Makofoed-6sn7 said:
We have a system, but it's an effing mess, and quite contradictory. You will see lots of types which differ only by filament voltage: here is but one example.
12C5: 12.6V @ 0.6A
17C5: 16.8V @ 0.45A
25C5: 25V @ 0.3A
50C5: 50V @ 0.15A
All four types are identical, except for the heater voltages. Now, you have the 50C5 and 25C5 with a heater voltage of 50 and 25 respectively. So far, so good. Now take a look at the 17C5. Its heater voltage is 16.8V, not 17V. So do they round up to the next whole number? Would seem so doesn't it? Then why is the 12C5, with a heater voltage of 12.6 not called a 13C5? If they use just the whole number part of fractional heater voltages, then why is not the 17C5 called a 16C5? It's already confusing. One final word about heater voltages: '0' always means a cold cathode. Any tube operating at heater voltages less than one volt (and there are some: CRT HV diodes) are always '1' types.
To make matters worse, you have types like the 35C5 (different characteristics) the 6BY7A (different characteristics, and a different tube from a 12BY7A (pentagrid converter v. a small signal pentode)) and, of course the 50L6 v. the 6L6. A 50L6 will poof immediately if used as if it were a 6L6 with a 50V heater. A 6L6 will be biased way into a highly nonlinear part of its characteristic if used as a 6V 50L6 in a circuit designed for the 50L6.
As far as the lettering goes, the rules are: no double letters, so that you won't see types like "6CC5". The lettering starts from 'A', except for power diodes where it starts with 'Z'. 'O' and 'I' aren't used at all, so as to avoid confusion with '0' and '1'. These letters have no significance. The type may end in a letter. If you see something like: 12BY7A, the 'A' means a "new and improved" type. Sometimes this involves performance enhancements. For the 12BY7A, that meant somewhat smaller internal capacitances, a higher g(m), and what's called a "controlled heater characteristic" for better operation with series connected heaters. Of course, a new letter appended at the end could mean something unrelated to electronic performance, such as using a different grade of glass.
The final number tells you how many active elements, so you have something like: 6C4.
1) Heater
2) Cathode
3) Control Grid
4) Plate
Unless there's a conflict, then a different number is used. For metal tubes, the metal envelope is considered an active element, so that single metal triodes always end with a '5', whereas glass single triodes end with a '4'. And anything internally connected is considered one element (i.e. beam plates/suppressors internally wired to the cathode).
As for how the 50L6 performs, it's really bad if used according to the most commonly specified Q-Point. This tube was originally intended for use with equpiment that ran straight off the 120V mains without power xfmrs. They were basically shooting for enough power to drive a reasonably sized loudspeaker, and weren't concerned with fidelity at all. Indeed, the OPT iron often used with the 50L6 (and the similar 35C5 and 50C5, and all its cousins) was poor in the extreme. You'd be lucky to get down to 200Hz with it (and that, too, was by design so as to quiet down an audio amp supplied from half wave recification and minimal DC ripple filters).
The 50L6 can sound much better if you get the screen voltage down (90 -- 100V) and the plate voltage up (150V or better). Like the 50C5, this will put it into a more linear region of the characteristic where it can sound quite good. However, this will require a power xfmr to get the necessary DC, and, of course, screen voltage regulation. Zeners, VR tubes, or active regulation are necessary for good audio performance. So far as commercial equipment, I know of none which used these tubes (50L6/50C5/35C5) and got it right.
I do have a viable design for 50C5 monoblocks that I believe will make outstanding computer speeks. However, it looks like the only way I'm going to get decent output iron for this is if I wind them myself.
Hello Magnus,
please don't forget that the European designator may also mislead you!
E.g.: The PL84 by no way is the 0.3 A series version of the EL84/6BQ5 valve. It's 6.3 V derivative is called EL86/6CW5, whereas there isn't a series heater equivalent to the EL84.
Best regards!
50L6GT are really nice in triode mode, very similar to 6W6
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
Last edited:
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.