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Author Topic: bias rite  (Read 2398 times)

MDV

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bias rite
« on: January 08, 2008, 11:14:06 PM »
Anyone know a UK dealer?
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HTH AMPS

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bias rite
« Reply #1 on: January 08, 2008, 11:49:04 PM »
I think Hotrox sell them.
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LazyNinja

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bias rite
« Reply #2 on: January 08, 2008, 11:54:02 PM »
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MDV

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bias rite
« Reply #3 on: January 09, 2008, 03:36:02 PM »
Cheers HTH, hotrox sorted me out with something basically the same.
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HTH AMPS

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bias rite
« Reply #4 on: January 09, 2008, 08:19:45 PM »
Glad you got sorted.  

btw, where in Cumbria are you? - is Newcastle too far to drive? - I can always bias your amp if you want.

 :twisted:
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MDV

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bias rite
« Reply #5 on: January 11, 2008, 06:46:30 PM »
Thanks dude, but I got the bias thing now and I'll try and do it myself.

If I fail, then yes, I'm a fraid newcastle is a fair trek when I dont drive, but thanks again for the offer.

Edit: Oh, I'm in Whitehaven: pretty much as far in cumbria from newcastle as you can get!
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MDV

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bias rite
« Reply #6 on: January 15, 2008, 01:05:53 PM »
OK, I got the biassy thing.

I did some biasing.

But I basically had to guess the plate voltage based on other engls (400 to 500V)

I think I biased it, judging by the sound, a little cool (its a bit grainy, it sounded searing and tighter with higher cathode current, but the numbers were getting pretty big and I got scared: I left it at 47mA)

The book that comes with it isnt explicit on the matter, but it SEEMS to be suggesting that I take the tubes out, turn the amp ON, and measure the plate voltage between the 3rd and 8th pin in the socket that the valve goes in.

I wasnt gonna do that without being sure.

Can I turn my amp on without valves in and have it not fry? Or should I figure out what points correspond to what pins in the socket on the reverse side of the PCB (amp construction snobs please remain silent on that!!!) with the valves in?
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LazyNinja

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bias rite
« Reply #7 on: January 15, 2008, 01:39:50 PM »
You measure the plate voltage with the tubes in. Clip the negative to ground (chassis) and measure the voltage on pin 3. Careful not to touch any other pins at the same time or you'll short the valve and blow the fuse. Scared the cr@p out of me when that happened to me :lol:

400-500V sounds about right though. It doesn't necessarily mean the hotter the better the sound so it could be that you biased it too high? I think high gain amps tend to have quite cold bias (mesa etc).
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MDV

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bias rite
« Reply #8 on: January 15, 2008, 02:26:42 PM »
Thanks Ninja. Very helpfull!

I'll do that and calculate the correct bias this time  :twisted:
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Oli

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bias rite
« Reply #9 on: January 15, 2008, 02:53:08 PM »
If you take out all the power valves, there is no issue with the transformer frying- there is no connection to both ends of the primary on the transformer, so you have no circuit (there will be a connection to one end of the primary, so be cafeful, you'll have a value of 300v-400v or so).

I would guess that you could use an amps preamp on it's own (without having any output to a cab), but pulling all the valves and using the FX send or the preamp out, and sending that to somewhere else.

Just be safe!!!

Read http://www.paulrubyamps.com/info.html#FirstPowerUp for more advice and technique on amplifier stuff, it is an essential article!
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jpfamps

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bias rite
« Reply #10 on: January 15, 2008, 04:59:52 PM »
It is perfectly safe to remove the power valves from a modern amp with solid state rectifier. If you want to measure the B+ with the amp on in this state I would clip the ground lead of you multimeter to the chassis and test the voltage on pin 3. I would NOT try poking both leads into the valve socket.

If your amp has a valve rectifier this method won't work as well. This is because the current drawn by the power valves causes a voltage drop across the rectifier, so without the power valves in place there will be a significant rise in the B+. In an amp with a solid state rectifier the voltage drop when the power valves are in situ is much less, perhaps  5 V or so, so you should be fine using the above method.

The more sophisticated bias probes also have the facility to measure the B+ as well as cathode current. I have made my own bias probe to do this however I would NOT recommend this as a project for someone with minimal experience of working with high voltages.
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MDV

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bias rite
« Reply #11 on: January 17, 2008, 07:20:44 PM »
Thanks jp, good to know.
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