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Biasing Tube amp


Black Dog

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So I have a Blackstar ID core 40 that is a decent little amp but I still want to get a tube amp, I think. I don't like the idea of having to take it somewhere for tube replacement and biasing. I had a Bugera that had a auto-biasing feature but I didn't like it so a ditched it. I want to try the Blackstar HT-5R but that requires biasing. Is it realistic to learn to do that or is it a really bad idea?

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There are some on this forum considerably more qualified to speak on this subject than myself. But the few I've biased myself weren't too bad to bias. Some amps have convenient bias points for the multi meter clips. Those I've had that don't I take to a close friend that does amp repairs.

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It’s possible to learn how to do it safely, but I think it would be foolish for someone to advise you to attempt this at this time given the level of electronics knowledge shown in the OP. That is not meant as an insult at all, just my honest opinion.

 

With that said, are you sure the HT-5r is adjustable bias? Its possible but unusual. Normally a 5W single ended amp is cathode biased which does not require a bias adjustment for new tubes. There are many cathode biased amps to choose from that would not require bias adjustments

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Biasing is overrated. Decent tubes will work just fine, most amps are cold from the factory for maximum tube life, and you won't really hear a difference after you bias decent tubes hotter to where it "should" be. Amps today are made to work in a window that happens to include the tubes made today. Biasing an amp was a bit more important when I was a kid I think.

 

rct

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It has two tubes, one I believe is cathode biased the other not.

The only tubes that you need to worry about biasing are the power tubes. A single ended amp has only one power tube. What I infer from what you are saying is that it probably has a 12AX7 preamp tube and probably a cathode biased EL84 or 6V6, in which case no bias adjustment would be necessary.

 

How long tubes last depends on the tube, the amp, how you use the amp, etc. There are too many variables to give a one size fits all answer

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Todays tubes, and by todays tubes I means since the late 90's or so, I use 200 hours of gigging. After that I change them. So I have the calendar with our gigs on it and I give each gig 5 hours to include the odd rehearsal with the good amp, which I do sometimes. A year maybe, three gigs a month or so. The amp itself matters too. My two Prosonics were each biased once in their lives, and easily did 200 hours each on a set of tubes. My last couple Marshalls, DSLs from the early 2000s, were never biased and did not eat tubes at all, I had the 401 for...8 years, rehearsed it regularly but didn't gig it, traded it with the original tubes still in it, still sounded great.

 

rct

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I bias my own amps (those with an easily adjustable bias) with a bias probe. It fits in the tube socket under the tube and, whean attached to a multi-meter, allows me dial in the bias I want with no danger of getting shocked. [biggrin]

 

And it can make a big difference in sound - especially in vintage amps. I once had a guy work on my Deluxe Reverb for some power supply noise I was having and he biased it super cold (around 20ma). It sounded terrible - like an ice pick. Once I got it biased back up in the 28ma- 30ma range it sounded great again.

 

Technically all tube amps are adjustable in bias - but those with cathode style bias or fixed bias usually have a bias resistor. To change the bias you have to change the resistor. I don't do those myself - but they also don't need it. I do have one Fender Princeton that seems cold and stiff, so I may need to consider adjusting the bias resistor in that one.

 

I don't think many of the new amps have an easily adjustable bias, and most small amps will never require it. But Badbluesplayer will be the real expert here.

 

As for durability, I have never carried spare tubes to a gig and I have never had a tube fail at a gig.

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...As for durability, I have never carried spare tubes to a gig and I have never had a tube fail at a gig.

 

I have to agree. I did have to change a pre once in one of my Twins, but I knew it was dodgy for weeks and could have done it at any time, didn't have to do it on the gig. I do carry spare tubes and fuses, old habit from back then. They stay in the car though. I've never taken them out, not in at least 25 years.

 

rct

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I'm definitely not an expert on tube amps, although I own two of them. So, I have a couple of questions and if there's a website(s) or book(s) that I can read to be better informed, I'd be happy to do so.

 

1. What does biasing a tube amp mean, exactly?

2. Is this something most people do themselves or take it to be done at a shop? I'm fairly competent at taking apart electronic items and putting them back together with no spare parts and having them work.

3. What tubes are most commonly used in a Marshall or does it vary by model?

 

I'd be happy to read more about the subject.

 

Thanks!

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I found this interesting thread on this very topic from a couple years ago:

 

http://forum.gibson....lackstar-ht-5r/

 

I went to the Blackstar website and they don't have any e-mail or phone contact info that I can find so I guess I'll just not worry about it and see how it goes if I decide to get one of these.

 

Thanks again for the replies.

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I biased my Marshall DSL15 head and it was easy since it has trim pots and a prong to easily check the bias voltage.

 

In this case there is a noticeably difference in sound, at 18V the amp sounded starved and at 23V it would turn into a fire-breathing beast. It took me a few tries but found the sweet spot.

 

Having said that, I would not dare bias an amp the old school way, meaning without the trim pots and prongs.

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I bias my own amps (those with an easily adjustable bias) with a bias probe. It fits in the tube socket under the tube and, whean attached to a multi-meter, allows me dial in the bias I want with no danger of getting shocked. [biggrin]

 

And it can make a big difference in sound - especially in vintage amps. I once had a guy work on my Deluxe Reverb for some power supply noise I was having and he biased it super cold (around 20ma). It sounded terrible - like an ice pick. Once I got it biased back up in the 28ma- 30ma range it sounded great again.

 

Technically all tube amps are adjustable in bias - but those with cathode style bias or fixed bias usually have a bias resistor. To change the bias you have to change the resistor. I don't do those myself - but they also don't need it. I do have one Fender Princeton that seems cold and stiff, so I may need to consider adjusting the bias resistor in that one.

 

I don't think many of the new amps have an easily adjustable bias, and most small amps will never require it. But Badbluesplayer will be the real expert here.

 

As for durability, I have never carried spare tubes to a gig and I have never had a tube fail at a gig.

 

What he said . msp_thumbup.gif

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yep. I was just going to mention them. very handy.

 

Yeah. Surfpup mentioned these earlier in this thread them so I went looking. Actually found that link in another website about biasing. You still have to "touch" the bias pot with your nylon screwdriver but it seems like it makes it much more safe overall. There's also some good info in the older thread about discharging the caps before hooking everything up. But obviously your still going to have the amp on when your making the adjustments. Anyway, it may all be moot if this amp doesn't really need biasing, which I'm still not sure about.

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I'm definitely not an expert on tube amps, although I own two of them. So, I have a couple of questions and if there's a website(s) or book(s) that I can read to be better informed, I'd be happy to do so.

 

1. What does biasing a tube amp mean, exactly?

2. Is this something most people do themselves or take it to be done at a shop? I'm fairly competent at taking apart electronic items and putting them back together with no spare parts and having them work.

3. What tubes are most commonly used in a Marshall or does it vary by model?

 

I'd be happy to read more about the subject.

 

Thanks!

 

1. Biasing a tube is kind of like adjusting the fuel-air mixture in your fuel injectors in your car. It kind of balances the voltages between the different parts of the tube so it will work properly when you step on the gas. There's a screen grid in there that's statically charged that keeps all the electrons from going crazy and jumping from the cathode (the minus charged thingy) to the anode (the plus charged thingy, also called the plate). You adjust the static charge on the grid and it controls how much current passes thru the tube. Like a gas pedal, kind of.

2. You can do it with a bias probe and a multimeter. You don't want to go poking around inside the chassis without some training, but the bias probes work well. If the bias is adjustable, you'll just be adjusting a pot with a screwdriver. The Eurotubes probe is good and their videos are great.

3. Marshall buys tubes from the major manufacturers and rebrands them as Marshall tubes. It will vary by model and era. I see lots of JJ tubes in modern Marshalls. They're good, especially the power tubes. There have been lots of failures of JJ preamp tubes in recent years though in certain kinds of circuits, and I'm shying away from them lately.

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3. Marshall buys tubes from the major manufacturers and rebrands them as Marshall tubes. It will vary by model and era. I see lots of JJ tubes in modern Marshalls. They're good, especially the power tubes. There have been lots of failures of JJ preamp tubes in recent years though in certain kinds of circuits, and I'm shying away from them lately.

 

For instance My Marshall DSL15H came with the following stock tubes:

 

V1 JJ - Branded Marshall in red print. (I compared this tube to other JJ tubes and indeed look the same inside).

V2, V3 and V4 Shuguang - Branded Marshall in white ink. (I compared these tubes to Ruby tubes that are selected Shuguang and they look exactly the same)

 

Power tubes are TAD (The Amp Doctor) not rebranded.

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Black Dog,

 

If you amp is distorted too easily or not enough output, you need to determine if biasing is a solution for you by someone who is a qualified technician. I would not attempt to work on high-voltage systems if you do not have the proper knowledge. As well, the purpose of biasing is to generally set the "idle speed" (think internal combustion motors on say a lawnmower or even an automobile) of the electronics (i.e. tubes). We all know how important it is to properly set this up... If the idle isn't just right, things aren't in balance as I like to term it. So improperly biased amplifiers will either have trouble amplifying (weak output) or begin to clip/distort (too much output). The goal is to let the small signal (i.e. the electric guitar output) "swing" (voltage vs. time) its greatest difference without distortion and the amplifier is "tuned" so to speak. Over-output can start to wear an amplifier's tubes prematurely as well. Under-driven just doesn't work for playing on. I don't believe this is good either for the electronics. Overall, don't play around with this if you don't know. It's high voltage, you can damage amp, and most importantly, hurt/kill yourself if you aren't confident what EXACTLY you are doing. As well, you aren't even sure if this amp needs biasing, so don't borrow trouble if you don't have it right now. See how it sounds and go from there. I just want to give the best advice on this subject that I can think of. Good luck!

 

Thanks NHC. I will proceed, as always, with the utmost caution and respect. msp_thumbup.gif

 

I'm just a do it yourself-er, whenever I can. It's ingrained in me. One of my most important memories as a child was when my dad gave me my first bike. The first thing he did, after teaching me to ride it, was to teach me how to maintain and repair it. That was mandatory. The same with cars and really everything else. Yeah, there are always things you can't do yourself (more and more as time goes on). But, I often find that with the right information, tools and approach, many things are easily doable.

 

Thanks.

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I will say that with the proper knowledge this job can easily be a DIY type of task. There are YT videos on this I believe that might give some insight how to do this... But my advice is to make sure there is a concrete plan on how and what to do before "exploring". I would hate to hear about someone damage their property and or cause injury/death due to lack of proper qualification. What I would do first is watch some videos, read some articles... do some research on the subject online. Get a feel if this task is too difficult for you to handle safely. Usually with home repairs, I go this route when I am entertaining something I have not done before. Most times, it lets me know if I need an experienced/qualified individual or not for the job. I do respect the DIY attitude that you have. I am the same way, but I have the humility to know when I need to ask for help as not to complicate the issue more than it already may be.

 

Just to be clear, I don't even own a tube amp. I am in the early information gathering stage here. It sounds like it will be about 2 years from the time I get the amp until the time I may have to worry about this. Worry not.

 

But I do appreciate the words of caution. This is why I asked about this here, for exactly this type of warning.

 

Thanks.

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