Hello all,

Well, I have finished replacing out all the paper type and electrolytic capacitors.

Applied power via a current limiter and was pleased to hear noise from the speaker.

With no smoke escaping and current draw within design specs, removed the current limiter to allow full power to be applied.

Was able to tune into a station and hear audio, very pleased.

But, after a few minutes the audio started fading away to next to nothing. Turned it off and allowed it to cool. Tried again and sure enough it did the same thing.

Isolated issue to the 6BV7 detector and audio driver tube, when replaced, the fading issue did not occur. Was able to repeat the issue by swapping the two tubes.

So, my question is, is this possible for a tube to degrade as it gets hot, or do I have an underlaying issue?

Regards

Lloyd

Could be just the 6BV7, as they were not a popular nor reliable tube in their day.
Incorrect bias to the control grid would cause problems after running for a while.
Would pay to check bias voltage and associated resistors, starting at the centre tap of the secondary winding on power transformer to G1.
But feel it's probably just the valve in this case.
BTW, I don't believe in just changing all the caps etc.
It generally is asking for problems that may not have been there before.
You sound experienced but I would suggest. A good visual inspection, replace obvious problem components, fire up with current limiting,
then fault finding to get unit going.
Then in the interests of safety, reliability and performance replace all paper caps and electros.
JJ

I've had 6BV7 valves that have gotten very hot and then malfunctioned. When I say hot, I mean very very hot. My stock of this valve is quite low, so I've resorted to making a bias adjustment so it runs cooler, and it then keeps working.

But if your valve is at normal temperature and is playing up then it's most likely had it.

Some members get quite irate when modifications are discussed, so it's something only as a last resort.

There should be little resistance [pun not intended] to such a modification if it is to preserve a radio and make it more reliable rather than let it overheat and destroy itself. Radio broadcasting in Australia turns 100 next year. We are approaching the stage where we will ultimately have to do what we can with the available parts that are left to keep these things going and this should be supported by all.

I personally have no problem with it. Turning a telly into a fish tank and throwing the innards on the tip is a different kettle of fish, again, no pun intended. This isn't preservation, just vandalism.

Thanks Brad.

Most radios with a 6BV7 are much the same: 6BE6, 6AU6, 6BV7, 6X4. There's a negative power rail which supplies the 6BV7 grid bias, and also the AGC base voltage.

The datasheet for 6BV7 specifies the power-rail resistor (transformer common to ground) which acts as a defacto cathode bias resistor, to be 90 ohms. Most radios use 100 ohms. However this generally leads to overheating (but not always). So I tend to increase this resistor to 150 or 200 ohms. The difference will be quite apparent, the 6BV7 will run noticeably cooler, and the HT may increase by as much as 50 volts because of the reduced standing current. I had a 6BV7 that got so hot that after a couple of minutes the cathode would short to G1, causing even more overheating. After the modification it works just fine.

I think that seeing these valves are not made any more we have to do anything to keep them working, especially when it only involves changing one resistor.

A possible side-effect is the changed negative rail causing the AGC to make the radio less sensitive. I haven't looked into this, but you might want to alter some resistors in the AGC circuit to get the best performance. I personally haven't noticed much difference with the modification, but it's up to you.

After having said all that, I've got one of those radios, and it was the only one fitted with 6BV7 to not have heating issues - in fact it works beautifully.

Thank you for the quick responses to my question and your suggestions. 😊👍

Yes, whilst I feel experienced and up for the challenge, I have much to learn from the more experienced persons as yourselves. I find them fascinating.

I offer the following points as answers.

When I first received the radio, I found most of the black paper type capacitors bodies were cracked, so changed them out for safety. Out of circuit testing found most of them still to be close to the stated capacitance, but two had gone very high, one was nearly twice (C10) what it should be?

The electrolytics had ESRs of 0.55ohms and 0.35ohms.

The 6BV7 was fingerprint removing hot, whilst the others were warm/hot to the touch. It also glows a lot brighter than the others.

Since getting it going, another observation has been that while running and listening to the only station I can currently receive, occasionally there will be a crackle and the volume will jump up briefly and then drop down again. I have been putting this down the signal coming under atmospheric disturbance and my extremely poor antenna, especially given if I touch the antenna lead it improves the reception. Or is this another possible issue with the radio itself?

On the subject of antennas, whilst I understand the best option would be to run a long wire external antenna, that option is not possible as my partner has forbidden it as it will detract from the visually, please appearance of the house.

So, I have been looking for the best option for something contained within my workshop, which is located at one end of the house structure, so I am not enclosed with a metal shed. Any suggestions.

Thank you all.

Lloyd

A couple of options for antennas

A loop antenna, plenty of designs available online. Some companies do make them as a kit or even completed

Run a wire around the skirting board or picture rail, inside your workshop

Find a new partner willing to put up with your radio addiction/madness.

May pay to check/replace C17 100pF inside 2nd IF tx.
Have found recently many 100pF micas with substantial DC volts on them to grow whiskas (int leaky) and cause exactly what your experiencing.
Crackling and by detuning the IF causing volume changes. Possibly cap across primary of 1st IF also. Another 100pF mica.
JJ

The 6BV7 was designed by AWV, It was never 2nd sourced by anybody although you will find some branded Philips.

I have seen that thermal runaway you describe. Later examples in the taller glass seemed to be more reliable.

Whatever the restoration purists say, 6BV7s are now very rare, I like the idea of using the 6GV8 circuit out of the Kriesler 99.

I have no problem at all with replacing old caps - especially ones that are cracked, or covered in wax.

Many old radios have gigantic 25μF 40v electros on cathodes, and even if they work I still replace them, as suitable ones are cheap and plentiful. It's only the high voltage ones that can be expensive or hard to get, and so it can be worthwhile attempting to resurrect old ones.

As for antennas, since we're talking AM, the antenna doesn't need to be a defined size or shape, anything random will do, the longer the better. You could try around the workshop near the ceiling, or run a wire to a large metal structure such as a garage roll-a-door, a gutter, or even a metal fly-screen. They all work well.

Volume erratic, could be the aerial connection, or a mica cap, as Johnny suggests. Any mica cap with many volts across is suspect.

It sounds like your 6BV7 might be too hot, so perhaps you could try the modification and you might find the old valve works again. You never know until you try.

Yes, it was very clever of the bean counters at Kriesler to suggest the designers use up the now redundant thousands of 6GV8's from B/W TV days.
And kept on the aging hard wire assembly personnel to produce the last valve radio in Australia.
And a top little radio as well.
JJ

Mica caps are not all the same. It is rare from the mid thirties to get failed Mica caps. However, in five+ decades the only mica caps I have seen failed in an IF were mechanical,due to instalation. Most in IF's have high voltage on them, not across them.

One particular design used by AWA & open, waxes & jacketed, is virtually indestructable. The Silvered Mica is one the have a huge amount of problems with in USA more so than here.

Some tubes had Silver on their pins and that can migrate. Dirty, corroded & ill fitting tubes can also crackle & hiss.

Most sets needing an external antenna were designed for 25 feet. Current one here is around 30 metres on poles, utilising traditional fenceing wire (2.5mm) and electric fence hardware. N.B. That stuff is good for around 10KV.

Marcc

I just found my notes of my 467MA. The aerial wire joins to the ferrite with a small clip - there's no solder. You might want to make sure the connection at that point is consistent.

Thank you all for the feedback and information, will digest and work through it.

Will follow up with findings.

Kind regards

Lloyd

Yes Kriesler were using the 6GV8 right up until the 49-7 (all solid state) TV appeared.

Thousands of 6GV8s were rejected on the TV line for vertical hold drifting off during warmup. Philips refused to replace them, claiming it was due to Kriesler's design. But they worked just fine in the audio stage of a radio.