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6V5GT

I found a bunch of these in my "Tube shed" and would like to do some experimenting. Yes, I know, use a different tube but I have quantities like Tubelab had in his storage, and oddities are sometimes surprising.

This is an oddball and I would like to give it a go with the cut and try approach.

The cathode, grid 3 and heater are connected together internally.

Has anyone out here experimented with this tube?
I have not found anything other than the datasheet online.
 

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6A3sUMMER

Member
2016-06-07 6:50 am
The data sheet gives 3 examples of single ended, and 2 examples of push pull. It seems to be capable of making a viable amplifier. The sound will largely depend on how you implement the circuit, negative feedback, and the output transformer. You could also triode wire the tube (and not even use negative feedback. You might be able to use the tube(s) in Ultra Linear too. With proper circuit design, you could also use Parallel Single Ended, or Parallel Push Pull for more power. The question is, do you want to stretch out where perhaps no one has gone before (where have I heard that on this forum before?).
 
Thanks 6A3sUMMER and Tubelab_com..

I was figuring fixed bias was the way to go, thank you for confirming.


Tubelab_com



I have been collecting tubes since the "sand state" revolution.


Back in the day when suppliers were dumping, I picked up a semi load for the cost of shipping...

I think it's time to push the limits and melt a few, and then back it off a bit for maximum effect.

No rare tubes will be harmed in the experiments to follow.
 
I was figuring fixed bias was the way to go, thank you for confirming.

Self (cathode) bias is possible. Employ dedicated, "floating", filament windings.

True "fixed" bias places a heavy burden on the matching of tubes. Assuming PP "finals", a very tightly matched quartet of tubes is necessary. While the bias voltage may be adjustable, a single setting has to work for all 4 tubes. :(

An interesting possibility is combination biased pairs. Each pair will have to, as is always the case, be closely gm matched, but minor cathode current differences are OK. Wire a pair's heaters in parallel (cathode to cathode) to a dedicated, "floating", filament winding. Connect the tied cathodes to ground via a 100 Ω/470 μF. RC bias network. The bulk of the total bias voltage comes from a C- supply. The 100 Ω resistor serves as a convenient test point for setting pair "idle" current. Having a portion of the total bias voltage be self generated makes for automatic correction of minor imbalances in pair cathode current.
 
True "fixed" bias places a heavy burden on the matching of tubes

When the cathode is tied to ground, and there is a pot controlling the negative voltage one each grid, the common text book term used is "fixed bias."

Whatever you want to call it, I am talking about grounded cathodes, and an adjustable negative voltage on each grid.

My preference for such a design is a mosfet follower tied directly to the grid and it's gate tied to the coupling cap and bias pot.

A possible schematic is in post #2 of this thread. Pictures of some metal 6V6's cranking out 25 watts are in post #61. I did get some glass 6V6GT's to make 31 watts at under 1% THD, but didn't process the pictures.

Tubelab Universal Driver Board, 2015 version

Note that there are 4 resistors connected to the bias pots. This allows for negative bias used with conventional G1 drive, or positive bias used for screen G2 drive or twin G1 + G2 drive. For conventional drive use R33,R36,R24 and R26 omit R25,R27,R34 and R35.

Do not place all 4 resistors unless you really know what you are doing and need them....not likely unless you need both positive and negative bias in the same amp.
 
When the cathode is tied to ground, and there is a pot controlling the negative voltage one each grid, the common text book term used is "fixed bias."

An "idle" current test point, typically a 10 Ω resistor, is placed between cathode and ground, when that sort of setup is employed. Checking must be done, when adjustability is present. The internal tie of heater and cathode is a problem for the setup. Additional thinking has me believing that separate, "floating", filament windings would work here too. The combination bias configuration I outlined above will be less costly to implement. Only 1 dedicated filament winding and 1 trim pot. per channel are necessary. Close matching of gm is in order, "come Hell or high water", for PP O/P tube pairs.