how much drive into an old tube amp w/ straight 4 x 6LQ6

[SmackDown]

Since I asked the question I've been scratching like a junkie trying to figure out how to connect with unit 399 and do a trial run of say 10 tubes (grinning)
and like Shockwave I got rid of my Nitro 300 but I could use tubes for my Varmint XL-1000, 3x 6KV6A and 6x 6LR6. I do have a 10 tube Phantom in storage so some 6MJ6's would work too. Unit 399 if it isn't a problem could you ask around and see if the tubes I mentioned are stocked and tell us if the places that stock those tubes ship internationally and could we depend on them?


3's

 

[mr_fx]

Well he is running it with a Magnum S-9 (45 watts out) and getting 450-500 out on SSB... not too shabby, he is running a fan which helps out a a lot I would think

 

[Jimbo165]

Tubes

Unit 399 I would like to have about 20 at that price please hook me up.

I have had 3 amps running 1 into 3 6lq6s and they all did 500w and it just did not seem to make much difference on how much I put into amps as far as deadkey or drive. I usally kept them down to 3w radio, 150w deadkey on amp so to get a little swing in the thing.
Thanks JIMBO165

 

[ExitThirteen]

I have a buddy that has an old grey 10 tube version of the Phantom 500, never used it on AM except for tuning up, bought it new. It still has the original D&A tubes in it yet, and he uses it on a regular basis. Runs about 15W PEP SSB in, gets about 500W out on my Bird 43A meter.

Sweep tubes will last if you don't push them, a 6LQ6 is rated at 30W max plate dissipation per tube. That being said, when you run SSB, you're only running about 20-30% cycle duty, so you're not going to burn them out near as fast as an AM'er, who runs more to the 70-80% area.

If you're running an amp that's 4-6LQ6's with NO driver tube, a rule of thumb that I follow is to keep your maximum input at or below 75% of the total maximum plate dissipation of all 4 tubes. So, 4 tubes x 30W per tube = 120W total max dissipation. Then figure 120W x .75 (75%) = 90W PEP. I would not recommend going over 90W PEP into the amplifier. If running AM, you'll want to run your deadkey about 25% of your max dissipation, or 30W max. Hope this helps.

(edit) Also, keep in mind that the amp may have to be setup a certain way to run that kind of input. Some amps use a swamping circuit to reduce input wattage; you can put 100W in, but you're not getting that full 100W to the tubes.



~Cheers~

 

[Shockwave]

I'm surprised no one has asked to see a picture of the power transformer as it could easily be the determining factor here. If this amp was made for 11 meters, it may not have a power supply capable of feeding the four tubes at full drive.

Some amps for this band just stuffed four tubes in a single stage because they didn't want to build a driver stage and the added tuned circuits. It makes for a rugged amp when driven as intended but makes no more power than a pair of tubes.

I'd drive it to the point of gain compression and back off a bit from there. For example, see if doubling the drive power from 15 to 30 watts PEP causes the output power to double. If it does, that's a good sign that it can handle the drive.

On the other hand if doubling the drive power only causes the output to go up by like 50%, you can be sure you're over driving the amp into flat topping the peaks and there will be distortion.

Amps like the GLA-1000 used the same tube compliment and with its decent sized power transformer would easily make 600 watts or more PEP. Others like Elkin used four tubes but were intended to be driven with 4 to 12 watts at around 200 watts output.

 

[Shockwave]

With 6LQ6's in it I'd drive it to the radio museum and buy an amp with transistors.

If you've seen the typical unfiltered transistor amp for this band on a spectrum analyzer and the typical unfiltered tube amp, you might change your mind. Even when driven within specs the tank circuit in the tube amp makes them much cleaner than dumping the output of a broadband transformer directly into an antenna.

The downside is replacing the tubes today is expensive and that's what should keep you away from them more than the fact they used TV tubes. Some sweep tubes are a lot more rugged than many might think. Their specifications can tell you much.

Some can withstand voltages of 7.5 KV and support currents of 1.4 amps for short durations during the scan line sweep. The 30 watt dissipation is continuous and without fan cooling. They can last for decades with decent cooling if they are not forced to amplify ridiculous carrier levels.

 

[Needlebender 13]

With 6LQ6's in it I'd drive it to the radio museum and buy an amp with transistors.

Awwww.......come on.....If it ain't glass....it ain't got class!!! And one thing is certian......NO transitorized amp, in the same class of a sweep tube amp of course, will EVER have the audio of tubes. That's just a given. If he can find some NOS tubes for that old girl, and replace those filter caps....she'll out talk an equal output transistoized amp ALL DAY!

 

[RatsoW8]

My comment was from 2012..there are even fewer 6LQ6's around now

 

[543_Dallas]

A sweep tube amp is like a mickey mouse fishing pole. The guys running those old amps with a dozen of tubes in them do seem to have a signature sound though. Raspy, over modulated and their power mic cranked up so high you can hear the neighbor's dog. Is that the audio you're talking about?