[Boatanchors] Tube reciever design banter

[Carl]

----- Original Message ----- 
From: <match at ece.utah.edu>
To: <boatanchors at mailman.qth.net>
Sent: Monday, December 21, 2009 12:18 PM
Subject: [Boatanchors] Tube reciever design banter


> Carl wrote:
>>
>> Well, if you are building it yorself and without a machine/model shop 
>> handy
>> a few corners have to be cut.
>>
>> So this is what Im suggesting based upon what Ive been working on for 3
>> years.:
>>
>> High intercept point pentode RF with an aggressive AGC curve, 6GM6 or 
>> 6EH7.
>> Dont bother switching it out. Use a switchable attenuator which may be
>> necesary for your 5kw neighbor.
>
> Yeah, we've discussed this before, however I found another that looks
> interesting. Have a look at the 6MU8 triode/pentode. Semi-remote cutoff,
> 9000 uM, 3.75 W plate diss., and an extra free triode section as a bonus.
> Designed around 1970 and I think it's a frame-grid. I like multi-section 
> tubes
> when it makes sense to use them. $2.43 right now at AES.


There are many nice tube possibilities at the end of the era altho many are 
the same tube with different pin outs or slightly fudged specs simply to 
generate captive audience sales.  However the 6MU8 pentode is a sharp cutoff 
and the 6LM8 isnt very exciting for a high performance receiver.


>
> There are only 3 semi-remote cutoff triode/pentodes that I've been able to
> find. 6AZ8 (which has a weird suppressor/heater connection), 6LM8, and
> 6MU8.
>>
>> Dual triode mixer in a Pullen circuit. 6ES8 preferred as its a very 
>> unique
>> variable mu tube.
>
> I've not yet tried a Pullen... I have an SP-600 that came to me with a 6J6 
> in
> the 1st mixer position, which I suspect may be a Pullen mod, but I've not
> looked at it to see.


It could be a regular dual triode mixer also, the Pullen hasnt received much 
attention until recent years.



>>
>> Crystal control front end. Heathkit rig xtals are dirt cheap and then you
>> can afford the other bands. 12AT7 oscillator/buffer.
>
> Good enough... if you want to follow Heaths plan.


Id prefer a premix style circuit but it aggravates the mechanical end. The 
Pullen and a carefully designed 6GM6 stage seems to hold up well under 
contest battle conditions with the tuneable preselector.



>>
>> You now have an absolutely bullet proof front end, NF around 4-5dB even 
>> with
>> a double tuned manually controlled preselector. Saves trying to find the
>> right components to get it all to track. I hate drive belts, gears and 
>> all
>> that mechanical crap. Beam deflection mixers are vintage history and the
>> only good one is getting real pricey.
>
> Amen... I've debated the beam deflection tubes for ages and decided they
> are just not worth the bother, with the possible exception of the product
> detector stage.
>>
>> This leads us to a potential weak link of a 500 kc wide IF using a pair 
>> of
>> Heath bandpass filters to sharpen the skirts. More cheap parts. However 
>> low
>> (minimum required) gain and hi Q tuneable preselector will keep overload
>> away.
>>
>> Heath doesnt believe in impedance matching so the mixer will drive a 
>> cathode
>> follower preceeding those LC filters.
>>
>> Next a low gain IF to support the Lamb style noise blanker and then to 
>> drive
>> the Heath 3395 kc xtal filters. More cheap stuff. This might be a good 
>> job
>> for a 6JH8 as a 6BA7 is overload prone. A dual control tube is needed 
>> here.
>>
>> A stage of 3395 IF stage followed by another 6ES8 Pullen fed by a (cheap)
>> Heath LMO and a buffer stage. Add a RIT with a varactor.
>>
>> That dumps to 455 or 500 kc where you have many choices of some serious
>> filters. 500 is harder to find but usually cheap. I have a 4 filter 8 
>> pole
>> module from a Racal cuz it was cheap. AM can bypass the filter and be 
>> gain
>> matched.
>
> Why not just choose a single IF freq of 9 MHz or so, use a high-quality 
> filter
> and have done with it? Good filters are available and you save another 
> mixer
> stage.


Lack of suitable filter availability for my requirements, no T Notch, no 
PBT. The Heath ones are readily available and can be cascaded if wanted to 
square up the skirts.


>>
>> A few more IF's to pick up the gain that was minimized earlier. Im a
>> stickler for gain distribution. Put it after mixers and filters.
>
> Agreed, mostly.
>>
>> Now we add passband tuning, and T Notch filter to that IF.
>>
>> Infinite impedance AM detector, 6ES8 Pullen SSB/CW detector, a real 
>> tuneable
>> BFO.
>
> Agreed, but I'm still not sure about that Pullen design...


Give it a try after digesting all thats been published. Ive implemented it 
with octal, loctal, and later tubes and performance is excellent to superb.



>>
>> Now it gets complicated. Two or 3 loop AGC using IF and audio derived
>> control. At least 3-4 choices of attack and delay.
>>
>> S Meter amp of course.
>>
>> The lowest distortion audio that you can build ending up with PP 6V6's at 
>> a
>> minum. Single ended audio sucks. Audio shaping for AM and a sharp filter 
>> if
>> you really need something more than 250-400 cycle xtal filters.
>
> AMEN BROTHER! Some otherwise good rigs make my ears hurt... but I
> think 6V6's are overkill. Lots of audio power that you don't need and lots 
> of
> heat.


I mentioned the reason yesterday and the location in the speaker cabinet. 
Its overkill for most which is why I offered a simpler option.


Right now I'm looking at an audio system using a pair of
> triode/pentode tubes originally designed for use as vertical osc/vertical 
> output
> tubes, like 6CX8's or 6EB8's. The pentode sections are wired for push-pull
> output, one of the triodes used as a phase splitter and the remaining 
> triode
> used as a gain stage. You can use a PA line transformer for the output 
> since
> you have no fear of core saturation in a P-P output stage. This should 
> give
> you 4-5 watts out in class A. If you really want more power out, then the
> same scheme using a pair of 6BM8's will giet you 6-7 watts out.

You can do it all with one Compactron but I wanted the classic look.

A pair of
> 6AK6's in P-P will still get you a couple of watts out if you want to 
> preceed
> them with a 12AX7.


There are as many choices as there are builders. Thats what makes it fun.


>>
>> Bells and whistles:  Output for a digital readout. I prefer a National PW
>> dial for a 0-500 direct readout.  Panadaptor output. Outputs/inputs 
>> to/from
>> a matching transmitter.
>>
>> No need for a RF gain control, run that stage at its best performing 
>> point
>> and use the attenuator if needed.
>>
>> All filaments on regulated DC. B+ 175V maximum and regulated. PS and 
>> audio
>> amp in seperate enclosure(s).
>
> Never understood why the lower B+, and I've learned to enjoy having my
> receiver all in one box, thank you very much. My shack is cluttered enough
> without yet another box and umbilical cord to hide somewhere.
>
> Hmmm... then again, the PS and audio can go in the speaker box, so maybe
> I'm being a little over-sensitive.


See above. I dont want a 20+ tube 2' high rack mount monster on the table. 
The PS and PP audio offers no benefits nor requires attention while 
operating so get them and the heat plus weight out of the way.

Its been known for decades that reducing B+ to the low level stages has 
minimal or beneficial effect on S/N, cuts down heat and tube noise in later 
stages. In the late 60's I had a HRO-5 and both the 180V and 250V supplies. 
I was amazed at how much better it worked at 180V even on 20M, so much of 
the hiss was gone. James Millen made the suggestion in a 30's QST that if 
you didnt need speaker volume it was better at 180V. It took manufacturers 
until the 60's to wake up, most likely led by Racal.


>>
>> This should all be pretty much modular allowing easy changes and repairs. 
>> Im
>> using SMA connectors and mini Teflon coax for RF.
>>
>> And the best for last, absolutely no rotary switches in any signal path, 
>> its
>> all done with miniature dip relays. No more compromises, cross coupling,
>> etc. Now you can have a nice user friendly panel layout for right or
>> lefthanders.
>
> A trade-off. Dip relays will bleed signal through when used in a high
> impedance path as well. Might be easier to control, might not.


Not with attention to details and the lack/minimizing of cross coupling far 
outweighs the work required. Hint: A PC board can be your worse enemy or a 
big help.

Carl



>>
>> Comments?
>
> Yeah, plenty  :-)
>
> Marvin
> KA7TPH
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