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Hi. I am driving a pair of 5881 in class AB2, with positive grid drive. When pushing this amp as hard as possible, actually letting it clip at some passages, I get this nasty sound now and then. It sounds like the speaker voice coil is loose and scratching against something, only very loud and pretty unfriendly harmonics wise.
Is this the sound of the tubes going into grid-cathode diode conduction, or whatever it is called?
Am I driving it with too high impedance? I use a 1:1+1 interstage and a 6922 runnning at 6mA as a driver. The gridresistors are high, 100kohms, because I want the interstage tranny to dominate. Lower resistances flattens the freq respons, but I prefer the sound of higher resistance.
Follow Ups:
Nikko
I think you mean its cathode biased.Thats some setup..What B+ voltage are you running on the 5881s? Typically class AB has some headroom where you shouldnt get a real hard clip as you speak of if its designed right.
Also why dont you eliminate the IT for a test and RC couple it and see what happens.This circuit you describe is typical on a lot of SET amps with the IT and a high transconductance driver.
One more question..Are you using this as a guitar amp? Its sounds like you were referring to one speaker and the 5881 is a popular guitar amp tube.
Thanks for reply. Yes this is for guitar, well guessed!
B+ is 360volts and the screens are at 300volt.
I love the sound of this amp and I think it's because of the 6922 driving the output with an IT. The 6922 provides reasonable low impedance for a slight positive grid drive, and the IT gives me the two opposite phases for PP operation.
The output is cathodebiased, and as it starts to clip, the cathodecap charges a bit, so there is some grid current going thru.
The nasty sound seems to happen around 200Hz, like when hitting the 1st string/7th fret together with another note. But I am having trouble duplicating on the bench so I havent found the real source of the problem. My guess is the grid is driven hard, but the driver cannot source enough current and there is some nasty grid-clipping added to the otherwise comfortable clipping of the output tubes.
I guess I should use a beefier IT and parallell a couple of 6922 and see if it helps, but beefier IT are expencive.
Hi,The input impendance of the output tubes goes from close to infinite down to 1 á 3 kohm once it goes into grid current.
Ideally the driving source - besides manage the power requriements (peak voltage times peak current at grid) - sould have a source impendance less than 100 to 300 ohm.
My guess is that a SEPP/SRPP/'my follower' 6922 driving the transformer would manage, and as a bonus lessen the DC over the primary of the IT (DC may also be a nasty thing).Give or take some personal opinions :-)
I agree if this were for HiFi, but the frequencyrespons I get when driving with higher impedance is very good for guitar.
I think I'll try a triode strapped 6V6 instead of 6922 as drivertube.
Hi,You could taylor the frequency responce by adjusting the inbalance of the SEPP and so adjust the DC through the IT and so get some transformer distortion (large signal core saturation) on lower frequencies.
:-)
> My guess is the grid is driven hard, but the driver cannot source enough current and there is some nasty grid-clippingI think you need a *much* beefier tube than the 6922 (and a beefy driver power supply) to drive this into A2 successfully. You'll probably need a sine-pulse generator to see what's happening on the bench.
Yes, I think I need something beefier than a 6922, and may try a triode strapped 6V6.
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