[Amps] L-PI amp network

Tue Dec 13 21:07:04 EST 2022

Hi Vic,

On 12/13/2022 8:26 PM, Victor Rosenthal wrote:
> 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.
>
Yep, this was one of my concerns
> 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.
Been doing it like that in the past, I guess, like most people.  I use 
my little 'AIM' box, which has served me well.  Within the last year, I 
got a nano VNA, so would be a good place to start.
>
> 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.
>
Thought about doing like that and maybe I still will, at least 
initially.  The physical layout of the two caps and where I'll put the 
tube make it easier for me to put the two coils about 60 degrees apart.  
Have latitude about where I can mount the tube, as I have to make a 
plate to cover the holes left by the 4-400 sockets.  If you can 
visualize it, the tube will be about 5 inches behind the tuning cap and 
loading cap is about 4-5 inches to the side of the tuning cap, so you 
can see how it would make an 'L' shape.  Tube to C1, turn hard left and 
go to C2.

I'm sure I could go diagonally between the tube and C2 with ONE coil and 
take the tap from that and go to C1.

I'm just looking for a better way...

(And, thanks for all the CWT QSO's.)

Steve, NR4M

> 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
>>>>
>>>> _______________________________________________
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>>>
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