jcm800 50 watt max dc current of OT (ClassicTone)

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Haze13
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jcm800 50 watt max dc current of OT (ClassicTone)

Post by Haze13 » Fri Jun 03, 2016 1:48 pm

Hello! I've been wondering... Every body bias the amp basing on parameters of the tube, but what about the Transformer? How much DC current it can handle? I use ClassicTone OT, but since it's direct replacement, it has to be like an original transformers.
So... How much?

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Re: jcm800 50 watt max dc current of OT (ClassicTone)

Post by Haze13 » Sun Jun 19, 2016 4:16 pm

I've got an answer from the ClassicTone about this and here what they told me:
The maximum primary DC current for our 40-18025 is 250mA (per half).

Do I understand it correctly? if pair of tubes will take like 250 on peaks, the transformer will hold...
So it's passable to use this transformer and to bias tubes higher if the tubes will take that... WITHOUT FEAR.
Like, a pair of 6550 with a bias higher than 70mA per tube :) Tubes love current, wright?

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Re: jcm800 50 watt max dc current of OT (ClassicTone)

Post by danman » Mon Jun 20, 2016 5:12 pm

I am not an expert on this matter so if I am wrong, someone please correct me. When you mention biasing at 70ma, that is only idle current that you are referring to. I could not find the 6550 in my RCA tube manual but I did locate the specs for EL34. They list the tube's maximum signal plate current at 143ma. This is the maximum current that the tube is able to pass under full signal load, but I doubt most amplifiers are designed to push the maximum. Again, I am no expert but I believe that this is the spec that you need to consider when choosing the proper PT for a circuit.

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Re: jcm800 50 watt max dc current of OT (ClassicTone)

Post by Roe » Tue Jun 21, 2016 7:22 am

yes, an the fuse should be max 0.5A slow blo
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Re: jcm800 50 watt max dc current of OT (ClassicTone)

Post by Haze13 » Wed Jun 22, 2016 8:28 am

Well... The RCA, Thung-Sol and other manuals for the 6550 state that this tube can give up to 20 watts single ended. I know for sure that there is no such amp, and the max what people could get from that tube is like 15 watts, and that's with an old tube. New tubes are very different and have different curves...

What I was thinking, is that when you build an amp, you look at the spec of the components, and for some reason, companies are not giving you the whole information about "Replacement transformers". The thing is, that when you biasing an amplifier you want as less cross-over distortion as possible. I've worked on a JCM800 BASS 4xEL34 (1992) and that "Knee" goes away when there is more than a 55mA at idle, but I don't know if the OT can handle this, because there is a max DC idle current that transformer can pass...

I have JCM800 2204, and I want to try different tubes, and there are options to run Hi-Power tubes with higher current and lower voltage, but the bottle-neck is a OT. How MUCH is enough current? If your OT can live happy with 70mA or more per tube at idle and tube can hold (25 watts for el34, 32 watts for KT77) than you can just use higher power tubes and bias properly... Tubes love current, so why not?. But I don't see that thing happen, because the only thing that people think when they bias amp is a Tube and the current of an earlier tube that was there. What is the point to use 6550 with 40mA at idle when it can take more?

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Re: jcm800 50 watt max dc current of OT (ClassicTone)

Post by Carbia » Wed Jun 22, 2016 11:39 am

There's many factors involved.

I've done bias adjustments with the multimeter in one hand and the scope in the other and it depends on the amp you're working.

I have a Fender Bandmaster that doesn't have crossover distortion with a VERY COLD bias, and, in fact sounds better with a cold bias adjustment (no very cold, but around the 55% of max plate dissipation) than with the usual 65-70%.

My Marshall, on the other hand, sounds better around 75% of plate dissipation, even with a little crossover distortion. I have to go to near 80% to be free of distortion, but sound worse that way.

To me, the rule is, set the bias where the amp sound the best if you're on the safety margin of power dissipation.

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Re: jcm800 50 watt max dc current of OT (ClassicTone)

Post by Haze13 » Wed Jun 22, 2016 2:02 pm

Fenders are really different amps! Twin Reverb Ri stays clean up to ~75 watts, the Dual Rectifier with 6l6GC's (bold setting, SS rectifier) begin to clip after ~55 watts and even less with a tube. Haven't tried it with EL34 so if the transformer was designed for EL34, may be it will stay clean even more.

Carbia, you are right about the listening... Every one have different taste, so I think it's possible to raise a current and listen if the tube will sound better/preferable and not being afraid to burn up the transformer, just to be careful:)

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