ulibub said:
Wow, Thats a cute amp, Those Mercury-recs look awsome!
I quite agree, This is exactly what I find with my Circlotron using the 6C33C-B, and their bias-setting...
They DO take an eternity to thermally stabilise...
i am not running mine quite as high volts, only around 190V, and two tubes per channel. I set current at 75-100mA, but only after around half hour of use. and use SS rectification
I dont bother with separate HT switching, Full HT is applied from cold, --I hear everyone screaming at this point!---
(My thinking being, if the tube is weak and cannot stand this, then let it die and be done with--I have had no failures yet through doing this over a couple of years,-- also knowing millions of tubed domestic-equipment TV sets etc survived for decades without 'Standby-HT' Switching/ or 'Cathode-Stripping' issues....Over several years, servicing tubed TV sets etc, I only found one genuinely low-emission PL509 O/P valve. Yes, I changed Many faulty tubes, for other internal failures, but Not from low-emission.....
Also, as there is full neg bias applied, cathode-current on warm-up is very limited anyway unlike a cathode-bias device, which relies on cathode-current to derive the bias...)
What Ive found on 6C33C warm-up, is current will slowly climb over 15 minutes, then over the next 15, will drop slightly, by around 10mA from the highest current.....
During some dissipation testing, I was able to run 400mA at 190V, (As this is well over the 60W P-Diss max, we did have a somewhat cherry-anode or two...!) BUT the same effect took place....Firstly, the current climbed slightly, then fell back, to a steady state, just above the set-value, almost as if the tubes were 'resisting Thermal-runaway'....I let them run like this for over an hour with no apparent problems, except a huge amount of heat, but No tendancy to runaway-These two tubes are still performing perfectly....
Careful burn-in for the big Russian triple-titter is really essencial to get long-life without 'drift' these valves seem to ba infamous for.
The way I do it, is Tube-Heaters Only, for around 3 days at 13.6V (Series-wired both heaters) then 1 day at 11V, still only heaters.
Fit them to amp, and run at normal bias for 1 hour, then at Max dissipation for 1-2 hours, Switch off, cool completely then re-set to normal bias...All done, and no drifting!
I'm another that has been running 6c33c's fixed bias for about 2 years without any problems.
The 6c33c is my favorite output tube, but the problem working with them is that it can be quite difficult to find Ideal transformers, both power and output. If you ever go with a single ended design there is a guy that sells single ended transformers for the 6c33c on ebay which are better than other transformers that I've paid twice as much for. But as far as push pull goes I haven't seen any transformers for 6c33c.
IMO diff phase splitters in general, just arn't very balanced. Sure you can manipulate them to work right/do the job, but it's pretty much bending a naturally unbalanced topology.
The 6c33c is my favorite output tube, but the problem working with them is that it can be quite difficult to find Ideal transformers, both power and output. If you ever go with a single ended design there is a guy that sells single ended transformers for the 6c33c on ebay which are better than other transformers that I've paid twice as much for. But as far as push pull goes I haven't seen any transformers for 6c33c.
IMO diff phase splitters in general, just arn't very balanced. Sure you can manipulate them to work right/do the job, but it's pretty much bending a naturally unbalanced topology.
Hi Alastair!
Thank you!
Yes, these are RCA-866 with huge bulbs.... beautiful gas-lights indeed... Looks very nice in the evening - the red glow of the tubes, the green magic bands (EM 800) , and the blue light of the gas lamps....
Originally, I planned to use Valvo 1701 (350V, 300 mA) two-way gas tubes, but they proved to be so incredibly noisy, that you even could hear (acoustically!!!) the ignition noise as a faint ticking sound when coming near to the tubes.... I never succeeded to get them really quiet, even with lots of chokes, damping resistors and the big PFC choke in front of the mains transformer - you always could hear the ticking high ignition noise in the tweeters.... So I replaced them with the 866s, and not to spoil one half-wave, I put a simple silicon bridge (B 250 C 1500) in front of them..... another kind of "hybrid" rectifier design.... One simple choke (10 mH) in the anode line of the mercury tubes, and they were silent....
....as I found, too... Set to 100 mA, one tube even climbs up to 130 mA, to decrease slowly then to 100 mA.. The others go onto 110-120 mA. Then half an hour has passed.... But it seems to me that this effect increases with rising plate voltage.
Very interesting.... Just, as this shouldn't burn away oxide particles on the plate... In new tubes, I see them often as white glowing points, and usually soon after there was a flashover just at this place - the particle finally vapourised, and I think that its gas cloud was the reason for arcing.... So I was concentrating on burn-in to get rid of these particles.
My amp is a completely symmetrical design (my pre-amp has symmetrical output). Pre-stage is a diff-amp, its two legs with ECC 82 in SRPP and CCS EF 184. Capacitor coupled to the driver, a mu-follower with ECC 99 generating the voltage swing for the power tubes and 2x PL 84 as cathode followers. Bias regulation for the power tubes is via the screen-grid voltage of these pentodes. 6C33CBs are directly coupled to the PL 84-cathodes, potential difference between the PL 84-cathodes (around +20 V) and the power-tube's grids (-130 V) is maintained by (no, don't scream, it works perfectly - I love to try new ways...!!!) OA2 gas stabilisers.... ECC 99 plate current is ca. 20 mA, quiescent current through the stabilisers is 15 mA. ECC 99 cathodes run through a bifilar neg. feedback winding on the OPT to their common bias resistor. In my experience, this single-pair circlotron sounds much better with a low-ratio OPT than in OTL mode. I can switch between these two modes.
my amp from above...
the mounting of the power tube sockets for forced-air cooling
The big transformer is the plate and bias transformer only (custom made by Sowter, UK) - heating transformer (toroidal) is below it under the chassis. In front of the transformer the big PFC choke (to compensate nearly 5000 microfarads in the 5 power supply rails...) with a magnetic shield. GZ34 bias rectifier is in the middle in front of it, with the four knobs for bias adjustment.
This was an immense project, it proved to be much more difficult than I expected at the beginning.... but it was worth every effort!
Uli
Thank you!
Yes, these are RCA-866 with huge bulbs.... beautiful gas-lights indeed... Looks very nice in the evening - the red glow of the tubes, the green magic bands (EM 800) , and the blue light of the gas lamps....Originally, I planned to use Valvo 1701 (350V, 300 mA) two-way gas tubes, but they proved to be so incredibly noisy, that you even could hear (acoustically!!!) the ignition noise as a faint ticking sound when coming near to the tubes.... I never succeeded to get them really quiet, even with lots of chokes, damping resistors and the big PFC choke in front of the mains transformer - you always could hear the ticking high ignition noise in the tweeters.... So I replaced them with the 866s, and not to spoil one half-wave, I put a simple silicon bridge (B 250 C 1500) in front of them..... another kind of "hybrid" rectifier design.... One simple choke (10 mH) in the anode line of the mercury tubes, and they were silent....
....as I found, too... Set to 100 mA, one tube even climbs up to 130 mA, to decrease slowly then to 100 mA.. The others go onto 110-120 mA. Then half an hour has passed.... But it seems to me that this effect increases with rising plate voltage.
Very interesting.... Just, as this shouldn't burn away oxide particles on the plate... In new tubes, I see them often as white glowing points, and usually soon after there was a flashover just at this place - the particle finally vapourised, and I think that its gas cloud was the reason for arcing.... So I was concentrating on burn-in to get rid of these particles.
My amp is a completely symmetrical design (my pre-amp has symmetrical output). Pre-stage is a diff-amp, its two legs with ECC 82 in SRPP and CCS EF 184. Capacitor coupled to the driver, a mu-follower with ECC 99 generating the voltage swing for the power tubes and 2x PL 84 as cathode followers. Bias regulation for the power tubes is via the screen-grid voltage of these pentodes. 6C33CBs are directly coupled to the PL 84-cathodes, potential difference between the PL 84-cathodes (around +20 V) and the power-tube's grids (-130 V) is maintained by (no, don't scream, it works perfectly - I love to try new ways...!!!) OA2 gas stabilisers.... ECC 99 plate current is ca. 20 mA, quiescent current through the stabilisers is 15 mA. ECC 99 cathodes run through a bifilar neg. feedback winding on the OPT to their common bias resistor. In my experience, this single-pair circlotron sounds much better with a low-ratio OPT than in OTL mode. I can switch between these two modes.
my amp from above...
the mounting of the power tube sockets for forced-air cooling
The big transformer is the plate and bias transformer only (custom made by Sowter, UK) - heating transformer (toroidal) is below it under the chassis. In front of the transformer the big PFC choke (to compensate nearly 5000 microfarads in the 5 power supply rails...) with a magnetic shield. GZ34 bias rectifier is in the middle in front of it, with the four knobs for bias adjustment.
This was an immense project, it proved to be much more difficult than I expected at the beginning.... but it was worth every effort!
Uli
Hi Ulli,
Very nice amp!!! Do you have a schematic, specially the output stage of your amp? I'm über interested in the way you connected the tubes to the feedback winding. Circlotron, low-ratio OPT schematic might just be what I'm looking for.
btw, nice Sarracenia plants. (I have also been into carnivorous plants)
Very nice amp!!! Do you have a schematic, specially the output stage of your amp? I'm über interested in the way you connected the tubes to the feedback winding. Circlotron, low-ratio OPT schematic might just be what I'm looking for.
btw, nice Sarracenia plants. (I have also been into carnivorous plants)
Hi Dr. Strangelove!
Yes, you'll certainly love this bomb of an amp....!
I don't have a "good" diagram, only a quick-and-dirty sketch, but I think it should be sufficient:
Uli's circlotron diagram big
small version
Oh yes, bias setting is over transistors..... but potentials didn't allow for using a regulator tube (maybe a low-voltage type like ECC 86) at this place. BF 471 needs hardly any cooling - a few square cm only. This kind of operation assures me plain pentode service (screen on cathode level for AC without using high-power potentiometers for bias setting) to obtain lowest output inpedance.
Driver is powerful enough to drive the power tubes far into grid current area - until the PSU fuses do an emergency break...
There is nothing special with the OA2 potential shifters.
OPT is a simple 160 VA mains toroidal type with 2x 115 V primary in parallel to 2x 15 V secondary in series. They are sitting below the chassis under the power tube sockets.
Negative feedback winding is 20 turns just wound bifilarly (bell wire....) around it by hand.
any questions?
Uli
Yes, you'll certainly love this bomb of an amp....!
I don't have a "good" diagram, only a quick-and-dirty sketch, but I think it should be sufficient:
Uli's circlotron diagram big
small version
Oh yes, bias setting is over transistors..... but potentials didn't allow for using a regulator tube (maybe a low-voltage type like ECC 86) at this place. BF 471 needs hardly any cooling - a few square cm only. This kind of operation assures me plain pentode service (screen on cathode level for AC without using high-power potentiometers for bias setting) to obtain lowest output inpedance.
Driver is powerful enough to drive the power tubes far into grid current area - until the PSU fuses do an emergency break...
There is nothing special with the OA2 potential shifters.
OPT is a simple 160 VA mains toroidal type with 2x 115 V primary in parallel to 2x 15 V secondary in series. They are sitting below the chassis under the power tube sockets.
Negative feedback winding is 20 turns just wound bifilarly (bell wire....) around it by hand.
any questions?
Uli
SY said:
I'm maybe overstating the caution. I like the look of the Borbely design, even though I was really going to do a PP, not SE... The article is good, and the design simple but supposedly with a good sound... It'll be a start.I find it difficult to get motived by boring-looking tubes...
Jeb-D. said:
Have a look here:
6c33c-PP-OPT
I also use their transformers (but not this one), also have had mains xfms custom wound there, they have a good reputation here in Germany, I can recommend them.
Uli
ulibub said:
I have used Sowter for other projects - they do 6C33 PP transformers,
Sowter PP output transformers
Sowter are very good, but very expensive... Having said that, I got the stuff for my Bevois Valley amp from them, and they were incredibly helpful - full of good advice and their product is excellent.
I designed and built a 6 x 6C33-C-B per channel circlotron about 12yrs ago. I had all sorts of problems with this design, including arcing during warm up that wiped out the woofer in one my speakers. I NEVER had a problem with thermal runaway at 140V/300mA per tube with fixed bias.
Had I known what some of these fellows know about burn in, regulated filament supplies and the like, I think my amp would have ultimately been successful. And it was a very good sounding amplifier despite the endless problems due to my bad design.
I think a long burn in is a major part of the issue - I didn't do it. I spent a lot of time matching tubes at 3 different current levels. I also noted the odd warm up behavior - I assume changes in the internal dimensions during warm up accounted for some of this behavior.
Make no mistake though there is a certain amount of risk in fixed biasing these tubes, but with care, and adequate burn in I believe it is surmountable.
Notice my avatar..
Good luck.
Had I known what some of these fellows know about burn in, regulated filament supplies and the like, I think my amp would have ultimately been successful. And it was a very good sounding amplifier despite the endless problems due to my bad design.
I think a long burn in is a major part of the issue - I didn't do it. I spent a lot of time matching tubes at 3 different current levels. I also noted the odd warm up behavior - I assume changes in the internal dimensions during warm up accounted for some of this behavior.
Make no mistake though there is a certain amount of risk in fixed biasing these tubes, but with care, and adequate burn in I believe it is surmountable.
Notice my avatar..
Good luck.
To me, too, arcing always was much bigger a problem than drifting. I also had to pay my fees to get all the experience I am sharing with you out there now.... The first tube lived for about 5 minutes before the mains fuses blew for the first time because it was arcing...... Other tubes lived for hours or - I really was lucky then - for weeks..... But someday, every tube seemd to flashover...... I noticed these fine white glowing particles on the plate surface, and then I was thinking about what a cloud of ionised gas would do inside the tube - greatly enhance conductivity. Forming kind of a thyratron at this place.....
I am surprised that you experienced arcing even at quite low voltages which are just in the optimal range for these tubes - I always was working at the upper limits - where ill effects should be more likely to occur. Seems to be a more general problem with large-surface oxide cathodes.
After I had lost my first two pairs of tubes due to arcing (surely it hurt loosing these tubes, but in the end, it was a financial loss of about 120 € - not too excessice though for such a big project), I started with burn-in - at about 100 V (using an isolation xfmr behind a Variac), connecting the grid to the cathode and turning the Variac up - until the plates just were starting to glow..... and then let it "roast" for about 4 hours, until I couldn't see any white glowing particles anymore....
But do this at a fire-safe place only - the tubes produce an immense amount of heat (thermal radiation!). A fan to provide some external cooling wouldn't be bad, too.
These tubes are working now since over 1 year in my amp, without any problems. The have run now for several hundreds of hours. So their electrode systems really need to be "clean" from any dirt or other particles.
Uli
I am surprised that you experienced arcing even at quite low voltages which are just in the optimal range for these tubes - I always was working at the upper limits - where ill effects should be more likely to occur. Seems to be a more general problem with large-surface oxide cathodes.
After I had lost my first two pairs of tubes due to arcing (surely it hurt loosing these tubes, but in the end, it was a financial loss of about 120 € - not too excessice though for such a big project), I started with burn-in - at about 100 V (using an isolation xfmr behind a Variac), connecting the grid to the cathode and turning the Variac up - until the plates just were starting to glow..... and then let it "roast" for about 4 hours, until I couldn't see any white glowing particles anymore....
But do this at a fire-safe place only - the tubes produce an immense amount of heat (thermal radiation!). A fan to provide some external cooling wouldn't be bad, too.
These tubes are working now since over 1 year in my amp, without any problems. The have run now for several hundreds of hours. So their electrode systems really need to be "clean" from any dirt or other particles.
Uli
A question for Ulibub.....
With your amp using the 'gas-tubes', are these in the Circlotron supply rails?....
What is the forward voltage-drop these tubes have, and at what current? I found out, that they are 300mA rectifiers, and I have a similar tube, the Osram GU50 mercury rec. which has a 250mA rating, with 1A pulse maximum...
I would like to try these out, but another concern is Ultra-violet light...
Do they emit quantities that could be considered harmful?
With your amp using the 'gas-tubes', are these in the Circlotron supply rails?....
What is the forward voltage-drop these tubes have, and at what current? I found out, that they are 300mA rectifiers, and I have a similar tube, the Osram GU50 mercury rec. which has a 250mA rating, with 1A pulse maximum...
I would like to try these out, but another concern is Ultra-violet light...
Do they emit quantities that could be considered harmful?
Hi Alastair!
Yes, they are.
Since they are one-way rectifiers, and not to spoil the other half-wave, I put a simple silicon bridge (B 250 C 1500) in front of them. 150 nF between the two AC leads. From + lead into a 10 mH toroidal choke, the Hg-tube 866A, 10 ohms, 100 uF, 22 ohms, 470 uf, Mosfet gyrator, 470 uF.
Originally I used these noisy Valvo-1701 two-way gas tubes with two silicon diodes forming a hybrid bridge.
I can use these big caps with the tubes because I have a big toroidal PFC choke in front of the mains xfmr which largely reduces excessive loading currents.
For Bias rectification I use a hybrid GZ 34 - 2x silicon-diode bridge.
They drop about 15 Volts current-independently (which is their great advantage). In my opinion, GU 50 is only another (national) name for the same tube. There have been several different names in different countries for this tube. In Germany I think this was RGQ 4/05. Heater rating is always the same - 2.5 V, 5 A. Since originally they were high-voltage rectifiers, current capalility depends largely upon voltage. They are rated 300-500 mA at 4 kV, 250 mA up to 10 kV (depending upon manufacturer). I have never seen any ratings for low voltages. But I think they could be used safely up to quite more than 0.5 A below 500 V - they have a very powerful cathode. Have a look on my home page www.uli.de/tubes/ , where I show some experience about their operation.
Yes, they do emit UV light, and since they are low-pressure devices, they emit most of their radiation in the invisible short-UV 254 nm (nanometers) line. Which indeed is very harmful - if - it gets out of the tube. But which shouldn't occur in minute quantities only in my opinion.... Since "ordinary" glass (but not quartz glass !!!!, which is completely clear down to 200nm, where the air itself starts to absorb - and forms Ozone) starts absorbing from about 300 nm on downward, it is almost completely "dark" to this 254 nm-line. So only long-wavelength UV-components should (and do) get out of the bulb.
Just to recall some of these values: the least visible dark violet light has about 350 nm of wavelength, everything shorter is invisible to the human eye. The visible greenish-blue light of these tubes is a mixture of many bands in the visible range (from 750 (dark red) - 350 nm) - but which transport only minor amounts of the emitted radiation energy - most of it being in 254 nm.
You can do a crude test of the absorption of your glass bulbs: Just hold a sheet of pure white paper close to the tube - if it seems to shine itself a bit in a bright blue colour (often you can see brighter and darker spots), then you see the fluorescence of the optical brighteners in the paper - stimulated by the emitted UV. I have one 866A from Haltron where the glass bulb itself flouresces in a greenish colour...
So perhaps sitting directly in front of the tubes and admiring them for hours shouldn't be recommended..... But I never have read anything about possible dangers from UV-radiation of rectifiers. Anyone else has?
So, to be on the safe side, I would propose to put a tube of acrylic or plexiglass around the mercury tubes - this material should be absorbing any UV-radiation quite effectively. But in fact, to be sure of that, a spectroscopic test should be made before use...
In the end, I would always keep in mind that there could be a possible danger, but I wouldn't worry too much about it. I would worry much more about breaking such a tube and contaminating my home with mercury.....
And never forget - any electron tube produces short wavelength radiation... An electron hitting a plate on 500 V also emits a part of its kinetic energy in short wave radiation, much shorter than UV - that's the reason also for the "magic limit" of 10 kV plate voltage, above which radiation gets so short-waved and powerful, that it penetrates the bulb and you have an X-ray emission out of the tube.....
But now I'm going off-topic..... 6C33C certainly presents no radiation dangers.....
Uli
Alastair E said:
Yes, they are.
Since they are one-way rectifiers, and not to spoil the other half-wave, I put a simple silicon bridge (B 250 C 1500) in front of them. 150 nF between the two AC leads. From + lead into a 10 mH toroidal choke, the Hg-tube 866A, 10 ohms, 100 uF, 22 ohms, 470 uf, Mosfet gyrator, 470 uF.
Originally I used these noisy Valvo-1701 two-way gas tubes with two silicon diodes forming a hybrid bridge.
I can use these big caps with the tubes because I have a big toroidal PFC choke in front of the mains xfmr which largely reduces excessive loading currents.
For Bias rectification I use a hybrid GZ 34 - 2x silicon-diode bridge.
They drop about 15 Volts current-independently (which is their great advantage). In my opinion, GU 50 is only another (national) name for the same tube. There have been several different names in different countries for this tube. In Germany I think this was RGQ 4/05. Heater rating is always the same - 2.5 V, 5 A. Since originally they were high-voltage rectifiers, current capalility depends largely upon voltage. They are rated 300-500 mA at 4 kV, 250 mA up to 10 kV (depending upon manufacturer). I have never seen any ratings for low voltages. But I think they could be used safely up to quite more than 0.5 A below 500 V - they have a very powerful cathode. Have a look on my home page www.uli.de/tubes/ , where I show some experience about their operation.
I would like to try these out, but another concern is Ultra-violet light...
Do they emit quantities that could be considered harmful?
Yes, they do emit UV light, and since they are low-pressure devices, they emit most of their radiation in the invisible short-UV 254 nm (nanometers) line. Which indeed is very harmful - if - it gets out of the tube. But which shouldn't occur in minute quantities only in my opinion.... Since "ordinary" glass (but not quartz glass !!!!, which is completely clear down to 200nm, where the air itself starts to absorb - and forms Ozone) starts absorbing from about 300 nm on downward, it is almost completely "dark" to this 254 nm-line. So only long-wavelength UV-components should (and do) get out of the bulb.
Just to recall some of these values: the least visible dark violet light has about 350 nm of wavelength, everything shorter is invisible to the human eye. The visible greenish-blue light of these tubes is a mixture of many bands in the visible range (from 750 (dark red) - 350 nm) - but which transport only minor amounts of the emitted radiation energy - most of it being in 254 nm.
You can do a crude test of the absorption of your glass bulbs: Just hold a sheet of pure white paper close to the tube - if it seems to shine itself a bit in a bright blue colour (often you can see brighter and darker spots), then you see the fluorescence of the optical brighteners in the paper - stimulated by the emitted UV. I have one 866A from Haltron where the glass bulb itself flouresces in a greenish colour...
So perhaps sitting directly in front of the tubes and admiring them for hours
shouldn't be recommended..... But I never have read anything about possible dangers from UV-radiation of rectifiers. Anyone else has?So, to be on the safe side, I would propose to put a tube of acrylic or plexiglass around the mercury tubes - this material should be absorbing any UV-radiation quite effectively. But in fact, to be sure of that, a spectroscopic test should be made before use...
In the end, I would always keep in mind that there could be a possible danger, but I wouldn't worry too much about it. I would worry much more about breaking such a tube and contaminating my home with mercury.....
And never forget - any electron tube produces short wavelength radiation... An electron hitting a plate on 500 V also emits a part of its kinetic energy in short wave radiation, much shorter than UV - that's the reason also for the "magic limit" of 10 kV plate voltage, above which radiation gets so short-waved and powerful, that it penetrates the bulb and you have an X-ray emission out of the tube.....
But now I'm going off-topic..... 6C33C certainly presents no radiation dangers.....
Uli
Thank you all for your comments - I'm actually going to try for a 13E1 or 6C33C SE design - keeping it simple - I have OPTs for both.
The thread has been fascinating - I didn't realise that the Russian tubes needed buring in, and that the number of options was so great. Re-reading parts of Rozenblit & Jones, van der Veen, Linsley Hood & the NEETS manual has been very worth while...
The thread has been fascinating - I didn't realise that the Russian tubes needed buring in, and that the number of options was so great. Re-reading parts of Rozenblit & Jones, van der Veen, Linsley Hood & the NEETS manual has been very worth while...
A 6C33C in a circlotron circuit... although I do not know it (the Circlotron circuit), I remember that their output impedance is lower than a 'standard circuit'. Maybe the 150R trafo from amplimo is intended for this kind of circuit!?
http://www.diyaudio.com/forums/showthread.php?postid=947113#post947113
http://www.diyaudio.com/forums/showthread.php?postid=947113#post947113
Absolutely! Forget about the UV. I don't think people are concerned enough about breakage of mercury-containing devices. A spillage would create a small hazardous zone in your home. I have tried to eliminate all mercury rectifiers from my shop. I still have one 83 in a tube analyzer, but that's about it, and the analyzer will on eBay sometime soon (with that tube wrapped in bubble-wrap while outdoors)!
ErikdeBest said:
That's possible very well. I use in my amp mains toroidal xfmrs as OPTs, 115 V : 30 V. So transfer ratio is about 4:1, and with impedances being transformed by the square of the transfer ratio, I get an impedance ratio of about 16:1. 16 x 8 Ohms = 128 Ohms. And since the impedance curves of my boxes are mostly a bit above 8 Ohms, I get quite exactly the same primary impedance value - 150 Ohms. I can switch my amp between OTL and OPT, and the difference is huge - 8 Ohms without OPT sound awfully.... I have one pair of 6C33CB per channel.
If I recall well, the circlotron output impedance generally is only one fourth of that of an "ordinary" push-pull amp. And 4 x 150 = 600 Ohms, this would fit as well....
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.