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I have a Decware SE84C & I know that it can use a single 6922 or 12AX7 for the input tube. My question is, how hard or easy would it be to modify an amp that uses 12AX7's to use 6922's also? If your like me & have a good assortment of 12AX7 & 6922 wouldn't it be nice to use either sometime?
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But the tube requirements are very different. Amplification factor is 33 for 6922 and 100 for the ax7. Max plate voltage is 300 for ax7 and 200 for the other. It seems the max plate voltage between the 2 might be a problem especially in the driver tube position of se amp.
I beg to differ. The signal connections are the same, BUT the heater connections are different. The 6922 expects 6.3 V across pins 4/5, while pin 9 is an internal shield. The 'X7 employs pin 9 as a CT on the heaters (pins 4/5 are the ends) and that allows either 6.3 or 12.6 V. to be used, with 4/5 being used for 12.6 V. (9 is NC). Tie pins 4/5 together and apply 6.3 V. to the composite and pin 9.
Eli D.
Edits: 12/16/16
Change the pin wiring and add a resistor?
As the two tubes are "different" in their biasing, plate loads/gain and filament pinout, switches and switched in-out parts would be necessary.
The OP, or anyone seriously interested in this implementation, should probably research the differing resistors required for cathode bias and plate loads for a given high voltage power supply. Also, finding noiseless, accommodating switches will not come easily. The gain is very different among these two tubes.
Perhaps, after much research, you might consider different tube types, like a 12AX7 vs. 12AT7 or 12AY7 vs. 12AT7 or 5751. A 12AY7 vs. 6DJ8/6922 might also be easier to implement.
Please also take note of a new breed of hybrid tube preamps and headphone amps, reasonably offered from Hong Kong and China, by companies like Indeed, Bravo, Little Bear, etc. Some of these can be switched easily and quickly to use either 6922 or 12AU7 tubes ! However, this is because their very low plate voltage/hybrid operation creates a unique combination of "gain with a very low output impedance." Thus, while a stereo headphone amp can be accomplished with one 6DJ8 or 12AU7 plus two FETs mounted on heatsinks, that same 32 Ohm to 600 Ohm output capable headphone amp can also be used as a very competent line stage preamp. Sound quality is surprisingly very good, but remember those tubes are now hybrid devices. So, it can be done...
Wrong about the DECWARE SE84. It can use either the 6n1p or 6922, which are pin compatible and not grossly dissimilar.
IIRC, Steve Deckert did a variation on the triode wired SE84 theme, using 12AX7 section voltage amplifiers and pentode mode "finals".
The 6922 and 12AX7 are VERY different in character. Circuitry designed for 1 type is incompatible with the other. Pin out differences are another "fly in the ointment".
Eli D.
Years ago I got an email from Steve Deckard, saying that I could use a 12ax7 as well and I have used a 12ax7 in the SC84! So I know it's possible I'm just wondering what it would take to do it.
So you have actually interchanged these tubes in your amp and it worked????? Whaaa!
I understand that some Decware amp designs over the years have used one or the other, but I can't find any information that you can swap those within one amp. What will you plan to do to be sure you can use either tube, since the 12ax7 and 6922 are very different from each other?
Thanks,
Jeff
Unless they have 2 sockets and you can use either/or, or a switch that will route the proper resistances and filament voltage. It seems like that would add way too much to the cost for the ability to swap.
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