Question for Octal Cornet

tinos5601 · « **on:** 10 Feb 2004, 12:39 pm »

Hello, I have a question for the safety margin of the cathode follower section of Octal Cornet.

I intend to use 5692 instead of 6SN7, and the specification indicates that the voltage potential between cathode and heater should not be higher than 100V. (For 6SN7 it's 90V)

I assume in real circuit it should be OK but since a simple voltage divider in HT section would do the job I would like to include it. However, I am hesitant because in Octal Cornet design it is not included. Certain design considerations may be behind this, and I would very much appreciate if Mr Hagerman would illuminate me in this regard.

Thanks,

Tae

hagtech · « **Reply #1 on:** 11 Feb 2004, 05:53 am »

Cathode can be 200Vdc above heater for 6SN7. Only 100Vdc the other way around. One reason this is a great tube. I haven't checked on the 5692. I would assume this is a rating you don't really want to exceed.

jh

tinos5601 · « **Reply #2 on:** 11 Feb 2004, 03:07 pm »

You're right... I found the original RCA 6SN7 spec where the rating was stated 200V. However, for 5692 - meant to be a sturdier substitute for 6SN7 - it's firm 100V both ways.

I've got the idea of this substitution because I have a couple of RCA red-base 5692's around, and in common cathode circuit they sound a lot better than Philips/JAN 6SN7 I currently use for Cornet. It'd also look better with 5692 since the other tubes are 5691 red's and 80 for rectifier.

May I use 5692 in Cornet with heater voltage floating about 70V above ground? If you'd say the hypothetical advantage of the substitution isn't worth the risk of tempering with ground scheme, (that's what I am afraid of) I'd drop the idea. Thanks in advance!

Tae

RogueAngel · « **Reply #3 on:** 11 Feb 2004, 03:54 pm »

I've been running an octal version for quite awhile now and have done a moderate amount of tube rolling. I started with the JAN Phillips and have moved on, as funds allow, to other tubes.

The best evaluation I have found on the various 6SN7 and 6SL7 types is at: http://home.att.net/~chimeraone/6sn7sound.html and http://home.att.net/~chimeraone/6sl7sound.html.

I have found these evaluations to be very accurate and would recommend trying some of the older tubes to hear what your Cornet can really do.

Hope this helps.
Bobbi

tinos5601 · « **Reply #4 on:** 11 Feb 2004, 06:21 pm »

Thanks Bobbi for the input and the links. Helped me a lot. I have tried to search the web for similar info but never seen this one.

I have seen VT229's and VT231's on top of various lists but unfortunately top of price lists, too. I have heard Sylvania 6SL7 is real good but also quite expensive... Some high-ranked tubes in the list are not terribly expensive (like GE black plate 6SN7) and that's good news.

It is just that I happened to have a couple of 5691's and 5692's, and they generally outperform 6SL7's and 6SN7's in my part bin. Maybe it's a better idea to get top notch NOS tubes than tinkering with the PCB... A couple hundred dollars and forget about it...

Tae

hagtech · « **Reply #5 on:** 13 Feb 2004, 07:14 am »

You could potentially (har har) float the heater supply. It would be a bit of work, but since you're already using octal tubes, maybe not that much more. Just a matter of lifting all of the grounded parts of the heater circuit. Then connect to a divider on the B+ line.

jh