External VFO driver for my BC 610

[N5AEA]

Question:  I plan to use a rice box as a vfo to drive the BC 610 at the Tuning Unit crystal socket.  How many watts would it take or how much would be to much?  I will be using a TS 480 that will go down to as low as 5 watts at 50 ohms resistive load.  A 50 ohm load will be placed at the crystal socket.
73
Ed  N5AEA

[KA2DZT]

Ed

I'm sure you'll get some detail answers on this, but my first guess would be that 5 watts into the crystal socket might be too much.  You may need to inject the 5 watts past the crystal oscillator circuit.

There are plenty of folks here who have a lot of experience with the 610.

Fred

[w5gw]

5 Watts into a 50 Ohm resistive load is just shy of 16 Vrms. I'd think something on the order of 2-4 Vrms is going to be better as a first guess.
An easy way to attenuate it would be to use a capacitive or resistive voltage divider.

good luck

Gary W5GW

[N8ETQ]

  Hey Ed,


       I drove mine with a Johnson 122 for a long
time. It worked fine. I have a couple FT-243 rocks
for 40 and just use the MO on 75. There not as bad
as they are portrayed to be. At least on 75. On 40
were up against the band edge so Xtals provide a
little piece of mind.

As Gary reports,

"5 Watts into a 50 Ohm resistive load is just shy of 16 Vrms."

Yes, and almost 45V P/P.

       I think the osc is just a 6V6 IIRC so 5 watts
would be way to much. If you pull the 6V6 or whatever
and capacitive couple to the 807 grids 5 watts may
still be a bit much, Never tried.  I passed the 122
along after being satisfied with the MO on 75. I have
FT-243 rocks for 3880 and 3885 as well but a lot of
guys are a bit off anyway with their Rangers and
Valiants to dodge the persistent Slop Bucket Splatter.

      GL and please keep us posted with the progress
on your 610.

73

/Dan

[N5AEA]

Thanks to all for the replies and good information.  I am thinking about building a 3db 30W attenuator that was designed by AD5X to be used between driver and amplifier.   But what ever I do I will post what ever action I take.
73 to all
Ed   N5AEA

[WD5JKO]

  Ed,

   You might post a schematic of the 1st stage oscillator circuit. Also what happens on a BC-610 if you key the thing without excitation? I remember hams like Ozona Bob, W5PYT (sk) who could fondly recognize a rig based upon how it keyed up, and how the carrier sounded with QSB. The BC-610 is famous for a richly modulated "Ker-Clunk" during key up, and with QSB the slight FM/AM on the Master oscillator made a nice relaxing hum. 

    Without seeing the circuit, I might just try this idea. Consider the 6 db pad. If the load on a 6 db pad is a short, the input side sees 2:1 SWR. If the load is an open, the input SWR is 2:1. So anywhere between a open and a short, the exciter sees < 2:1 SWR. So then follow the 6 db pad with a toroidal core transformer which provides two purposes. First you need to find a turns ratio that gives you whatever drive voltage you need to the BC-610 for a given RF out of the exciter, and the other purpose is to have ground isolation to eliminate a ground loop. 

Jim
Wd5JKO

[W2PFY]

 Well I am sure there are many on here who have more experience than I, but I did run mine driven with an IC 746 and used a fifty ohm resistor across the socket to keep the Icom happy but it's not looking for wattage, but rather for voltage, so therefore a VFO output is where to go. Transistor devices that I am familiar with are all fifty ohms and you would need a matching network to get the voltage to where you need it to drive it in place of the crystal. Now I suppose there are newfangled mosfets that would have the required voltage you would need, but I am a dumbbell when it comes to that stuff!

[AB2EZ]

A few additional comments:

You are trying to transform a crystal oscillator into an RF voltage amplifier. The existing crystal oscillator is an RF voltage amplifier... with positive feedback added to produce oscillation. So one of your objectives is to remove, or substantially reduce, the positive feedback (see below).

I believe that we are all assuming that the BC-610 crystal oscillator is of the type where the crystal is between grid and ground of the associated tube. E.g. the type of crystal oscillator one sees in a Johnson Ranger, a DX-20, and many other transmitters of that vintage.

If necessary, you can place the 50 ohm transceiver load in parallel with a voltage step up transformer. E.g. a ferrite core of the type we use to make class E transformers .... 1 inch o.d. and 1.125 inches long, type 43 material... 3 turns on the input side and 6 turns on the output side. As long as the impedance (i.e. the capacitive reactance) looking into the crystal socket of the BC-610 is 200 ohms or more, the impedance seen by the transceiver will be: 50 ohms in parallel with -jX, where X is greater than 50 ohms. Meanwhile, the peak voltage across the crystal socket will be roughly twice as large as it would be without the step up transformer. With 5 watts of drive, that should produce a peak voltage (not rms) of roughly 2 x 22.4V = 44.8V across the crystal socket... which would be more than sufficient. (Actually 22.4V peak is probably more than sufficient). Typically, the drive (whether one uses a step up transformer or not) would be coupled to the grid (crystal socket) via a 1000pF series capacitor; and one would include (if it isn't already part of the oscillator circuit) a (roughly) 27k ohm resistor between grid and ground. This results in "self biasing" of the grid with respect to the cathode... and therefore makes the circuit less sensitive to the amplitude of the grid drive signal. If the peak drive voltage applied to the grid is (for example) 22.4V, and if the grid-to-ground resistor has a value of 27k ohms, then the average grid current will be 22.4V/27k ohms = 0.83mA, and the grid-to-cathode voltage will swing between 0V and -44.8V.

It is a good idea to remove or substantially reduce the feedback that is associated with the circuit when it works as a crystal oscillator. For example, if the feedback is obtained by using a small value capacitor between the cathode and ground... as in the Johnson Ranger, the DX-20, and other transmitters of that vintage... the usual approach is to use a SPST switch to add an additional .01uF between the cathode and ground... to cut the gain around the feedback loop to essentially zero. Just driving the crystal socket with a relatively low impedance source (e.g. 50 ohms or 200 ohms) may or may not sufficiently reduce the gain around the feedback loop... although it will probably keep the phase shift around the feedback loop below 360 degrees ... and thus prevent oscillation. Therefore, it is a good idea to bypass the cathode to ground (or otherwise open the feedback loop) when converting a crystal oscillator to an RF voltage amplifier.

Stu

[N8ETQ]

  Ed,

   You might post a schematic of the 1st stage oscillator circuit. Also what happens on a BC-610 if you key the thing without excitation?

   
   As long as the "Bias" rectifier (5R4 on the Mod deck) is good, Nothing.
If that stage is bad Overload relay can trip.

[AB2EZ]

The schematic of the BC-610 oscillator with one tuning unit plugged in, and with "switch 15" set to the crystal oscillator position (posted above) is on page 52 of the BC-610 manual that can be downloaded from the BAMA web site.

To drive the BC-610 transmitter from a 5W output transceiver:

You could just plug the output of the transceiver (with a 50 ohm termination across it) into the crystal socket, and see if it works. However, I would suggest the following:

a. Terminate the transceiver with a non-inductive, 50 ohm load, rated at 25W continuous... as you planned to do.

b. Add a 0.01uF capacitor between the cathode, pin 8, of the 6V6 "oscillator" (now being transformed into an RF voltage amplifier) and ground. You can use a SPDT switch to open this capacitor's ground-side path to ground when you want to return to crystal operation.

c. Make sure there is no crystal plugged in.

d. Connect the transceiver's output (with the 50 ohm load) between ground and the grid, pin 5, of the 6V6 via a 1000pF series coupling capacitor.

[Alternatively, for step d., you could plug the transceiver's output into the crystal socket... but there will be some loss of signal due to the small value of C47]

The 22.4V peak RF sine wave produced by the transceiver across the 50 ohm load should work just fine. The self biasing produced by the combination of the existing 33k ohm resistor (R1) and the 1000pF series coupling capacitor... between the transceiver's output and the grid of the 6V6... will result in a grid-to-cathode RF voltage that swings between about 1V and about -43.8V. This will be enough to properly drive the 6V6. In fact, this is roughly the same grid-to-cathode RF voltage swing as the 6V6GT experiences when it is working as a crystal oscillator.

Stu

[flintstone mop]

An uglier way to excite the BC610 could be done with a low power antenna tuner connected to the grids of the final and tune for minimum SWR to the transceiver and only adjust RF power to obtain the necessary grid current. Or go to the RF driver tube and make the same adjustments.
The Johnson VFO is a really nice unit.

Fred

[KB2WIG]

"The Johnson VFO is a really nice unit."

Yes it is....  I'd suggest, from a purely black crinkle, FB OB standpoint, is to use a BC 221. It will fit in with the colour scheme.

klc

[N8ETQ]

  Ha Ha Ha...

       This came out after the "E"  Model was done but this is
one of the BC-610 VFO's that was shipped with the GRC-26.

       A little "Bulky" but nice.

/Dan

"The Johnson VFO is a really nice unit."

Yes it is....  I'd suggest, from a purely black crinkle, FB OB standpoint, is to use a BC 221. It will fit in with the colour scheme.

klc

[N5AEA]

Thanks again to all for the wonderful ideas you have given me and for my "continued education".

Reply to Stu, AB2EZ:
The VFO interface to my Globe Scout 65A was via the Xtal socket and the osc. also was a 6v6 except it is a tuned plate osc.  However, the Scout specs required a VFO output of 10 to 45 volts but I assume that was really Vpp.

Your voltage transformer idea is one I like.  In fact, I had already purchased a T-106-2 core to build a 1:4, thinking that I could afford to have some core loss with 5 watts.  If this works I could build one  that is switchable from 1:1 to 1:9 in steps.

Your a, b, c, & d steps make it easy and think I will try that first.  All I need to do at this time is see if the plate supply is going to fire up the 250 TH.

Per your step "b", I am unable to add the .01 from pin 8 to ground.  I don't have access to under side of RF shelf.  However, in the Tuning Unit I can short L14 to ground.  This will take one side of the Xtal socket to ground along with C44 & C1 that are connected to pin 8.

Per your step "d", the other side of the Xtal socket goes directly too grid pin 5.  Be advised that there is no C47 between Xtal and grid.  This only exist in TU-47, a unit I will never use.

I will add the 1000pF coupling external to the Tuning Unit as a source coupling direct to grid.


Reply to Jim  WD5JKO:
Long term I plan to use my FT 101B that will be my receiver and exciter.  In that case I will use your idea of a 6db or 3/6/9 db attn. and a xformer.  I don't think the 101b will be happy to run at 5 watts, but I don't know.

73    Ed
N5AEA

 

[AB2EZ]

Ed:

Modified step b: shorting the top of L14 to ground (or shorting the bottom of the crystal socket to ground) should work. This won't remove the feedback (associated with the impedance, from cathode to ground, of the 250pf capacitor, C44); but the phase shift around the feedback loop, at low HF frequencies, will be -270 degrees... and, the magnitude of the gain around the loop at low HF frequencies will be less than 1*. Hopefully, the stage won't exhibit parasitic oscillations.

Modified step d: okay

*The impedance of the 250pF capacitor at 3.875MHz is -j164 ohms. The transconductance of a 6V6GT is approximately 4000 micromhos. Therefore the gain around the feedback loop at 3.875MHz will be approximately j0.66.

Stu

Thanks again to all for the wonderful ideas you have given me and for my "continued education".

Reply to Stu, AB2EZ:
The VFO interface to my Globe Scout 65A was via the Xtal socket and the osc. also was a 6v6 except it is a tuned plate osc.  However, the Scout specs required a VFO output of 10 to 45 volts but I assume that was really Vpp.

Your voltage transformer idea is one I like.  In fact, I had already purchased a T-106-2 core to build a 1:4, thinking that I could afford to have some core loss with 5 watts.  If this works I could build one  that is switchable from 1:1 to 1:9 in steps.

Your a, b, c, & d steps make it easy and think I will try that first.  All I need to do at this time is see if the plate supply is going to fire up the 250 TH.

Per your step "b", I am unable to add the .01 from pin 8 to ground.  I don't have access to under side of RF shelf.  However, in the Tuning Unit I can short L14 to ground.  This will take one side of the Xtal socket to ground along with C44 & C1 that are connected to pin 8.

Per your step "d", the other side of the Xtal socket goes directly too grid pin 5.  Be advised that there is no C47 between Xtal and grid.  This only exist in TU-47, a unit I will never use.

I will add the 1000pF coupling external to the Tuning Unit as a source coupling direct to grid.


Reply to Jim  WD5JKO:
Long term I plan to use my FT 101B that will be my receiver and exciter.  In that case I will use your idea of a 6db or 3/6/9 db attn. and a xformer.  I don't think the 101b will be happy to run at 5 watts, but I don't know.

73    Ed
N5AEA

 

[KD6VXI]

I've had no problems running a 101 at less than 10 watts of carrier.

One of the nice things about screen mod.   They are almost infinitely adjustable.

--Shane
KD6VXI