Almarro a318B

[Enochrome]

Greetings,

I have an Almarro a318B that uses 6c33c tubes. The right channel began making popping sounds. Before that it would not bias down to 180 (the required bias) and could not be biased under 220. After the popping sound, the channel stopped making sound altogether.
I ordered new tubes. Installed and ready to bias, the right channel is now not reading at all?
The tube powers on but does not read. I figured a fuse had blown but I cannot find another fuse inside besides that main fuse located outside the chassis near the power cord.

Incidentally, I had this problem before and took it to a tech and he fixed it and said that it was a fuse. He said it was hard to find. I noticed when hooking up the amp after getting it back that the bias was still high.

Any ideas? Anyone familiar with this amp, that might know where the fuse is or if there is an additional one at all?

Thanks

 

[JonSnell Electronic]

There is a 1R fuseable resistor in the cathode circuit for the output valves. With a DC voltmeter there should be about 75Volts on the cathodes which equates to about 1250mV across the 1R resistor. The Bias needs checking frequently!

 

[JonSnell Electronic]

This may be useful to you, a bit old hat but who knows;
http://www.diyaudio.com/forums/tube...imple-modifications-improved-performance.html

 

[tubetvr]

about 1250mV across the 1R resistor

I wouldn't believe so, 1250mV equates to a current of 1.25A which is way too much, around 200mA which would be 200mV over the 1 ohm resistor is a typical operating point for 6C33C in SE configuration.

It does't have to be something wrong of a 6C33C that doesn't bias correctly in this amplifier, 6C33C spread very much, (the electrode distances are so small it is difficult to keep them exact). I posted a graph earlier showing grid voltage variation from below 40V to almost 80 V for the same current.

The problem when you buy just 1 or 2 tubes is that you never know how they will behave.

Aside of this issue there is a probably a broken fuse or connection somewhere, I am not so familiar with the Almarro but some fuses looks like resistors with wires, try to trace the connection from the anode of the 6C33C through the output transformer and all the way to the power supply, the fuse must be there somewhere.

 

[JonSnell Electronic]

I wouldn't believe so, 1250mV equates to a current of 1.25A which is way too much, around 200mA which would be 200mV over the 1 ohm resistor is a typical operating point for 6C33C in SE configuration.

It does't have to be something wrong of a 6C33C that doesn't bias correctly in this amplifier, 6C33C spread very much, (the electrode distances are so small it is difficult to keep them exact). I posted a graph earlier showing grid voltage variation from below 40V to almost 80 V for the same current.

The problem when you buy just 1 or 2 tubes is that you never know how they will behave.

Aside of this issue there is a probably a broken fuse or connection somewhere, I am not so familiar with the Almarro but some fuses looks like resistors with wires, try to trace the connection from the anode of the 6C33C through the output transformer and all the way to the power supply, the fuse must be there somewhere.

Whoops should have typed 125mV!

 

[tubetvr]

Whoops should have typed 125mV!

I just saw now that you wrote fuseable resistor, maybe that is the key to the problem and should be easy to check by removing tube and measure resistance of the 1ohm resistor.

The poster wrote about biasing to 180 which I interpreted as mV over the 1 ohm resistor.

 

[Enochrome]

Thanks everyone! That sounds like what the problem is. I have no experience with tube circuitry and have reservations working on it due to safety reasons. I will take it to a repair shop. I just wanted to know what the issue was before hand. I should learn how to correct this issue because it would be the most frequent problem due to bad tubes. Do you think it might be something beyond bad tubes that is causing the bias to drift?

Does anyone know a reliable source for 6c33c's? I have been only able to find them on Ebay.

I really like this amp (except for the heat) so I would like to keep it in top form.

Thanks again! This site is great.

 

[SoCratz]

I end up buying them off Russian/Ukranian sellers on eBay. The prices is OK and when I have gotten a bad one, I have not had any problem with them sending me a replacement. Domestic sellers who offer fully tested tubes that are matched are asking for $100-$140 each tube.