I didn't see that upside-down wiring problem earlier. I did review your drawings, don't know how I missed it.

Jack

[zaphod_phil]

What schematic do these references come from? Could you please post it?

Thanks!

[TriodeLuvr]

It's the schematic the OP posted early in the thread. I think there's also a partial located one or two pages back that shows these parts.

Jack

[The4thWatcher13]

OK guys, I'm fried. I had no idea how much work building an amp, any amp, actually is. It's my first time building one and it's been a real learning experience in almost every area. I'm eternally grateful to all of you who have taken the time to assist, guide and critique this project. So now it's all wired up and the first thing I noticed was that it wasn't pulling snot for current. I had a 120VAC lamp in series with the mains. I removed it hence the hole next to the switch on the front. With the lamp gone I've got current all right..looks like it's pulling almost 2A at startup and the 1.25A fuse that was installed during the mock-up went south. I don't think it was the lamp!
So now, as Jack so kindly forewarned me about, begins the de-bugging stage. I'll start after dinner tonight but for now pictures, updated schematic and and a wacky wiring diagram!
(..don't ask about the reverse labeling on certain parts, it's a trauma I'm still dealing with to this day!) Back in a few!

[The4thWatcher13]

OK, already found one mistake: C6/C7 are supposed to be 0.01uF and I grabbed 0.001uF. ...lovely.