Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: BobL on July 30, 2024, 11:04:12 am
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I am rebuilding an existing amp into a Princeton w/o Reverb or Trem (based on the Robinette layout) and really trying to use things I already have where possible.
I will be using Deluxe transformers, so if there are any things I need to consider because of that, let me know - I think I have the voltage figured out w/ the combo of rectifier and tap used from the PT.
The chassis I will be building into is a low powered tweed twin chassis, so space is at a bit of a premium, and my first question is about the bias board, and if I could move the bias board to the end of the main board as this helps me with mounting, or if I need to plan to keep that bias board away from the main board due to noise, etc?
Here is what I was picturing doing:
(https://www.boblefevremusic.com/temp/Princeton2Build/bias_placement.jpg)
Second question:
I am not 100% sure if I can wire the center tap the way I have it layed out here - I am using 4x 22uF caps vs. a single can cap since this is what I have, and it doesn't involve punching holes in the chassis. So first question - is this location for the B+ node caps ok, and second question, is the center tap wiring correct here (going to the common ground at 2), or should I be going to the negative end of node A directly (at 3)?
(https://www.boblefevremusic.com/temp/Princeton2Build/center_tap_connection.jpg)
I do this infrequently enough that I have to relearn things every time, so any guidance is much appreciated!
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1. Sure it's ok to mount the bias ckt on the mainboard. Hoffman does it for his Princeton Rev.
2. The 5.3 vac center tap, the HV center tap and the main filter cap negative ends should all tie to the power supply grounding point. I think that was what you were asking. Those connections are shown on Robs layout as well the + lead of the bias cap, power tube pins 1 and 8 and the bias resistor ground are shown attached there.. Your filter caps will be mounted on the board instead of a cap can, but it is wired the same.
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Use a separate grounding point near the input jacks for the preamp grounding.
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Here's how Hoffman does it on his Princeton Rev .. note you can remote the bias adj pot or use a trimmer like shown here.
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1. Sure it's ok to mount the bias ckt on the mainboard. Hoffman does it for his Princeton Rev.
2. The 5.3 vac center tap, the HV center tap and the main filter cap negative ends should all tie to the power supply grounding point. I think that was what you were asking. Those connections are shown on Robs layout as well the + lead of the bias cap, power tube pins 1 and 8 and the bias resistor ground are shown attached there.. Your filter caps will be mounted on the board instead of a cap can, but it is wired the same.
Cool - I thought what I had done was okay, but wanted to make sure I wasn't missing something.
Use a separate grounding point near the input jacks for the preamp grounding.
Yeah, that's my little black wire going off to the right of the board! :)
Thanks, guys!
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About ready to get started on this build (once my new reading glasses arrive... when did I get old?)
I was hoping someone might be willing to check my work on this overall layout and let me know if there are any things I should consider doing differently?
EDIT: Oh, the bias pot I have currently mounted in this amp (I am rebuilding from existing) is 10k - guessing I can use a 22k or so resistor in place of the 10k coming off of the bias pot and that should get me into the range I need to bias?
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I built that amp a while back. Just a note: expect B+ to be pretty high. Mine ran about 450. I used JJ power tubes. Guy has been playing it for about two years now with no issues. I have some marked up schematics for records. Will share what little knowledge I have. Good sounding amp.
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Look at the B+ filter cap C node, it's not feeding anything.
Your grounding needs to be altered. The last B+ filter cap, D node, should be grounded with the preamp input jack ground.
Node C, because you have -FB, should be grounded with the PI.
The PT B+ CT should be grounded directly to the 1st filter cap's ground only. Then run a wire from there back to the chassis power amp ground.
Read this on grounding if you haven't yet.
https://www.valvewizard.co.uk/Grounding.html (https://www.valvewizard.co.uk/Grounding.html)
And you don't have the 6V6 power tube grid stopper R's, 1K5 and screen grid R's, 470R on the 6V6 tube sockets.
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I just looked at the Rob Rob Princeton lite, no verb/trem, layout.
He's using a 4 section filter cap can. Your using 4 separate filter caps, which is better for grounding, separate ground for each cap.
This is why your groundings off. I would not use Rob Rob's grounding, I would use Merlin's grounding.
And you have a 5Y3 for your rectifier tube, B+ dcv will be low. You are going to need a GZ34/5AR7 or a 5U4, depending on your PT.
I'm really surprised Rob Rob has no screen grid and grid 1 stoppers.
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You have a faux CT for the heaters, 2x100R and you have the 6.3acv wind CT grounded.
You don't use both, 1 or the other.
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Look at the B+ filter cap C node, it's not feeding anything.
I think this matches the layout on Rob's site? Is this just to get the right voltage drop at D?
Your grounding needs to be altered. The last B+ filter cap, D node, should be grounded with the preamp input jack ground.
Can this tie to the preamp ground buss just to the right, or does it need to go all the way to the lug by the input on its own?
Node C, because you have -FB, should be grounded with the PI.
The PT B+ CT should be grounded directly to the 1st filter cap's ground only. Then run a wire from there back to the chassis power amp ground.
I think I follow... center tap to ground side of node A, wire from A to the lug.
Less clear on node C grounding with the PI and where that would be?
And you don't have the 6V6 power tube grid stopper R's, 1K5 and screen grid R's, 470R on the 6V6 tube sockets.
I was just following what was in the original schematic here, but more below...
I just looked at the Rob Rob Princeton lite, no verb/trem, layout.He's using a 4 section filter cap can. Your using 4 separate filter caps, which is better for grounding, separate ground for each cap.This is why your groundings off. I would not use Rob Rob's grounding, I would use Merlin's grounding.
Are you saying run individual wires for the ground of each node? Seems that was alluded to in the previous post...
And you have a 5Y3 for your rectifier tube, B+ dcv will be low. You are going to need a GZ34/5AR7 or a 5U4, depending on your PT.
I'm really surprised Rob Rob has no screen grid and grid 1 stoppers.
On one of his mod layouts he has the grid stop and screen resistors... I can add those easily enough.
I had it in my head that I wanted the 392VDC that this classictone deluxe PT would give me with a 5Y3, but I think you are right... I want as much voltage as I can get from this, which would be a 5AR and give me 406VDC? Is this close enough? Attaching info on the PT...
RE: the 2x 100R and the CT being grounded... I think the red/yellow is actually my high voltage CT to ground mislabled in my layout.
I do appear to have a 6.3V CT on this PT as well, green/yellow wire... so I could just run that to ground and omit the 100Rs.
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I just looked at the Rob Rob Princeton lite, no verb/trem, layout.
He's using a 4 section filter cap can. Your using 4 separate filter caps, which is better for grounding, separate ground for each cap.
This is why your grounding's off. I would not use Rob Rob's grounding, I would use Merlin's grounding.
You need to read the Merlin link I posted on grounding. Then it will be much easier to answer your grounding questions.
I can write it all out, but Merlin already did that. And you'll learn more if you read it all for yourself and then ask questions. You want what Merlin is calling a buss ground with local stars.
The way you have the grounding will probably be noisy/buzzy. Merlins grounding will quite the amp way down. New build way worth doing it now then having to go back and try and re-wire the grounds to quite the amp down. You won't like that.
https://www.valvewizard.co.uk/Grounding.html (https://www.valvewizard.co.uk/Grounding.html)
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I just looked at the Rob Rob Princeton lite, no verb/trem, layout.He's using a 4 section filter cap can. Your using 4 separate filter caps, which is better for grounding, separate ground for each cap.This is why your groundings off. I would not use Rob Rob's grounding, I would use Merlin's grounding.
Are you saying run individual wires for the ground of each node? Seems that was alluded to in the previous post...
Yes.
You ground each B+ filter cap with the circuit grounds from the circuit that filter cap feeds. They call it a local star. Each B+ filter cap has it's own star ground. Then you string/daisy chain the individual stars to with a wire. So local stars bussed together with a buss wire.
Example 1st preamp tube grounds, tubes input grid R - the grid leak R, the cathode (K) R, the input jack ground and the B+ filter cap ground that feeds that tube.
Your going to end up with 2 chassis grounds for the ac circuit. 1. power amp ground, and 2. a preamp ground.
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Willabe is right about grounding. You'll build better amps with just a few basic principles. The valvewizard.com has a good brief tutorial. It seemed really daunting when I first looked at it. But it will really pay off in reduced noise and reliability all through the amp. Read some and ask some questions and you'll have the gist of it in no time. :icon_biggrin:
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More on grounding. Go to EL34 world and search for chassis grounding. Find Doug’s grounding drawing. Ground as shown except use a pre amp grounding buss grounded on a lug at the input jack instead of soldering to back of pots. All others ground including B+ cap ground on a lug near B+ cap. Pre amp caps ground on pre amp bus. Ground 120 volt mains on a lug near where the power cord enters the chassis.
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Thanks all -
Ok, so... I read the Merlin grounding document... I understood... a good chunk of it. :D
Some basic principles I am taking away from this:
I should try to have the grounds be based on the section of the amp the current is looping through, but be cautious about doing this when it would result in long ground wires, which could also be noisy. Node A is my reservoir, and B is a power amp smoothing cap, feeding the power tubes, and should share a common ground on the power amp end of the amp. Node D is feeding V2, and that's why it should share a ground with the preamp circuitry that node is feeding, so should go to my preamp ground bus. More on Node C below...
Questions:
I'm not 100% clear on why there is nothing attached to the positive end of Node C? Is that to replicate the filtering scheme from the version of this amp that has trem/reverb and needs more nodes to feed those?
The Wizard text talks about creating local stars, then tying those ALL to the same ground point connection, but it seems more common to see a ground for the power amp near the power amp section, and a second ground at the input end for the preamp section (which I'm understanding to include Node D now). I'm unclear what the best way to actually do this in the context of the amp would be? Is the disconnect I have there that I need to think of the preamp and power amp as their own separate amps w/ their own separate 'single' ground?
The chassis grounding on el34world says do NOT do star grounding, so seems at odds with the wizard article... and also shows the two ground points, but that may be answered by the question above...
My understanding is that I would run a separate wire to my power amp ground from nodes A, B, (and C?), and then ground node D to the preamp ground - should this be a short wire that runs from the negative end of D to the preamp bus, or a longer wire that goes to the preamp grounding lug (also connected to the bus)?
I also wanted to go back to this comment: "Node C, because you have -FB, should be grounded with the PI." - perhaps this means that nodes C and D should be tied together and then go to the preamp ground, or possibly go individually to the preamp ground?
Thanks for helping me figure this out. My brains are leaking out just a little bit...
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Your getting it, but not all yet. I'll explain as best I can so you have enough to look for in Merlin's drawings. Look how he wires each node.
Lets get this out of the way 1st;
Fender use a 'random chassis' ground system. Fender used a random ground system. Many most Fender amps are pretty quite. But with a new build or a complete re-build want a 'wired' ground system.
And with that 'wired' ground system, there's a difference between 1.wired star ground system, 2. wired bussed local star system. We want 2, bussed local star with Doug's split buss. (See below.)
With a star grounding system, each B+ filter cap has all the grounds for the circuit it feeds grounded with that single B+ filter caps ground wire. It makes a single local ground star. Then they hook up a wire to that local ground star and do the same with all the other local ground stars and run all the ground star leads together to 1 single chassis ground. That's a chassis star ground.
With a bussed local star system it's similar, but each local ground star is hooked up to a common buss wire, 1 at a time, in the order of the circuit, end to end in the chassis. Then the buss wire is grounded to the chassis at 1 end and 1 end only. If you ground it at both ends you get a ground loop and it will buzz. Merlin with the local star buss system only uses 1 chassis ground, at the input jack.
Doug is saying to use the buss grounding, not the single star ground. Doug then splits the buss into 2 sections; 1. Power amp 2. Preamp and has each grounded to it's own chassis ground. He does this so the current in the power amp ground can't modulate (cause noise/buzz) in the lesser current tubes.
What we're trying to do is reduce chassis grounds so there's not all these different ground paths going through the chassis crossing each other/disturbing each other. The less chassis grounds the better.
The power amp has the most current in the amp because of the power tubes. This is why we isolate the reservoir filter cap and PT B+ CT from everything else, because this makes it THE noisiest ground in the amp. To deal with this we wire the PT CT directly to the reservoir filter cap's - lead, then run a wire from there to the chassis ground. That way it can loop all it want's without disturbing any other tube. See Merlin's drawings. See Merlin's drawing.
Power tube as an example; A tube pulls current up from ground through the tubes K (cathode) goes out through the tubes plate, then back through the OT's CT back to the B+ reservoir filter cap's + lead, back down through that cap, out it's - lead back to the power tubes K. And round and round it goes. All tube circuits we work with work this way.
My understanding is that I would run a separate wire to my power amp ground from nodes A, B, (and C?), and then ground node D to the preamp ground - should this be a short wire that runs from the negative end of D to the preamp bus, or a longer wire that goes to the preamp grounding lug (also connected to the bus)?
Look at Merlin's drawings. Look how he wires each node.
Local ground star preamp tube example; 1st preamp tube grounds; tubes input grid R - the grid leak R, the cathode (K) R, the input jack ground and the B+ filter cap ground that feeds that tube.
You ground each B+ filter cap with the circuit grounds from the circuit that filter cap feeds. They call it a local star. Each B+ filter cap has it's own local star ground. Then you string/daisy chain the individual stars to with a wire. So local stars bussed together with a buss wire.
Your going to end up with 2 chassis grounds for the ac circuit. 1. power amp ground, and 2. a preamp ground.
(The 3rd wire on the power cord, usually green safety ground wire, has it's own chassis connection. Nothing else gets connected with it. It's not part of the ac circuit grounds.)
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I should try to have the grounds be based on the section of the amp the current is looping through, but be cautious about doing this when it would result in long ground wires, which could also be noisy.
Fender and Rob Rob (copying Fender) have all the filter caps at the PT end of the chassis. Some Fenders have them in a 'dog house' on the back of the chassis near the PT. Rob Rob went with a multi filter cap can close to the PT.
But with a 'wired' local star buss ground system, you move the filter caps to be located close to the circuit they feed. Keeps the filter cap wires short. Doug does this too.
If you do this you can move some of the resistors/caps on the board towards the PT end and closer to the tubes and pots they are wired too, ie, shorter hook up wires. That's a very good thing.
Look at Merlin's drawings. Look how he wires each node.
I'm not 100% clear on why there is nothing attached to the positive end of Node C? Is that to replicate the filtering scheme from the version of this amp that has trem/reverb and needs more nodes to feed those?
Yes, pretty much.
The B+ filter caps; 1. Reservoir filter cap, 2. Screen grid, 3. Phase inverter (PI) 4. Preamp.
Look at Merlin's drawings. Look how he wires each of these nodes.
I also wanted to go back to this comment: "Node C, because you have -FB, should be grounded with the PI." - perhaps this means that nodes C and D should be tied together and then go to the preamp ground, or possibly go individually to the preamp ground?
You ground the PI with the power amp IF the amp has a negative feed back loop (-FB). This is because the -FB loop comes from the OT secondary/output. If no -FB, you ground the PI with the preamp. (See Merlins drawings.)
Look at Merlin's drawings. Look how he wires this node.
I need to say; I know Rob Rob is very knowledgeable on tube amps. With that; I don't know why Rob Rob put the PI on the D node, it's a lower dcv. I'd move the PI to the C node, higher dcv, cleaner, more output and less on the D node. You don't want too many tubes on 1 filter cap, causes stability problems.
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I had it in my head that I wanted the 392VDC that this classictone deluxe PT would give me with a 5Y3, but I think you are right... I want as much voltage as I can get from this, which would be a 5AR and give me 406VDC? Is this close enough? Attaching info on the PT...
The Fender schematic shows 215dcv on the 6V6 plates.
Edit: Should be 415dcv, Willabe
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Ok, thank you for taking the time... that was a lot of information...
Doug is saying to use the buss grounding, not the single star ground. Doug then splits the buss into 2 sections; 1. Power amp 2. Preamp and has each grounded to it's own chassis ground. He does this so the current in the power amp ground can't modulate (cause noise/buzz) in the lesser current tubes.
I'm confused by some of this... it is almost clicking but I can't quite put it together, which is frustrating.
A = reservoir
B = power amp filter
C = nothing right now, but you are suggesting the PI
D = all of the preamp?
Seems the key things are, have a bus, and connect that bus to a single chassis point (or one for each section, but make sure that the connections to the buss occur in the order of the circuit flow (is that the right way to phrase that?) rather than just the physical location of the components on the board. Additionally, you are saying that the best case would be to move the filter caps on the board to be as close as possible to the part of the circuit/tube they are related to...
The less chassis grounds the better.
Got it.
Power tube as an example; A tube pulls current up from ground through the tubes K (cathode) goes out through the tubes plate, then back through the OT's CT back to the B+ reservoir filter cap's + lead, back down through that cap, out it's - lead back to the power tubes K. And round and round it goes. All tube circuits we work with work this way.
Look at Merlin's drawings. Look how he wires each node.
Your power tube example is a good representation of what makes this difficult for me to wrap my head around. I do this infrequently (the last thing I built was in 2021), and as a result, I'm trying to learn enough to accomplish what I want in terms of a useable amp, but because my exposure to the circuits and technology is infrequent, it's a lot of information to try and take in at once each time I do it. Anyway... also my layout is upside down from Merlin, which confuses me. :)
You ground each B+ filter cap with the circuit grounds from the circuit that filter cap feeds. They call it a local star. Each B+ filter cap has it's own local star ground. Then you string/daisy chain the individual stars to with a wire. So local stars bussed together with a buss wire.
So I want a local star for each section of the amp, and then a wire from each local star to the bus, and the bus to ground.
Your going to end up with 2 chassis grounds for the ac circuit. 1. power amp ground, and 2. a preamp ground.
So what I need to determine is which grounds belong together on each local star, and then what order in which to attach those to the bus, and then run the bus to the appropriate chassis grounds for preamp/power amp?
...green safety ground wire, has it's own chassis connection....
Yep, already done.
Ok, so if I take a crack at what I *think* is correct here, it looks like the attached... what'd I get wrong? I am sketchy about the ground bus between A and B, and the location of the ground wire for D as well... also a lot of the rest of it. :D
Changes:
1. CT -> - lead of A
2. A & B bussed and then to ground.
3. PI using C instead of D (I think), and C ground sharing the ground for the PI... I think.
4. Preamp using D, and moving the D ground to the bus where the V2A connects?
It seems you are suggesting that the filter cap for D could move down the board in between the PI section and the V2A section, which would also move the PI stuff closer to the power tube connections - am I understanding that correctly? The CT for the 6.3V winding can go to ground as shown?
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I had it in my head that I wanted the 392VDC that this classictone deluxe PT would give me with a 5Y3, but I think you are right... I want as much voltage as I can get from this, which would be a 5AR and give me 406VDC? Is this close enough? Attaching info on the PT...
The Fender schematic shows 215dcv on the 6V6 plates.
I'm not seeing that? I'm seeing 410v on the plates?
Chart in my PT documentation is attached... the fender schematic looks like 340v AC with the rectifier putting out 420V with a 5U4GB and feeding that to node A...
My PT seems to only be able to put out a max of 406v B+ from the rectifier with a 5AR4?
Have I completely lost the plot here? I might be way confused...
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The Fender schematic shows 215dcv on the 6V6 plates.
I'm not seeing that? I'm seeing 410v on the plates?
Very sorry, yes, 410dcv or 415dcv depending on which Fender BF 6V6 amp you look at.
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Your transformer voltages are specified for 120 ma load (Deluxe Reverb). A Princeton Reverb amp without reverb and trem will draw much less than 120 ma so the amp voltages will run higher than predicted.
You may like that or you may want to use the r/w taps to correct for over voltage.
Heater voltage will run higher than expected too so check that after you get the tubes in but before you start playing:-)
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Your transformer voltages are specified for 120 ma load (Deluxe Reverb). A Princeton Reverb amp without reverb and trem will draw much less than 120 ma so the amp voltages will run higher than predicted.
You may like that or you may want to use the r/w taps to correct for over voltage.
Heater voltage will run higher than expected too so check that after you get the tubes in but before you start playing:-)
If the heaters are too high, how would I handle that? I would imagine I'd need to shunt some tiny amount of voltage to ground?
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If the heaters are too high, how would I handle that? I would imagine I'd need to shunt some tiny amount of voltage to ground?
That's a different wind on the PT. It may be fine, have to wait and see.
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Why did you change the Rob Rob design from a multi cap can to using 4 single e-caps?
I don't like multi cap cans because of grounding, but that's just me. Many guys use them and their builds are quite.
Then you could just go with most of his layout. Just move the -bias over to the main board like you did.
And if your B+ is too high you can try using a 5U4. That PT has a 3A supply at 5acv.
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Why did you change the Rob Rob design from a multi cap can to using 4 single e-caps?
I don't like multi cap cans because of grounding, but that's just me. Many guys use them and their builds are quite.
Then you could just go with most of his layout. Just move the -bias over to the main board like you did.
And if your B+ is too high you can try using a 5U4. That PT has a 3A supply at 5acv.
I am building the amp into a chassis that I originally built a 6G3 into, didn't like, and then built a version of a Benson Monarch into - I am trying to use existing components I have (so I already have these 4 single caps) and avoid cutting new holes in here while I have things like all the tube sockets etc wired already. Hence the transformers I'm using as well.
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Ok, thank you for taking the time... that was a lot of information...
:icon_biggrin:
A = reservoir
B = power amp filter
C = nothing right now, but you are suggesting the PI
D = all of the preamp?
Close, but no; A reservoir is 1 of the power amp filters, so is B, screen grid, screen grid is in the power tube, so it gets grounded with the power tube ground.
A= reservoir, B= screen grid, C= PI, D=preamp.
On most amps the power tubes have 2 filter caps, 1 for the PT CT that feeds the tubes plate and the other for the power tubes screen grid. On this amp it has a split load PI = V2b. Rob Rob is calling V2a preamp. But it's the driver for the split load PI. I'd call it part of the PI. A split load PI has no gain so it needs a driver before it.
So; A = PT CT going to power tube plates, B = power tube screens, C = split load PI and it's driver = V2b/V2a, D = preamp V1a/V1b. That's how I'd do it. (PT CT = power transformer center tap.)
Seems the key things are, have a bus, and connect that bus to a single chassis point (or one for each section, but make sure that the connections to the buss occur in the order of the circuit flow (is that the right way to phrase that?) rather than just the physical location of the components on the board.
Yes.
But almost always when you do the layout, "the connections to the buss will occur in the order of the circuit flow rather than just the physical location of the components on the board." They will/should be the same. And yes you phrased it right.
Additionally, you are saying that the best case would be to move the filter caps on the board to be as close as possible to the part of the circuit/tube they are related to...
Yes, and for node C and node D, I would use a small modern radial e cap, instead of an axial lead cap. They are much smaller and less $$, so they work great for this. The current demand for the PI/driver V2 tube and V1 preamp tube is very low. So these small caps are more than enough. You need them at 450dcv and either 20uF, for V2, but you could go as low as 10uF or 8uF for V1. Some guys like a lower uF cap for the preamp, they say it's more touch sensitive.
Your power tube example is a good representation of what makes this difficult for me to wrap my head around.
I was trying to get you to see the 'current loop' in the tube circuit. It's just to get the grounding reasoning. That 'current loop' is how we determine what's wired into the local ground star.
You ground each B+ filter cap with the circuit grounds from the circuit that filter cap feeds. They call it a local star. Each B+ filter cap has it's own local star ground. Then you string/daisy chain the individual stars to with a wire. So local stars bussed together with a buss wire.
So I want a local star for each section of the amp, and then a wire from each local star to the bus, and the bus to ground.
Yes, very close. But not for each section of the amp, but for each B+ filter cap node. That will mostly line up with each section of the amp most of the time.
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So what I need to determine is which grounds belong together on each local star, and then what order in which to attach those to the bus, and then run the bus to the appropriate chassis grounds for preamp/power amp?
Yes. :icon_biggrin:
Like I said before the order is already there in the chassis/board layout. That is, if it's a good layout. :think1: And Rob Rob's is fine.
Ok, so if I take a crack at what I *think* is correct here, it looks like the attached... what'd I get wrong? I am sketchy about the ground bus between A and B, and the location of the ground wire for D as well... also a lot of the rest of it. :D
Changes:
1. CT -> - lead of A
2. A & B bussed and then to ground.
3. PI using C instead of D (I think), and C ground sharing the ground for the PI... I think.
4. Preamp using D, and moving the D ground to the bus where the V2A connects?
Close.
1. Yes, but..... see below.
2. Yes.
3. Yes.
4. No.
On 1. You moved the PT CT to the correct place, the A node filter cap - lead. But, move the chassis power amp black ground wire from the A node filter cap eyelet to the B node (screen grid) filter cap eyelet. That's all, just move that 1 black wire.
On 4. You have the C node B+ connected to the right place now, V2b, but you still have it's negative lead wired to the preamp ground buss. Disconnect V2b from the preamp ground buss at the junction of the 47R NFB Tail resistor and the 56K PI Tail Load R. But leave the black ground wire from that junction of 47R NFB Tail resistor and the 56K PI Tail Load R going to the - lead of the screen grid node C cap. Now connect node C's ground lead eyelet to node B's ground eyelet. It should now be; B+ node A, node B and node C negative lead eyelets are daisy chained together, with the black power amp ground wire connected to node B's ground lead eyelet, and run over to the power amp chassis ground.
Next; string that same B+ C node wire over to V2a, which is the PI's driver. Now you have to;
1. Disconnect the PI's driver ground wire (V2a) from the preamp ground buss at the junction of the 1K5 R and the K 25uF/25v bypass e-cap.
2. Disconnect the black D node ground wire at the same junction of the 1K5 R and the K 25uF/25v bypass e-cap.
3. Now run that same V2a ground junction wire over to the 47R NFB Tail resistor and the 56K PI Tail Load R. So that V2a and V2b are daisy chained/grounded together.
4. Lastly, connect node D's - ground lead to the preamp buss.
Now let me see the layout drawing after you do all this.
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It seems you are suggesting that the filter cap for D could move down the board in between the PI section and the V2A section, which would also move the PI stuff closer to the power tube connections - am I understanding that correctly?
Close, I would move the C node cap between or close to the V2a/b R's/C's and move the D node cap close to the V1a/b R's/C's. I'd use the small radial caps I already wrote you about.
Amp lead dress; Try to keep the grid lead wires short, it's better to have to plate wires longer if needed. Once the signal goes through the coupling/blocking cap it is now a grid wire. Grid wires are very sensitive to picking up noise and bug a boo problems. So you try and move the coupling cap closer to the next tube stage it feeds if possible, and leave the plate wires long. K wires are usually not a problem.
This way you often don't need to use shielded wire for the grids.
The power tube plate wires in that layout are pretty long. At the very least you should twist them tightly together. The PI is probably putting out enough signal to be able to do this and not need shielded wire, but you still might need it.
If you move the C and D node caps, then you could turn the V2b circuit 90 degrees counter clockwise. That would shorten those power tube grid wires a lot.
The CT for the 6.3V winding can go to ground as shown?
Yes.
For clarity;
Power amp chassis ground;
1. Power tube caps, PT CT cap
2. Screen grid cap.
3. Heater CT ground.
4. -grid bias circuit ground.
5. If amp has -FB loop, then the PI ground.
And some say;
6. Power tube K's.
7. Speaker jack/OT ground lead. You have to use an isolated speaker jack to do this. OT has most of the current running through it. Akin amps talks about doing this.
And all the control pot grounds should go to the preamp buss, not to the back of the pots. Input jacks should be isolated from the chassis too.
Do you really need all this? Probably no. Part of it depends on how much gain the amp has, more can be more noise problems.
But the more you do these things as good amp build layout design, the more you nip things in the bud from being a problem.
I do a lot of these things on any new build or complete re-build. Many/most guys don't.
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Why did you change the Rob Rob design from a multi cap can to using 4 single e-caps?
I am building the amp into a chassis that I originally built a 6G3 into, didn't like, and then built a version of a Benson Monarch into - I am trying to use existing components I have (so I already have these 4 single caps) and avoid cutting new holes in here while I have things like all the tube sockets etc wired already. Hence the transformers I'm using as well.
Ok, that's fine, makes sense.
I think you can finish this. But, and not to discourage you;
That it can be fairly easy to go with a proven amp circuit and layout. And yet many guys still have problems doing that. :dontknow:
But if you don't do that, change things, then you need a whole lot more understanding of amps and grounding and lead dress and layout, etc.
When I've built an amp, I've spent 10 to 20 times more time on the computer working on the circuit changes, mods, grounding, chassis, eyelet/turret board, layout for all these things, with all the parts in my hands so I can measure them and lay them in/on the chassis and eyelet board, etc., than actually soldering it up. Not an exaggeration. But I've never had a problem at start up, not that I can remember. (Only 1 time, I had a bad preamp tube socket.)
I understand guys just want to get to soldering and get the amp built, but, then they often have problems with things, sometimes a lot of things. And they then have to spend hours trying to get the build to work. :think1:
That is what I try to avoid with spending the time I need to get it right on paper 1st.
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Oh for sure - that's what I'm hopefully doing a decent job of now - I appreciate all the guidance to try to get this right to begin with! I wish I had the time to mess with this stuff more often than I do so that the knowledge would stick better - it's interesting, and I always learn new things as I do each build (I think this is the... 7th one I've built?)
I really don't need additional hobbies between 2 bands, riding a ton of mountain bikes, skiing, and raising kids... but I am interested in most things. :)
I will work through your previous posts and update here... thanks again!
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Now let me see the layout drawing after you do all this.
Attached. I'm understanding what is going on now... hopefully I understood what you were telling me to do there.
Why does the ground for A/B/C attach at B and not at A? You had previously mentioned isolating A since it was the nosiest part, and I'm wondering how that plays into this setup.
If this looks good I will look at how those C/D nodes could be moved.
The power tube plate wires in that layout are pretty long. At the very least you should twist them tightly together. The PI is probably putting out enough signal to be able to do this and not need shielded wire, but you still might need it.
Yeah, the layout isn't really representative of the space of things in this chassis, which is pretty tight... are you talking about the control grid wires from pins 5/1 that go to the coupling caps? Looking at what is currently wired in this amp they are probably 5 - 6" long. The plates are connected to the OT on pin 3, right?
If you move the C and D node caps, then you could turn the V2b circuit 90 degrees counter clockwise. That would shorten those power tube grid wires a lot.
Ok, seems like that's what you are saying here... if I moved D to the other side of that PI/V2B group and rotated the components...
Would it make sense, layout wise, just speaking positionally on the board, to go V2B stuff -> Node C ->V2A stuff -> Node D -> right side of the board as is?
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Grounds are now wired right. :icon_biggrin:
Why does the ground for A/B/C attach at B and not at A? You had previously mentioned isolating A since it was the nosiest part, and I'm wondering how that plays into this setup.
Reply #16;
Power tube as an example; A tube pulls current up from ground through the tubes K (cathode) goes out through the tubes plate, then back through the OT's CT back to the B+ reservoir filter cap's + lead, back down through that cap, out it's - lead back to the power tubes K. And round and round it goes. All tube circuits we work with work this way.
The power amp has the most current in the amp because of the power tubes. This is why we isolate the reservoir filter cap and PT B+ CT from everything else, because this makes it THE noisiest ground in the amp. To deal with this we wire the PT CT directly to the reservoir filter cap's - lead, then run a wire from there to the chassis ground. That way it can loop all it want's without disturbing any other tube.
See Merlin's drawings. Go back and read what Merlin says about it.
The power tube plate wires in that layout are pretty long. At the very least you should twist them tightly together. The PI is probably putting out enough signal to be able to do this and not need shielded wire, but you still might need it.
The long orange and red wires from the PI coupling caps to the 1k5 grid stopper R.
If you move the C and D node caps, then you could turn the V2b circuit 90 degrees counter clockwise. That would shorten those power tube grid wires a lot.
Ok, seems like that's what you are saying here... if I moved D to the other side of that PI/V2B group and rotated the components...
Would it make sense, layout wise, just speaking positionally on the board, to go V2B stuff -> Node C ->V2A stuff -> Node D -> right side of the board as is?
Yes. Look at Merlins drawing of a small preamp tube and it's filter caps grounding.
Look at how Doug puts a big filter cap on a eyelet/turret board. I like using the small radial e caps for this.
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Ok... how do we feel about something like this?
I have some radial caps, but they are all 50v, and I'm also not sure what benefit they would have on this particular board... seems like using the caps I have would work fine?
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Yes that's it. :icon_biggrin:
Now save that, and see if you can play around with moving the C node over too.
Look at some of Dougs layouts, he hangs the filter cap off the back of the board, it's another option.
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Yes that's it.
Now save that, and see if you can play around with moving the C node over too.
Ok... I'm a little less confident about this one in two spots:
1. The grounding around the - lead of C... I think I have it functionally equivalent now?
2. The leads from the + lead of C to the board... do I need to daisy chain like this, or should I have two leads directly from the + node to the two spots on the board C feeds, or is this the right way?
3. confirming that the V1B K ground is correct to go to the power amp ground and not the preamp ground? I know you had said to do that, but I'm not totally clear on why, as that is fed by D, yeah?
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Node C was never used in the original PR. Hoffman followed the original schematic and didn't use it either. The only thing node C does is provide a lot more filtering (ripple reduction) for node D. With that in mind maybe just follow the original design but ground the node C cap with the node D cap since you have that option. Remember, the original used a 4-pole cap can and did not have the option of separate grounds. Fender sold thousands and thousands of successful clones have been built using that original circuit.
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1. is right, 2. is right.
3. is wrong.
The ground wire in the #1 green box connected to the 1K R going to the #3 green box move the 1K R end to the preamp buss, leave the #3 green box connection alone. So #3 green box eyelet V1b 1K5 K R and 25uF/25v bypass cap are now connected to the preamp buss. Next disconnect the D node filter caps ground lead from the preamp buss and move it to the #3 green box eyelet with the V1b 1K5 K R and 25uF/25v bypass cap. Should now be D node filter cap ground lead, V1b 1K5 K R and 25uF/25v bypass cap all 3 in #3 green box eyelet going over to the preamp buss.
Now disconnect the C node filter cap - lead from the PI 1K R, gray wire in the layout drawing.
V2b pin 8 K wire move from the .1 uF filter cap over to the PI 1K R. This will shorten that wire a little.
Last thing, I know I said to turn the PI 90 degrees counter clockwise, but;
Since you have moved both c/d filter caps you now have moved the PI over towards the power tubes enough to put the PI caps/R's back like they were. The grid wires will be shorter now. And the orange and red wires will be about the same length.
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And fix your V2b pin 6 wire in your layout, it got bumped a little. :laugh:
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I think this is what you described - I also had a number of V2 leads that needed to be lined up better...
I guess the C/D filtering difference here vs. the stock princeton is a setup... I don't want to change the character of the circuit too much, so that'd be something to consider... if I did use D for everything ala the original circuit, I think I'd need to fix that preamp ground coming off of the V1B K wire to go to the preamp bus as you described above for this one?
I have build the stock princeton reverb circuit before, and definitely one of my favorites. Hrmm.
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Looks good.
Except, flip/flop the orange and red power tube grid wires at the top ends of the .1uF coupling caps, they got moved. That way the 2 grid wires will be about the same length (and the -FB should be correct). And when you run them over to the power tube sockets, tightly twist them together for noise cancellation.
But I forgot a ground. Sorry.
Ground the junction of the -FB 47 ohm tail R, it's labeled NFB Tail, and the 56K R, labeled PI Tail load, to the C node filter cap - lead eyelet.
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I wouldn't put both 12AX7's on the D node cap and nothing on the C node cap.
If you have a small choke from the old build, use it instead of the 1k8/2w B+ dropping R.
And, shorten that last B+ dropping R by moving it to the left side of the last 39K/2w dropping R and the 100K V2a plate load R.
It's buggin me. :icon_biggrin:
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Like so?
I do not have an extra choke. I guess I could run the node C/D wires to the board on top of the board and if I want to try running everything off of D after hearing it they'd be easier to get to... but curious how this will sound.
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C node ground is good now.
Orange/red wires are good.
And, shorten that last B+ dropping R by moving it to the left side of the last 39K/2w dropping R and the 100K V2a plate load R.
Sorry I meant to say shorten that last B+ dcv feed wire. Put that 39K B+ dropping R back in between node B and node C like it was.
Now take out the wire that goes from the PI 56K plate load R to the V2a plate load 100K R. Now run a wire from the V2a 100K plate load R eyelet over to the left side of the last 39K/2w dropping R eyelet that's directly below the .02uF/630v coupling cap.
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Oh, you mean this... that makes more sense... this was sorta what I was asking yesterday - I wasn't sure if that was ok or not...
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Yes! :happy1:
That's it. :bravo1:
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Guess I should build it, eh?
Again, all the guidance has been very much appreciated, and it has been an interesting process. Thanks you!
I have a lot of tear down to do before I can actually start putting this together. Slightly daunting... solder sucker is gonna earn its keep.
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The grounding looks really good.
Before building, you could also remove most of the components that say reverb in front of them. It's making my OCD go off, and I'm not OCD.
If space is an issue you can also place a lot of the tone stack components across the pots, instead of on the board.
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The grounding looks really good.
Before building, you could also remove most of the components that say reverb in front of them. It's making my OCD go off, and I'm not OCD.
If space is an issue you can also place a lot of the tone stack components across the pots, instead of on the board.
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...
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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
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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!
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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.
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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.
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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. :laugh:
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Yep. JJ's or EH. I don't use anything else.
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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).
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I dropped the 39k down to 33k
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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.
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EH advertises that the 6v6EH performs flawlessly up to 475 volts.
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EH advertises that the 6v6EH performs flawlessly up to 475 volts.
Ok, new to me. :dontknow:
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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.
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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?
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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.
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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.
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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. :laugh:
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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?
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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
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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
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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.
(http://www.boblefevremusic.com/temp/Princeton2Build/princeton_build_1.jpg)
(http://www.boblefevremusic.com/temp/Princeton2Build/princeton_build_2.jpg)
(http://www.boblefevremusic.com/temp/Princeton2Build/princeton_build_3.jpg)
(http://www.boblefevremusic.com/temp/Princeton2Build/princeton_build_4.jpg)
(http://www.boblefevremusic.com/temp/Princeton2Build/princeton_build_5.jpg)
(http://www.boblefevremusic.com/temp/Princeton2Build/princeton_build_6.jpg)
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What are the voltages here?
There's something very wrong with your 3rd stage, so this is a good place to start.
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1. 250
2. 249
3. 60.5
4. 249
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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.
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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.
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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...
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I'm subscribed. Just to find out why the ground reference is + 35V.
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I found some radial 22uF 50v caps, so I replaced that 25uF axial cap and 1.5K resistor... didn't really affect anything.
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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.
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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.
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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. :laugh:
Would the next step be to pull the preamp tubes and test voltages w/o those in there?
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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. :laugh:
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...
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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. :laugh:
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
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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.
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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?
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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.
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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.
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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/princeton_test.m4a)
https://www.boblefevremusic.com/temp/Princeton2Build/amp_noise.m4a (https://www.boblefevremusic.com/temp/Princeton2Build/amp_noise.m4a)
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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. :think1: 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?
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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. :think1: 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!
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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%?
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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.
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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...
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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. :laugh:
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. :dontknow: 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.
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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...
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Oh, and I figured out the oscillating noise was due to proximity to a wireless router. Moved it away from that and no issues! :)
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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.
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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.
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Okay. I will need to get back to this later in the day, but will post those when I'm able. Thanks!
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My multimeter just has one DCV setting and it auto calibrates depending on signal...
Auto ranging is fine.
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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...
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It's this black wire in the layout...
That would be the power tube K ground connection.
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...
Yep, snip it out. Fixed. :icon_biggrin:
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You forgot to remove the two grounding wires from pin 8 of the power tubes when you added the 1Ω bias sense resistors. You must remove two wires to completely fix. Now your meter should read correct mV on both pins 8.
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SEL49, yes we found that.
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...
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You forgot to remove the two grounding wires from pin 8 of the power tubes when you added the 1Ω bias sense resistors. You must remove two wires to completely fix. Now your meter should read correct mV on both pins 8.
SEL49, yes we found that.
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...
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SEL49, yes we found that.
You found the yellow wire(shown as black on the layout) that connects v4 pin 8 to V3 pin 8. I don't recall anyone ever mentioning the black wire that connects V4 pin 8 to chassis ground.
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SEL49, Yes it was supposed to be in the last quote I posted in the reply to you, but the layout picture didn't come with it.
He says "it's the black wire in the layout...." Then he says "I guess the resistor going to ground is taking the place of that wire now."
It's in reply #99.
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Spent the day riding mountain bikes from 10K ft down through incredible changing aspens. Hell of a day.
Got home, snipped out those wires, and set my bias. Easy peasy. I have to have the pot all the way down to get to just under 70%, but since this amp will get used at low volume at home, I'm not stressed. I guess I could change out the resistor if I ever need to get lower w/ different tubes, but stuff is sounding pretty good, so I think I'm all set.
Thanks, everyone! Very much appreciated. Learned some stuff. Made some mistakes... learned from those too.