Metropoulos Forum

Hi,

I'm going to order and build a JTM45/100, but I have a few open questions, so I will ask you all helpful guys for finding solutions the next few weeks.

My topic today is about power trannies, especially the one from Brian Wallace and the one from Metroamp. Perhaps you can help me, which one to buy.

How does the plate voltage and the bias affect the tone and feel of the amp?

I'll put Shuguangs KT66 into this amp and read often in this forum that these tubes like it to be biased hot (about 80% of max plate dissipation).
So all the following calculations are reffered to 80%.

The metroamp PT has 530V (or the newer version 500V?)
and the Marstran has 490V (and 560V optionally for GEC tubes).

That does mean you have to bias your tubes to 38mA (for 530V)
If you have 500V the current would be 40mA

Using the Marstran PT the bias should be set to 41mA.

Some things I kept in my mind:
The lower the voltage, the longer the live of your tubes.
The higher the voltage, the stiffer the feel, lesser the sag.

So that's the theory. But can anybody explain to me how these 3 different voltages and bias settings affect the tone and response of the amp?
Is it audible, to hear a difference for Shugs KT66, if biased them to 38mA or 41mA?

Thanks for your reply

Dom

Dom wrote: I'll put Shuguangs KT66 into this amp and read often in this forum that these tubes like it to be biased hot (about 80% of max plate dissipation).
So all the following calculations are reffered to 80%.

When I set up my 45/100 for initial testing with tubes, I'm going to aim for ~30mA, less than 60% of max plate dissipation. I'll be using the MetroAmp 3" PT, which is rated at ~520v. I intend to use the Shuguangs as well, and will also grab a set of the new Russian Gold Lions.

Sounds like you have your heart set on 80%. I see no reason!

What people say about tubes and what you should do are two different things. A lot of the time I find myself preferring the sound closer to 60%. I have fancy signal generators, an oscilloscope, dummy loads, and can set biasing in the anal crossover manner, but that doesn't make an amp sound it's best, either.

Sure, some tubes sound better hotter or cooler, but you simply cannot take what someone else has said and take it to be true, independent of your own experience and reason. And ESPECIALLY not if what they say is 80% regarding biasing. Talk to a few tube vendors and if they're greedy, they'll tell you to go ahead and bias at 80%, hoping to sell you a new set when you inevitably blow the ones you've got at 80%, or if they're honest, they'll tell you that you've created a tube eating monster, and to cool it down.

If you're running 520v on the plates and you bias up your Shuguangs to 80%, the question would be, then, how long they'd last. 80% at 520 would be somewhere around 46mA. Even using your suggestions of 40 or 38mA, which are closer to 70% @ 520v, could be hard on the longevity of your tubes.

Next thing, GEC's are rated as a design max of 500, and an absolute max of 550. If you examine the data sheets carefully, you will see that they aren't designed for these high voltages either. Sure, you hear things about the Dickinson amp at 560 and the one Young Brothers' 45/100 at 625, but good GEC KT-66's didn't cost $400 a pair in Jimi's time, and AC/DC, having sold 150 million albums, can afford to blow a quad or more of GECs. GEC's, when you look at them beside modern KT-66's, are robust in construction and certainly seem well made in terms of quality. GEC's are well known for lasting 6000 hours, but that is within reasonable operating boundaries. Given the right circumstances a GEC will die, too. Although the GEC might be good for more than their rated maximums as specified on the data sheet, you'd have to ask yourself HOW much you'd want to exceed those suggestions with such an expensive tube or set of tubes. Will it sound better? Will someone claim it is stiff, and less 'saggy'? Perhaps. But, for how long, and at what cost?

To answer another one of your questions, three mA of change in bias isn't going to move mountains in terms of tone. The common rule of thumb is that most people, unless you change it right in front of them so they can hear the change first hand, can't hear a difference until about 5 mA.

Either the Metro or Marstran will be fine; you have to check exactly what that PT is giving you and then use that to bias. I think your figures are quite high, that is all.

Remember, if you need a "Super Amplifier" decal in gold to adorn the back panel of your 45/100, check out the chassis/panel thread:

http://forum.metroamp.com/viewtopic.php?t=16515

Thanks Will for your info.

For my 50W Plexi RI I like the sound more, if the tubes (EL34) are biased hotter.
I agree also that a difference of 2 or 3 mA is nearly imperceptible.
Bu I started at 32mA and ended up to 42mA. And the more the current increased, the better I liked the tone.
So perhaps I was alittle bi naive to think , that KT66 would act the same way.

I started this question regarding the issue higher voltage---cooler bias, lower voltage---higher bias because I want to know, if there's soundwise a difference running a PT with 490V an a little hotter bias or a PT with 520V and cooler bias.
But if Shuguangs act different, this question is obsolete.

And why is the Marstran PT (120$) such cheaper than the Metro PT (175$)? I don't think, the Marstran is lower in quality.

Cheers
Dom