Metal (or not) triode with top cap usable as CF - Have you a type ?

[kagliostro]

Only for aspect reason

Which will be a triode with topcap to be used as CF after a 6J7 tube ?

Thanks

[HotBluePlates]

Any tube could be used as a cathode follower. The only limitation will be the voltages in your circuit, the resulting voltage at the cathode, and the heater-to-cathode voltage limit of your chosen tube.

[shooter]

heater-to-cathode voltage limit

fwiw, I ran into a situation where I was close to that limit, so I switched DC for filaments which gave me more *range* if you will. 

[PRR]

6J7 triode-strapped is a fine cathode follower.

6J5 will also work. No cap.

RCA RC-14 also suggests 6C5 6P5 as 6.3V lo-Mu triodes for general use.

For most audio cathode followers, "any" of the RF/IF tubes would work. That means the middle stage of millions of radios, so there is a good supply. 6K7 etc.

What usually won't work-out well, unless you need an exceptionally strong cathode-follower, is dual-tubes with common cathode. 6F7 and many others.

[kagliostro]

Thanks to all

I didn't considered the use of a pentode connected as triode .... very good idea

--

Looking for a metal triode with topcap on my tube junk box I've find a pair of 6S7

Do you think they can be used as CF if triode connected ?

What about the use of one 6S7 as preamp pentode ??? (thinking to a 5C1 circuit)


http://tdsl.duncanamps.com/link.php?target=09259818

Thanks

Franco

[Mats]

Hi !
Perhaps this will work, not a topcap.
Found this searching, an 6SF5
/Mats
and with topcap, 6F5 tube

[sluckey]

Franco, why do you want to use a tube with a grid top cap? There will be a big voltage on the grid of a CF. Most grid caps I've seen in use were an uninsulated metal clip. I suppose you can find a ceramic or phenolic cap to fit a grid top cap?

[kagliostro]

Thanks Math & Steve

Steve, I've some of those ceramic top cap

I was thinking to something like a 5C1 with a 1625 (or 807) power tube and a 6J7 tube as V1

initially I was thinking to a 6SJ7 but then as to have all tubes with topcap I've think to the 6J7

I was also wondering to add a CF after V1 as to use an FMV ToneStack and that, if possible

using also for this a (metal ?) tube with TopCap as to make uniform the look

Franco

[PRR]

S>> why do you want to use a tube with a grid top cap?

K> for aspect reason

In Maine I think we would say "for appearance".

"Aspect" is good English but tends to be used more technically. "Aspect Ratio" is length-to-width proportion, important in structural problems. "The financial aspect of the project" does mean "the appearance of the money" {does the money make sense?}, but often means the project is in financial trouble (the money situation is ugly).

> big voltage on the grid of a CF.

Yes, but usually a lot of resistance.

Mats' posted plan will be over 50 Megs to about 100V. A 2uA shock. Probably won't even feel it. Almost safe enough for open-heart surgery.

Another plan I saw here today had a 5.6Meg+1.5Meg divider. 1.2Meg to about 60V, 50uA shock. Safe enough through skin.

He "could" direct-couple out of a tube with 100K plate resistor to 300V. That's 3mA, which is actually "legal" (shock-protection GFIC/RCB devices trip at higher level) but enough to sting, and worst-case enough to stun a weak heart.

It needs consideration. It is easy to design it non-dangerous.

The conventional "hat" for grid-caps is a metal cup which is a tight fit on the top-cap ring. IIRC, there is a US-size copper tube fitting which is close-enough after a little saw and hammer. The metal tube shell is grounded via pin 1. The cap adds shielding on the grid lead, essential for the microphone input stage (not so much on a power tube driver).

The 807 power tube top-cap is the big danger. 400 Volts exposed in an open cabinet can send you to Heaven without a ticket.

I was going to say all GRID-cap tubes are metal, but I thought of an exception. 1D8GT is a diode-triode-pentode in glass. Top-cap is pentode grid. Pentode is a "power" pentode (0.2W), needs 5 to 9V of drive, so the top cap probably does not need shielding. (Especially since the main use was battery beach-radios, far away from hum sources.)

[DummyLoad]

I was going to say all GRID-cap tubes are metal, but I thought of an exception.

6j7gt

http://www.ebay.com/itm/Radio-Tubes-6J7GT-6J7-Greylock-NOS-/350684768670

--pete

[DummyLoad]

i actually built this close to 5 years ago on my breadboard. all metal tubes - ALL of them. 2204 ckt. w/ james stack. killer tone. ditched it because 6SF5's like to become very micro-phonic.

the 1614 is an industrial/military 6L6 with an ICAS plate dissipation rating of 25W: 21W in CCS. IIRC, the amp i built as shown in the attachments was a about 30W.

http://tubedata.milbert.com/sheets/049/1/1614.pdf

if you can find type 1620, they are a low micro-phonic 6J7.

http://tubedata.milbert.com/sheets/049/1/1620.pdf

the 1603 is another 6J7/57 variant - only it's glass, low micro-phonic, but it's on a 6 pin base.

http://tubedata.milbert.com/sheets/049/1/1603.pdf

--pete

[PRR]

> 6j7gt

You are of course correct.

Not just "Greylock" (who?)..... Tung-Sol made 6J7 in three types: metal, 12 gauge bulb, 9 gauge tube.

Hytron had an early line of glass replacements for metal tubes: 6A8GT 6J7GT 6K7GT 6K8GT (and 6J5 without cap). However these were "supplied with metal shields".

[DummyLoad]

> 6j7gt

You are of course correct.

Not just "Greylock" (who?)..... Tung-Sol made 6J7 in three types: metal, 12 gauge bulb, 9 gauge tube.

Hytron had an early line of glass replacements for metal tubes: 6A8GT 6J7GT 6K7GT 6K8GT (and 6J5 without cap). However these were "supplied with metal shields".

i have a tung-sol 6J7GT WW2 MRO labeled box: can't find it to photograph, but found the pair of 6SF5GT MRO boxed tung-sol though, one is fried. don't have the heart to pitch it.

here's one on eatbait.

http://www.ebay.com/itm/like/252026424042?ul_noapp=true&chn=ps&lpid=82

--pete

[DummyLoad]

6R7 med. mu triode & diodes

http://tubedata.milbert.com/sheets/049/6/6R7.pdf

6Q7 hi mu triode & diodes

http://tubedata.milbert.com/sheets/049/6/6Q7.pdf


--pete

[kagliostro]

Many Thanks for the useful info PRR and Pete

a special thanks to PRR for the language clarification

In Maine I think we would say "for appearance".

"Aspect" is good English but tends to be used more technically. "Aspect Ratio" is length-to-width proportion, important in structural problems. "The financial aspect of the project" does mean "the appearance of the money" {does the money make sense?}, but often means the project is in financial trouble (the money situation is ugly).

---

A particoular tube with both grid and plate on topcap

(I was looking for it on 2015 at some Ham Fest, also in Germany, but no chance to find it)

http://www.mif.pg.gda.pl/homepages/frank/sheets/049/2/2C22.pdf

http://www.mif.pg.gda.pl/homepages/frank/sheets/095/6/6C8P.pdf



Franco

[kagliostro]

This morning I was to a small HamFest

and I've seen this babies      for sale at a very low price (€ 5.00 for 6 pieces)



I don't know what I'll do with it, but tey have a so nice look ans seems brand new .......

5 are Ken-Rad, the other has no brand but maybe National

If I'm not wrong they are the same of 2C22 so I think I can consider 1/2 of 6SN7 ....

Franco

[PRR]

It's a 6C5 with VERY short plate and grid leads, for 300MHz work (which is not easy on Octals).

So yeah, half a 12AU7/6SN7.

How are you going to avoid confusing the plate and grid caps? (Boom!) I think in the original war-radio, a small coil of stiff wire and some caps sat right ON the tube, and maybe hard to get wrong.

OTOH, the socket is easy to wire...

[kagliostro]

How are you going to avoid confusing the plate and grid caps?

Looking to the datasheet seems that it will not be difficult

only you must put the holes for the wire of Grid and Plate on the chassis on the right place respect the socket key

and it will become impossible to connect it wrong  (wires must be strictly of the required lenght and no more)



Franco

[Greg]

Ok, I'm cross threading a little here but PRR's idea of a copper end shield cap is a cool one. Actually a 7/8  is a perfect fit. Some cheap top hat bakelite cap from China can serve as an insulator between the tin connector and the copper cup and be glued (with a little sanding) at the bottom of the copper cup. A small hole can be drilled through the copper cup and some small Gibson style-braided coaxial wire will make contact through the copper shield. The braided shield can be grounded at the metal shell pin or eleswhere. Of course, the copper could be painted any color for a good «aspect» ;)   

[kagliostro]

That is a cool idea, I was thinking to do something like that


Franco