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  #1  
Old 19th April 2016, 04:21 PM
Tweeky Tweeky is offline
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Default gm-70 help

I have a Liang 845 amp with 300B and 6J4P drivers. It has been converted to GM-70 output tubes and fitted with Tentlab Heater supplies and 6AC7 tubes. One channel is currently down. Failure symptons were loud cracking/shorting noises with visible arcing under pre-amp pcb followed by total channel failure. Arcing appeared to be in area of 220uF cap connected to 6AC7. Any suggestions would be welcome.
Also, I have read that the cathode resistor value should be changed when the 6J4P is changed to 6AC7 but to what value?

Thanks in advance

Steve
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  #2  
Old 19th April 2016, 04:52 PM
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andrew ivimey andrew ivimey is offline
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Default Re: gm-70 help

I'll be the pessimist here - A Liang amp is max-ed out in the current and HT supplies. It will get too hot and components will fail eventually. One person I know stuck long legs on the underneath and mounted two fans to cool things down. The amp lasted four years. Two others I have know have lasted about three years before the PSU conks out.

It is a big chassis and very heavy amp but it is still too small for the complexity and budget of such a manufacture.

I'd rip it apart and build a simpler GM70 SE amp. For example, my GM70SE is smaller but has a completely separate 'off-board' 800volt power supply. There is only one choke loaded input/driver valve high Mu (12GN7). As the circuit stands it is very good at this voltage. I have tried 450 volts to over a thousand and this seems right to me. I mounted the output transformers on the bottom of the amp chassis giving me room for a couple of 6amp switched mode computer (switchable fan assisted) PSUs. Things get comfortably warm but nothing is stressed and the amp really is well used, well worn and appears to be a very happy amp.
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Old 19th April 2016, 06:32 PM
Tweeky Tweeky is offline
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Default Re: gm-70 help

Andrew,

Thanks for your quick response. A total revamp is beyond my capabilities. My amp has been in this configuration since December 2009 so has lasted pretty well so I would like to resurrect it in its present guise if possible.

Regards

Steve
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Old 19th April 2016, 06:43 PM
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pre65 pre65 is offline
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Default Re: gm-70 help

Quote:
Originally Posted by Tweeky View Post
Also, I have read that the cathode resistor value should be changed when the 6J4P is changed to 6AC7 but to what value?

Thanks in advance

Steve
I'm sorry to say this, but if you need to ask this question then you don't really understand enough about how things work.

Changing the cathode resistor depends on the current you need (in ma) and the bias voltage you need at the grid (G1) in V.

Once you know what these should be then ohms law will tell you the cathode resistor value.
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Old 19th April 2016, 07:03 PM
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Default Re: gm-70 help

I looked at the data sheets for both the valves you mention, and they are not straightforward replacements. The pin layout is different for a start.

It would suggest that the cathode resistor for a 6J4P is 68 ohm and one for a 6AC7 is 160 ohm but DONT take those values as gospel.

A diagram of how the 6AC7 is wired may help.
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Old 19th April 2016, 07:16 PM
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Default Re: gm-70 help

I have built a GM70 amp in the past (similar to Andrews) and currently use the GK-71 in triode mode which is very like a GM70, but not identical.

I'll try and help you if I can, but it depends on your understanding of the amp and the modifications carried out.

As a start, what is the value of the cathode resistor at the moment ?
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Old 19th April 2016, 08:18 PM
Tweeky Tweeky is offline
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Default Re: gm-70 help

Philip,

The 6AC7 replaced the 6J4P with no wiring changes. I can follow circuit diagrams ok but don't have lots of technical knowledge as you obviously do, having followed your numerous projects with interest over the years. I will try to attach the original circuit diagram for you. There is a 470 ohm resistor in parallel with a 220 microfarad capacitor connected to the cathode of the 6AC7 tube.

Regards

Steve
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Old 19th April 2016, 08:37 PM
Tweeky Tweeky is offline
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Default Re: gm-70 help

Philip,
Having checked the pcb, on my unit r102 has been replaced by a direct link.

Regards

Steve
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Old 19th April 2016, 08:44 PM
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Default Re: gm-70 help

Right, there seems to be more than 1 data sheet for the 6J4P, and the one I looked at was different wiring to 6AC7, but I've found another data sheet that says the pin out is the same, so that clears up that matter.

As you say, the cathode resistor is 470 ohm, BUT there is also a 110 ohm resistor in series with it.

Sorry to say this is outside of my area of experience without more information regarding the original operating points of the 6J4P and the currents and voltages involved.
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Old 20th April 2016, 09:33 AM
Tweeky Tweeky is offline
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Default Re: gm-70 help

Philip,
Ok thanks. Perhaps someone else can help?

Regards

Steve
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