starting work on the amp

[KD6VXI]

Did I not just post the same question,  on a 3cx3000 project?

Deja Vu?

I believe YOU where the one who broke it down and gave me the maths I was looking for!   Lol

--Shane
KD6VXI

[KD6VXI]

On the subject of ceramic heater elements,  this is Exactly what Henry used in my 3cx3000 based 2000D (single phase,  one heater element)  and the 1500D (8877), also factory single phase.

This is for B plus.   Filament is it's own power supply with a large ceramic glazed wirewound left in line continuous for both inrush and setting of fil V.

They do work.   These only have 4 uF of capacitance along with the choke.

--Shane
KD6VXI

[Opcom]

I forgot about that. Maybe I should go back and look at those elements. Too many variables floating around in my head I guess.

[Opcom]

The RF stage is going to be built up from a 3CX3000 Henry RF generator. At least that will be the tube mounting deck.

The RF thingie on the back of a Henry RF power generator - this picture is not of mine but on one the e-place. This would be the power metering device?

Can these be repaired? mine is in bad shape and has some parts missing (no 'bird?' slugs of course.. long gone), Anyone got a diagram of this thing?

[W7TFO]

That is a generic Bird line section.  Not much of a diagram, as the slug wiper hooks to the screw-on RG-58 receptical center pin.

Disassemble yours, either ultrasonic clean or soda blast and reassemble.  The slugs for that are pricy.

73DG

[Opcom]

Now to divert temporarily to the other project, Jacob's hi-fi musical tesla coil, which is important not just beacause it is cool but also because we are together doing both projects and progress has to be made on both.

So at this point, to briefly describe the coil - it is a regular coil but driven by parallel 4-400 tubes in class C (like any transmitter). The coil is not run by RF feedback oscillation, but will be driven at the grid.

The screen voltage is 400-500V supplied by a regulated unit. The screen voltage will be modulated to put audio on the system. That seems fairly easy, its power supply is the kind that is pretty easy to modify. The plate supply is about 3000-4000V.  The grid supply will need to be somewhat regulated and provide a swing from about -200 to +200V. The supply we have is OK but isn't lending itself well to the mods. For this reason we will be rebuilding part of it.

Basically the 4-400 grid is a problem because it has to be biased and also driven. The frequency is 70KHz so it is almost audio, yet audio gear is not going to work for this frequency.

The choices we want to consider are the two diagrams. Both use 6 parallel 6l6s, they are already in the power supply.

One is to use the tubes as a pass device, the other use them as a voltage amplifier. Both would have about a 1K resistor as the 'load'. Would either of these drive a 4-400 grid well? It seems so, due to the low resistance of the 1K load.

[K6IC]

It's needed to get the controls and soft start working before going forward with the power supplies. Instead of waiting to find possibly unobtainable 7 pin ceramic sockets (like for the 1625)  ...

Hi Patrick,

Great work,  you may have found 7-pin sockets,  and the ones in the following Link probably come from Asia ...   but:

https://www.tubesandmore.com/products/socket-7-pin-ceramic-1625

Real,   vintage sockets are probably in the junque boxes that many of us have.   Know of none in stock here.

73   Have Fun!   Vic

[KD6VXI]

I'm not sure your ultimate goal (as much off the shelf / Homebrew / what),  but......

This seems like a perfect spot for Frank's (GFZ)  mosfet driver,  no?

--Shane
KD6VXI

[Opcom]

It's needed to get the controls and soft start working before going forward with the power supplies. Instead of waiting to find possibly unobtainable 7 pin ceramic sockets (like for the 1625)  ...

Hi Patrick,

Great work,  you may have found 7-pin sockets,  and the ones in the following Link probably come from Asia ...   but:

https://www.tubesandmore.com/products/socket-7-pin-ceramic-1625

Real,   vintage sockets are probably in the junque boxes that many of us have.   Know of none in stock here.

73   Have Fun!   Vic

I'm not sure your ultimate goal (as much off the shelf / Homebrew / what),  but......

This seems like a perfect spot for Frank's (GFZ)  mosfet driver,  no?

--Shane
KD6VXI

I have moved ahead with Octal sockets, not wishing to delay progress. There are enough now to build the electronic brain for the amp project.

The Tesla project is aligned to vacuum tube technology, so no Mosfets for now.

[w4bfs]

just looking at drive choices schizmatic .... may need some refinements ... need a second filter choke for each bipolar power supply or dc voltage and ripple amount will differ...the 4-400 will need up to +290 V swing above bias setting so I would go to capacitor input anyway.... need to run both driver sections as a cathode follower remembering that grid current will be provided on + going swing .... a single 6l6 should be able to provide the 25 or so mA needed but a higher voltage tube would be better, say a russian 5881 or el34....you can raise the cathode resistor value to avoid unnecessary current dumping .... definately put the 6l6 on separate fil supply due to large swing.... make each 4-400 grid negative bias separately adjustable via the  cathode follower... interesting

[Opcom]

Here's a more accurate diagram of the 6L6s and shows the three 4-400s. The bank of 6L6s is already wired like so in the power supply. The + and - 220VDC supplies are hypothetical - -one for bias and one for a high current source of DC to be used as drive to the 4-400's. The 6L6s already have a isolated filament voltage. The cathode follower is shown - I agree this would be more responsive, low impedance. Also, the basic version of the load is shown, a Tesla coil. That is already almost made.

The bias of -220V should be enough to keep the 4-400s cut off. the three pots - could be tapped resistors or big WW pots, could control the drive to each of the 4-400s. Would this work in lieu of trying to control the bias individually?

The +220VDC should be enough to provide the +290V swing above the bias setting of -220VDC.

I do not understand the current dumping statement. Do you mean the discharge of the capacitance of the grids of the 4-400s?

[w4bfs]

hi Patrick ....tnx for including the schemat soze can see what you are trying to do ... the 6l6 grids will need to go 65 V or so more negative than the cathode to cut off the tubes to set the condx shown .... this means an additional bias supply for what is shown ....no biggie but is needed .... big V swing for the 6l6 grids .... good use for single ended step up interstage xfmr.... the 47 ohm cathode resistors help the six pack 6l6 share the load ... the drive pots to the 4-400's as shown are not the most effective way to perform this job and are what is driving the need for the six pack 6l6 as the pot values are low to get the required drive currents .... it would be better to move the balancing adjustment to the 6l6 grid side like done in Arrl handbook screen modulator 6l6 cathode follower ckt

[Opcom]

The idea came from this image.

1K was a random value but considering six 6l6s it could be handled. But it is not necessary to have too much current in the resistor- the 4-400 grid should only draw current when the grid is positive and that comes from the cathode follower. I still don't understand about current dumping and the cathode follower's cathode resistor. At 70KHz, what is there to happen?

I suppose my idea is that with the 4-400's all cut off by a voltage of -220V or so, the only difference/alignment need, as I understand it, should be their individual merit, that is the amps per grid volt applied. I can see that if i use two 6L6 per 4-400 grid, some balancing can be done. But that is more of a refinement after some very basic tests. For now I don't want to worry about it, and just pretend there is only one 4-400 driven by a giant 6L6 cathode follower.

So, about the cathode load resistor, the thing I had in mind is like the attached image. The cath. resistor in there does not really have to take any current - whenever the cathode follower is not conducting, it just supplies bias to the driven grid. Ok that is audio, and class B. 77KHz and class C bias is not a far jump.

The whole article which is about a driving a push pull class B stage is thoroughly explained starts on page 138 here in the AGH-AM (128MB large file)
http://69.36.164.11/kb/goth/AGH-AM-027p.pdf

Wouldn't this work OK?  Driving the 6L6 and supplying it with bias could be simplified - It looks like the 6L6 could be driven without grid current.

[w4bfs]

wow that was a big file    ....lotsa good ole stuff there..

yes, I think that is a refinement from the first idea ... since the cathode follower is dc connected to each 4-400, I would split it up and I think a single 6l6 should easily take care of a single 4-400 but this will require 3 driver transformers ... the sticky wicket is the grid swing required for the 6l6 cathode follower ...roughly 350 Vp ....no big deal since the cf should not draw grid current, just use a step up transformer that is good at 77 kHz (probably a toroidal) ... another possibility is to split up the carrier (77 kHz) and audio to different tube sections (grid modulated vs screen modulation) .... this would tend to simplify things

the 15 K resistors would dump much less current than the aforementioned 1 K resisotrs ... use just low enuff to keep the waveforms clean

[Opcom]

was thinking of a 6BQ6 for driving the 6L6 grids. Such plate swing is real easy for that tube and it needs only a low screen voltage. It makes more sense to drive the grids like that than to try and float a low voltage (70V p-p) grid driver on the 6L6 cathode line. At 77KHz it gets difficult to keep good waveforms due to capacitances and coupling to large floating power supplies. Transformer would probably be OK but not too willing to go that route yet.

[Opcom]

Here's what I came up with for bias and a voltage amplifier. I'm sure it's in no way final but may be a place to start. There should be a way around two plate transformers. Maybe the "economy" power supply hookup. The lower-most transformer is for bias and can be low current.

So, the 6BQ6 used class A as a voltage amp, 6L6s biased off unless drive is present, same for the 4-400s. The text on the left gives the voltages relative to the cathodes with no signal applied.

I didn't work on anything this weekend. Was too tired after excessive WED-FRI work stuff.

[w4bfs]

looks like some good work here ....one question .... how does the 6bq6 screen get down to -32 V quiescent ? .... maybe another resistor to negative supply ? I don't have much experience with tubes operated with bipolar supplies ... I can see that the cathode is way negative....

 may need a bit of grid resistance in the 4-400's to equalize their grid currents


n.b.  been thinking about this today (now 5 pm) and the screen voltage  for the 6bq6 could be just -32 v ..... maybe a zener diode  or mv 0A2 to regulate it .... a bit of bypassing as well for cleanup 

[Opcom]

It's a good idea to regulate the screen voltage. An 0A2, referenced to the -220V rail, which is at the bottom of the 6BQ6 cathode resistor, would give a 112V screen voltage. This may be low but the goal is to run only 15mA plate current in the 6BQ6, so it could be sufficient. Returning the VR tube to the cathode would mess up the bias considering the VR tube should pass 10-20mA depending on screen current. The 6BQ6 bias will have to be lower with 112V on the screen instead of 150V but I don't have any idea how low, because I do not have any screen voltage curve for the 6BQ6/6CU6.

I agree on equalizing the 4-400 grid currents - that may be the least of the worries if wiring is too long as they like VHF.

[Opcom]

On a different note - -some friends dropped off a nice looking Systron Donner spectrum analyzer. I have not had one of that brand before but it looks like it does everything and then some. Covers 0.01 to 40GHz. Said to be working but in need of periodic de-ox-it on some of the controls. Must be old, maybe 1971? no picture of this seems to appear on the www.

Model 763  ? - yes.

PN 110225
NSN 6625011224858 possibly 6625-01-122-4858
110594

The units are marked as:
4701A sweep unit
4745B preselector
4809BK tuning unit
1335A variable persistence display (HP)

[Opcom]

Friday night!! what does that mean? Tomorrow Saturday I can do some more work on the RF amp..

Last time, basically I worked on Jacob's stuff - but now thanks to you all input I think we have a decent schematic for him to work with and he can start to build that from the Lambda DC supply.

1.) The nice industrial grade surface-mount Octal relay sockets came in, along with some 1-Ohm 100W resistors to help make up an inrush limiter. The goal is to wire up the control system as much as possible. Then the tube HV rectifiers need hookup and the SS rectifiers need mounted up on panels.

2.) I spent a lot of time in 'Front Panel Designer' drawing the cut-out shapes for the big old Westinghouse/RCA meters I want to use. The project's supposed to be low budget 'junkbox' but there's ample justification from a time and effort standpoint to have a panel machined with the unusual cutout shapes. I think there are 8 meters and four will just fit across a 19" panel. It may give the amp that Classic Transmitter look.

3.) Lastly, I have been looking at VFDs and really don't know which brands are better/worse, except that Allen Bradley is one of the best and those with no names are of lower quality.

The blower motor's 1HP. Possibly an inexpensive 2HP Chinese VFD for $110 would be reliable as an Allen-Bradley unit rated just for the 1HP requirement?
http://www.ebay.com/itm/NEW-1-5KW-2HP-VFD-7A-220V-SINGLE-PHASE-VARIABLE-SPEED-DRIVE-VSD-DRIVE-INVERTER-/141441065981?hash=item20ee8b63fd:g:rswAAOSwv2JXwO2z

[KJ4OLL]

Hi,
I used a TECO-Westinghouse FM50-101-C VFD on mine, works great.
http://www.factorymation.com/Products/FM50_115V/FM50-101-C.html
73
Frank
KJ4OLL

[KD6VXI]

Interesting on the VFDs.

Any residual receiver noise,  and if so,  how'd ya fix it?   Ferrite?


--Shane
KD6VXI

[Mike/W8BAC]

You might consider these for your project Pat.

[KD6VXI]

Themz seckzi!


--Shane
KD6VXI

[Opcom]

Those are super! The HV should be 4800, I would not dare re-scale nice meters like that. Or is it acceptable?  They definitely predate the 3CX3000 era!