phase inverters

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jbrew73
Junior tube assistant
Posts: 109
(4/3/04 12:31 am)
 phase inverters
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 of the common PI's like the long tailed pairs used in vox,marshall,and fender which one can take the most input without distortion and which the least? also which ones would have the highest and lowest outputs with a given input signal?

i've been doing a little experimenting lately with the vox type and it seems to me the PI is over driving more than the power tubes. this would lead me to believe a good part of the vox sound comes from the PI and not the power tubes as most people suggest. i built one of dougs listening probes a while back but had forgotten about it until tonite i will give it a try tomorrow and listen to the circuit a little more carefully to see what is going into overdrive 1st the PI or power tubes.

i should add that the preamp in the amp i built is the normal channel only so i don't think preamp distortion would come into play here.
 
ganzonimx
Hey get your own solder
Posts: 456
(4/3/04 11:34 am)
 Re: phase inverters
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 That is a Q that I was thinking about for some time... a would also like to know more about it... specially in a setup with 6500's ore KT88.

I see in amp with 6550's ore KT88's there is mostly a driver stage after the PI, I guess to get the voltage swing needed to push the tubes sufficiently...

I would appreciate if anybody could shed some light on that.

Cris
 
jbrew73
Junior tube assistant
Posts: 110
(4/3/04 2:07 pm)
 Re: phase inverters
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 ganzonimx brought up a good point about the amout of voltage needed to drive different tube types. i built an ampeg jet amp and tried it with 6v6 6l6 and kt66 tubes. it seemed to overdrive at the same point with each set of tubes although it had a quite different tone and volume depending on the tubes used.
so my question now is how much voltage does it take to overdrive various tubes? i realize plate voltage, bias,etc. will detemine this to some extent but if someone could rank the popular tubes say from 1-10 would be great.
 
6G6
Junior tube assistant
Posts: 152
(4/5/04 7:41 am)
 Re: phase inverters
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 That sounds like a good question to me.
Has anyone got any good answers or at least opinions?
 
PHATamps
I only work on tube amps
Posts: 434
(4/5/04 10:09 pm)
 Re: phase inverters
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 The Vox LTP is set up to really hammer the output tubes. The down side is they tend to produce more hiss (anybody else ever notice this?)

Randall Aiken has a great article at his site. Click "Tech Info", then "Designing Long Tail Pairs..."
 
Lucid Alice
Senior tube assistant
Posts: 191
(4/8/04 11:10 pm)
 Re: phase inverters
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 AH! PHAT you've hit it! When this subject came up I knew I had seen a site with a lot of PI info. I thought I had saved a link to it, but couldn't find it.

What it doesn't answer is the how hard you need to drive various power tubes. It sure would be nice to find some sort of yardstick for PI output/Power Tube input requirements to drive the tubes into their sweetspot.

We do know that the fender and marshall circuits do that very well, but when it comes to rolling your own... what are the magic numbers or the formula? There are many examples, and even programs, for preamp and tone stack calculations. How can you know if your marriage of PI and power tubes should work or not?

 
GroundhogKen
Forum Moderator
Posts: 2343
(4/9/04 7:16 am)
 Re: voltage swing
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 I believe by definition class AB amplifiers never have the grid voltage positive with respect to the cathode.

So if you bias voltage is -36v, your phase inverter output can swing up to +/- 36 volts--so the grid voltage would swing from -72V up to 0V. That's the maximum and there are other factors to consider.


Ken
 
 
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