Ive been meaning to try good old-fashioned Autobias for an OTL for a couple of years.
I finally settled on a pretty basic simple Inv. Futterman having two gain stages and a concertina phase-splitter.
I was looking to make it reasonably simple, and not have the usual problems of bias drift and runaway sometimes associated with OTL using the 6C33C
I built this before Christmas, a Stereo 'Breadboard', and its been used every day.
Sounds wonderful and has been utterly reliable. At a guess, its done around 400 hours.
(Yup, I like to TEST my breadboards!)
I was surprised when I built it, that the 'worst DC offset' with my most mis-matched 6C33 was only 0.25 volt with an 8 ohm load...
With a reasonably matched pair this is on average 20mV!
It has two 6C33 per channel, and all heaters series wired and running off the O/P +B transformer AC side with an additional 'underwind' to take the 55V to 49-50....
I'm only running the O/P stage with plus/minus 100 V supplies as I'm using a common or garden 500VA 0-55 0-55 toroid with a couple of extra turns added to the existing windings.
Ive not checked the actual O/P power, but reckon on around 15W ish. Not a lot, but I have a pretty small listening room....
I finally settled on a pretty basic simple Inv. Futterman having two gain stages and a concertina phase-splitter.
I was looking to make it reasonably simple, and not have the usual problems of bias drift and runaway sometimes associated with OTL using the 6C33C
I built this before Christmas, a Stereo 'Breadboard', and its been used every day.
Sounds wonderful and has been utterly reliable. At a guess, its done around 400 hours.
(Yup, I like to TEST my breadboards!)
I was surprised when I built it, that the 'worst DC offset' with my most mis-matched 6C33 was only 0.25 volt with an 8 ohm load...
With a reasonably matched pair this is on average 20mV!
It has two 6C33 per channel, and all heaters series wired and running off the O/P +B transformer AC side with an additional 'underwind' to take the 55V to 49-50....
I'm only running the O/P stage with plus/minus 100 V supplies as I'm using a common or garden 500VA 0-55 0-55 toroid with a couple of extra turns added to the existing windings.
Ive not checked the actual O/P power, but reckon on around 15W ish. Not a lot, but I have a pretty small listening room....
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Congratulations Man 
I`m not the big fun of Futterman/Inverted Futterman Amp topology,anyway......
You done Nice variation of `Inverted Futterman` OTL Amp topology.
Bet you that this your small ~15 w OTL Amps sounds 1000 times better than any~ 15w WOPT or SS based Amps
With only ~18w idling per each 6c33c plate ,this output tubes will last for really long time.
With your Autobias configuration for each 6c33c is `sacrificed` maximum available output peak power of Amps performance but on the other hand actually rises the Amps reliability for sure.
Just wonder, since you are not show the PSU schematic:did the transformer CT secondary of B+and B-(+- 100V)goes to the ground or is `floating`?
Best Regards
I`m not the big fun of Futterman/Inverted Futterman Amp topology,anyway......
You done Nice variation of `Inverted Futterman` OTL Amp topology.
Bet you that this your small ~15 w OTL Amps sounds 1000 times better than any~ 15w WOPT or SS based Amps
With only ~18w idling per each 6c33c plate ,this output tubes will last for really long time.
With your Autobias configuration for each 6c33c is `sacrificed` maximum available output peak power of Amps performance but on the other hand actually rises the Amps reliability for sure.
Just wonder, since you are not show the PSU schematic:did the transformer CT secondary of B+and B-(+- 100V)goes to the ground or is `floating`?
Best Regards
I'm pretty sure the same idea could be used for any OTL topology, certainly a circlotron could use it...
Yes, it may have the drawback of a lower power O/P but the benefits IMO far outweigh the loss...
After all, How often do you use full power?
The plate-diss currently is 14W, --100V Minus whats dropped over the cathode-resistor (at the moment, 18V) so that makes around 14W ish...
It definately runs cooler than other designs Ive made.
The PSU for the O/P stage ground is connected to main ground of the amp. Its not floating. Just a pretty simple plus/minus grounded supply..
I would like ideally to raise the supplies to the O/P stage to around 140V and trim the cathode-resistors to give 180-190 mA keeping the P-diss down to around 20-25W
This will give more headroom, although when playing as it is, there's no apparent issues even when played loud.
--It just quietly and gracefully goes into clipping at the limits of its power.
--Looks like I'll have to add some more turns to the Toroid!
Initially when I built it, I used quite large coupling-caps to the G1 of 6C33, (10uF) but this gave me blocking/pumping effects, when payed at full output --on certain tracks only though!
I tried 0.22 and eliminated that effect, with little if no loss in LF performance in listening tests.
Yes, it may have the drawback of a lower power O/P but the benefits IMO far outweigh the loss...
After all, How often do you use full power?
The plate-diss currently is 14W, --100V Minus whats dropped over the cathode-resistor (at the moment, 18V) so that makes around 14W ish...
It definately runs cooler than other designs Ive made.
The PSU for the O/P stage ground is connected to main ground of the amp. Its not floating. Just a pretty simple plus/minus grounded supply..
I would like ideally to raise the supplies to the O/P stage to around 140V and trim the cathode-resistors to give 180-190 mA keeping the P-diss down to around 20-25W
This will give more headroom, although when playing as it is, there's no apparent issues even when played loud.
--It just quietly and gracefully goes into clipping at the limits of its power.
--Looks like I'll have to add some more turns to the Toroid!
Initially when I built it, I used quite large coupling-caps to the G1 of 6C33, (10uF) but this gave me blocking/pumping effects, when payed at full output --on certain tracks only though!
I tried 0.22 and eliminated that effect, with little if no loss in LF performance in listening tests.
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Since 6c33c tube generaly is not suited well for `deep` A2/AB2 OTL operation(like 6h13c) this 6c33c tube autobias is very good for lower eficiency A1/AB1 Amp operation.It is posible to use some sort of dual mode bias-(autobias + fixedbias) with DC coupled CF drivers to,for improved OTL output stage power efeciency.
And Yes,pure autobias like you did cood be good for 6c33c OTL Circlotron tf course than that topology includes grids coupling capacitors,with all sonics(coupling capacitors) drawback of that design aproach.
Agreed with you: OTL Amps usualy dont run for extreme output power but for Best of the Best Amps sonics !
30 W(150V/0,2 A) for 6c33c plate steady state bias is Apsolute Maximum for optimum tube performance and longest tube live(My expirience),since your Amps run far from this plate Maximum
dont wory,they well live amazing long,just check the tube heater pins & sockets pins contacts for burn out (corosion) sometime.
Best Regards
And Yes,pure autobias like you did cood be good for 6c33c OTL Circlotron t
f course than that topology includes grids coupling capacitors,with all sonics(coupling capacitors) drawback of that design aproach.Agreed with you: OTL Amps usualy dont run for extreme output power but for Best of the Best Amps sonics !
30 W(150V/0,2 A) for 6c33c plate steady state bias is Apsolute Maximum for optimum tube performance and longest tube live(My expirience),since your Amps run far from this plate Maximum
dont wory,they well live amazing long,just check the tube heater pins & sockets pins contacts for burn out (corosion) sometime.
Best Regards
Banat,--
You have some very valid points. I initially did think of a combined 'dual-bias', using a combination of auto and fixed, but I was looking to make a Really simple, no nonsense amp,--Maybe the sort of thing a less experienced DIY'er could try if he's interested in OTL, Something nice and basic that works well and a pretty simple PSU...
I think there's more to be gained from the basic design as it is, with a somewhat higher +B supplies to the 6C33's but still retaining the 'reliability' of the autobias format
--I really should try this and do some proper power measurements, Getting the time these days is the problem!
Currently, this is in bread-board format, and needs work on the PSU, currently, I have just a little hum if you listen right at the speakers.--a 'fault' of the topology, and too little smoothing.
Over the years, Ive esperienced the issues of poor heater-pin contact with certain sockets, in fact I have two different types on this current amp.
One Russian white square ceramic type, --which has had a very hard life in other projects in the past, and I'm using three of the brown phenolic type sockets.
These brown ones grip the pins Very firmly, its Much harder to remove valve from this type socket than the ceramic type....
--The top of the socket in contact with the valve has gone black, but the pin contact is still very firm
Ive had to tighten the ceramic type a few times over the years (in other projects), and one of its heater connections had gone very loose on the rivet...
Heat and time will tell how the phenolic type last!
I should have added, that the 6C33C-B I'm using are well burned in and pretty well seasoned, they probably had about 100 hours on them at the time I made this amp.
I believe Burning in of these valves is Mega important....
You have some very valid points. I initially did think of a combined 'dual-bias', using a combination of auto and fixed, but I was looking to make a Really simple, no nonsense amp,--Maybe the sort of thing a less experienced DIY'er could try if he's interested in OTL, Something nice and basic that works well and a pretty simple PSU...
I think there's more to be gained from the basic design as it is, with a somewhat higher +B supplies to the 6C33's but still retaining the 'reliability' of the autobias format
--I really should try this and do some proper power measurements, Getting the time these days is the problem!
Currently, this is in bread-board format, and needs work on the PSU, currently, I have just a little hum if you listen right at the speakers.--a 'fault' of the topology, and too little smoothing.
Over the years, Ive esperienced the issues of poor heater-pin contact with certain sockets, in fact I have two different types on this current amp.
One Russian white square ceramic type, --which has had a very hard life in other projects in the past, and I'm using three of the brown phenolic type sockets.
These brown ones grip the pins Very firmly, its Much harder to remove valve from this type socket than the ceramic type....
--The top of the socket in contact with the valve has gone black, but the pin contact is still very firm
Ive had to tighten the ceramic type a few times over the years (in other projects), and one of its heater connections had gone very loose on the rivet...
Heat and time will tell how the phenolic type last!
I should have added, that the 6C33C-B I'm using are well burned in and pretty well seasoned, they probably had about 100 hours on them at the time I made this amp.
I believe Burning in of these valves is Mega important....
You are 100% right about this square ceramic Russian 6c33c tube socket, with fork type contacts.I seen everybody use them as original,(Profesionals & DIY) for 6c33c OTL aplication,but this ceramic sockets is not just reliable as Original 6c33c brown type (phenolic),but ceramic one is just OK for 6c41c tube. Example:
If Amps expirience some accidental mechanical shock,lets say during the Amps move from one room to other, 6c33c tubes will fall down from their sockets and broke
With this brown type Russian socket this is simple imposible,+ elecritcal & thermal contact with tube pins is superior !
About your autobias OTL amps again :Some electrical measurments have to be done(in one of future day),until than Enjoy
Finaly Only what is count on the end is the final judgement of Your Ears,nothing else !
Best Regards
Ps: Mr Karsten(Atmasphere) explained somewhere on this DIY site one very good method for 6c33c tube precondition,which is prety important for future normal OTL Amp operation.
If Amps expirience some accidental mechanical shock,lets say during the Amps move from one room to other, 6c33c tubes will fall down from their sockets and broke
With this brown type Russian socket this is simple imposible,+ elecritcal & thermal contact with tube pins is superior !
About your autobias OTL amps again :Some electrical measurments have to be done(in one of future day),until than Enjoy
Finaly Only what is count on the end is the final judgement of Your Ears,nothing else !
Best Regards
Ps: Mr Karsten(Atmasphere) explained somewhere on this DIY site one very good method for 6c33c tube precondition,which is prety important for future normal OTL Amp operation.
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You might be interested to know that I have just opened a Wireless World archive page on my website, and the first item is a 1957 article on transformerless valve amplifiers.
The Wireless World Archive
I hope that's useful.
The Wireless World Archive
I hope that's useful.
About 6c33c-b tube socket again,there is picture of four Russian 6c33c-b socket type.
Type 1,2 and 3 is just OK, type 4 is with fork type contacts,not bad but previous three types is much better and reliable.
Type 1,2 and 3 is just OK, type 4 is with fork type contacts,not bad but previous three types is much better and reliable.
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Type 3 with ceramic base is superior one for 6c33c tube,but type 4 is just OK for 6c41c tube,filament current is only 2,7A(6,3V) and dont run the tube socket filaments contacts to the limit of endurance,released tube heat is lower to,
6c41c is about 1/2 of 6c33c tube and is just fine for OTL.
Anyway there have to be good air ventilation all around the tubesocket & tube placed on OTL Amp chassis,whatever tube socket is used for !
Best Regards
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Hello Alistair E,
Very nice design! You mentioned using the 6C41C in place of the 6C33C. I did this with the 'Tim Mellow' OTL design and they work perfectly well at 100mA. with 140v +&- HT. I think they are best with 8 Ohm speakers, 4 Ohm are too low for the 6C41's . You mentioned that you have a hum problem (which you will be looking at). My previous OTL,s using a similar circuit to yours (but with independant biasing of each O/P valve) also hummed slightly. This is the startling thing about the Tim Mellow design in that there is no hum - due I suspect, to the use of LTP's throughout. Might this be an option for you if it proves difficult to cure?
I'll probably be building one of your design soon.
Thanks,
Kind regards,
David.
Very nice design! You mentioned using the 6C41C in place of the 6C33C. I did this with the 'Tim Mellow' OTL design and they work perfectly well at 100mA. with 140v +&- HT. I think they are best with 8 Ohm speakers, 4 Ohm are too low for the 6C41's . You mentioned that you have a hum problem (which you will be looking at). My previous OTL,s using a similar circuit to yours (but with independant biasing of each O/P valve) also hummed slightly. This is the startling thing about the Tim Mellow design in that there is no hum - due I suspect, to the use of LTP's throughout. Might this be an option for you if it proves difficult to cure?
I'll probably be building one of your design soon.
Thanks,
Kind regards,
David.
Hi David!
--Thanks for the nice words...
'Hum' is an issue of the 'Futt' design I believe. I was thinking of making up a simple hum-filter with maybe a bipolar or MOSFET and fit between the first and second smoothing-electrolytic in the PSU. Currently, Ive only got 6,600uF in each rail.
Not a great fan of the Futterman design myself, but I thought I would try one out, having never heard one, and seemed like a good time to try the autobias dodge too, simplifies the PSU a little!
I'm not using anything special component wise, ordinary cheapo metal-film resistors, grid-stoppers of 220 ohm, --100 ohm on 6C33c-- (G.S. not in above diagram) and Russian K40Y-9 PIO, 1000V
--I must admit, I was impressed with the sound from it, particularly in the clarity and bass sound,--always thought the bass would be lacking in a Futterman but no.....
--Might sound a strange thing, but sometimes you may hear a track you've not heard before and have 'difficulty' in distinguishing exactly what the singer is saying, in some parts, especially backing-singers....
Dunno who else has found this.....
--Not with this amp, Its just as if the singer is there in front of you....
--Summit my Circlotron isnt quite up to....
(the above statement hints that I have inferior hearing,--It aint that, I think its more my 'slow' brain!)
Hum is only noticable if you shove your ear to the speaker, OR its a very quiet night (which is rare here!) between music tracks...
Not really an issue, but I'm intending to do something with it...
Ive played with the 6C41c before a few times. It seems like a lot of these big tubes, they need a Lot of burn-in. A few years ago, I got 8 from fleagay to play with, plugged them into my then breadboard circlotron, altered heaters, set bias, but they sounded awful....
--This was straight out of the box new, still with blue print markings on....
Left them idling in the amp with no signal for a couple of hours and re tried....
Completely different beast! From appearing almost distorted initially, after a few hours sitting idling they sounded really sweet...
I might just do that this evening, I need to re-wire two sockets on the heater side and add a 25V 4A toroid to do it....
(I use series-wired heaters..)
--Thanks for the nice words...
'Hum' is an issue of the 'Futt' design I believe. I was thinking of making up a simple hum-filter with maybe a bipolar or MOSFET and fit between the first and second smoothing-electrolytic in the PSU. Currently, Ive only got 6,600uF in each rail.
Not a great fan of the Futterman design myself, but I thought I would try one out, having never heard one, and seemed like a good time to try the autobias dodge too, simplifies the PSU a little!
I'm not using anything special component wise, ordinary cheapo metal-film resistors, grid-stoppers of 220 ohm, --100 ohm on 6C33c-- (G.S. not in above diagram) and Russian K40Y-9 PIO, 1000V
--I must admit, I was impressed with the sound from it, particularly in the clarity and bass sound,--always thought the bass would be lacking in a Futterman but no.....
--Might sound a strange thing, but sometimes you may hear a track you've not heard before and have 'difficulty' in distinguishing exactly what the singer is saying, in some parts, especially backing-singers....
Dunno who else has found this.....
--Not with this amp, Its just as if the singer is there in front of you....
--Summit my Circlotron isnt quite up to....
(the above statement hints that I have inferior hearing,--It aint that, I think its more my 'slow' brain!)
Hum is only noticable if you shove your ear to the speaker, OR its a very quiet night (which is rare here!) between music tracks...
Not really an issue, but I'm intending to do something with it...
Ive played with the 6C41c before a few times. It seems like a lot of these big tubes, they need a Lot of burn-in. A few years ago, I got 8 from fleagay to play with, plugged them into my then breadboard circlotron, altered heaters, set bias, but they sounded awful....
--This was straight out of the box new, still with blue print markings on....
Left them idling in the amp with no signal for a couple of hours and re tried....
Completely different beast! From appearing almost distorted initially, after a few hours sitting idling they sounded really sweet...
I might just do that this evening, I need to re-wire two sockets on the heater side and add a 25V 4A toroid to do it....
(I use series-wired heaters..)
Hi Alistair,
Interesting points you made there. As you say, the 'burn in' period is essential with these valves. My little 6C41C amp. has settled down nicely and sounds excellent, but definitely needed the burn in. Looking forward to hearing how you get on with your mods.
Kind regards,
David.
Interesting points you made there. As you say, the 'burn in' period is essential with these valves. My little 6C41C amp. has settled down nicely and sounds excellent, but definitely needed the burn in. Looking forward to hearing how you get on with your mods.
Kind regards,
David.
Absolutely! Go for it....
I would try them myself, but dont have any....
I do have some really tired 6080's somewhere....Maybe I'll try 'em sometime....
Possibly, you may want individual cathode-resistors per section, set up for whatever you feel is the correct bias for that tube...
--Unfortunately, that would mean individual cathode-bypass caps too--Possibly not a bad thing.....
The more I play it, the more I like it...
To me, it really adds the drama and realism to the music not often found these days, especially from SS stuff....
Dramatic tracks like 'Cry me a river, Micheal Buble' sound absolutely tremendous, very powerful,--although not to everybodys taste....
Well, Ive re-wired the sockets so that I can use the 6C41C, and also the 6C33C with only having to change the heater supply-voltage....
Currently on the 6C41C using all the same bias parts in the cathode-circuit, Ive got 150mA bias on average at a little under 100V supplies, giving a P-diss of 12 ish W. (About half the max of the 6C41C)
Sound appears so far to be the same as with the 6C33C, although Ive not used it for many hours yet, or at high power-levels...
The heat is definately lower though!
Testing comes next....
Currently on the 6C41C using all the same bias parts in the cathode-circuit, Ive got 150mA bias on average at a little under 100V supplies, giving a P-diss of 12 ish W. (About half the max of the 6C41C)
Sound appears so far to be the same as with the 6C33C, although Ive not used it for many hours yet, or at high power-levels...
The heat is definately lower though!
Testing comes next....
Well, Its played faultlessly now for a month more using the 6C41C For normal listening levels in my reasonably small room, its more than enough...
Sound is still very good and the valves completely stable.
Week or two ago, I scored some British made sockets, at a local ham-fest, which look very similar to the Russian ones, but the metal of the 'grips' for each pin is about twice the gauge and the small spring-things clipped round them are stronger too....
Got a set of four for a bargain price of 2 quid each,--Perfect!
Time will tell if they last better than Russian or other types though....
Its about time I committed this amp to a proper build, the breadboard is getting a little dusty now!
Sound is still very good and the valves completely stable.
Week or two ago, I scored some British made sockets, at a local ham-fest, which look very similar to the Russian ones, but the metal of the 'grips' for each pin is about twice the gauge and the small spring-things clipped round them are stronger too....
Got a set of four for a bargain price of 2 quid each,--Perfect!
Time will tell if they last better than Russian or other types though....
Its about time I committed this amp to a proper build, the breadboard is getting a little dusty now!
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