Separate names with a comma.
Discussion in 'Shock Brother's DIY Amps' started by James Knox, Nov 22, 2020.
Thats awesome man - thank you for that great detective work!
Thank you Brother for this! You connected a bunch of dots for me and helped me understand “fixed bias” a little Better. I wasn’t aware of the 3-terminal voltage divider method. I need to study this a bit more. I already installed the board and wired up all the tube sockets so I don’t think I wanna to pull it oit to re-run the underboard jumper from the 220k junction.
Hey, I hope at least the part about resistor values was helpful. I didn't realize you already had the board mounted. You'll be totally fine to run a 2-lug 'variable resistor' -- as I say, lots of smart folks like it for bias. Hey, I used it (after getting some super-pro advice) on my 5G9. A nice detail somebody mentioned is to jumper the empty third lug back to the wiper -- I *think* so if the wiper fails to contact the track, it keeps the 'tail' resistor (politely called the safety resistor, sometimes called the 'idiot' resistor) in the circuit.
I’m down to the last couple connections here!
1. where do I connect Pin 8 on the Power Tubes. It looks like the schematic has them going to ground? Do I just put a ground lug on the socket mounting screw and a short jumper? Never done this before.
2. Where do I elevate my Heater CT? I usually attach it to Pin 8 of a Power Tube.
Maybe @Lowerleftcoast @jsnwhite619 or @mjcyates @JuneauMike @Mr Ridesglide
might check in later... I know they have build Harvard’s.
Don't know if this helps but this from an @mjcyates harvard build a few years ago.
I have seen that one. I just went ahead and grounded them at the tube socket. I’m not sure why Both 6V6 tunes need to be connected together if they are just grounded. And not sure why a jumper need to go to the Power Side ground buss.
and at the end of that build thread there is this...
Which is different than his other one. Note the Cathodes (pin 8) go to ground with a 1 ohm resistor. Not sure what that is. I think I’ve read that some guys measure their bias on the tubes this way.
As I understand it, you hook mini grabbers on them and then dial in your 10K pot. Never seen it never done it but I'll be starting my Harvy soon so that will change. I'm right over your shoulder on this cool build.
yeah I'm pretty sure that last diagram is how it ended up. Most of this stuff I just do because people who know more about this stuff than I do say to do it!
So I went ahead and ran the cathodes to ground...
So I could try out my bias circuit...
starting the startup procedure so I can get some readings.
The build is looking good mate!
The precision 1R resistors allow you to measure the current in mA. Simply put the neg lead on the chassis and the positive on the tube side of the resistor and your measurement will be the current draw of that tube in mA. I think they are a no brainer and save a little bit of maths when setting bias.
Heh, I sense some questions here. This is just my take, and I'm the opposite of an expert, so read with one hand in your back pocket. I'm not sure I have the questions straight, so bear with me (unrolls large map and small chart....)
Why does fixed bias run the cathode to ground?
Think of it the other way around. Basically, every cathode goes to ground. *Cathode bias* adds a resistor in the path to elevate the cathode voltage -- that is, to give electrons a slope to ski down. Fixed bias takes the direct route; instead of rigging to get positive voltage on the cathode, fixed bias puts *negative* voltage on the grid to create the slope -- and you've been working on building that slope recently!
Why can't I elevate my heater voltage to the cathode in this case?
See previous answer. No elevation there any more. If you want to elevate heater voltage here, Google "Valve Wizard Heater/Filament" and search for the term "elevation." But it's not so easy now; most fixed bias amps don't do it.
Why do both cathodes go to ground if they're also tied together?
Are they tied together? The schematic shows a single ground arrow on a single line, but ground is ground (mostly) -- instead of tying them together via a long wire to one ground, it's easy and elegant to run two short wires and let the chassis tie them together -- that is, if you ground them *locally.* Next....
Why do they then also have a jumper to the power amp ground?
Do they, *also*? I don't see one on the schematic or layout. When you *do* see one, think of it as a step to avoid multiple grounds through the chassis. But in that case you **don't** want to ground locally. Luckily, this extra step isn't a huge deal, so lots of fine amps ground the cathodes locally.
Why do some stick 1% 1R resistors in the middle of the ground?
What @BigDaddy23 said -- simple way to measure plate current (as cathode current) which is the simplest way to measure bias on a fixed bias amp. The neat trick is that if I=V/R, and R=1, then I=V, and so you just measure mV with your meter (say 22mV) and use the same number (as 22mA) in bias calculation.
Related question: If you ground locally, do you include pin 1 and 2, as shown in the Fender layout?
No. Why? Well, pin 2 is obvious; back then, Fender ran 6.3 V in through one heater pin and sent it to ground through the 'other' heater pin. Pin 2 ain't ground these days! But pin 1? That dates back to when pin 1 on these tubes was tied to the metal shell -- it stopped being ground when glass bottles came in. You could tie pin 1 into your local ground (you can read how it has obscure theoretic advantages) but it's really not needed, not tidy, and *definitely not* OK if you use pin 1 as an unanchored pin to mount a grid stopper or screen resistor on the socket.
^^ you sir, are a great communicator.
Thank you @King Fan - You have a way of demystifying Amp Building Magic!
Some of those questions I had regarding the grounding came from comparing Leo’s Layouts and Schematic with other builders. Knowing there are different was of skinning the cat, I’m trying to Find my way. It’s a balancing act sometimes between “old Fender Charts have errors” and “more modern approach” by smart articulate builders. If I don’t understand what the modern builder is doing, I know it’s safe to default to Leo. At least the amp will work, lol.
I’m learning how genius Leo really was. If there is a better, modern way I want to know the “why” behind it. I’m Thankful for you and many other Shock Brothers who take the time and effort to explain things and answer questions.
So my voltages this morning with 120vac at the wall and a NOS 5Y3 are:
The range on my 10K bias pot with a 27k resistor gives me -39V to -30V, which yields 353v to 326v to my JJ 6V6 plates.
Are these the correct ranges?
What am I shooting for?
Do I need to change the resistor?
Rob Robinette has an excellent article on setting bias.
Your education is coming along nicely!
I would like to stress... at this point in your education you really need to take the time and follow along with the schematic. It is kinda like a roadmap. Have the schematic in front of you and just like a map look at the schematic and find that part of the circuit on the layout and in the amp. I can appreciate you may want to hurry and hear this new build. Take the time and use this opportunity to advance your schematic reading ability.
Currently you could not see, from looking at a layout, the difference of pulling bias voltage from the PT bias tap or pulling from the PT HT. You now know the resistor values of the voltage divider that makes the bias voltage circuit are quite different depending on where the initial bias voltage is obtained. Compare what you now know with the schematic. Find other amp schematics for both types of initial bias voltage and compare.
There are different ways to create the negative bias circuit. Some have a different filtering capacitor arrangement. Some use a variable resistor. Some use a pot as a voltage divider. It is much easier to decipher the differences by comparing schematics.
You have pondered Heater CT elevation. With the Cathode Bias Circuit like the 5E3 and 5F1 it is convenient to find roughly +20DCV on pin 8 of the power tube. May as well elevate the CT because it is so easy to do. With a fixed bias Harvard there is no convenient location.
But wait a minute... your Harvard just created a low voltage source for the negative bias. It didn't need a low voltage tap from the PT. Can positive low voltage from the PT be created in a similar manner? The answer is yes, but I'll let you look up the *how to elevate a heater CT with fixed bias amps* on the internet. I think the Valve Wizard writes of such things.
Do you need a elevated CT in the Harvard? Is it worth the effort?
Well the schematic says -31volts but that figure doesn't mean a lot. We want to go by the tube bias %PD. Shooting for some range on either side of the pot will allow for different tubes to fit somewhere on the range. You may want to try paralleling a 470k resistor with the 27k with alligator clips. That would make an equivalent of ~25k resistance. (Instead of the 470k resistor you could clip in a 1M (or 500k) variable resistor (pot) to dial in a desired resistance for that 27k resistor. Of course replace the pot with a resistor or replace the 27k entirely with the desired total resistance.)
I’m going to be doing a little more study on Biasing to see if I can answer my own questions. So much to contemplate! In the meantime, I want to report that the Amp sounds fantastic. I am plugging it into my 5F2A cabinet which is Deluxe size with a 12 Eminence C-Rex Speaker. Nice wooly clean to just-over-the-edge break up. This is a fantastic Classic Rock (Stones, Petty, etc.) and Blues Amp. Reminds me more of a Tweed Bassman than a Tweed Deluxe. Tactile response is amazing and using the 1/10th Power Switch gives me everything I wanted From a Champ but warmer, chewier, more organic.
I would say it is my current favorite amp, but isn’t every build our current favorite amp when we have finished, lol. Here are some of the finished Gut Shots...