Why don't more amps have external bias pots and test points?

[jpage]

It costs me $35 total to change power tubes because I don't have to pay somebody $70 to hold my hand. Cost of doing business.

[teemuk]

You're changing your power tubes ALOT.

Maybe he's biasing them too hot.


...so which one is it? The feature saving up $1K or costing it?

[jpage]

Originally Posted by teemuk

Maybe he's biasing them too hot?

Nope. They are set up perfectly. Because unlike what techs want you to believe, it's not rocket science. I can actually bias them hot or cold and decide which I prefer. What an idea!!!

Originally Posted by teemuk

...so which one is it? The feature saving up $1K or costing it?

Definitely costing. Because I save so much $$ on 'bench' fees, I can actually be proactive and ensure my amps are performing at their best.

I really cannot believe there is anyone arguing this. Do you people pump your own gas?

[WesKuhnley]

Originally Posted by jpage

Nope. They are set up perfectly. Because unlike what techs want you to believe, it's not rocket science. I can actually bias them hot or cold and decide which I prefer. What an idea!!!



Definitely costing. Because I save so much $$ on 'bench' fees, I can actually be proactive and ensure my amps are performing at their best.

I really cannot believe there is anyone arguing this. Do you people pump your own gas?

Do you top your tank off every 20 miles?

YMMV...and don't get me wrong...biasing is not rocket science, I'm here to encourage anyone who knows how to do it properly. You could probably halve the number of times you change powertubes, and not notice a single bit of difference in performance of the amp, and save even more money!

[Geoffb]

Hi Guys,
Although I'm now retired, I've been fixing valve amps since the 60's and I can't recall ever hearing the term "bias the valves" until it became "fashionable" in the late 70's early 80's.
Yes, if you want a pure sine wave from your amp then bias the pa valves to reduce distortion, but most of the early 60's amps were never biased and that's the 60's sound most people want !
Most amps only have one bias pot which varies the voltage on the pa valve grids to set the current flowing through both valves ( 2 valves in push-pull ) to a value recommended by the manufacture. Unfortunately this does not ensure the current through each valve is the same. But this is why we spend lots of money buying "matched" valves I hear you cry! Have you ever measured the impedance or DC resistance the each half of the output transformer? I have measured many over the years and most are in the range from 10% to 25% different. This means that even with matched valves the current flowing through each half of push-pull output stage will be different.
Therefore unless you modify the circuit to include a bias pot for each valve, don't wast your hard earned cash on matched valves, just enjoy the valve distortion.
I'll leave you with one final thought given to me from my old college tutor ( good memory hey!): if you can't see distortion on a scope you can't hear it !
.
Geoff.

[Blue Strat]

^ I'll admit that we've gone way overboard on matching and biasing, but we also have fewer unexplained tube burnouts and tonal changes from one pair of tubes to the next.

It's about knowing what you've got and how it affects tone and longevity. It absolutely can't hurt to know where your power tubes are idling, how far out of match they are, and correlate it to the tone you got from the last pair of tubes.

[scottl]

Geoff, the Dumble style amps I have here all have a PI trimmer. This allows you to compensate for the output transformer "mismatch", as well as the PI tube triodes and power tubes. You can see the distortion clearly on a distortion analyzer scope.

When it is set correctly, there is a very noticeable amount of bloom and upper harmonics that jump off the sustaining notes. This is a fact.

So, knowing how your tubes are idling, is not a bad thing ever. Keep in mind, the older 60s and 70s tubes were far far superior than todays stuff. The variations from tube to tube were a lot less, and they lasted longer.

Regardless of what the manufacturer specified, setting the tubes bias to where YOU like it, is better than not setting it at all.

All that matters is that everyone I know can hear the difference between a 40% and 80% biased valve. Why not get yours into the normal range for maximum life and tone?

Originally Posted by Geoffb

Hi Guys,
Although I'm now retired, I've been fixing valve amps since the 60's and I can't recall ever hearing the term "bias the valves" until it became "fashionable" in the late 70's early 80's.
Yes, if you want a pure sine wave from your amp then bias the pa valves to reduce distortion, but most of the early 60's amps were never biased and that's the 60's sound most people want !
Most amps only have one bias pot which varies the voltage on the pa valve grids to set the current flowing through both valves ( 2 valves in push-pull ) to a value recommended by the manufacture. Unfortunately this does not ensure the current through each valve is the same. But this is why we spend lots of money buying "matched" valves I hear you cry! Have you ever measured the impedance or DC resistance the each half of the output transformer? I have measured many over the years and most are in the range from 10% to 25% different. This means that even with matched valves the current flowing through each half of push-pull output stage will be different.
Therefore unless you modify the circuit to include a bias pot for each valve, don't wast your hard earned cash on matched valves, just enjoy the valve distortion.
I'll leave you with one final thought given to me from my old college tutor ( good memory hey!): if you can't see distortion on a scope you can't hear it !
.
Geoff.

[Paratrooper]

Why don't more cars have external ignition setting points? ;-)

[VacuumVoodoo]

Originally Posted by Paratrooper

Why don't more cars have external ignition setting points? ;-)

All modern cars have them. You need a laptop and some software. Piece of cake.

[scottl]

Since this was brought up before..... Geoff's point that he just put the tubes in and that is the sound of rock and roll, is nonsense. And he dwells on the matching of the tubes and not the bias adjustment. We are discussing bias adjustment here. For every amp that you retubed blindly that sounded amazing, you probably had several that sounded off. Maybe that is why people would always wonder why one of their amps sounded great and another of the same amp did not?

Imagine replacing the distributor cap (older car of course)and not timing the car. It may not even start. Why anyone is arguing the point that biasing the tubes is some excessive gear geek obsession is BS. Especially with today's crap. Tubes 20% mismatched from one another.....

Now I know that some companies like Mesa, don't have bias adjustments. However, they sell matched sets that are in a certain tolerence range from where they spec the amp. So in essence, they are fixed bias with replacement tubes chosen to be in a certain dissipation range. For the record, everyone I know that has modded a Mesa to allow exact adjustment, has felt it improved the amp a lot.

Back to ignition timing, yeah, it is easy to plug a device into the OBD II port and tweak to your hearts content! My M3 is modded to maximize power with 93 octane. It is set from the factory for 91 octane. On the east coast we have 93. I'd like some free horsepower! It added 15hp on the dyno. You all should know that most cars have piezo knock sensors for the block, and in some cases, individual cylinders (as in the M3). The car's computer is adjusting the timing for optimal programmed parameters at all times. Obviously older non computerized car's had easy to set timing that any backyard mechanic could set with simple handtools. Without risk of 500VDC killing them.....

Originally Posted by Paratrooper

Why don't more cars have external ignition setting points? ;-)

Originally Posted by VacuumVoodoo

All modern cars have them. You need a laptop and some software. Piece of cake.

[teemuk]

Back to ignition timing, yeah, it is easy to plug a device into the OBD II port and tweak to your hearts content! ... The car's computer is adjusting the timing for optimal programmed parameters at all times. ....any backyard mechanic could set with simple handtools.

Keywords = device, computer, mechanic.

Basically the point is: the adjustment still needs expertise (usually a mechanic knowing what he's doing), special devices and tools (laptops with proper interfaces and software or perhaps more old-fashioned strobe guns), or perhaps the adjustment is simply handled with an automated microcontroller circuitry (like amp biasing should be). Still, not nearly as same as having an adjustment screw or pot in the main console resulting into bunch of people adjusting the ignition according to their personal preference of engine's tone at various revs.

[VacuumVoodoo]

Originally Posted by teemuk

Still, not nearly as same as having an adjustment screw or pot in the main console resulting into bunch of people adjusting the ignition according to their personal preference of engine's tone at various revs.

Not the same, but it's a lot of fun, and LOUD:

[Koop]

The same Renault F1 V10. This tune may be more recognizable for some of us. I don't know how to embed a video so here's the link:

http://www.youtube.com/watch?v=MGEqlNU30Tg