Using sweep tubes for modulators

[W9BHI]

Hello all,
I have read here on the forum that 6146's don't make good audio modulators.
I have a Valiant II and was wondering if sweep tubes like 6KD6 or 6LQ6 and the like would be a better choice?
Any opinions?
(I just put on my flame-proof suit)
Thanks,
Don W9BHI

[WA1GFZ]

many have used 6DQ5.

[W9BHI]

Would any circuit changes be necessary for a 6DQ5?

Don W9BHI

[W1RKW]

I may be wet behind the ears here but you probably can't triode connect a 6146 easily since the cathode is tied to one of the grids internally assuming that's the way it should be connected.  A 6KD6 or 6LQ6 sweeper can be triode connected since there's no internal connection between the cathode and G3.  I 'm not sure what the effect would be if one used a 6146 in a triode connected fashion. I 'm assuming that's your intention but I have my paper towel ready.

[W9BHI]

The thought never crossed my mind about using the 6146 in a triode configuration.
But I'm open for suggestions.
Maybe the sweep tubes could be configured that way?
I am asking these silly questions here on the board cause I don't know that much about tube circuits.

Don W9BHI

[kb3rdt]

wa1hlr use 811's

http://www.amwindow.org/tech/htm/valiantbytron.htm

[KE6DF]

There is one AM boat anchor that used 6DQ6's as modulators.

The screen grid was driven with the control grid grounded (along with the cathode) I think.

That probably would work with other sweeps as well.

In general, sweep tubes run with a fairly low screen voltage, so driving the screen is possible with reasonable voltage swing.

I believe you might need some negative bias on the control grid to use the 6DQ5 -- and perhaps some others -- in screen driven mode.

Dave

[W8ACR]

If I were going to put in completely new tubes, I'd use 809's.......No screen circuitry, and at 660V plate voltage, you could probably run them zero bias. Doesn't get much easier than that. You could use the same filament transformer, whereas you'd probably need a bigger one for 811's. 809's would give you 120 watts or so of sine wave audio output...should be enough for a valiant.

You would need new tube sockets of course, and you may need a different mod tranny.

I have a Viking II that will soon have a homebrew outboard speech amp and 809 modulators. Probably gonna use a Triad M12AL mod tranny.

Ron

[IN3IEX]

HI, I use a pair of 6KM6 sweep tubes in my 80W modulator.
They work well. Great modulation reported with no feedback in the circuit.
http://frank.pocnet.net/sheets/049/6/6KM6.pdf
If you look at the curves, they are very regular.
http://frank.pocnet.net/sheets/127/6/6L6GC.pdf
6L6 and 807 are not so regular.
"Modern" sweep tubes can give a lot of peak current and work very well for audio.
For 200mA current we see not less than 50V anode voltage for the 6L6, control grid positive (class A2)
For 200mA current we see not less than 10V anode voltage for the 6KM6 and the control grid is still negative (class A1).
There is e net 40V gain in power supply voltage efficiency.

6KM6 and most modern sweep tubes have multiple fin anode structure (you see inside a sort of box instead of a plane plate).
This special anode is not present in 807, 6146, 6550, 6CA7, EL34, 6DQ5, 6JB6 and older tubes. It was invented for sweep tubes for color TV.
This special anode makes a difference, and the difference can be seen in the curves.
I get a lot of audio power in class AB1 from the 6KM6s at 600V and very simple driving circuitry.

Try them or any other "modern" color TV sweep tube.

Hints:
If you already have a pp 6146 modulator class AB1, just rewire the sockets, keep the variable -g1 polarization, reduce the screen voltage with a voltage divider.
Start with 50% (say two 10k 7W resistors) then check the cathode current for about 20mA current each tube. See if you can reach that value with the g1 bias trimmer. If not, modify the screen voltage divider (higher screen voltage -> higher cathode current). Use a fixed 10 ohm cathode resistor each tube for measuring cathode current.
I did this tuning on the fly and I do not remember the exact values in the circuit. I could repeat the tuning in 5 minutes if necessary....
Remember that 6KM6 is Novar, but most suitable Russian sockets are european Magnoval, the base looks the same but pin diameter is different. It is necessary to adjust the contacts in the socket with a screwdriver.

[The Slab Bacon]

Most sweep tubes are low impedance devices. (preferring lower plate voltage but being able to carry more plate current.)

this combination can work out very well for some of the low to medium power transmitters. The whole trick is just figgering out how you want to drive them and bias them.

There is one AM boat anchor that used 6DQ6's as modulators.

The Gonset G-76 did that trick with the grounded control grid and fed the drive into the screens. I guess they ran something like class-B or AB2. It used a pretty beefy driver and driver / audio outpoot transformed to do it. IIRC, the driver was a 12AB5 or something similar. About 5 watts of drive or so. I thought that circuit was a bit odd, but they worked well and sounded good. (I had 2 of them at one time)

[iw5ci]

why are the 6146 not good for modulators?
i have good results with a push pull couple of 6146 as modulators.

[kb3rdt]

The link I sent says about useing sweep tubes!

[The Slab Bacon]

why are the 6146 not good for modulators?
i have good results with a push pull couple of 6146 as modulators.

You know, that is a very good question. The 6146 has always been maligned as a modulator, but no one ever says why. I have a pair of 6146Bs in my Valiant, and the work fine and sound quite good. I have also put pairs of them in a few other radios and gotten very good results. I have an Apache with a pair in it as modders. The original mod transformer was blown and the transformer I had was better suited to a pair of 6146s than EL-34s so I used them. It also sounds quite good and makes a ton of audio.

The buzz for modders has always favored BIG triodes for their linearity. But a little negative feedback wrapped around a tetrode also sounds good. I think the feedback ladder scares some people to use.

[KE6DF]

why are the 6146 not good for modulators?
i have good results with a push pull couple of 6146 as modulators.

I think what happens is someone with a reputation says the 6146 is a bad audio tube, then someone else quotes the first guy, and then lots of people report that the 6146 is not a good audio tube in articles and on Web posts.

And most, of not all of them, including perhaps even the first person in the chain has ever actually built an audio amplifier with 6146's.

But, when anyone does a little Internet research, you see all the posts saying the tube is bad for audio so that appears to be the consensus.

There are some audio types saying they have had success with it on the audio forums.

There were also some commercial tube audio amps built -- particularly with the 6146B.

Acrosound offered an ultralinear transformer for 6146B amps.

So, I'm not sure there is a whole lot of truth in this rumor -- if you use good design and proper negative feedback.

If Bill Orr had published a design article claiming good results with 6146 modulators then we would have the exact opposite and there would be lots of posts saying what a great audio tube the 6146 is.

Isn't the Internet is fun?

Fluoride causes cancer and autism.

Sugar is toxic.

Coconut oil causes heart desease. Oh, sorry that has changed and now it's very benficial.

Dave

[k4kyv]

6146s were used in a lot of slopbucket  rigs from the 60s through the 80s in the final output stage.  Other rigs, like the FT-101E series used sweep tubes not because they were better, but because at the time they were much cheaper than 6146s.

Nowadays, sweep tubes carry a premium price, much more $$$ than 6146s, with the drop-off of sales volume that accompanied the demise of hollow-state TV technology. Articles have been published on converting FT-101s to 6146 finals, but I have seen  reports that they don't work as well as the original 6JS6s.

Regardless, if they worked well enough as class AB1 leen-yars for slopbucket to be used by major manufacturers  like as I believe Collins, they should work equally well as class AB1 modulators for AM.  Of course, back in that era hardly anyone gave a crap about what their slopbucket audio sounded like. Good audio quality on phone was supposed to be blasphemous and un-amateur radio like. The space shuttle sound was the order of the day.

BTW, for audio use, it doesn't matter that the deflection plates/suppressor grid is internally connected to the cathode.  They are connected together internally in the 807, and yet it has been used as a high-mu zero-bias triode for many years.  Run a 20k resistor between screen and control grid, and feed the audio to the screen.  That configuration is even recommended in the ARRL Handbook tube section.  Not sure, but like many of their other tube chart listings, that data may have been taken right from the RCA tube manual.

What you don't want is to run a tetrode or pentode in grounded grid linear service with the suppressor/DP tied to the cathode.  That introduces enough internal capacitive feedback to cause self-oscillation, the avoidance of which is one of the reasons triodes are usually run grounded grid in linear service to begin with.

Sweep tubes should be good for delivering high positive peaks from the modulator, since their peak emission current rating is by necessity high. In TV horizontal deflection service, they are essentially used as pulse modulators, which calls for massive peak power in a saw-tooth wave, even though the average power output is low. Sort of like miniature versions of the 304-TL, which was used as a radar pulse tube.  That's why sweep tube leen-yars worked pretty well for slopbucket if you were careful to talk in a normal voice and didn't run a lot of processing, but had the reputation of going belly-up with a sustained whistle into the mic, or driving them too hard into the flat-topping region.  They could deliver the p.e.p., but couldn't handle much real power.

[Jim, W5JO]

Mo,  W5KD used a B&W 5100 that was slightly modified by increasing coupling caps.  The mod tubes in it and the B model were 6146s.  A number of people raved about his audio.  I think the assumption is mythical if they are operated properly.

[KM1H]

Since the 807/1625 can be connected as a low mu triode I see no reason that the 6146 cant be used that way also.

Carl

[Opcom]

Don't forget the 6CD6GA and 25CD6GB.
G2 volts 175 max, so does well in the plate swing department
understated plate dissipation ability,
rated plate voltage 700V,
is cheap like $6.00 because audiophools don't want them,
is cheap like $6.00 because  they don't do well in CB linears,
they have shapely curves,
rated 200mA DC cathode current (more than a 6550).
They will conduct 230mA with 60V on the plate (Class C TV service).
The later ones have better plate-meat.
AB1 pair will do 50W
Screen driven in zero bias 'special class B" they are extremely linear.

I just don't want these little Octal workhorses to be ignored.

[kb3ouk]

I think I got some 6CD6s in my tube stash out in the shed. Might try building a pair of those modulating a 1625 (got one of them out there too, that's why) or use another pair of 6CD6s in the final as well. Wonder how well that would work?

[VK7ZL]

I run a pair of QE05/40's modulating a pair of 1625's.
Works well, no complaints.

Bob

[AB3FL]

I have 6DQ5s in my valiant


Tom - AB3FL

[W9BHI]

What changes did you have to make to put the 6DQ5's in?

Don W9BHI

[kf4qkr]

[I have read here on the forum that 6146,s don't make good audio modulators.]         I think that might an AM urban legend. I hope so any way. I am building a home brew modulator for my BC-457 using 6146,s with a little negative feedback.I have seen them used in a cathode modulator and they sounded good to my ears.

[Opcom]

I think I got some 6CD6s in my tube stash out in the shed. Might try building a pair of those modulating a 1625 (got one of them out there too, that's why) or use another pair of 6CD6s in the final as well. Wonder how well that would work?

I've never seen one in RF service, only in class C as TV horizontal sweep amplifiers (fundamental about 100Khz) and in medium voltage power supplies as pass regulators. 160M and 80M sure, but at some point there may be issues. Compare the interelectrode capacitances to RF tubes like the 6146 and also look at one visually to best consider the internal inductance of the connections to cathode, grid, and plate. Those tubes because they were not intended for RF may not all be uniform in respect to construction.

[Opcom]

[I have read here on the forum that 6146,s don't make good audio modulators.]         I think that might an AM urban legend. I hope so any way. I am building a home brew modulator for my BC-457 using 6146,s with a little negative feedback.I have seen them used in a cathode modulator and they sounded good to my ears.

I used 6146's for audio and they sounded great. The dynamotor power supplies were the limiting factor but I was running about 50 watts from a pair.