Thanks DL. This is my 1'st complete scratch build _including_ the chassis and face/rear plate and layout.
I follow your bias/balance much better in layout mode. You have 2, 22k's in series feeding the balance and from the balance wiper to the bias wiper you have another. The range pot is easy to understand. How did you come up with 22k values? What type of range are you expecting?
The 1'st pair of 22K R's are in parallel that are feeding the opposite ends of the balance pot. It's to give the balance pot something to work against, if you leave them out the balance pot wont work. Their junction is feed from the term intensity pots wiper. If this amp didn't have the -bias vary trem this junction would be feed from the raw -bias.
The 3'rd 22K going from the bias balance pots wiper to 1 end of the -bias range is there to make sure there is always some -bias left even if you turn it all the way down. If you need more -bias you can make this R larger since it's in series with range pot and it's the
-bias path to ground.
Note that the way the the bias/balance is wired up, if either the balance pots wiper or the range pots wiper ever lifts (stops working) -bias will remain as a safety.
I think I need to see if I can add a -bias safety R to the term intensity pot in case it's wiper ever goes wonky. Not sure if you can or if you need 1 or 2 safety R's?
The values for the R's and pots for the bias/balance are from KOC's TUT books. There what he likes to use and IIRC it's for the overall impedance of the -bias supply, to keep it low?
For the -bias range, the Fender schemo calls for -28dcv (the PT has a 60v bias tap), so with 25%, +/-, I'll need to go from -21dcv to -35dcv. But since this is a bias vary term that range might not be enough to let the term over come the -bias of the output tubes?
Also, I noticed you are running your CT to pin 5, then to your cap ground to yet another cap, then tied into the bias after the diode and 470. What is the purpose?
The B+ CT is going to pin 5 so I can twist all 3 B+ PT HT secondary wires together (I'm using it as a solder post), to help cut down on the electrostatic and electromagnetic field radiated from them, ie, noise, + now I have a ground wire to twist with the rectified B+ wire from pin 8 of the rectifier tubes socket (for the same reason) to the 1'st 2 HT B+ filter caps, 22uF each in parallel @450v = 44 uF @ 450v.
The 1'st -bias caps ground is tied to the B+ HT CT and 1'st pair of HT B+ filter caps ground because the -bias comes from a tap on the B+ wind, so any noise from the HT B+ filter caps charging current on the HT B+ is _also_ on the -bias tap. This also lets me run this ground wire with the -bias tap wire as a twisted pair, for the same reason stated above. This is a very noisy ground and KOC, Merlin and others wire it up this way. These grounds go together as their own ground star node _only_ and _then_ they run a wire to the next ground star node and so on. If this PT had a CT on the heater wind it would go there too. This is so the B+ filter caps charging current circulate by themselves and do not affect other circuits by modulating their grounds.
The 2'nd -bias caps ground goes to the screen ground star node to separate it from the HT B+ ground star nodes charging currents noise.
I'm trying to use twisted pairs or twist 3 when/where ever I can, some guys don't and their very happy with their amp builds.
There is 1 hitch in the giddy up with this. If I'm getting this right, KOC likes twisting wires to cancel noise and IIRC, Merlin says to keep the B+'s ground rail wire _away_ from the B+'s positive rail wire, so the ground wires noise is not induced back into the B+ (positive) rail.
Again this is just my understanding/interpretation of KOC's and Merlins wired ground star/node scheme. Different guys wire their amps ground differently with good/great results, no doubt.
Our host Doug has his own grounding scheme that _many_ guys here have used on _many_ amps and report great results.
Any and all corrections of what I'm saying are welcome.
Brad
