5150 II tube head with hot bias

[costis_n]

Originally Posted by p0ultry

Right, but like I said it is modded to allow adjustment on the bias as they come biased very cold stock. This was done by an amp tech of the previous owner, not by myself, so unfortunately I dont have those specific schematics. But in general, if my tubes are a fixed bias of 37mA, and I used the same tubes to replace the 6L6's currently in there, would I have to adjust the bias again when the new tubes are installed? Its basically the difference between taking it in to have someone pop new tubes in for me at substantial cost, or just removing them and replacing them myself?

I think you didn't mention that it's been modded. Does the mod have any means to check current at each tube (for example a low-Ohm resistor on pin 8 to ground) ?

[p0ultry]

Originally Posted by costis_n

I think you didn't mention that it's been modded. Does the mod have any means to check current at each tube (for example a low-Ohm resistor on pin 8 to ground) ?

doh! you are correct, i ommitted that part. honestly, I do not know the answer to that question. I know the bias on 5150II/6505+ is adjustable stock, but not as hot as some like it. the mod basically makes it so you can bias hotter.

[famousmockingbird]

Plate voltage is also a variable in figuring out plate dissipation. How do you know the tubes are pulling 37ma? Do the power tubes share a common low ohm resistor in series from cathode to ground? You are not giving us enough information to help you.

[p0ultry]

Originally Posted by famousmockingbird

Plate voltage is also a variable in figuring out plate dissipation. How do you know the tubes are pulling 37ma? Do the power tubes share a common low ohm resistor in series from cathode to ground? You are not giving us enough information to help you.

yikes, the more the experts ask, the more I know that I dont know..its sounding like I may just want to take it in.

I have no physical means of proving the mA draw, just the information that was provided by the seller.

Thanks anyway guys.

[costis_n]

But how much would a tech ask you for doing this for you? (excluding the tubes)

[p0ultry]

Originally Posted by costis_n

But how much would a tech ask you for doing this for you? (excluding the tubes)

Not cheap. For labor, probably in the neighborhood of $100-150 USD, then figure about $40 per tube.

I had assumed, perhaps prematurely, that since the bias was already set, if I just needed a new set up power tubes, would I be able to just pop out the old set, pop in the new, and then power up without having to re-bias anything, assuming that I was able to procure identical manufacturer/model. Its sounding like there are a bunch of other factors to consider.

[costis_n]

150$ !!! I want to immigrate!

Seriously, you sure can pop in a new set. Espexially if they re the same make, nothing can go too wrong. Just buy them matched, and don't lose your sleep anout that.

Do some homework, too, seems it will really pay off

Tales From The Tone Lounge:Biased Opinions

[p0ultry]

Originally Posted by costis_n

150$ !!! I want to immigrate!

Seriously, you sure can pop in a new set. Espexially if they re the same make, nothing can go too wrong. Just buy them matched, and don't lose your sleep anout that.

Do some homework, too, seems it will really pay off

Tales From The Tone Lounge:Biased Opinions

wow, thanks! that link had a lot of great info, most of which I will have to read after work it seems. cheers!

[kevinkr]

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