IF tube oddity: 6CB6 and 6AG5

[JohnCT]

My Andrea TVL16 was originally designed with the three IFs to use 6CB6 tubes. The inside label and the appropriate SAMS call them out as 6CB6 for all three IFs.

In my example, I found the first IF to be a 6CB6, but the second and third IF tube sockets have 6AG5s installed instead - all tubes original to the TV (RCA 274 ID and date codes match all the rest of the tubes).

Closer inspection of the tube compliment chart pasted inside the cabinet shows that the original label was rubber stamped over and the second and third IFs show that the 6AG5 was indeed spec'd for those sockets in a later revision.

I have new tubes of both varieties but the TV does NOT want them swapped. Any swapping leaves a very bad ringing and other picture issues. Apparently, the gain and capacitance differences between the tubes means that a re-alignment would need to be done if I wanted to put all 6CB6s in or all 6AG5s.

It's interesting that only the second and third IF were changed to the 6AG5, not the first IF - so I assume this revision wasn't done for tube availability reasons.

Near as I can tell, the 6AG5 is a little lower in gain. Would it be correct to assume that the designers decided a little less gain was preferable in the second and third IF slot in this TV?

Would there be another reason this was done?

Any insight appreciated.

John

EDIT: looks like they're wired differently internally. I had seen a sub book where they would interchange, but apparently that data is wrong. So I guess it's not just alignment, Andrea must have wired the chassis differently for the revision. Still wonder why they would have changed tubes midstream.

[Kevin Kuehn]

Riders lists both 6CB6 and 6AG5 for 2nd and 3rd video IF's. There's also a note that "late models contain additional 6AU6 AGC tube similar to model VL-19". I'm assuming with the addition of AGC they decided they didn't need as much gain, or possibly the later simply works better with AGC.

[Penthode]

The 6CB6 and 6AG5 are similar construction tubes. The 6AG5 predates the 6CB6 and you will generally find manufacturers switched from the 6AG5 to the 6CB6 in the early 50's. The only difference I recall is the elimination of the internal link between suppressor grid and cathode in the 6CB6 which may have helped raise the transconductance.

Because the IF stages are stagger tuned, swapping even tubes of the same type can lead to a change in IF response. Certainly swapping a 6CB6 for a 6AG5 or give versa would require at least realignment or at most minor change in circuit design.

[Tom Albrecht]

Penthode,

I used to think that changing the gain of a particular stage in the IF would affect frequency response, but I'm not sure that's the case. If you simply think of each of the tuned circuits as independent filters, and each of the amplifiers as linear wideband amplifiers, the frequency response and gain end up being completely independent, I believe. You can put all the gain up front, at the end, or interspersed however you like. The amplifiers, of course, also serve to buffer the filters from one another so that the tuning of one doesn't affect the neighbors, so you can't really put all the gain up front, but for illustration purposes, I believe it's correct.

[dieseljeep]

Originally Posted by JohnCT

My Andrea TVL16 was originally designed with the three IFs to use 6CB6 tubes. The inside label and the appropriate SAMS call them out as 6CB6 for all three IFs.

In my example, I found the first IF to be a 6CB6, but the second and third IF tube sockets have 6AG5s installed instead - all tubes original to the TV (RCA 274 ID and date codes match all the rest of the tubes).

Closer inspection of the tube compliment chart pasted inside the cabinet shows that the original label was rubber stamped over and the second and third IFs show that the 6AG5 was indeed spec'd for those sockets in a later revision.

I have new tubes of both varieties but the TV does NOT want them swapped. Any swapping leaves a very bad ringing and other picture issues. Apparently, the gain and capacitance differences between the tubes means that a re-alignment would need to be done if I wanted to put all 6CB6s in or all 6AG5s.

It's interesting that only the second and third IF were changed to the 6AG5, not the first IF - so I assume this revision wasn't done for tube availability reasons.

Near as I can tell, the 6AG5 is a little lower in gain. Would it be correct to assume that the designers decided a little less gain was preferable in the second and third IF slot in this TV?

Would there be another reason this was done?

Any insight appreciated.

John

EDIT: looks like they're wired differently internally. I had seen a sub book where they would interchange, but apparently that data is wrong. So I guess it's not just alignment, Andrea must have wired the chassis differently for the revision. Still wonder why they would have changed tubes midstream.

Rumor has it that there was a shortage of certain tubes during the Korean war. All sets built at the time had different tubes in place, especially Admiral, as they were the largest TV builder at the time.
The tube changes were rubber stamped on the chassis next to the socket.
It was strange to see a 6H6 instead of a 6AL5 in a newer set.