50 watt home build doesn't like 120v!

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50 watt home build doesn't like 120v!

Postby grasshopper on Sat Apr 09, 2016 4:58 am

I'm coming close to the end of my first amp build. I am through most of the debugging, but I have one problem I need help solving before I can put her in a head cabinet and claim victory. The amp is unstable at 120v. It runs well when plugged into the variac at ~75v, but when run at 120v the volume begins to swell (oscillations?) until it sounds like the speaker will blow and the amp must be turned off. I have taken detailed voltage measurements all over the amp and have the data in excel format. I have an old oscilloscope, good DMM, and some other test equipment. I would greatly appreciate any help!

The first video is of the amp going to failure- after powering on, and switching off of standby, the volume begins to increase like normal but doesn't stop increasing. The next two videos are of the amp at ~75v in operation.

[youtube]https://youtu.be/OqstYe4VdJs[/youtube]
[youtube]https://youtu.be/-QoYdLazCX0[/youtube]
[youtube]https://youtu.be/d4qDlGhvYcI[/youtube]

Here are some images of the amp.
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Re: 50 watt home build doesn't like 120v!

Postby tracynorton on Sat Apr 09, 2016 3:42 pm

double double triple check your wiring on mains primary....if it works on 75vac...my guess is you've wired it to the j@p tap
dump those MM trannies.....i mean it...don't make me come over there :x
:POd: :mrgreen: :bday: :old: :tantrum: :tantrum: :tantrum: :whistling:

seriously fire it up at 75v and check the heater voltages to confirm this
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Re: 50 watt home build doesn't like 120v!

Postby grasshopper on Sun Apr 10, 2016 12:15 am

Well as much as I would love for this to be the problem, it doesn't seem to be it. I thought about mentioning in the initial post that I had this thought myself and double double checked it, but I went ahead and tripled checked it. The first EL34 reads 4.1v, the last ecc83 reads 4.0v. 75/120=.625, 4.1/6.3=.65, so close enough. Giving the amp 120v should get the heaters to about 6.3v with a reasonable drop at the end of the line. I uploaded a new video that better illustrates the issue I'm having. It appears as a hum shortly after the amp turns on, and only at 120v. I'm crossing my fingers and hoping it's not a lead dress issue that only presents at 120v when the em fields around the wire are larger. :dunno:

It is coincidental that in the video the test signal stops as the humming begins. The signal continues coming through clean after switching off of standby but then the hum builds 5-10 seconds after.
[youtube]https://www.youtube.com/watch?v=ThqDBnRcHwo&feature=youtu.be[/youtube]

I also uploaded a copy of the spreadsheet showing voltages at different points in the amp. It's set up as stated voltage (as read from the schematic), stated adjusted (adjusted proportionally for 75v), and the (actual) measured voltage. I hope this helps. http://s000.tinyupload.com/?file_id=83517351569699829517

Also, what's wrong with Mercury Magnetics?
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Re: 50 watt home build doesn't like 120v!

Postby tracynorton on Sun Apr 10, 2016 12:27 am

grasshopper wrote:Well as much as I would love for this to be the problem, it doesn't seem to be it. I thought about mentioning in the initial post that I had this thought myself and double double checked it, but I went ahead and tripled checked it. The first EL34 reads 4.1v, the last ecc83 reads 4.0v. 75/120=.625, 4.1/6.3=.65, so close enough. Giving the amp 120v should get the heaters to about 6.3v with a reasonable drop at the end of the line. I uploaded a new video that better illustrates the issue I'm having. It appears as a hum shortly after the amp turns on, and only at 120v. I'm crossing my fingers and hoping it's not a lead dress issue that only presents at 120v when the em fields around the wire are larger. :dunno:

It is coincidental that in the video the test signal stops as the humming begins. The signal continues coming through clean after switching off of standby but then the hum builds 5-10 seconds after.
[youtube]https://www.youtube.com/watch?v=ThqDBnRcHwo&feature=youtu.be[/youtube]

I also uploaded a copy of the spreadsheet showing voltages at different points in the amp. It's set up as stated voltage (as read from the schematic), stated adjusted (adjusted proportionally for 75v), and the (actual) measured voltage. I hope this helps. http://s000.tinyupload.com/?file_id=83517351569699829517

Also, what's wrong with Mercury Magnetics?

Are you in LA....bring it over here.
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Re: 50 watt home build doesn't like 120v!

Postby grasshopper on Sun Apr 10, 2016 12:34 am

Sadly, no..Ohio. But I do have a microphone/webcam setup that I've hooked up on the bench before. I'd be happy to set up a virtual troubleshooting session if you'd be up for it.
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Re: 50 watt home build doesn't like 120v!

Postby tracynorton on Sun Apr 10, 2016 12:59 am

grasshopper wrote:Sadly, no..Ohio. But I do have a microphone/webcam setup that I've hooked up on the bench before. I'd be happy to set up a virtual troubleshooting session if you'd be up for it.


nope, if you ain't gonna come over and drink beer and fiddle w/ yer amp....deal's off

Post the datasheets for your PT, and the EXACT schematic you built to.
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Re: 50 watt home build doesn't like 120v!

Postby grasshopper on Sun Apr 10, 2016 2:24 am

I realized I have left out some pertinent information. I have been calling it a Zodiac 50 because I built the power and output section from this zodiac schematic http://oi65.tinypic.com/za3qp.jpg. Everything else is the same as your schematic. The power transformer datasheet is here http://i42.tinypic.com/15pkfnm.jpg.

The main difference I found centers around the grids of the output tubes (pin 5). The Zodiac 30 schematic shows the grids going through 22k and 220k resistors, then straight to ground. The Zodiac 50 schematic shows the grids and their 22k and 220k resistors going back to some components off one of the rectifier plates. Mine is set up as the 50w. Is the difference here fixed vs. cathode biasing? If you believe this is the problem I will remove the link between the grid resistors and the stuff in the power section and put it to ground, as your schematic shows.

I was surprised to see how cheap airfare is since the price of oil has dropped! I found a roundtrip ticket from Cincinnati to LA for $178. That's just insane. If we get this thing working, who knows, maybe I'll ship it to you and catch a flight to come jam :jam:. I owe you at the least a case of beer!
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Re: 50 watt home build doesn't like 120v!

Postby tracynorton on Sun Apr 10, 2016 2:55 am

grasshopper wrote:I realized I have left out some pertinent information. I have been calling it a Zodiac 50 because I built the power and output section from this zodiac schematic http://oi65.tinypic.com/za3qp.jpg. Everything else is the same as your schematic. The power transformer datasheet is here http://i42.tinypic.com/15pkfnm.jpg.

The main difference I found centers around the grids of the output tubes (pin 5). The Zodiac 30 schematic shows the grids going through 22k and 220k resistors, then straight to ground. The Zodiac 50 schematic shows the grids and their 22k and 220k resistors going back to some components off one of the rectifier plates. Mine is set up as the 50w. Is the difference here fixed vs. cathode biasing? If you believe this is the problem I will remove the link between the grid resistors and the stuff in the power section and put it to ground, as your schematic shows.

I was surprised to see how cheap airfare is since the price of oil has dropped! I found a roundtrip ticket from Cincinnati to LA for $178. That's just insane. If we get this thing working, who knows, maybe I'll ship it to you and catch a flight to come jam :jam:. I owe you at the least a case of beer!


if you built it fixed bias like the 50W...it should be fine. What plate dissipation did you bias it at?
grid resistors are fine, as well as 220k to bias voltage. I really need to see a build schematic....not just two or something else you bodged together.
c'mon out. Sierra Nevada Pale ale is my brand. you gotta get away from all them white folk.
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Re: 50 watt home build doesn't like 120v!

Postby grasshopper on Sun Apr 10, 2016 3:01 am

I have not biased the amp. In fact, I have never biased an amp. :oops: I can work on a build schematic for you, but the only difference I'm aware of is the one I pointed out, with the grids of the output tubes. Other than that it was built from your schematic. I'll start putting one together for you, but I've never drawn a schematic of this complexity (I bet that has you grinning lol). It will take me a few days.
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Re: 50 watt home build doesn't like 120v!

Postby pdf64 on Sun Apr 10, 2016 4:44 pm

For a given set of operating conditions (ie plate, screen grid and control grid / bias voltages, WRT the cathode) plate current of an actual tube (as opposed to a nominal standard bogey tube of that type) may vary significantly, eg +/-20%.
Hence plate current should be checked and control grid / bias voltage adjusted as required to ensure it is suitable.
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Re: 50 watt home build doesn't like 120v!

Postby Unit_1 on Sun Apr 10, 2016 5:53 pm

biasing is super easy, you measure the b+, then put it through an equation

(tube power *.7)/b+

and that will give you the mA to set the tube at

so for el34 (on my plexi clone) it's

(25 watts *.7) = 17.5

17.5 / 440 volts = 0.397 or about 40 mA, I like it at around 36....

Ohm's law states that at 1 Ohm, mV and mA are equal. So instead of measuring mA you put a 1% 1 Ohm resistor in between tube ground and ground. Then you set your multimeter to mV, hook the ground probe to the chassis, and touch the positive probe to the tube side of the resistor so you're measuring across that 1Ohm resistor.

be sure to turn the trim pot all the way to maximum negative polarity voltage before turning anything on so the tube is running at it's coolest. You can measure this before putting the tubes in so you know you're turning the trim pot the right way.

I think of it this way. The tubes are running on positive voltage. So to cool them off, you need negative voltage. That's why the higher the negative voltage, the cooler the tubes run. This is a simplification of what's actually happening, but it makes it easier for my tiny mind to grasp, lol...
Last edited by Unit_1 on Tue Apr 12, 2016 9:49 pm, edited 1 time in total.
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Re: 50 watt home build doesn't like 120v!

Postby grasshopper on Mon Apr 11, 2016 1:46 am

I am familiar with the cathode resistor method..on paper anyway. I'll be glad to get some first hand experience with it. I ransacked my bench tonight looking for some 1-ohm resistors but came up short. I'll have to pick some up tomorrow and report the findings. Thanks for the replies fellas!
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Re: 50 watt home build doesn't like 120v!

Postby Unit_1 on Mon Apr 11, 2016 4:06 am

Remember they have to be the super accurate 1% resistors, the blue ones! Very important.
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Re: 50 watt home build doesn't like 120v!

Postby grasshopper on Mon Apr 11, 2016 8:33 pm

Ok. I picked up the 1 ohm 1% resistors today and put them in place between the cathode and ground. Also, to make adjustments I installed a trim pot on the bias supply resistor. Initially the amp was biased a bit cold, about 31mA. I would guess this to be within range but I went ahead and adjusted it up to 36mA. I'm still having issues at anything over ~75v. :crybaby:
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Re: 50 watt home build doesn't like 120v!

Postby Unit_1 on Mon Apr 11, 2016 11:10 pm

I couldn't access your spreadsheet, my system warned me of potential bad-ness so I aborted

What is your B+? If it's not 440v you need to re-run the calcs in the equation.

btw, it's a nice looking build and I totally love Mercury Magnetics. I use it in all my builds since they're close enough for me to pick up in person and I get 'the good guy' discount of around 50% (which you get after buying 25 trannies) :mrgreen:
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