Viking II modifications

[WD8BIL]

If y'all are going to use the low voltage supply for the screens be sure to relay switch that voltage. You don't want screens on with no plate voltage.

[N6YW]

If y'all are going to use the low voltage supply for the screens be sure to relay switch that voltage. You don't want screens on with no plate voltage.

Got it!
I have several able bodied relays in my arsenal. I should just relay switch both Plate and Screen supplies simultaneously then.
Nearly complete with the speech amp mod. 2 more hours max.

[N6YW]

Speech amp is finished, for now. I installed the tube underneath because the top side is a Pain to replace tubes from. Telefunken ECC83 and a Mullard 6C4 will do the work.
Onward.

[N6YW]

It works.
The audio is sufficient to drive the transmitter into 100% modulation. However, the audio response is restricted as evidenced by the sucky interstage transformer. I swept the audio bandwidth with my audio oscillator from 50 to 4000 cps and the result was not gratifying. The audio peak occured at 400 cps and dramatically dropped off at a high of 2500 cps. The low end dropped off  around 150 cps. I then monitored my transmissions with my 756 Pro and using my trusty D-104 non-amplified mic, it sounded okay but not great.
I definitely need to address the coupling to the modulators. This is an easy fix with a good transformer, but is likely best addressed with RC coupling rather than a lossy transformer.
At least I was able to re-energize the transmitter after decades of non-use and repair the numerous issues that had plagued it. I found the reasons why the calamity happened in the first place... poor cricket.
I am pleasantly surprised to find that the single ended style filament arrangement does not induce any hum into the carrier or the audio. Very quiet. The carrier looks great on the 475 scope and the 756 panoramic display.
If the 807 is much like a 6L6, then I would assume biasing them properly would be a very key component in getting the audio to really swing and sound natural. Just thinking out loud here.

[kb3rdt]

I Like run mine 210 mils plate bout 90 watts out but i seen mine do 110 watts no smoke 45 min key down into dummy load played a mp3 player into the input mic jack i said passed my test!

[KB3DKS]

  I ran the screen supply thru the other half of the plate switch and use a bnc relay to attenuate the rf drive. That way I can switch that relay for spotting/zero beat. My intention is to put in a solid state relay for the plate trans and a regular one for the screen supply. Make push to talk easier and less beating on the plate tranny with zero switching.
Wonder what triode strapping the 6CA7s would do?
Bill

[WD5JKO]

The audio is sufficient to drive the transmitter into 100% modulation. However, the audio response is restricted as evidenced by the sucky interstage transformer. I swept the audio bandwidth with my audio oscillator from 50 to 4000 cps and the result was not gratifying. The audio peak occured at 400 cps and dramatically dropped off at a high of 2500 cps. The low end dropped off  around 150 cps.
If the 807 is much like a 6L6, then I would assume biasing them properly would be a very key component in getting the audio to really swing and sound natural. Just thinking out loud here.

Billy,  Good deal getting this beast up and running.

   As to that transformer (interstage), there a some things you can do to make a crappy transformer perform better than its designer ever imagined. I will name a few:

1.) Resistance load the secondary. One circuit on the AM Window used 39K grid to grid on the 807's. Might try that , or 47K. Within class Ab1 swing, the driver tube will only see a varying reactive load and then adding the swamping resistor will dominate this causing the driver to behave better so long as it can provide power to the resistor.

2.) Choke couple the primary and capacity couple the audio to the transformer to the driver plate. This gets rid of the magnetizing DC current and its detrimental effects that restrict a large portion of the transformer BH curve. To do this you need something like a > 10 Henry choke. These can be pretty small at current ratings of < .03A.

3.) If you have a surplus of gain, add a negative feedback circuit from one 807 grid to the 6C4 input. Maybe Taylor for 6db gain reduction. The circuit on AM window by 'Ed' applies NFB across two stages (6C4 + 807's). I'd be hesitant since phase shift across two transformers might lead to instability. Also the 6C4 cathode is bypassed eliminating any NFB. Many of those circuits on AM window are best used for ideas only. So take audio from one 807 grid (whichever provides NFB), capacitor isolate the DC, and load with something like a 100K pot to ground. Then take the wiper (NFB adjust) to the grid of the 6C4 with a series resistor like 470K to sum the NFB with the audio from the driver stage.

Jim
WD5JKO

[N6YW]

Thanks for the tech tips. When I get to my computer, I'll take a look at those ideas in detail. Also relating to the 807 modulators, I plan on measuring both tubes to see what condition they are in and how much resting current they're drawing. Its possible that the bias isn't correctly optimised for best audio. I will scope the output of the Mod Trans to see if any crossover notch distortion is present.

[N6YW]

The audio is sufficient to drive the transmitter into 100% modulation. However, the audio response is restricted as evidenced by the sucky interstage transformer. I swept the audio bandwidth with my audio oscillator from 50 to 4000 cps and the result was not gratifying. The audio peak occured at 400 cps and dramatically dropped off at a high of 2500 cps. The low end dropped off  around 150 cps.
If the 807 is much like a 6L6, then I would assume biasing them properly would be a very key component in getting the audio to really swing and sound natural. Just thinking out loud here.

Billy,  Good deal getting this beast up and running.

   As to that transformer (interstage), there a some things you can do to make a crappy transformer perform better than its designer ever imagined. I will name a few:

1.) Resistance load the secondary. One circuit on the AM Window used 39K grid to grid on the 807's. Might try that , or 47K. Within class Ab1 swing, the driver tube will only see a varying reactive load and then adding the swamping resistor will dominate this causing the driver to behave better so long as it can provide power to the resistor.

2.) Choke couple the primary and capacity couple the audio to the transformer to the driver plate. This gets rid of the magnetizing DC current and its detrimental effects that restrict a large portion of the transformer BH curve. To do this you need something like a > 10 Henry choke. These can be pretty small at current ratings of < .03A.

3.) If you have a surplus of gain, add a negative feedback circuit from one 807 grid to the 6C4 input. Maybe Taylor for 6db gain reduction. The circuit on AM window by 'Ed' applies NFB across two stages (6C4 + 807's). I'd be hesitant since phase shift across two transformers might lead to instability. Also the 6C4 cathode is bypassed eliminating any NFB. Many of those circuits on AM window are best used for ideas only. So take audio from one 807 grid (whichever provides NFB), capacitor isolate the DC, and load with something like a 100K pot to ground. Then take the wiper (NFB adjust) to the grid of the 6C4 with a series resistor like 470K to sum the NFB with the audio from the driver stage.

Jim
WD5JKO

Jim
Thank you. As to the swamping resistors, the 47K's would be directly to the grids of the 807's just like many AB1 designs, right? There is also a post stating that disconnecting the secondary center tap and using two 10k bias divider resistors feeding the secondary leads to supply grid bias.
I am curious as to the actual layout for the inductor mod you suggested for the driver tube.
Really interesting information here, learning a lot from this project.

[N6YW]

I am contemplating substituting the transformer with this PI arrangement for feeding the 807 modulators.
With the other 12AX7A in the circuit, I wouldn't need the 6C4
What do you guys think?

[KA2DZT]

Where did you come up with that circuit??   Are you sure it will work??  The .1uf cap coming off the one grid to the 100ohm resistor shorts the signal to ground.

I also see other issues with the circuit.

Fred

[N6YW]

Where did you come up with that circuit??   Are you sure it will work??  The .1uf cap coming off the one grid to the 100ohm resistor shorts the signal to ground.

I also see other issues with the circuit.

Fred

Fred
This is a standard "Long Tail Pair" phase inverter used in many audio amplifier circuits. Most notably, Fender guitar amplifiers in most of their "AB763" designs. It is basically a phase inverter stage that has gain over the usual distributed load phase inverter. The .1 cap coming off the one grid is there to isolate the DC present on the grid so the feedback signal from the output stage can couple a sampling back into the phase inverter.
Hence, we have "negative feedback".
Because the speech stage of the transmitter is just an audio amplifier, I see no reason why it wouldn't work. How well it works is the question here.

[KA2DZT]

OK, I think I see how it works,  I overlooked the common cathodes.  I'll have to try that set-up to see how well it works.

Thanks for posting it.

Fred

[N6YW]

Some questions regarding the use of an external modulator.
I have an Eico 730 driver modulator that I have finished restoring. By itself, it sounds fantastic especially after performing some mods to improve the audio. My question is, how would I go about properly interfacing the 730 to drive the RF stage of my Viking II?
Here is the schematic of the 730:

[WD5JKO]

Billy,

   I see three major shifts in your approach in the same number of days. Realize that all have merit, and I believe each could be crafted into a very satisfactory modulator. You asked me a question a few days ago, and I am now just getting back to this. Since then you have moved on twice so I will just save the keystrokes.

  The Eico modulator might do a fine job. You will want to key it with the Viking, and wire it in as if it was the internal modulator. This is manageable, but could get tricky. Make sure it wires into the Phone-CW switch so that the modulation transformer secondary is shorted when in CW position.

Jim
WD5JKO

[N6YW]

Jim
You are correct!
I'm not convinced that what I have done so far is optimal.

[W1REA]

I agree mostly with the others, but I want to emphasize just how hard it is to drive 6L6/807 grids in AB2. Keeping in mind that the plate resistance of the 12AX7s is pretty high and the grid drive gets really squashed on the peaks due to grid current of the 807s, you need something herkier than a 12AX7. A cathode follower driver is in order here, an afterburner so to speak! The tube you use should be a 6SN7 (or maybe even a 5687) running on as much as 450-500V on the plates (especially if yoy ss, i hintend to use a good amount of NFB). Guitar amps almost never use AB2, even the powerful Ampeg SVT series. True Hifi amps built in the fifties never ran AB2 that I can recall. This why everybody keeps harping on the interstage XFMR thing. Yeah, I know a single ended class A driver has alot of 2nd distortion and it's a PITA to find a decent XMFR, but they ARE still available new from Hammond and other mfgs. The MOSFET circuits the others have mentioned are an elegant solution to the dynamic load problem of the 807 grids drive discussed here. In my dusty files of tube amp schematics, I have some suitable driver circuits I can send if you desire.
Good luck,  Lenny

[N6YW]

I agree mostly with the others, but I want to emphasize just how hard it is to drive 6L6/807 grids in AB2. Keeping in mind that the plate resistance of the 12AX7s is pretty high and the grid drive gets really squashed on the peaks due to grid current of the 807s, you need something herkier than a 12AX7. A cathode follower driver is in order here, an afterburner so to speak! The tube you use should be a 6SN7 (or maybe even a 5687) running on as much as 450-500V on the plates (especially if yoy ss, i hintend to use a good amount of NFB). Guitar amps almost never use AB2, even the powerful Ampeg SVT series. True Hifi amps built in the fifties never ran AB2 that I can recall. This why everybody keeps harping on the interstage XFMR thing. Yeah, I know a single ended class A driver has alot of 2nd distortion and it's a PITA to find a decent XMFR, but they ARE still available new from Hammond and other mfgs. The MOSFET circuits the others have mentioned are an elegant solution to the dynamic load problem of the 807 grids drive discussed here. In my dusty files of tube amp schematics, I have some suitable driver circuits I can send if you desire.
Good luck,  Lenny

Lenny
Seasons greetings and thank you for the offer. I am always open to anything that helps my thirst for knowledge and radio fun. I am getting ready to fire up the Gold Dust Twins for the first time on Christmas Day.
After that, the Vike gets all of my attention again. I wish I could find the original nickle core Hammond transformer just to see how it sounds. I guess at this point anything will be better than the one that resides in the rig now.
Cheers.

[kb3rdt]

Now I like to know how you can tell if it's AB1 or AB2!

                                                         Thanks Carl

[W1REA]

Good question! How can you tell whether or not the  amp is running AB1 or Ab2? AB2 IS AB1 for probably 90% of a sine wave. It's just a little before the crest, the actual crest and a little after the crest that grid current flows. The major difference between the classes is just how hard the amp is driven. How one measures this isn't so simple though. I doubt seriously that you could see any grid current flowing with a DVM because the duration of the pulses is short. When I had access to one, I used to use a current probe on an oscilloscope to see the pulses. Current probes just aren't everywhere and good ones can cost more than the scope!
However, in a driver that doesn't have enough scroat to drive the grids hard enough under grid current is easy to see on an ordinary scope with conventional voltage probes (where the ground clip has to get grounded). First, remove the output tubes. Connect the scope probe tip to the grid of the output tube socket. Connect a variable level tone generator to the amp input. Turn on the amp and raise the tone level while watching the scope. At some level, the tops of sine waves will begin to square off and that is the unloaded clipping point. Make a note of this voltage. Now repeat this with the 807s in their sockets. If your driver can't deliver almost the same undistorted voltage as before, you are into the grid current region (AB2) complete with bad distortion. If the clip voltage is the same, it indicates that you aren't quite getting out of AB1 and into AB2. This could caused by not enough driver voltage swing. If the peak clip voltage stays almost the same and you are getting 120 watts out with 700 volts on 807s, you have one potent driver! In practical tube drivers, one never sees this. Without NFB, it is almost impossible to not see some driver droopage during the grid current flow. I haven't actually scope tested any of the MOSFET drivers, but their very low on-resistance promises them to be practically constant-voltage source drivers.
How can you tell if you are using AB1, AB2 or B? Consult the scriptures (tube manuals), my son! There are guys like WA1HLR (the Timtron) that can quote the operating voltages of the various tubes and their various classes like chapter and verse. A guy that has some pretty good tutorials on tube amps is: www.funwithtubes.com.
Merry Christmas and Happy New Year!   Lenny

[N6YW]

Good question! How can you tell whether or not the  amp is running AB1 or Ab2? AB2 IS AB1 for probably 90% of a sine wave. It's just a little before the crest, the actual crest and a little after the crest that grid current flows. The major difference between the classes is just how hard the amp is driven. How one measures this isn't so simple though. I doubt seriously that you could see any grid current flowing with a DVM because the duration of the pulses is short. When I had access to one, I used to use a current probe on an oscilloscope to see the pulses. Current probes just aren't everywhere and good ones can cost more than the scope!
However, in a driver that doesn't have enough scroat to drive the grids hard enough under grid current is easy to see on an ordinary scope with conventional voltage probes (where the ground clip has to get grounded). First, remove the output tubes. Connect the scope probe tip to the grid of the output tube socket. Connect a variable level tone generator to the amp input. Turn on the amp and raise the tone level while watching the scope. At some level, the tops of sine waves will begin to square off and that is the unloaded clipping point. Make a note of this voltage. Now repeat this with the 807s in their sockets. If your driver can't deliver almost the same undistorted voltage as before, you are into the grid current region (AB2) complete with bad distortion. If the clip voltage is the same, it indicates that you aren't quite getting out of AB1 and into AB2. This could caused by not enough driver voltage swing. If the peak clip voltage stays almost the same and you are getting 120 watts out with 700 volts on 807s, you have one potent driver! In practical tube drivers, one never sees this. Without NFB, it is almost impossible to not see some driver droopage during the grid current flow. I haven't actually scope tested any of the MOSFET drivers, but their very low on-resistance promises them to be practically constant-voltage source drivers.
How can you tell if you are using AB1, AB2 or B? Consult the scriptures (tube manuals), my son! There are guys like WA1HLR (the Timtron) that can quote the operating voltages of the various tubes and their various classes like chapter and verse. A guy that has some pretty good tutorials on tube amps is: www.funwithtubes.com.
Merry Christmas and Happy New Year!   Lenny

Lenny...
It's official, you rule.
Thanks in abundance and have a cool yule.

[kb3rdt]

Thanks!

Marry Christmas and Happy New Year!

                                       Carl

[KM1H]

Put a regular old 0-10ma meter in series with the bias going to the driver xfmr CT. If there is absolutely no needle movement you are in AB1, if it barely wiggles on voice peaks you are in AB2 and if it moves more than that you are splattering everywhere.

Any analog VOM or VTVM can also be used as an indicator.

Carl

[W1REA]

Dear Carl,
Gotta love the old analog meters! Even though I first learned to use them in the sixties, I sometimes have to be reminded of what they can do that can't be done with a DVM! I'm also somewhat embarrased to see that you provided an answer in a couple short sentences which took me a treatise to cover! I sometimes think that my two semesters of "Technical Writing" in college has gotten the better of me.
Thanks, Lenny

[KM1H]

They also sub for an oscilloscope to see AC ripple on a B+ line....if it moves its bad. Ive used a Simpson 260 or Triplett 630 as a multipurpose tool more than once usually because Im basically lazy and it was the simplest way