KRIS Big Boomer amplifier input tune....

[CorruptHawkeyez]

OK, I'll start off with the story from the beginning. So I have received a Kris Big Boomer box and have been messing with it for quite some time now. When I first got it, it had a hum in xmit, but was getting full power. So after having a friend of mine replaced the filter caps and fixed some other odds and ends, after putting it back together I was only getting half the wattage. So I took it back to him to check the tubes. I was there while he was looking it over and he was explaining to me that he thought that the factory had made it so all the tubes were 6LQ6;s. Well, to make this long story short, he had put the driver tube, 6JG6A into a 6LQ6 slot and a 6LQ6 into the driver slot. The 6LQ6 was getting its voltage in the driver slot due to the driver slot also having a top cap; but the driver tube was not getting plate voltage in its slot as the 6LQ6's plate voltage comes from the top cap; but the 6JG6A tubes plate voltage is thru pin 9 Well, I finally figured out that issue and now the tubes are in their correct slots.

For a little while I have been running a RCI2950 into the amp and I was getting around 275w peak out of it on AM. The 2950 has been detuned to be able to run a low drive amp. Well, today I was playing around with a Cobra 29 LTD chrome and installed variable watts on the RF gain and slapped a capacitor and resistor in there to make her swing. Hooked this radio up to the amp and now I'm seeing a little over 325w peak on AM. But I noticed on the AVG scale I'm getting backwards swing. Also noticed the SWR light on the radio coming on when I modulate. Which probably means I'm getting some reflect and the radio and amp are probably not in tune with each other. With the watt meter after the amp right now I'm not getting any reflect back from the antenna.

My question is this. If I put the watt meter between the amp and radio and it shows a high reflect under modulation. Can I adjust the input tune in the amp to reduce my reflect? What effect, if any, will this have in output watts when adjusting the input tune? I already know the amp has an input tune and know which one it is.

 

[nomadradio]

If you have the "old" Big Boomer, there is an adjustment for matching the amplifier's input impedance to (nearly) 50 ohms. It's worth a try to see if it helps.

The "revised" or late version left this out. Just feeds straight into the driver tube. This tends to cause a SWR of 2 or 3 to 1 on the radio when the amplifier is keyed.

Improving the impedance match between the radio and amplifier tends to improve the power output.

Makes life easier on the radio's final circuit. When the Big Boomer was designed, most base radios had tubes in them. A high SWR on the amplifier's input was not a big deal. Solid-state radios are less forgiving about this.

Making the amplifier's input side perform better did not make the amplifier sell better. Probably why the late version left out the parts to match the input side to 50 ohms. Save a buck on each of 500 units, and you look like a hero to your boss.

73

 

[CorruptHawkeyez]

OK, so I may have figured out whats going on. The tube that I thought was a 6JG6A for my driver is possibly a 6LQ6. The plate voltage goes to pin 9 under the socket, but it also has a plate cap coming off of it that goes to the top of my driver tube. The 6JG6A is not supposed to have a plate cap. All tubes in the box are also the same size.Therefore, this makes me believe that my driver is also a 6LQ6. So if I have 1 6LQ6 driving 3 6LQ6, could that possibly overdrive the 3 tubes?

here's another strange thing that happened. if you read above, one of the tubes was swapped with the driver tube and I was only getting about half of what I am now. Could it possibly be that the tube I had in the driver slot before is a weak tube?

 

[nomadradio]

If the amplifier has four plate-cap connectors on the top side, it has been modified. This would have been done by taking the pin 9 connection loose from the driver-tube socket, extending it above the chassis deck and adding a fourth plate cap for the driver tube.

If it has a plate-cap clip for only three tubes, you need a 'baldy" driver tube like the 6JG6, 6JT6 or 6KV6.

The 6LQ6 should not work at all without the plate cap attached.

As for weak tubes, I would first look at the Plate Tune controls. Each one should show a peak with the plates somewhere in between the two extremes of rotation. Those controls have no mechanical end stop, and will rotate a full 360 degrees.

If you get the wattmeter peak with the plates either fully meshed, or all the way apart, this is part of your problem. This means that the control went as far as it could and did not actually reach a resonant peak. Changing tubes can disrupt the peak setting of a Plate Tune control. If you see either the driver or final Plate Tune control peaked at one extreme, the coil attached to that control needs to be adjusted, and bring a proper resonant peak back to that control.

There is an easy way to tell this without peeking inside at the plates. Turning either Plate Tune control one full turn should always exhibit TWO PEAKS in one full turn. If you see only one peak, this tells you the control went to one extreme end of its adjustment range. When that happens, it simply did not turn "far enough" to resonate the coil attached to it.

Adjusting the coil will typically bring it back into range, so that you see TWO peaks in one full turn of the knob. They don't have to be terribly far apart from each other, but you need to see TWO of them in one turn of the knob.

A Plate Tune control that can't reach that resonant peak properly will hold back the power, even if the tubes are good.

And if you don't have access to a tube tester, this just muddies the water a bit more.

73

 

[CorruptHawkeyez]

With this amp there is a Tune and a Drive.. I usually tune it first with the Tune knob until I hit peak and then detune slightly...which is just a little shy of half way around...then I tune the Drive knob which is just a little more than halfway around and then detune slightly with it.

Are you saying with each knob, I should be seeing 2 separate moments when the needle hits peak when doing a full revolution?

 

[nomadradio]

Whups, remembered that one wrong.

Doesn't have a Load control on the front, only on the inside.

That "detune" thing is a good way to overheat the tubes. Old wive's tale. If this one had that "Load" control, this is the knob that advice would apply to, and NOT a knob marked "Tune".

Both the Tune controls on this one should be peaked.

For max.

And if you don't see two separate peaks in one full turn of the knob, it's time to have a look at the plates on that control to see if the plates are fully apart or fully together where they appear to show that false peak.

That will indicate what adjustment must be made to the coil attached to that control.

73