Dropping resistors and voltage to preamp tubes.

[Megachunk]

Hi all, long time.

Tubenit has been helping me with a layout (see attached) I just got together today, (35 watt 2 6L6 jazz amp, deluxe-like with more power, no choke) and I've made some adjustments to the cap setup, opting for a cap board instead of cans this time. Using the same resistors, I seem to have too high voltage on the preamp tubes. The resistors I have in now are 4.7K between A&B, a 33K between B&C and a 56K between C&D. They seem high, yet still the voltages seem too high. They are
 
on V1:
pin 1: 208V
pin2: 0
pin3: 1.594V
pin6:205V
pin8:1.66V

V2:
pin1:352V
pin2:0v
pin3:3.88V
pin6:302V
pin7:24V
pin8:62V


Any help would be much appreciated.I'm wondering if I need to adjust all resistors in the B+ rail or if my issue is somewhere else. Thanks in advance. While testing, I realized I had a bad output tube (read voltage, but no current - swapped sockets to verify) - Would this affect voltages later in the power rail?
Megachunk

[phsyconoodler]

I would bump the first dropping resistor up to 10k and measure the voltages right at the filter caps.Then you can decide where to gom from there.
  Nothing is written in stone.Experiment and see what it does.

[jjasilli]

Would this [dead power tube] affect voltages later in the power rail?

Yes, I think so.  The power tubes draw current even at idle.  Without current draw, you're reading B+ voltage with no load, or only partial load.  That would result in higher B+ voltage than when under proper load. 

The preamp tubes draw almost no current, so it takes a large resistor to get a voltage drop in the preamp (Ohm's Law).  However hi preamp voltage, within reason, may be desirable in a jazz amp, because it gives more headroom and less break-up or overdrive. 

[Megachunk]

Cool, I'll buy that.

Think I'll wait to get new power tubes to re-evaluate the resistor values in the rail. I think the 10K in between a&b is a great idea and would be a good place to start. Thanks for that.

Less breakup and overdrive - exactly.

Thanks a bunch, guys.
Mike G
Megachunk

[RicharD]

>I realized I had a bad output tube

This has to be fixed first to really hear what the amp sounds like.  With 1 output not working, you're only hearing 1/2 the waveform.  I'm betting it sounds brittle & harsh.  Your preamp voltages don't really look out of line to me.

>Less breakup and overdrive - exactly.
Ah..... I think you should play with your cathode bypass caps and maybe not worry about the DC voltages.  Right now you have all 3 preamp stages bypassed.  I'd leave stage 1 bypassed with a 2.2uF (I love that value for V1).  I'd unbypass the 2nd stage altogether.  The 3rd stage you have a .68uF bypass.  This is gonna cut a lot of bottom end and for a Jazz amp, this may not be what you want.  I'd try a 15 or 22uF cap there.

-Richard

[Megachunk]

Yeah, it buzzed and hummed a bit. Not a good tone.
I do have a 25/25 in the second stage. This was what I had from the first build. Maybe that's what keeps the bottom end in there. That and the C100 Jensen speaker we have in there. But, by "unbypass" you mean remove the cap and resistor and run straight wire, correct?
Thanks for the tips, I can use them all!
Mega.

[RicharD]

>But, by "unbypass" you mean remove the cap and resistor and run straight wire, correct?

Nooooo!  Leave the cathode resistor  (gotta have that) but remove the bypass cap.  For experimentation, disconnect just the ground of the cap.  Grab a clip lead so you can A/B bypassed vs unbypassed. 

[tommytornado]

You mentioned this was going to be used as a jazz amp and that you wanted the amp to be as clean as possible.  I was going to suggest using a large grid stopper on the grid of the PI (which cleans up split tail PI's very nicely) and while looking at your layout, something didn't look right.  Someone correct me if I'm wrong....??  But...  It looks like in an effort to save space on the board, your PI is wired up incorrectly.  On a cathodyne PI the output of pin 8 comes directly off the cathode and the drawing has it going through a 1.5K resistor.

PI grid resistor comes after the .022 coupling cap you have between pins 1 & 7 on V2.  You can use anything between 56K and 1M.  I tried this on a 5F4 I just finished (using a 56K, I didn't want to clean it up a ton) and was pleased with the results.  I'm also seeing folks on other boards doing this to their cathodyne that are very happy with how it cleaned up the harsh distortion when the amp is cranked.   

[Fresh_Start]

FWIW 2.0 to 2.5ma seems to be a reasonable assumption for current consumption by a 12AX7 in my experience.  That should help you guestimate dropping resistor values on the power rail.  Remember that current is cumulative as you move toward the PT.  Of course, measuring the current  (volts/resistance) in your amp as is always works better than a guestimate.

As far as a "jazz amp" goes, my build for a guy who plays fairly clean, classic jazz had one channel with the first triode set up as follows:
220K plate resistor
2700 ohm cathode resistor
2.2uf cathode bypass cap

2nd stage (after tone stack) was 100K/1.5K||6.8uf

3rd stage 100K/1.5K||25uf

With 2.2K on the cathode of the second stage like you've got, the equivalent bypass cap would be about 4.7uf.  Of course, you're using a very different tone stack so you might want no bypass at all on that second stage like Butter suggested or something higher than 4.7uf.

If your goal is BF Deluxe-like gain & tone, my guess is that you need a voltage divider betwen the second & third triodes to drop some signal.  The typical Blackface amp has roughly a 95% cut with the 3.3meg resistor and the reverb pot section.  Someone here has done a Princeton Reverb with 6L6s, so you might search for "Princeton 6L6".

About the phase inverter, there's a 1.5K resistor off pin 8 of V3 which should be 1K.  Tommy is right about the wiring of Pin 8 on the PI. 

Also, if you want "less breakup and overdrive", you want more voltage going to the phase inverter.  There was a long debate about the "Stokes mod" for the Princeton Reverb circuit here recently.  The idea behind that mod is to increase the clean headroom from the PI by boosting its supply voltage.  Instead of a 22K dropping resistor between B & C on the power rail, you might try something like 4.7K and then use something like 47K as a starting point for the resistor between C & D. 

Also, I'd suggest bumping the value of the filter caps for nodes B & C up to at least 22uf.  For the 6L6 screens (B), you might even want 32uf or higher.  I'm afraid that the 16uf filter caps will yield less and flabbier bass but could be wrong about that.

Hope this helps,

Chip

[Megachunk]

Thanks guys.

This was a working amp, but we're swapping chassis and trying to make the layout a bit quieter. He loved the tone and used it for jazz and rock/pop gigs. It wouldn't breakup til around 7 or 8.

The voltages dropped a bit to where they were, now that I did get some power tubes in there and got the thing biased, but when I turn up now, I get a squeal at about 5 or so. Tried swapping the OT leads, and same thing. Could the PI wiring be the culprit? How would one rewire it if it could be the issue? Tommy could you draw me a quick layout if its not too much trouble? Dont want to do too much to the amp. May change the tone altogether and I'd like to keep it as close as possible. Of course better would be preferable!

**UPDATE**

Moved wires around on V1 and squeal/noise went away. Can max volume. Clean, and quiet. May play with values with owner to make to his liking. Guitar sounds nice - I am using 6L6GCs. I think the others were 5881's. But it seems not as loud this time and more gainy. Want to try to clean up and will try the above tips one at a time. Will report voltage and current on power tubes shortly.

Thanks so much again, guys.
Megachunk
MikeG.

[Fresh_Start]

Glad you got it sorted out for the most part.

To fix the PI layout, take the .02 cap between 13 & 14 and connect the 14 end to pin 8 of V3 instead.  I still think that 1.5K ought to be 1k.

My guess is that your PI is VERY unbalanced right now with one power tube working a lot more than the other.  That could explain the lower volume and more gainy sound.  Also, the NFB wouldn't work right if the PI is unbalanced, possibly leading to oscillation.

I do wish you'd told us this was a working amp the customer/friend used before and pretty much liked the way it was.

Cheers,

Chip

[Megachunk]

Cool.

The .02's are actually .1's now and have been since the old build.

If I connect that to V2 (There's only 2 preamp tubes) pin 8, I'm just not sure where what's left over goes.  Everything else stays the in same place? Via the 2 resistors coming off 7 and 8 or directly to pin 8 with the other resistors still attached?? Just not visualizing it. But I'm sure its affecting the amp. Let me know if there's any way you can draw it out for me. I'm a bit of a dyslexic when it comes to this stuff.
Thanks again,
MikeG.

[Fresh_Start]

Mike - here ya go. I was talking about V3, not V2.

Tommy - does this address your comments?

Chip

[Megachunk]

I can see clearly now! Thanks a bunch. I realized I have no NFB connected right now anyway. That would probably make it a bit more gainy, too, I'd reckon.

I meant v3 which I was referring to as v2. Sorry.

Out to the lab!

Thanks again!
Mega

[Megachunk]

So far.

Me and the owner played through it and determined its very quiet. Before he had trouble with loudness with the volume on 1-2 and now we're talking over it at 6, where its breaking up in a non musical flabby way... I had this issue in the beginning on the first build, and fixed it via some reviewing. I saved the whole thread of this amp's first build from the forums here and am printing them out to see if I can review it and fix the output ish first. That solved just about all my problems the first time. But this volume thing is kicking me around this time. I'm about to rewire it to an older schematic/layout I have of it.

Tried bypassing the tone stack to get volume. V2pin2 to the .01 cap coming off V1-6. very small jump in volume, but volume control when turned up made volume decrease. ??? Also added grid resistors (33k) to V1 and V3.

I am looking at the layout at where the high voltage comes in before the bias, and on my amp it comes through a 470 Ohm 5 watt resistor into a 1N4007 then through one 50uF 50V cap to ground. Don't know if this will affect anything. Finding subtle differences in the 2 builds.

Thanks for all the help, guys.
MikeG.
Megachunk

[jojokeo]

V1 bypass caps should make V1 1st triode stage fully bypassed for best low noise / noise rejection performance "as a rule of thumb".

For lowering B+ to where you want w/out guessing take your measurement and subtract how much you want to lower it to get your actual dropping volts needed, then divide by the (remaining) circuit's current.
example: B+ of 350, want to lower to 300 and your V1 is last tube which uses ~2.5mA
350-300=50, 50v/.0025=20k ohm resistor
For power rating: 50v drop across it means 50 x 50/20,000=0.125 Watts x 2 (for safety factor) = 0.25 watt resistor. But, at 300 Vdc you want to use a 1/2w resistor not a 1/4 watt because of voltage rating.

[Megachunk]

Thanks man, I can use all the math I can get! Gonna measure that rail again as soon as I get the output thing solved.

I'm still not getting full power yet. Thing is baffling me and I'm going to get back to it soon. I've tried to combine 2 layouts and its really confused things. Back to the lab!

Thanks jojokeo!

MikeG.

[FYL]

For power rating: 50v drop across it means 50 x 50/20,000=0.125 Watts x 2 (for safety factor) = 0.25 watt resistor. But, at 300 Vdc you want to use a 1/2w resistor not a 1/4 watt because of voltage rating.

Voltage ratings are across the resistor. Any modern leaded resistor would be OK with 50 v - or even up to 200 v - across it.

But we're dealing with a power supply, so you should exclusively use flameproof resistors, which usually come at 1 watt and more.

[jojokeo]

Something of a lesson here: I RARELY use anyone's layout or if it's solid state I will NEVER use anyone's pcb board unless I go through every component path myself to verify everything. But, in the end it's a waste of time and better when I just do it myself.
I've made this mistake before from well published books, etc. and find incorrectly done stuff all of the time. Plus, I find many incorrect ways or things being done or better ways to do them.

The positive to be taken is that it helps you learn and be better in reading and transposing circuits and components. The more practice you get the better you will become at it just like reading a newspaper.

Putting together two layouts has to be confusing and increases the odds of screw-ups especially if you're not very good at it in the first place or to even spot errors from one of the originals. In this situation it's always best to make your schematic putting everything together first and double checking for problems. Then make your layout to the schematic which should be straight forward from there, again double checking it all before you start building.

It's the setbacks that always make you better in the long run.

[jojokeo]

For power rating: 50v drop across it means 50 x 50/20,000=0.125 Watts x 2 (for safety factor) = 0.25 watt resistor. But, at 300 Vdc you want to use a 1/2w resistor not a 1/4 watt because of voltage rating.

Voltage ratings are across the resistor. Any modern leaded resistor would be OK with 50 v - or even up to 200 v - across it.

But we're dealing with a power supply, so you should exclusively use flameproof resistors, which usually come at 1 watt and more.

It's not the voltage drop across the resistor we're talking about that only influences the size of resistor to use. Two factors are power dissipated and the voltage rating of the resistor itself. Most 1/2 watt resistors are only rated for a maximum working condition of 350 volts max., 1/4 watters are 250v max.

[Megachunk]

Point taken. And I got it working, BTW.

Reread my schematics this time instead of referring to my layouts, and got the output I was looking for. Now I can concentrate on its tone and getting better voltages on the preamp tubes. I'm going to retake the voltages tomorrow and go from there.

I love when stuff doesn't work, because I have a need to make it work. Especially if I'm the one who jacked it in the first place! Thanks all for your help - It's all used at one point or another.

Megachunk

[FYL]

It's not the voltage drop across the resistor we're talking about that only influences the size of resistor to use. Two factors are power dissipated and the voltage rating of the resistor itself. Most 1/2 watt resistors are only rated for a maximum working condition of 350 volts max., 1/4 watters are 250v max.

Working voltage is calculated *across* the resistor, ie. actual voltage drop.

[bnwitt]

Something of a lesson here: I NEVER use anyone's layout ...

Wow!  That rule would have caused me to do a lot of unnecessary work over the years.  Leo Fender's layouts are phenomenal and most work fine.  There are many other layouts out there just as good too.  I will admit I've found some layout errors in other folks work, but I'd rather start with something done and check it against the schematic for corrections rather then start from scratch if I don't have to.  Frankly when I do layouts myself, I make mistakes all the way through until I finally get em right.  The Hoffman Princeton Reverb had so many revisions from other folks catching mistakes  I can't remember them all.  Other folks layouts are a great resource especially if amps have been built from them.  Hoffmans layouts are rock solid.

[jojokeo]

Wiring those heaters up w/ one leg to ground, putting all those caps together, using no shielding w/ long grid wires being an antenna & non-insulated jacks, not the best grounding schemes, etc. Even Leo himself improved and upgraded from his prior mistakes or flaws. He was not perfect as people may want to believe.

[bnwitt]

Well then you just go ahead and wire those heaters up w/ one leg to ground, put all those caps together, use no shielding w/ long grid wires being an antenna, use the non-insulated jacks, follow the terrific grounding schemes, etc... pilgrim. If that floats your boat to blindly follow along then please get in line and don't change a thing from 50 years ago. Even Leo himself improved and upgraded from his prior mistakes or flaws. He was not perfect as you might want to believe. No offense directed at you, please.

I never said I was doing any of that.  Everyone in here knows I'm the king of change and don't have an ounce of Holy Grail circuit worship in me.  Let me tell ya something, I never do anything blindly.  But, I don't have a problem trusting other peoples layout when it produces a great amp.  Boy I love how you say "No offense directed at you, please." after a diatribe of insults.    You need to lighten up friend.  No offense directed at you either, please.   

[jojokeo]

My sense of humor doesn't transfer over from the keyboard very well sometimes, my apologies.

When people take the time to etch their own boards, drill them out, populate them, wire them, etc...and found there were mistakes because it was taken for granted that it be correct because of who it was that made it or that it was in a book so it must be so? Then you may understand? Same w/ wiring up an amp and/or making a nice tag board either w/ eyelets or posts. Except the great thing w/ tube amps and it's circuitry is that they're so easy to modify and change when mistakes are made. Circuit boards are not. My electronics started w/ solid state circuits many years ago, and I made those mistakes in the trust dept. and it cost me time, money, frustration, etc...but it made me a good circuit reader and maker. Others should be encouraged to do things on their own as well for a variety of reasons.

One last thing, I've seen almost everyone here make mistakes for one reason or another. Nobody here is impervious as we're all human and why pencils have erasers. Even guys like Tubenit, Geezer, PRR, Hotblueplates, Sluckey, etc...everyone has put down something that wasn't correct at one time or another in an explanation, schematic or layout. Sorry guys for naming names but my point is proven as you are some of the most respected guys in here and "creaters of great amps." But, sorry that still doesn't mean I would blindly copy any of your layouts w/out going through them first.  

[Fresh_Start]

Point taken. And I got it working, BTW.

Reread my schematics this time instead of referring to my layouts, and got the output I was looking for. Now I can concentrate on its tone and getting better voltages on the preamp tubes. I'm going to retake the voltages tomorrow and go from there.

I love when stuff doesn't work, because I have a need to make it work. Especially if I'm the one who jacked it in the first place! Thanks all for your help - It's all used at one point or another.

Megachunk

WHAT did you fix?

Did you make the change to the phase inverter wiring we suggested?

Sorry, but it's a bit frustrating to put some effort into helping another amp builder and not know what worked in the end.

Cheers,

Chip

P.S. Copying, or at least starting with, a "known good" schematic and layout makes a hell of a lot more sense to me than starting with a blank piece of paper.  Thanks again for the Princeton Reverb layout BNWitt! 

[jojokeo]

P.S. Copying, or at least starting with, a "known good" schematic and layout makes a hell of a lot more sense to me than starting with a blank piece of paper.  Thanks again for the Princeton Reverb layout BNWitt!  

I agree. Use "good" schematics to go by if needed or use them as a reference as long as they are atleast checked or looked at to make sure there are no errors is all I'm getting at.
Megachunk fixed his own problem because he has the experience and electronic background to read schematics and layouts of which he combined 2 together and where his issue was in. That being correctly interpreting and understanding what is going on, therefore what to do when he saw the error. This thread is not a case of simply "following a known good schematic" as you say. This is my point - that if other's would try to better learn & develop these skills, then many would be better able to find and fix their own problems as well. Maybe they wouldn't even have the problem in the first place because again, it would have been spotted before it occured.

[bnwitt]

jojokeo I just can't seem to find the logic in your posts.  Megachunk didn't blindly copy anything.  In fact, he stepped out on his own and attempted to blend two different amps together.  You're the one who threw in the never copy layouts remark from left field.  It seems you're on some mission from God or something.  And my point is that there are many Fender and other layouts that can be recreated exactly as drawn and fantastic sounding and well designed amps will be the result.  To say "never follow anyone's layout"  is very poor advice so I hope you didn't mind that I refuted that advice.  Ah but it is clear you did and you seem to feel you are the king of all tube knowledge.  Also your assertion that you didn't insult me is a jokeo.

"Well then you just go ahead and wire those heaters up w/ one leg to ground, put all those caps together, use no shielding w/ long grid wires being an antenna, use the non-insulated jacks, follow the terrific grounding schemes, etc... pilgrim. If that floats your boat to blindly follow along then please get in line and don't change a thing from 50 years ago "

Now you didn't just point out Leo's flaws, you had me going ahead and performing all of the tweed era dumb things "blindly" in your rant.  That was pretty damned personal and insulting.  Then you tell Chip he's "stickin' his nose in.."  This is a forum sir.  Contributing opinions is what a forum is all about.  I will say it again.  You need to lighten up.  Unless of course you hung the moon pilgrim.

Using a well vetted layout to build and amplifier is a good thing for new amp folks to do to sharpen their build skills and to help them understand how it is done correctly.  Not the evil thing you make it out to be.

Then you post this paragraph...


"This subject reminds me of my old school days when slackers just want the answers given to them rather than working out the problems themself. What do they do when they're on their own after school is over? Likely work at some dead end job being told what to do all day.  


I read something like that and I think "this fellow has issues."

[jojokeo]

jojokeo I just can't seem to find the logic in your posts.  Megachunk didn't blindly copy anything.  In fact, he stepped out on his own and attempted to blend two different amps together.

Megachunk fixed his own problem because he has the intellegence and electronic background to read schematics and layouts of which he combined 2 together and where his issue was in.

*You still aren't reading correctly - that IS what I said.

Please lets stop all of this crap. Again, I asked politely that you not take any offense. You and skippy want to keep pressing this issue for some reason? Am I a bad guy to promote self reliance and learning? I have learned from past mistakes made by others whether in books, magazines, or places like this online in copying things directly produced. As a result, I learned to do things on my own. I don't experience the problems that can happen from this any longer. I have learned better design, layout, wiring, grounding, etc techniques as a result. I am only trying to offer a bit of sage advice based on my past failures and/or successes that may prevent simple mistakes like what Megachunk experienced. When combining copied circuits the chance for the kind of errors I'm referring to and is my point in all of this are multiplied greatly. Now, I don't exaclty know if he did copy his personal or other's layouts but for my point on this issue, that doesn't matter.

Again, please let this go. I don't want to argue w/ you as even though I don't know you, I respect you and your opinions and work here. Same w/ Fresh_Start/Chip. We're all only trying to help others in the big picture. Peace guys okay? 

[Megachunk]

BOY???!!

Guys, I'm a guitar player who fixes his own amps because I'm a poor working musician. I love fixing stuff, but electronically, am green as a new banana. I built me some amps and friends like them and have me build one for them, but I'm no engineer or electriconics guy - although I hang out with them at home and here on the boards. So honestly, I'm looking for answers from the smart guys. I can't design a circuit, let alone REALLY understand what I'm doing. But neither could the women wiring up Fender amps back in the day, but they're some of the best sounding today. No need for any of that crap above - The amp's working, you guys helped me fix it and thats good enough for me.

What I did, is I took the schematic from the first build and REALLY ran over it, finding discrepancies between the new layout and schematic - It was VERY different from the newer layout I was working from. The point of this chassis transfer was to quiet the amp down - Had a slight hum which is gone now, too. I rewired the preamp tubes as per the schem and changed a resistor that was over valued (the one from the gain stage with a 47R to ground off it - It was a 33k and changed it to a 56ohm) I usually work from layouts, because they're easier to read for me. So in reviewing the old schematic, I wired it up according to it, and used the PI rewire and some of the cap/resistor chages in the gain stages and it sounds great - Sounds like it did even though we changed a few things.

My amps have never worked right the first time. I almost like it! I'm never in a rush or hurry and have no time lines so troubleshooting keeps my brain sharp-Like memorizing lyrics (I sing about 3-4 hours straight every night.) Tubenit is the man. He spent time drawing me layouts and schematics and answering me back real fast. It's guys like him that make this board great. I feel like I know these folks and have for a long time (Tubenit, Sluckey, Doug, etc) I have no real time to learn everything about this stuff. I just want my amps to work. And I find I'm learning tons whether I like it or not.

Boy I didn't mean to open a can o worms. But since I did, why don't we all just go fishin'?
Thanks guys.
Megachunk

[HotBluePlates]

Even guys like ... Hotblueplates ... everyone has put down something that wasn't correct at one time or another in an explanation, schematic or layout.

Guilty. Multiple times. 

But I hope I learned something from all the mistakes.

[Megachunk]

OK, back to the amp!

It sounds real good now, very jazzy bassy clean. Nice, and I think he'll like it. But one thing I do notice, is the volume does nothing til about 2 or so. It's slightly quieter than it was, (with 6L6GC's instead of 5881's?) but I think he'll like that too, because before it had a volume jump between 1-2 that was hard to get under control for certain rooms. I would like to know why that may occur.

Thanks for the help again guys. It is greatly appreciated
Mike
Megachunk

[HotBluePlates]

I don't know exactly. Most pots seem to do it to some extent.

It might be an artifact of the resistive track and how it jumps from all-off to barely-on. Maybe there's a physical aspect to it.

I never think of it as a big deal, because to me, if I'm focused on the valoume range from 0-2 I probably am using an amp with too much power. There's a way to shift the action of the volume control towards the lower-volume range, but it limits the maximum output from the volume control. The method is to insert a resistor between the previous coupling cap/circuit and the "top" of the volume pot. The end effect is that your output from the volume pot is always somewhat less than the signal from the previous stage even if the volume is full-up. How big a resistor to use is figured by trial and error (maybe 100k for a 1M pot, maybe more, maybe less).

That still won't cure the jump in volume from all-off to slightly-on, but makes that jump appear at a slightly lower volume.

[jojokeo]

Glad you have things going well. It almost seems like you were using a linear pot there instead of a log pot? It could be a change from your current value is all that's needed or a slight adjustment to the resistor going into it if it's got one creating a divider with it. If you would like to leave everything be, you could put a small value resistor between lug 3 and ground of that pot. If you go large enough on this one, when you turn the control down all of the way down, you'll still get a small signal sounding through therefore the control won't go entirely all of the way "off". If you do this and this happens, then go back down to that last smaller resistor and it should be perfect.

[Megachunk]

Thanks doods.

The layout I used is at the top of the thread, but its not exact. I'll go through it, insert the new proper values I have now, and repost it so you can see what I have going. The tone stack is exactly the same, though.

Thanks,
MikeG.

[Megachunk]

Update!

My voltages are back to normal, seemingly, and everything sounds good, but the hum is kinda back. It gets louder when I turn the volume all the way down, but when I turn it up to volume, say 3-4, it almost goes away and is faded out and replaced by slight amp hiss. When I used a clip lead to experiment with the tone on the second stage 2.2uF cap, hum got louder when disconnected and went back to hiss with it connected. Any ideas, I'd be more than happy to check them out. I'm using 2 12ax7's and 2 6L6GC's, FWIW.

Measuring new voltages and noting them today. Then will do layout corrections.

Thanks guys
Megachunk

[Fresh_Start]

Louder hum as you turn volume down might be a bad ground connection between the pot & ground?

Just a thought..

Chip

[Megachunk]

Thanks man check that today.

Making some other adjustments as well. I did notice this: I posted 2 pix, power1 and power2. Power1 is how I have it now and power2 is how it is on the schematic. Could not having the 2 other diodes going to ground behind the ac cause a problem with hum? Is it necessary to have it like that, or is the way I have sufficient? I want to say Sluckey gave me the info to hook it up like this, but I could be wrong. He's helped me with all my amps, so it gets to be a blur now.

Gonna work on the new schematic and the values today and will post when completed. Thanks for all the help, guys.
Mike G
Megachunk

[sluckey]

Power1 cannot work,,, unless you have a center tap connected to ground on the HT winding that you forgot to show.

Power2 is correct and will work as drawn. But it won't work if you have a center tap connected to ground.

[Megachunk]

Yes, there is a center tap, I would have to say, being that this is the setup I used on the previous chassis. I also didn't have the 2 100ohm resistors off the pilot to ground until yesterday, thinking that may have caused some hum. They're on there now - It works with them or without with no change in hum level.  I'm going to write all my values down and test it out before I change anything else.

Thanks man,
MikeG. 

[Megachunk]

New layout and the values I have now currently.

Sound is strong, loud, fat, and clean. Still that hum, though! It's slight, amp's usable, but I can hear it still. Gonna try it on a known grounded circuit. If anyone see's anything amiss, please holler! It's pert close compared between the amp and this layout. That 10K between C & D is actually a 56K.

Thanks again!

Megachunk

[tubenit]

Does it hum significantly less without the ground lifted off the James Tone Stack?  

Are you using Hoffman's grounding scheme?  Make sure the power B+ filter caps and preamp B+ filter caps are grounded separately.

http://www.el34world.com/charts/grounds.htm

With respect, Tubenit

[Megachunk]

Hums the same.

They are not grounded separately. Going out to do that now.

Thanks Tubenit!

MikeG.

[jojokeo]

Not only what tubenit said but two other things that get overlooked is the way the output tranny secondary is hooked up and where it's ground goes to, especially when you have a feedback loop going on. There's huge amounts of current here and it's important that the secondary leads go directly to the output jack and that they have no sensitive preamp wiring near them. Then the output jack's ground goes to the power supply cap's negative lead ground point. Also do you have the power transformer center tap grounded at exactly the negative lead/point of the resivoir cap (the first power filter cap)? This is also where huge amounts of current are and if not at the same place then you could be having these currents going into the chassis between these two points as well? These above problematic area's issues can be more sensitive when using standard jacks and not isolation jacks. These items are key to prevent oscillations and noisey ground loops.

[Fresh_Start]

Mike - Please forgive me if I seem to be beating a dead horse.  At this point I'd prefer to send you a PM but can't attach a file to a PM.  Please try the change shown in the attachment.  A push-pull amp should be humcancelling, especially with negative feedback.  However, the humcancelling won't happen if one tube is getting a significantly stronger signal than the other.  

Before breaking out the soldering iron, do you have an oscilloscope?  If you do, you could compare the signals on each power tube's grid (pin 5).  I'll bet one is bigger than the other.  You might be able to measure it with a good DMM but I'm not sure about that.

Take a look at the schematic and layout for the 5E6-A Bassman.  The cathode of the phase inverter is tied directly to the coupling cap for the lower of the two power tubes.  BTW your earlier value of 1.5K for the resistor tied to that cathode (pin 8) matches the Bassman.

One last thought is that your latest drawing shows the leads going to the power tube grids looping up to the top of the board and around the left-hand end.  If that's how the amp is built, shortening up those leads so they go more directly to the grids should help reduce noise.  Just remember to keep sensitive grid wires perpendicular to and away from (horizontally or vertically) everything else but especially heaters, plates and screen grid leads.

Hope this helps,

Chip

[Megachunk]

I rerouted those grounds on the first 2 caps to the CT of the OT's grounds. Then the last 2 caps to the rail along the board, which connects to the backs of the pots. MUCH quieter. Standard hum and hiss, now.

I do have the heaters out of the way, but I'll shorten up those leads going to the grids, as they do loop up. I have a scope, but its old and needs tubes before I did anything remotely accurate, let alone knowing how to even use the darn thing. But I can figure it out, I'm sure. I have the plate wires twisted together for a short run. I'll take a pic of what I have sofar and post it in a bit.

Thanks for all the tips and links. guys. He's going to come by tonight, so I'll let you know how he digs it. Once its in the cab and away from my flourescent worklight it'll be ready to record!

Mega

[tubenit]

I rerouted those grounds on the first 2 caps to the CT of the OT's grounds. Then the last 2 caps to the rail along the board, which connects to the backs of the pots. MUCH quieter. Standard hum and hiss, now.

Excellent! Hoffman's grounding scheme has always worked for me.

With respect, Tubenit

[Megachunk]

Well, there you go.

Amp's quiet as a mouse, we changed the gain stages around and played with the EQ (bass and treble). Mounted beautifully in the cab. Sounds wonderful in the cab with the Jensen. He's stoked. I'm stoked. Took it with him. I'll have to have him bring it back to take pics. I ended up putting the .68uF/2.2k in the first stage, then the 25uF/1.5k where the .68 was. And Fresh Start, I used your mod to the layout as well. It all came together. My final layout is posted here too, and I'm going to go through the schematic next. I gained more know-how on this project than any other, and what I did get out of it, is a layout and schematic that can be reproduced almost exactly.

Hot damn tamale, thanks a bunch all you guys. Once again, the Hoffman clan is the best. You cant get this stuff in a  classroom.

Megachunk
Mike Glendinning