[Amps] L-PI amp network

[Victor Rosenthal]

I faced this problem when building a 4CX1000A amplifier.

The GM3SEK calculator gives you a value, but the stray plate-to-ground 
capacitance is an important variable and it is hard to determine. I 
measured it (with an AADE LC meter) with the tuning cap at minimum and 
the main tank coil disconnected, but I realized that connecting the tank 
coil would change everything.

I just took the value for the small L before the pi network that popped 
out of the spreadsheet as a starting point and used the method of 
connecting a carbon resistor equal to the calculated plate load 
impedance from plate to ground (power off) and adjusting for a match 
with an antenna analyzer connected to the output (today I would use my 
NanoVNA). The adjustment of the small coil was critical.

Finally I operated the amp into a dummy load and "scientifically" fooled 
around with it until I got acceptable efficiency and everything "felt 
right."

One way of doing this is not to add an additional coil but to just tap 
the plate tuning capacitor on the first turn of the tank coil instead of 
at the very start. You can make a sliding clamp so that it's easy to 
adjust the tap location; when you are done testing you can permanently 
solder it in place.

I know this isn't the kind of answer you are looking for, but I was 
unsuccessful in finding a better way.

Keep in mind that when you do this you lengthen the parasitic "tank 
circuit" and increase the risk of VHF parasitics. So you should check 
carefully that the amp is stable when you are done.

73,
Victor, 4X6GP
Rehovot, Israel
Formerly K2VCO
CWops no. 5
http://www.qsl.net/k2vco/

On 14/12/2022 0:36, Steve Bookout wrote:
 > Hello all,
 >
 > (First part is background, so you know a more 'overall' of what I'm 
doing.)
 >
 > I've been intending to finish a long-ago-started mono band amp for 10 
meters, and I think I regained the enthusiasm I need to complete the 
project.
 >
 > I had started out going to use a pair of grounded grid config 4-400's 
only BECAUSE I HAVE A BUNCH.  That was then, this is now. I've recently 
found a treasure trove of 8877's and have determined that I have SIX of 
them!
 >
 > With the exception of one, all were unknowns, so I 'cooked' them on 
fil voltage only for a day, and then applied the B+.  My test platform 
as a HB 8877 40 meter amp that I built in the mid 80's. It still uses 
the same tube.  Peter Dahl 3000 volt 6 KVA transformer, for 4000 volts. 
  22 watts drives it to 1500 out; ~ 600 ma/.030 grid.
 >
 > Over the period of about a week, I cycled all of the unknown tubes 
thru this amp.  They all were similar, taking 22 to 27 watts of drive 
for the same 1500 watts out;  550 ma to 600ma @.030 ma grid.
 >
 > That was the background for my real question for the group.
 >
 > Based upon GM3SEK's PI/PI-L calculator, I'm working with a plate load 
impedance of ~3000 ohms.  I know from my own experience, and that of 
others, that it's a pain to get the plate inductor 'right' so that it 
all works and tunes, as it should and make power with reasonable efficiency.
 >
 > I have never used an L-Pi, but I basically understand the idea.
 >
 > IS THERE A RECOMMENDED TOOL OUT THERE, WHICH WILL HELP ME FIGURE OUT 
MY TWO "L's"?
 >
 > Using GM3SEK's calculator, I see there is a line, 'Lead inductance 
(total from tube to tank, but excluding suppressor)', which I have 
played with.  Adding my 'L' inductance there, say '.6uh', does reduce 
the inductance associated with PI, and it does raise the value of the C1 
capacitor.   ( I am using a 100 pf /15 KV vacuum variable and right now 
as I write this, I don't remember it's minimum C, but it's in the range 
of what I think you should see; maybe 5 pf?)  Using this, I was able to 
come up with about 15 pf, vs 4 pf in a standard PI configuration.
 >
 > Suggestions on if this would be an accurate way to figure this out? 
Or, another/better way?   And, what should I be looking for as maybe a 
ratio of the 'L' vs the 'L' in the PI network.   As I recall, I think 
I'm looking at about 1.7 uh.
 >
 > I would appreciate any constructive comments, or even precautionary ones.
 >
 > 73 de Steve, NR4M

Victor

On 14/12/2022 1:23, Steve Bookout wrote:
> Hello Jim,
> 
> 4kv.  It's about 4.2 kv, no load.
> 
> On another point, for the group, 'somewhere, sometime', in more than one 
> place on the web, I've seen where people recommended using 2.0 as the 
> 'K' factor, instead of 1.5-1.8, etc.  This was specific to 8877's.  I 
> used 2.0, as I very, very rarely use a microphone.
> 
> Just wondering
> 
> Steve NR4M
> 
> On 12/13/2022 6:01 PM, Jim W7RY wrote:
>> What plate voltage are you going to have on the tube of choice Steve?
>>
>> Jim W7RY
>>
>>
>> On 12/13/2022 4:36 PM, Steve Bookout wrote:
>>> Hello all,
>>>
>>> (First part is background, so you know a more 'overall' of what I'm 
>>> doing.)
>>>
>>> I've been intending to finish a long-ago-started mono band amp for 10 
>>> meters, and I think I regained the enthusiasm I need to complete the 
>>> project.
>>>
>>> I had started out going to use a pair of grounded grid config 4-400's 
>>> only BECAUSE I HAVE A BUNCH.  That was then, this is now. I've 
>>> recently found a treasure trove of 8877's and have determined that I 
>>> have SIX of them!
>>>
>>> With the exception of one, all were unknowns, so I 'cooked' them on 
>>> fil voltage only for a day, and then applied the B+.  My test 
>>> platform as a HB 8877 40 meter amp that I built in the mid 80's. It 
>>> still uses the same tube.  Peter Dahl 3000 volt 6 KVA transformer, 
>>> for 4000 volts.   22 watts drives it to 1500 out; ~ 600 ma/.030 grid.
>>>
>>> Over the period of about a week, I cycled all of the unknown tubes 
>>> thru this amp.  They all were similar, taking 22 to 27 watts of drive 
>>> for the same 1500 watts out;  550 ma to 600ma @.030 ma grid.
>>>
>>> That was the background for my real question for the group.
>>>
>>> Based upon GM3SEK's PI/PI-L calculator, I'm working with a plate load 
>>> impedance of ~3000 ohms.  I know from my own experience, and that of 
>>> others, that it's a pain to get the plate inductor 'right' so that it 
>>> all works and tunes, as it should and make power with reasonable 
>>> efficiency.
>>>
>>> I have never used an L-Pi, but I basically understand the idea.
>>>
>>> IS THERE A RECOMMENDED TOOL OUT THERE, WHICH WILL HELP ME FIGURE OUT 
>>> MY TWO "L's"?
>>>
>>> Using GM3SEK's calculator, I see there is a line, 'Lead inductance 
>>> (total from tube to tank, but excluding suppressor)', which I have 
>>> played with.  Adding my 'L' inductance there, say '.6uh', does reduce 
>>> the inductance associated with PI, and it does raise the value of the 
>>> C1 capacitor.   ( I am using a 100 pf /15 KV vacuum variable and 
>>> right now as I write this, I don't remember it's minimum C, but it's 
>>> in the range of what I think you should see; maybe 5 pf?)  Using 
>>> this, I was able to come up with about 15 pf, vs 4 pf in a standard 
>>> PI configuration.
>>>
>>> Suggestions on if this would be an accurate way to figure this out? 
>>> Or, another/better way?   And, what should I be looking for as maybe 
>>> a ratio of the 'L' vs the 'L' in the PI network. As I recall, I think 
>>> I'm looking at about 1.7 uh.
>>>
>>> I would appreciate any constructive comments, or even precautionary 
>>> ones.
>>>
>>> 73 de Steve, NR4M
>>>
>>> _______________________________________________
>>> Amps mailing list
>>> Amps at contesting.com
>>> http://lists.contesting.com/mailman/listinfo/amps
>>
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