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Old 20th March 2008, 10:47 AM   #1
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Default 807 AB2 oscillation maybe?

Hi,

Sorry if this post is a bit brief but im at work.

i have designed an AB2 guitar amplifier using 807's ive tested all up till the 807 grids as working perfectly.
When i connect the 807 grids i get a waveform like the one in the picture attached.

It works fine up until about 20v peak to peak on the grids then increasing it further gives the distorted waveform and a whistley sound.
When the distortion occurs the neon shunt regulators on the screen of the 807's flicker like theyre drawing a lot of screen current?

HT is about 460V, bias is around -23v on the 807 grids giving 3V across their 47ohm anode resistors (64mA per tube), screens are nominally at 300V.

2*12ax7 gain stage => 12ax7 LTP => MOSFET fixed bias and drive using IRF820's => 807 grid.

Any ideas because im a little stumped so far?
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Old 20th March 2008, 11:02 AM   #2
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Can you post a schematic?
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Old 20th March 2008, 11:28 AM   #3
SHiFTY is offline SHiFTY  New Zealand
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does the screen supply have decoupling for each valve: i.e a resistor to each screen?

Is it fixed bias?
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Old 20th March 2008, 11:41 AM   #4
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Thanks for replies,
Ill draw up a schematic in about an hour, hopefully i can squeeze it into the picture size limit.. that always gives me grief.

I have no resistors on the screens i just have the voltage dropping resistor for the shunt which is 6K.
i.e. both screens are connected directly to the node which is between the top of the neon stack and one end of the 6K dropping resistor.

The bias is fixed by the mosfet drivers whose sources are retuned to an adjustable negative supply.
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Old 20th March 2008, 12:29 PM   #5
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Heres schematic,

-supply for 12ax7's is 260V

-6L6 is actually 807

--sorry for dodgy symbols i had to whip it up fast

-from the phase inverter onwards i only drew one half of the output stage because it is identical.

-D3 and D2 represent VR150 regulators

-the inductors represent the output transformer primary

-D4 goes to 460v AC
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Old 20th March 2008, 12:54 PM   #6
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Re: flickering VR tubes...could your B+ be sagging enough during transients to briefly fall below the necessary strike voltage for the tubes?(WAG)

It looks like you aren't employing grid stoppers. They may help prevent any oscillations.

Are you employing any global feedback?
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Old 20th March 2008, 01:15 PM   #7
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Thats an interesting point, i havent checked the B+ when driving with signal. Ill look at it tonight.
I have got 300ohm stoppers on all the 12ax7's, and 1k on each mosfet gate, i read it is best not to put stoppers on the 807's in AB2 as the grids draw current and youd get a reduction in grid volts when going from AB1 to AB2 on the signal peaks?

Should i have one side of my OPT connected to the signal ground? at the moment the speaker and secondary are floating.
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Old 20th March 2008, 01:51 PM   #8
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Quote:
Originally posted by Craig405


Should i have one side of my OPT connected to the signal ground? at the moment the speaker and secondary are floating.
You need to ground your OPT secondaries. I ground mine to the chassis at my PSU ground point. I'm surprised that you aren't getting any hum leaving them floating.

...a grid stopper may be a necessary evil if you are getting an oscillation.

...no feedback?
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Old 20th March 2008, 02:05 PM   #9
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Yes forgot to say, no feedback.
Right ill ground the secondary, suprisingly little hum actually!
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Old 20th March 2008, 07:26 PM   #10
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According to your schematic, you aren't using any grid stoppers. Those 300 ohm resistors (68k on the first stage) are in the wrong place to act as stoppers, they need to be between the grid resistor and the grid itself and soldered to the grid pin with a very short lead. I doubt very much if that would be the cause of your grief, though, since the 12AX7 has very low Gm.

If you're sharing the screen regulation between two tubes, then there is only 26.67mA available before the screen voltage will drop below 300v. Since each screen will draw approx. 10mA at full power, that leaves only 6.7mA for the VR tubes, which have a minumum current of 5mA. My guess would be that the VR tubes are dropping out of regulation at maximum power - which would make them flicker!

Try reducing R24 to 4.7k, allowing 34mA total current. That will be able to provide 10mA per screen and still have 14mA left over for the VR tubes. I don't know what the no-signal screen current would be but even if it dropped to zero, those VR150 tubes could handle the full 34mA OK (max is 40mA). While you're at it, add a screen stopper to each tube of 150 ohm, because parasitics there are a possibility.

Better than just using those puny VR tubes on their own, though, would be to amplify the regulation voltage with a source follower, using an IRF820.
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