Low power Musing 40 with parallel/cascade idea

[dwinstonwood]

First, thanks everyone who's taken the time to advise me with this project. Willabe, I hope I didn't come across as being argumentative. I really value your input. But, I've found a process that seems to suit the way I want to enjoy building these things. 

Unfortunately, I'm having trouble with Duncan's PSUD2 app. I'm missing something key it seems.

Here's a config that I've set next to Hoffman's 6V6 Plexi B+ voltages. My voltages in PSUD2 are darn close, but I just don't think I'm using the app the right way. I tried to put an 85ma load on the end. Any input would be great.

Thanks everyone!

[sluckey]

I tried to put an 85ma load on the end. Any input would be great.

That's not right. The bulk of that 85mA load only goes as far as node A (plates). Then the rest of the load is distributed across the remaining nodes. The final node probably only supplies 1mA or 2mA current.

I think you are wasting time on PSUD2 unless you are having fun. Reading this is a lot more useful to building and understanding power supplies...

     http://www.valvewizard.co.uk/smoothing.html

[dwinstonwood]

Thanks, I agree sluckey. It does seem like a waste of time! I'll sit back and and re-read Merlin until it clicks.

In the end I suspect I'll be taking actual voltage readings at the nodes and swapping out resistors until I get them close. I've done it that way before. But, I do want to more or less aim for the voltages on Hoffman's 6V6 Plexi schematic - specifically his C, D, and E numbers.

And, Merlin's Preamp book has been my poolside reading material this summer. 

[dwinstonwood]

Well, one last bit of noodling with a 2ma load...

This one has an 8.2K, a 4.7K, and a 2.2K. Pretty close.

[acheld]

I am interested in modeling, but have no experience.   Sluckey is correct. 

While the power supply is modeled, the loads represented at nodes A,B,C,D,E are not.   That's the part that has always confounded me. 

The current draw for each node is fairly easy to guesstimate, but what I have trouble knowing are the conditions of that current draw (the resistance and inductance), eg the power loss in each node.

LOL, maybe I need to go back to school. . . or read Kuehnel's book on modeling, which is sitting on my desk.

[dwinstonwood]

acheld, I hear you. I also have Kuehnel's Circuit Simulation book siting around here somewhere... unread.

And, I have no idea what I'm doing with software.

The soldering is almost done. I just need to finish wiring up the Hot switch, which means I need to make a shielded cable and probably remove the switch up out of the chassis to get to it. No problem.

Then, I'll go over every component and connection a few times to find mistakes.

As far as the B+ is concerned, I'm just going to change dropping resistors until I get what I want.

[Willabe]

Double checking a build;

https://el34world.com/Forum/index.php?topic=17701.0

[dwinstonwood]

Thanks Willabe. I'll print out some layouts.

And, I just found error #1. I left off the 33K "slope" resistor.

[PRR]

PSDU is for the MAIN power node. Like a radio transmitter.

It does not cater to the long chain of filtered loads we use in guitar amps. It does not have a way to attach loads all over.

This is not a problem because we can readily hand-estimate voltage drops and ripple reduction. Practice! (Also plagiarize.)

[dwinstonwood]

Thanks PRR! And, some time back you did show me how to calculate voltage drops and current simply from the info on classic Fender schematics. I need to work out a few of them to get a better hang of it. 


But... PSUD2 did let me insert current taps after each node. However, I don't know if it's actually modeling what I want to see:
(the graph is only showing the preamp nodes)

[PRR]

OK. But you are not running it long enough to estimate final values (you won't play much in 1 second after turn-on). And you are hiding the number-table which is often the best way to read ripple.

[dwinstonwood]

10 second run time
10K, 10K, and 4.7K dropping resistors

Is RMS the column I should be looking at?

[dwinstonwood]

I've been over it and over it. I think the soldering is complete.
I went with 20K - 10K - 10K dropping resistors.

[Willabe]

Looks real nice. 

But the 3rd (green) wire/safety ground for the power cord should have it's own dedicated chassis bolt. 

And for the quietest operation, the PT B+ CT, red/yellow wire, should go directly to the B+ OT CT/screen cap cans ground lug, and leave the black wire going from the cap can ground lug to your chassis ground bolt. (It should be without the screen cap, but your wiring is next best thing.) And it should be twisted in with the 2 B+ red wires on the way to the cap can.

Both are in here;     

http://www.valvewizard.co.uk/Grounding.pdf

[dwinstonwood]

Thanks Willabe. I thought I had followed Merlin's scheme. I copied the diagram in the attached image. But, I'll go back over it.

OK, after a test power up with the dim bulb tester I plugged it straight into the wall.

No Tubes Installed.

Filaments: 6.9VAC

Bias range on Pins 5: -49.1VDC to -83.3VDC and -49.3VDC to -83.5VDC

Obviously, I need to change bias circuit resistors to bring this down to a 6V6 range. I have a 100K coming off of the PT secondary into the diode right now. Maybe I should try a 220K?

But, this is all I have time for right now. I have to go earn a living.

[dwinstonwood]

Willabe, I'm not 100% clear on your post. But, this is what I did:

PT CT, Cap Can Negative, Bias Circuit, and 6V6 Cathodes all connected to one star. This is how I understood/misunderstood Merlin's diagram.

Merlin says to try grounding the Filament CT at different places to find what's best; and, the IEC Earth ground gets its own bolt.

Please take a look at my layout detail below and let me know what should be moved around.
Thanks!

[sluckey]

I would just try it as is. If you have hum, then deal with it.

[dwinstonwood]

Thanks sluckey.

But, sluckey, if you're still there! I changed the bias resistor to a 220K (that's what's on Hoffman's schematic) and my negative bias voltage range is now -47.2VDC to -25.58VDC. Is that an OK range for 6V6's? This is with no tubes in the amp.
Thanks!

[sluckey]

perfect

[dwinstonwood]

cool

[sluckey]

I don't like having the power ground connected to that pot buss bar. If you decide you don't like your ground scheme, this is how I would do it...

[dwinstonwood]

sluckey, actually the buss bar is floating; it's not connected to the pots. It captures the star grounds and runs them all to the input jack - the only ground point (except for the power earth and the filament CT grounds). This is how I interpreted Merlin's method.

[sluckey]

sluckey, actually the buss bar is floating;

Yes, I know that. But the Vol, Gain, Mid, and Presence pots are connected to the buss. I was suggesting take the power grounds off the buss. Then cut off the excess buss bar that extends past the presence pot. This is easy to do. But I still think you should try the amp just as you have it before making ***ANY*** changes to the grounding.

[dwinstonwood]

Thank you sluckey. I got confused.

That sounds like a good plan. Who knows, it might be quiet enough as it is.

[PRR]

10 second run time

Why start from zero time? How fast can you start playing?

If you start from several seconds, nothing happens different in second 10 than second 1. In fact one cycle tells all. That's 20mS at 50Hz line frequency. (Altho I have 60Hz on tap here, I use 50 for calcs because any mental-math is easier, and to be conservative.)

Is RMS the column I should be looking at?

No. Here, "Diff" is peak-to-peak ripple.

[dwinstonwood]

Wow, so the choke/capacitor (LC filter) reduces ripple to about 1/100th.
In the world of guitar amps is is 0.27V ripple considered good, average, or noisy?
Thanks for the help with this.

[shooter]

fwiw;
I shoot for <50mV ripple on the 1st preamp tube (V1)
I shoot for <500mV up to the PA tap
In SE I try for <2V at the PA tubes, <4V on PP

[dwinstonwood]

Thanks shooter. So, the sim would indicate 270ma ripple on the 6V6 plates. If I get what your saying, that's not bad?
When I get home I'll see what it predicts for V1.

Speaking of V1, I saved some expense on this amp with the blank chassis and parts I had on hand, so I bought a TAD 7025WA for V1, and a TAD 12AX7A-C for V2. These are supposed to be quiet tubes. And, since this is the highest gain amp I've built I figured it might make a difference.

[shooter]

Make it work, then make it different 


I'll wait for full power meter readings, but 270 is real good at the power tubes

[sluckey]

So, the sim would indicate 270ma ripple on the 6V6 plates.

No. You have the current loads in the wrong place. The ***BIG*** load (plates) is on the left side of choke. There will be several volts of ripple on the plates. Not uncommon to see 10V ripple.

But that's not a problem in a push/pull circuit with balanced tubes because the P/P action cancels the ripple. Big problem for SE amps though.

[dwinstonwood]

No. You have the current loads in the wrong place. The ***BIG*** load (plates) is on the left side of choke.

Of course, the first node connects to the OPT and then to the plates. It's been a long day.

[sluckey]

Hey, you got a lot done. Why ain't you playing guitar?

[dwinstonwood]

I'm at work. It's not an amp-friendly place.

But, now I'm wondering if I should go with the F&T 50uF/50uF instead of the 32/32? Well, I'll wait and see how it sounds.

[dwinstonwood]

Hmm... a 50uF/50uF can would squeeze the ripple down from 22V to about 13V.

It's an easy part to swap if I think I need it.

[sluckey]

Hmm... a 50uF/50uF can would squeeze the ripple down from 22V to about 13V.

It's an easy part to swap if I think I need it.

You still have the model set up wrong. I1 (plates) should be about 50mA. I2 (screens) should be about 10mA. These are more realistic numbers for your fixed bias P/P 6V6.

[dwinstonwood]

Dang!! 

OK, well that brings the ripple down to about 15V on the plates with the 32/32 cap, and 138ma on the screens.

With a 50/50 it's 10V plate ripple and about 62ma on the screens.

[PRR]

10%-15% ripple On The Plates is no big deal. Plate is a very high impedance so most of the ripple is across the tube, not across the load winding. (Why we like pentodes.)

Screens are much more sensitive.

Which is why you GOTTA put them right, in the sim and in the amp.

Frankly I think you have about 3 pounds more filtering than you need. COMPARE to other amps known to be satisfactory. With modern high-uFd caps, plate-chokes are unknown and screen chokes getting rare.

[dwinstonwood]

Alright, I'm moving slow here because this always gets my heart rate up.

All tubes installed.

Plate to cathode voltages: 379.5VDC and 381.0VDC.

I set the negative bias voltage to -20V and -18.5V respectively. This is on the cool side - around 60% - which is ok for now.

I did disconnect the NFB. When I first turned on the amp with it connected the speaker made a pop and I could just barely discern a very high frequency ringing. I guess I'll have to swap the OPT wires.

I'll get relaxed and plug in a guitar.

BTW, I'm not hearing any hum, but I haven't turned up the volume control, either.

[Willabe]

Willabe, I'm not 100% clear on your post. But, this is what I did:

PT CT, Cap Can Negative, Bias Circuit, and 6V6 Cathodes all connected to one star.

Different people wire things differently and still have a very quite amp. I'm just going by Merlin, Kevin O'Connor, PRR and others say on this. It works for me, you can wire it up as you think best. What Sluckey said will work fine. 

What you have now is very good, probably wont notice any difference in noise by moving this 1 wire. But for me, I do this to help eliminate a possible problem. If my build has noise, then I know it's probably not from this ground star. And I look elsewhere. And since it's a new build and your in there, I would do this.

The 1st B+ filter cap has to deal with the most ripple and filter the most current in the amp. This is the noisiest B+ node in the power supply. That's why Merlin and many other are detailed in how to wire it up.

Merlin could have been a little clearer in his drawing on this. What you missed is (see attachment below) he shows 2 wires coming from the rectifier, 1+ / 1-  going to the 1st filter cap. The rectifier  -  /negative  wire going to the filter cap negative lead IS the CT of the PT. (If it was a SS FWB it would be the FWB's ground.) So he shows the rectifier - lead going directly to the filter can - lead. There's NOTHING else connected there. That way all the current flowing through that filter cap can go round and round without disturbing any other B+ filter node. Then he runs a separate wire from the 1st filter cap - lead/rectifier - lead over to the tubes K, bias star.

In the pic below, the red arrow points to your CT red/yellow wire and the purple arrow points to your black wire that goes from your cap can ground over to your ground buss.

So I'm saying leave the black wire as you have it now. Move the red/yellow CT wire from the ground buss back over to the cap cans ground lug. This way you will have it wired up so that 1st B+ filter node can go round and round in a loop all by it's self. Your black wire from the cap can to the ground buss isolates all that current flowing in that loop from the rest of the ground buss.

Merlin says to try grounding the Filament CT at different places to find what's best; and, the IEC Earth ground gets its own bolt.

Yes, but he doesn't' say to try grounding the heaters with the IEC ground.

Nothing else should be wired to the IEC ground bolt.

[Willabe]

10%-15% ripple On The Plates is no big deal. Plate is a very high impedance so most of the ripple is across the tube, not across the load winding. (Why we like pentodes.)

Screens are much more sensitive.

This is why Merlin, KOC and others don't like to use cap cans. ^^^^^^ 

Can't separate the grounds on that double cap can feeding both the PT CT, which feeds the power tube plates and the screens. So now you could have that 1st filter caps current/ripple modulate the screens ground.

Yes many, many amps have used cap cans and many, many amps don't have a wired ground, they have a random chassis ground and don't have drastic noise problems, but why take the chance? I figure any noise you I eliminate or reduce is a step in the right direction. That's why I wire grounds up like Melin and KOC recommend. It's not that much more work.

High gain amps seem to be more prone to grounding wiring schemes, but the theory still stands.

Although, I think you'll be fine.

[dwinstonwood]

Thanks Willabe! You cleared that up. I will move the PT CT from the star over to the can's negative lug.

But, I wired something else up wrong. I'm not getting any sound at all. Just dead silence. I don't have time to track it down today, but I'm off tomorrow. I suspect it might be the mini switch that I goofed up. I might just bypass the switch and wire the hot on all the time. I already own enough clean, lower gain amps, anyway.

But, first I'll trace through everything a fourth time.

[dwinstonwood]

I moved the PT CT and I ditched the mini toggle.

Maybe some wiser eyes can glance at this and spot my error(s). Sorry the text is upside-down, but it's easier for me to look at the pots when they're right-side-up like this.

Thanks! I'll have all day tomorrow to work on it and pull my hair out.

[dwinstonwood]

I was worrying that I might have damaged my OPT, but I'm measuring about 204 Ohms on each primary to center, and "0" Ohms across the two secondaries. From what I've been able to research that's normal, I think.

Maybe there's a grid that's not referenced to ground, or a cathode, or bungled pot wiring. I hope I find whatever it is. Anyway, after I go over all the wiring again I'll take voltage readings on all of the tubes. Maybe that will show something.

[dwinstonwood]

I didn't have time yesterday to make a version of the schematic without the hot switch. Here it is.
I've been over my wiring many times, I've compared it to Hoffman's hot switch schematic and layout. I can't find my mistake.
I'm going to take voltage readings on all the tubes next.

[shooter]

spend a couple minutes verifying continuity on Input jack and speaker jack, AND associated grounds

[sluckey]

Schematic looks OK. I bet the amp is not wired exactly like the schematic.

[dwinstonwood]

Schematic looks OK. I bet the amp is not wired exactly like the schematic.

Yep!!! It wasn't until I started taking voltage readings that the mistake stood up and slapped me in the face.

I had left out the ground connection for the V2 cathodes (see attached image)! 

I soldered in that wire and WOW! This thing has some gain. It's also very quiet; I'm not hearing any hum or hiss.

>> I'm editing this to say that with the gain/master (whichever it is) up past midway there is a slight bit of hiss, but not enough to hear when playing. No hum at all. It's a quiet amp. So, thanks y'all for all the help with the grounding scheme!

[dwinstonwood]

Here are my voltages.

V1:
pin 1 - 154.5
pin 6 - 157.5
pin 3 - 1.45
pin 8 - 1.38

V2 (cathode follower):
pin 1 - 129.4
pin 6 - 233.4
pin 3 - 0.857
pin 8 - 130.7

V3 (LTP):
pin 1 - 183.1
pin 6 - 176.8
pin 3 - 30.17
pin 8 - 30.17

V4 (6V6GT):
pin 3 - 379.3
pin 4 - 380.6
Plate to Cathode - 379.1

V5 (6V6GT):
pin 3 - 378.9
pin 4 - 381.4
Plate to Cathode - 379.5

Bias is set to an average of 21mA across the 1 Ohm resistors for about 65% dissipation.

I'll post a final schematic and layout (with a board template) later today. Thanks again Willabe, sluckey, PRR, shooter, and others for guiding me through this! Maybe the thread title should be changed to "Hoffman 6V6 Plexi with Hot Switch" to reflect the actual build?

[dwinstonwood]

Here's my finished schematic with my voltages, and the layout.

[sluckey]

Looks great!