B+ question

[J Rindt]

I put together an old Magnatone, 2x6V6, cathode bias, 12" combo. It is coming along pretty well, but there is a discrepancy of 25 volts B+ on my power tube plates. The difference follows the socket, not the tube. This amp was original from 1948, had not been played in many years, and saw a bit of water at some point in its life. So I did not even hot this amp up in its original condition. I just started doing a cap job, 3 wire cord, and replacing a bunch of crappy old parts.
Now that it is running, I have this issue. The OT Primary seems OK. From CT to each end is 187 and 168, with 356 Ohms side to side. So what can cause the B+ to vary this much.? Have I missed something obvious.....
Thank You

[jjasilli]

If there's supposed to be, say, 250VDC on the plates, a 25V difference is only 10%.  The OT primary windings could be off by that much.  187 - 10% = about 168.  Bingo.  You don't have a problem.   

[J Rindt]

So it is something obvious.?
I do not mean to be an idiot, but is this just a simple case of Ohms law.?
There is a voltage drop across the DC resistance of each side of the OT.? And with the OT at an imbalance.........
Thanks for your help.
I Appreciate It

[sluckey]

The difference follows the socket, not the tube.

There are some possibilities other than just an unbalanced OT. What are the voltages on the control grids? Screen grids? If the cathodes have separate resistors, what are the values and what voltages? Even the socket can be a factor.

 All are pretty easy to rule out. If it still looks like the OT imbalance is causing the difference, I'd just ignore it if it sounds OK and isn't running hot.


 

[J Rindt]

The difference follows the socket, not the tube.

There are some possibilities other than just an unbalanced OT. What are the voltages on the control grids? Screen grids? If the cathodes have separate resistors, what are the values and what voltages? Even the socket can be a factor.

 All are pretty easy to rule out. If it still looks like the OT imbalance is causing the difference, I'd just ignore it if it sounds OK and isn't running hot.

I will get that info ASAP....
I do not have a whole lot of experience with cat biased amps. Would I have more "control" over the bias of the power tubes if I used a separate cat resistor for each 6V6.?
Thanks

[sluckey]

Would I have more "control" over the bias of the power tubes if I used a separate cat resistor for each 6V6.?

Yes, but that's really overkill for a guitar amp.

[jjasilli]

Per TUT1, KOC, you can compensate for bias control in a cathode biased amp with a pot that adjusts the signal level to ea power tube.   I think you'd need a scope on the PT secondary to balance the final waveform.

OTOH it's probably a capital offense to "perfect" a 1948 Magnatone amp.

[J Rindt]

The power tube plates are 366 and 340.
The screens are only 315, do I need to raise that.?
There is a 260 Ohm cat resistor and it is dropping about 27 volts.
On the grid of the power tube with the lower voltage, I see 12VDC when I meter the grid, but if I hold the probe there, it drains away the VDC. Do I have a bad coupling cap coming from the PI.?
Thank You

http://i111.photobucket.com/albums/n143/zzmoore/Maggie192001.jpg
http://i111.photobucket.com/albums/n143/zzmoore/Maggie192002.jpg
http://i111.photobucket.com/albums/n143/zzmoore/Maggie192003.jpg

The difference follows the socket, not the tube.

There are some possibilities other than just an unbalanced OT. What are the voltages on the control grids? Screen grids? If the cathodes have separate resistors, what are the values and what voltages? Even the socket can be a factor.

 All are pretty easy to rule out. If it still looks like the OT imbalance is causing the difference, I'd just ignore it if it sounds OK and isn't running hot.


 

[sluckey]

On the grid of the power tube with the lower voltage, I see 12VDC when I meter the grid, but if I hold the probe there, it drains away the VDC. Do I have a bad coupling cap coming from the PI.?

That would be my first suspect. Disconnect the end of the cap that connects to the power tube grid. Turn on the amp, and measure the voltage between the dangling end of the cap and chassis. If the cap is leaky, you'll have positive dc on the dangling end.

It's not a bad idea to replace every coupling cap in that old amp, especially since it's been swimming.

PS... Check the output tube grid resistors too.

[J Rindt]

Well, the Mallory 150's are only a few years old, but I did something stupid on that side of the PI when I "rebuilt" this amp. That may have corrupted the integrity of that one cap.
Anyway.......yeah, I will pull it and see if there is VDC getting through.
What about the screen VDC. Is it OK for the screen and plate VDC to have that disparity between them.?
Thanks

[sluckey]

I didn't look at the pics until just now. Looks like a complete rebuild under the hood? Screens are fine.

[OldHouseScott]

Looking at pic 3, I'm not seeing any grid to ground resistors? Maybe I'm just not recognizing them? Looks like there might be one resistor from one output tube going to a pin on the PI? Or maybe that's a ground tab?

[J Rindt]

http://f1.grp.yahoofs.com/v1/MHTRTfkbsC9AAWBP2kfu5wNATMsRtl5Fs2t8AoJ3SkFwJBQuKa6RhyeH2ZEShSpyAtXx-_lTeFuaUXjts_7H0LPm7jlOACIG2xzBzL0tMs0/Schematics/Magnatone_M192-5_schematic.pdf

Grid resistors......
This is kind of a crazy schematic. What is up with that 100k going to ground off of Pin 3 of the PI.?
I changed that bad coupling cap. Let me check voltages again, and I will get right back to you. Got a lot of irons in the fire at the moment.
Thanks

[sluckey]

What is up with that 100k going to ground off of Pin 3 of the PI.?

Huh???

[J Rindt]

If you look at the schem I attached previously, you will see a 100k, after the Pin 3 coupling cap of the PI. On the schem it goes to ground at the same spot as the power tube cat resistor. Mine was broken and that was causing all my troubles. I did not notice it until I changed that .1 mic cap on the same Pin 3. Now the amp sounds a lot better.
My plates are at 404
Screens are at 376
Cat resistor is dropping 24.2 across a 270 ohm.
Now I have some questions.....
1.  Is it a good idea to add screen resistors to an amp like this.? Maybe the same values as a BF Fender perhaps.?
2.  Perhaps I should try to drop the B+ a bit.? Maybe 25-30 volts with some Zeners on the Power Tranny High Voltage CT.? Correct me if I am wrong, but I do not see an advantage to having that much B+ on the 6V6 tubes.
Thanks Again

What is up with that 100k going to ground off of Pin 3 of the PI.?

Huh???

[OldHouseScott]

Your schematic link doesn't seem to work for me. Can you try uploading the file to this site? Use the Additional Options button when posting and attach the schematic.

[J Rindt]

Sorry about that.
I tried to post the schem from My Documents, but I get an error that says the file is too big. It is 502KB. I am not a computer person at all. Is there another way fro me to post this. It is a 192-5. Do not know another way to link to it.
Thanks

[sluckey]

I knew which resistor you meant. I just wanted you to be sure it was not connected "to ground off of Pin 3 of the PI".    

That pair of 6V6s is idling at 34 watts! That's way too hot. Adding screen resistors is a good idea. Dropping B+ will help cool down the dissipation. I don't like the zener method but others do. Are you using a NOS 5Y3? If not, get one. It'll probably drop the 25-30 volts you mentioned. Increasing the value of the cathode resistor is another way to cool the tubes down.

OHS, that same schematic is at the bottom of this page...
http://home.comcast.net/~seluckey/amps/pila/pila.htm

[J Rindt]

Your a genius sluckey (and the rest of you guys too).
That is my amp and the correct shem, yes.
My amp does not have the field coil speaker any more. But I guess this newer schem takes that into consideration.
OK, so I will add some screen resistors.
Regarding B+...... how do you like to deal with that.? I have different value 5W zeners that will do the job, but is there a "better" way.?
And yes, I am using a NOS Sylvania 5Y3.
Thank You

[J Rindt]

Wait a minute......that is your handle at the bottom of that page. The guy who did all that nice work is a friend of yours.?
What kind of hole did he cut to be able to use that nice strain relief for the power cord.? I had to rig up a combination of things to keep the new cord from pushing in or pulling out. Strain reliefs have kind of a funny shape. Will they work in just a regular "round" hole.? I am trying to remember what the back of a BF Fender looks like. Is it more of an oval that the power cored and strain relief go into.?
Thanks

[sluckey]

Regarding B+...... how do you like to deal with that.? I have different value 5W zeners that will do the job, but is there a "better" way.?

NOS 5Y3 for starters. If you're already using a NOS 5Y3, then use the zeners. It really all kinda depends on what you are doing. Are you restoring that Maggie or are you just rebuilding to put in another cab?

Look on page 2 of this pdf for another idea...
http://home.comcast.net/~seluckey/amps/misc/Amp_Scrapbook.pdf

[sluckey]

That project was done by Bill Creller, a member of this forum. I just hosted the web page for him. Where do you live?

[J Rindt]

Sorry, thought I had my location in my profile.
I am on the West Coast.
Kind of right between San Francisco and Sacramento off of HWY 80.
USA CA 94585
I have the original cab. It is in good shape considering a birth date of 1948. I was born in 1960. And I thought I was old.....
I will take some pics of the outside and post them here in a few hours.

That project was done by Bill Creller, a member of this forum. I just hosted the web page for him. Where do you live?

[Willabe]

Schemo says 318vdc on the plates, with 5Y3. Your getting 404vdc with a real 5Y3, and your running the 6V6's on the hot side, which should drop the plate voltage all by it's self.

Are you sure that PT hasn't been changed at some point? I know wall voltage is higher these days then it was when it was built, but since your already running the 6V6's hot, something seems off.       

That's almost a 25% increase, seems a little high for the original PT, but I could be wrong. If you were running the OP tubes on the cool side, then I could see it, but your not.


            Brad         

[J Rindt]

I have Hemmed and Hawed, gone Bask and Forth with this for a few months. I even bought a new PT from Edcor to get lower voltage, but i decided to save it for another build.
The PT sure looks original, but with an amp this old, a PT could be 40 years old and not be close to original. I also bought a choke and thought I might go the input choke-filter route. In the end, I figured it is not worth it. Thought I would just try some Zeners to get the B+ under control, get the amp sounding good, and put it up for sale.
I will post some pics later tonight. Right now I am off to school.....
Thank You All

[jjasilli]

I'm still fiddling with the arithmetic.  Wall volatge in 1948 was probably 110.  Now it's probably 120.  110 : 318 = a factor of 2.9; 2.9 X 120 = 348.  +/- 20% = 417.  Looks to be reasonably within spec to me @ 404.  But if this anaylsis is good, then heater voltage may be 120/110 = 1.1 X 6.3 = 6.9V.  I would check the heater voltage to see if this is correct.  If yes, then you may want to either: a) drop the heater voltage; or b) use a bucking transformer to drop the wall voltage down to 110.

[Willabe]

Yes jjasilli, --but--- thats not taking in a count he's running way hot on the OT tubes, so that should drop the plate  dcv. all by it's self.

      
           Brad      

[J Rindt]

I am not sure how to figure the current draw for the Zeners.....
But I am dropping 35 volts of B+ with a 20 Volt and 15 Volt, 5 watt Zener.
They get way too hot to touch. How do I figure how much power they are dissipating.?
Thank You

[sluckey]

Your power tubes are idling at 90ma. Just figuring an additional 10ma for the other tubes gives a nice round number of 100ma or 0.1a. This is your load current and is also flowing thru the zeners. So, your 20v zener with .1a flowing thru it is dissipating 20*0.1=2 watts. The 15v zener is dissipating 1.5 watts.

Zeners do get hot even when operating at half their rating. Are your zeners the stud mounting type? If so, put them on a heat sink using proper insulating mica washers and shoulder washers.

Have you considered using a bucking transformer? What does your filament voltage actually measure? Now that the B+ is lower, remeasure your plate and cathode voltages on the 6V6s and let's see if the power tubes are running cooler. You may still need to increase the value of that cathode resistor.

[J Rindt]

OK.....
Zeners look like a "normal" diode with a band on the cat side. I have them on terminal strips.
Heaters are at 6.91 VAC
Cat voltage is 21.8 across a 10 watt 250 ohm.
Plates are 375 and 376
Screens are both at 348
Thank You

[jjasilli]

Heaters are at 6.91 VAC  OK, that proves my analysis.  The heater voltage is high but not horrible.  My personal solution would have been a bucking tranny to get the heater voltage down to 5.5 - 6.3.  The B+ voltage would of course drop too.  Then tweak from there.  But since you've already gone the zener route, maybe leave well enough alone.

[sluckey]

You're still idling at 30.9 watts. I'd increase the cathode resistor to run cooler.

[PRR]

> Heaters are at 6.91 VAC

That's not a panic for the heaters, but everything else is high too.

The amp was designed very-hot for 1948's 110V walls. Now you got 120-124V in walls, it is time to add a Bucking Transformer.

Get a 120V:12V 1A transformer. Wire the 120V side to wall through a lamp-limiter! Connect one end of the 12V side to one end of the 120V side. Use clip-leads to attach a voltmeter to the other ends of each winding.

Stand back. Power on.

The lamp-limiter should not glow. The voltmeter will show either 132V or 108V. If 132V, power-down, swap the 12V wires, and try again.

Check again without lamp-limiter (that was just to protect against major screw-up).

Now you have a dandy 122V-110V converter for 1940s amps. I suggest you put it in a box and give it a proper 3-pin modern wall-cord, with an outlet for the 110V. You can mount it inside the cabinet and plug the amp to the box, the box to the wall, or leave it loose to serve all your 1940s amps.

[J Rindt]

I hate to say, but because of where I live, there are NO parts houses for the last 30 years. Would Radio Shack stock a tranny like that.?
Also......I think I follow you, but just to be sure.....
The tranny has a 120 volt primary and a 12 volt secondary. I wire one end of the 120 to one end of the 12. Then I take my "output" from the remaining leads of the 120 and 12 windings. Depending if my arbitrary connection happens to be aiding or opposing. I will see 132 or 108 (round numbers) at the volt meter.
Do I have that right.?
Then I should maybe enclose the whole mess in a tidy enclosure of some sort....
So the biggest question for me is where to get one. Mouser perhaps....or as I said, would Radio Shack stock these.?
Thanks Again

[sluckey]

Did you look on page 2 of this pdf...
http://home.comcast.net/~seluckey/amps/misc/Amp_Scrapbook.pdf

[J Rindt]

Oh wow.....OK, sorry about that. And you even give a Radio Shack part number. Let me give them a call.
Thanks

[J Rindt]

OK....so I built the 12 volt wall dropper. Nice tool and it helps. But I still am too hot where bias is concerned. I cannot seem to bet passed about the 14 watt mark, unless I use Zeners also. And then I can not get past about 10.5 watts at idle.
It seems like changing the cat resistor is just a vicious circle. I either get less B+ and more current, or more B+ and less current.......with the disipation remaining about the same.
I wonder if, in my case, I just simply need to get a new PT that is more of a 250-0-250 kind of thing on the secondaries.? Though I am not sure if somebody makes a PT like that, that will run 2x 6V6, 2x Octal preamp tubes, and a tube rectifier........
Thanks

[jjasilli]

It seems like changing the cat resistor is just a vicious circle. Yes, that tends to be true with cathode bias.  A larger cathode resisitor not only limits current draw, but also boosts plate voltage, while increasing cathode voltage.  The net result is that the tube's bias keeps re-balancing itself around the same wattage dissipation.  The solution is to change the supply voltage.

I'm not getting your numbers.  WITH the bucking tranny AND NO zeners, what is the plate dissipation for EACH POWER TUBE individually?  Please show how you calculated it.

[J Rindt]

Maybe I am doing the wrong math......?
The plates are at 359.
The cat resistor is at 21.
The cat resistor is 270 ohms.

21/270 = .0777777
.077777/2 = .0388888
 
359-21  = 338

338 x .038888 = 13.15 watts at idle.
Am I doing this wrong.?
Thank You

[jjasilli]

Good.  Now check tube specs:  max plate diss. for ea tube:  old 6V6 = 12W; close enough; for more modern 6V6GTA = 14W.  No problem.  

[J Rindt]

Good.  Now check tube specs:  max plate diss. for ea tube:  old 6V6 = 12W; close enough; for more modern 6V6GTA = 14W.  No problem.  

Hey......
As a matter of fact, all tubes are NOS Except for the Power Tubes. They are modern production Tung-Sol 6V6.
I realize they are kind of a 14 Watt tube, but I thought 13.2 watts at idle would still be Way Too hot. Do you think I am more worried about this than I need to be.? I can handle tube life being short, as long as the amp sounds/operates well.
So what do you guys think.....13.2 Watts. Still too much.?
Thanks

[jjasilli]

I do think your bias is hot, but OK.  PRR seems to agree per his post above.  The proof is in the pudding. Test for redplating with your guitar, or better: signal generator @ 200 - 800mV > amp > dummy load.