Interesting tube gain observation

[emrr]

I’ve been bouncing between a couple of old RCA tube line amps for mix bus lately. 

  As you’d expect, putting a fully variable T attenuator on the output of one channel to trim gain is necessary for center balance, given the gain controls are 2dB/step.  I’m using an unloaded decade attenuator after a loaded output stage which gives steps smaller than stated, 0.1dB on the label. 

The later amps with 9 pin tubes and feedback loops requires frequent gain trim checks.  Up or down a click or two on the T. 

The earlier amps with 6 pin tubes (pre-octal) and no feedback have never needed readjustment, and are stable within minutes of turn on.  I keep checking, and they are solid. 

This is the opposite of what I’d expect.  Total gain pretty similar. 

I wonder if this is related to new manufacture versus very old NOS tube materials differences. 

 

[bluebird]

Very interesting! My first guess is that the metal cases act as heat "feedback" in some way. Both keeping in heat and radiating it at the same time. Keeping gain constant with regards to room temperature. Or not. don't know.

 

[emrr]

Very interesting! My first guess is that the metal cases act as heat "feedback" in some way. Both keeping in heat and radiating it at the same time. Keeping gain constant with regards to room temperature. Or not. don't know.

Actually all glass tubes here.  But good thought. 

 

[mjrippe]

Doug, when you say "new manufacture" are you referring to the tubes themselves, or something else?  Obviously you could try NOS tubes for the novals...

 

[emrr]

Doug, when you say "new manufacture" are you referring to the tubes themselves, or something else?  Obviously you could try NOS tubes for the novals...

Current manufacture tubes. 

 

[bluebird]

Actually all glass tubes here.  But good thought. 

Oh my bad! You got any pics of these things?

 

[ruairioflaherty]

The later amps with 9 pin tubes and feedback loops requires frequent gain trim checks.  Up or down a click or two on the T. 

Tempco of the feedback resistors?

 

[emrr]

Tempco of the feedback resistors?

or ALL the resistors.    That seems likely.  The later is all carbon, the earlier is all wirewound. 

 

[mjrippe]

or ALL the resistors.    That seems likely.  The later is all carbon, the earlier is all wirewound.

Well, yeah.  I'm sure there are a number of component differences between the two units that would be likely to account for drift.  That's why I was wondering how you figured it was the tubes.  BUT, I would still be interested if NOS tubes made a difference because I don't think new manufacture are as good either.

 

[emrr]

Overall my assumption would have been that NFB would have compensated, but it did not.  That is one of many reasons it's used.

 

[ruffrecords]

Overall my assumption would have been that NFB would have compensated, but it did not.  That is one of many reasons it's used.

A lot depends on how much NFB is used. With only 6dB of NFB any drift will only be halved, With 20dB is will be reduced to a tenth. What are the relative amounts of NFB in the two designs?

Cheers

Ian

 

[emrr]

A lot depends on how much NFB is used. With only 6dB of NFB any drift will only be halved, With 20dB is will be reduced to a tenth. What are the relative amounts of NFB in the two designs?

Cheers

Ian

Sorry I apparently left out ‘no feedback’ in regards to the earlier amps.  Will have to look at the feedback amps again.   

 

[DaveP]

It could well be down to chemistry.  The first half of the 20th century was built on growing knowledge of Inorganic Chemstry  (Chemistry of the Elements).  The second half became the study of Organic Chemistry like plastics and then Biochemistry.

The triple carbonate emission paste on the cathodes was probably purified to a much higher standard than in later years.  Every tube manufacturer wanted as much gm as they could get to beat the competition.  You could liken it to the R&D effort put in by Intel and others to raising the clock speed, who cares about that any more apart from gamers?  Like all industries, once a basic quality and mass market is established, it becomes down to price.

As a former inorganic chemist myself, I saw expertise and technique being lost as people concentrated on newer industries.

DaveP

 

[Squeaky]

Dave P - I agree, it could well be chemistry. However, your description of the branches of chemistry leaves a bit to be desired. I would prefer to suggest that organic chemistry is involved with substances containing C-H bonds. I understand that us organic chemists delve more deeply into the "black arts" than other chemists and so our discipline can be more easily misunderstood (we are the creators of new substances).

From an industrial perspective (I believe that you are an industrial chemist), I would think that organic chemistry (for the moment setting aside natural product chemistry and medicinal chemistry - both branches of organic chemistry) was probably greatly advanced out of the discovery of oil in the 19th century. For example, early knowledge gained out of the study of aromatic compounds led to developments of dyes. Pity, though, those poor buggers who thought it was safe to wash their hands in benzene.

Out of politeness, you should probably have also made mention of physical chemistry. Traditionally there were three main branches of chemistry: physical, organic and inorganic.

Cheers,

Nick.

 

[DaveP]

Yes, you are right of course, but I try to give short answers to the point people are interested in, rather than give a detailed history of Chemistry.  I also do not wish to downgrade organic chemistry, but to illustrate that organic chemistry became more interesting to chemists.  Much the same as people stopped working on tubes when transistors came along and then in discrete transistors when chips became the vogue.

DaveP