I have an RCA VRA-121 phono radio, has FM ...very nice and can't wait to get it working ... and it is interesting because the radio frequency chassis uses all miniature tubes but the power supply/amplifier chassis has an octal 6SC7 plus the 6F6's and 5U4 ....but I am really wondering..how is it that tubes can go from big 4-5 pin /octals etc...(with grid caps too)like I.E. a 6K7....to a tube without grid cap (6SK7)....and then to small miniatures....? Are the miniatures really as good as the octals/glass or metal...and would there be more problems with miniatures because of the closer pins.....,or not at all...hmmmm???

Simple---Tube tech was advancing, especially during WWII, towards continually smaller envelopes. Also, because so many tubes were made during the war there was a lot of surplus available. So, companies designed their circuits on what they had available, modifying the tube types in the circuit as they ran out. That is why you will see mixed sizes used, especially after the war.

:Simple---Tube tech was advancing, especially during WWII, towards continually smaller envelopes. Also, because so many tubes were made during the war there was a lot of surplus available. So, companies designed their circuits on what they had available, modifying the tube types in the circuit as they ran out. That is why you will see mixed sizes used, especially after the war.
:

Yep. I had a Rogers Majestic that had 7-pin sockets riveted over the original octal socket holes. The 1946 Stromberg Carlson 1100 table radios have 6 octal tubes, later years have 4 mini tubes and octal power tube and rectifier. By 1950 or 51 that same cabinet and similar chassis has 5 mini tubes and a selenium rectifier.

The mini tubes are very reliable, generally run cooler, and in the mid-fifties, began to appear mounted sideways on a vertical circuit board. This eliminated the need for a dial cord. I don't think big old octals will work that well on their sides.

:Simple---Tube tech was advancing, especially during WWII, towards continually smaller envelopes. Also, because so many tubes were made during the war there was a lot of surplus available. So, companies designed their circuits on what they had available, modifying the tube types in the circuit as they ran out. That is why you will see mixed sizes used, especially after the war.
:

Yep, the march of vacuum tube technical progress was very quick in the first half of the 20th century- similar to that seen in personal computers decades later. (Around 1990 I bought a PC hard drive that had a staggering ONE MEGABYTE capacity. PC drives of roughly the same physical size and cost are now in the terabyte range- a memory-density growth factor of 1 million.)

Vacuum tube technology was greatly advanced in the USA via the proximity-fuze "smart shells" used in WW2 artillery. This was a very efficient secret weapon that was only possible through the development of tiny vacuum tubes with so little internal mass that they could survive the hundreds of g's of acceleration of being fired from a cannon and still function to allow the ranging circuitry within the fuze to detect that it was near its target and detonate the shell's high explosive charge. Other electronic equipment such as tactical radios still used the large octal-based tubes; it has been suggested that this was intentionally done to conceal the existence of this new generation of tiny vacuum tubes that made the incredibly lethal artillery shells possible.

Hi All,
Interesting discussion.
Looking at reliability and all, it appears that Octal tubes represented the peak of vacuum tube design.
They had two problems, though, size and cost. With 7 and 9 pin miniature tubes, radios and more importantly televisions could be made smaller.
Those smaller tubes are not more reliable. The sockets don't grip those little pins as well as octal sockets do.
Later tubes, even smaller, dealt with this problem by soldering directly. By then tubes had become so reliable that soldering them in became an attractive option.
Octal tubes at that point could have been manufactured to be that reliable, but by then they were obsolete.
It seems like cost and size drove technology away from octal, not reliability concerns.

Best Regards,

Bill Grimm

:Hi All,
: Interesting discussion.
: Looking at reliability and all, it appears that Octal tubes represented the peak of vacuum tube design.
: They had two problems, though, size and cost. With 7 and 9 pin miniature tubes, radios and more importantly televisions could be made smaller.
: Those smaller tubes are not more reliable. The sockets don't grip those little pins as well as octal sockets do.
: Later tubes, even smaller, dealt with this problem by soldering directly. By then tubes had become so reliable that soldering them in became an attractive option.
: Octal tubes at that point could have been manufactured to be that reliable, but by then they were obsolete.
: It seems like cost and size drove technology away from octal, not reliability concerns.
:
:Best Regards,
:
:Bill Grimm
:I have heard some audiophiles say that octal preamp tubes like the 6SN7/6SL7 have more bass response than miniatures like the 12AX7???? I wonder if this is true or not...
:

::I have heard some audiophiles say that octal preamp tubes like the 6SN7/6SL7 have more bass response than miniatures like the 12AX7???? I wonder if this is true or not...
::
:
:
Why would it be true? A vacuum tube, be it large or small, does not function as an acoustic enclosure. Amplifying audio frequencies is a piece of cake for virtually any device- it's the RF frequencies where things start to get hinky.

Hi Chris,
I had not heard that about audiophiles.
Most octal tubes have problems above about 30MHz. The plastic in their plugs starts to conduct at these frequencies. Thus you have tubes such as the 6SB7Y whose base is constructed of a yellow material that won't conduct at these frequencies.
One would expect the characteristics of a 6SN7/6SL7 to go into the tank in the VHF range. Maybe that is what they like?
Then again I can't imagine getting VHF through a speaker, not to mention the audio output transformer. Never mind.

Base response may be related to feed back. The base notes may vibrate the smaller tube easier than the big hulking octal. The differences among many of these tubes is just mechanical, size and socket reliability.

All the Best,

Bill Grimm

Bass response (as well as over-all response) is heavily related to impedance (plate resistance), and if impedances are different when used in the same circuit, you will have different bass response, gain, etc.

Where I notice this a lot, since I don't deal with 12AX7s, etc., too much, is when I swap a dual-plate 2A3 for a 45. Being essentially 2 #45 tubes, the plate resistance is going to be lower, and will have more driving power, especially since output transformer impedance goes down with frequency, and so a lower plate impedance will do a better job of driving the transformer at low frequencies than a higher impedance one.

Tubes can also have different sounds because of things like inter-electrode capacitance, though this is usually very small at audio frequencies, and also the shape of the plate, elements, etc.

I have some 6V6 tubes that sound more mellow and some more sharp (mid-range). The difference isn't always that much, and the circuit can be modified to compensate, but it is noticeable.

Another interesting example of bass response is in my Crosley 1117. It is designed for 6K6 tubes, which have an approximate plate resistance of 95K. Switching to 6V6 tubes notably increases bass response, and they have a plate resistance of 66K. Switching to 6L6 tubes makes for overwhelming bass response, and these tubes have a plate resistance of 22K.

Where the plate resistance is lower, the tube will be able to better dominate the load resistance, where-as if the plate resistance is higher, the load resistance, being the same as before, will load down the tube more, and the tube will have less control of the voltage at the load resistance.

Hi Thomas,
Excellent description.
Great seeing you back.

Best Regards,

Bill Grimm

Thanks! :D

I check in now and then.