Princeton Build Questions

[Willabe]

I don't think I want to remove those things as the design replicates the existence of those aspects of the original design so that the overall character of the amp feels more like an amp w/ the trem/reverb... but turned off...

Yes, that's right. He's trying to keep the circuit drain  load without the verb/trem the same. So gain is the same.

Edit; drain added load,... So gain is the same. Willabe

[nickv]

Great thread! I just built this exact amp design about a month ago. I wish I had known some of the info here presented here about the grounding schemes and B+ nodes. It still sounds great and is plenty quiet, but I'm sure it could be better.

I have a question about the voltage dropping resistors in this design that I think also might be relevant to BobL.

In his description of the amp, Rob Rob notes that "The three voltage dropping resistors were increased to compensate for the deletion of the four reverb and tremolo triodes. Since we have four less triodes pulling current through the voltage dropping resistors they all need to be increased to get the same voltage drop across them. The first 1k dropping resistor is increased from 1k to 1.8k. The second and third are increased from 18k to 39k."

For my power supply, I'm using a Hammond 290AX (325-0-325) with a JJ 5AR4 rectifier tube, and I'm getting 445V B+ (which I realize might be a little hot - I have a 5U4GB on the way, which the Hammond PT can accommodate with its 5V 3A tap). 6V6s are biased at ~22mA (right around 70% dissipation). My build has both 12AX7s (new Russian-made Tung-Sols) powered from the D node.

With this setup, I'm getting:
A node (B+1): 445V
B node (B+2): 436V
C node (B+3): 317V
D node (B+4): 201V

Seems like the 12AX7s are pulling ~3mA, so I'm getting a little too much voltage drop with a ~116V drop across each of the 39k dropping resistors. This leaves me with 201V at the D node, rather than the target 240V that the Fender schematic lists. This also leaves me with 154V at the PI plate, and ~125V at the V1a/V1b and V2a preamp plates.

Would it be worth it to reduce the value of the power dropping resistors a bit to get a D node (B+4) voltage that is closer to 240V?

Thanks!

[Willabe]

445dcv on the 6V6 plates is high. I see your getting a 5U4, hope that brings it down to 420dcv or a little lower.

This leaves me with 201V at the D node, rather than the target 240V that the Fender schematic lists. This also leaves me with 154V at the PI plate, and ~125V at the V1a/V1b and V2a preamp plates.

Would it be worth it to reduce the value of the power dropping resistors a bit to get a D node (B+4) voltage that is closer to 240V?

Yeah, 125dcv @ V1a/b is a little low. Fender schemo shows 190dcv. And when you lower the B+ string with that 5U4, it's gonna be even lower.

You can play with the dropping R's for the preamps tube B+ cap nodes. But wait until you get the 6V6 plate dcv where you want it, then dial in the B+ for V1 and V2/PI.

[labb]

FWIW: I built this amp for a buddy of mine in 2020. Voltages are as follows: 6V6 plates 452 VDC, 6V6 screens 448 VDC, PI plate 185 VDC, 3 rd (V2A) plate 158 Volt DC, Pre amp 2 (V1B) plate 155 VDC, Pre amp 1 V1a) plate 155 VDC. We have discussed lowering the Voltages but he doesn't want to touch it. Been playing it for 4 years this July. He is in a band that plays and practices regularly. Running JJ power tubes.

[Willabe]

Voltages are as follows: 6V6 plates 452 VDC, 6V6 screens 448 VDC, .......Running JJ power tubes.

That's why your getting away with 452dcv on the 6V6 plates, can't do that with real 6V6's. Not long anyways. 

[labb]

Yep. JJ's or EH. I don't use anything else.

[nickv]

Thanks! Hopefully the 5U4GB will get it in the sweet spot.

Out of curiosity, assuming labb used the same value of dropping resistors that I did, what would cause the differences in the PI/preamp plate voltages between our amps? Just variation in current draw from the 12AX7s?

I had thought that you assume ~1ma current draw for each preamp when designing, which makes the math make sense for the original 39k dropping resistors x2. From the original schematic, going from 400V at B node (B+2) to 240V at D node (B+4) is 160V drop, which assuming a combined 2ma current draw from both of the 12AX7s, equals out to 80k of resistance (or close enough to the 39k x2 in the schematic).

[labb]

I dropped the 39k down to 33k

[Willabe]

Yep. JJ's or EH.

Your missing the point, JJ 6V6's are made differently and IIRC are the modern made 6V6's that can take higher dcv.

I would not run EH 6V6's that hi, no way. 

[labb]

EH advertises that the 6v6EH performs flawlessly up to 475 volts.

[Willabe]

EH advertises that the 6v6EH performs flawlessly up to 475 volts.

Ok, new to me. 

[AlNewman]

I think I'm ok on space.


I don't think I want to remove those things as the design replicates the existence of those aspects of the original design so that the overall character of the amp feels more like an amp w/ the trem/reverb... but turned off...

You had mentioned space was an issue in your 1st post.

I'm not good with layouts, so I just assumed all the spare parts weren't connected to anything.  Can you draw a schematic based on your layout?

Looks like you're building an ab763 normal channel.  All of the spare parts included in your layout are based on on ab763 with a recovery stage off of the reverb.  Your design doesn't include that recovery stage.  If all those parts are connected, I think you may be disappointed.

[nickv]

Don't want to hijack the thread, but there's been so much good information on grounding schemes for this specific circuit.

I've already built the amp, but I'm trying to incorporate some of the grounding layout described in this thread into my existing amp. Mainly just route the HT CT to the node A filter cap negative terminal, and move the node D filter cap ground to the pre-amp ground bus bar.

I've attached my original layout, and then the updated grounding layout.

Does this look right?

[pdf64]

It looks like 6V6 cathode current will be flowing via the input socket shield?
If so I suspect it's not good practice.
I suggest to keep current loops contained in wires, especially the heavier current ones, eg add wire from 6V6 cathodes to the reservoir cap -ve turret.

[mresistor]

JJ6V6S is a large plate pentode that can handle 500vdc and is rated up to 70ma per tube.  I can attest to that. I accidently biased a 6V6 amp thinking that it was 6L6s and then played the amp for a while. 
No harm done.  I think they are constructed more like a 5881.

[Willabe]

JJ6V6S is a large plate pentode that can handle 500vdc and is rated up to 70ma per tube.  I can attest to that. I accidently biased a 6V6 amp thinking that it was 6L6s and then played the amp for a while. 
No harm done.  I think they are constructed more like a 5881.

Yeah, those are not 6V6 specs, not even close. 

[BobL]

I finally have this thing wired up and after checking my work I'll be ready to see how it sounds... I did have one question:

The layout has a ground wire coming from the lamp to the common power amp side ground - the previous circuits I had in here did not include this, and I'm wondering if it is necessary and even desireable to do this if I don't have the lamp socket itself isolated? It seems to me like it could be a potential source of ground loop noise since it is a self grounding socket?

[BobL]

Ok, here's where I'm at with this build. Currently using a GZ34 and the 660 windings on the PT, but I could try different rectifiers to try and get closer to the expected starting voltages.


I have checked all wiring and connections multiple times, and I'm feeling pretty confident I have it wired as per my layout (minus that ground from the lamp).


I get sound through the amp, and it's musical, but it is *very* quiet... with the volume on 10 it is low talking volume.


Here are my voltages with the amp idled and controls all the way down:
A: 439 (I guess this is my OT center tap value as well)
B: 430
C: 249
D: 188


I'm not sure if the larger than expected voltage drop at C and D has something to do with us using C instead of giving all the filtering to D and attaching all board components to that on the regular schematic?


V1:
1: 114.3
2: (-2.4mv)
3: 1.18
6: 113.8
7: 0
8: 1.086


V2: (these seem problematic, I think?)
1: 60.3
2: (-32)
3: (-32)
6: 190
7: 7.79
8: 2


V3:
3: 437
4: 429
5: (-52.4)
6: 428
8: 0 - I have a 1ohm 1 watt to ground on this pin because this was how I was planning to bias, but obviously I'm missing something here


V4:
3: 440
4: 432
5: (-53.3)
6: 432
8: 0


So... I've got zero drop from the OT center tap to my plates... is this wrong?
CT to Plate resistance:
V4: 199 ohms (I'm reading this right, right? .199 with MM automatically setting itself to K ohms? I just wanna cover my bases here)
V3: 185 ohms


Are those really high? Something seems pretty wrong here...

Not entirely sure what to look at next, so any guidance is greatly appreciated! Attaching current layout for reference

[stratomaster]

Ok, here's where I'm at with this build. Currently using a GZ34 and the 660 windings on the PT, but I could try different rectifiers to try and get closer to the expected starting voltages.

Don't worry about this for the time being. You have bigger problems.

I have checked all wiring and connections multiple times, and I'm feeling pretty confident I have it wired as per my layout (minus that ground from the lamp).

Be prepared to continue checking. Correctly wired amps rarely misbehave in this way.  Upload a clear photo and we can check with you.

I get sound through the amp, and it's musical, but it is *very* quiet... with the volume on 10 it is low talking volume.
This is very wrong.  While I don't find Princetons to be loud (and I judge those that use them with attenuators) it should be moving a good bit of air.

Here are my voltages with the amp idled and controls all the way down:
A: 439 (I guess this is my OT center tap value as well)
B: 430
C: 249
D: 188


I'm not sure if the larger than expected voltage drop at C and D has something to do with us using C instead of giving all the filtering to D and attaching all board components to that on the regular schematic?
Ohm's law. Voltage drop equals current times resistance. If you increased the current draw at that node without changing the resistance your voltage drop across the resistor will increase. I don't understand what meant by "giving all the filtering to D ..."

V1:
1: 114.3
2: (-2.4mv)
3: 1.18
6: 113.8
7: 0
8: 1.086


V2: (these seem problematic, I think?)
1: 60.3
2: (-32)
3: (-32)
6: 190
7: 7.79
8: 2
Pins 1-3 represent your third gain stage (which you could probably eliminate entirely if you got rid of the 3.3M/470k divider that proceeds it, but that's a different conversation).  These numbers should be similar in ratio to your V1 triodes.  If you indeed have negative voltages here then you wired something very wrong.  The only way I can imagine these voltages is if you somehow wired the bias supply to this triode.
And how this amp makes any sounds if you did that is beyond me.  This is where we'll need clear photos. 

V3:
3: 437
4: 429
5: (-52.4)
6: 428
8: 0 - I have a 1ohm 1 watt to ground on this pin because this was how I was planning to bias, but obviously I'm missing something here
Manually set your meter to mV.
Your auto ranging is probably not picking up the few mV there.  Plus your bias is set very cold at -52v. The tube is barely conducting meaning you're probably pretty close to 0v at the cathode. 

V4:
3: 440
4: 432
5: (-53.3)
6: 432
8: 0


So... I've got zero drop from the OT center tap to my plates... is this wrong?
again you tubes are barely conducting with the bias set that cold. No current= no voltage drop. ∆V=IR

CT to Plate resistance:
V4: 199 ohms (I'm reading this right, right? .199 with MM automatically setting itself to K ohms? I just wanna cover my bases here)
Manually set the range if you know where it'll be. But yes, 0.199*1000 = 199
V3: 185 ohms


Are those really high? Something seems pretty wrong here...

Not entirely sure what to look at next, so any guidance is greatly appreciated! Attaching current layout for reference

[BobL]

Thanks for the feedback! Attaching photos. Totally probable/possible that even with rechecking I'm missing something. It looks a bit messy as things are very cramped in this chassis and I was building into some existing stuff. I have tried to take care with keeping wires away from each other even if things are a bit of spaghetti in some ways... let me know if I need to capture anything differently... and thanks in advance.

[stratomaster]

What are the voltages here?

There's something very wrong with your 3rd stage, so this is a good place to start.

[BobL]

1. 250
2. 249
3. 60.5
4. 249

[mresistor]

There appears to be a kink or unneeded sharp bend in this wire also..   it appears to also have a burn mark on it..  and it's over a 2-3 watt resistor which isn't good.

[stratomaster]

Ok we've confirmed the plate voltage measurement is indeed real.  Check the value of that resistor between 3 and 4. It's doing heavy lifting.

How about these (blue)?  If the voltage from earlier is real, that cathode bypass cap is likely toast.

[BobL]

Ok we've confirmed the plate voltage measurement is indeed real.  Check the value of that resistor between 3 and 4. It's doing heavy lifting.

How about these (blue)?  If the voltage from earlier is real, that cathode bypass cap is likely toast.

5. 23.3
6. (-31.4)
7. (-3.4mV)
8. (-34.5)
9. (-35.3)


I did have a bad solder joint at 9 earlier, FWIW.

I replaced the kinked wire just to see... wasn't that kinked, but looked that way from the angle and a little melt, so I replaced it.


The cathode bypass is the 25uf w/ the 1.5k resistor from 6 - 8?  I pulled these from a previous build and had to remove other value resistors from them, so they might've gotten a bit toasty... I would not be shocked if there was something up with one of those...

[AlNewman]

I'm subscribed.  Just to find out why the ground reference is + 35V.

[BobL]

I found some radial 22uF 50v caps, so I replaced that 25uF axial cap and 1.5K resistor... didn't really affect anything.

[stratomaster]

Check for continuity between the two points with the yellow arrows. If there isn't any add the jumper shown in green. This is over by your bias pot, for orientation.

I think you left your bias supply with no ground reference then referenced your screen node to the floating bias.

[stratomaster]

I found some radial 22uF 50v caps, so I replaced that 25uF axial cap and 1.5K resistor... didn't really affect anything.

It wouldn't. I was just worried that if your ground was actually at 0v then the positive side of the cap was effectively reversed and exposed a substantial negative voltage.  Since it wasn't, and the cathode side of the cap was more positive than your "ground" side, then the cap was just fine. 

[BobL]

Check for continuity between the two points with the yellow arrows. If there isn't any add the jumper shown in green. This is over by your bias pot, for orientation.

I think you left your bias supply with no ground reference then referenced your screen node to the floating bias.

Aw crap... yeah. I was looking at how to get all the ground attachments on there and saw I had that in the bin and assumed (whoops) that it was bridged... but it isn't.


Wired that... turned the amp on... as soon as it warmed up it started *screaming* with the volume all the way down. High frequency feedback style screeching + some lower frequency buzz/moan... it uh, solved the volume problem, I guess? Painfully loud. 


Would the next step be to pull the preamp tubes and test voltages w/o those in there?

[AlNewman]

Wired that... turned the amp on... as soon as it warmed up it started *screaming* with the volume all the way down. High frequency feedback style screeching + some lower frequency buzz/moan... it uh, solved the volume problem, I guess? Painfully loud. 

That's just Murphy's Law.
Try switching the OT leads between the plates of your power tubes.

...and if the noise goes away, then check voltages...

[BobL]

Wired that... turned the amp on... as soon as it warmed up it started *screaming* with the volume all the way down. High frequency feedback style screeching + some lower frequency buzz/moan... it uh, solved the volume problem, I guess? Painfully loud. 

That's just Murphy's Law.
Try switching the OT leads between the plates of your power tubes.

...and if the noise goes away, then check voltages...

Ah, yep, I knew I'd run into this before. That fixed the noise, though at idle the amp seems kinda noisy. Power tubes seem smoking hot...


Voltages now:


A: 394
B: 364
C: 240
D: 181.5


V1:
1: 111.8
2: -2.6 mV
3: 1.126
6: 110.8
7: 0
8: 1.048



V2:
1: 160
2: unstable? noisy
3: 1.3
6: 192
7: 17.22
8: 49.7


V3:
3: 376
4: 363
5: -17.28
6: 365
8: 1 mV



V4:
3: 379
4: 364
5: -17.3
6: 365
8: .6 mV

[stratomaster]

Set the bias for -34v at pin 5.  It's way too hot now.

-34v is the schematic value for a Princeton Reverb.  This value probably isn't the best value for your amp, but it'll be safe while you continue to work out the bugs. Come back and set preferred bias after you've sorted the amp out.

V2A might be pretty well trashed from the midwired ground.  Wouldn't be a bad idea to swap it out and label it as suspect.

[BobL]

Set the bias for -34v at pin 5.  It's way too hot now.

-34v is the schematic value for a Princeton Reverb.  This value probably isn't the best value for your amp, but it'll be safe while you continue to work out the bugs. Come back at set preferred bias after you've sorted the amp out.

V2A might be pretty well trashed from the midwired ground.  Wouldn't be a bad idea to swap it out and label it as subject.

Yeah, I guess I need to swap out the bias resistor. I don't have enough range to take that much off with my pot. I'm a little hazy on how the math would work there and what value I should try... I think I have a 22k there now?

[stratomaster]

Set the bias for -34v at pin 5.  It's way too hot now.

-34v is the schematic value for a Princeton Reverb.  This value probably isn't the best value for your amp, but it'll be safe while you continue to work out the bugs. Come back at set preferred bias after you've sorted the amp out.

V2A might be pretty well trashed from the midwired ground.  Wouldn't be a bad idea to swap it out and label it as subject.

Yeah, I guess I need to swap out the bias resistor. I don't have enough range to take that much off with my pot. I'm a little hazy on how the math would work there and what value I should try... I think I have a 22k there now?

It's just a resistive voltage divider. The math is easy to look up and sort out for yourself.  Draw out what you have currently.  It'll help add clarity.

Edit: I noticed you have a fixed value resistor across the adjustment divider. Where did you get this layout?  Lift one end of the 22k resistor attached to the bias filter cap at the very end of your board (or just clip it out) and check your range again.  Take the tubes out for checking range.

Usually it's one or the other, a fixed resistance to ground or a voltage divider.  Not both, and not in parallel with each other.

[BobL]

Set the bias for -34v at pin 5.  It's way too hot now.

-34v is the schematic value for a Princeton Reverb.  This value probably isn't the best value for your amp, but it'll be safe while you continue to work out the bugs. Come back at set preferred bias after you've sorted the amp out.

V2A might be pretty well trashed from the midwired ground.  Wouldn't be a bad idea to swap it out and label it as subject.

Yeah, I guess I need to swap out the bias resistor. I don't have enough range to take that much off with my pot. I'm a little hazy on how the math would work there and what value I should try... I think I have a 22k there now?

It's just a resistive voltage divider. The math is easy to look up and sort out for yourself.  Draw out what you have currently.  It'll help add clarity.

Edit: I noticed you have a fixed value resistor across the adjustment divider. Where did you get this layout?  Lift one end of the 22k resistor attached to the bias filter cap at the very end of your board (or just clip it out) and check your range again.  Take the tubes out for checking range.

Usually it's one or the other, a fixed resistance to ground or a voltage divider.  Not both, and not in parallel with each other.

That was in the Princeton bias board layout that I grabbed from Robinette... I built the full AA1164 circuit once previously, and did something similar, though I think I had to update the value of that resistor...


EDIT: Oh, but then looking at the full layout for his PR w/o reverb/trem, I see that it's set the way you are suggesting, without that 22K.


My bias cap is 50uF, not 25uF, fwiw.

[BobL]

Ok, clipped that resistor and it hit the bias right at -35 w/o touching anything, so plugged in a guitar and... we have ourselves an amp!


I am getting a low volume oscillation with some small static bursts that would be nice to dial out if I can... not sure where that is coming from.  This is something I clumsily referred to before, but the changes that we made in this thread to the filtering, basically putting V2 on node C, rather than having everything coming from node D is one thing I wonder about with this noise, and if changing those connections to originate from D might reduce it?

I tried two different V2 tubes since you wondered about the state of that tube after this, but there was no discernable difference between the two tubes.


Attaching a clip of the noise, and a clip of the amp played (at about 1 on the dial since it's so early, and it seems like a pretty loud amp)... some nice sparkle on those highs. I'll try to look at fine tuning the bias here in a bit. Thanks for the help!


EDIT: I guess I should also chopstick some wires around and see if anything there is contributing...


https://www.boblefevremusic.com/temp/Princeton2Build/princeton_test.m4a


https://www.boblefevremusic.com/temp/Princeton2Build/amp_noise.m4a

[Willabe]

I am getting a low volume oscillation with some small static bursts that would be nice to dial out if I can... not sure where that is coming from.  This is something I clumsily referred to before, but the changes that we made in this thread to the filtering, basically putting V2 on node C, rather than having everything coming from node D is one thing I wonder about with this noise, and if changing those connections to originate from D might reduce it?

You can change it, but I don't believe that's the problem. And this amp has -FB loop. So the PI should be grounded with the power amp not the preamp. You move the PI back to the preamp, node D, then it should be grounded with the preamp. But the -FB comes from the OT secondary. It'll probably work but, it's not the best practice.     It's also better decoupling from each stage, each 12A_7 has it's own filter cap.

Merlin is very clear on this and so is Kevin O'Connor/ TUT books.

Can we see pictures of the inside of the chassis? 

[BobL]

I am getting a low volume oscillation with some small static bursts that would be nice to dial out if I can... not sure where that is coming from.  This is something I clumsily referred to before, but the changes that we made in this thread to the filtering, basically putting V2 on node C, rather than having everything coming from node D is one thing I wonder about with this noise, and if changing those connections to originate from D might reduce it?

You can change it, but I don't believe that's the problem. And this amp has -FB loop. So the PI should be grounded with the power amp not the preamp. You move the PI back to the preamp, node D, then it should be grounded with the preamp. But the -FB comes from the OT secondary. It'll probably work but, it's not the best practice.     It's also better decoupling from each stage, each 12A_7 has it's own filter cap.

Merlin is very clear on this and so is Kevin O'Connor/ TUT books.

Can we see pictures of the inside of the chassis?

Pics are a few posts up!

[BobL]

Ok, so a couple updates.


The V2 Grid wire on pin 2 seems to be the thing that is very sensitive to noise... it is making that pretty long run. Would it make sense to try and use a shielded wire here?


Looking at the bias, and help me figure out if I'm doing this right, please...


Plate voltage is 427V. OT CT is 435V. Plate to CT resistance is 186 ohms. Current is 43 mA? Is that right?  If I plug those into the robinette calculator, it thinks I'm way hot at 133% of tube rating... my pin 5 measurement is -34.9V.


Am I missing something or doing something weird here, or should I be cutting my bias by half to get down to 22-ish, which the calculator says is 70%?

[Willabe]

I don't think that's bad at all. Is that a Tele with single coil PUP's? (And a B bender?) Got a guitar with HB's? And the chassis is open, put a metal shield over the chassis opening, cookie sheet, and see if that helps.   

The V2 Grid wire on pin 2 seems to be the thing that is very sensitive to noise... it is making that pretty long run. Would it make sense to try and use a shielded wire here?

V2 pin 2 is the input grid for the PI. So maybe yes. But......

Start from the input jack, V1 pin 2 is a long run, and it has the lowest signal in the amp. So it's the easiest to corrupt from air born signal/noise. I would try shielded wire on V1 1st, see how that does, then if needed use shielded for the next grid, V1 pin 7. Keep going until you knock out what you find offensive.   

Looking at the bias, and help me figure out if I'm doing this right, please...

Plate voltage is 427V. OT CT is 435V. Plate to CT resistance is 186 ohms. Current is 43 mA? Is that right?  If I plug those into the robinette calculator, it thinks I'm way hot at 133% of tube rating... my pin 5 measurement is -34.9V.

Am I missing something or doing something weird here, or should I be cutting my bias by half to get down to 22-ish, which the calculator says is 70%?

Don't you have 1 ohm R's on the 6V6 K's? What are they reading? Get that down to ~20mA each, you can always go higher.

I have no idea what your doing here. You don't need plate to CT ohms for anything here.

Rob Rob's bias calculator all you have to do is pick the tube your using and put in the plate dcv, then hit the calculate button. Your looking at the class AB Fixed Bias. I would try 60% dissipation 1st.

[BobL]

The V2 Grid wire on pin 2 seems to be the thing that is very sensitive to noise... it is making that pretty long run. Would it make sense to try and use a shielded wire here?

Yes, and from the input jack to V1 is a long run, lowest signal in the amp. I would try shielded wire on V1 1st, see how that does, then if needed use shielded for the next grid. 

Looking at the bias, and help me figure out if I'm doing this right, please...

Plate voltage is 427V. OT CT is 435V. Plate to CT resistance is 186 ohms. Current is 43 mA? Is that right?  If I plug those into the robinette calculator, it thinks I'm way hot at 133% of tube rating... my pin 5 measurement is -34.9V.

Am I missing something or doing something weird here, or should I be cutting my bias by half to get down to 22-ish, which the calculator says is 70%?

I have no idea what your doing here. You don't need plate to CT ohms for anything here.

Rob Rob's bias calculator all you have to do is pick the tube your using and put in the plate dcv, then hit the calculate button. Your looking at the class AB Fixed Bias. I would try 60% dissipation 1st.

I am trying to understand Rob Rob's bias instructions, but I'm clearly missing something. I tried to follow his instructions for biasing, which said to measure the plate voltage, the OT center tap voltage, and then with the amp off/discharged/unplugged the CT to plate resistance in ohms. Divide the voltage drop by the resistance to get the current... but it s coming in way high. So I'm not understanding something here.


Can you help me understand the physical steps of this process better? I put in 418V as my plate voltage and it says 60% is 20.1 DC milliamps. If the method I described above (from here: https://robrobinette.com/How_to_Bias_a_Tube_Amp.htm - "Measure Bias With the Output Transformer Resistance Method") is not what I should be doing to calculate my bias current, then I don't think I understand what I am supposed to be doing to calculate the current. I think in the past I have used the 1ohm 1watt resistor from pin 8 to ground to calculate bias, but I get no reading in this amp that way, so... I feel like I have all the pieces but one, and that's the one that is going to make this process make sense...

[Willabe]

I am trying to understand Rob Rob's bias instructions, but I'm clearly missing something.... Can you help me understand the physical steps of this process better? ...."Measure Bias With the Output Transformer Resistance Method"

I wouldn't use that method. Makes my head hurt just reading it. 

I have well over 50 tube amp books and have been hanging around here for almost 15 years.

I don't think I have ever seen anyone figure bias settings like that. Never. I have no idea why he put that in on bias.   I'm sure it works but you don't need it. Use the calculator, easy.

I think in the past I have used the 1ohm 1watt resistor from pin 8 to ground to calculate bias, but I get no reading in this amp that way, so...

What's your meter set to? Should be set to dcv, lowest setting. You probably set your meter to mA. That's why it wont work/give a reading. We install 1 ohm R's so we get; 1 mdcv = 1mA

Yes, use the 1 ohm power tube K R's, that's why you put them in.

Pick the tube type, put in the plate dcv, hit calculate button. Read what it says for Class AB Fixed Bias and set the bias using the adjustable bias pot with your meter hooked up with gator clip leads across the 1 ohm 6V6 K R. Set the meter for mDCV.

[BobL]

Ok, good. It was making my head hurt too.  What you described is what I was planning to do all along, and it was only when that wasn't working that I started looking at other things.

My multimeter just has one DCV setting and it auto calibrates depending on signal... but I get essentially nothing in mV across that resistor (reading 3mV) and the value doesn't change as I turn the bias pot.  The plate voltage does change when I turn the pot, and the pin 5 reading changes as well.


I keep feeling like I'm missing something here...


This is what my MM looks like when reading across cathode-> ground resistor, and here is also a pic of the resistor itself. I'm not sure what I'm doing wrong.  I do feel like the last time I built a Princeton (like 6 years ago, so a bit hazy) that I may have had similar issues, and people just had me go off of that pin 5 reading and the amp worked happily forever...

[BobL]

Oh, and I figured out the oscillating noise was due to proximity to a wireless router. Moved it away from that and no issues! :)

[Willabe]

1st! Take out the power tubes and leave them out until we get this straightened out.

Now, gator clip the meters black/ground lead to the chassis ground and the red lead gator clip to pin 5 out put tube socket. Turn the bias pot full CW then full CCW, post both of those -dcv. 

[Willabe]

It looks like there's a yellow wire soldered to pin 8 on the power tube socket along with the 1 ohm K R? Looks like it goes back over to the other 6V6 socket's K?

Nothing should be there except the 1 ohm K R.

[BobL]

Okay. I will need to get back to this later in the day, but will post those when I'm able. Thanks!

[Willabe]

My multimeter just has one DCV setting and it auto calibrates depending on signal...

Auto ranging is fine.

[BobL]

It looks like there's a yellow wire soldered to pin 8 on the power tube socket along with the 1 ohm K R? Looks like it goes back over to the other 6V6 socket's K?

Nothing should be there except the 1 ohm K R.

It's this black wire in the layout...


But now that I look at that in the context of this conversation I guess the resistor going to ground is taking the place of that wire now, isn't it, and that's why I get no reading...