Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: EW57 on March 13, 2013, 11:39:53 pm

Title: Taking preamp output directly from the phase inverter?
Post by: EW57 on March 13, 2013, 11:39:53 pm

Greetings, the following question should paint a fairly clear picture of how weak my understanding of the topic is:  Is it possible (and advisable)  to have a preamp output come from the phase inverter (with no power amp circuit/OT/speaker load)  to feed an external poweramp, maybe through a interstage transformer?

I became curious of this after reading various posts regarding Marshall/hiwatt/etc  phase inverter distortion posts, specific web pages discussing phase inverters,  (http://www.guitaramplifierblueprinting.com/files/Phaseinverter.pdf (http://www.guitaramplifierblueprinting.com/files/Phaseinverter.pdf)) and even a (relatively) unanswered posing of the same question. (http://74.220.215.222/~neampfes/ampgarage/forum/viewtopic.php?t=12386&start=0&postdays=0&postorder=asc&highlight=&sid=9935961d47748b3c9e77c8e5f3437885 (http://74.220.215.222/~neampfes/ampgarage/forum/viewtopic.php?t=12386&start=0&postdays=0&postorder=asc&highlight=&sid=9935961d47748b3c9e77c8e5f3437885))

I did some searching for preamp only circuits that would include a phase inverter without much luck.

Thanks!

Title: Re: Taking preamp output directly from the phase inverter?
Post by: PRR on March 13, 2013, 11:46:40 pm

Signal out of phase inverter is 30V-50V, MUCH greater than you could possibly need or want for a line out.

Signal IN to the PI's driver is more like 1V-3V. That's a good (hot) voltage, but the current is slim to none.

You want a fat cathode follower.

Title: Re: Taking preamp output directly from the phase inverter?
Post by: EW57 on March 13, 2013, 11:56:00 pm

Thank you, that follows most of the "preamp only" circuits I've seen. Apparently any sonic differences resulting from the inclusion of the PI don't justify the added complexity, cost, effort required to decrease the levels back down to acceptable level.

Title: Re: Taking preamp output directly from the phase inverter?
Post by: sluckey on March 14, 2013, 07:30:48 am

Quote

Apparently any sonic differences resulting from the inclusion of the PI don't justify the added complexity, cost, effort required to decrease the levels back down to acceptable level.

All you need is two 1/2 watt resistors to make a 50:1 voltage divider. Maybe 220K plus 4.7K. Take the output across the 4.7K.

I've never seen this done but it certainly can be easily/cheaply done.

Title: Re: Taking preamp output directly from the phase inverter?
Post by: HotBluePlates on March 14, 2013, 07:44:51 am

I've always looked at the phase inverter as the first stage of the power amp, not as the last stage of the preamp. It's really there to take the preamp signal and deliver what push-pull output tubes need to work.

Title: Re: Taking preamp output directly from the phase inverter?
Post by: mat janssen on March 14, 2013, 10:45:27 am

I did it with a transformer.
You can see it on my website which is still under construction.  (PA Poweramplifier)
http://amplifiers.is-an-engineer.com/ (http://amplifiers.is-an-engineer.com/)
This is a test site. When ready it gets an original name.

Title: Re: Taking preamp output directly from the phase inverter?
Post by: 6G6 on March 15, 2013, 09:11:03 am

Ahh,
This one makes me feel better.
I was going to answer pretty much what PRR said.