Troubles with EL84 class A output stage - diyAudio
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Old 3rd June 2012, 09:57 PM   #1
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Default Troubles with EL84 class A output stage

Hey folks - first post here and just wondering if anyone could offer some direction. I am building a low power tube guitar amp without following any particular design. I'm working on the output stage which is single ended, class A with one EL84.

For what its worth, the previous stages before the output stage operate correctly. Also, I do get dirty little bursts of guitar output from the speaker if I strum the strings and/or have the volume pot up enough.

My schematic is shown below with measured voltage values and I'm looking to have the following observations explained:

1. There doesn't seem to be any current flowing through the EL84. I know the middle grid voltage should be a bit lower than B+ but with a 4.7k resistor I'm only dropping 0.1V across it. Bigger resistor I guess?
2. My grid voltage fluctuates like crazy when I probe it with the multimeter (between -50V and -11V??).
3. I'm using a Hammond 125CSE OT using the 10k primary and 16 ohm speaker. However when connected into the circuit, I measure 194 ohm primary and 1.6 ohm speaker!!
4. During the initial heat-up process, I get a large AC burst out of the speaker which quickly decreases in amplitude and frequency down to nothing.
5. I know the EL84 ONLY uses 6.3Vac for the filaments but I started building by using 12.6Vac per my 12AX7 tubes. But I notice once the standy switch is closed and main power delivered, the heater voltage of the EL84 actually drops itself down to 6.3Vac automatically!!

Thanks for any help in advance.

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Old 3rd June 2012, 10:04 PM   #2
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Take a good look at your schematic around the EL84 and you will soon understand!
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Old 3rd June 2012, 10:15 PM   #3
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Well, I chose my 470 ohm resistor to set cathode bias at -10V which should give me ~22mA current through the tube, correct?

I get that the tube is obviously in cut-off because the grid voltage is so negative relative to the cathode. But I can't explain why it's so negative, or why it's so unstable.
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Old 3rd June 2012, 10:20 PM   #4
7N7 is offline 7N7  United Kingdom
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No grid leak? Usually 470k.

Paul
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Old 3rd June 2012, 10:28 PM   #5
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the control grid of your el84 is floating ...
add 470k resistor from el84 control grid to ground (connect between 0.1uF and 5.6k)
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Old 4th June 2012, 12:50 AM   #6
Fenris is offline Fenris  United States
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1. No current because, as others have pointed out, there is no resistor that references the EL84 grid to ground. 200K to 470K should work.
2. See above.
3. If you're just measuring the transformer, that's the DC resistance of the copper coils. The AC impedance of the speaker reflects back to the primary of the transformer with the proper impedance.
4. Magnetic coupling maybe? Maybe the current through the EL84 is fluctuating before it settles down into cutoff.
5. As the heater warms up, resistance increases. If you overvolt it, it will overheat and resistance will increase quickly. Increased resistance will let less current flow, so it won't go into thermal runaway, but it will use ALOT more power. It should stabilize at somewhere above 6.3v. Your power transformer could be undersized and the voltage could be dropping for all the heaters, which would be bad for the input tubes. I'd recommend putting a 8 ohm 10 watt resistor in series with the EL84 heater. Otherwise your EL84 lifespan could be thrillingly short.

BTW, you're running pentode mode with no feedback? That should be...interesting. If it were me, I would ditch the 4.7K resistor between G3 and B+ and go with a 100 ohm between G3 and the tube anode to run in triode mode. Less power, but gives the tube greater internal feedback.
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Old 4th June 2012, 02:13 AM   #7
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Does the EL84 filament still light? 12 volts is enough to toast it pretty much immediately, so no current draw. Before you do anything else, find yourself some 6.3 volts, which can also be used for the 12AX7 (wire pins 4 and 5 together as one side and use pin 9 as the other side) and add a grid leak resistor. Should make you a nice guitar amp.

All good fortune,
Chris
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Old 4th June 2012, 02:36 AM   #8
kevinkr is offline kevinkr  United States
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Per forum policy this really belongs in the Instruments and Amps forum, see the headers at the top of tubes/valves and I&A for an explanation of why.

[/moderation off]

As has been mentioned you really need that 470K grid resistor, once that has been addressed you will have plate current, and likely the input transformer will fry shortly thereafter.. You either need a proper power transformer available at Edcor at a reasonable cost or at least a separate 6.3V transformer for the 6BQ5 filament - the fact that it drags the voltage down to 6.3V in combination with the switcher is a good indication that the transformer is massively overloaded.

A traditional transformer based supply will provide more of what you are looking for in terms of guitar amplifier performance..

Here is one possibility which if used in conjunction with something like a 6CA4 rectifier tube would probably be pretty decent. Use duncan amps psud2 to design the input filter: http://www.edcorusa.com/p/604/xpwr013_120

(Note that there is also a 240V version as you provided no location information. Please provide as that will help people to suggest appropriate parts and vendors.)

and http://www.duncanamps.com/psud2/index.html
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Old 4th June 2012, 04:15 AM   #9
taj is offline taj
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The volume control at the input is redundant as it's in series with your guitar's volume control. If you move it to between the 2 triode sections of the input tube and use a 500k or 1M pot, you'll end up with a typical guitar amp gain/volume control.

..todd
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Old 4th June 2012, 05:08 AM   #10
kevinkr is offline kevinkr  United States
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