Can you please give us some description of what´s in the schematic?
What tubes, what voltages?
PS: I have a 2x115V 1kVA transformer at home, waiting for some action😀
T8 OTL
For those who might be interested, I have added some pics to my website of my recently completed Bruce Rozenblit T8 OTL monoblock (there are two of them--the other is identical to the one shown). See link. 😀
For those who might be interested, I have added some pics to my website of my recently completed Bruce Rozenblit T8 OTL monoblock (there are two of them--the other is identical to the one shown). See link. 😀
Hi,
Nice work, Chris.
This caught my eye when browsing your site:
As far as I know Techzotic doesn't "make" any tubes but maybe you can tell us more?
Cheers,😉
Nice work, Chris.
This caught my eye when browsing your site:
As far as I know Techzotic doesn't "make" any tubes but maybe you can tell us more?
Cheers,😉
Yeah. nice work!
How about PL36 (25E5, I think) as output tubes for an OTL?
They seem to be capable of quite high anode currents and most important, they´re cheap and easy to find😀
I´m trying to think outside the box here to see if I can find something interesting, ie, acheap and easy way to get started with OTL´s.
I realize that I might come up with nothing, but here´s a few more questions:
1: The classic Futterman amp with 8 x 12B4, how much does put out into 8 ohms load?
2: Can those 12B4´s be replaced by 4 x 6C19 or 2 x 6AS7 (per channel)??
3: Has anyone actually tried that choke loaded 6AS7 SE OTL that shows up here and there at this forum?
A lot of people seems to have a lot of opinions about it but I´ve never read about someone who has actually built one.
I´m quite attracted by it´s simplicity and I have good experiences from choke loaded followers.
How about PL36 (25E5, I think) as output tubes for an OTL?
They seem to be capable of quite high anode currents and most important, they´re cheap and easy to find😀
I´m trying to think outside the box here to see if I can find something interesting, ie, acheap and easy way to get started with OTL´s.
I realize that I might come up with nothing, but here´s a few more questions:
1: The classic Futterman amp with 8 x 12B4, how much does put out into 8 ohms load?
2: Can those 12B4´s be replaced by 4 x 6C19 or 2 x 6AS7 (per channel)??
3: Has anyone actually tried that choke loaded 6AS7 SE OTL that shows up here and there at this forum?
A lot of people seems to have a lot of opinions about it but I´ve never read about someone who has actually built one.
I´m quite attracted by it´s simplicity and I have good experiences from choke loaded followers.
Techzotic
Frank-
Well, that's right--they don't make tubes, period, since they seem to have disappered from the face of the Earth! Along with a quantity of my hard-earned cash.😡
I learned my lesson--never again purchase anything from overseas unless the vendor is VERY reputable, and known to me!
Frank-
Well, that's right--they don't make tubes, period, since they seem to have disappered from the face of the Earth! Along with a quantity of my hard-earned cash.😡
I learned my lesson--never again purchase anything from overseas unless the vendor is VERY reputable, and known to me!
Hi,
You could.
With all the nice triodes floating around + those dreaded anode caps....
Roughly half of what it put out into 16 Ohms, for which it was designed and it would still be a mismatch.
So, if you need to drive a 8 ohm speaker I'd double the amount of output tubes and perhaps relax the NFB a little.
See 1:
The only person I can think of is Scott Nixon and you'll probably need a custom made choke anyway.
Sorry to hear about that, Chris.
Cheers,😉
You could.
With all the nice triodes floating around + those dreaded anode caps....
Roughly half of what it put out into 16 Ohms, for which it was designed and it would still be a mismatch.
So, if you need to drive a 8 ohm speaker I'd double the amount of output tubes and perhaps relax the NFB a little.
See 1:
The only person I can think of is Scott Nixon and you'll probably need a custom made choke anyway.
Sorry to hear about that, Chris.
Cheers,😉
My main problem is that I can´t decide if I´m going to do some experiments on low power SE designs (cheap and fun) or to grab my balls and build something serious based on my 1 kVA transformer and a bunch of PL519 or similar (not cheap...)
Hehe!
When I talk about my speakers I mean the mid/high section of my dipoles. The midrange driver is a Scanspeak 8545 with 87 or 88dB sensivity, the tweeter is a bit higher but it´s atteunated.
For the moment I use a 12B4A SE amp (1W or so)for this section and it works fine, that´s why I´m a but curious about low power SE OTL´s.
It seems that Transcendent (sp?) gets 1,5W out of four choke loaded 6C19´s. The choke is a 0,32H 600mA 10ohm piece, and I think Hammond has quite similar stuff.
When I talk about my speakers I mean the mid/high section of my dipoles. The midrange driver is a Scanspeak 8545 with 87 or 88dB sensivity, the tweeter is a bit higher but it´s atteunated.
For the moment I use a 12B4A SE amp (1W or so)for this section and it works fine, that´s why I´m a but curious about low power SE OTL´s.
It seems that Transcendent (sp?) gets 1,5W out of four choke loaded 6C19´s. The choke is a 0,32H 600mA 10ohm piece, and I think Hammond has quite similar stuff.
Hi,
Unless you suspect you're listening to the OPTs now....either way it's going to be that or a coupling.
Or maybe you can go overboard and use a dual rail supply and set the output fo zero offset.
Or, better still, build the big'un with the PLs...
That's what you'll be doing after a while anyway...I can tell the itch is aching already...😀
Cheers,😉
Unless you suspect you're listening to the OPTs now....either way it's going to be that or a coupling.
Or maybe you can go overboard and use a dual rail supply and set the output fo zero offset.
Or, better still, build the big'un with the PLs...
That's what you'll be doing after a while anyway...I can tell the itch is aching already...😀
Cheers,😉
Well, here's one idea for a WF based dc coupled OTL. I kept the maximum supply voltages low enough to be able to run 100ma bias per output tube if 6AS7G's are used and not risk cathode/filament breakdown on the 12au7 which is doing double duty. Some neon bulb protection between the grid and cathode of the driver stage might be a good idea here. The TBD values on the differential input are to allow empirical adjustment of the cross coupled differential input for the best OL gain, since all the voltage gain is in the first stage. No offset adjust or bias adjust scheme is shown but something workable is not too difficult to figure out. I've shown an approx 26 db gain feedback network without stability compensation with gain decreasing to unity at dc. Some ideas for possible enhancements are to split the functions of V3 to allow increasing the driver voltage swing by maybe connecting its plate to a voltage higher than +100 volts. Then R8 could be significantly decreased in resistance and split to allow bootstrapping, if desired. C3 can be chosen to give a little lead compensation at a critical frequency or just made large enough to bypass R10 across the audio range. Again, this is a somewhat speculative circuit in certain areas - it is substantially different in its topology in most areas compared to the dc coupled OTLs I have built for myself which are, for instance, more complex and operate their outputs class AB1 edging into AB2 territory.
Attachments
Btw, most of these supply voltages can be derived from doublers and triplers right off the same transformer secondaries that supply the output rails, except for the +150V. It's possible to connect the plate of the nonloaded half of the input stage directly to +200V or +300V, but dc offset stability with varying mains voltage might be slightly degraded.
Ok, that simplifies things a bit.
A few questions:
What happens to the DC offset when the tubes age?
How many 6AS7´s are required to get any kind of performance?
A few questions:
What happens to the DC offset when the tubes age?
How many 6AS7´s are required to get any kind of performance?
It shouldn't be too susceptible to offset drift with tube aging, but I would readjust a few times until new tubes settle in, then check & adjust as necessary every 6 months or better. At the input stage, depending on the tracking of both halves of the (upper and, particularly, lower) twin triode and setting it for a relatively small bias voltage (around a volt) between the grid and cathode of the input tube handling the input and feedback signals should help quite a bit here (It does for my OTL and hybrid prototype circuit). If the input stage OL gain is sufficient, and the diff input stage halves are at about the same bias points, the offset should stay within a few dozen millivolts. For instance, suppose the combined bias of the driver and output stage shift ten volts (quite a lot), then, if the OL gain of the input stage is effectively 400, the offset shift due to this would be 10V/400, or 25mV. There may be a few dozen millivolts shift during the first half hour warmup. Using regulated DC filament supplies for the input and driver tubes will likely minimize offset shift due to mains variations (apparently less important for the fully balanced dc coupled OTLs I have built which have AC on all their tube filaments).
The White follower output will limit power quite a bit compared to class B. Maybe 5-10 watt into 8 ohm with 4 6AS7G in a WF pp? With 6, you would probably double that.
Of course, I'm speculating somewhat here about the available power, but these seem like reasonable values, given 100mA bias per 6AS7G.
The White follower output will limit power quite a bit compared to class B. Maybe 5-10 watt into 8 ohm with 4 6AS7G in a WF pp? With 6, you would probably double that.
Of course, I'm speculating somewhat here about the available power, but these seem like reasonable values, given 100mA bias per 6AS7G.
Fuling, thanks for that info about the Transents choke loaded
otl. About that .32h choke, I have a 2 kilo spool of # 16 magnet
wire that measures about 5 ohms and 100 mh. This is on the factory spool, using a better form factor the inductance would be higher. So using an air core inductor here might not be out of the
question.
otl. About that .32h choke, I have a 2 kilo spool of # 16 magnet
wire that measures about 5 ohms and 100 mh. This is on the factory spool, using a better form factor the inductance would be higher. So using an air core inductor here might not be out of the
question.
Had a thought about the schematic I posted earlier today:
If you set R2 to 0 ohms, make R6=a much smaller R1 (probably a few dozen ohms/# 6AS7G's) and and connect R3, instead of to -100V, to the wiper of a, say, 50K 2W potentiometer that is adjustable between -100 and -200 volts, you can probably get significantly more power out of this modified WF, since little of it will now be wasted across the series resistors. Then the potentiometer will also act in effect as the bias setting pot for the amp.
If you set R2 to 0 ohms, make R6=a much smaller R1 (probably a few dozen ohms/# 6AS7G's) and and connect R3, instead of to -100V, to the wiper of a, say, 50K 2W potentiometer that is adjustable between -100 and -200 volts, you can probably get significantly more power out of this modified WF, since little of it will now be wasted across the series resistors. Then the potentiometer will also act in effect as the bias setting pot for the amp.
Woody -
I'm guesstimating you should be able to get into the 320mH, 10 ohm or lower area with around 3-4 Kg of 16awg using an optimal form factor. Maybe have 5Kg available just to be safe🙂
I've wound chokes like this on a spool I made from a PVC DWV end cap sided by two 3/8" or thicker plywood discs with 1 or 2 radial slots in the disc opposite the end cap flat to thread the wire start and end through. I found that by smearing a layer of silicon caulk on the inside of each disc, I can even impregnate the windings in place with hot epoxy or medium vis CA and still get the end caps off since the glue tends not to stick to the flexible caulk. Then the resulting choke can be mounted on a flat surface with a screw through the hole in the end cap, if desired.
I'm guesstimating you should be able to get into the 320mH, 10 ohm or lower area with around 3-4 Kg of 16awg using an optimal form factor. Maybe have 5Kg available just to be safe🙂
I've wound chokes like this on a spool I made from a PVC DWV end cap sided by two 3/8" or thicker plywood discs with 1 or 2 radial slots in the disc opposite the end cap flat to thread the wire start and end through. I found that by smearing a layer of silicon caulk on the inside of each disc, I can even impregnate the windings in place with hot epoxy or medium vis CA and still get the end caps off since the glue tends not to stick to the flexible caulk. Then the resulting choke can be mounted on a flat surface with a screw through the hole in the end cap, if desired.
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