2x 813 tUbes grid driven...LOW PWR ON 15/10M

Hallo all, Its been some time scince I last posted on this old amp i was working on. I am now at the point where I am getting 500w out on 80/40m and 400w out on 20m, but 15m its 200w and 10m about 150w. I have tried many things to try and coax more power out on the two higher bands.
I have rewound the 10m coil with higher Q, thicker wire dia, length etc
moved the coil closer to the plate cap
tried different tapping positions
earthed the plate cap direct to chassis near the tubes with 10mm copper strapping
used 10mm coper strapping from bandswitch to 10m coil. and from coil to dc block caps
rewound parasitic choke coils with silver plated wire and cut back on inter connecting lengths

I am now out of ideas short of reposition the large lf coil vertically to minimise stray capacitance where ever I can.


Can someone suggest whats happening and where is all the power going...probably the anodes, but its not glowing red. The anode meter shows very little dip when trying to load the amp about 50mA.

Thanks
John

 

Any number of things can be the problem.
too much stray inductance / capacitance, poorly matched input / output circuits.
Keep in mind the tube is only good to 30MHZ and getting it to play well on fifteen meters maybe a bit of a challenge.

 

thanks for the reply, yes I realize I will need to tighten losses.

 

You're not the only one that has problems getting power out on 10 meters. Read another posting on the net that the builder was running 1KW out on 80,40 and by the time you get up to 10 it was putting out 500W. His efficentcy is better than yours and he decided that 500W was good enough and left it there. So it's good that you are seeking to resolve the loss with your amplifier. The drive to the tube is one place that may also require attention to loss. It could be too low of a drive to power the amplifier to full output. So, what is your plate voltage and when you are at maximum output on 10 meters what is your plate current?
73
Gary

 

Hi, The plate voltage on keydown is 2250v with plate currrent on load at 305mA and 20w drive.

 

hi.take a look at you rf jock.perhaps dont work well at 10m.thats means,rf go back to sopply.so take a look there.sorry about my english.73

 

Hmmmm...maybe the plate choke is causing the problem. It could be it's natural resonant point. You can tell that by using a Grid (or Gate) Dip Oscillator. If that's the problem then you need to replace the choke and see if that doesn't help.Just an idea to try.
Good luck.
73
Gary

 

Thanks everyone, seems to be pointing in that direction. Would a tuned input on 15/10m help or not.

 

Hallo again, well its all seems to be pointing to the plate choke now, as I have tried just about everything else. I am going to build an inductance meter and measure the inductance of this choke first . I could probably short out some sections one at a time and see if any difference on 10m.

73
John

 

A tuned input is good for your rig if it wants to see 50 ohms at the input of the amplifier. I found that getting the impedance matched made it easier to get proper operation from the amplifier. The rig is happy and the amplifier is happy, so you have nothing to lose (unless you have a really lossy tuned circuit).
The plate choke's actual value can be useful to know but that won't necessarily tell you if it is resonant at any frequency. Try a different plate choke and see if it changed anything. If the amplifier puts out more power then you're headed in the right direction.
Good luck
73
Gary