Life expectancy of tubes vs heater voltage.

Help Support GroupDIY:

MaxDM

Well-known member
Joined
Oct 23, 2018
Messages
147
I'm sure they did not worry about tube life so much, if a bit less noise, and the avoidance of a separate winding for the first stage could be avoided.

Tubes were plentiful and they had the money to replace them often.
 

Winston OBoogie

Well-known member
Joined
Jun 3, 2004
Messages
2,543
Location
UK.
Other than the person/people involved in the design, who knows whether they worried about tube life.

By all accounts, dead air or an avoidable technical problem wasn't taken lightly at the BBC.
I think if another winding on a transformer would serve the amplifier better and avoid a premature valve failure, I suspect it wouldn't have been an issue to provide one.

Anecdotal evidence I read somewhere on here: Old BBC Radio Broadcasting Equipment and Memories - Home Page indicated that, while the BBC were initially concerned about valve amp failure in the desks they built - to the extent that there were spares for each amplifier which could be switched over to immediately - actual memories of a failure or issues with the amps seem to be very rare.

These things served a long life, I've seen a picture or two somewhere that showed an OPA/9 still being used in the 1980's which is kinda crazy really! :D
 

Winston OBoogie

Well-known member
Joined
Jun 3, 2004
Messages
2,543
Location
UK.
Hey Max,
the folks active in the microphone sub forum would be better equipped to answer than me, but yes, what you say seems correct to me.

A few on here have played with higher than the usual 60V polarizing voltage and found no issue. My question would be if the capsule had any tendency to stick or collapse but, I have no experience.

My guess is that the BBC would have been using the M7 in mid and far field positions rather than close mic, so that would change things a little from how most use M7's in U47 type mics.

Interesting question though :)
 

Tubetec

Well-known member
Joined
Nov 18, 2015
Messages
3,004
I have a few pages from a BBC training manual which gives a good overview of the valve era gear , especially the mic amps , it only contains simplified circuit diagrams , I'll try and dig up the reference numbers as theres a vast amount of non audio related stuff to wade through in the site .

Ive tried applying above 60 volts to a capsule I have and sure enough it starts to deform , the drill holes in the back plate become visible on the membrane as it colapses. its temporary as soon as the voltage stops the membrane unsticks itself .
 

MaxDM

Well-known member
Joined
Oct 23, 2018
Messages
147
I have a few pages from a BBC training manual which gives a good overview of the valve era gear , especially the mic amps , it only contains simplified circuit diagrams , I'll try and dig up the reference numbers as theres a vast amount of non audio related stuff to wade through in the site .

Ive tried applying above 60 volts to a capsule I have and sure enough it starts to deform , the drill holes in the back plate become visible on the membrane as it colapses. its temporary as soon as the voltage stops the membrane unsticks itself .

Yes, it depends on the capsule. The U87 capsule was made for 45 (?) volts, while the U87a and U67, which are very similar capsules, use 60.

Not sure about an M7, but it should do something to the sound, compared to the normal voltage, perhaps in the bass?
 

Tubetec

Well-known member
Joined
Nov 18, 2015
Messages
3,004
Some of the more interesting stuff has the prefixes ,
GPA general purpose amplifier
LIM limiter
OBA outside broadcast amp
AMC mic amps
Extra stuff in the dedicated Microphone section as mentioned already
other great stuff in the pre 1960's section under A,B,C,D amplifiers
 
Last edited:

Winston OBoogie

Well-known member
Joined
Jun 3, 2004
Messages
2,543
Location
UK.
Going back to that BBC OPA/9 - I was looking at data sheets and equivalents to the EF37A and, the ME1400 and its military/Gov. supply CV432 came up.

Interesting to read that the ME1400 is claimed to be a selected low grid current version of the EF37a for use in volt meters or photocell units etc., and the datasheet shows heater voltage at 4.5V for operational characteristics. Whereas the CV432 data sheet uses 6.3V heater as a base line.
 

Tubetec

Well-known member
Joined
Nov 18, 2015
Messages
3,004
An interesting example of undervolting, maybe the ME1400 would make a good candidate for a low noise preamp , good examples seem to be a little cheaper than regular EF37a's which are gone upto a saucy price .
 

Winston OBoogie

Well-known member
Joined
Jun 3, 2004
Messages
2,543
Location
UK.
An interesting example of undervolting, maybe the ME1400 would make a good candidate for a low noise preamp , good examples seem to be a little cheaper than regular EF37a's which are gone upto a saucy price .

That's what I was thinking too, or the CV432 if it's also flying under the radar of audiophools 👍
I didn't see any at the 2 places I usually get old tubes from, where are you seeing them?

Edit: I see there are some CV432's on Ebay. I like the military livery of them, with the pheon symbol showing it's "property of the Crown".

It's all very Bletchley Park 'Colossus' looking.
Not inexpensive, but cool :)

 
Last edited:

Winston OBoogie

Well-known member
Joined
Jun 3, 2004
Messages
2,543
Location
UK.
Of course, when we're done with building Mk. 1, we'll need to hire a couple of nice ladies to operate each preamp
 

Attachments

  • 1080px-Colossus.jpg
    1080px-Colossus.jpg
    201.4 KB · Views: 11

Tubetec

Well-known member
Joined
Nov 18, 2015
Messages
3,004
Was just going to point the link to ebay .

I have a few mullard EF37a's in my stash , the red shoulder type , There seems to be another variant the CV5080 spec'd for weapons grade usage , a special series of tests is listed in the data sheet

 

Winston OBoogie

Well-known member
Joined
Jun 3, 2004
Messages
2,543
Location
UK.
Thanks. I'll check the datasheet but, wonder if there's any real difference between the Navy/Admiralty CV5080 and the Army/Intelligence Service CV432?

Was thinking of trying CV432's when I get time and funds, I'm thinking an OBA/9 and V41 mash-up.

EF37a's are generally a bit noisier than a good EF86, but they seem better in other areas like distortion spread. I'd rather give up a little noise and get the rest tbh.
 

Latest posts

Top