6KT8 pin 5 is pentode G3 and heater. With G3 tied to Cathode, any concerns?

[rob_h]

The 6KT8 Triode/Pentode tube pin 5 is tied to G3 and to one side of the heater.  If G3 is tied to the Cathode, the Cathode is tied to the heater supply.  Are there any problems or concerns, like not connecting the heater supply to ground through resistors, etc.? 


Thanks,

[sluckey]

If G3 is tied to the Cathode, the Cathode is tied to the heater supply.

I would not connect the cathode to G3 in this case. There could be hum issues. Having the heater winding referenced to ground either thru a real or artificial center tap would give G3 a good ground reference.

[HotBluePlates]

...  If G3 is tied to the Cathode, the Cathode is tied to the heater supply.  ...

You don't do that, because:

... pin 5 is tied to G3 and to one side of the heater.  ...

Obviously, the manufacturer assumed you would tie one side of the heater to ground (like early 50's Fender one-wire heater setups), or have a low-impedance path to ground (like 50-100 resistors making an artificial center tap, or a real center tap).

The RCA data sheet also says pin 5 is the internal shield, along with being one end of the heater and G3. I'd say that's more support for assuming the manufacturer intended you to ground that pin.

Do not tie the Heater/G3 to the cathode.

Are there any problems or concerns ...

A.C. on your electrolytic cathode bypass cap may make that cap explode, especially if cathode voltage is less than the heater a.c. voltage.

Besides, the important thing is not tying G3 to the cathode, but that it be a substantially low voltage compared to the plate and screen, and similar in voltage to the cathode or ground. The heater voltage is probably fine (and probably too small in the face of the screen/plate voltage to do harm to the output signal).

[rob_h]

So, with no connection between G3 and Cathode, what would be best?
1) Ground one side of the heater supply at its first exposure, then run the heater wires to all tubes, making sure that the Grounded side is tied to pin 5 of the 6KT8?
or
2) Run the heater wires as normal, and then ground pin 5 at the socket?
or
3) Another way?

Thanks!

[sluckey]

So, with no connection between G3 and Cathode, what would be best?

Just connect the heater pins 4 and 5 across the filament winding with a CT on the filament winding. If you don't have a real center tap, use two 100Ω resistors connected between each filament lead and ground. That's all. DON'T GROUND PIN 5. DON'T GROUND ONE SIDE OF THE HEATER SUPPLY.

That info applies to using AC voltage to heat the tubes. But if you will be using DC to heat the tubes, then ground one side of the dc supply.

[PRR]

Pick another tube.

[HotBluePlates]

Pick another tube.

      

That's what I was thinking, but I assumed you have a very compelling reason to use this t.v. tube...

[rob_h]

With respect, the GE description is "The 6KT8 is a general-purpose triode-pentode tube".  

Edit:And yes, even though GE calls the 6KT8 a “general purpose” tube, RCA lists it in the RCA RC-30 1975 in 3 separate TV applications, and now a reverb tube too. :)

The application is a reverb driver/recovery amp. The pentode is 2.5Wa, the triode is 1Wa with 100 gain.  
The compelling reason is that I wasn't satisfied with the 6U8A characteristics for driving my 4FB21AC reverb, so, after reading a post with a question on the 6KT8, I thought I'd try it since I have a couple.  I drew up the curves using the 6KT8 and they seemed to work beautifully.  The pentode driver shows a full +- 6ma , from 1.5 to 13.5ma.  For the recovery, the triode has a gain of 79 of its 100 rating and a low harmonic distortion.  That was until I noticed the oddity in the grid 3 and heater connection.  In the post about the 6KT8 it was said that its triode had a bit higher gain with less noise than the 12AX7 because of its higher mu and lower plate resistance, and for the recovery amp that sounded like a good thing.  
Anyway, thanks again for your help.

[darryl]

Another possibility is the 6DX8/ECL84. This was used in a vintage Australian guitar amp, the Moody GA40. The reverb tank was a 4FB2A1C.

[rob_h]

Thanks so much! I have a few of those too. I'll check it out.
Working with Merlin's basic "cheap current amp" design, all I need for the driver is a good 12ma p-p that doesen't run too close to cutoff, and maintains low harmonic distortion, output symmetry, and a reasonable gain on the recovery section.
I'll draw it up using the 6DX8 and see how it works out.
Thanks again for the new info!

[DummyLoad]

Pick another tube.    

6AC10?

--pete

[rob_h]

I drew up the graph for the 6AC10, but its characteristics don’t lend their selves to the +-6ma required for the reverb driver, current amp, circuit. 

Univox U-1000 uses a 6AN8-A pentode and triode as its reverb driver amp.  This tube has the same G3-to-heater connection as the 6KT8.  I drew up the pentode driver graph and it doesn’t work nearly as well as the 6KT8 and costs 5 times as much. 
The Gain of 100 of the 6KT8 is higher than just about any of the other triode/pentodes and is a plus when amplifying a 14mv peak, per PRR, max voltage out of the reverb tank.  This one will be biased at about 79, x1.4mv, =1.1vout max.  Found what I was looking for: a one tube reverb for a high impedance tank, transformerless, with a low harmonic distortion driver and a high gain triode recovery amp. 

Yes I was looking for a pentode/triode, I drew up these, plus 5 or 6 more: 6AN8, 6AW8A, 6BR8A, 6EA8, 6GH8-A, 6JV8, 6LC8, 6bm8, 6GW8.  None of them has a pentode with characteristics that will work well with the +-6ma requirement, and a high mu triode.  I must say, however, that I only used the graphs provided in the data sheets and didn’t draw up different graphs with different screen voltages.  I would think that some of the other triode/pentodes could be configured to work if the screen voltages were experimented with, reduced, for the desired results. 

Thanks for the advice on the heater/G3 circuit. 

[sluckey]

Univox U-1000 uses a 6AN8-A pentode and triode as its reverb driver amp.  This tube has the same G3-to-heater connection as the 6KT8.

6AN8 G3 does not connect to the heater.

[DummyLoad]

None of them has a pentode with characteristics that will work well with the +-6ma requirement,

http://tubedata.tubes.se/sheets/084/6/6MF8.pdf