6414 tube instead of 12AT7?

[Jon L]

It seems 6414 tube is a slightly lower Mu version of 12AT7 with similar specs, kind of like 5751 instead of 12AX7.  The black plate Raytheon 6414 looks pretty cool.  Would there be any issues replacing 12AT7 with 6414 in Paramour II?   

[Paul Birkeland]

You should have no issues, as the Paramour is cap coupled. 

[Len]

You should have no issues, as the Paramour is cap coupled. 

What issues would exist if it were direct coupled?

I'm still working on trying to understand what is required for a good direct coupled circuit.

Thanks.

[Paul Joppa]

I'm sorry, but the 6414 is NOT "slightly lower Mu version of 12AT7 with similar specs". It has 2/3 the mu and 2/3 the plate resistance, takes 50% more heater current, is rated for no more than 2/3 the maximum voltage and 2/3 the maximum plate dissipation. The maximum of 200 plate volts is the killer, it is NOT suitable for use in the Paramour where the shunt regulator has 350v on the plate.

And before someone else points this out ... yes the 12AT7 is rated for 300 max volts. I think that's OK since the shunt reg does not see peaks above the regulated voltage - but 17% over is a far cry from 75% over voltage! And I want to drop the Paramount regulated voltage to 300 volts in the next revision.

[Paul Birkeland]

Oops, so this would operate in Paramour I, but not Paramour II?

[Paul Joppa]

All Paramours (with no shunt regulator) are probably OK. From the published curves, it looks like you would want to raise the bias voltage to 2.5v (aTL431 chip in place of the LED as in Paramount perhaps).

[Paully]

So what voltage does the Paramount run 12at7's at?

[Paul Birkeland]

The plate of the 12AT7 sees 375v maximum in the Paramount, although that amount is for a short duration.  Once the regulator kicks in, that voltage drops down to 350 and sits.  I would say any 12AT7 substitute rated for at least 300V should work fine (the absence of AC on the regulator plate allows for some flexibility).

[Paully]

So if the 12at7 is rated for 5-10,000 hours how long would you run one at 15-20% over before changing it out to avoid failure in the amp?

[RPMac]

If the plate of the Paramount 12AT7 is at 350V, wouldn't that put the plate of the direct coupled 2A3 at over 500V?

[Doc B.]

There are two triodes in the 12AT7. The plate of the 12AT7 triode in the Paramount that is in the shunt regulator runs at 350V. The plate of the second triode that works as the input stage and couples to the output tube grid sits at around 195V, not 350V.

[paulw]

I once bought a pack of 5 NOS National 12AT7's and was I surprised when I opeded the box!  Obviously not as they were marked - so a bit of reasearch provided the answer

Wonder if I'll ever find something to plug them into!

Paul