Electra-Print 300BDRD

[Salsero]

Hi,
Several questions about the electra-print DRD design

http://www.electra-print.com/300bdrd.php

1. Has anyone used the more economical Hammond or Edcor transformers for these amps? They are not exactly the same specs as the electra-prints. If so, do you have the schematics, I am very curios about these amps given their simplicity and non-capacitive coupling.
2. Can one set up a DRD amp in ultralinear? Would it work.
Thanks in advance
Salsero

[Mickeystan]

I built a single channel mono block of Jack's amplifier this past winter. I am currently gathering parts to build the other channel.

I built mine using a mix of parts, some made by Jack Elano and some by others. The driver plate choke, the HV choke and the filament choke I purchased from Jack. The power transformer I had custom made by Edcor. The output transformer I used was one of a pair I was given for Christmas (Transcendar 3k SE) units. Other than these differences, I followed Jack's design to the letter.

The results have been very impressive and I am totally happy with DRD 300 design. I highly recommend this amplifier but it is not cheap to build.

At this point, I have most of the parts for my second channel and am looking forward to turning my Hi Fidelity mono system into a stereo.

I have been amazed at how well this amplifer drives my very inefficient JBL 100 Century speakers in my electronics build area. I have yet to take the amplifier upstair to run it on my intended speakers, (Klipsch Forte II's). The JBLs are only 78 db/meter sensitivity and the Klipsch's are 99 db/meter. My 300b mono amp on the JBL 100 Century will nearly hurt my ears already. Thus, if you do not have the most sensitive speakers, I would not let that discourage you too much. You can always upgrade speakers later.

Doug S.

[Salsero]

Thanks for your reply. MY main issue with the amp design is the very high voltage it operates makes getting the power transformer rather cumbersome instead of allowing some standard off the shelf parts. Therefore, I wonder if running the output tube in Ultralinear mode may help power output while allowing a lower operating voltage which maybe attainable with an off the shelf tranformer.
If it could be run in UL, what voltage would I need at the power transformer to keep at least 10W power output?
Thanks again
Salsero

[JLH]

Originally posted by Salsero
Thanks for your reply. MY main issue with the amp design is the very high voltage it operates makes getting the power transformer rather cumbersome instead of allowing some standard off the shelf parts. Therefore, I wonder if running the output tube in Ultralinear mode may help power output while allowing a lower operating voltage which maybe attainable with an off the shelf tranformer.
If it could be run in UL, what voltage would I need at the power transformer to keep at least 10W power output?
Thanks again
Salsero

The output tube is a 300B triode. You cannot run a triode in Ultralinear because it does not has a screen grid. What are you talking about?

[Salsero]

You are right, my bad, I have been looking at too many schematics and mixed them up. However, now that you mention it, could a DRD design be used in a UL output stage? If so, could at KT88 tube be used?
Thanks again
Salsero

[Mickeystan]

Salsero,

Jack's direct reactance drive design requires the higher plate voltage on the 300B in order to elevate the filament of the 300B which is the source of the electron emission as this tube is a direct heated cathode design. The elevated cathode is at a level which establishes the delta cathode to plate voltage for the 300B at approx. 400V. With the cathode elevated to approx. 350V it has enough potential to support the direct reactance drive circuit made up by the choke loading of driver. This topology is what allows the direct part of the reactance drive work (e.g. no coupling capacitor in the audio path between the driver and output stage. Hope this helps in the understanding of the design.

Doug S.

[leadbelly]

Originally posted by Salsero
However, now that you mention it, could a DRD design be used in a UL output stage? If so, could at KT88 tube be used?

Yes. There's many variations of this out there. AFAIK, the basic DC coupled choke loaded driver part came into fashion with the original Free Lunch design and web page. Then many variations around this developed, like the drive circuit + ultrapath (DRD), or drive circuit + ultrapath + parafeed, etc. If you search on "free lunch" you will many different circuits with different tubes.

[leadbelly]

http://home.earthlink.net/~jeremyepstein/freelunch.html

"Excited by what I heard, I showed this circuit to the JoeNet pundits and rapidly found that some other people had been working towards a similar topology. One builder, Brian Clark, had been calling his version the "Monkey On A Stick," which sounded to me like some sort of obscene carnival snack (kind of like a super-disgusting corn-dog.) I was directed to John Broskie's "TubeCAD Journal" on the Web - he had also drawn up the Monkey circuit as a thought-experiment in a discussion of the Loftin-White amplifier. Doc Bottlehead's "Afterglow" circuit, published in the above-referenced Sound Practices article, was similar, but used a solid-state constant-current source instead of a choke. Jack Elliano of ElectraPrint has a design that is essentially the same except for the Ultrapath connection of the final power supply bypass cap. Christian Rintelen, always a pacesetter, has been listening to a similar amp for a while, designed in 1996 by his friend Reto Luigi Andreoli. And of course, there was the Komuro voltage-doubler version Clippy showed me that was the inspiration for mine. It appears that there have been a few different designers, then, contemplating this sort of arrangement, although most have been using stacked power supplies, while my amp uses only a single supply. The Monkey is more unconditionally stable than my design: the placement of the cathode resistor tap ensures that the 2A3 will have bias present even if the driver tube draws no current or is pulled out entirely. Mine does not have this feature* but I enjoy the sound of the amp best with all the tubes in it anyhow, so this does not concern me. Refer to the diagram (Figure 2) below for a comparison between the two topologies."

[Salsero]

Thanks for the replies, this is getting to the bottom line. The choke seems to work as a current source (I think, I am still getting to understand tubes). Therefore, is there a 300B design using a solid state current source as a coupling stage - if so, will this bring down the voltage requirements while keeping retaining the input to output coupling characteristics of a DRD? Hopefully, this is not a dumb question.
Thanks
Salsero

[dsavitsk]

The choke allows the driver to swing higher than its power supply. If you try to do a loftin white with a current source instead of a choke, it will increase your PS voltage requirements as a CCS can't swing so much.