I have a pair of these http://store.triodestore.com/tf65wscta48o.html .
They are currently hooked up to a set of Reflektor 6p6s triode strapped(looks like 6N6C on bottle) which are similar to 6V6GT's . The circuit design is similar to the "Compact Amp" http://www.triodeel.com/compact.html which is fairly well known.
I am curious if I can run these OPT's in a SE format with the pair of 6p6s's run parallel , which currently draw 64ma combined. The OPT is rated for 80ma per side, so if run SE that would be a total of 80ma as the windings would be in series.
Would this be doable, or would I run into overtaxing the OPT ?
I understand I will have to rebias, etc, but if I can keep the current below the 80ma rating, would this work , or would I have to run a Parafeed type circuit ?
I am curious how the SEPT would compare to the Compact amp design.
Thanks for any help.........................Blake
They are currently hooked up to a set of Reflektor 6p6s triode strapped(looks like 6N6C on bottle) which are similar to 6V6GT's . The circuit design is similar to the "Compact Amp" http://www.triodeel.com/compact.html which is fairly well known.
I am curious if I can run these OPT's in a SE format with the pair of 6p6s's run parallel , which currently draw 64ma combined. The OPT is rated for 80ma per side, so if run SE that would be a total of 80ma as the windings would be in series.
Would this be doable, or would I run into overtaxing the OPT ?
I understand I will have to rebias, etc, but if I can keep the current below the 80ma rating, would this work , or would I have to run a Parafeed type circuit ?
I am curious how the SEPT would compare to the Compact amp design.
Thanks for any help.........................Blake
The short answer is no. That transformer is made to have balanced amounts of current drawn across the halves of the primary. If you parallel the tubes, you will suddenly be drawing an unbalanced current, and the primary is not gapped for that use.
If you use parallel feed topology, you can try this type of arrangement, but there are other factors that will contribute to poor performance.
If you use parallel feed topology, you can try this type of arrangement, but there are other factors that will contribute to poor performance.
The long answer is no also. Any more than 5 microamps of DC will seriously eat up your core's working zone and cause poor AC signal, bass response. Along with this will come saturation that will cause ugly noises to emit from your speakers.
Sticking a 1 ufd expensive cap in series with the plate will block the DC and combine with the unknown capacitance of the OPT to cause the tubes all sorts of problems. Adding a plate choke will just add to the woe.
A parafeed set up is a VERY complex animal. There is no data sheet or information source, other than the RDH 4 section about RF amplifiers, that I have been able to find on these.
Doc Bottlehead's designer is justly proud of his skill with these things.
So, purchase as good a SE OPT as you can find and afford. Start with the usual suspects, Hammond and Edcore, find out what you do and don't like about the redesign and then spend some serious money for much better OPT's. Somewhere in there you will equal the current amps performance, except for distortion.
Bud
Sticking a 1 ufd expensive cap in series with the plate will block the DC and combine with the unknown capacitance of the OPT to cause the tubes all sorts of problems. Adding a plate choke will just add to the woe.
A parafeed set up is a VERY complex animal. There is no data sheet or information source, other than the RDH 4 section about RF amplifiers, that I have been able to find on these.
Doc Bottlehead's designer is justly proud of his skill with these things.
So, purchase as good a SE OPT as you can find and afford. Start with the usual suspects, Hammond and Edcore, find out what you do and don't like about the redesign and then spend some serious money for much better OPT's. Somewhere in there you will equal the current amps performance, except for distortion.
Bud
talkin just about principle , without checking exact xformers and other numbers - you can ;
Ub via primary halve to anode
cathode via other primary halve to gnd ;cathode must be AC bypassed to gnd.
naturally - primary CT must be interrupted
but - there is another issue - is half of primary adequate for your SE
Ub via primary halve to anode
cathode via other primary halve to gnd ;cathode must be AC bypassed to gnd.
naturally - primary CT must be interrupted
but - there is another issue - is half of primary adequate for your SE
The short answer is, yes.
The first long answer is, to use a string of incandescent bulbs at the opposite side of the tranny, to draw the current equal to the idle current of your SE output tube. Gain in tranny price/quality, but loss of power for balance.
The second long answer is, to use a CCS instead of a bulb. No loss of a singal power, but loss of B+ power still.
The thirt long answer, a conter-modulated CCS. More of power from a single SE tube, price/performance ratio of the transformer improved.
There were several such concepts discussed here; some on paper only, some SIMMed, some built as working prototypes.
The first long answer is, to use a string of incandescent bulbs at the opposite side of the tranny, to draw the current equal to the idle current of your SE output tube. Gain in tranny price/quality, but loss of power for balance.
The second long answer is, to use a CCS instead of a bulb. No loss of a singal power, but loss of B+ power still.
The thirt long answer, a conter-modulated CCS. More of power from a single SE tube, price/performance ratio of the transformer improved.
There were several such concepts discussed here; some on paper only, some SIMMed, some built as working prototypes.
Zen Mod "talkin just about principle , without checking exact xformers and other numbers - you can ;
Ub via primary halve to anode
cathode via other primary halve to gnd ;cathode must be AC bypassed to gnd.
naturally - primary CT must be interrupted
but - there is another issue - is half of primary adequate for your SE "
Could you provide a schematic /drawing ? Not sure I follow this description.
Some things have changed since I first posted. Still using the same OPT, rated for 80ma per side, but I only want to run about 30ma SE thru it. Will I still saturate the core ?
Bud P commented that as little as 5 micro amps will cause problems. How could anyone even run a PP setup with cathode bias if this was the case ? 5 microamps of imbalance is very little, and I imagine is not too uncommon.
AM I nuts thinking this ? I understand that the closer the tubes are balanced in a PP setup the better it will work, but 5 micro amps seems impossibley small unless you are using a CCS to bias them and /or are constantly monitoring the current .
Thanks...........................Blake
Ub via primary halve to anode
cathode via other primary halve to gnd ;cathode must be AC bypassed to gnd.
naturally - primary CT must be interrupted
but - there is another issue - is half of primary adequate for your SE "
Could you provide a schematic /drawing ? Not sure I follow this description.
Some things have changed since I first posted. Still using the same OPT, rated for 80ma per side, but I only want to run about 30ma SE thru it. Will I still saturate the core ?
Bud P commented that as little as 5 micro amps will cause problems. How could anyone even run a PP setup with cathode bias if this was the case ? 5 microamps of imbalance is very little, and I imagine is not too uncommon.
AM I nuts thinking this ? I understand that the closer the tubes are balanced in a PP setup the better it will work, but 5 micro amps seems impossibley small unless you are using a CCS to bias them and /or are constantly monitoring the current .
Thanks...........................Blake
Nihlist,
5 micro amps of DC in a SE is a different animal from 5 microamps with an alternating AC current dragging it below zero every half wave. Yes there will be some slight saturation in a PP with imbalanced DC flowing, in alternate directions. Again, this is not like having DC current flow setting your core into an offset polarization.
Heck, your PP OPT is probably poorly made enough that it will withstand 20ma of DC current. Try it out. It's not likely to catch fire right away! If it starts getting warm to the touch within two minutes you have a problem, but mostly any problem willl just be poor sound quality. But, if the mfg. didn't pay very close attention to closing the E/I gaps for the lams, you might be home free.
Bud
5 micro amps of DC in a SE is a different animal from 5 microamps with an alternating AC current dragging it below zero every half wave. Yes there will be some slight saturation in a PP with imbalanced DC flowing, in alternate directions. Again, this is not like having DC current flow setting your core into an offset polarization.
Heck, your PP OPT is probably poorly made enough that it will withstand 20ma of DC current. Try it out. It's not likely to catch fire right away! If it starts getting warm to the touch within two minutes you have a problem, but mostly any problem willl just be poor sound quality. But, if the mfg. didn't pay very close attention to closing the E/I gaps for the lams, you might be home free.
Bud
Wavebourn, actually this would be a great idea to have SE OPT of reduced size. Let's say that we have two primaries: the first one for our audio use HV etc. The second one would pass a current sufficient to balance the magnetization of the audio primary. It does not need to be HV. Like this the iron would not saturate.
It sound too easy. What's the probem ?
Thanks,
D.
It sound too easy. What's the probem ?
Thanks,
D.
Why complicate things for the fellow??
The answer is NO, if you use it like a standard transformer.
The answer is YES if and only if you can A) remove the DC from the
primary windings or B) run the exact amount of DC coming from the tube in opposition (by some circuit trick means) to said DC, thereby cancelling the DC offset in the core.
The easiest way to do this is with what is called "parafeed" in this scenario there is a choke feeding the output tube DC and a cap feeding the OPT with AC signal.
_-_-bear
PS. there is a third option, that is to take the transformer apart, remove the lams and reassemble the E lams together, not interleaved, and then gap the I lams... but i'd go with the parafeed first and second one or more circuit tricks as a solution, sort of just buying a new OPT for SE operation.
The answer is NO, if you use it like a standard transformer.
The answer is YES if and only if you can A) remove the DC from the
primary windings or B) run the exact amount of DC coming from the tube in opposition (by some circuit trick means) to said DC, thereby cancelling the DC offset in the core.
The easiest way to do this is with what is called "parafeed" in this scenario there is a choke feeding the output tube DC and a cap feeding the OPT with AC signal.
_-_-bear
PS. there is a third option, that is to take the transformer apart, remove the lams and reassemble the E lams together, not interleaved, and then gap the I lams... but i'd go with the parafeed first and second one or more circuit tricks as a solution, sort of just buying a new OPT for SE operation.
Last edited:
"Sound Practices issue #16 "
Ummm, that won't work. The 10 mF cap is shorting out the audio in the transformer.
There are quite a few ways to do SE using PP, as have been mentioned, but the simplest by far is to just cut the cap feeding audio to one of the output tubes in the amp. Leave the tube's DC biasing intact, so the undriven tube becomes the counter balancing DC current source for the OT. (biasing for both tubes must be increased to class A operation, which may cause tube dissipation problems and power supply overheating problems) Since the remaining tube will be driving the speaker alone, the speaker should be moved from the OT's 8 Ohm tap to the 4 Ohm tap.
Ummm, that won't work. The 10 mF cap is shorting out the audio in the transformer.
There are quite a few ways to do SE using PP, as have been mentioned, but the simplest by far is to just cut the cap feeding audio to one of the output tubes in the amp. Leave the tube's DC biasing intact, so the undriven tube becomes the counter balancing DC current source for the OT. (biasing for both tubes must be increased to class A operation, which may cause tube dissipation problems and power supply overheating problems) Since the remaining tube will be driving the speaker alone, the speaker should be moved from the OT's 8 Ohm tap to the 4 Ohm tap.
Smoking amp is quite right about that, in order for this to work and not short the ac signal out entirely there would have to be a huge amount of leakage inductance between that winding and the others.
When you wind two windings on a common core you have mutual inductance between the two, and what you do to one is reflected to the other by the degree to which they are coupled.
So shorting one winding effectively at least partially shorts all of the other windings on the common core depending on the degree to which they are coupled. A 10uF capacitor across a fairly high Z primary winding should constitute a pretty good short over most of the audio bandwidth.
Incidentally I have successfully used PP OPTS in small SE amplifiers as have several others. The key is that the flux in the core for the combined quiescent current and the audio signal cannot exceed that of the AC component present in the original transformer. (In PP application) This often means that a PP transformer rated for 20Wrms might be good for a couple of watts SE at best. Some work much better than others.
I used a pair of PP transformers salvaged from a sadly mis-treated Eico HF-81 (6BQ5 amp) that was not restorable in a 45 DHT SE amplifier that was my first (very inexpensive) foray into SE territory - I still have that amplifier considerably more than a decade later whilst many other "technically superior" amps have come and gone. It was a deliberate mis-application to save some coin, but worked extremely well in the end - so well in fact that I have not felt compelled to change them out. This amplifier is a perennial favorite at every single audio event it has ever been shown at. The output stage runs at 36mA - 38mA fixed bias with 250V on the plate, and produces a whopping 1.6Wrms (4K primary Z based on the way the secondary is connected) - it sounds great, limited primary Z does limit power bandwidth on the bottom to about 30Hz though.. The transformers do not run warm, and in all of these years and thousands of hours of use nothing untoward has EVER happened.
http://www.kta-hifi.net/projects/amp_page/45dht/45_dht.html (Despite the comment that the amplifier had found a new home, it was returned to me after about a year in exchange for a more powerful 2A3 based amplifier a few years ago. I'm quite happy to have it back. Space was at a premium and that was the main reason it left.)
All that said good SE transformers are much cheaper and far more readily available than they were in 1998 when I designed and built this amp. (Featured on my site for the curious.) So I would say if the budget is not too tight to get the proper SE transformer.
Last edited:
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.