[Amps] L-PI amp network

[Jim W7RY]

Steve..

So... the minimum capacitance of the tune cap is a problem on 10 meters....

I would use about a 100 pF cap MAX for the tune cap (vacuum cap is 
absolutely not needed).
Then L1 is 2.3 uH (most likely a bit less)
C2 (loading cap) is 86 pF
L2 the "L" coil is .67 uH

If you raise the Q of the tank to 15-18, then the load cap needs to be 
larger capacitance. But not much.


Alpha used a dual tune cap in their amps. That way, the minimum 
capactiance was signigitly lower on 10 meters with the smaller cap. I 
can verify this is true, when I designed and built my homebrew amplifier 
with 2X 3cx800s. This arrange mad the amp super easy and fairly broad to 
tune on 10 meters. And ran 1500 out on RTTY contesting.

It is a real sweet amp, 100 % stable and extremely reliable.

An 8877 for a single band 10 meter amp would be a really nice amp. Just 
right for a contest station. Easy to tune, and super reliable.

73, Jim W7RY


On 12/13/2022 8:18 PM, Jim W7RY wrote:
> Here is what I come up with... ~3000 ohms plate load impedance.
>
>
> Oh..... I cant attached to this group!
>
> But you can see it Steve since I copied you directly. The results are 
> highlighted in yellow.
>
> 73, Jim W7RY
>
>
>
> On 12/13/2022 5:23 PM, 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
>>>
>> _______________________________________________
>> Amps mailing list
>> Amps at contesting.com
>> http://lists.contesting.com/mailman/listinfo/amps
>

-- 
Thanks and 73, Jim W7RY