Hi everyone!
I've had several vitage tube amps and modified them but I'm not at the range of expertise to evaluate every aspect of them. Since SMPS are available (please disregard tha tin the discussion, I've got several analog and digital PS available), the major factor in cost is the output transformer. They are (compared to other transformers) very expensive, and I'm aware the saturation is a big issue, my question is, why not use regularily available ones? I have to admit, I am not aware what transformation values are exactly needed nor do I know about the typical inductance of sait OT.
Capable and reputable OT (~40-100W) come at 150-400€, but here's my question, if you can get 100V audio transformers for as low as 2,50€ (in bulk even cheaper), isn't it possible to buy a bunch of these and compensate for the current by parallel/serial connect them? Probably counter inductance by R-C or other circuits? I mean, for 400€ you could get ~160+ of them.
Please don't laugh about that, I'm often able to tell what's a problem in an amplifier circuit but regarding tubes I've got little practical experience in output transformers and what's really required or the 'usual range' of output transformers.
So my question is, (besides saturation), what's the problem of mass parallel/serial 70/100V transformers and why it isn't there any example on the net for it (or rebunking it) since it could shave the cost down to about 1/10s? I know, that it's probably looking at it very naively but I haven't found any reputable stance on it.
I've had several vitage tube amps and modified them but I'm not at the range of expertise to evaluate every aspect of them. Since SMPS are available (please disregard tha tin the discussion, I've got several analog and digital PS available), the major factor in cost is the output transformer. They are (compared to other transformers) very expensive, and I'm aware the saturation is a big issue, my question is, why not use regularily available ones? I have to admit, I am not aware what transformation values are exactly needed nor do I know about the typical inductance of sait OT.
Capable and reputable OT (~40-100W) come at 150-400€, but here's my question, if you can get 100V audio transformers for as low as 2,50€ (in bulk even cheaper), isn't it possible to buy a bunch of these and compensate for the current by parallel/serial connect them? Probably counter inductance by R-C or other circuits? I mean, for 400€ you could get ~160+ of them.
Please don't laugh about that, I'm often able to tell what's a problem in an amplifier circuit but regarding tubes I've got little practical experience in output transformers and what's really required or the 'usual range' of output transformers.
So my question is, (besides saturation), what's the problem of mass parallel/serial 70/100V transformers and why it isn't there any example on the net for it (or rebunking it) since it could shave the cost down to about 1/10s? I know, that it's probably looking at it very naively but I haven't found any reputable stance on it.
Typical 70/100V transformers cannot handle DC and must be isolated from the tube standing current.
You could build a push-pull amp or investigate parafeed. Good parafeed amps have been built with some pretty ordinary output transformers. Bottlehead in Seattle started out this way I think?
The challenge then becomes how to provide sufficient load for the tube (parafeed choke or current source).
I don't know how you'd compensate for the current by parallel/series connection of multiple transformers. The capacitance of the transformer winding might start to become a problem.
Not trying to put you off, it's worth trying - please report back!
Cheers, Mike
You could build a push-pull amp or investigate parafeed. Good parafeed amps have been built with some pretty ordinary output transformers. Bottlehead in Seattle started out this way I think?
The challenge then becomes how to provide sufficient load for the tube (parafeed choke or current source).
I don't know how you'd compensate for the current by parallel/series connection of multiple transformers. The capacitance of the transformer winding might start to become a problem.
Not trying to put you off, it's worth trying - please report back!
Cheers, Mike
ICG,
Nice idea, but with limitations.
Those generally are Public Address transformers, and Elevator Music transformers.
No DC on the primary for single ended use (could use for a Parafeed output stage).
And Generally No "Center Tap" for push pull.
May, or may not, be frequency response limited.
Not specifically designed to be usable in global negative feedback circuits.
100Vrms, 140V peak, when driving it as a 1k load impedance (using the transformer as a 1k load) = 10 Watts RMS.
Etc.
Put a bunch of them in series, series parallel, or parallel, and what do you have?
Only some extensive measurements of a single one, and then some calculations can give a guess of the possible results.
Do not forget the capacitances of the windings to the laminations, using lots of them will multiply the capacitance by 1 x "the number" of transformers.
There are several reasons why consumer Hi Fi Stereo amplifier manufacturers went to solid state, here are 2:
No Expensive output transformer
No Heavy output transformer
Nice idea, but with limitations.
Those generally are Public Address transformers, and Elevator Music transformers.
No DC on the primary for single ended use (could use for a Parafeed output stage).
And Generally No "Center Tap" for push pull.
May, or may not, be frequency response limited.
Not specifically designed to be usable in global negative feedback circuits.
100Vrms, 140V peak, when driving it as a 1k load impedance (using the transformer as a 1k load) = 10 Watts RMS.
Etc.
Put a bunch of them in series, series parallel, or parallel, and what do you have?
Only some extensive measurements of a single one, and then some calculations can give a guess of the possible results.
Do not forget the capacitances of the windings to the laminations, using lots of them will multiply the capacitance by 1 x "the number" of transformers.
There are several reasons why consumer Hi Fi Stereo amplifier manufacturers went to solid state, here are 2:
No Expensive output transformer
No Heavy output transformer
I don't know about Bottlehead but paralelling a bunch of OT should be able to handle it for ~30-50W output? I mean, at ~160 per side (or even a fraction of it) the saturation because of the DC can't be that bad? Can it?
At the sheer number of transformers, an R-C compensation should not result in a that high cost per capacitor/resistor. Maybe I'm off with my numbers but my calculation said I'd need just ~20€ of parts for that. Again, I might be overseeing something really important or am ignorant of some fact but the inductance seems so far the biggest issue - which could be compensated by paralleling enough OTs. What I am overseeing?
Thank you but so far it's just a thought experiment (and having a bunch of 100V transformers laying around).
If you are designing a tube amplifier from scratch, use an output transformer that is designed for the job at hand.
You will have lots of other problems to solve that will take your time; and no time to solve all the problems with trying to use those 100V transformers.
You remind me of my early miniature 6 transistor AM radio. It had an interstage/splitter transformer [worked with DC in the primary],
and a push pull output transformer.
Those transformers were light and inexpensive, but they were not Hi Fi.
A Blast from the Past!
You will have lots of other problems to solve that will take your time; and no time to solve all the problems with trying to use those 100V transformers.
You remind me of my early miniature 6 transistor AM radio. It had an interstage/splitter transformer [worked with DC in the primary],
and a push pull output transformer.
Those transformers were light and inexpensive, but they were not Hi Fi.
A Blast from the Past!
These series-parallel small xfmrs ideas come up regularly, and they don't generally work out well, since you need equal split primaries for DC balance or expensive inductors for parafeed. Inductance of windings all on the same core go up as turns squared, but with separate cores you only get addition. Capacitances go out of control as well.
The only hope are "Circlotron" type amplifier designs. Which look ingeniously - attractive on paper until you price out -TWO- floating B+ supplies. Heavy too.
Totem pole tube designs also can work here, but they have their own long list of design problems.
Then there is the D. Berning switchmode output scheme which has an even longer list of technical problems.
OTL can heat your house with lots of parallel tubes.
The Greinacher multiplier circuit ( or Cockroft-Walton circuit), in resonant form, can perform impedance conversion economically, similar to the D. Berning design. Just a bunch of capacitors stacked up with an inductor. But requires extensive design knowledge. Was mentioned here like 15 years ago. No interest developed. Tube Amps are supposed to be simple.
I would suggest just getting some decent OTs with your favorite tube type primary impedance, like maybe 4K or 6.6K, and putting them on a board with Banana plugs so you can move them from project to project as needed. You can only listen to one set of amplifiers at a time anyway. (excepting complex multi-amped speakers )
The only hope are "Circlotron" type amplifier designs. Which look ingeniously - attractive on paper until you price out -TWO- floating B+ supplies. Heavy too.
Totem pole tube designs also can work here, but they have their own long list of design problems.
Then there is the D. Berning switchmode output scheme which has an even longer list of technical problems.
OTL can heat your house with lots of parallel tubes.
The Greinacher multiplier circuit ( or Cockroft-Walton circuit), in resonant form, can perform impedance conversion economically, similar to the D. Berning design. Just a bunch of capacitors stacked up with an inductor. But requires extensive design knowledge. Was mentioned here like 15 years ago. No interest developed. Tube Amps are supposed to be simple.
I would suggest just getting some decent OTs with your favorite tube type primary impedance, like maybe 4K or 6.6K, and putting them on a board with Banana plugs so you can move them from project to project as needed. You can only listen to one set of amplifiers at a time anyway. (excepting complex multi-amped speakers )
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So what 100 volt transformers do you “have lying around”? What wattage and taps? A trafo that has a factor of 4 wattage tap available has a center tap for push pull. It might be a little off center if they tried to correct for ohmic losses in the wire, but would be close enough to “work”. As a rule you end up with better performance with those little 50 VA Antek power toroids turned around backwards because they are inherently balanced, but free or already paid for is better than $20 plus shipping. Can‘t expect miracles but its possible to make it work. The lower the wattage the higher impedance you get anode to anode.
There was an article on the internet showing the use of line output transformers in a tube push-pull setup, but can't find it now.
PS Found it: https://www.ozvalveamps.org/optrans.htm
PS Found it: https://www.ozvalveamps.org/optrans.htm
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I was hunting around for that article posted by @jcalvarez before I saw his post. The interesting take away from that article is that the power of the line output transformer is optimal around 50VA; above that and the potential number of use cases starts to drop.
Probably the best budget solution for OPTs, with lots of empirical evidence on this site, is the use of power toroids with two primaries and secondaries (e.g. 115V + 115V, 12V + 12V, 50VA). You would have to dig around for the impedances and so on, but at least 2 posters here use them a lot in their builds.
The problem I see is that after spending a lot of hours fabricating something that is safe, robust, and designed to perform at a high level, then skimping on the OPTs is counter productive.
If you buy from a respected provider, like Toroidy, then they do not need to cost a disproportionate amount to be high quality (albeit not superlative quality), especially if you buy them with the basic finish, and not domed and chromed.
I have had the best success with putting a lot of research into auction adverts, including going to other countries. Sometimes a good pair of OPTs can be in an otherwise scrap amplifier (e.g. some Sansuis, Pioneers). I stumbled across a pair of Partridge 8K PP+UL transformers in a job lot of old transformers. I fitted them to a Tubelab SPP and it is hard to describe properly, but music that can be background on an amp with cheaper OPTs, just demands to be listened to in the foreground with the Partridges.
It is actually best to find good OPT candidates at a reasonable price, and then to develop the project around them.
Probably the best budget solution for OPTs, with lots of empirical evidence on this site, is the use of power toroids with two primaries and secondaries (e.g. 115V + 115V, 12V + 12V, 50VA). You would have to dig around for the impedances and so on, but at least 2 posters here use them a lot in their builds.
The problem I see is that after spending a lot of hours fabricating something that is safe, robust, and designed to perform at a high level, then skimping on the OPTs is counter productive.
If you buy from a respected provider, like Toroidy, then they do not need to cost a disproportionate amount to be high quality (albeit not superlative quality), especially if you buy them with the basic finish, and not domed and chromed.
I have had the best success with putting a lot of research into auction adverts, including going to other countries. Sometimes a good pair of OPTs can be in an otherwise scrap amplifier (e.g. some Sansuis, Pioneers). I stumbled across a pair of Partridge 8K PP+UL transformers in a job lot of old transformers. I fitted them to a Tubelab SPP and it is hard to describe properly, but music that can be background on an amp with cheaper OPTs, just demands to be listened to in the foreground with the Partridges.
It is actually best to find good OPT candidates at a reasonable price, and then to develop the project around them.
Single ended:
You could try parafeed design, load your output tube(s) with a CSS or big choke and capacitively couple it to your output transformer(s).
I think it will be hard to find the 100v transformers to make impedance match good here.
Single ended push-pull:
Again capacitivily couple the output stage to one or more 100V output transformer.
It works quite well in my experience (one of first things i built)
Looking for EL86 SEPP or murray amplifier might help get you started here.
Circlotron:
Here you will have the added complexity/cost of the dual power supplies needed per output stage.
It will allow you to make a true push pull amplifier without the need for a center tap on your output transformer.
Also, looking at OTL topologies and adding a 100V (auto-)transformer might allow you to make them work a little more efficient and with less output tubes.
IMHO:
If you have the parts: try it, have some fun and gain some experience.
If you dont have them: start out with something more traditional. Buy some output transformer for 50-100 euros and get soldering 🙂
You could try parafeed design, load your output tube(s) with a CSS or big choke and capacitively couple it to your output transformer(s).
I think it will be hard to find the 100v transformers to make impedance match good here.
Single ended push-pull:
Again capacitivily couple the output stage to one or more 100V output transformer.
It works quite well in my experience (one of first things i built)
Looking for EL86 SEPP or murray amplifier might help get you started here.
Circlotron:
Here you will have the added complexity/cost of the dual power supplies needed per output stage.
It will allow you to make a true push pull amplifier without the need for a center tap on your output transformer.
Also, looking at OTL topologies and adding a 100V (auto-)transformer might allow you to make them work a little more efficient and with less output tubes.
IMHO:
If you have the parts: try it, have some fun and gain some experience.
If you dont have them: start out with something more traditional. Buy some output transformer for 50-100 euros and get soldering 🙂
Over the years i have bougth several mediocore output transformers, no more. There are reason why good transformers cost so much. Now i have many door stopper.
I've had great success using Antek Toroidal transformers as output transformers. I used the 25VA for an amp where the output power is less than 15W. Plenty of headroom. There have been discussions on this forum about the integrity of the isolation as the max voltage in the OT is considerably higher than it would see in PT duty. I can happily confirm that my 6360 PP amp runs just fine on 300V B+. No sparks yet. These transformers were purchased for $US13 each and the amp is a delight to listen to.
Half decent OPT's don't have to be expensive, I've built some nice amps using OPT's from Primary Windings in the UK. They are typically about £70 each. I get a cheap toroid power tfmr off Ebay, say £30 & rewind it for HT & heater windings, total cost about £50 discounting my time. Same goes for chokes, get an old EI tfmr bang a load of turns on = cheap choke or just wind the power tfmr for extra current and use loads of capacitance.
Total PSU & stereo OPT's for under £200.
Andy.
Total PSU & stereo OPT's for under £200.
Andy.
Despite their disadvantages, i. e. no air gap, low primary inductance, and low primary voltage capability, two 100 V line transformers might be used in a PP amplifier when connected in series. Norman Crowhurst comes to mind. The transformers need to have a tap labelled four times the miminum wattage which becomes the center tap. Connect each pair of related secondary terminals in parallel. Connect the CT of the 1st transformer to GND, it's primary common tap to the cathode of tube A, it's lowest wattage tap to the cathode of tube B. Connect the 2nd transformer's CT to Eb, it's common primary tap to the plate of tube B, it's lowest wattage tap to the plate of tube A. Power pentodes need a blocking capacitor from their screen to their cathode, while the screens are fed via resistors from the other plate. And last but not least consider the massive drive signal swing demandments.
Best regards!
Best regards!
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I thank you all for the replies (and sorry I haven't responded yet).
To the questions: I've got ~50 100V transformers which got 50W / 25W / 12,5W / 6,25W taps and I could get ~200 for <2€ each, ~500 for <1€ each. That's still a significant ammount of money but surely cheaper than a good OT - but surely not equally good at the same time. I thought about it and while it's very tempting, it's probably not worth the effort and wait time and selection of the transformers it if I just want to build 1-3 amps and I think that's saving on the wrong place, I'm meanwhile convinced ordering renowned OTs will result in a better sounding amp with much less variables and much more predictable results within a much shorter span of time. I will continue to experiment with the big pile of audio transformers but with a very low priority.
Again, thank you all for the replies, valuable input and resources!
To the questions: I've got ~50 100V transformers which got 50W / 25W / 12,5W / 6,25W taps and I could get ~200 for <2€ each, ~500 for <1€ each. That's still a significant ammount of money but surely cheaper than a good OT - but surely not equally good at the same time. I thought about it and while it's very tempting, it's probably not worth the effort and wait time and selection of the transformers it if I just want to build 1-3 amps and I think that's saving on the wrong place, I'm meanwhile convinced ordering renowned OTs will result in a better sounding amp with much less variables and much more predictable results within a much shorter span of time. I will continue to experiment with the big pile of audio transformers but with a very low priority.
Again, thank you all for the replies, valuable input and resources!
There was a project that appeared in a hobby magazine a while back that attempted to run push-pull 6L6s with a 100V line matching transformer, the 'Currawong':
http://messui.polygonal-moogle.com/valves/currawong.pdf
Long story short, it's a fun experiment, but you won't ever get more than a few watts output. The Currawong also kinda spoiled its raison d'etre by incorporating a bunch of extra silly 'features' that made it way overcomplicated, instead of focussing on output stage performance and producing a low-cost, easy circuit that other people might actually build. C'est la vie.
You can buy 'proper' output transformers relatively cheap from China/Aliexpress, and although they won't be amazing, they're probably a better option than trying to work around 100V line txs. And you can always upgrade later. I recently bought some Chinese 5W SE transformers for just a few dollars and they're... fine! Nothing special, nothing to complain about either, for the money.
http://messui.polygonal-moogle.com/valves/currawong.pdf
Long story short, it's a fun experiment, but you won't ever get more than a few watts output. The Currawong also kinda spoiled its raison d'etre by incorporating a bunch of extra silly 'features' that made it way overcomplicated, instead of focussing on output stage performance and producing a low-cost, easy circuit that other people might actually build. C'est la vie.
You can buy 'proper' output transformers relatively cheap from China/Aliexpress, and although they won't be amazing, they're probably a better option than trying to work around 100V line txs. And you can always upgrade later. I recently bought some Chinese 5W SE transformers for just a few dollars and they're... fine! Nothing special, nothing to complain about either, for the money.
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There once was an Australian website on the Alltronics MT-1115 line transformer, describing a UL PP amp with a pair of 6CM5/EL36's. Sadly, it appears to be not live anymore.
Best regards!
Best regards!
From the specs mentioned: 100V primary taps ; 50W, 25W, 12.5w, 6.25W and assuming an 8 Ohm secondary.
The 6.25W primary tap will have the most primary turns. W = V*V/R so 6.25W = V*V/8 so Vsec = 7.071 Volts, turns ratio = 100V / 7.071V = 14.142 ratio
Primary Z = 14.142 x 14.142 * 8 so primary Z = 1600 Ohms
The 25W tap is nominally the center tap.
A 1600 Ohm P to P primary is rather low for P-P tube loads, paralleling up P-P tubes could fit 1600 Ohm, but 4 tubes for only 6.25W is kinda a waste.
Are there any frequency specs for this OT? You likely will have to go to even lower Watts to get the low freq. end down to something approaching HiFi.
With a Circlotron circuit you only need half the primary turns, and you can series connect the primaries endlessly as well, since there is no DC problem. No problem getting the primary Z up to what you want, or to go further for a lower LF response. HF response could be OK, since only half the total turns are needed with Circlotron mode.
The 6.25W primary tap will have the most primary turns. W = V*V/R so 6.25W = V*V/8 so Vsec = 7.071 Volts, turns ratio = 100V / 7.071V = 14.142 ratio
Primary Z = 14.142 x 14.142 * 8 so primary Z = 1600 Ohms
The 25W tap is nominally the center tap.
A 1600 Ohm P to P primary is rather low for P-P tube loads, paralleling up P-P tubes could fit 1600 Ohm, but 4 tubes for only 6.25W is kinda a waste.
Are there any frequency specs for this OT? You likely will have to go to even lower Watts to get the low freq. end down to something approaching HiFi.
With a Circlotron circuit you only need half the primary turns, and you can series connect the primaries endlessly as well, since there is no DC problem. No problem getting the primary Z up to what you want, or to go further for a lower LF response. HF response could be OK, since only half the total turns are needed with Circlotron mode.