Booster Amp for low powered SET

[eeyore]

Currently finishing off the DRD running the 45 tube, and according to design, it gives around 1.5 W of max power. I want to give this amp a little more flexibility and would like to find a design or come up with a simple design for a booster amp to give it more power to mate with 88 dB speakers (i.e., Jordan JX92s).

Came across one design in an old issue of VTV using 8 Svetlana tubes. I would like to use a KT88 in triode mode, as a singular push-pull pair. For that, I assume I will need an input transformer, to take the 8R tap from my DRD. What is the best way to determine the design of the transformer? I also assume that if I use a pair of KT88 in push/pull, that I can get away with a standard OPTX that is designed for a KT88 push/pull pair.

Anyone out there shed some light on designs and/or concepts?

[Richard Ellis]

I'm a bit confused, you state it is an SET...and you want to reconfigure your already completed amp to a push-pull type?
If indeed it is an SET, a more simple redesign could be parallel output configuration perhaps two 45s' or three....It is going to of course load down the driver stage........Do you have a schematic handy we can see?
____________________________________________Rick.. ...

[eeyore]

Sorry, perhaps I needed to provide more details. Currently, the amp is a DRD45 design from www.electra-print.com. It is a SET. However, I would like to, on occasion, use this amp with a lower efficiency speaker, and hence would like to provide an amp that will 'boost' the power of the SET. I have a half working KT88 p/p amp with parts lying around and thus would like to explore the possibility of using the KT88 to boost the power on occasion.

[JoshK]

I've heard of booster amps, but never seen a design of one. The pass amp with no gain, think its the F5 or something like that could work. A similar idea from tubes could be worked out. Think low/unity gain amp. If your DRD amp has 16 ohm output, I'd use that tap to keep voltage up.

[DigitalJunkie]

I vaguely remember seeing a schematic for one.

IIRC, It was just your basic PP output stage,with an input transformer,instead of the gain/PI stage(s).
Something like a small 10W PP output transformer might work for the input.
You could even supply -Vg1 bias through the input transformer center tap.

[DigitalJunkie]

Here's the basic idea..

(Sorry for my fantastic paint skillz!)

[revintage]

Why not go for sand? You would have to order a custom OPT from Jack though. My guess is it should be in the ballpark of 3k:16+16ohm(64ohmCT). Don´t mind the component values, this is just a quickie.

[kmaier]

A booster amp is a nice idea, I'm guessing you're looking to retain the "45 magic" sound but once you use it drive a push-pull stage you won't have much of that left.

DJ posted a familiar booster schematic, this was quite popular back in the 50's for getting high-power drive (like several hundred watts) for AM modulator use (look at some 50's vintage ARRL manuals).

You could get a custom driver transformer, but that gets expensive. The idea of using a small low powered PP OPT could work but you'll probably need to use a voltage divider to drop the voltage swing down. Also note that any output noise from your 45 amp would also get amplified here, so resultant S/N may be unacceptable.

You may well find a more cost effective (and quieter) solution is to just build a simple input/driver circuit for the push-pull amp and live with it. I'm certainly not trying to discourage you from going down this path, just pointing out some possible pitfalls.

Regards, KM

[Sheldon]

Originally posted by eeyore
Currently finishing off the DRD running the 45 tube, and according to design, it gives around 1.5 W of max power. I want to give this amp a little more flexibility and would like to find a design or come up with a simple design for a booster amp to give it more power to mate with 88 dB speakers (i.e., Jordan JX92s).

Came across one design in an old issue of VTV using 8 Svetlana tubes. I would like to use a KT88 in triode mode, as a singular push-pull pair. For that, I assume I will need an input transformer, to take the 8R tap from my DRD. What is the best way to determine the design of the transformer? I also assume that if I use a pair of KT88 in push/pull, that I can get away with a standard OPTX that is designed for a KT88 push/pull pair.

Anyone out there shed some light on designs and/or concepts?

If it's just for occasional use, no need to get complicated. Power of 1.5W into 8R translates into an output voltage of 3.7RMS. Just make a voltage divider out of two 4R power resistors, to half the output voltage, and drive any amp designed for 2VRMS input - tube or solid state (or adjust the divider appropriately for higher sensitivity amps, so that you retain the clipping characteristic of the 45). You can try 4 to 32R, or higher for the total divider string to sample different load lines for the 45, if you like. Think of your amp as an inefficient pre with plenty of current drive.

Sheldon

[kevinkr]

I've been toying around with the idea of a 211 based SE booster amplifier for occasional use with my 300B SET.. (Might make it modular so IT unplugs and can be replaced with a standard driver stage as I desire.) I can't see why a PP triode connected KT88 scheme couldn't work and sound relatively good. It would certainly be a unique conversation piece and should be a very interesting build.

I am sure Edcore could build you a suitable IT at reasonable cost. Ratio might need to be something like 1:10+10 assuming KT88 and slightly less than 4Vrms available from the 45 SET.. Interestingly enough source impedance is quite low (800 ohms grid to grid, 200 ohms grid to ground) so you should have lots and lots of bandwidth.

I'd encourage you to try it. I am finding a lot of value in old, and often discarded ideas. (The Japanese have been much quicker at recognizing these things than we have.)

I am slowly discovering that transformer coupling works in a lot of places where I wouldn't expect it to be as good as it has turned out to be.

I would probably use fixed bias, no feedback, and a relatively high B+.. (Say 450V - 500V) Noise and signal will both be amplified by this approach so you need to assure that the SET amplifier used is quiet enough in normal use to produce good results when used with a booster amplifier. You should be able to achieve about 30Wrms using this approach.