Changing a Pre-amp tube on a Hybrid Head

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post.aux.fader

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Hey all. I've got myself a Marshall VS100H, the old valvestate series head. I bought it used, and the thing is starting to sound a bit funky, plus I don't know how old the pre-amp 12AX7 is. I was thinkin of getting a tung-sol 12ax7 and was wondering if I can pop the old one out and slap the new one in, badabing, badaboom, no biasing or other such things I have no clue how to do.




Thanks duders!
 
No biasing needed, just pop the old one out and the new one in. Biasing is only needed for power tubes, and even then, there are some (myself included) who feel it is overrated. The Tung-Sol is a good choice. Are you talking new Tung Sol or NOS Tung-Sol? Because the vintage ones aren't that much more expensive and they destroy the newer ones. Just a thought.

BTW..I think a 1980's Sylvania 12AX7 would absolutely smoke in that particular amp, and they are only about $15-20 for NOS. JMHO.
 
Another tube that sounds really good in the old Valvestate amps is the Tesla/JJ ECC83S. http://www.triodeelectronics.com/12ax7tesla.html The best thing to do is buy 3 or 4 different brands of tubes and test drive each of them. The differences in the tubes is subtle so take your time testing them. You can never have too many spare 12aX7 tubes.
 
No Bias adjustments for any preamp tubes,..... and they very rarely go bad,..... or wear out,... so don't just throw it out,....they are really not taxed that much, and hardly at all in a half breed amp,..... IE valve pre, solid state power,....and , by the way, Mr.Poe sir,.. you are absolutely wrong about biasing,.... In some amps, especially, you MUST adjust the bias every time you re-tube the power section.....I know this is especially true of some boutique amps,.... the DR.Z amps sound horrible untill you do this adjustment, and so do Bogners,... and the Rivieras are yet another,....

This is VERY important in obtaining the last drop of tone from your hard earned dollars,....

Good power tubes are not cheap,.... and biasing your amp after new ones are installed not only makes the amp sound better,... it helps to lengthen the life of your tubes....


Yes, in most cases, NOS,.. that is New Old Stock,. tubes generally have more balanced tone than the new stuff,...

there are several places to find those,... just Google Vacuum Tubes, and you will see,....

Steve
 
Thatupstateguy said:
No Bias adjustments for any preamp tubes,..... and they very rarely go bad,..... or wear out,... so don't just throw it out,....they are really not taxed that much, and hardly at all in a half breed amp
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Most preamp tubes have a fixed life at around 5,000 hours. The cathode current is probably is no different whether in a hybrid design or not, unless the filament are starved.

http://www.pentalaboratories.com/tubeworks.asp
 
and , by the way, Mr.Poe sir,.. you are absolutely wrong about biasing,
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Yes...and no. It really depends on the amp and the quality of tubes you are using. If an amp is well made...and I mean really well made...the the bias should be stable. If the tubes you are using are consistent, then biasing isn't really all that important. Let me give you an example...

I have a Fargen Bordeaux amp. Ben Fargen is as good an amp maker as there is. The amp came with 6L6GCs. Now if I remove those tubes and buy a new set of JJs or Svets or something else...I would agree that the amp will need baised. But that is because the tubes are all over the map as far as quality and consistency are concerned.

Now, here is the opposite end.

I take that same Fargen Bordeaux and set the amp up with a killer set of NOS RCA Blackplates (or equivalent). If I replace those tube with the same type of NOS tubes I normally don't have to do any biasing. This is just my experience. Good tubes don't fluctuate...new tubes do.

Biasing has become important because of the poor quality of new tubes. There isn't a tube made today that can stack up to an RCA Blackplate or a Tung-Sol 5881, or a Mullard EL-34. Hell, even the Sylvanias/Phillips of the 80's are light years ahead of modern tubes. The Phillips 7581A is absolutely indestructable. trust me...I have tried..LOL!!!

Dr. Z makes an absolutely splendid amp. I know, I have owned several. Bogner and Rivera are also amazing. And an amp does need biased (and other things) if you are changing tube *types* (Ie 6L6 to 5881, or KT66, etc...). But in my experience, biasing is only necessary because we are using junk for power tubes and need to compensate. I have had a 1970 Marshall SL since 1980. I have never biased it, and it has never needed it. Of course I have only had to change tubes 3 times in 30+ years . Mullard XF1 EL-34s will do that to an amp. I also never had to bias my silver face Fender Twin.

There are a boatload of amp gurus who will tell you that certain amps actually sound better when the bias is slightly off.

You make several excellent points in your post. But I am of the opinion that a lot of biasing that is done is really not needed. Are there instances where it is important? Absolutely. But if you buy quality tubes and have an amp that is devoid of problems, biasing every 6 months is a waste of time and money. Put the money toward good tubes instead.

JMHO
 
One thing to be aware of. Resistor and capacitor values change as they age. This causes a change in the plate voltage which directly affects the cathode current at the power tubes. Thus, the bias values will change over time no matter how consistent you're tube selections are. This is true for fixed bias amps as well. BTW, fixed bias amps can be somewhat adjustable by reading the plate voltage on the particular amp and matching your tube values to best suit the range of bias you're looking for. Even the highest quality tubes have varying values. This is just a fact of life with tubes and the way they are manufactured. This is why they are sold as matched sets. The point is, your bias should be checked whenever you change your power tubes. Besides keeping your sound as good as possible, it could save the life of your new tubes or even your power transformer if you have failing components.

Back to the original point of the thread...Pre-amp tubes don't get biased. If that particular amp is sounding funky, it is probably the pre-amp tube. Read web sites for different tube manufacturers. You'll be able to select a tube that is 'warmer' or with higher gain values to suit your taste and playing style i.e. 12AX7C or 12AX7A or ECC83 etc etc. If in doubt, replace it with whatever comes stock from the factory.
 
One thing to be aware of. Resistor and capacitor values change as they age. This causes a change in the plate voltage which directly affects the cathode current at the power tubes. Thus, the bias values will change over time no matter how consistent you're tube selections are.
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Excellent point.

Even the highest quality tubes have varying values. This is just a fact of life with tubes and the way they are manufactured.
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Yes..but the degree of variance is nothing when compared to modern tubes which can vary widely. Remember, tubes today are made almost explicitly for amplifiers. Older tubes were made to perform functions where people lives could be on the line (ie. military applications) so the degree of tolerance was incredibly high.
 
So what you are saying is, there is no need to bias an amp unless you do something to it that would make you need to bias it.

You can get by with just changing your tubes with Groove Tubes, as long as you use the same rating, because you are replacing it with the same thing. That is their big selling point. It has less to do with biasing being over-rated than the fact that tube quality sucks.
 
So what you are saying is, there is no need to bias an amp unless you do something to it that would make you need to bias it.
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For the most part...yes. But again, lots of people feel differently about this. Do what your ears tell you to do. Other people in this thread have brought up very compelling argument for the opposite school of thought. All that I know is that biasing didn't become an issue until tube quality fell of the face of the earth. I seriously doubt that Hendrix or Clapton, etc..had their amps biased in the 60-70s...and their tone was just fine IMHO. :p

You can get by with just changing your tubes with Groove Tubes, as long as you use the same rating, because you are replacing it with the same thing. That is their big selling point. It has less to do with biasing being over-rated than the fact that tube quality sucks.
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In theory...yes. The problem is that even Groove Tubes quality isn't good enough to rely *completely* on this principle. GT is essentially a reseller of other companies tubes. They take tubes from other makers and rebrand them with the GT logo. Thier selling point is that they are more selective than most tube companies, but they are still selling the same Russian/Chinese tubes.
 
They are selling the same tubes, but they test them and rate them. It's not that their tubes are any different, they just plug them in and test them. The rating system works pretty well.

BTW, I know a bunch of old amp techs that worked for people like Richie Blackmore, Tony Iommi, etc... and they did check the bias every time. If it worked out that the bias didn't change, fine, but it was checked. I don't know about Hendrix and Clapton specifically, but it was not an unheard of practice in the late 60's and early 70's.
 
They are selling the same tubes, but they test them and rate them. It's not that their tubes are any different, they just plug them in and test them. The rating system works pretty well.
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True..but I have had more than one GT go south after a few hours. Not their fault as it is impossible to predict.

BTW, I know a bunch of old amp techs that worked for people like Richie Blackmore, Tony Iommi, etc... and they did check the bias every time. If it worked out that the bias didn't change, fine, but it was checked. I don't know about Hendrix and Clapton specifically, but it was not an unheard of practice in the late 60's and early 70's.
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Unheard of....no...but it certainly wasn't nearly as widespread as it is today.

BTW...the simple solution to all of this is to get a cathode biased amp like my *other* Fargen. A cathode biased amp has a huge tolerance and will run just about any tube that will fit the socket without the need for biasing or other such nonsense...LOL!!

Great thread guys...
 
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