Hi,
A few years ago I bought some old Tektronix transformers, including two 120-0140-00 from 545 oscilloscopes. Massive beast, ~500VA with five HV windings and several high current heater windings. Great for big, complex tube projects except for one major issue: the mechanical buzz they make. Removing the end bell helps a little bit, adding the two AUX windings to the primaries helps a to lower the flux density helps a little bit, soaking the windings with varnish didn't help at all.
Today I got this bright idea and lowered the input voltage drastically, from 230V to 160V by adding a "bucking transformer", resulting in a transformer quiet enough to use in hifi projects.
(Of course the secondary voltages went down with a factor 0,7 but that is not necessarily a problem, there's still plenty of HV available and the heater windings can be connected in series and and then either adjusted with voltage dropping resistors or rectified and filtered to clean 6,3Vdc)
This whole "bucking transformer" concept is new to me and raises a few questions: Would it be safe (and legal) to just put the bucking transformer in the amp, as an integrated part of the PSU? Most hits on google shows bucking transformers as separate units, in their own boxes connected between the amp and the outlet.
My next, more disturbing idea, is to use one of the HV windings to do the same job as the bucking transformer. The 120-0140-00 has five secondaries ranging from 113 to 195V and the datasheet states "All windings are insulated for 600Vdc unless otherwise stated". The 545 scope has several stacked PSUs fed from separate windings so I think it would work just fine but there's something about this idea that leaves a nasty itch in my mind.
Best regards,
Daniel
A few years ago I bought some old Tektronix transformers, including two 120-0140-00 from 545 oscilloscopes. Massive beast, ~500VA with five HV windings and several high current heater windings. Great for big, complex tube projects except for one major issue: the mechanical buzz they make. Removing the end bell helps a little bit, adding the two AUX windings to the primaries helps a to lower the flux density helps a little bit, soaking the windings with varnish didn't help at all.
Today I got this bright idea and lowered the input voltage drastically, from 230V to 160V by adding a "bucking transformer", resulting in a transformer quiet enough to use in hifi projects.
(Of course the secondary voltages went down with a factor 0,7 but that is not necessarily a problem, there's still plenty of HV available and the heater windings can be connected in series and and then either adjusted with voltage dropping resistors or rectified and filtered to clean 6,3Vdc)
This whole "bucking transformer" concept is new to me and raises a few questions: Would it be safe (and legal) to just put the bucking transformer in the amp, as an integrated part of the PSU? Most hits on google shows bucking transformers as separate units, in their own boxes connected between the amp and the outlet.
My next, more disturbing idea, is to use one of the HV windings to do the same job as the bucking transformer. The 120-0140-00 has five secondaries ranging from 113 to 195V and the datasheet states "All windings are insulated for 600Vdc unless otherwise stated". The 545 scope has several stacked PSUs fed from separate windings so I think it would work just fine but there's something about this idea that leaves a nasty itch in my mind.
Best regards,
Daniel
I would expect much greater primary to secondary insulation than just 600 volts.
A permanent bucking transformer should be just fine. Although I would insulate it from the chassis.
I would also check to see if it makes less noise under load. That could allow for less bucking voltage.
The other possibility would be to use two of the transformers with the primaries and secondaries in series.
A permanent bucking transformer should be just fine. Although I would insulate it from the chassis.
I would also check to see if it makes less noise under load. That could allow for less bucking voltage.
The other possibility would be to use two of the transformers with the primaries and secondaries in series.
600Vdc insulation perhaps is a bit low, yes. We usually se 4000V pri-sec insulation in modern equipment.
I have tried the transformer in a sweep tube PP prototype running four big tubes in class A and the hum/buzz was still present, unfortunately.
What I'm thinking (this is only on a very theoretical level) is to use the 306 turns /195V winding closest to the primaries as a bucking winding, and then leaving the 215 turns /137V winding outside it unused as a "safety barrier" between the mains voltage and the other secondaries. Maybe with one end of the winding connected to mains earth? Just thinking loud here.
The windings are made with rather thick, impregnated paper/tape and a part of me thinks this would propably be much safer than any 5 dollar Asian made transformer/SMPS made today, but still...
I have tried the transformer in a sweep tube PP prototype running four big tubes in class A and the hum/buzz was still present, unfortunately.
What I'm thinking (this is only on a very theoretical level) is to use the 306 turns /195V winding closest to the primaries as a bucking winding, and then leaving the 215 turns /137V winding outside it unused as a "safety barrier" between the mains voltage and the other secondaries. Maybe with one end of the winding connected to mains earth? Just thinking loud here.
The windings are made with rather thick, impregnated paper/tape and a part of me thinks this would propably be much safer than any 5 dollar Asian made transformer/SMPS made today, but still...
The idea of using both transformers connected in series is strangely attractive. As far as I can tell it would be perfectly safe, and I think connecting as many secondaries as possible in parallel between the two transformers would force them to operate in balance with each other? OTOH, as long as the transformers are identical, which I assume they are, they should be equally balanced with the secondaries in series. This is uncharted territories for me, but it would be nice to be able to use them without having to live with the sound of a bumblebee in a beer can.
My next project is a 6B4G SET, two Tektronix 500VA power transformers would at least provide some visual impact...😎
My next project is a 6B4G SET, two Tektronix 500VA power transformers would at least provide some visual impact...😎
Are you wiring the primary input to pin 1, pin 3 to A, C to B, D to 2, and the other side of the line to pin 4? The other version would change 3 to D and A to 2. The correct way will yield the lowest output voltage.
Simon: I don't have the transformer in front of me right now but your description sounds familiar. I followed the wiring instructions on the sticker on the transformer, adding the AUX windings in series with the primaries lowered the unloaded heater windings from 6,7V to 6,4V (or so, I don't remember the exact numbers). Messing with the AUX windings has a marginal effect on the mechanical noise, in fact there are miniscule traces of it left even with 162V into the 234V primary windings but not enough to be annoying.
I've used these transformers in various prototypes over the last few years an the buzzing has always been rather constant, always a bit too loud to be acceptable.
If I must use one of these transformers "as is", I guess it could be mounted on rubber dampers inside som kind of box with thick walls.
For the uppcoming 6B4G SET project it could actually be benificial to cut all the voltages down to 60-70% or so with a bucking transformer, since 4,5-5VAC windings connected in series for 9-10VAC would be much easier to rectify into clean 6,3Vdc supplies than 6,3VAC windings and the HT would only be in the 300V range.
I've used these transformers in various prototypes over the last few years an the buzzing has always been rather constant, always a bit too loud to be acceptable.
If I must use one of these transformers "as is", I guess it could be mounted on rubber dampers inside som kind of box with thick walls.
For the uppcoming 6B4G SET project it could actually be benificial to cut all the voltages down to 60-70% or so with a bucking transformer, since 4,5-5VAC windings connected in series for 9-10VAC would be much easier to rectify into clean 6,3Vdc supplies than 6,3VAC windings and the HT would only be in the 300V range.