DIY Audio was kind enough to remind me that it is my birthday today, so I decided to celebrate by starting a thread 🙂
My brief flirtation with a B&H FoS 626 (unconsummated) has left me curious about the PC97, of which this amp contained several. Looking at Merlin B's discussion about how he uses it in a pre-amp design, it looks like there should be a fairly smooth transition from the clean region where he is using it into high-gain, high-distortion territory. They are cheap, so I plan on experimenting.
The only thing that worries me is that this is one of them new-fangled frame-grid tubes, and may therefore be prone to microphonics. Which in turn led me to ask myself if there are ways to forestall this problem (in a combo design) by acoustically protecting the pre-amp stage(s): I'm imagining things like an isolated box-within-a-box, or even an "air chassis" where the tube sockets are hung from a butyl-rubber spider the way that microphones are often hung. Just curious to know if anyone ever went down this path to overcome microphonics.
My brief flirtation with a B&H FoS 626 (unconsummated) has left me curious about the PC97, of which this amp contained several. Looking at Merlin B's discussion about how he uses it in a pre-amp design, it looks like there should be a fairly smooth transition from the clean region where he is using it into high-gain, high-distortion territory. They are cheap, so I plan on experimenting.
The only thing that worries me is that this is one of them new-fangled frame-grid tubes, and may therefore be prone to microphonics. Which in turn led me to ask myself if there are ways to forestall this problem (in a combo design) by acoustically protecting the pre-amp stage(s): I'm imagining things like an isolated box-within-a-box, or even an "air chassis" where the tube sockets are hung from a butyl-rubber spider the way that microphones are often hung. Just curious to know if anyone ever went down this path to overcome microphonics.
Hi Zonnebloem,
happy birthday! 🙂
Please note that a frame grid construction is not "per se" especially prone to microphonics. Mostly it is a matter of very high gain in a stage - and frame grid valves were just designed for very high gain. But often enough you don´t need to go for very high gain. F.e, frame grid pentodes like EF184 work very nicely in first stage position of guitar amps, if you design for a more or less "usual" stage gain.
Suspensions for tubes in high gain stages were used commercially indeed - for example in RIAA preamps using EF86.
Attached you find two suspension drawing examples from the Ludwig Ratheiser "Das große Röhren-Handbuch", page 141. The "foam rubber" one is of the damping type, while the soft springs plus the weight of the iron shielded base type form a classical mass - spring decoupling system. Guess which one works better and keeps its properties longer 🙂
Kind regards, Tom
happy birthday! 🙂
Please note that a frame grid construction is not "per se" especially prone to microphonics. Mostly it is a matter of very high gain in a stage - and frame grid valves were just designed for very high gain. But often enough you don´t need to go for very high gain. F.e, frame grid pentodes like EF184 work very nicely in first stage position of guitar amps, if you design for a more or less "usual" stage gain.
Suspensions for tubes in high gain stages were used commercially indeed - for example in RIAA preamps using EF86.
Attached you find two suspension drawing examples from the Ludwig Ratheiser "Das große Röhren-Handbuch", page 141. The "foam rubber" one is of the damping type, while the soft springs plus the weight of the iron shielded base type form a classical mass - spring decoupling system. Guess which one works better and keeps its properties longer 🙂
Kind regards, Tom
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Hi Tubes4e4,
as I understand it the thing about frame-grid tubes is that the high gain is achieved (at least partially) by reducing the distance between grid and cathode - and that means that if the grid does vibrate then the amplitude of the vibration is more significant in relation to the grid-cathode distance. OTOH the grid can be wound more tightly so maybe the excursions are smaller and the whole thing cancels out, apart from shifting the range of frequencies most likely to cause acoustic feedback upward.
Great diagrams, the "sprung" suspension looks great but it will be fun working out how to build it 🙂 The idea of electromagnetic shielding (Dynamo-blech) which doubles as extra mass is appealing.
Ratheiser is talking about "NF"-feedback (LF in English), i.e. sub 300Hz and frankly I don't see a rubber grommet being enough to deal with that.
Thanks, this has given me a lot to think about!
All the best, Chris
as I understand it the thing about frame-grid tubes is that the high gain is achieved (at least partially) by reducing the distance between grid and cathode - and that means that if the grid does vibrate then the amplitude of the vibration is more significant in relation to the grid-cathode distance. OTOH the grid can be wound more tightly so maybe the excursions are smaller and the whole thing cancels out, apart from shifting the range of frequencies most likely to cause acoustic feedback upward.
Great diagrams, the "sprung" suspension looks great but it will be fun working out how to build it 🙂 The idea of electromagnetic shielding (Dynamo-blech) which doubles as extra mass is appealing.
Ratheiser is talking about "NF"-feedback (LF in English), i.e. sub 300Hz and frankly I don't see a rubber grommet being enough to deal with that.
Thanks, this has given me a lot to think about!
All the best, Chris
Hello Chris,
The "NF" terminus in German language usually maps just to the "AF" range (audio frequency). Not sure "LF" is really ment here.
But you are right of course that the foam rubber approach is much inferior to a carefully tuned mass - spring system. Not to speak of long term stability - we all know how foam rubber looks like after a few years. Add the elevated temperature near tube circuitry and one will get a deteriorating mess soon 😱
Tom
The "NF" terminus in German language usually maps just to the "AF" range (audio frequency). Not sure "LF" is really ment here.
But you are right of course that the foam rubber approach is much inferior to a carefully tuned mass - spring system. Not to speak of long term stability - we all know how foam rubber looks like after a few years. Add the elevated temperature near tube circuitry and one will get a deteriorating mess soon 😱
Tom
Don't confuse potential for something and it actually happening. Just because a certain type tube may be microphonic more often than some other type, doesn;t mean it will be so in YOUR application.
Perhaps some wider reading may help you appreciate the topic better, and what your practical options could be.
https://dalmura.com.au/static/Microphonics%20in%20valves.pdf
https://dalmura.com.au/static/Microphonics%20in%20valves.pdf
Thanks everybody. From trobbins' article it is clear that the problem is most acute in the range 1kHz-10kHz rather than in the LF range strictu sensu. (At least for an EF86, but the physical dimension of a PC97 or E88CC are in the same ball-park). i also found an interesting discussion here: Rubber or silicone gaskets for tube socket bases to reduce vibration? - Music Electronics Forum.
Conclusion is that there are some simple and cheap precautions I can take to reduce the risk of microphonics becoming a problem in the first place, and if they don't prove enough then there are still plenty of things to try which don't involve a complete re-design.
Specifically I have concluded that while the classical mass - spring decoupling system appeals to my intellectual side, my inner engineer goes for silicone rubber, flexible wiring, and maybe an o-ring - and embraces extra mass that might be provided by shielding etc..
Great stuff.
Conclusion is that there are some simple and cheap precautions I can take to reduce the risk of microphonics becoming a problem in the first place, and if they don't prove enough then there are still plenty of things to try which don't involve a complete re-design.
Specifically I have concluded that while the classical mass - spring decoupling system appeals to my intellectual side, my inner engineer goes for silicone rubber, flexible wiring, and maybe an o-ring - and embraces extra mass that might be provided by shielding etc..
Great stuff.
It seems unlikely to me that the Bell and Howell 626 film projector contains several PC97's. This projector was introduced in 1953 ( Bell and Howell 16mm Projector Models ), while the PC97 was introduced in 1962 ( PC 97, Tube PC97; Rohre PC 97 ID3446, Triode, vacuum ).