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In Reply to: RE: Any approach we can make 6h30 warmer and richer? posted by Eric Chan on January 30, 2017 at 03:44:02
High gm types, like the 6n30p, are particularly vulnerable to parasitic oscillation. Parasitic oscillation at VHF or higher frequencies can cause "hard" sound. Use carbon comp. grid stoppers of substantial value and 100 ohm carbon film stoppers on the plates.
Eli D.
Edits: 01/30/17Follow Ups:
I second Eli's recommendation to use grid stoppers and plate stoppers.
I suggest that you look elsewhere in your circuit to soften the sound. Capacitors and resistors are candidates.
Dave
Thanks everyone for all inputs
The Rp of 6h30 is 820 ohm. Any gain or output impedance impact if I add 100 ohm to its plate as plate stopper then ?
Small RF chokes won't add resistance. Something like 15 turn spiral of 18 ga wire, 1/4" diameter.
"The Rp of 6h30 is 820 ohm"
At what operating point?
Yours?
Va Ia Vg1 Gm Mu Ra (Rp)(ohms)
50 5 -2.8 7.1 15.2 2100
50 10 -2.3 9.8 15.9 1600
50 20 -1.5 12.3 16.3 1300
90 10 -4.7 8.4 15.3 1800
90 20 -3.9 11.7 16.0 1400
90 30 -3.1 13.3 16.2 1200
120 10 -6.5 7.6 14.9 2000
120 20 -5.6 10.9 15.7 1400
120 30 -4.8 12.9 16.1 1300
120 40 -3.9 13.3 16.3 1200
180 10 -10 6.5 14.5 2200
180 20 -8.9 10.0 15.3 1500
180 40 -7.1 13.3 16.0 1200
250 10 -14.4 5.8 14.4 2500
250 20 -13.2 8.7 15.0 1700
250 30 -12.1 11.0 15.4 1400
250 40 -10.5 12.5 15.8 1300
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
300-ohms?
Thanks!
I would give 300 ohms a try and if that doesn't stop the oscillation (if that's what he's hearing) then make it bigger.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
This brings up an interesting parallel to the HFAC filament discussions. Elsewhere it has been insisted that we cannot hear the effects of HFAC since the frequency is well above what humans can here. If this logic is to hold true then surely we cannot hear an oscillation in the mHz range yet people claim the effects of an oscillating tube as a bright or harsh sound.
what gives?
dave
Oscillation usually happens with grid current...and I once saw a 12BY7 LTP go singing away at a few tens of MHz...and the sonics were not very good at all...:) Now if it is a semi-regular oscillation, triggered by some of the program, getting harsh is quite likely, though you're not hearing the oscillation, it is the other side effects that present...:)
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
Good point Dave, I'm curious if HFAC heating has any IM artifacts in the audio band...
Often the problem with RFI in a circuit is when it gets rectified. When that happens any modulations become audible.
Preventing the RFI in the first place is the better move...
If high frequency (above our hearing range) is just steady HF that's one thing.
If a circuit is falling in and out (parasitic) of oscillation at a rate that we can hear that's a different thing.
Just a thought.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
I've seen that so many times. This is one of the reasons it's so silly when people say engineering is unimportant.
--------------------------
Buy Chinese. Bury freedom.
IMO/IME, in DIY projects, it's best to assume parasitic oscillation will be present and appropriate prophylactic measures taken, before a single connection is soldered into place.
Darned tootin, engineering matters. 1st, get the Physics right! Then, you know the field you're "playing" on. If Newton, Einstein, Maxwell, et al, say you are smoking loco weed, move on.
Eli D.
used to smoke loco weed in Newton...or was that eat Newtons after smoaking loco weed?
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
Eli D.
IM distortion products inside the audio band are (IMO) a likely candidate.
Remember, IM distortion, even in small amounts, is highly irritating.
Eli D.
and finding spurious 'puffs' of oscillation is difficult at best.
Stoppered adequately, I have not found anything wrong with them.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
Warm and rich can be measured. Looking at system frequency response, distortion, and oscillation will tell you a lot.
You may have a system which is too bright being driven well. Sometimes designers counteract the fact that their circuits are being driven by something whimpy that cannot overcome miller capacitance and cable losses, so they make stages bright to compensate. This is not a bad thing in that in can make the total system flatter. When you put a good drive circuit in the mix it can make things bright. You may have to tame the frequency response to warm things up. In olden times there were knobs with the words "Treble" and "Bass" on them. Those were crazy times.
You are making and listening to a system, not a component.
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