Help for newbie

[Whiting]

Hi All,

One of two amps which i purchased is not working ( came as a spare ). It appears to have some problems with resistors in the power section ? Both are the same make and model ( Linmark YA-130 A ) and appears to be Japanese built.

Can any one suggest what type and size / specs of resistors to replace them with and where in Australia can i purchase them ?

Tubes in amp are 6BQ5 x 2 - 12AX7 x 1 and 6CA4 x 1. From the little research i've done on the internet it appears to be single ended.

This would be my first step to refurbishing the one that's not working. Image 05 is of the one that works, the one that doesn't work has different items ( long sqare ceramic types ) and appears to have previous work done.

Thanks in advance.

[tubewade]

http://imageshack.us/

Use thumbnail for forums option.

[hotbottle]

Both Dick Smith Electronics (and Tandy - same stuff now), and also Jaycar Electronics sell resistors that can be used in valve circuits. Most resistors can be used, but I tend to not buy the really small low wattage ones, as they look silly, are hard to read, and there is a small danger of exceeding the voltage ratings. 1/2 or 1 watt resistors are what I normally use, but in power supply circuits, sometimes you need higher wattage parts.

I can't see your picture but...

The long square ceramic types you describe will be 5 watt or 10 watt usually, and their values and power ratings will be written on one of the sides.

[Whiting]

Thanks for your reply Hotbottle.

I've tried to attach images twice now without any success, however i'll try again now.

If anyone has any help on attaching images, that would be good.

Do you think the white ceramic one will be ok at about 5 or 10 watts in the required size ?

The ones that are in the working unit are approx 30mm in length, gray in colour and are hollow in the middle. Brand name on them is King with sizes 2 K Ù and the other is 100 Ù however not sure of the wattage.

[tubewade]

The 30 mm ones sound like 7 watt resistors. There would be no harm in using 10 watt ceramic ones. Then you must determine what damaged the original ones.

[hotbottle]

I agree, that those resistors are probably less than 10 watts. I would measure the voltage across each of them (be careful of the high voltage) on the working amp, and so calculate the power they are handling, then choose the next size up when you buy replacements.

[Whiting]

Thanks all for your replies.

I changed the resistor which was getting hot with a 2.5 k ohm of similar physical size to the one in the amp that works, however it is still getting hot.

As you pointed out Tubewade, there is obviously some other issues with some other part of the unit.

Should I replace the 5 watt resistor from the capacitor to the rectifier valve with a 10w rather than a 5 watt 100 ohm.

Does anyone have any other suggestions where to now, other than giving up and taking it to a technician, which would defeat the purpose of trying to learn something and improve the sound of this little amp.

Thanks for your help

[hotbottle]

You really need to measure the voltage on the same resistor in the good amp. Not only can you calculate from that, the power rating that you need to use, you can compare voltages with the other amp to try to find the problem.

[tubewade]

I would expect the 2k5 resistor to be part of an RC filter for the screen supply for the 6BQ5s and the anodes of the 12AX7s. Are you familiar with Ohm's law and can you calculate power from measured voltage across a known resistance? If the resistor is located where I think it is the current through this resistor should be around 15mA or so, which would result in a voltage drop of 38 volts. This is only 0.6 watts. If it is substantially more than this there is a short somewhere and I would suspect most likely a leaking filter capacitor. If you are comfortable checking voltages at various points in the amplifier this would be most helpful.

Wade

[Whiting]

Hi Hotbottle

With just a multimeter ?