AL-811H plate and load capacitors

[N0SQ]

I'd like to replace the air variable capacitors with vacuum variable capacitors. I don't see any capacitance values in the manual so I'm wondering if anyone knows of any vacuum capacitors that'll replace the air capacitors. From looking at some websites it appears that I would need some other method to determine settings, also, since the vacuum capacitors are listed as multi-turn capacitors. I have to replace the plate capacitor so I figured I'd replace the load capacitor also.

[AD4U]

No offense intended - why?  Vacuum caps will not increase efficiency.  About the only reason to use one is they allow a greater range of capacitance (ie: lower minimum capacitance for a given size).

Dick  AD4U

[N0SQ]

No offense intended - why?  Vacuum caps will not increase efficiency.  About the only reason to use one is they allow a greater range of capacitance (ie: lower minimum capacitance for a given size).

Dick  AD4U

I've been told that they are much less likely to arc.

[W1QJ]

Really, don't even bother doing this!

[N0SQ]

Really, don't even bother doing this!

Why not? It solved a friends problem.

[K8AXW]

No offense intended - why?

I have to ask "why" too.  Replacing the air variables with vacuum variables is a construction nightmare!  I put vacuum variables in my homebrew linear so I know whereof I speak!

First of all, brackets have to be fabricated to hold the caps in the proper plane.  This means drilling mounting holes in the chassis with the possibility of interference from other components under the chassis.

Second, a strap connector must be fabricated for the rear terminal of the cap and flat copper strap has to be rerouted because it isn't likely the original wiring will reach.

The caps are multiturn and require a turns counter which cost about $50.00 each. 

Mounting the turns counters to the panel and aligning it with the cap shaft is a major undertaking itself.

The turns counter shaft and the cap shaft will no doubt be of different diameters so a coupling for each will have to be fabricated.

There's a 50-50 chance the turns counter will change numbers opposite the increase/decrease in capacitance.  (Since I couldn't remember this on occasion I had to fabricate a special tag to remind me)

After all of this what have you accomplished other than ruin the panel of a fine linear?  Zip!!

However, if you really want to do this, good luck.  No doubt a phone call to the factory will get you the cap size info.  (I can't imagine that this information isn't on either the parts list or the schematic)

[N0SQ]

It doesn't look like it'll be a simple task to remove the air variable capacitor either. I figured that since it'll be a PITA to replace the cap I might as well add another improvement. But, if it's really that much of a problem I won't bother. A friend had an arcing problem with an air cap and solved it by replacing it with a vacuum variable (he has a different amp, BTW).

Not to mention that it was a PITA to convert my AL-811H amp to a grounded grid amp.

Obviously, it wouldn't be the first time that a mod would be problematic. I run into these issues at work also - I'm an electronic tech with 30+ years experience in communications electronics so I've had my share of issues with electronic equipment.

[W8JX]

I can see the merit of vacuum variables in a harsh or hostile environments but in a amp in shack? As far as concerns of possible arc over this can easily be addressed through the use a air variable with larger plate spacing. 

[N3JBH]

Convert to a grounded grid amp What was it before ?

[N0SQ]

Convert to a grounded grid amp What was it before ?

It had resistors in the grid circuit. They burned up so I installed the mod that eliminated the resistors and tied the grids directly to ground. The mod was developed by the original designer of the amp.

[N3JBH]

Oh ok now i understand... Yes i seen Tom's mod page my 811-H seems t been good for man years with the tubes it has. So i have not done it to mine and i think when i do have to replace the tubes i do a totally different thing with  the amp's power supply and scrap it as a 811-A amp

[N0SQ]

I can see the merit of vacuum variables in a harsh or hostile environments but in a amp in shack? As far as concerns of possible arc over this can easily be addressed through the use a air variable with larger plate spacing. 

Well, I'd welcome a recommendation for an air cap with greater spacing. It'll probably be much cheaper than a vacuum cap.

[KX5JT]

No offense intended - why?  Vacuum caps will not increase efficiency.  About the only reason to use one is they allow a greater range of capacitance (ie: lower minimum capacitance for a given size).

Dick  AD4U

Vacuum capacitors allow GREATER POWER HANDLING.  THIS is why they are used.  HIGH POWER APPLICATIONS. Yes, they will take a much higher power than a breadslicer before arcing.

[W1QJ]

Find out why the amp is arcing in the first place.  When you look at the design of the amp you'll see that the spacing on the variables are fine and because they arc is not because the spacing is inadequate.  Wider spacing will lend itself a little better to prevent arcing but I'd prefer to find out why the arcing occurs in the first place.  First off you didn't  "change" your amp to grounded grid by just grounding the grids instead of using a resistor.  The amp was still grounded grid with the resistors. That said, by grounding the grids directly you increased the gain of the amplifier.  That means you should be able to get X amount of power output with lower drive than you did before.  Also, increasing the gain of the amp would increase in the likelyhood of the tune cap to arc when the amp is driven in an out of resonance condition.  Any time you increase the drive level during loading the amp is now in an out of resonant condition as soon as you key down again.  The amp will arc if you have added enough increase in drive to make the out of resonant condition bad enough.  You will either have to add less drive each time you increase drive or advance the load control a little bit before you key down again.  Once you understand what is actually happening you can eliminate certain problems without going through adding unnecessary modifications.  Changing bands with vacuum variable caps on both load and tune is a nightmare.  Sure, you can probably make a log table to cut the time down in switching bands but you'll probably go through a set of tubes finding all the right settings on each band with vac variables.  Keying down too long with 811 tubes is what kills them.

[K8AXW]

If you amp is a commercial amp then theoretically the cap values and plate spacing are correct.  If it's arcing, find the cause as QJ suggests. 

If you increase the plate spacing then you will probably shift the problem to the next vulnerable component like the bandswitch.  Smoke that one and "you in a heapa trouble boy!"