After a very long hiatus, I'm finally finishing this BDT preamp build.
Tests just now are producing it's first sounds but I have to check out how I have the pot wired. I'm also just using a pair of headphones for testing so the sound is pretty low because I'm using the output transformer designed for driving into an amplifier.
Schematic:
It's a really interesting & unique topology using Beam Deflection Tubes which were originally designed for colour TV as colour subcarrier demodulators. here's what John S says about these tubes: "they just happen to be one of the most linear amplifiers ever made and behave like differential amplifiers with balanced in and out (they make great phase splitters too), with one really neat feature: the gain can be adjusted with a DC voltage on the control grid." So what we have is a balanced tube version of the lightspeed with gain! Main thread on this is here
I'm looking at various possibilities for using this pre-amplifier:
- A headphone amplifier if a suitable output transformer is used - I'll be trying a Toroidal PS transformer as John has suggested dual primaries/dual secondaries - secondaries of 24V will give about 100ohm output impedance - depending on how low the headphones' impedance is secondaries of 12V or 10V can be tried. This should make a really nice H/P amp and can easily be run in balanced configuration.
- I'm also thinking of being able to switch the output transformers so as to use this as the front end for a Susan Parker Zeus amplifier. This will be an interesting configuration I reckon.
- I'll also be using this as a separate pre-amplifier!
Has anybody else built one of these?
Tests just now are producing it's first sounds but I have to check out how I have the pot wired. I'm also just using a pair of headphones for testing so the sound is pretty low because I'm using the output transformer designed for driving into an amplifier.
Schematic:
It's a really interesting & unique topology using Beam Deflection Tubes which were originally designed for colour TV as colour subcarrier demodulators. here's what John S says about these tubes: "they just happen to be one of the most linear amplifiers ever made and behave like differential amplifiers with balanced in and out (they make great phase splitters too), with one really neat feature: the gain can be adjusted with a DC voltage on the control grid." So what we have is a balanced tube version of the lightspeed with gain! Main thread on this is here
I'm looking at various possibilities for using this pre-amplifier:
- A headphone amplifier if a suitable output transformer is used - I'll be trying a Toroidal PS transformer as John has suggested dual primaries/dual secondaries - secondaries of 24V will give about 100ohm output impedance - depending on how low the headphones' impedance is secondaries of 12V or 10V can be tried. This should make a really nice H/P amp and can easily be run in balanced configuration.
- I'm also thinking of being able to switch the output transformers so as to use this as the front end for a Susan Parker Zeus amplifier. This will be an interesting configuration I reckon.
- I'll also be using this as a separate pre-amplifier!
Has anybody else built one of these?
Do you find any problem with partition noise? A BDT has two sources of partition noise. One is the normal g2 (called accelerator in BDT) like a pentode, the other is partition between the two anodes. The second would be the worst. This noise is not a problem when a BDT is used for its intended purposes, but I wonder if it is noticeable in this audio application?
I'm still in preliminary testing so too soon to say - I'm using headphones to test at the moment as my main amp is out with a reviewer. I'll be trying to optimise the sound tomorrow & will post my findings but the sound I heard on my headphones had no signs of noise. JS had two issues with these tubes
- one, they are sensitive to magnetic fields so need to be located away from the PS trafo.
- two, I think he said that they are a bit sensitive to vibration.
I have to test both of these aspects as I have not taken any special precautions in these areas, yet!
This is the first time I've heard about partition noise - have you used these tubes before in the TV field?
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Interesting tube. My first question of course was how linear is it without all the sand on top? I was curious to find out what might be achieved by more normal methods so I did some searching. From Mr. Swenson's write up here...
Tube DIY Asylum
it sounds like it might not be all that linear. It would be interesting to see a comparison. He also mentions that it is susceptible to magnetic fields and to heater hum (thus DC heaters). I like the idea of being able to control the gain via grid voltage (remote control via. SS pot) but it looks like you have to beat the tube into submission with some pretty big hammers. 🙂
Makes me wonder what other odd ball tubes might also be repurposed.
Tube DIY Asylum
it sounds like it might not be all that linear. It would be interesting to see a comparison. He also mentions that it is susceptible to magnetic fields and to heater hum (thus DC heaters). I like the idea of being able to control the gain via grid voltage (remote control via. SS pot) but it looks like you have to beat the tube into submission with some pretty big hammers. 🙂
Makes me wonder what other odd ball tubes might also be repurposed.
Well I've briefly listened to it without the mosfet cascodes & I don't hear any non-linearities - that's not to say there are none -again this was through headphones (32ohm) so the vol was down. There's a max gain of 4 without the mosfet cascodes. Not sure if it has to be beaten into submission to linearise it though. The cascodes only require a little CCS & a couple of parts - not difficult. The main build-up work is in the PS but with some boards these are not too difficult. I built mine on perf-board before I got pcbs
I think he also mentions using 6DJ8 tubes which he used in a low power amp along with the 6AR8 front end - here's what he said "For the fun of it I also built a power amp just using BDTs, you can get a couple watts out of it. I used a 6AR8 driving a 6DJ8 (with the cascode on the 6AR8 to get enough gain), it sounds fantastic on very sensitive speakers. It would probably make a really nice headphone amp but I've never actually built one in that configuration. The headphone amp could be built with just a 6AR8 per channel. "
I think he also mentions using 6DJ8 tubes which he used in a low power amp along with the 6AR8 front end - here's what he said "For the fun of it I also built a power amp just using BDTs, you can get a couple watts out of it. I used a 6AR8 driving a 6DJ8 (with the cascode on the 6AR8 to get enough gain), it sounds fantastic on very sensitive speakers. It would probably make a really nice headphone amp but I've never actually built one in that configuration. The headphone amp could be built with just a 6AR8 per channel. "
The BDT has signal on only one of its deflectors. It might perform better with a symmetrical input. If only one input is available, I would try tapping R17 to get the same signal amplitude (maybe 1/2 since the output will double with both deflectors working) as on the active input, and connect that to the grounded deflector (cap coupled with a high value R to ground for 0 V DC). You could also try that with both deflectors, as long as these positive feedbacks have less than unity gain (adjust tappings for a little less than unity gain).
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I'm using both inputs (balanced), smoking-amp but thanks for the advice for SE use as I will want to connect it SE at times - I don't really follow what you are saying though, - could you diagram it?
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You just split R17 into two series resistors. Their sum approx. the same resistance as the present R17, but the smaller resistor/larger resistor should = (input signal V)/(2X present output signal V) at the BDT plates. Smaller resistor goes to ground as before. Tap point gets a cap to the presently grounded deflector. A high R resistor from that deflector to ground maintains 0 DC level on the deflector still.
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Ok, I got it but the Vout on the plate changes based on the vol control, I think - so does this negate your idea?
I seem to remember JS stating that it will work better in balanced mode
"but the Vout on the plate changes based on the vol control, I think - so does this negate your idea?"
No problemo, input and output levels track.
No problemo, input and output levels track.
The input signal on the active deflector isn't changed by the vol. control though - this is not like a conventional volume control where the input signal goes through the vol pot & is attenuated. This volume is controlled by changing the DC voltage on pin 6 (the control grid). This is one of it's advantages & makes it the tube equivalent of the SS Lightspeed but a lot more flexible & with gain if wanted.
It could be called a "preamplifier in a bottle"!
So the signal level on the active deflector will not be attenuated by the vol control, just the output on the plates. I don't know if your mod still stands in this case?
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No, but I have thought about using them for receiver mixers - see my website. Note that equivalent noise resistance figures given there are for mixer applications via grid drive; deflector drive will be worse because the gain is lower.
Partition noise happens when each electron in a stream has to randomly choose which way to go. It is the main source of noise in pentodes, as the cathode current divides between anode and g2. I believe a BDT will have extra partition noise due to the current dividing between the two anodes too. It may be that in a line stage it is still too low to be a problem.
I don't know - I haven't gotten that far yet 🙂
But a couple of points:
- AFAIR, JS said the matches between these tubes was very good.
- these tubes are so cheap $7, I bought 4 of them - should find 2 matching
- you can always have a balance control (a remote one would be nice)
- this seem far easier than all the matching that goes on with LDR devices in the SS lightspeed
I will report my results as I make progress!
In practise, I haven't heard any noise yet & JS has not reported anything - actually quiet the opposite - I seem to remember him saying that he thought he had hooked it up wrong because there was no sound, it was dead quiet, no hiss no hum!
He has run this "up to 90V PP with a different transformer without any increase in distortion"
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I just found his write-up when he changed output transformer from Lundahl LL1660 to Dave Slagle wound ones:
Re:
"The input signal on the active deflector isn't changed by the vol. control though "
Oh, I see what you mean. In that case it won't stay balanced as the volume control is changed. Would require a tracking pot in the deflector attenuator (R17,R17'). Not too practical that way.
"The input signal on the active deflector isn't changed by the vol. control though "
Oh, I see what you mean. In that case it won't stay balanced as the volume control is changed. Would require a tracking pot in the deflector attenuator (R17,R17'). Not too practical that way.
Thanks, I suspected that might be the case!
I don't think there's a problem with the current schematic except the battery is connected the wrong way around - pos should be to ground & neg driving the control grid! Also JS tells me gate stoppers R9, R10 should be up around 4K to kill any chance of mosfet oscillation.
I also substituted P4NK60SZFP mosfets for the IRF820 - I'm sure lots of others would do - it's advisable to use insulated types as there is 130V on these
I don't think there's a problem with the current schematic except the battery is connected the wrong way around - pos should be to ground & neg driving the control grid! Also JS tells me gate stoppers R9, R10 should be up around 4K to kill any chance of mosfet oscillation.
I also substituted P4NK60SZFP mosfets for the IRF820 - I'm sure lots of others would do - it's advisable to use insulated types as there is 130V on these
Oh I forgot to add this quote form JS showing the versatility of this concept:
An externally hosted image should be here but it was not working when we last tested it.
I have just done a quick noise calc. Assume 100k equivalent noise resistance (guess, but probably OK within a factor of 2) you get 5.5uV in a 20kHz bandwidth. If your input signal is 100's of mV then this won't be noticeable, athough it is more noise than a well-designed conventional preamp.
This type of circuit should sound clean on signal peaks as the deflector can handle 10V of signal. A typical line stage will struggle with more than a few volts, and some poorly designed ones will fall over before then due to poor biassing.
I hope this design does not become too popular, as we radio amateurs want to keep the price of BDT down so we can use them in RF equipment!
This type of circuit should sound clean on signal peaks as the deflector can handle 10V of signal. A typical line stage will struggle with more than a few volts, and some poorly designed ones will fall over before then due to poor biassing.
I hope this design does not become too popular, as we radio amateurs want to keep the price of BDT down so we can use them in RF equipment!
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