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?
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?
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.....
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.....
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.
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.
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.
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.
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
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
eeyore said:
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
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.
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.
Booster Amps that will keep the SE sound:
http://www.diyaudio.com/forums/showthread.php?postid=873393#post873393
These use grounded gate to preserve the accuracy of the SE current output waveform and use Circlotron variants to reflect the load characteristics back to the SE amplifier. This is a form of anti-triode technology that drives the P-P stage with truly accurate complementary signals (not just your usual complementary drives, but here the resultant outputs are accurate).
Don
http://www.diyaudio.com/forums/showthread.php?postid=873393#post873393
These use grounded gate to preserve the accuracy of the SE current output waveform and use Circlotron variants to reflect the load characteristics back to the SE amplifier. This is a form of anti-triode technology that drives the P-P stage with truly accurate complementary signals (not just your usual complementary drives, but here the resultant outputs are accurate).
Don
DigitalJunkie said:
I'm sure that thought occurred to the original poster as well, but perhaps he, as I do, had something else in mind. 😀
kevinkr said:
Mebbe, mebbe not. But others listen in too, so no reason not to have a bunch of options to choose from - simple to wild. I qualified my suggestion with "if it's just for occasional use". It requires the least modification.
But what are the requirements or goals? Any good low distortion amplifier will pass through the distortion profile of the 45 amp - as seen across a resistive load. But it won't necessarily sound the same as a speaker loaded 45 amp.
If you want a single ended amp, just put a fairly high resistive load across the 45 OPT, and take the signal from the plate - for a choke loaded driver stage to a single ended higher power output stage. Again, requires almost no modification, but you maybe need something more than a single 300B or similar, to drive 88dB speakers.
If the goal is to preserve the sound character as much as possible, including the reactive speaker components, it seems that Don's suggestions would get closest to that. (Don, you got a FET version that will give more than power doubling?)
If the goal is to try something that's creative, different, you have the parts for, you think will look cool, whatever goal you have in mind, then of course the field is wide open
Sheldon
Thanks for the input and replies. Sheldon's idea sounds the simplest, and from I understand from his description, is effectively to have a output power attenuator to connect direct to another higher power amp. For argument sake, if I have a Chipamp LM3886, I could use a 4R/4R or 4R/6R or 4R/8R network (or similar) to adjust to the sensitivity of the Chipamp. With this idea, is the 45 amp effectively a very large and over-powered preamp?
Interesting ideas are always fun to explore. I'd like the idea of a 211/845 SET to boost the power. Without changing the 45 amp, would I be correct in assuming that I'd need another transformer that effectively steps up the output transformer of the 45 so that it provides enough voltage swing to the 211/845? Or would it be better to have a set of terminals that took the signal prior to the output transformer of the 45 and fed direct into the 211/845 which would effect be like a 21//845 amp driven with an IT where the driver stage is the 45 amp? Would this read something like this, that a switch would switch between the two;
6AN4 -> 45 -> OPTX -> Speakers
6AN4 -> 45 -> IT -> 211/845 -> OPTX -> Speakers
That would be a very interesting idea and an interesting build. Given the world economy, electricity prices, etc, have the ability to not switch or use the 211/845 SE all the time would be beneficial, whilst being able to still use a SET amp.
Interesting ideas are always fun to explore. I'd like the idea of a 211/845 SET to boost the power. Without changing the 45 amp, would I be correct in assuming that I'd need another transformer that effectively steps up the output transformer of the 45 so that it provides enough voltage swing to the 211/845? Or would it be better to have a set of terminals that took the signal prior to the output transformer of the 45 and fed direct into the 211/845 which would effect be like a 21//845 amp driven with an IT where the driver stage is the 45 amp? Would this read something like this, that a switch would switch between the two;
6AN4 -> 45 -> OPTX -> Speakers
6AN4 -> 45 -> IT -> 211/845 -> OPTX -> Speakers
That would be a very interesting idea and an interesting build. Given the world economy, electricity prices, etc, have the ability to not switch or use the 211/845 SE all the time would be beneficial, whilst being able to still use a SET amp.
eeyore said:
Yes. With only 1.5W of power, you don't need massive power resistors either - 5 watters should do fine. Try this idea first. Then, if you want to get more creative, you can try other methods mentoned - the sky's the limit.
I don't think it makes much sense to use a transformer to step up the voltage from the OPT. Why step down, then back up? If you want to go with an interstage transformer, I think it would be better to just switch out the OPT and switch in a purpose built interstage transformer.
An alternative simple approach is to use your OPT as a plate choke and take the output from the 45 plate to drive the bigger tube. The choke loaded 45 should give you enough voltage swing. Now, normally you wouldn't go to all the expense of winding up an OPT to use as a choke, but in this case you've already got it, and it's already mounted to your chassis. It should have adequate inductance for the job (check with Jack), and it's already properly gapped for your 45 operating conditions. You could use something like 1K across the OPT secondary (maybe Kevin has some suggestions here) to eliminate any possibility of insulation breakdown from high voltages developing in the unloaded secondary.
Sheldon
One other possibility.... building off Sheldon's idea of taking the drive direct from the 45 plate. Take a look on the Magnequest website at the Seth PP2A3 amplifier. It uses a center-tapped Magnequest choke to act as a phase splitter and drive the push-pull output pair. You could take the output from the 45 plate via a capacitor and drive the choke directly. This would "sorta" give you a parafeed output to the booster amp and keep things pretty simple.
Regards, KM
Regards, KM
Hello ,
and here a circuit from the past .
Regards , Alexander .
P.S.: It is from the Fonar site with many , many old circuits .
and here a circuit from the past .
Regards , Alexander .
P.S.: It is from the Fonar site with many , many old circuits .
An externally hosted image should be here but it was not working when we last tested it.
- Status
- Not open for further replies.