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redplater 16th November 2010 10:32 PM

Phase inverter/power section
 
After building a good number of tube guitar and bass amps, I'm getting ready to start my first audio build. I'm looking at something in the range of 40-50 watts in a push-pull monoblock amps.

My question is, being a guitar amp guy.....the signal from the guitar preamp typically goes into a long-tail phase inverter into the power tubes. Usually, the phase inverter is a 12ax7 or 12at7.

I have yet to see this arrangement in an audio amp. While I have seen the occasional long-tail phase inverter, there's always some other driver stage in front of it. Why?

Any suggestions for schematics that are simple with few driver stages before the phase inverter?

I'm also building a grounded-grid preamp....do I need a driver stage in between this preamp and the phase inverter?

Thanks!!

Eli Duttman 17th November 2010 12:25 AM

1 Attachment(s)
Think in terms of the total gain needed and the 2 VRMS a "standard" CDP yields to achieve the power O/P desired.

The GG preamp has some gain. That might be enough to make "El Cheapo" style circuitry (schematic attached) work with the KT88s or 6L6GCs you will have to use. If a CDP directly drives an "El Cheapo", the 12AT7 LTP can produce enough gain only for "12" W. types and 7591s.

redplater 17th November 2010 01:53 AM

Sorry, but I'm a lowly builder of guitar amps...I don't know any of these acronyms: VRMS, CDP or O/P. However, I did kinda gather that you don't think I'll get much bang for my buck without putting a little more juice in the front end of the power section.

So, with that in mind...do you have any opinions of the Dynaco Mark III? It looks like a pretty simple build and should not take me too far from my Marshall/Fender power section/comfort zone that I'm used to.

Eli Duttman 17th November 2010 03:50 AM

VRMS = Volts root mean square.

O/P = output.

CDP = compact disc player.

The "El Cheapo" splitter/driver is highly competent. FWIW, I think that between the gain of the GG preamp and buffering the LTP with source followers you would have a satisfactory setup. Things could get very interesting if you dragged Soviet surplus 6Π3C-E (6p3s-e) tubes slightly into Class "AB2" with Tubelab's PowerDrive. The Russian tube is an affordable, good sounding, close match for the 6L6GC.

Miles Prower 17th November 2010 05:25 AM

Quote:

Originally Posted by redplater (Post 2367520)
I have yet to see this arrangement in an audio amp. While I have seen the occasional long-tail phase inverter, there's always some other driver stage in front of it. Why?

Vixen Main Schemo

Have a look at that. The grid drivers make for easy, DC connected, fixed bias adjust for the finals. That gets rid of the sonic penalties of capacitor coupling to the final control grids. Overdrive can't put an additional negative voltage on the grids when a transient turns on the GK parasitic diode. Since you have current sourcing, the 807s can slip a bit into Class AB2 transparently, making for better clip behaviour.

It also gives much better current sourcing to avoid the problem of slew limiting at the higher frequencies.

It presents the 6SL7 LTP a much friendlier load (Hi-Z, Lo-C) than would the 807s.

artosalo 17th November 2010 09:34 AM

Quote:

there's always some other driver stage in front ..... Why?/
One amplifier stage is needed before the phase splitter so that the sensitivity on the whole amplifier is sufficient ( 300 mV to 500 mV).

One reason to the need of additional gain is that in HiFi-amplifiers some amount of negative feedback is used (typically 10 dB to 20 dB) and this lowers the sensitivity with same amount.

Quote:

Any suggestions for schematics that are simple with few driver stages before the phase inverter?
Below is one. This uses 6L6G, 6P3S-E or 807 tubes ( I have tested all with this design). The power output with 450 V anode voltage is about 35 W, but even 50 W is available with some 550 V.

Essential points are to have separate bias adjustments for both output tubes as well as AC-adjustment at the phase splitter. By these adjustments the output tubes can be "matched" to minimum distortion.

http://kuva.termiitti.com/image/14088.gif


http://kuva.termiitti.com/image/14088.gif

redplater 17th November 2010 09:53 AM

Artosalo, Could you post the power supply for this schematic?

Thanks!
David

artosalo 17th November 2010 09:54 AM

I forgot to ask about one detail:

Quote:

I'm also building a grounded-grid preamp....
Such amplifier has very low input impedance.
What is the reason for such construction (grounded-grid) ?

redplater 17th November 2010 10:05 AM

I have heard good things about this preamp.

Transcendent Sound Grounded Grid Reviews

artosalo 17th November 2010 10:57 AM

Quote:

Artosalo, Could you post the power supply for this schematic?
Here it comes. There are some additional components, like DC-filament supply for pre-amp, which was not in the amplifier schematic I sent earlier. This power supply was designed for a integrated stereo amplifier, and therefore so much stuff.

The transformer is from Italy: inMADout Transformers



http://kuva.termiitti.com/image/14101.gif


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