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Michael_B

NAD. New Tubes. Need to Rebias?

Question

I bought a Marshall '62 Bluesbreaker reissue, purportedly in mint condition.  While the amp hasn't yet arrived, yesterday I unexpectedly received new tubes from the seller.  He chose to ship it without tubes.  Am I correct in understanding that I now need to have the amp rebiased?

My local shop has a three week turnaround on rebiasing and a shop in Denver (90 min. drive with traffic) says they can turn it around in a week.  Is rebiasing something I can easily do myself with rebiasing probes?  Do most/many tube amp owners know how to rebias their own amps?

Finally, was shipping without the tubes and sending new tubes a nice gesture or a dick-move that should be treated with some suspicion?  'Cause if I have to drive 90 mins each-way, twice, and wait a week to use the amp, it feels more like a dick move than a nice gesture.

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Posted (edited)
16 hours ago, tbonesullivan said:

Or.. go for cathode-biased amplifiers?

This is exactly one of the reasons why I have/use 18 watt Marshall clones, as their power tubes are cathode biased...I'm a 'home' player, so I definitely don't need anything bigger (they're LOUD when I want them to be with no effort beyond turning up the volume), plus they have decent enough headroom for my needs when played clean.  A Tweed Deluxe (or clone) is cathode-biased, too, the problem I have with those is that I wanna turn 'em up to 12 (check the control panel, that's what it says!) and go full Neil Young.  I suppose you can use them for clean somehow, but I can't help but turn Tweed Deluxes up, they sound too good cranked! 

Edited by crunchee
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That's pretty much the argument that Randall smith makes for having fixed resistor bias supplies on Mesa Boogie amps. I got my bias rite so that I could make sure I wasn't torching or freezing my amps, not so that I could mess around with the bias all the time. Others say they "bias for sound", but I don't have that level of hearing. I would think it's more of a feel thing anyway.

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13 minutes ago, tbonesullivan said:

Others say they "bias for sound", but I don't have that level of hearing. I would think it's more of a feel thing anyway.

You can hear the difference between a cold and hot bias, and feel the difference to a lesser extent. 

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On 3/12/2019 at 3:54 PM, tbonesullivan said:

Or.. go for cathode-biased amplifiers?

My only issue with a lot of the amps that have "built in" bias adjustment lights or indicators, is that they are often based on NGV, which is the same thing that you get from external test points, which just isn't a very accurate way of getting a bias reading.

Some of the amps do it right and have 1 ohm resistors from cathode to ground and the bias probes are connected to these.  That way if you measure the voltage it will give you the approximate current in mA.  Bias probe points done right are a very nice feature especially if the bias adjustment is on the outside of the chassis.

I've installed 1 ohm resistors to a lot of guys amps and it makes biasing a snap.

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On 3/13/2019 at 10:08 AM, tbonesullivan said:

That's pretty much the argument that Randall smith makes for having fixed resistor bias supplies on Mesa Boogie amps. I got my bias rite so that I could make sure I wasn't torching or freezing my amps, not so that I could mess around with the bias all the time. Others say they "bias for sound", but I don't have that level of hearing. I would think it's more of a feel thing anyway.

I've installed bias pots on a couple of my old Mark II boogies and been very happy about it.  The 6L6 boogies are biased way too cold for me.  What's strange though is that some of the el84 boogies are biased way too hot. Go figure!

You can tell a difference in the sound too.  Just put your bias rite on it and play until you like it and make sure that's not  biased too hot with the meter. Occasionally I found a warm bias to sound too muddy and backing it off even a little bit sounds better.

 

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Posted (edited)
On 3/5/2019 at 6:41 AM, Michael_B said:

I bought a Marshall '62 Bluesbreaker reissue, purportedly in mint condition.  While the amp hasn't yet arrived, yesterday I unexpectedly received new tubes from the seller.  He chose to ship it without tubes.  Am I correct in understanding that I now need to have the amp rebiased?

My local shop has a three week turnaround on rebiasing and a shop in Denver (90 min. drive with traffic) says they can turn it around in a week.  Is rebiasing something I can easily do myself with rebiasing probes?  Do most/many tube amp owners know how to rebias their own amps?

Finally, was shipping without the tubes and sending new tubes a nice gesture or a dick-move that should be treated with some suspicion?  'Cause if I have to drive 90 mins each-way, twice, and wait a week to use the amp, it feels more like a dick move than a nice gesture.

Yes you will need to get it rebiased.

The bias pot is on the inside of the amp for that one.  You could learn to do it, but I won't recommend it unless you can get someone to show you how to do it safely.  There are DC voltages in excess of 450 volts inside the amp and if you touch them it can kill you!! Repeat, the amp has to be ON to check bias and when you are probing inside a running amp and slip and touch the wrong thing , it can kill you!  Don't do it unless you have an experienced tech teach you.

If I had sold that amp, I would have bought new tubes, rebiased it, then listed it for sale and sold it.  I think you are right to be disappointed with this situation. He probably had nice tubes  in it that he wanted to keep.  It isn't right in my opinion.

Edited by texwest
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