Troubles with EL84 class A output stage - Page 2 - diyAudio
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Old 4th June 2012, 05:14 PM   #11
rongon is offline rongon  United States
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Location: Across the river from Rip's big old tree...
Yup.

Also, guitar pickups are high impedance. I think the vol pot on a typical Gibson guitar is 500k. That 10k pot on the input of the amp would load down any normal electric guitar pickup. It would probably sound dull and flat, lifeless.

I hope that helps.
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Old 4th June 2012, 09:51 PM   #12
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Quote:
Originally Posted by payloadde View Post
the control grid of your el84 is floating ...
add 470k resistor from el84 control grid to ground (connect between 0.1uF and 5.6k)
what he said

also I would change the 470k of first gain stage to 1M if you have passive pickups
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Old 5th June 2012, 02:37 AM   #13
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Thanks everyone - I got a chance to tinker tonight and unfortunately still having stability issues.

I updated the following based on your feedback (image below):

1. Added grid leak resistor (tried connecting to the control grid as well as between the 0.1u cap and 5.6k, and also tried values of 470k and 300k)
2. Put stage in triode mode by connecting 100 ohms from the middle grid to the anode.

With these changes, voltages still fluctuate like crazy (including control grid voltage - this measures anywhere from -10V to -24V and the cathode which fluctuates from +5.5V to +13V), though I'm at least getting current through the tube (though that fluctuates too - anywhere from 11mA to 37mA is what I measured).

Now, tube current/conduction changes with its input signal, correct? I'm noticing I get a continuous low frequency thump out of the speaker after the tubes warm up...maybe the amp is just very noisy for some reason and this is the cause of the bias fluctuations? Just a thought.

And this is the PT I'm using - I have a hard time believing I'm overloading it if it can really source up to 4 amps. I also have a 3 amp fuse which has never blown.
24V, 4A Center Tapped (12-0-12) Transformer-MPJA, Inc.

Thanks again in advance for any help!

Click the image to open in full size.
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Old 5th June 2012, 08:44 PM   #14
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To obtain triode mode you would directly connect the plate and the screen. If you want it switchable take a three pole switch and on the center lug connect to screen, the plate gets connected to one side of the switch, and the screen supply would be on the other side of the switch.

Where are you getting your B+? How is it filtered? I have a hard time reading the power supply schematic.
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Old 5th June 2012, 09:11 PM   #15
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C2 would be your screen supply and would go on one side of the switch, the other side of the switch to the plate, and the center of switch would go the screen of EL84 in series with a 100 ohm resistor.

Also I would change the cathode resistor to 250 ohms otherwise you will have a cold bias especially with a large signal the valve self bias' itself colder. Reading anywhere from 10 to 12 volts dc across 250 ohm cathode resistor is in the ballpark.

If you don't have a center tapped secondary on power transformer you will have to use 4 diodes in a bridge rectification configuration.
Attached Images
File Type: jpg EL84.jpg (202.0 KB, 90 views)

Last edited by famousmockingbird; 5th June 2012 at 09:16 PM.
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Old 5th June 2012, 10:05 PM   #16
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So the way I have the 100 ohm wired in my last post is correct for triode mode "only". I'm not yet at the point where I care to make the modes switchable, I'd just like "a mode" to work first.

Below is how I am generating my B+. It's a cheap alternative because the PT I'm using is 4A but very cheap and I feed 16Vdc into this module to get 200Vdc (45mA current, but it's enough).
Click the image to open in full size.


I have it filtered exactly as shown in my first image post with the capacitors n such. I assume I need to try adding more filtering to ensure my power rail is absolutely stable - that might keep my bias voltages more stable as well?
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Old 5th June 2012, 10:25 PM   #17
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I don't think that device will give you the results you are looking for. Also if you haven't fixed your heater configuration for the EL84 than you are asking for some headaches.

There is a reason good iron is costly

Last edited by famousmockingbird; 5th June 2012 at 10:29 PM.
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Old 6th June 2012, 12:12 AM   #18
jjman is offline jjman  United States
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Mount the 470k to the other side of the 5.6k to have the 5.6k act as a stopper, which is it's purpose. Leave the grounded side of the 470k grounded.
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Old 10th June 2012, 08:06 PM   #19
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Quote:
Originally Posted by jjman View Post
Mount the 470k to the other side of the 5.6k to have the 5.6k act as a stopper, which is it's purpose. Leave the grounded side of the 470k grounded.
I moved this resistor and tried many values ranging from 100k up to the max 1Meg.

Now I am having an issue with the speaker motorboating. I posted a video here for those interested to hear the actual sound (it's kind of funny - and sorry my phone didn't turn the view sideways) --> DIY tube amp motorboating - YouTube

When I play guitar through the amp, it's like I'm fighting the motorboating to get it out of the speaker. I have to strum hard enough or it's like the guitar signal is eaten up by the motorboating.

As I was troubleshooting, I noticed that if I touch my Fluke 115 multimeter to the plate of the very first Preamp tube (12ax7), the motorboating "goes away"; that is, it lessens in amplitude greatly but also seems to rise in frequency greatly. The noise is much less noticeable, but the speaker is still humming pretty good. I didn't capture this part in my video, but any explanations for this behavior? What is my meter doing to the circuit that would help stability?

My power supply voltage is very steady but I've added more filtering caps without any noticeable benefit.

From what I've read, motorboating could be caused by the grid leak resistors being too high which causes current to not be returned to ground quickly enough as it comes out of the tube...but all my values are the standard 470k, etc...
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Old 10th June 2012, 09:26 PM   #20
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Decoupling cap is not decoupling properly due to high impedance in preamp is my guess. Do you have an esr meter?
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