looking for pp amp schematic without nfb

[Curly Woods]

6336 availablity

Hi Robert,
Here is a direct link to the article which is on my site:

http://kta-hifi.net/projects/amp_page/300b_amp/30WPushpull 300B Amplifier.pdf

You just needed to dig a little further.

Here is the page the link lives on. There is a little additional information here plus some pictures.

http://kta-hifi.net/projects/amp_page/300b_amp/300bpp.html

The full schematics are at the end of the article. Note that this was written for VTV originally and at a time when no one considered using 300B in PP amplifiers so you may find the opening paragraph a bit bombastic.. I know I do many years after the fact..

THD in this design using 300B's is under 1% all the way up to 25Wrms.
NFB is not required unless you need a lower output impedance. i.e. higher damping factor and the rated output power.

Kevin,

The 6336 seems to be difficult to find at reasonable prices. Can the Russian tube 6C33C be substituted for this in your power supply design?

 

[ray_moth]

NFB is not required unless you need a lower output impedance.

Damping factor is the most important reason for NFB for many people; hoever, there are times when other benefits of NFB are worth having, e.g:

* stabilization of overall gain (the reason NFB amplifiers were first developed); it could be useful if both channels of a stereo amp maintain the same gain on a long-term basis, regardless of tube ageing.

* flattening of the frequency response.

* reduction of distortion.

Good damping is most important in the bass (NFB) while detail is most important in the middle and treble (no NFB). This being the case, maybe biamping offers the opportunity to get the best of both worlds?

 

[kevinkr]

Yes you can definitely substitute a 6C33 for the 6336, however in my experience the 6C33 is usually not a whole lot cheaper than the 6336. You could use a pair of 6080 or 6AS7G in parallel however use 50 ohm 5W resistors in series with each plate to assure current sharing. This option ought to be very economical as well.

Incidentally quite a few of these amps have been built over time, without any serious problems being reported back to me. I still have the originals, and unit to unit gain matching is just fine. (Probably better than a couple of tenths of a dB.)

Distortion is extremely low, reaching 1% at 25W, and 2% at 30Wrms - and its all low order. (predominantly 3rd with a little 2nd iirc)

Note that well matched output tubes are required to avoid hum on the outputs if AC heating is used as I did. Hum + Noise in this design if layed out with care and well matched tubes are used should be less than 2mVrms, and it should be mostly 60Hz from the heaters. Obviously dc could be used, if so I would recommend constant current heater operation - it allegedly sounds better, and based on my unscientific approach to the matter it at least seems to.