20v3 comes to life

[kc2hgc]

Hi everyone,

Steve here just want to give an update on the 20v3 and say thank`s to all the replies on the bias supply thread.
some of the problems are low power out 120 watts on low 240 watts on high but I guess that`s fairly good for a rig that was DOA.

also mod bias pots are not working right looks like one tube is conducting more than the other.Plate of one tube getting red
I ll do some cold & hot checks.
Rf amp only puts out 240 watts 3100 volts on the plates 640 on the screens 120- on grids 3.5 vpp of drive from the 807.

However the test tone sounds ok on the receiver and power supply's don't seam to be over heating .
Let me here what do you guys think?

73 Steve

[flintstone mop]

Be careful with the H.V. Do not operate it very long in this condition. Something is very wrong, the way you are describing its operation. Check out the wiring from the schematic.
The drive does not seem like it is enough.
Fred

[kb3ouk]

First, the screen voltage is a little high for AM (should be 500 volts) and the bias is too low (should be -220 volts). The way you are running it now, it's probably running close to class AB₁, since the specs for AB₁ grid driven RF linear service are 3000 volts plate, 750 volts screen, and -130 volts bias. According to the datasheet, the peak RF grid voltage in class C plate modulated service should be 290 volts, grid current for a single tube should be 12 mA. Sounds to me like the finals are way underdriven.

[KA3EKH]

Have you converted it to Ham frequencies or is it still on AM Broadcast band? One good thing about the 20V is there is lots of documentation on how to convert it over out there although I have never been a big fan of those transmitters and the way the tanks are packaged. In broadcast service it’s not uncommon to see the tubes getting a small amount of red but that’s at full rated power.

[flintstone mop]

Have you converted it to Ham frequencies or is it still on AM Broadcast band? One good thing about the 20V is there is lots of documentation on how to convert it over out there although I have never been a big fan of those transmitters and the way the tanks are packaged. In broadcast service it’s not uncommon to see the tubes getting a small amount of red but that’s at full rated power.

I had a 20v2 and it was easy to move up to 160M. Plenty of documentation to get it up there. There may be some cut and try to get the exciter to give more drive to the finals. You need plenty of drive to make it happy. There were a lot of ops on AMFONE who can rattle off the specs for what is needed to get your TX in the ballpark. K2PG, Phil comes to mind
It should make 1100W at full power. Modulation for a lot of positive peaks is another matter. Extremely nice audio.
I never liked 4-400's...The screen voltage is another hurdle to deal with.
Good luck with the TX.
Fred

[kc2hgc]

Thanks for replying guys,

Ok we'll the transmitter is still at the broadcast band 1450 am
Fred I think your right about the drive and Ill recheck the HV wiring.

I'll also look into the voltages on the tubes.
Here's what I found there is 24 ma of grid drive I put the scope on the frequency monitor output and got a 3.5 v-pp at 1450 hz looks low to me.

This may be Y there's only 240watts out Or I could have screwed up the HV wiring but if so
The plate voltage would be off and it's not. Its at the 3150.

I think we all agree there's not enuff  drive
I'll keep every one posted thanks for the replies!

73 Steve