[Amps] MLA-2500B Question

Tony King - W4ZT amps at w4zt.com
Tue Mar 29 11:46:59 EST 2005 

Agreed, cost is an issue for the manufacturing world as it is for 
individuals but this isn't about the cost.

Once again, back to Tomm's question: I believe it is not a good idea to 
depend on a rig's inboard tuner to match the input of the amp through 
some length of coax.  I don't think there is any flywheeling going on 
through that length of coax and as a result there is poor IMD 
performance and poor efficiency. Of course no one else has expressed an 
opinion on that question so I may be the lone soul that feels this way.

73, Tony W4ZT

Will Matney wrote:

>Tony,
>
>The very reason they don't have them is cost. They build the amps as cheap as possible to make the most bux. For amateur amps, this means an extra bandswitch and a tuned circuit for each band. The way most manufacturers see it, leave them out and let the transceiver tune to it. Thus, more money in their pockets. That's just my opinion on the subject and might be wrong.
>
>Best,
>
>Will
>
>*********** REPLY SEPARATOR  ***********
>
>On 3/29/05 at 2:40 AM Tony King - W4ZT wrote:
>
>  
>
>>Will Matney wrote:
>>
>>    
>>
>>>Tony,
>>>
>>>Even the old 10-11 meter sweep tube amps had a tuned input, or at least
>>>      
>>>
>>the better ones of them. They even knew it was better than not having any
>>at all. Most of those used a tuned transformer feeding either the
>>cathode(s) or the grid(s). Same thing applies to any amp IMHO.
>>    
>>
>>>Best,
>>>
>>>Will
>>> 
>>>
>>>      
>>>
>>Hi Will. We agree that's what "should" be, but that's why we're having 
>>this discussion. The very subject line of this thread and the fact that 
>>the MLA-2500B doesn't have tuned input and the AL-811 does is enough to 
>>verify a great difference in commercially built amps.  Not to mention 
>>all the home brew amps that don't have tuned input. I submit that it is 
>>very likely that there are a great many more hb amps without than with 
>>tuned inputs. We are talking amateur bands, HF, amplifiers. That's why I 
>>asked that question. Only answer so far is yours.
>>
>>But... to Tomm's question, since the three of us agree we should have 
>>tuned input, then we would really like to see, hear and read the in 
>>depth discussion about "why" and "where" tuned input.  We all hear those 
>>amps without the tuned inputs, driven to their limits and wider than the 
>>barn door.  If you haven't you haven't listened lately.  I think it's 
>>fair to seek knowledge about tuned inputs and I don't think that it will 
>>do any of us any good to say they "should" use them without giving good 
>>technical reasons.  In trying to understand if the flywheel effect 
>>really does exist to any benefit when using a transceiver's tuner with 
>>some number of feet of coax between it and the amp we may find that it 
>>is ill advised to do so.  I don't know but I do want to know.
>>
>>I have a gut feeling there are a lot of folks that don't want to know 
>>the real answer to this question. Maybe it's  because it will make their 
>>amp look bad.  That's not really the point but if it does, then couldn't 
>>this discussion help them make it better? I don't want to toss mud and 
>>neither does Tomm, but we, and I'm sure others, would like to see the 
>>subject addressed by our friends here  that have a good understanding 
>>and grounding in the subject.  There's certainly more to this than an 11 
>>meter sweep tube amp discussion. There are hundreds of amateur HF amps 
>>on the Internet shown without tuned inputs. There are a lot more than 
>>that on the bands that sound terrible when you listen away from the 
>>operating frequency. Of course if it sounds good on your frequency why 
>>should you care about people on either side of you? Maybe that's another 
>>technical discussion ;~)
>>
>>Just to set things straight, Tomm and I both have two hole GS-35B amps 
>>in construction. My tuned input is in construction. Tomm has a valid 
>>question before deciding what to do with his.
>>
>>That said... what about it guys?  Where are the guys that know about 
>>this subject? I know Rich Measures feels strongly  about tuned input 
>>but, Rich, what about that length of coax and tuner issue?  Doesn't 
>>sound like  a flywheel to me. How's that going to make the amp act?   
>>What do you think? Anyone else? Subject filters on?
>>
>>Thanks for your time.
>>73, Tony W4ZT
>>
>>    
>>
>>>*********** REPLY SEPARATOR  ***********
>>>
>>>On 3/28/05 at 10:57 PM Tony King - W4ZT wrote:
>>>
>>> 
>>>
>>>      
>>>
>>>>Tomm Aldridge wrote:
>>>>
>>>>   
>>>>
>>>>        
>>>>
>>>>>This is an interesting thread to me not because of the AL-811 vs
>>>>>MLA-2500 but because I am designing and building an amp. If I
>>>>>understand this correctly, using a TS570 transceiver to drive my amp
>>>>>implies I really don't need a tuned input circuit as I can just use the
>>>>>built-in tuner of my rig to achieve a good enough match to the input of
>>>>>the tubes. That saves me money and complexity.
>>>>>
>>>>>However, some on this list advise that the proximity of the output C of
>>>>>the tuned input to the cathode of the PA is very important for its
>>>>>"flywheel" or energy storage effect during the wide swings in impedance
>>>>>during a cycle of operation.
>>>>>     
>>>>>
>>>>>          
>>>>>
>>>>I have gotten the same advice / impressions. A tuned input adjacent to 
>>>>the cathode appears to be the only true way to get a good flywheel 
>>>>effect.  I suppose, if you didn't care about improving the IMD that it 
>>>>might not matter.  I hope that  some of the experts will chime in on
>>>>        
>>>>
>>this.
>>    
>>
>>>>   
>>>>
>>>>        
>>>>
>>>>>My question is, which strategy should one employ and what are teh pros
>>>>>and cons? Assuming, of course, a built in AT such as in the TS570.
>>>>>     
>>>>>
>>>>>          
>>>>>
>>>>There is a good link at <http://www.somis.org/SB220ci.html> where Rich 
>>>>discusses the need for a good tuned input.  This article is about the 
>>>>SB-220 but the idea is the same. Go to the link and scroll down to 
>>>>"Improving Input SWR" about two thirds of the way down.  I think that 
>>>>will help us understand why it's more complex than matching the radio 
>>>>output to the cathode input.
>>>>
>>>>My questions to the group are these:
>>>>How many of those running homebrew amps use a tuned input? How many are
>>>>not?
>>>>Now how many of those running a COMMERCIALLY manufactured amp are NOT 
>>>>running a tuned input?
>>>>
>>>>   
>>>>
>>>>        
>>>>
>>>>>Regards,
>>>>>
>>>>>KD7QAE
>>>>>Tomm
>>>>><snip>
>>>>>     
>>>>>
>>>>>          
>>>>>
>>>>73, Tony W4ZT
>>>>
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>>>>
>>>>        
>>>>
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>>>      
>>>
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