Thunderbolt 305 Tuning & Operating

[WB2WIK]

Yes, the grid bias voltage isn't very high, and of course it is "negative" in polarity (with respect to chassis ground).

This circuit is often called "super driven cathode," in that the cathodes are driven but the control grid is not grounded, it's biased with DC and grounded only for RF (with capacitors). An "even more super-driven cathode" circuit biases both the control grid and the screen grid, which can provide tons of gain (like a grid-driven amp), but drive is applied to the cathodes.

It would have been better for them to just use triodes, ground the grids directly, and bias the cathodes. But "back in those days," 6LQ6s were only about $1.99/each and many "cheapie" amps were built using them and similar television horizontal output tubes, because they were all cheap. Pity, because at that same time a brand new RCA 811A, which is a real "transmitting" triode, was only $9.95 and was equivalent to 2-3 of the sweep tubes, and probably more linear. But people were doing silly things.

[KF5OBS]

People will always do silly things if they can maximise their profit, unfortunately. It seems like working on tube amps is one of the easiest things I will put down on my "have done" list soon. I don't know why they appear to be black art at first but after reading into it I am confident to be able to turn that box into something useful. Now, I have plenty of experience in frying MOSFETs, how sensitive are tubes in relation to MOSFETs?

EDIT: I just found a box full of some PL8503 tubes I got a while back. Are they any good for an amplifier? Judging by their size I suspect they are overkill.

[WB2WIK]

Not familiar with PL8503, but PL means "Penta Labs," who used to be a manufacturer many years ago. No longer. They're still in business; in fact, they're about six blocks from my office in Chatsworth, CA -- but they no longer manufacture anything.

Nothing black art about tube amps. I designed and built dozens of them. If you look up the 73, Ham Radio and CQ Archives on line, you'll find a lot of articles I wrote on the design and construction of tube amps. A lot of them, from 250W to 1500W output, for various bands.

Tubes aren't sensitive to ESD or EMP and in those ways are much more resiliant than FETs. However, you can still damage them! "Good tubes to use for ham radio amplifiers" are, I think, ones that are very difficult to damage like 3-500Zs, 3CX1200A7/D7, even 572Bs. One of the "best tubes ever" for ham amps in the HF-VHF range that can run lots of power and is difficult to damage is the 3CX3000A7. Its "little brother," the 3CX1500A7/8877 is a very good tube but easier to damage by excessive grid current.

My first homebrew HF amp was one I built at age 16 in 1968 when in high school, and was a 4-400A with 2500V on the plate and ran about 750W output power. I couldn't afford a new tube, so I picked up a used one for $25. That amp still works fine and the same tube, which was manufactured in 1961, is still in it.

[K9STH]

Here are the specifications on the 8503:

http://tubedata.tubes.se/sheets/084/8/8503.pdf

They were designed for pulse modulator service. As such, I don't have any idea as to how they would work at 28 MHz.

Glen, K9STH

[KO6WB]

Tubes are infinitely most forgiving then MOSFET power transistors are. There some types of tubes that are fuzzy and lose it if operated incorrectly. There are time limits on how long you can make a mistake with a tube unlike the MOSFET's that pop in a microsecond tubes can survive incorrect operation in the seconds and sometimes the minutes of time. It'll depend on what perimeter you exceed and the type of tube construction. Sweep tubes do not have a high dissipation on the plates and as a result they cannot withstand a long duration of tuning at full power. They also cannot endure any high duty cycle mode without a substantial reduction in power output. They do work well with the PEP signals of SSB. The tuning time for most sweep tubes is about 5 seconds or less. Doing that will improve the operational life time a lot.
Tubes seem to handle higher SWR easier then any solid state unit can (at least for now). Tubes can also load into a greater variation of impedances. The tube inputs are subject to varying changes while tuning the plate and load and as such for a modern solid state rig a tuned input is used. Most of the older tube final rigs have little or no trouble loading into an amplifier that has just straight cathode feed.
Hope this helps
73
Gary

[WB2WIK]

Here are the specifications on the 8503:

http://tubedata.tubes.se/sheets/084/8/8503.pdf

They were designed for pulse modulator service. As such, I don't have any idea as to how they would work at 28 MHz.

Glen, K9STH

I see from that it's a hydrogen Thyratron. Probably not a good candidate for an RF amplifier.

I'd make a lamp out of it.

[WB2WIK]

Tubes seem to handle higher SWR easier then any solid state unit can (at least for now). Tubes can also load into a greater variation of impedances.

That's mostly due to the fact that tubes are high voltage devices so their plate impedance is very high compared with 50 Ohms, and the output matching network won't transform reflected energy back into them.

Unfortunately, SS devices including FETs have a very low output impedance and the output matching network is usually a broadband transformer and not a critically tuned network.

[KF5OBS]

I'd make a lamp out of it.

Rather not after looking at the filament current I thought those tubes were used as driver tubes on a VHF TV station. At least I am pretty sure that is where I pulled two of these out and the rest were in a box around that old transmitter.

[WB2WIK]

I doubt it. Probably, and more likely, used as switches for the transmitter.