Building a new amplifier

[ka1tdq]

I tore apart my dual 3-500 amp because it was constructed in such a way that the HV could be accessed if a person were determined to. Too dangerous for little children to be around.

I'm looking at building another tube amp though, this one with a much better enclosure. I don't want to stick to the conventional 3-500 pair though either. How about the 3CX1500 (8877)? The price is acceptable. I'm wondering about the tube's availability in the future. Are there any rumors of it becoming obsolete?

How about other tube options?

Jon
KA1TDQ

[W1ITT]

Many years ago, I built an amplifier using a 4X500.  Knowing that I might get bored and want a new toy sometime down the road, I cut a large square hole and mounted a 6 inch square aluminum plate over it with plenty of screws.  The tube socket sat on that.  The idea was that when I wanted a tube transplant, all I had to do was unbolt the socket plate and bolt in another to utilize some new tube socket.  I left room around the filament transformer to hold something different that might be larger.
I ended up dismantling the whole thing much later and using parts in other places but, with a little forethought, you can build a reconfigurable amplifier that would potentially hold an 8877, GS35B, 3-500Z or many other bottles. 

[w1vtp]

Just a couple of comments: doesn't the 3CX1500 have a relatively low grid dissipation power spec?  Also on the GS-35 is the socket for this excellent tube hard to come by?

The idea of a linear that has a sub-chassis that will accommodate different tubes is an excellent one - one that Ameritron successfully employs.  One only then needs to pressurize the sub chassis.

Al

[KD6VXI]

3 washers and bolts with nuts make an excellent socket, if grounded grid is OK.   Works on the 35b as well as other tubes.

--Shane
KD6VXI

[N2DTS]

I had an RF deck that started out as four 813's in grounded grid, then went to four 4x150's, then went to four 4-125's.

My 813 rig has the tube sockets mounted on a plate with holes in it, almost like a screen, air flows up past the tubes, but any socket or sockets could be put on the plate.

I think its a very bad idea to try and make anything that runs any power or voltage small in size.

And for protection, I just put all the high power stuff in 6 foot tall cabinets with doors that lock.
In addition, you can kill the ckt breaker feeding the shack and even lock it out.

I had many nice cabinets at work I tried to get rid of here, they all went to the scrap people.

[Opcom]

Serious concerns about preventing the gear from being turned on by children or the occasional snooping person could be helped by a simple key switch in series with the coil of a "main" contactor that controls all of the juice beyond the point where the power cable enters the cabinet. 

Alternately, a toggle switch up high in the top panel of a 6 FT cabinet out of reach of children could help. One of those military-type switch guards would be a nice touch and require a deliberate effort.

Get tricky and mount a reed switch behind an aluminum panel and have a little V shaped iron bracket on the front of the panel. A small round magnet salvaged from a miniature speaker, placed on the bracket with a pole against the panel, would stick in place there on the bracket and also close the reed switch on the other side of the panel. When not in use the magnet could be stuck to the side of the cabinet or anywhere else. No one would know and no key to fret over.

The contactor and line input fuses etc could even be enclosed in an internal small metal box into which the power cable goes after passing through the cabinet strain relief, so that if the cabinet door is opened there is no live 120 or 240V to be touched, maybe only 24VAC or low control voltage, fused.

Nothing is foolproof though.

[ka1tdq]

That's true... nothing is foolproof.  But, I think this time I'll build it into a communications cabinet that locks.  I already have the 220 mains box that I can lock out. 

After giving more thought to the tube selection, I'm again leaning toward another dual 3-500 design.  It's cheaper and this'll be the third one that I've built.  I could incorporate again what I did with the first one for cooling.  I used chimneys and pumped air directly to each tube socket using PVC piping and a super quiet bathroom fan. 

I had considered buying a solid state Ameritron amp, but I just can't settle for 125 watts-ish carrier. 

Jon

[Detroit47]

3 washers and bolts with nuts make an excellent socket, if grounded grid is OK.   Works on the 35b as well as other tubes.

--Shane
KD6VXI

Where have I seen this before?

[W1ITT]

Al...W1VTP
One tube socket for the GS35B is made by Raco, VA3YP.  I bought mine from him on Ebay for around $100, plus or minus.  Raco is a real craftsman, the thing is chock full of nice finger stock, and it even looks pretty.  He doesn't always have one listed, but they are available, as well as sockets for some of the other Russian tubes.  I run into him in the 80m DX window occasionally.
Norm W1ITT

[wb3eii]

Here is a box with most of the work already done for you, all really needed is a cheap way to get it from 3000 miles away.

http://www.ebay.com/itm/C120906-Henry-Electronics-3000D-RF-Generator-w-Eimac-3CX3000A7-Power-Triode-/391319393387?hash=item5b1c74146b:g:SugAAOSw~bFWMVJp

[ka1tdq]

Yeah that's nice! A little far away though.

There's a ham here locally that I sometimes buy stuff from. He can probably source a good cabinet for me. It's amazing. The hamfests here aren't as big as the ones in the NE, but I've managed to accumulate two BC-610 plate transformers, and a Henry 2k filter choke.

Jon

[W6TOM]

 Gilroy is less than an hour from here.

 Having been to the Fall NEARFEST 3 times now I will say it is bigger than anything here in California but we have more swaps.

 Enjoy your posts, you build some interesting things.

[Opcom]

Unless there are objections, why not add a third 3-500Z? So many amps have two, and then it's a little marginal for high power high duty cycle use.

[ka1tdq]

Now there's an idea!  My components will definitely handle the extra load.  I'll just use vacuum variables this time around. 

I have an LDG autotuner for the input, so that's easy.  My only question would be about the output pi inductor.  What value do I use?  A pair of 3-500's has an output impedance of 2500 ohms, and using 3 would be less.  I'm using a BC-610 plate transformer, so Eplate is about 3100 volts. 

I laid awake last night thinking about how to construct this, and it's definitely doable.  Just put the plate choke in the center of the three tubes. 

Jon

[w4bfs]

....  Just put the plate choke in the center of the three tubes.  

maybe consider putting plate tune vac cap in middle ....

Amforum/index.php?topic=40736.msg297329#msg297329

[n2bc]

The plate load resistance is a function of Plate E and I at your planned operating power.   There are lots of PI net calculators around.

Adding a third 500Z will impact the output capacitance & therefore the value for C1.  Figure the total output C of the tubes, then subtract from the calculated C1 value... with three 500Zs it will likely go negative on the higher frequencies.

Corrected the following:

The trick for this is to add a small inductance between the plate choke and plate blocking cap. This will have minimal effect on the lower frequencies but will offset the tube(s) output C on the higher freqs.

Have fun!    Bill  N2BC

 

[w1vtp]

On the topic of safety:  My 2 x 813 hb transmitter had micro switches on all doors.  These were wired in series with a contactor that had a hold-in circuit.  To bring up the transmitter a "start" button was pressed which engaged the contactor. After that the hold-in circuit kept the contactor engaged.  If at any time a door was opened, the line going into the whole transmitter mains was broken.  One had to close any door that had been opened, hit the "start" button and power was restored.

This had an additional benefit where if we had a brown out,, the contactor would disengage and therefore any main voltage swings were not imposed on the transmitter.

[KF7WWW]

Jon

If you want to go with another tube. Just contact me.. I have quite a selection you might want to build with.. Glass or finned ceramic. Have both..

[K1JJ]

Hi Jon,

After watching you put the last 3-500Z amplifier together with 2X4s I figured it would only be a matter of time until you tore it down and started over...     I've done the same thing in many different ways.  

Check out the YC-156 tube. It's similar to a 3CX-5000A7 but slightly different fil requirements.  Both amplification factor of 200.  No socket required and uses 225 watts to light the fil and is probably the cleanest tube on the market. They used it for precision MRI work.  

With 3.5 KV on it, you can run it VERY conservatively at 2500 W out and achieve a 3rd IMD approaching -45 dB or higher, depending on how good your driver is.  CLEAN!   It's like an 8877 on steroids.  It would just laugh at AM KW service.

I know of someone who may part with a good pull since he never built an amp with it yet.

T

[Donnie SWL]

I thought the YC156 was the same as a 3CX15000B7 but had a 5000w cooler not a 3CX5000...

[ka1tdq]

I just went to a hamfest this morning and picked up a couple extra 3-500 sockets. Sometimes the parts you get dictate the project, so I'm going to go with (3) 3-500's. It would be nice to go with higher power, but my antenna system wouldn't handle it anyway. 350 watts-ish carrier is plenty for me.

I also got a nice ceramic form to build a nice plate choke on.

Jon

[K1JJ]

I thought the YC156 was the same as a 3CX15000B7 but had a 5000w cooler not a 3CX5000...

Yep, this is true. They are the same construction inside.  Amazing.  

The 3CX-15000B7 is very hard to find and costs too much new.

The 3CX-5000A7 is similar except  the fil is slightly lower at 175 watts.

A 3CX-5000A7 or YC-156 / YC-179  are great choices.  All clean, low fil and amazing performance.   The good thing is that the fil requirements, being an indirectly heated cathode, is efficient.  So if we run the amp at low power, no big fil waste like with a directly heated cathode.  The drawback is it needs a good 6 minutes of warm-up time before use..

T

[K1JJ]

I just went to a hamfest this morning and picked up a couple extra 3-500 sockets. Sometimes the parts you get dictate the project, so I'm going to go with (3) 3-500's. It would be nice to go with higher power, but my antenna system wouldn't handle it anyway. 350 watts-ish carrier is plenty for me.

I also got a nice ceramic form to build a nice plate choke on.

Jon

The 3-500Z is also a very clean tube and three in parallel will do nicely.  Be sure to use air chimneys this time with a pressurized chassis or sub-chassis.

T

[ka1tdq]

I have a plan to do just that. A friend back east told me that Coleman oil lanterns can work as chimneys. I'll have to Google the dimensions. If not, I'll just get the regular chimneys.

Jon

[K1JJ]

I have a plan to do just that. A friend back east told me that Coleman oil lanterns can work as chimneys. I'll have to Google the dimensions. If not, I'll just get the regular chimneys.

Jon

I once used lantern chimneys for a pair of 813s and it worked FB.

However, the key is be sure to find a set of lantern chims that will taper in at the top of the tube so that the air gets pushed into the plate cap structure. Otherwise the air will run straight past the glass without cooling the plate cap and seals.  This requires a precise height to the glass. I would just spend the money on real 3-500Z chimneys.  

The correct Eimac 3-500Z chimney is slightly shorter than a 4-400A chimney to compensate for the tube height difference - by about 3/4", so get the 3-500Z type to get the right air flow.

http://amfone.net/Amforum/index.php?topic=20186.0;wap2



ebay:

http://www.ebay.com/sch/i.html?_from=R40&_trksid=p2050601.m570.l1313.TR0.TRC0.H0.X3-500Z+chimney.TRS0&_nkw=3-500Z+chimney&_sacat=0

T