Driving Power Tubes, How Much Input Level

[JammyDodger]

Hey All,

I've been researching ways to reduce the output volume level on an amp and have come up with a question. There seem to be three main ways to get overdrive from an amp. They are

1 - Attenuator on output, allows for preamp and power amp tube distortion
2 - VVR - Allows for preamp tube and / or Power Amp tube distortion
3 - PPIMV - Allows for preamp tube distortion

O.K. - so here is my question. How do you know how much signal it will take to drive the power amp tubes into distortion? I want to make sure that my preamp section has enough power to overdrive the power tubes.

Cheers, Mike

[Cliff Schecht]

You have to drive the grid with enough signal to swing it at a higher potential than the bias voltage voltage. For fixed bias this means that for a negative grid bias of 30V you med to swing the signal near 0V before you start clipping. With cathode bias you need to swing the grid higher than the cathode voltage. So take your bias voltage, double it and know that this is about the minimum p-p swing you can have before clipping in the power stage occurs.

Also realize that you can't clip the power stage negatively, or at least you don't want to. Clipping the power stage of a SE amp gives you almost entirely even harmonics which usually sounds fantastic (if done right).

[PCollen]

Hey All,

O.K. - so here is my question. How do you know how much signal it will take to drive the power amp tubes into distortion? I want to make sure that my preamp section has enough power to overdrive the power tubes.

Cheers, Mike

The signal level required depends upon the plate voltage applied to the power tubes and OT characteristics, and also the bias point/voltage. And you could cause the PI to overdrive before the power tubes, depending on amp component values. You'd need an oscilloscope to really check this.

[Cliff Schecht]

Yeah I should have mentioned that I was speaking mainly about se amps, push pull class a/b is a different story because of the requisite phase inverter.

[M Fowler]

There seem to be three main ways to get overdrive from an amp.

The fourth way.

There is another way to reduce output volume by using a 25kL pot with a 33k 3w resistor across the pot between the tube and PI.

This works very well in amps like Marshalls and the TW Rocket circuit.

Haven't had a chance to try this in many others amps; however.

[Alexo]

..another way is to use giant screen resistors on the power tubes.

A couple years ago, I built an amp with 5881's, using the RCA datasheet specs of 360 V on the plates and 270 on the screens. I used a big 22K or 33K R in the power supply to drop the 360 down to 270.

According to the datasheet, this setup should yield about 25 watts with a 6K6 load. I recently hooked the amp up to a scope and found it only put out 8 watts at most! Even though there was a reservoir cap after the screen dropping resistor, that giant dropping resistor sank the screen voltage down to close to 200V when presented with a healthy signal. The extra current form the 6K6 load may have played a role too.

Not only does this clip off your signal swing early on the output tubes, but as the screen voltage drops, the requisite bias voltage for 70% dissipation (or whatever) also drops, meaning the tubes suddenly find themselves biased pretty cold if you're using fixed bias.

The result is greatly reduced power, insane compression and a lot of crossover distortion. It was actually pretty fun!

I guess you could implement this by using say... a stereo 25K, 2 watt (or more) pot between the screen supply and the screens.