Dynatron RG24 Marlow

[zenith1950]

Got one of these in very good condition, and fired it up with a variac. All went well until a puff of smoke came from the transformer, which wiped out the rectifier valve and the resistor that is connected between the HT centre tap and ground. I assume this is to provide some negative bias.
I cannot get a diagram for this, and have tried various values between 100 ohm and 500 ohms. These work but the resistor gets very hot.
Transformer voltages are all OK and well balanced, and the amplifier works very well. All electrolytics have been replaced.

Help please

[John Caswell]

Hi Zenith1950,
Further information please. The various units should have small metal labels affixed giving the type nr. of the unit, e.g T10A, LF11 etc.
I suspect faulty electrolytics here.
Photos will help immensely

John

[Station X]

The Dynatron RG24 has been discussed in the forums before:-

https://www.vintage-radio.net/forum/...ad.php?t=44627

In that thread it was suggested that the circuit might be similar to the RG21. I have looked at the RG21 circuit, but cannot see the resistor you mention.

If the resistor does control the bias voltage to the output valves, then you could check what value of resistor gives the correct voltage, then chose one with a suitable power rating.

[Station X]

Can you identify the overheating resistor in this picture and tell us what type of rectifier valve is fitted.

[zenith1950]

The rectifier valve is a GZ34S and is new. The electrolytics have been changed.
The component in question is between the two green power resistors at the right hand side.
My model does not have these two resistors fitted.

Thank you all

[Station X]

The red wires on pins 2 and 8 are the heater supply to the rectifier. Given that the pink wire emerges from the same grommet I'd say it's a centre tap on the heater winding. A continuity check would prove this. The pink wire goes to pin 5 of the rectifier, which is a spare tag and thence to chassis via the resistor.

If the tap is on the heater winding its function is to reduce hum.

Was there nothing left of the resistor to enable its value to be determined?

[ms660]

I could be wrong but the component between the two enameled resistors shown in the picture looks like a thermistor.

Lawrence.

[Station X]

Thinking about it, there wouldn't be an earthed centre tap on a rectifier heater winding as with a GZ34 rectifier the winding would be at full HT potential.

If it's a tap on the HT winding I'd expect to see a fuse in the centre tap connection.

[zenith1950]

Thank you for your replies

The pink wire on the rectifier tag is the HT centre tap.
Resistance 74 ohm one way and 72 the other, but when powered up an equal 270v each way with the transformer just luke warm.

HT at the rectifier is 190v.

Output ecl86 around 170v anode

Thanks again

David

[ms660]

From what I can make out from the picture, the HT is fused on the cathode side of the rectifier, a red wire from the rectifier's cathode goes through a hole in the chassis at the top right to a fuse mounted on the top of the chassis, to the left of that hole is another hole where a red wire returns from the other side of the fuse and connects to the reservoir capacitor and the HT filter resistor.

Lawrence.

[Station X]

Well spotted. Hopefully the OP can confirm this. If so the overheating component must have originally been a thermistor as you said. NTC to reduce in rush current or PTC to reduce current in the event of a fault?

[ms660]

Ref. Post#1:

Looking at the picture that's been posted the bias for the output valves appears to be cathode derived (180 Ohm cathode resistors)

The more I look at the mystery component the more it looks like a thermistor as previousely stated.

Some CZ type thermistor specs here:

http://www.electrojumble.org/DATA/ST...s_CZ_Types.pdf

You could arguably replace it with a resistor but it will dissipate a fair bit of power so a resistor should be rated accordingly, as a stab in the dark I would try a 150 Ohm resistor and measure the HT and then juggle to suit.

Lawrence.

[Station X]

This eBay item looks similar:-

http://www.ebay.co.uk/itm/Vintage-60...p2047675.l2557

No sign of a fuse, but that's definitely an NTC thermistor.

[Station X]

Another one:-

http://vintage-radio.net/forum/showthread.php?t=24022

[ms660]

Maybe the fuse and holder was fitted as an afterthought by someone, the holes where the red wires go through to the fuse appear to have some burr material around them, also no grommets fitted.

Some pics of the OP's amp might clarify.

Lawrence.

[ms660]

In the 1st pic posted, I woudn't mind betting that the two WW resistors and the tag strip aren't original, they look too new, why would the manufacturer fit limiting resistors when the thermistor is already doing that.

Lawrence.

[zenith1950]

Thanks for all this information.
Although the thumbnail is the same model as mine. Mine does not have a fuse.
I have just powered up with the ecl86's removed. 'resistor' ran cool.
Then changed all of the coupling capacitors (they checked OK on capacitor tester!)
The 'resistor now runs sigificantly cooler.
There ate a couple of electrolytic decouplers. Will replace them also.
Then will need to source a thermistor.

Thanks for all the help. Will repost when capacitors fitted

David

[ms660]

This "might" be the original type of thermistor:
Lawrence.

[zenith1950]

All caps replaced now and amplifier running OK.
Transformer is just warm. Very grateful that this was taken out.
Our consumer box RCD trips out when I use my variac and that probably was the cause; applying full power too soon.
Last time I take a chance!
Anyone know where I can get a thermistor?

Thanks again for all of your help

David