Otl

[fdegrove]

Hi Mohan,

Long time no see.

I agree with what you're saying in general and as you may know I built a copy of the GA Rosenblit amp for a friend years ago.

It was modded, but everything was pretty straightforward and sounded great.

So far I have not come up with any new ideas for eliminating the cap at the output in a SE OTL. Apologies Frank.

It can be done from what I gather BR is saying but it makes for more complexity in the circuit.

Zout is always a major concern with OTLs but if it's for home use and DIY its not that hard to come up with a 16 Ohm speaker though.

Frank, have you had any experience with 13E1. I love the looks of this tube. No longer in production and spares are becoming hard to find. Should I believe this?

Never, ever saw a single one but I know what you mean it's any OTL lovers' wet dream valve....
I don't know of one single dealer carrying these so, unfortunately, yes, you'd better believe it.

Maybe some of our British members know of a source or two? Who knows...

Cheers,

 

[316a]

Immortal 6C33

Paul, thanks for waking me. I have a lot of respect for Bruce ever since I read his article in Glass Audio some years ago. From memory, this was a no nonsense straight forward design based on Futterman topology. The power supply could be better.

My earlier post in this thread states my present preoccupation. So far I have not come up with any new ideas for eliminating the cap at the output in a SE OTL. Apologies Frank.

6C19PI is more or less equivalent to one half of a 6AS7. 6H30PI needs to be driven.

All of us know that a high transconductance tube such as 6C33C needs a large cathode area. This is because manufacturers use the same oxide coating on the cathodes. More R&D is required. But then again the Russians are waking up to current trends such as scale of economy, market volume and hence the returns in dollar terms. Sense of perfectionism and the pride that goes along with it has temporarily disappeared over the past two decades.

Thickness of cathode coating is not uniform and this translates to uneven emission. How do we know that the entire cathode area is uniformly heated? Directly heated triodes have similar problems, non uniformities in the composition of the cathode (Thoriated Tungsten), pitting, contamination as the tube ages and so on.

All this gives us some sort of explanation on variabilities that are inherent in the manufacture of modern electron tubes. During my brief experiments in the early nineties, I preferred to operate the 6C33C at about 30W dissipation and at a transconductance of around 20mA/V.

Yes, the Russian Septar sockets do conduct heat. I had to mount spacers and suspend these sockets below the chassis. A fan would help but then all the effort that I put into a low noise power supply will become an exercise in vain!

316A, I would love to have some more of those sockets, but keep some spares. I have cracked quite a few when removing tubes while the sockets are rather hot.

Frank, have you had any experience with 13E1. I love the looks of this tube. No longer in production and spares are becoming hard to find. Should I believe this?

Mohan

At 30w your 6C33 are going to (virtually!) last for ever . My door is always open , if you need any sockets drop me a mail .
13E1 are very easy to find in the UK but are quite expensive , in the early-mid 80's large quantities of these were manufactured , plenty to go around . Try Retrovox who may be your local 'downunder' 13E1 supplier 🙂

316a

 

[fdegrove]

Hi,

Patrick Turner did some cooking with the 13E1 as well:

13E1 SEUL.

Cheers,😉

 

[gnomus]

A real OTL amp may be the best thing. But, I am not so sure.

What is better at the output a cap or transformer? Because of the really high quality caps you can buy now like teflon, copper foil or insert your favorite here. This would seem to be lesser of these two evils. But, then output transformers technology has also vastly improved too. I am not so sure now in terms of sound which would be better. In price the cap may be a cheaper solution. But, then again I have seen some really expensive caps. Nelson pass of Passlabs said that "cap distortion is nothing in comparison to the output device be it tube or transistor". I do not know if output transformers fall into this realm too. Meaning they have improved so much that it too has less distortion than the output device. To me the real problem is the ouput device. What good is an OTL if the tubes are not as linear and thus do not sound as good as a 300B with a transformer output? I do not know if this is true. I am just trying to decide what would be the best tube based amp topology to build. I am leaning towards selecting based on the output devices quality running in class A. I would like to hear other thoughts on this to help me decide.

 

[316a]

Choices

Class A operation rules out OTL straight away ! Maybe a new posting under tubes may help 🙂

316a

 

[Mohan Varkey]

Thanks Frank. Yes, I am becoming slower by the day – age perhaps or is it a state of mind or too many priorities to run to all the time – I do not know.

Perhaps you could throw some more light on what BR says about eliminating the output cap in an OTL. Does this involve solid state devices? If so I do not want to know about it. Rectifiers are the only exceptions in my designs.

Patrick and myself participate in the Melbourne Audio Club forum. He has only a handful of 13E1. Perhaps, I should take up 316a’s offer. It is good to know that there are stocks around.

316a, I made the 6C33C amp some years ago and gave this away to a pensioner who loves classical music and could not afford a valve amp for his Klipschorns. Personally, I did not take to this amp despite a few tries to overcome the high input capacitance of the tube. I tried neutralisation but then feedback produces uninvolving music – kills dynamic contrasts.

Paul, I would prefer a transformer to a capacitor at the output. In an OTL we are looking at large capacitance values and so this could be quite expensive if you are looking at film caps.

By the way, 300B curves look linear in print. However, let us for a moment think about the load line that you have drawn on a set of curves. We all know that a loudspeaker impedance changes with frequency and that your output transformer has a 20:1 turns ratio. Now let us think about what happens in real time when music is played. As the impedance of the speaker changes, the load reflected back to the primary and thus to the 300B is magnified in proportion - square of the
turns ratio. In other words, the load line rotates up and down swinging from the operating point. In other words, there is no perfectly linear device available to us in real time. We just have to cop it on the chin as we forgive ourselves for all of our inconsistencies. There is a way to minimise this and that is by selecting a tube that has low internal resistance. There is no win win situation here, only better compromises.

Mohan

 

[fdegrove]

Hi,

Thanks Frank. Yes, I am becoming slower by the day – age perhaps or is it a state of mind or too many priorities to run to all the time – I do not know.

I hope I wasn't suggesting any of that but is, yes we're not getting any younger...Still smashing the "jonkies" though.

Whatever we lose physically, we make up with experience....

Perhaps you could throw some more light on what BR says about eliminating the output cap in an OTL.

He's referring to SE OTLs and how to eliminate that dreaded coupling cap.
Short of reading his mind I'd say a bi-polar supply should be able to get rid of it?

Rectifiers are the only exceptions in my designs.

Ditto.

Personally, I did not take to this amp despite a few tries to overcome the high input capacitance of the tube. I tried neutralisation but then feedback produces uninvolving music – kills dynamic contrasts.

You need plenty of current drive with this valve, it can be done and then they shine. If not they sound too dull to even bother with.

Paul, I would prefer a transformer to a capacitor at the output. In an OTL we are looking at large capacitance values and so this could be quite expensive if you are looking at film caps.

Depends on Zo of the amps.
If it's low cap couple, if it's too high with respect to load go with a xfromer. Both can sound good but do not make for a true OTL.

By the way, 300B curves look linear in print.

Stop, in print, period.

There is a way to minimise this and that is by selecting a tube that has low internal resistance. There is no win win situation here, only better compromises.

Persackly, but oh so little gain so all of that needs to be done at the input stages...

Hold on, has anyone seen curves of a tube in a SEPP configuration?
No? Ahhh...

What are the curves we usually look at anyway?

Mucho to do muchissimo to learn I reckon?

Clevor Trevor strikes again...kidding,😉

 

[fdegrove]

Hi,

6C19PI is more or less equivalent to one half of a 6AS7.

Well it seems you made the same wrong assumption as I foolishly did in another post regarding the 6C19pi.

It's not a baby 6AS7G, at best it's a baby TFK ED8000.

I thank BR for pointing this out to me, he's absolutely right.

Now, solving the DC on the output of a SE OTL isn't too difficult ( thanks once again Bruce R.) but it complicates matters a little on the PS side.

If the interest on that is still alive, I'm more than willing to explain how, unless BR or someone else objects....I don't want to rob someone's livelyhood.

Cheers, 😉

 

[gnomus]

Burce's grounded gride pre-amp uses a negative supply. No FETs. But he states in his book that an "alternatvie would be to use an FET as a constant current source. This would eliminate the need for a negative supply entirely." Not an expert in tubes so do not know if this is what he is thinking.

By the way the grounded grid per-amp looks really good. Simple design and not expensive to make. Never heard one. But, if perform like what he claims it's a real winner.

 

[fdegrove]

Hi,

Not an expert in tubes so do not know if this is what he is thinking.

The idea is not to lose a negative supply but to add one, for the SE OTL that is.

This supply should be well regulated and adjustable so that the DC voltage at the cathode can be balanced out.

I don't know about the GG preamp but see no reason why it wouldn't sound fine.

Cheers,😉

 

[Mohan Varkey]

Frank, can you shed some more light on how DC can be eliminated without capacitors? Perhaps a small sketch of the topology would help. If this is Bruce’s idea, we can give him due credit.

Mohan

 

[fdegrove]

Hi,

Frank, can you shed some more light on how DC can be eliminated without capacitors? Perhaps a small sketch of the topology would help. If this is Bruce’s idea, we can give him due credit.

Bruce was kind enough to shed some light on it in an e-mail.

Actually it's rather simple in concept:

I suppose you've seen CFs with a bipolar supply before, if you don't look at Joe Rasmussen's IGC with a CF in front of the chip.

JLTI

Now make that negative regulated supply adjustable so you can compensate for the voltage drop due to Rp and adjust for 0 VDC at the output.
It's simple and brilliant.

With SE designs we still have the entire PS in the signal path though, so a high quality supply is still very much a necessity.

Cheers,😉

 

[Mohan Varkey]

Thanks Frank. I had thought about this for some time. Do you think that the circuit would require constant tweaking in use to maintain 0 volts.

Mohan

 

[fdegrove]

Hi,

Do you think that the circuit would require constant tweaking in use to maintain 0 volts.

No, not at all.
In practice I'd include some testpoints though.

One should try to keep the circuit close to 0 VDC at the output but even at 100mV across the output terminals I wouldn't panic.

It wouls be really clever to have automatic voltage tracking at the output which is not unthinkable to do.

Cheers,😉

 

[NickC]

Frank
how about using a servo to controlled the regulated negative power supply. By comparing the output of the otl with a preset value, it would compensate for dc in the output thus having very low dc on the output which in turn avoids constant tweaking to maintain the 0 volts

 

[gnomus]

If your going to use high efficiency horns. I have read that 50mv or less is desired.

 

[tubetvr]

If your going to use high efficiency horns. I have read that 50mv or less is desired.

Interesting, where did you read this? and why should this be more important with high efficiency horns?, I would think it should be less important as the speaker membrane movement is less in a horn and any fixed displacement of the membrane as when a DC voltage is applied therefore must be compareably less important then in other types of speakers.

I have not heard any differences in sound with DC voltage between 0 to 200mV applied to my Lowther Fidelio horns.

BR Hans

 

[fdegrove]

Hi,

and why should this be more important with high efficiency horns?,

One has got nothing to do with the other but I think I know where this comes from...Either way it's wrong.

I have not heard any differences in sound with DC voltage between 0 to 200mV applied to my Lowther Fidelio horns.

One of the reasons BR didn't put a OCL SE OTL on the market is that he knows all to well how paranoid people can get when it comes to a little DC on the output...
Can't say I blame him.

Cheers,😉

 

[gnomus]

I most likely read it on Passlabs' forum. I know Mr Pass has stated that 100mv or less is acceptable. He was not talking about horns but just in general for most speakers. He may have mentioned 50mv for horns but I can not remember. From what I understand dc offset causes the speaker cone to push out. I think mostly in the bass. The cone starts with an excursion and is not in its normal postion at rest. Horns are more efficient and tend to have little movment. So, to me less dc offset is needed in a horn to move the cone a tiny amount. This tiny amount of excursion may be significant because the cone moves so little to begin with. It's just a guess. Anyway there is no reason to have 200mv. Most circuits have ways to trim out dc at the output. I have built Passlabs amps. There balanced designs and the output devices are matched. And what dc was there I could trim out with a resistor to levels of 10mv or so.

Tubes are new to me as far as construction and understanding. And I tend to like tubes over almost all of the solid state I have heard. To learn electronics it was easier to start with solid state. And Passlabs' forum is a good place to start. Mr. Pass is very active in it and has tremendous expertise. His circuits are simple and use the fewest parts. I am tainted by the minimilist ideas there. OTLs are a natural choice for me as you can do away with the output transfomer. Which I see as a big slab iron that is in the way. The problem is finding a simple effective OTL design.

 

[tubetvr]

Horns are more efficient and tend to have little movment. So, to me less dc offset is needed in a horn to move the cone a tiny amount. This tiny amount of excursion may be significant because the cone moves so little to begin with. It's just a guess.

Horns are more efficient, yes that is correct but not for DC. If a speaker element is used either in a horn, reflex or closed box it will have exactly the same efficiency for DC, that is for a certain DC voltage the speaker cone will move the same amount regardless where the speaker element is installed.

as you write in horns the speaker cone is heavily loaded so for AC above horn cutoff the cone movement is less then in other type of speakers, this means that a speaker mounted in a horn is less sensitive for eventual displacement due to DC voltage then if the same element is used in a closed box or reflex cabinet.

Example: if DC offset gives say 2mm offset displacement it is more of a problem if the cone in a closed box need to move +- 3mm, then if same element used in a horn need to move +- 1mm as there will be more "movement margin" available in the horn application.

I agree that it should be easy to limit offset voltage to less then 200mV and in my case I just wanted to test what effect it would have as I agree with Frank and others that the issue of offset voltage is vastly exaggerated, however normally I aim for offset voltage less then 25mV.

The problem is finding a simple effective OTL design.

It is difficult to find any OTL design which is effective, (as in efficient) and most designs are quite complicated compared to transformer coupled amps, if you however want to try building an OTL amp I encourage you to do that, for me after building my OTL there is no way I would go back building a transformer coupled amp.

Regards Hans