RH807 / 6L6GC g2 issues

[AmadeusMozart]

Just completed a RH807 build using 6L6GC tubes. Going through the testing phase I notice that at times the g2 is having a reverse current.

I am testing because apparently modern tubes are not of the same level of quality as those of days past - the spacing of the g2 wires is critical in power beam tubes and the design specs for g2 varies from manufacturer to manufacturer. This can lead to reverse g2 current and since KenRad speaks of not using a voltage divider for the 807 but using a proper stable g2 voltage. As a result we may be in deep water with the RH807 implementation.

As far as I gather the RH807 is highly optimized but the variations possible in the g2 current will cause imnsho a wide variation of results when using tubes that exhibit a varying amount of g2 current (even negative).

( There seems also to be a wide variation of the plate disspiation for the 6L6GC , some are as low as 19 watts, others go as high as 30 watts. )

I want to keep this amp running for a very long sime so I want to ensure that the design is "amended" so no nasties can happen to the unwary when replacement of the finals is needed.

Those of you have more knowledge than myself: Would you a) stabilise the g2 voltages using a shunt regulator and give g2 a fixed volatge (seems to be some amount of interaction when current through g2 changes or b) switch to UL?

What will the pro and cons be of each, in particular what will switching to UL do to the drive requirements (is is going to require more drive) since UL may be the simplest solution.

(the PSU has enough spare capacity for a shunt regulator so no worries there but if I can I like to stay away from solid state)

On another note: Would it be possible to drop the B+ voltage and decrease the impedance of the opt without disturbing too much of the performance? I've got the James 9123HS OPT presently sitting at 5K but would like to go to the 2.5K at 275 ~ 300V.

Thanks
AM

[Matt.B.H.]

Can you post a link to the RH807 amp?

Cheers Matt.

[aardvarkash10]

here

[SHiFTY]

UL is certainly a lot easier to implement than a regulated screen supply. And that original design (pentode) doesn't exactly have a lot of decoupling between the screen and B+! I would at least add a cap to ground from the end of the 10K resistor.

You could also easily experiment with triode mode. Triode is a lot less sensitive to OPT impedance so you try changing the transformer impedance to 2.5K.

Whatever sounds best really.

[AmadeusMozart]

Originally Posted by SHiFTY

UL is certainly a lot easier to implement than a regulated screen supply. And that original design (pentode) doesn't exactly have a lot of decoupling between the screen and B+! I would at least add a cap to ground from the end of the 10K resistor.

You could also easily experiment with triode mode. Triode is a lot less sensitive to OPT impedance so you try changing the transformer impedance to 2.5K.

Whatever sounds best really.

Decoupling is done in the 8 W PA system in Radiocraft Libary #33 publication (5K with 8uF). But that does not take into account that the current may go negative so am loath to persue that matter.

Triode mode gives me too little output,

Thanks for the suggestion and keep them coming,

AM

[aardvarkash10]

whats to lose by tying a zener string (say, 175 to 200v) to ground from the end of the 10k? Now you have a simple stabilised g2 voltage at next to zero cost and complexity - if it doesn't add value, ditch it. Bypass if and as desired.

[AmadeusMozart]

Originally Posted by aardvarkash10

whats to lose by tying a zener string (say, 175 to 200v) to ground from the end of the 10k? Now you have a simple stabilised g2 voltage at next to zero cost and complexity - if it doesn't add value, ditch it. Bypass if and as desired.

That is one solution I have been navel gazing about but then started thinking of going one step further and turning it into a proper shunt regulator.

However I wonder if the missing bypass cap was deliberatedly left out to achieve something by the fluctuating g2 screen current, I don't know.

edit: one of the tube manuals mentioned that the 6L6 was designed for having an optimum working of g2 with a voltage between 250V and 300V.

AM

[aardvarkash10]

suck it and see...

[AmadeusMozart]

Perhaps more try it (we do not want to suck on something that has 250 - 300V on it do we? Can be very lethal) and then measure it and listen.


(PS I know of a true happening from a guy fresh out of school who asked his boss how to check if the mains sockets were connected up properly. Boss replied to stick two nails in and if it gave a tingle it was OK. The bloke did just that and died and boss went up for manslaughter. Moral of the story: never assume that the other knows what they are doing when it concerns electricity)

AM

[aardvarkash10]

I credit anyone posting in here with more braincells than that... Especially the kiwis. Got my doubts about the Diggers :-)