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NAD. New Tubes. Need to Rebias?


Michael_B

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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What was the problem with the old tubes?  Were they doing anything weird or funky, or did he break one by accident?  Or were they a set of tubes that he wanted to keep for whatever reason (i.e., a brand or make not available for sale anymore)?  Granted, you're gonna need to replace tubes eventually, but this does make things inconvenient to say the least.

BTW, the Marshall Bluesbreaker is AKA a Model 1962, and the 1962 model number doesn't have anything to do with the year of manufacture or issue that I know of, it's just the number that Marshall assigned this particular amp model way back when, just like the Model 1960 4x12 cab doesn't have anything to do with the year 1960.  You're not the first person to think it did have something to do with the date, you can thank Jim Marshall for that!

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Even if I had whatever proper biasing gear is needed, I'd don't know if I'd watch a YouTube video or two or read a quick meter manual, get my confidence boosted falsely and then be DIY playaround guy in a chassis filled with live and potentially deadly AC voltage.

Keep in mind too, a high percentage of shit that crosses my guitar repair bench is stuff where a guy watched a YT vid, became an overnight "expert" and the result is he's now paying me to not only correct the initial issue but his good-intentioned fuckery that worsened the issue exponentially.

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I had a RI BB and I don’t think mine had a bias pot, I took it in to get turned back to Beano specs with KT66s so I didn’t do it. 

Biasing is easy if there is a pot and bias measuring points. It’s all about a math equation. Without the bias points or a bias pot, it is a lot more difficult. A 3 week turn to bias an amp is ridiculous. The shop must be up to its ears in business.

Not a dick move to ship without tubes. Depending on how tight the tube sockets are...or how loose, he may have done you a favor. I would have simply wrapped them in bubble wrap and taped them in the back on the amp and packed paper in the cavity. Lots of people ship without tubes installed to ensure they don’t rattle loose and shatter inside the amp.

Are you sure those tubes haven’t already been biased for the amp? 

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Some think it's better to ship amps without tubes; definitely not a bad thing. It's not really necessary, though.

As for biasing, it's not difficult, but could be deadly if you're not careful. There are some good Youtube videos out there, but it's best to leave it to someone more experienced if you have any doubts about your ability.

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Thanks, everyone, for the replies. I won't mess with biasing, based on the advice here.

1 hour ago, Jakeboy said:

Are you sure those tubes haven’t already been biased for the amp? 

I have no idea.  The tubes that he sent came directly from Tube Depot.

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IMO, sending you a full set of new tubes was definitely not a dick move.  Seems like a very nice gesture.  The original tubes may have been at the end of their lifespan or inferior quality. 

I'd install the new tubes and play the amp for a while.  If you think the amp sounds too harsh, cold, hot, or whatever, take it to shop and have the bias checked and adjusted to your taste.  In the meantime, you might want to watch some YouTube videos to see if you can get comfortable with doing it yourself.

 

 

 

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Does the reissue still have the individual tube bias resistors, or did they change them to trim pots. Most of what I see regarding the reissues mentions a "vertical bias trim pot" as well as the hum balance control.

I will say that even more recent Marshalls like my JCM2000 TSL have TWO bias trim pots, which are actually adjustable from the outside of the chassis. So, hopefully it has adjustable resistors, and not fixed.

As for why the seller sent new tubes instead of sending ones in the amp? Well, they were probably nice tubes, and rather than do the usual move of putting "Safe" tubes in the amplifier, he just sent you some.

As for safety, get some thick rubber gloves as well as a plastic handled screwdriver, or even a chop stick, depending on the type of bias trim pot that is used. ANd DO NO TOUCH anything inside the amp.

I got a weber bias rite years ago, and honestly, it has probably saved me hundreds of dollars in tech servide fees. I got the one that has both Octal and Nine pin, so I can adjust just about every time of power tube. The voltage and current readings mean simple math and no having to even go inside the amp except to adjust the trim pot.

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1 hour ago, tbonesullivan said:

As for safety, get some thick rubber gloves as well as a plastic handled screwdriver, or even a chop stick, depending on the type of bias trim pot that is used. ANd DO NO TOUCH anything inside the amp.

Excuse me while I re-bias my amp 😆Ld9EPXu.png

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2 hours ago, tbonesullivan said:

That's the idea!  I mean, there is always the "rough and ready" way to do it: measure the current across the standby switch.

No doubt... I've been working with electricity most all my career & I have tremendous respect for it & trust me, I've accidentally shorted quite a few high voltage/current circuits in my career... I send all my tube amps out to professionals for service... to me, it's not worth the risk of potential shock injury/death

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Unfortunately a video on how to drain filter caps is good for people to know, but won't help much.  When biasing, the amp has to be running while you adjust the bias.  You have to be VERY careful while probing around inside a running amp.  I recommend extreme caution when the bias pot is inside the amp.  Fender really did it right compared to other manufacturers.  On the old amps the bias pot is on the outside of the chassis and you can easily use one of those bias meters on the tube sockets and adjust the bias without even opening up the chassis.

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On 3/9/2019 at 10:19 AM, texwest said:

Unfortunately a video on how to drain filter caps is good for people to know, but won't help much.  When biasing, the amp has to be running while you adjust the bias.  You have to be VERY careful while probing around inside a running amp.  I recommend extreme caution when the bias pot is inside the amp.  Fender really did it right compared to other manufacturers.  On the old amps the bias pot is on the outside of the chassis and you can easily use one of those bias meters on the tube sockets and adjust the bias without even opening up the chassis.

I ended up giving it over to the local tech.  Despite telling me that he's three weeks out, he turned it around in less than a day.

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I have a long plastic screwdriver for adjusting bias pots.  I also use a chopstick as well, for those that have the little "handle" you can turn (carvin amps).

Marshall amps, at least my TSL100, have external bias trim pots, one for the PUSH tubes and one for the PULL tubes.  If a soldering iron is required, it goes right to my amp guy.

Things like cleaning pots, cleaning jacks, etc, that's stuff I'll do myself. I'm also one of those who does basic electrical work around the house. Replacing sockets, fixtures, etc.

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On 3/9/2019 at 11:19 AM, texwest said:

Unfortunately a video on how to drain filter caps is good for people to know, but won't help much.  When biasing, the amp has to be running while you adjust the bias.  You have to be VERY careful while probing around inside a running amp.  I recommend extreme caution when the bias pot is inside the amp.  Fender really did it right compared to other manufacturers.  On the old amps the bias pot is on the outside of the chassis and you can easily use one of those bias meters on the tube sockets and adjust the bias without even opening up the chassis.

Agreed on all counts.  The easiest amp to adjust I've come across is the Carr Mercury.  It has receptacles for a stadard multimeter on the exterior and the manual provides the recommended range.  No math needed!

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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.

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This is probably too late but thought I’d comment anyway...

I have worked on a lot of Marshall’s, and what I can tell you is that MArshall ships their amps biased cold. Safer for them than going hot and having customer issues.

if the amp feels excessively stiff you may want to rebias it, but if it sounds and reacts to your satisfaction then don’t worry about it.

Interesting story... I once had a guy bring me a JCM800 reissue that “wouldn’t keep up”. I checked the bias and it was waaay cold. I couldn’t back off the negative voltage enough to get the bias where it needed to be so I started checking the divider on the bias supply. Marshall had put a resistor in there that was high by about a 200% factor.  I gave him the amp back and next time he saw me he said “I don’t know what you did to my amp, but it really opened it up”.  One of his playing buddies asked me if would do the same “mod” to his amp. 🤣

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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! 

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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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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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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.

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