Push-Pull Without a Phase Inverter

[mijomo]

After reading the a post titled "Self Inverting Push Pull EL84 Amp" that you can find here: http://www.freestompboxes.org/viewtopic.php?f=2&t=15276
I recalled an article called "Push-Pull Without a Phase Inverter" which can be found here: http://www.ampbooks.com/home/amp-techno ... -inverter/

Now I was wondering if there are any amps that have been commercially produced that use such a design? I recall reading somewhere that Hiwatt may or may not have used "Common-Cathode Coupling Phase Inversion", does anyone know if this is or is not true?

Also, if you've designed something push/pull with no phase inverter I would love to give it a listen. In the first link I have listed above the author links to a video of the amp he build and I have to say that I'm impressed that such a simple circuit can produce such a good sound.

I would like to find out more about this design so I can build one for myself. If you know of any negatives to this method of phase inversion I would love the hear your opinion(s) about why, how, or why it should not be done.

Ciao & Cheers!

[soma_hero]

http://home.comcast.net/~stphkeri/magnatone_109.png

Used signal from the screen of the first power tube to to the grid of the second.

I built this power section into an amp once to cut down one tube. Just not enough volume, and the sound wasn't distinct enough to be cool for cool sake

info here: http://www.tubecad.com/index_files/page0020.htm
and http://www.ax84.com/bbs/dm.php?thread=129856

[printer2]

Without a phase inverter the design has been compromised (not to say each PI has its own compromises).

Field coil amp section from an 30's radio.

[img:720:897]http://i406.photobucket.com/albums/pp14 ... 18a860.jpg[/img]

First you have to run the amp in Class A otherwise the second output tube gets a distorted signal. Usually you get lower power than you would with a PI design. Used to be done to cut costs or save space.

[mijomo]

http://home.comcast.net/~stphkeri/magnatone_109.png

Used signal from the screen of the first power tube to to the grid of the second.

I built this power section into an amp once to cut down one tube. Just not enough volume, and the sound wasn't distinct enough to be cool for cool sake

info here: http://www.tubecad.com/index_files/page0020.htm
and http://www.ax84.com/bbs/dm.php?thread=129856

Thanks!
Those links were some good reading.
Is the low volume thing inherent in these style circuits?

[mijomo]

Without a phase inverter the design has been compromised (not to say each PI has its own compromises).

Field coil amp section from an 30's radio.

[img:720:897]http://i406.photobucket.com/albums/pp14 ... 18a860.jpg[/img]

First you have to run the amp in Class A otherwise the second output tube gets a distorted signal. Usually you get lower power than you would with a PI design. Used to be done to cut costs or save space.

Interesting.
I think I read somewhere that the older department store amps would skip on parts to save money. I knew they made some amps that were transformerless, now I have to try and find some designs without the whole phase inverter stage.

Thanks for your comment!

[printer2]

Without a phase inverter the design has been compromised (not to say each PI has its own compromises).

Field coil amp section from an 30's radio.

[img:720:897]http://i406.photobucket.com/albums/pp14 ... 18a860.jpg[/img]

First you have to run the amp in Class A otherwise the second output tube gets a distorted signal. Usually you get lower power than you would with a PI design. Used to be done to cut costs or save space.

Interesting.
I think I read somewhere that the older department store amps would skip on parts to save money. I knew they made some amps that were transformerless, now I have to try and find some designs without the whole phase inverter stage.

Thanks for your comment!

The firefly is PI'less.

Department store amps would usually be SE rather than P-P, cheaper. I think the transformerless amp was a Philips thing. Did not take off, I have two of these tubes, see last page.

http://frank.pocnet.net/sheets/168/7/7695.pdf

[Ken Moon]

I once built a tiny 2-tube P-P amp using 2 ECL86s, with a small interphase transformer doing the PI duties. It worked fine, but not really an improvement over using another tube, just a few less components.

[M Fowler]

Another example of no PI tube in the Magnatone Melodier 109

Sorry didn't see the eariler reference to the Magnatone 109.

[Andy Le Blanc]

I got a rig with kt88 and 50w opt, works but the trade off is distortion.
I always thought it would be great for slide or as a harp amp, it gets all saxophone-ish in an interesting way.

fun simple circuit

[renshen1957]

After reading the a post titled "Self Inverting Push Pull EL84 Amp" that you can find here: http://www.freestompboxes.org/viewtopic.php?f=2&t=15276
I recalled an article called "Push-Pull Without a Phase Inverter" which can be found here: http://www.ampbooks.com/home/amp-techno ... -inverter/

Now I was wondering if there are any amps that have been commercially produced that use such a design? I recall reading somewhere that Hiwatt may or may not have used "Common-Cathode Coupling Phase Inversion", does anyone know if this is or is not true?

Also, if you've designed something push/pull with no phase inverter I would love to give it a listen. In the first link I have listed above the author links to a video of the amp he build and I have to say that I'm impressed that such a simple circuit can produce such a good sound.

I would like to find out more about this design so I can build one for myself. If you know of any negatives to this method of phase inversion I would love the hear your opinion(s) about why, how, or why it should not be done.

Ciao & Cheers!

Hi,

As mentioned earlier, Magnatone had the Screen Phase Inverter, which was lifted from a Philco Radio Corp design of circa 1947. According to the late Gar Gillies of Garnet Amp fame the circuit was best at very moderate power levels (appropriate for a Radio at listening level) but not much else.

The Cathode Coupled design was used originally by Gibson with a Pair of 6V6 tubes, Class A in more than one amp. Gar Gillies wrote that the circuit was used in amps which produced between 4-15 Watts, 15 Watts can be quite loud with an efficient speaker. For that matter 4 Watts can have LE at the door with a speaker that is 1 Watt = 112+ dBs.

Garnet Amps produced a CC PP amp for the beginner market using two 50C5 Tetrodes and two 12AX7. All the heaters wired in series with the AC mains.

The design dates from around 1973 for their Private label (dept store) and RAM line. It was a 4 Watt Class A design. This amp sought inspiration from Radio, too. It used the same power tetrodes found in some of the later power transformer less radios known collectively as the American Five Radios. The Garnet RAM P-T 10 schematic from Jan. 1974 is also Power transformer less. Gillies like Fender was a radio repairman back in the day. Unlike Leo, Gar knew how to make Guitar amps, PA systems, etc. plus he worked and listened to musicians and their needs. (Hence the Herzog that was developed and used on American Woman).

The Firefly design is a self-split design. Kevin O'Conner uses a self split PP 12A_7 design in TuT Vol 5 for a reverb instead of a SE Xformer. Less hum, smaller transformer, etc. According to KOC, self split PP dates from the time of the telegraph.

Not exactly related but of interest (no tube is used in these PI)

Transformer coupled Phase splitting was used in some old, old low power Gibson amps. Transformer cost real money so other companies didn't copy Gibson's lead. Impedance Coupling was a variation of this principle used by Garnet in his earliest versions of the BTO. Like his fellow Canadian Traynor, Garnet was interested in Clean amps, but Impedance coupling only works to a maximum clean sound, which in the 1960's Rock and Roll era wasn't always a virtue.

Best regards,

Steve

PS The scanner is on the blink so I can't upload examples.

[printer2]

As mentioned earlier, Magnatone had the Screen Phase Inverter, which was lifted from a Philco Radio Corp design of circa 1947.

The above portion of a schematic I posted was from a '37 Philco radio I salvaged.

[img783]http://i406.photobucket.com/albums/pp14 ... GRadio.jpg[/img]

[img1010]http://i406.photobucket.com/albums/pp14 ... peaker.jpg[/img]

[renshen1957]

As mentioned earlier, Magnatone had the Screen Phase Inverter, which was lifted from a Philco Radio Corp design of circa 1947.

The above portion of a schematic I posted was from a '37 Philco radio I salvaged.

Hi,

This the schematic I pick up off the web the last time PP w/o PI was discussed and Philco 41225 came up.

I have included a Magnatone 109 schematic, too.

For those curious about the sound of the 109, Youtube has a video posted

http://www.youtube.com/watch?v=tA7q0FWY4Q0

Best regards,

Steve

PS My reference source may have had a typo it's just as easy to type 47 by mistake as 37. Then again the author has passed away no way to find out. Check out the power tubes, 41, used in the schematic are more typical of the 1930's than post WW II.

[barfoden]

Hi,, there is one very cheap asien made guitar amp that uses such a stage..

Designed by James brown who once worked at peavey and made the peavey classic series..

He has designed the kustom tube amps.. The defender series.. There is one that is called the defender v15, or 15h. I have bought such an amp and was intrigued as it only had one 12ax7 tube. First I thought maybe it has one gain stage and then a cathodyne splitter on the second triode. Nope.. then maybe a parallel SE amp.. Nope it is a "series" push pull where the input of one tube is grounded.

It seems to run the stock tubes pretty hot,, According to my calculations measuring the cathode voltage on its 91 ohm resistor i get around 14.6W plate dissipation at idle (I subtracted 10% screen current). That seems hot.

One user changed the chineese stock tubes to EL84M with a higher power rating (114W) and he said the amp has really high headroom for
cathode biased dual EL84 design..

[Structo]

I have seen interstage transformers do the inverter duty.

[rdjones]

Yes the output tubes will run hot, Class A.
That's because this type of inverter only works if the first tube amplifies the whole waveform.
If the waveform goes into cutoff the second tube looses it's drive.
So at or near clipping the whole scheme falls apart.
There's some inherent imbalance in the system which depends on the individual tubes characteristics.

It also doesn't clip gracefully, that is unless you like that particular sound.

It works best with small easily driven tubes such as EL84.