Clear and Smooth tone V1 Tube

[jazzkritter]

Gain? Nope. Amplification Factor? Yeppers.


I’ve never ceased being amazed at the FUD (Fear, Uncertainly and Doubt) around tubes thanks to the folks that sell them. Or write books about them, right AP? Go to any tube store web page… they will quote verbatim the RCA Receiving Tube Manual text, but then leave out the measures that matter. Like in the table above. But, they’re selling tubes, right? I learned tube theory for my first amateur radio license in 1966 (WN2RJD). I’m not an EE, but schooled/worked as an ET. I’m sticking to researchable fact, but any opinion/comments by me are marked IMHO.


We are going to go to the source of all things tube, the RCA Receiving Tube Manual (RTM RC-30, 1968). But first you need to know:

• The numbers in that chart above are not gain but Amplification Factor, taken from the RTM. Incorrectly labeled by the tube seller. “Amplification Factor” (AF) and “Gain” in the RTM are two very different measures. AF is the ratio of input to output voltage. You can objectively electronically measure AF. Gain OTOH is a measure of increase of signal input to output. The gain calculation will vary on signal type, load, frequency, etc. Thus not an ‘engineering’ measure nor necessarily repeatable. Neither are useful without the Transfer Conductance or Plate Resistance.
• Transfer Conductance (TC) is the resistance between the input and output sides of a tube. Looking at the AF and TC provides a better indication (IMHO) of what gain you can expect, but again gain can not be accurate by definition. Plate Resistance is the potential resistance to current flow between the plate and cathode. Yes they are inverse.
• Remote cut off vs Sharp cutoff. Unless marked below all are Remote cutoff. This means they are non-linear devices (and in most cases not designed primarily for audio). The Sharp cutoff tube is a linear device designed from the start for high fidelity.
• Tube design goals. You will see below that most of our favs were not designed for the audio world. To me (IMHO) that has to account for some sonic differences between tubes, but there is no formulaic proof I’ve seen.
• Tube construction.You know there are consumer, industrial and MIL SPEC tubes. I bring this up for two reasons: First are construction parameters; second is at the end in the 5751 comments. People will say (correctly) the coatings were thicker, different formulas, the internal bracing different, the getters different. IMHO there are examples where MIL SPEC tubes do sound different. The Bendix 5992 (6V6 EQ) was designed for ICBM guidance. Spec’d to 500 Gs to 80,000 ft. Try that on your TAD 6V6. No argument on line they make a beautiful sweet 6V6. Now going $250/tube. I thought buying in at 50$ in 2003 was sick. (5751s were 17$!)
• Mu = TC x Plate Resistance (PR) Mu is a rough gain range (low, medium, high). All three interact: mu, TC and PR. See RCA wouldn’t dare spec gain, it’s too individual use dependent: low, meds high. They attempt a relationship between TC and PR but the best we get is an approximation.
• I have intentionally left out the design operating frequency of these tubes. Most are topped at 140-150MHz, in the Radio Frequency spectrum and sort of out of our hearing range..

Now let’s see what the RTM thinks about our beloved preamp tubes from their objective view. All this beginning page 412 of RC-30 1968. (5751 is placed at the end.)


The RTM recommends the following tube types to be used as as audio amplifier tubes: 7199, 12AU7, 12AX7.


12AT7 AF 60; High mu, TC 4000-5500 micro mhos (uMhos). PR 12000-15000 Ohms
Design: FM/TV radio frequency converter, higher transconductance and Lower V Gain.
RTM Replacement 12AU7


12AU7 AF 17-19.5(!); medium mu, TC 2100-3200 uMhos, 6250-7500 Ohms
Design: VHF RF tube advised for uses in phase inverters, push pull audio amp, multivibrators, and vertical oscillator.
RTM Replacement 12AX7


12AX7 (spec’d as twin 6AV6).
Designed for audio in car radio amps of the 30’s.
Specs are:
6AV6. AF 100; Medium mu, Trans 1750 uMhos. PR 62000-80000 Ohms
No replacement listed.


12AY7 AF 40; Medium mu, TC 1750 uMhos, 22800 Ohms
Designed as a first stage high gain audio amp (Note it is not sharp cutoff)
No replacement listed


7199 AF 17; High mu, TC 2100 uMhos, 400000 Ohms (.4 MegO)
Sharp cutoff pentode.
RTM design quote: “High quality, high fidelity audio equipment particularly for phase splitters, tone control pre amp, and high gain voltage amp”. (No, jk, it’s not an industrial tube, as I stated. My bad.)




5751 AF 70; High mu, TC1200. uMhos, PR 58000 Ohms
RTM lists 5751 as a Mil spec 12AX7 (but AF of 70 vs 100). Design: phase inverter or high gain amp in avionics and industrial controls. Built for extreme environments in avionics and industrial control; Meets every spec for extreme environments, vibration, shock, heat, that the RTM has.


So we see our 5751 friend although designed for amplification was designed for control signal amplification. As it is MIL SPEC hardened, we know we are getting a tube here, much as the 5992, that has had construction elements aimed at survivability. Does that give us a tone difference?

And how cool… if you wanted detailed 5751 specs in 1960 you needed to apply for access to MIL-E1-10A/12A/238A by contacting the Director of the Armed Forces Electro-Standards Agency (AFESA), Fort Monmouth, New Jersey. Out in the public by 1968.


This was a lot of fun to research. I learned a few things and flipped my assumptions about the 7199! We guitar players have been really lucky to have watched the tube world twist and survive.

[Herbie]

Tubes can be a rabbit hole. And it can be a pricey rabbit hole.

I solved this by joining the camp that uses most available brand new tubes, no exotic NOS or vintage ones. If the sound of the amp depends on a rare tube You are in trouble when Your precious tube breaks in a strange city and there is no the same rare tube available.

But 5751 is worth trying!

And nowadays the no-tube amp is a very good option!