viking ii with solid state driver

[kc2etm]

hi i have a question i have a solid state 50 watt 8 ohm amp it feeds a 8 ohm to 500 ohm transformer then feeds an utc ls10 transformer 500ohm primary 60k secondary in two sections that feeds into my 807's through the 100 ohm resistors my question is this it sounds great but seems weak not to have much drive i see the 807 turning purple i am showing swing on the pep meter my scopes shows me 100 percent mod amp is cranked all the way up  but it just sounds weak in the headphones do i need different resistors feeding the grids? do i need a bigger amp? is it just me? are my headphones fooling me? anything i can add in the driiver chain to improve it?

thanks
fred

[WA1GFZ]

Try eliminating the second transformer. Drive the 807s with the 500 ohm winding.

[The Slab Bacon]

Also make sure that you have a DC path to ground (or the biass supply) from the center tap of the winding driving the 807 grids.

Even in AB2 it shouldn't take more that a watt or 2 to drive those grids.

[WA1GFZ]

you can feed bias into the grids with a pair of 1k resistors if the transformer doesn't have a CT. Make sure you bypass the junction of both resistors or the CT of the transformer.

[The Slab Bacon]

you can feed bias into the grids with a pair of 1k resistors if the transformer doesn't have a CT. Make sure you bypass the junction of both resistors or the CT of the transformer.

Ah Yes, the "artificial center tap" . We have used that for many different applications, and it usually works OK-FINE.

But if he didn't, and the tubes arent getting any biass or at least a "DC path to ground" they will get real uncomfortable quickly and also do strange things.
If he doesnt have the path to ground his driving audio will become the only biass that those tubes see.

[kc2etm]

The utc transformer is center tapped on the secondary but if i eliminate that transformer
As suggested my 500ohm transformer has no center tap how should i fashion an artificial
Center tap

Thanks
Fred

[kc2etm]

Sorry didnt see the post about 1k resistors as ct so when you say bypass the junction of the resistors or ct do you mean a bypass cap to ground? Because i have nothing on the ct of the current setup

Thanks

[WA1GFZ]

Yes bypass the junction or CT to ground. I put a 10 watt ZENER to regulate the bias in my V2. I think it was around 32 or 36 volts if I remember. The dual transformer is way too much transformation ratio. 40 volts P-P to each grid
will drive the heck out of them.

[Opcom]

The virtual center tap also works on 70V line transformers enabling their use as low cost drivers. The lower the "wattage tap" selected, the higher the turns ratio.

[Gito]

Fred,

when You wrote You get 100% modulation,but it seem weak,I think the problem is in the tone,You have less middle frequency,if You have a tone control maybe You can make it"stronger".

I think artificial ground ( using an R) can only be used Class A ,or AB1 when there's no grid current.only needs voltage
Maybe if the R is changed with audio choke,than it can be used for class AB2 and class B.

Gito

[WA1GFZ]

Excessive transformers most likely messing up frequency response. Yes any grid cuttent through the resistors will change tube bias. So you want to keep the resistance value low. At 1K 1ma is 1 volt bias change.

[The Slab Bacon]

Sorry didnt see the post about 1k resistors as ct so when you say bypass the junction of the resistors or ct do you mean a bypass cap to ground? Because i have nothing on the ct of the current setup
Thanks

When you say "nothing" do you mean it is floating with nothing at all hooked to it? ?
If so, then you do not have a DC path to ground or the biass supply. In this case the tubes will definately not be happy at all. You MUST have a DC path to ground (or the biass supply) for the control grids.

That may also explain why you have to crank up the audio drive to attempt to produce some mudulation. If you are running them in AB1, all you should have to do is tickle the grids with a little voltage to make them go.

[kc2etm]

When i say nothing i mean no capacitor to ground or anything
The three wires of the secondary goes two wires to the grids then the ct goes
To the center wire that the old transformer went to nothing to ground unless
There is a bypass cap on the other end of the harness there was none
On the original interstage driver transformer so i didnt add one

[kc2etm]

So in poking around r15 r16 r17 area i found sh4 to be broken in half i see it in the manual pictures but not on the schematic it says in the parts list 5.2ohm shunt can i use a standard resistor to replace this sorry if this a stupid question but im new to these tube rigs
The colors on the broken part are green brown gold gold

[WA1GFZ]

yes, green brown is 5.1 ohms though.

[steve_qix]

If you have that kind of power available, triode connect the 807s using the patented    QIXonic method of connecting the grids together.  Use a string of 5  12V zener diodes (to start) between the screen and the control grid of each tube, such that the control grid voltage is lower.  Bypass this with a healthy electrolytic capacitor (470uF or something like that) to even out the peak current passing through the diodes.  Put a small resistor in series with the capacitor.

Run a 5k (or something reasonable) resistor from the control grid end of the zener string (the negative side of the zener string) off to a negative bias supply of some kind.   Connect the drive directly to the screens.  The center tap of the transformer secondary is connected to good bypass cap (if a bias supply is needed), or ground if not. I don't actually remember if any negative bias (or forward bias) is required to get the tubes to pull a little idle current.

When I did this, I used a direct-coupled audio driver, and the modulation was almost class E quality.  There was feedback around from the secondary of the mod xfmr back to the input, just FYI.   I am using this driver with my Valiant RF deck with the home brew 811 modulator (to drive the 811 grids).

Anyway, another idea..... it does work quite well.

[ka1tdq]

Steve,

I drew out what you described and I think I got it right.  I've attached a photo. 

How do you determine what what -bias you need for the control grid? 

I added an electrolytic and resistor on the cathodes to pull the idling current with the secondary of the audio transformer connected to ground. 

I'm interested in using this method but using a pair of 6L6's instead for a mobile rig.  I'll probably use a pair of 7984's in the final (low plate Z, but that okay too). 

Jon
KA1TDQ

[steve_qix]

The circuit looks pretty good except for the cathode bias.  That is definitely not a good idea with power amplifiers, unless they aren't expected to deliver lots of power.  So, just ground the cathodes.

I need to dig around in my "lab" and find the original zener strings I used, and tell you exactly what they are.

This circuit may not work for 6L6s and other tubes, because their characteristics may be very different, and therefore a different method of triode connecting such tubes may be in order.

Regards,

Steve

[ka1tdq]

Thanks Steve for the feedback.  Don't look too hard for those zener strings because I'm just tossing around ideas in my head for my next project.  I might do something totally different.

I found the top to a small barbecue grill in the dumpster at work and I'm trying to figure out what to do with it.  It would make a great chassis.

Jon
KA1TDQ