Hi,

I am new here and I have got a valve radio.

It has 4 valves, they are 6AN7, 6N8, 6M5 and 6V4.

When the radio works, the glass temperature of the 6M5 is near 120C, I am a bit concerned it is too hot.

Is this normal for valve radio?

Thanks

Kevin

G'day Kevin,

The 6M5 is a power amplifier valve and whilst I am not sure of the exact temperature valves can reach I know that these valves and some rectifier valves can get quite hot.

I wouldn't worry about it unless there are other problems developing such as smoke or an electrical smell.

It is good practice to keep valves clean though.

The 6M5 is one valve that when I hear its name I immediately think "issues!" I've have one of them crack its glass envelope. I researched this some years ago when I was trying to find one for my Kriesler Duplex (6M5s are not easy to find these days). Here's what I found on an audiophile forum:

"The 6M5s had a bit of a problem, they tended to draw grid current when they started to get old. I had 2 radios today, where the grid was about 4V positive. The cathode voltage went up to about 11Volts and the tube took a lot more anode current. It also got very hot. It is not a very nice looking tube, specially when used for a long time. The inside goes black and outside the anode inspection holes, a silver layer builds up."

The cracked one that I replaced was also showing signs of silver migration on the glass between the pins which would have created shorts.

6M5's are never a pretty valve and as you say GTC nearly always have issues. Being a uniquely Australian valve they're getting very hard to find. A search of this forum will show up previous threads with suggestions on how to use alternative valves, my favourite is the 6BQ5 referred to by Monochrome625 in this thread;

http://vintage-radio.com.au/default.asp?f=3&th=446#1644.

As long as pin 9 isn't used as a tag of convenience it's possible to wire the chassis to accept either a 6M5 or 6BQ5. In my opinion 6BQ5's sound a bit tinny but they are a good substitute and still being manufactured.

Re the temperature of your valves, it's quite normal for the rectifier and power valves to be too hot to touch but 120 degrees does sound very hot. The only way to know for sure is to check all the voltages and current draw against spec.

Hi Guys,

Thanks for the info.

Just wondering if anyone measured their valve temperature before?

I will try to check the voltage of the tubes.

There plate and screen voltage in spec, do I need to measure the grid voltage too?

What is the common reason for tubes to run hot?

Regards

Kevin

The more current the more heat. The cathode current is important here. Either measured across the cathode resistor or the bias resistor if it's biased that way. Once all values are known Ohms law will give you the answer.

I've never measured the temperature and with so many variables it's probably irrelevant. As Brad mentioned, smoke or a burning smell is very bad!

Hi Scraps,

Thanks for the info.

It is a Fleetwood 1003 radio.

I got the schema from the web.

There is no cathode resistor I think. The cathode is connected to the ground. So it is not easy to measure the current.

Any easy way to do that?

This my first valve radio so bear with me.

Regards

Kevin

R13 should be a 160 ohm 1 watt resistor that comes off the centre tap of the transformer and goes to the chassis. The circuit doesn't specify the total HT current but it should be no more than about 50 mA which would give you 8 volts across this resistor. If it's much higher than that, you have a problem and shouldn't run it until you find the cause - damage will result! All other voltages on page 1 should be within about 10%
If not already done, it would be a good idea to replace the two electros, C17 and 18 as well as any wax paper caps particularly C9, C10, C14 and C15.

Cheers,

Warren

Thanks Warren,

I have replaced all the capacitors and will check the voltages tonight.

Regards
Kevin

I've just had another look at the circuit, there should be 6.7 volts across R13 which would make it about 42mA.

If the valve is not embarrassed by being red in the face, that is at least a positive.

The recommended Rk for 6M5 250V is 170 with a current draw of 36mA + 5.2mA Screen 41.2mA = 7V, that is the grid self bias.

If you do not want to totally rewire and cannot get a 6M5 clear pin six and tie it to the cathode (3) and use the video valve 6CK6 which needs -5.5V grid and has the suppressor on pin6 whereas 6M5 is an internal connection.

If the 6M5 grid leak is connected to the CT. The voltage between CT and chassis (chassis is positive relative to CT)
should be the recommended volts for the 6M5 grid.

Thanks Warren and Marc,

I measured the voltage across the R13 and it is -6.73v, so it is ok.

The only voltage measured that is a bit low is the screen one. According to the spec they should be 55v, they a bit low at about 47v. And also the R2 and R3 connected together should be 25k, I checked and they are 38k!

Will this low screen voltage causes any problem like overheating valves?

Regards

Out of spec is out of spec and the screen grid can impinge on performance.

In my book if a resistor is 10% or more out, it lands in the bin. It is a moot point that considerable time, rework & frustration, is to be saved, by checking resistors & tossing out the duds as you change the caps.

Marc

Hi , I have read the 6GW8 can be used with minor wiring changes.

I plan to do this if the 6M5s are not good in chassis I repair.

Cheers
Vic

Changed out spec resistors and the difference is I don't need to turn the volume control up as much to get the same speaker volume.

And also the screen voltages for the 6An7 and 6N8 are up and at the upper limit of the spec.

the voltage across R13 that is the fixes bias for 6M5 is higher now at -7.1 voltage.

I measured the temperature again and it is now 140C at 2/3 position from the bottom.

don't know what to do about it next?

Also noticed the aluminium piece on the top of the back cover changed colour at the position where 6M5 pointed to. so it must be hot before.

Really feel uncomfortable to use the radio as it is.

Is changing the tube the only option?