[Boatanchors] Tube reciever design banter

[match at ece.utah.edu]

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 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.
> 
> 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.
> 
> 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.
> 
> 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...
> 
> 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. 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. 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.
> 
> 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.
> 
> 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.
> 
> Comments?

Yeah, plenty  :-)

Marvin
KA7TPH