The AM Forum

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

many have used 6DQ5.

Would any circuit changes be necessary for a 6DQ5?

Don W9BHI

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.

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

wa1hlr use 811's

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

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

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

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.

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.


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)

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

The link I sent says about useing sweep tubes!

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.

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

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.

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.

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

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.

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

Bob

I have 6DQ5s in my valiant


Tom - AB3FL

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

Don W9BHI

[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'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.

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.

Yea, that's the main problem is having an identical pair, I have at least 4 or 5 of them in a box in my stash, and I think only 2 of the 4 or 5 are identical.

That's a good point.

I've noticed with 6BG6's, particularly the straight sided GA versions, that some tubes have the elements near the base with the whole top part of the envelope empty.

Other tubes have the elements near the top of the envelope with very long leads up from the base.

One would think the tubes with the elements near the base would be more stable in RF applications.

Use the high up element tubes for audio.

A low element example:

http://www.ebay.com/itm/One-NEW-CHANNEL-MASTER-brand-Japan-6BG6GA-tube-07-/260999617184?pt=LH_DefaultDomain_0&hash=item3cc4ca1aa0

A high element example:

http://www.ebay.com/itm/Radio-Tubes-6BG6GA-6BG6-RCA-Test-98-93-PAIR-/350556135668?pt=Vintage_Electronics_R2&hash=item519ec620f4

The 6CD6's of different brands also show the same differences.

dave

QST had an article about sweep tube modulators around the early 60's and the 6GJ5 years. Likely a McCoy article iffn I remember and a few types were tested.

Carl

I know that Nick VE3OWV has used triode connected 6146s as modulators in his DX100. It sounded good.

In my experimenting I have found what the other say is true, 6146's, 807's and 1625's all make excellent modulators as long as you bias them right and use the right iron and a little feedback.

Why would you choose tubes that are scarce and expensive?   The 6DQ5 is the least expensive sweep tube I have seen so economics makes it a good choice, but how long will that last?

I see 6DQ5s nos at hamfests in Ontario for reasonable prices. So if you had a bunch of them already it might make sense to use them. I do remember working Les W1FAT sk in the 80s and he used them as modulators in his modified Johnson Valiant. He used a S9 driver transformer with the arrangement.

6DQ5s are bas-ass! At least as far as all of the octal based sweep tubes are concerned. Several on here have had sucess with them as modders. They also are good as RF finals all the way up to 10m. The Gonset G-76 uses one as a class-C modulated final making 50w of resting carrier, and the Hammarlund HX-50 SSB transmitter uses one as a linear final at 50w of slop bucket.

Just FWIW......................

The original article using the 6GJ5's and the 6GW6's as zero-bias modulators was by George Hanchett, W2YM in February, 1962 QST. Lew MoCoy had an article later in that year adapring the "Novice Gallon" to plate modulation with a similar circuit.

RCA datasheet for the 6146 says use fixed bias in modulator service, cathode bias is not recommended. Wonder if that might be what gave the 6146 a bad reputation as a modulator tube, people trying to use cathode bias on them?

I guess my memory is holding up better than many my age ;D :o

My GSB-100 also uses a 6DQ5 and sounds real nice on AM thanks to it being a phasing rig.

Carl

6GJ5's - Yup those were early sweep tubes and I think that was what was in my old NCX-3.

6DQ5's and 6CD6's would last for 5-10 years in a TV set. How many hours a year was put on a TV set back in the day?


Well that's it. Now the audiophools will be 'selecting' those tubes.

Using audio forums and usenet to compound their general ignorance in such matters, they'll come to desire the ones best suited for RF as appropriate for reproduction of "airy highs". Shortly thereafter, the tube vendors will begin devising the appropriate eBay prattle.

In my experience, the closer the screen grid is run toward its maximum voltage rating or dissipation, the more potential there is for overheating problems in cathode biased circuits. I never ran them with cathode bias.

Hmmmmm................

In the old days of 6DQ5s, 6DC6s, 6BG6s, etc they were called "horizontal output tubes". Somewhere later they became known as "sweep tubes"? ? ? ? ?

Kinda like "at what point does a large boat become a small ship? ? "  ;D  ;D 

I think the term "sweep tube" originated in ham circles as slopbucket gained a foothold, and in CB circles as "leen-yars" came into vogue.  In the TV industry they remained "horizontal output tubes" for as long as hollow-state TV technology was used.

I have a Valiant II that uses 6DQ5s in the modulator.
It was built up by Dave KE1AV, Tim HLR may recall what went into it.
It makes over 100% modulation, no problem. Has the 3 diode neg peg limiter.
Should you hear me on the air, I am running that rig.

Kicks butte, imo.

The thing about sweeptubes (from "horizontal sweep") is that they are good for a ton of peak current.
More than something like a 6146 or EL34.

Very good for modulators, and in a smallish package too.

                        _-_-bear

I would really appreciate it if you could find the information on the change to the 6DQ5's.
The diode limiter info would be cool also.
Thanks,
Don W9BHI

Three Diode Negative Peak Limiter info here.

http://www.amwindow.org/tech/htm/3diodeka.htm


From the Valiant mods article

Brent, W1IA has gone one step further and swapped the 6146 modulators with type 6DQ5 TV sweep tubes. You will need to rewire the tube base to do this but it will give you much more audio punch from the tubes. You will probably want to change the meter resistor in the cathode circuit of the modulators to a value of .202 ohms. Do this by cutting it in half & butt splicing into it with some copper hook-up wire so you can reconnect to the tube socket. It's made of nichrome wire & will not solder. Your meter will now read 500 mA full scale. This is needed to measure all your new audio punch.

The entire article and schematic are at the link below.

http://www.amwindow.org/tech/htm/valiant/valiantmods.htm

Depending on the date and the technical level of the discussion, they have been called many things.

I make no claims. What is the order of propriety?

horizontal deflection amplifier
horizontal output tube
horizontal switch
H.O.T.
sweep tube
trace amplifier

?what else?

I always called them "horizontal output tubes" to customers.

"sweep tube" is vernacular but the term may have been first used in "ham tips"-like publications. I think it's slang from tube designers or application engineers writing application notes.

It may have then been seized upon by ARRL handbook authors who assumed the term was proper because of its origin and the majority of whom cared little for the tubes' noble service in television sets.

The use of "sweep tube" in association with an RF application implies that the speaker means the high powered horizontal deflection amplifier, not the small vertical deflection amplifier.

What was the earliest date the term "sweep tube" was seen in a publication?

If it was first coined by a vacuum tube design engineer specializing in products for magnetic deflection applications, would that legitimize it?

A tube by any other name would still glow as bright.
-Rosie Playt

on my Val i ain't the modulator section was completely bulldozed and replaced... new iron too... fewer tubes...

                  _-_-bear

WBear !!  Tons of peak current  at very low anode-cathode voltage is fine for class C too !! But this is another story.
In Italy we say: "do not touch that key".





~~~~~ class E?  ... sorry,  my alphabet ends at class C~~~~~~

Beautiful article this month in Electric radio on a 6146 Class AB modulator. 6SJ7, 6J5 and a pair of 6146's for 100 Watts. For those of you who do not get this fine mag, bang head with soft 813.

I found the article about the 125 watt modulator using push-pull 6146's. It is from April, 1958 CQ Magazine.

6146s lived in many SSB finals. A linear RF tube should be fine at audio.
The 6DQ5 has twice the peak emission so it will haul more peak current.
I ran a pair as switch tubes in my V2 PDM rig for many years and the tubes never got tired.

PP 6146's were also in many ARRL HB's