304TH/304TL in grounded Grid

[ke7trp]

I am starting to gather some parts for an amp. I have some 304s, a tall Bud case, An RF deck for a 4-1000 with no tube, some chokes and a plate choke.  I also have a few coils and some bread slicers and vac caps.

I researched 304 amps on the internet and found a note from w8ji saying that its not possible to run a 304 in grounded grid.  I wondered if this was true.

I was bored and reading a 1956 QST magazine the other night and found what I was looking for.  A 304TH amp in grounded Grid.

This amp design was made to sit behind the common 100 to 150 watt AM/CW transmitters.  They mention the Collins 32 and say a typical Driving power is 125 watts but more is fine.

It was suggested that this was a better way to go rather then use a power att but lower drive to a different tube. I think they where thinking of the thunderbolt.

They also mention the obtained 800 watts AM OUTPUT. They also mention that you cant overdrive the amp and any excess power will just flow through the tube and be added to the amps output.

The article said that if you used a TL tube instead the drive would go up and you could run a full 200 watts into the amp and get more then a 1000 output for a single tube.  Not bad.

They also say that you need NO neutralizing cap at all and the amp runs stable.

Sounds like a Dream.  Anyone run these tubes in Class B grounded Grid in the past?
Any reasons not to do this and to use a more modern tube?

I was thinking this would make a nice amp for rangers, valiants and the like. Maybe a pair? 

It sure would look cool with a glass panel having a couple old 304s lit up! Runing a nice clean valiant into this thing would just be FB. Thought of painting it in matching colors.

[WD5JKO]

Clark,

  There was an old Bill Orr article about this in one of his old radio Handbooks (1947?).

I looked at it, and I recall details similar to what you describe. It was for one 304TL.

With a pair of 304TL's in G-G, IIRC the spec's come out like this:

Gain 6 db (4X)
Drive 400W PEP, or 100W carrier on AM
Actual Drive to the tubes, 50W total
Feedthrough power 350 watts
Output ~ 1500W PEP
Pi Net output, no neutralization

A pair of 304TL's could take a 100W class AM exciter (DX100, Viking II, etc.), and boost the signal to legal input without the need of an attenuator.

Using am AMP like this (class C) in the old 1KW input days, it was possible to run 1KW DC input and have an RF output greater than the DC input..

Jim
WD5JKO

[The Slab Bacon]

Just FWIW, It is most likely doable except for 1 small problem.

As long as the driving inpoot signal is applied between the grid and cathode (filament) The tube really doesn't care which one is grounded, but.........

304TLs require a lot of negative biass to tame them and keep them happy and controlable. (kinda like a bunch of bikers) So, you will find it necessary to keep the grid at RF ground potential and still apply some biass to it. (look at the Orr skiz closely). This is really no big deal to do, but it adds the requirement for a biass supply and complicates the grid circuit a little. The simplicity of a triode grounded grid amp is by far one of it's virtues.

Also, the 304TL is one of the few low-mu triodes that will run AB1 or 2 depending on where you set the biass at, this could also be somewhat of an advantage (depending on what you are driving it with. As it would need less driving power than full class-B.

If you allready have a deck made for a 4X1, why not go with it. They are a really nice, friendly, non complaining bottle and work very well in grounded grid. (and they can be found used cheaply.)

Hmmmmm................ a 2-holer G-G 4X1 amp would be SCHWEEEET! ! ! ! ! ! ! ! ! !

Just my $.02 worth............

[Bill, KD0HG]

I agree with the above comments, the 304 is not a good choice for a GG linear. With a 100 watt + filament and amps of peak emission, it's really designed for Class C operation. (Or radar modulator-LOL)

Other downsides: A single 304TL in GG is going to need about 200 watts of drive to play for some 850 watts out. 170 or so watts is ending up as feedthough energy from the exciter, add it to the 850 watts for about 1 KW out. This is talking Class C GG.

The big problem is that with so much feedthrough power, you can never modulate the amplifier and get 100% modulation. At full power, 850 watts of the output can be fully modulated, but the nearly 200 watts of feedthrough power won't be modulated at all.

Use a 304 in grounded cathode, class C neutralized, blow a lot of air on it and its connectors, drive the snot out of it and 1200-1400 watts of AM carrier per tube is achievable at 90% efficiency. Grounded grid won't play with that tube. Or use a 4x1 for a linear, a much better choice.

Also consider the plate voltages required and your parts stash. The 304 is happy at 2.5 KV, you need way more plate volts to get a 4x1 to boogie.

Bill

[K3YA]

I assume you are contemplating a GG AM linear amplifier rather then plate modulated.  In that case the tube's plate dissipation is going to be the limiting factor and a 304 is only rated at 300 watts.

The KW of dissipation of a 4-1000 is a big advantage.  They look almost as good as a 304TH when glowing orange too.  I have a GG 4X1 running at 5500V and it does a fine job on AM.  However, anything bigger then a lightly loaded Ranger will overdrive it.   

[KM1H]

A 304TH/TL with lots of air and big radiating anode caps will easily dissipate 500W. As an AB1 GG linear it will be pretty clean BUT inefficient when just tickled with enough drive to not get into the grid current region.

A modern style filament choke and tuned input will reduce drive and improve linearity.
Ground the grid directly and feed the operating AND standby bias thru the filament transformer CT.

Use a PTT relay terminal to shut down 2 of the filaments when not yakking, not recommended for VOX.

And finally dont believe anything JI claims without checking other sources with more credibility and actual engineering credentials.

[ke7trp]

Thanks for all the responses. 

The 4-1000 idea is out as of right now, I dont have a tube handy and I cant run a Valiant full bore into the thing. So its pointless to have around here Frank.  If I needed a high power amp, then that would be a great idea man.

But for an amplifier used in AM mode with a 50s style transmitter, The 304s might be the way to go.  You can run full bore into the amp and get legal limit out.

Jim, Thanks for the specs. Thats the type of amp I was looking for. 

Carl, I never thought of turning a filiment off. Neat

Bill.  I guess its the lost effiency that you cant give up?  If so, I understand that but the amp is for use behind a 400 watt pep transmitter and there is just to many ways I can think of to get that to play without making a huge amount of power I cant run.  I could build a 3000 tube amp and then run the valiant into that but then would have about 7500 watts on the output

I dont know.. I just happen to have some parts and one of these years, I want to build something up that I can use. This sounded like a good idea when I read that article.

C



 

[WA1GFZ]

Trade the 304s for a 4-1000A. then you can have a real RF tube that will play up to 10 meters. 304s belong in modulators. I tried running my 4-1000A at 5500 volts, it is a sight to behold.

[ke7trp]

Sounds good. What do I use to Drive the 4-1000?

C

[The Slab Bacon]

Sounds good. What do I use to Drive the 4-1000?
C

A ranger would be the bee's knee's. there is a guy that used to be on 3885 all of the time with a ranger driving a home brew 4X1 amp. He is a member here, but I cant remember his call.

In my homebrew 4X1 transmitter (running class-C) I use a single 6146 (also class-C) with around 450v on the plate to drive the grid of the 4X1 final.
The RF section tube layout is: VFO > 6CL6 > 6146 > 4-1000A. I easily get 45mils of grid drive with -150v on the grid of the 4X1.

[ke7trp]

I dont have a ranger to use frank. I have a valiant and a globe champ. Thats why we are talking about using 304s in gg.

I have amps here that a ranger can drive. Dont need another man.

[WA1GFZ]

At 3KV on the plate 110 watts will drive a good tube to 1500 watts out. jack the plate voltage and you get more pep power. 10 or 11 dB of gain.
But it will work up to 10 meters. I once got 1700 watts out on 10 meters with lots of plate volts.

[W2VW]

An amp like you describe would require over legal limit DC 1 KW input power do make anything over 300 watts carrier. Back before 6/1991 or whenever that would have been a curiosity and not a project many would build.

On top of that low and medium mu triodes require extra steps to keep from oscillating in cathode driven service. The advantages of cathode drive evaporate except for inverse feedback.

GFZ offers a solution which will work. Use a 4-1000. Take it a step further and put screen and control grid potentials on it. That will keep it from exceeding control grid dissipation and becoming a doorstop.

A Valiant's RF out shouldn't be amplified unless something is done to clean the crap up. An outboard adjustable DC supply to the final stage would allow sorely needed dynamic range for the mudulator plus allow tune-up power level. After tuning the amp for full bore, lower the DC to the Valiant final and turn down the audio gain. Others have managed similar setups without hacking the mechanicals since you like to leave things factory fresh.

Any linear amplifier project planning comes down to what kind of power supply one can build.

[WA1GFZ]

Dave I never hurt a 4-1000A running GG. I used it for years as an afterburner for a SB401 that ran 100W PEP and on CW. I would not run much more than 30 watts carrier on AM.

[ke7trp]

 
An amp that only takes 30 watts of am carrier input is completely useless to me.  I have half a dozen amps here that will work for a ranger or any other low power transmitter. Building another would be like a third nipple.  Its just not needed.

I wonder why everyone is talking about higher power, lower drive amps here. Its been done a million times over. I thought I was clear at what I was thinking of doing.

What we are talking about here is an amp that one can place behind a 100 to 150 watt AM rig and amp to the legal limit. 

sorry for bringing this up. Lots of great ideas here for SSB amps and low power DX60s though!

C

[KE6DF]

How about a pair of 833's?

Greater gain than 304th's.

There is even a linear circuit out there somewhere, but I believe that circuit is grid driven -- so back to low drive power.

But, GG on 833's should do what you want.

I've toyed with the idea of building and 805 GG linear for use with a 100w PEP (25w AM carrier) rice box.

Seems like tubes that make great modulators should make good RF linears also.

Dave

[W7TFO]

 Building another would be like a third nipple.  Its just not needed.

Ah, the dreaded "supernumerary mammary" syndrome !

Then again, would they be linear nipples ?

73DG

[ke7trp]

833s are neat. But the 304s dont need neutralizing and i have 2 pair of them now. I would have to buy some good 833s somewhere alot with a way to mount them.

C

[WA1GFZ]

so you buy 3 resistors and make an attenuator. Imagine how clean a 100 watt modulator is at 25 watts. 304 is a crap RF tube and will give you all kinds of trouble a lot motre trouble than reducing the output of a valiant.

[KM1H]

Carl, I never thought of turning a filiment off. Neat

The 304 is constructed so that if wanted 2 of the 4 filaments can be turned off right at the socket. It now becomes a 152T.

At 3KV on the plate 110 watts will drive a good tube to 1500 watts out

In GG? Thats dreaming as its only 1800W input and will barely make 1000W out with a new tube.

On top of that low and medium mu triodes require extra steps to keep from oscillating in cathode driven service. The advantages of cathode drive evaporate except for inverse feedback.

Not really if basic construction practices are used. You dont use zip cord to ground the grid or 50's input and layout designs.

304 is a crap RF tube and will give you all kinds of trouble a lot motre trouble than reducing the output of a valiant.

I disagree. Why not let him try and find out? At least it doesnt need obscene voltages, expensive socket and chimney and REALLY careful attention to detail plus parasitic suppression if operation above 40M is wanted. The 4X1 is a very squirrely tube to the first time builder.

[KE6DF]

so you buy 3 resistors and make an attenuator. Imagine how clean a 100 watt modulator is at 25 watts. 304 is a crap RF tube and will give you all kinds of trouble a lot motre trouble than reducing the output of a valiant.

Well, you don't want to go the the trouble of building something that won't work.

But, we aren't engineers working at a paid design job.

We are hobbiests doing stuff for fun.

So, I'd say try it.

An amp with 304's glowing through windows sounds cool to me (or should I say hot).

Attached is a datasheet for a 6C21 -- another Eimac radar tube even larger than the 304TL.

The datasheet shows operating conditions for Amateur GG linear service.

Dave

[ke7trp]

Sweet tube. Now that gets the juices flowing.  It looks like a 450TL.  But medium mu. I have a new 450 TL here that someone Drilled a hole in to make a display pice.   

I know a ham that has a 450TL in grounded Grid Class C with a two 100THs as modulators.  Its an amp for an apache.  OMG is that combo loud.  Up there with the loudest radio I have ever heard.  Just blow you out of the chair loud.  What a sweet sight it is to see that 450 lit up.

C

[W2VW]

On top of that low and medium mu triodes require extra steps to keep from oscillating in cathode driven service. The advantages of cathode drive evaporate except for inverse feedback.

You dont use zip cord to ground the grid or 50's input and layout designs.

Where did you get that line Karl?

It sounds exactly like an eham 6 meter conversion review  

[KM1H]

And another thing...with a pair of TL's and the Valiant you will still need at least a 3dB attenuator if you want to run it tuned up properly.

Figure on 300-350W carrier as an AM linear and a good fan. The filament pins also need cooling but a small slow speed muffin fan is fine or one of those phonograph motor fans as used in the SB-200.

Once the rebuild is finished here the old 2X 250TH amp will have a pair of 6C21's and the existing 304TL modulator can be cranked up to where it will do justice to the tubes.

 

[kb3ouk]

I seem to recall seeing somewhere that 833s don't do so well in grounded grid service.