oscarhuang
Well-known member
- Joined
- May 22, 2020
- Messages
- 159
You are right! I don’t wanna keep the secret. This is a Western electric 408A. A 270 OHM resistor series to filament.
Microphonics in VF14 and U47
- Thread starter tomas.borgstrom
- Start date
Help Support GroupDIY Audio Forum:
IIRC Oliver A. posted an inside picture of opened up VF14 and another tube kind of like it on a different forum in the past.
It might have been an older version of the following forum
https://www.tapatalk.com/groups/prorecordingworkshop/?sid=4a19801d4e06a894cf9794eb0d61f20b
some more links
http://www.wagner-microphones.com/vf14_background.htm
http://www.phaedrus-audio.com/VF14T.htm
https://realgearonline.com/thread/2225/telefunken
It might have been an older version of the following forum
https://www.tapatalk.com/groups/prorecordingworkshop/?sid=4a19801d4e06a894cf9794eb0d61f20b
some more links
http://www.wagner-microphones.com/vf14_background.htm
http://www.phaedrus-audio.com/VF14T.htm
https://realgearonline.com/thread/2225/telefunken
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- Joined
- Mar 11, 2014
- Messages
- 252
This is inside EF14
. 
. 
I have a broken VF14 here. I'll ask the owner if I'm allowed to open it.
Thanks Tom ,,
the structure looks more like a 6V6 in terms of size , is it glass to metal seals where the terminals pass through the tube casing ?
They obviously seal off the vacuum via the tube that sticks down at the end .
Amazing stuff ,
I found these sockets destined for wire ended valves ,
A few of them would make a handy burn-in/test rig for wire ended submini's ,
Ive emailed to get a price .
Most submini tubes have 6.3v heaters , theres no getting away from that if you have a U47 that needs a new VF14 you might have to consider a tube a different heater voltage/circuit .
Maybe theres a technological solution ,
Could a buck down Switcher be incorporated inside a VF14 style metal can and base to provide a wire ended submini tube with 6.3v DC from the U47's 105v supply ?
Maybe Thor might be the right man to ask .
the structure looks more like a 6V6 in terms of size , is it glass to metal seals where the terminals pass through the tube casing ?
They obviously seal off the vacuum via the tube that sticks down at the end .
Amazing stuff ,
I found these sockets destined for wire ended valves ,
A few of them would make a handy burn-in/test rig for wire ended submini's ,
Ive emailed to get a price .
Most submini tubes have 6.3v heaters , theres no getting away from that if you have a U47 that needs a new VF14 you might have to consider a tube a different heater voltage/circuit .
Maybe theres a technological solution ,
Could a buck down Switcher be incorporated inside a VF14 style metal can and base to provide a wire ended submini tube with 6.3v DC from the U47's 105v supply ?
Maybe Thor might be the right man to ask .
rock soderstrom
Tour de France
here you can get these sockets for small money
https://groupdiy.com/threads/socket-for-5840-8-pin-subminiature-tube.81899/post-1058752
soapfoot
Well-known member
Remember that in a U47, the heater supply divides down to make the bias supply... so anything you do had better be very quiet, and you have to make the adjustment for bias if you change the heater voltage.
Generally easiest to do what most substitute tube U47s do... send a separate filament supply and adjust the voltage divider to get the proper bias voltage
Generally easiest to do what most substitute tube U47s do... send a separate filament supply and adjust the voltage divider to get the proper bias voltage
- Joined
- Mar 11, 2014
- Messages
- 252
To get a decent sound with 408a in U47 i found that you have to do more than just adjusting the heater voltage. I increase the cathode resistance to get more second harmonics and lower the plate resistance to get a better low end. Unfortunately lowering the plate resistance can't be done between the tube and the socket. You have to parallell the 100K plate resistor (installed in 1 minute and removed in 10 seconds). The different brands of 408a do not sound and behave the same so I haven't found a general circuit yet.
Many 408a are noisy and can be very microphonic (I still don't consider it desirable) and you need a donor metal tube to fit it in the U47 socket so producing a good replacement tube is not cheap (but definately a lot cheaper than $800 Telefunken USA and $??? VF14)
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The 1780 ohm resistor sheds a lot of heat into the casework and drags down the HT supply to where it wants to be for a VF14 .
By the time you've saved all that wasted energy you should have enough to make 6.3v 150ma from a slightly higher HT voltage with less load on the supply .
For the sake of argument lets say we decide to under volt a 6.3v heater to 5.2v , we arrange to drop the extra 1.1V across a resistor in series with the lower end of the heater/cathode , that cuts cold inrush current to the heaters in half .
A postage stamp sized pcb could take the place of the dredded 1780 ohm resistor .
By the time you've saved all that wasted energy you should have enough to make 6.3v 150ma from a slightly higher HT voltage with less load on the supply .
For the sake of argument lets say we decide to under volt a 6.3v heater to 5.2v , we arrange to drop the extra 1.1V across a resistor in series with the lower end of the heater/cathode , that cuts cold inrush current to the heaters in half .
A postage stamp sized pcb could take the place of the dredded 1780 ohm resistor .
Agreed Soapfoot ,
doing it quietly might be a whole different matter entirely,
Plenty of food for thought about quiet switchers around just now as well though,
The 408a dates back donkeys years , even though the data sheet says low noise low vibration etc that pertains to the standards of the day . Repeater amps wont typically be subject to vibrations ,so even very microphonic samples will work fine.
Probably the only reason the 408a was chosen was similar heater voltage , it doesnt really bear any other resemblance in its characteristics to a VF14 and as weve heard they vary a lot from sample to sample .
doing it quietly might be a whole different matter entirely,
Plenty of food for thought about quiet switchers around just now as well though,
The 408a dates back donkeys years , even though the data sheet says low noise low vibration etc that pertains to the standards of the day . Repeater amps wont typically be subject to vibrations ,so even very microphonic samples will work fine.
Probably the only reason the 408a was chosen was similar heater voltage , it doesnt really bear any other resemblance in its characteristics to a VF14 and as weve heard they vary a lot from sample to sample .
Theres some interesting bits here ,
http://www.phaedrus-audio.com/VF14M_story.htmHes come up with his own plug and play solidstate replacement ,
after giving up with the 408a .
http://www.phaedrus-audio.com/VF14M_story.htmHes come up with his own plug and play solidstate replacement ,
after giving up with the 408a .
A little off topic
I have built a number of solid state replacements that work at tube voltages and...
Remember the U47 has a series power supply that is loaded at about 40 mA so it acts like a shunt supply with the .55mA plate current
Tube voltage solid state often does not exactly sound like a tube. I think of it like the best of a tube and the best of solid state.
I have a solid state drop in circuit for the VF14 drawn and simmed.
Two plug ins for pentodes like a EF86 one in a 250 like circuit the other in MXL revelation built and installed
A 120 VDC solid state circuit in a 47 like body.
A solid state add in circuit built and installed in a 6AU6 C800G like circuit build installed that helps out the C800G tube.
And a number of other circuits drawn up or simmed waiting to be built
Debating on what to do with the designs. Tube voltages open you up to liability.
I have built a number of solid state replacements that work at tube voltages and...
Remember the U47 has a series power supply that is loaded at about 40 mA so it acts like a shunt supply with the .55mA plate current
Tube voltage solid state often does not exactly sound like a tube. I think of it like the best of a tube and the best of solid state.
I have a solid state drop in circuit for the VF14 drawn and simmed.
Two plug ins for pentodes like a EF86 one in a 250 like circuit the other in MXL revelation built and installed
A 120 VDC solid state circuit in a 47 like body.
A solid state add in circuit built and installed in a 6AU6 C800G like circuit build installed that helps out the C800G tube.
And a number of other circuits drawn up or simmed waiting to be built
Debating on what to do with the designs. Tube voltages open you up to liability.
Thats has a musical ring it has to it ,
Power tubes that have been thrashed about in the back of an amp for years get a distinctive metalic sizzle , the electrodes and grids rattle .
The quickest and easiest way I find to test a tube for microphonics is hold it close to your ear an give it a sharp tap with your finger , any resonances are clearly audible , loose or shaky electrode likewise make themselves obvious .
It may not be very scientific but it allows me get a quick idea of which tubes are likely have lowest microphony out of a pile .
Theres a standardised physical test to determine an electronic component or devices susceptabillity to vibration/shock ,
it involves dropping a known mass in a repeatable manner . B&K spec their sound measurement gear to withstand a certain force of impact for a given noise level deflection on the scale at max gain .
Power tubes that have been thrashed about in the back of an amp for years get a distinctive metalic sizzle , the electrodes and grids rattle .
The quickest and easiest way I find to test a tube for microphonics is hold it close to your ear an give it a sharp tap with your finger , any resonances are clearly audible , loose or shaky electrode likewise make themselves obvious .
It may not be very scientific but it allows me get a quick idea of which tubes are likely have lowest microphony out of a pile .
Theres a standardised physical test to determine an electronic component or devices susceptabillity to vibration/shock ,
it involves dropping a known mass in a repeatable manner . B&K spec their sound measurement gear to withstand a certain force of impact for a given noise level deflection on the scale at max gain .
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k brown
Well-known member
Reminds me of the (probably true) theory that the 'superior' sound of LPs is actually a euphonic effect of needle vibrations resonating through the vinyl and back to the cartridge again.
It's just marketing and hype of Record Labels trying to make a few more bucks in an age where CD's don´t sell.
Vinyl is very limited and inferior in all the technical specs, and sounds that way.
It was never chosen as a consumer format because of quality it was adopted because it was very cheap to produce and very fast to fabricate in big quantities.
I was always puzzled by how little attention there used to be to the sound of tremolo bar springs in strat style guitars. People were ranting about some very questionable aspects of guitar sound for decades. I can hardly remember anyone mentioning super obvious "built in spring reverb" in these. Nowdays people seem more aware of it, probably because of the obsession with gating, and overproducing guitars.
k brown
Well-known member
The theory was put forth by audiophiles and audiophile magazine editors, not by record labels - and was way before the CD era. Folks were theorizing about why in many cases LPs sounded better than the same releases on R2R tape (despite the surface noise).
I wonder if people were actually aware that masters for LPs are way different.
