UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio and TV Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Other Discussions > Homebrew Equipment

Notices

Homebrew Equipment A place to show, design and discuss the weird and wonderful electronic creations from the hands of individual members.

Closed Thread
 
Thread Tools
Old 15th Mar 2018, 8:39 am   #21
FrankB
Heptode
 
Join Date: Oct 2005
Location: Olympia, Washington, USA.
Posts: 663
Default Re: Pre-amp for Golden ears

In the "Good Old Days" here we used to put lead pipe sections around the tubes to damp out microphonics, (And to shield them). They even made heavy metal chrome plated "Top hats" for the early tubes like the 01A's.
This practice continued well into the 60's on some brands of TV sets, where they used the lead pipe to shield and eliminate microphonics in TV tuner tubes.
FrankB is offline  
Old 15th Mar 2018, 9:20 am   #22
Craig Sawyers
Dekatron
 
Craig Sawyers's Avatar
 
Join Date: Nov 2010
Location: Oxford, UK.
Posts: 4,941
Default Re: Pre-amp for Golden ears

Quote:
Originally Posted by kalee20 View Post
Quote:
Originally Posted by Craig Sawyers View Post
For what it is worth, the resonant frequency of a spring/mass (which is what an isolation mount for a valve is) is given by (1/2.pi).root(g/B)

Where g is acceleration due to gravity (9.81m/s^2) and B is how much stretch there is in the isolation springs. Therefore the resonant frequency depends only on the stretch of the suspension springs.

So if the springs stretch by 1 cm (10^-2 m), the resonant frequency is 5Hz, which looks fine. If they stretch by 1mm the resonant frequency is 16Hz, which is probably still OK.
It's a bit of a misleading formula, this - the resonant frequency depends on the spring constant and the mass. Nothing to do with gravity, the resonant frequency will still be the same on the moon. But! The Spring constant is also the extension divided by the force to produce said extension, hence the g/B calculation also works.
Not at all misleading. Sure the regular formula is given by the spring constant and the mass. But the extension of the spring is dependent on gravity, the mass and spring constant, which leaves the equation I quoted.

The only reason to look at it this way, is when vibration isolating something, you only need to look at how much the spring is stretched and that is enough to give you the resonant frequency without knowing anything at all about the mass and the spring.

Just as another for instance, the bed plate of an old SME arm used to have grommets, so the arm was isolated in some way from the plinth or arm board. The mounting screw is required not to compresses the grommet, so the grommet compression is only by the arm mass itself. Suppose that is by 0.1mm (which seems plausible). That means that the vibration isolation cuts in at 50Hz. Of course there are more complex rocking modes too, and for that reason the grommets were often removed and the bedplate screwed hard to the plinth. Current SME arms are bolted hard down by design.

The only (classical) way of working out the local gravity, for example on the moon, is to use a pendulum.

Craig
Craig Sawyers is offline  
Old 15th Mar 2018, 12:04 pm   #23
Craig Sawyers
Dekatron
 
Craig Sawyers's Avatar
 
Join Date: Nov 2010
Location: Oxford, UK.
Posts: 4,941
Default Re: Pre-amp for Golden ears

Sorry - dropped a power of ten. The SME grommets cuts off over 500Hz, NOT 50Hz!

Oops
Craig Sawyers is offline  
Old 15th Mar 2018, 1:01 pm   #24
GrimJosef
Dekatron
 
Join Date: Sep 2007
Location: Oxfordshire, UK.
Posts: 4,310
Default Re: Pre-amp for Golden ears

Quote:
Originally Posted by Craig Sawyers View Post
... The only (classical) way of working out the local gravity, for example on the moon, is to use a pendulum.
I guess you mean in terms of the base units (otherwise you could use a calibrated force, and we'd be back to a spring balance). If you substitute the base unit of electric charge for the base unit of time, which is needed for the pendulum test, then in principle you could use a version of Millikan's oil drop experiment to measure local gravity (if you trust the permittivity of free space). Or if you prefer current to charge then you could build a current balance. It would be a good deal less fiddly to use a pendulum though .

Cheers,

GJ
__________________
http://www.ampregen.com
GrimJosef is offline  
Old 15th Mar 2018, 2:17 pm   #25
Craig Sawyers
Dekatron
 
Craig Sawyers's Avatar
 
Join Date: Nov 2010
Location: Oxford, UK.
Posts: 4,941
Default Re: Pre-amp for Golden ears

If you want the true nature of non-relativistic gravity, you need a gravity gradiometer
Craig Sawyers is offline  
Old 16th Mar 2018, 12:50 am   #26
PJL
Dekatron
 
Join Date: Sep 2005
Location: Seaford, East Sussex, UK.
Posts: 5,997
Default Re: Pre-amp for Golden ears

If I have read it correctly, for low noise the article recommends a step-up transformer to raise the signal level as high as possible and use cascoded triodes to reduce the input capacitance to avoid roll-off from the increase in source impedance?
PJL is offline  
Old 16th Mar 2018, 10:10 pm   #27
kalee20
Dekatron
 
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 7,060
Default Re: Pre-amp for Golden ears

Quote:
Originally Posted by Craig Sawyers View Post
Quote:
Originally Posted by kalee20 View Post
Quote:
Originally Posted by Craig Sawyers View Post
For what it is worth, the resonant frequency of a spring/mass (which is what an isolation mount for a valve is) is given by (1/2.pi).root(g/B)

Where g is acceleration due to gravity (9.81m/s^2) and B is how much stretch there is in the isolation springs. Therefore the resonant frequency depends only on the stretch of the suspension springs...
It's a bit of a misleading formula, this - the resonant frequency depends on the spring constant and the mass...
Not at all misleading. Sure the regular formula is given by the spring constant and the mass. But the extension of the spring is dependent on gravity, the mass and spring constant, which leaves the equation I quoted...
...and having read your full post, I agree: yes, the resonant frequency can be calculated knowing only the extension (or compression) due to the mass; yes it's easy, and yes it's convenient!

I'm now off to crawl into a corner where I can calculate the resonant frequencies of a few spring-isolated motors and sub-chassis!
kalee20 is online now  
Old 17th Mar 2018, 1:13 am   #28
tritone
Banned
 
Join Date: Nov 2014
Location: Derry, Northern Ireland, UK.
Posts: 167
Default Re: Pre-amp for Golden ears

Quote:
Originally Posted by joebog1 View Post
Yes tritone I have used damper rings. Read the advertising!! Ones costing as much as 50 quid work much more gooderer than 20 quid ones. A fellow that brought me an amp for repair had them stuck all over the glassware in the amp. After careful examination I discovered that ordinary hydraulic "O" rings even triple the output power of your 1960 Vauxhall, if you fit them to the car radio!!! They prevent punctures as a sideline as well. Well you dont want punctures in your whitewalls, do you?.

The bloke was convinced they worked, I couldnt tell any difference
Hey, Grumpy ole' man! Life goes on! I'm pretty sure that diabolic resonance will traverse with you forever!

Goodbye
tritone is offline  
Old 17th Mar 2018, 1:40 am   #29
joebog1
Banned
 
Join Date: Jun 2015
Location: Mareeba, North Queensland, Australia
Posts: 2,704
Default Re: Pre-amp for Golden ears

My apologies tritone, twas nowt more than my feeble attempt at humour, to share some fun over something I have tried and found no difference in the microphonics. Other members have quoted lead pipe, which I have seen, but its something almost impossible to obtain in Australia, the only place I have seen it was old lead covered, paper insulated telephone cables that were being ripped out and replaced with PVC when I was a kid, probably 60 years ago.

Again, my apologies

Joe
joebog1 is offline  
Old 17th Mar 2018, 7:06 am   #30
Radio Wrangler
Moderator
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,800
Default Re: Pre-amp for Golden ears

Air springs can give an escape from the deflection relationship. The force they exert is proportional to piston area and gas pressure, but the spring rate involves the volume behind the piston. Add in active control and the world is your mollusc of choice.

David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is online now  
Old 17th Mar 2018, 8:41 am   #31
Craig Sawyers
Dekatron
 
Craig Sawyers's Avatar
 
Join Date: Nov 2010
Location: Oxford, UK.
Posts: 4,941
Default Re: Pre-amp for Golden ears

Quote:
Originally Posted by Radio Wrangler View Post
Air springs can give an escape from the deflection relationship. The force they exert is proportional to piston area and gas pressure, but the spring rate involves the volume behind the piston. Add in active control and the world is your mollusc of choice.

David
Same equation. Imagine the piston sealing a chamber at atmospheric pressure. Now you apply a mass. The piston moves downwards to balance the mass with an increase in pressure in the chamber. That distance is B in my original equation.

I guess your comment comes from the design of optical honeycomb tables, which are fitted with a number of vibration isolation options for the legs, including active control. Last one I specced was an 8' x 4' with super-invar skins, and earthquake bump stops on the legs. That was for an astronomical spectrometer on Gemini South. Unfortunately big telescopes are sited on mountain tops, which are generally in earthquake zones - so immunity is built into all aspects of design. I was system engineer for that, and Maggie Aderin (now the Sky at Night host) was project manager.

Craig
Craig Sawyers is offline  
Old 17th Mar 2018, 9:00 am   #32
Radio Wrangler
Moderator
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,800
Default Re: Pre-amp for Golden ears

There are all sorts of methods of trying to approach the ideal force-source of infinite compliance much as there of creating a current source of infinite impedance.

Air spring arrangements rarely have cylinders long enough for their piston to move to their true unloaded position which would have been the reference position from which deflection would have been measured to use that equation, so the presence of an end stop means that the active range is only a segment of a much larger curve, and the spring constant can be made low considering the mass being supported. Once active control is introduced, the rules can be changed.

Supporting a mass and having some freedom to choose the natural frequency is a classical engineering problem.

In this case, I guess a positioning servo would have to be implemented with valves

David (running!)
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is online now  
Old 17th Mar 2018, 10:45 am   #33
Craig Sawyers
Dekatron
 
Craig Sawyers's Avatar
 
Join Date: Nov 2010
Location: Oxford, UK.
Posts: 4,941
Default Re: Pre-amp for Golden ears

If it does not move to its unloaded position, you get the same result by using the ratio of cylinder pressure to atmospheric pressure. So if the cylinder pressure is 3 bar, the relevant ratio is B = 3 and hence the resonant frequency is about 0.3Hz. That will be quite high Q and hence legs have quite careful viscous damping using oil and orifices, which means that the legs start to operate (ie to attenuate) at about 1Hz.

It is more complicated than that, because they have to deal with horizontal excitation too, so the legs also have pendulum mechanisms too (also damped) to handle that.
Craig Sawyers is offline  
Old 17th Mar 2018, 11:02 am   #34
GrimJosef
Dekatron
 
Join Date: Sep 2007
Location: Oxfordshire, UK.
Posts: 4,310
Default Re: Pre-amp for Golden ears

Quote:
Originally Posted by Craig Sawyers View Post
... I guess your comment comes from the design of optical honeycomb tables, which are fitted with a number of vibration isolation options for the legs ...
Unsurprisingly perhaps, these are often bought by optics (usually laser) specialists. The table company salesman helps them pick the right one and he/she usually knows what they're doing. So things start out well. But once the optics folks are left on their own their lack of mechanical engineering experience can start to show. I've seen any number of these lovely and expensive tables with, say, noisy motor-driven optical chopper wheels clamped onto them. Or a laser's cooling water pipes, driven by a noisy pump, secured to the table top - just one more example of a lovely isolation job being completely 'shorted out'. The last time I used a chopper with an optical table I mounted it on a separate gantry. The only issue then was that the table's legs (steel) had a different thermal expansion coefficient than the gantry's ones (aluminium). So if the room temperature varied the chopper moved up and down relative to the laser beam ! There's more to this mech eng stuff than meets the eye.

Cheers,

GJ
__________________
http://www.ampregen.com
GrimJosef is offline  
Old 17th Mar 2018, 11:17 am   #35
Radio Wrangler
Moderator
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,800
Default Re: Pre-amp for Golden ears

Quote:
Originally Posted by Craig Sawyers View Post
If it does not move to its unloaded position, you get the same result by using the ratio of cylinder pressure to atmospheric pressure. So if the cylinder pressure is 3 bar, the relevant ratio is B = 3 and hence the resonant frequency is about 0.3Hz. That will be quite high Q and hence legs have quite careful viscous damping using oil and orifices, which means that the legs start to operate (ie to attenuate) at about 1Hz.

It is more complicated than that, because they have to deal with horizontal excitation too, so the legs also have pendulum mechanisms too (also damped) to handle that.
I'm not sure about that, at least not in the terms I'm thinking of.

If we have some sort of air suspension system supporting a mass and the piston has some area and there is some pressure in the cylinder so the pressure times area balances the gravitational force of the mass.

If I move the pass up and down a little, the pressure in the cylinder changes due to the change in volume of the space the gas has to occupy. This creates a restoring force which acts to put the piston and mass back to its equilibrium resting position once I've stopped poking it. The proportional to deflection restoring force (for small deflections) and the mass create the classical mass-spring resonator and the resonant frequency is now set by the mass and the spring coefficient.

If I now connect or add a large air reservoir to the pressure side of the cylinder, and ump it all up to the same pressure as before. I have the same pressure, I have the same mass, I have the same piston area.

What has changed is that when I poke the mass and deflect it a little, I change the volume of the pressure side of the piston by the same amount, but due to the much greater connected volume the change in pressure is now far less, and that means the restoring force is far less and therefore the springiness is far more compliant. This drops the resonant frequency of the the system.

If I took the mass off of the piston and the cylinder was long enough, the piston would move far higher to its new equilibrium position than it would have before I added the air reservoir, because there is more air that needs to expand before the pressure comes down to the rest position.

The resonance is controlled not by the spring force, but by the slope of the force/deflection curve at the operating point.

I think we're just visualising different things in different terms.

It would be one heck of a preamp with air sprung valve sockets with say a 1Hz natural frequency and little dampers. Don't tell the audiophiles - they'd only argue over which gas sounds best.

David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is online now  
Old 17th Mar 2018, 11:34 am   #36
Craig Sawyers
Dekatron
 
Craig Sawyers's Avatar
 
Join Date: Nov 2010
Location: Oxford, UK.
Posts: 4,941
Default Re: Pre-amp for Golden ears

Quote:
Originally Posted by Radio Wrangler View Post
It would be one heck of a preamp with air sprung valve sockets with say a 1Hz natural frequency and little dampers. Don't tell the audiophiles - they'd only argue over which gas sounds best.
David
We're going to just have to disagree on the meat of the air leg discussion, or we'll completely subvert the topic of the thread

I love the last comment though; fill it with helium because of the speed!

Craig
Craig Sawyers is offline  
Old 17th Mar 2018, 6:57 pm   #37
tritone
Banned
 
Join Date: Nov 2014
Location: Derry, Northern Ireland, UK.
Posts: 167
Default Re: Pre-amp for Golden ears

Quote:
Originally Posted by joebog1 View Post
I need to isolate the valve base (s) from the chassis, so that I wont suffer physical "modulation" of the valves. (6BQ7A )

I am asking the hundreds of thousands of hours of experience within this forum, for other ideas.



With many thanks in advance,
Joe
No worries Joe! from my previous post my friend!

I have an idea please don't shoot me down as it may sound a bit crazy . I don't know if something like this has ever been done before.

Ok, here I go, (phew),,

Would it be possible to construct some kind of valve base that could magnetically 'float' and have zero physical contact with the chassis! Probably need some mechanical ingenuity, I rekon but with perserverance, why not?

I see those neodynium magnets for sale in all shapes and sizes and they are very powerful for their size, my only concern is, would they (magnets) interfere with with the electron stream within in the valve.

Just an idea Joe!
tritone is offline  
Old 17th Mar 2018, 7:46 pm   #38
VT FUSE
Hexode
 
Join Date: Aug 2010
Location: Malvern, Worcestershire, UK.
Posts: 340
Default Re: Pre-amp for Golden ears

Here goes nothing..... or maybe not? What about submini valves like the American 6112 and its Eastern Bloc clones ie 6n17B.

Designed with rigid low mass structures in order to resist microphonics,much easier to construct an anti-vibration mounting than for a valve with higher mass/insertion force AND poor contact (possibly)via pins eliminated.

Maybe a different angle to consider

Last edited by VT FUSE; 17th Mar 2018 at 8:04 pm.
VT FUSE is online now  
Old 17th Mar 2018, 8:35 pm   #39
tritone
Banned
 
Join Date: Nov 2014
Location: Derry, Northern Ireland, UK.
Posts: 167
Default Re: Pre-amp for Golden ears

+1 ^^

Happy St Patricks Day forum!
tritone is offline  
Old 17th Mar 2018, 11:06 pm   #40
joebog1
Banned
 
Join Date: Jun 2015
Location: Mareeba, North Queensland, Australia
Posts: 2,704
Default Re: Pre-amp for Golden ears

I do have minature valves that are easy to mount (a simple grommet) and they are designed for very high G forces. I don't know however if they are microphonic or not. As yet I haven't made anything using them so no experience whatsover. They don't need bases at all as they are wire ended, just solder them into the circuit. I suppose one could "roll up" the leads to make a sort of isolation spring as well. I will have to look at them again I think.

Joe
Attached Thumbnails
Click image for larger version

Name:	minature.jpg
Views:	60
Size:	38.2 KB
ID:	159418  
joebog1 is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 6:41 pm.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2024, vBulletin Solutions, Inc.
Copyright ©2002 - 2023, Paul Stenning.