Some may remember the 6BV7 was used in the 1956 Lt General from Radio & Hobbies. It ran exceedingly hot and burnt me a few times on that build.

In the end I couldn’t stand its failure rate so I got rid of it, the valve that is.

Interestingly later versions of the 6BV7 had longer glass envelops similar to the 6M5, I guess they were trying to fix a heat issue!

What are others experiences with the 6BV7?

This valve has been mentioned a number of times recently.

My experience with it is that the manufacturer often used insufficient bias, leading to overheating, excess power consumption, internal distortions and finally, er, death.

Adjusting the bias correctly (by whatever means) fixes those issues and makes it as reliable as any other valve. And it won't burn your fingers.

Given its rarity, I've only seen one of the taller variety.

QUOTE: This valve [6BV7] was made [by AWV] for domestic radio receivers. It contains a sensitive high slope power pentode to drive a loudspeaker. It was used in place of two valves, the first Audio Frequency (AF) stage and the power output stage.

It also contains two diodes, one for detecting the AF from the Intermediate Frequency (IF) stage, the other to provide an Automatic Gain Control (AGC) voltage for previous stages.

Usually the diodes were in the valve used in the IF stage or sometimes the first AF stage.

It was not a popular valve and had limited production.

As Quoted by The Manufacture July,1952

“Ventilation.

The envelope of Radiotron 6BV7 becomes very hot in operation, and free circulation of air around the
valve is necessary. “


Radiotronics July, 1952

In my build I had 225+DC on the plate of the 6BV7 and it was biased at -4.5v. so that would have been just under 30ma plate current,
that should have been pretty conservative data for the 6BV7 in the 1956 LT General.

I'll put up a picture of the relative size of the taller version, I'm still of the opinion they increased the bulb size because of heat issues.

Yes, just enter '6BV7' in the search forum on this site to get an earful!
I am no fan of the valve from personal experience.
Fred.

If you want to stick with that valve & its a heat issue & most output tubes & rectifiers run seriously hot; Consider fan forced air from a CPU fan, under it.

Or increase the bias.

An interesting stat - the rectifier and output valves were usually located side by side and at the rear of the radio to allow their heat to dissipate. All valves get hot but these two got hotter than most. It was an important consideration since many radio cabinets were inflammable.

Hot tubes and their placement, for me has been a constant problem. Many have a bad habit of destroying their sockets, which I replace with ceramic. On plastic sets they can eventually cause the plastic above them to lose plasticity & crack or distort.

I had dealings with a HMV 42-71where the last three tubes were inline & too close to the transformer. That act had cooked all of the insulation & surface wrapping of the transformer. Irrespective of what magnetic effects there may, or may not have been.

After repairing the damage, I ended up putting a heat shield behind & close to the tubes to act as a heat shield for the transformer. I often think chimneys & fans are now the answer to some of these issues. A scaled down version of what goes on with the PA of tube transmitters & some power supplies.

Getting back to the 6BV7...

In a 1952 appnote AWV recommended under-running the 6BV7.
Running at 250 volts with -4 volts bias the valve draws 40mA, which equates to 10 watts dissipation. No wonder it gets hot!
According to the appnote, with 180 volts plate and -4 volts bias it draws half the current (20mA) and delivers 2 watts vs 4.
The reduction in undistorted power output amounts to a 3dB difference, something that will be barely audible.

So, if your radio is stuck with, say, 250V B+, you can reduce the current the 6BV7 draws by increasing the bias.

Doubling the value of the back-bias resistor will do that.

Halving this to 20mA takes the power dissipation to 5 watts, with a corresponding heat reduction in both the OP valve and the rectifier. As well as general stress reduction right through the radio.

I would expect changes to the back bias to have an effect on all tubes. You may need to recalculate & add a conventional cathode resistor, for self bias. The voltage drop across the back bias resistor will tell you the current for the whole set: Unaltered.

Back bias is a diagnostic tool. All tube currents must be correct for it to be correct.

Marcus, the 6BV7 is a very high slope bottle. So doubling the back bias resistor will not double the total back bias as you might think, just increase it slightly, probably by about 1 volt max.

I could give you a more accurate figure but the available published curves for the 6BV7 are very sparse.

The AGC standing bias (depending on how it's implemented) will be tapped down anyway. There will be a small shift but not enough to noticeably affect the performance except maybe on very weak signals, because on reasonable signals the AGC predominates.Or it should if the radio is aligned properly.

Ian

Is this what you are talking about?

The 6BV7 Radio Valve

I am not intending to double the back bias. I was considering applying that to the other tubes only to maintain their design parameters and adding conventional self bias to the 6BV7 by adding a cathode resistor and bypass cap.

That would require reworking the bias resistance for them.

Much simpler to just double the back bias resistor and get the result with far less modification.