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OTL SE - Help Wanted with Idea

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poynton

poynton

Member
Joined 2005
Hi.

One of the 'classic' designs for a simple OTL amplifier is shown below :-

http://www.audiodesignguide.com/otl/otlse.jpg

However, this is not true OTL as it uses chokes as cathode loads.
The chokes could be replaced with resistors but this would not be an efficient idea.

My idea would be to replace them with a current source. (As I have seen used as plate loads etc.) See link :-

An externally hosted image should be here but it was not working when we last tested it.


Anyone care to comment whether this would work or even if it has already been tried in this manner ??

Many thanks.

Andy
 
J

JPPP

Member
Joined 2004
Hi, I would like to know too if somebody has tried this. There is a webpage containing this schematic with a note that it has actually been tested with excellent result. But no other info.

http://www.tubebuilder.com/schematic3.html

I just build open baffle speakers with Goodmans Axiom 201's (16ohm, 98db/m) and I do happen to have 8 pieces of 6as7 tubes, so I guess a low power OTL would be a nice project at this stage.

yours, Jussi
 
rdf

rdf

Member
Joined 2004
Doesn't replacing the choke with a CCS cause you to lose half the available voltage swing? I don't know for sure but I think it's analagous to replacing a plate choke and losing the ability to swing plate voltage over B+. If so a simple solution would be to use a positive and negative supply for the output stage. A clever rabbit might additionally be able to make use of the servo philosphy behind SY's Heritcal Line Stage and eliminate the output cap.
 
Fuling

Fuling

Member
Joined 2001
Swing is not the issue here, current is.
One idea that I´ve been thinking about but never tested is to load the cathode followers with a dynamic (Aleph-type) current source. This would roughly double the output power from a given number of output tubes, though I can feel that it would be so much of an hybrid amp that it would be better to replace the tubes with another Mosfet
:D

Anyway, I have a box of 6C19 low Rp triodes and a pair of suitable cathode chokes so one day I´ll build myself an SE OTL similar to Rosenblitz´s design.
 
tubetvr

tubetvr

Member
Joined 2003
qtron.se
Note that the max allowed continous anode current for each section of a 6080 is 100mA, here the current is 250mA! Also the power dissipation of each section is 165V * 0.25 = 41W which is way over the allowed 13W for each half. It is a recipe for disaster where the anodes will be glowing and the tubes will burn out in a very short time.

There is no shortcut to a class A OTL design, either you will get very low power or you will burn out the tubes.

Regards Hans
 
poynton

poynton

Member
Joined 2005
Geek said:
Hi Poynton,

One thing you have to be careful of with CCS-loaded cathode followers like your posted schematic is they like to turn into power oscillators at the drop of a pin unless the load is a fair stable resistance.
Click to expand...



HI.

Thanks for the input.

Do you know if this type of arrangement has been tried before as an output stage or as intermediate stages ??

Andy
 
poynton

poynton

Member
Joined 2005
tubetvr said:
Note that the max allowed continous anode current for each section of a 6080 is 100mA, here the current is 250mA! Also the power dissipation of each section is 165V * 0.25 = 41W which is way over the allowed 13W for each half. It is a recipe for disaster where the anodes will be glowing and the tubes will burn out in a very short time.

There is no shortcut to a class A OTL design, either you will get very low power or you will burn out the tubes.

Regards Hans
Click to expand...

Hi.

Thanks for the answer.

I am aware of the numbers on the diagram which seem to possibly go against the published data for the 6080.

However, the circuit was developed in 1952 and, as noted in the post from JPPP, does work !! ( presumably without self-destructing ).

The idea obviously requires work.

Andy
 
tubetvr

tubetvr

Member
Joined 2003
qtron.se
The idea obviously requires work.
Click to expand...

Don't get dissapointed if you will find out in the end that it will selfdestruct. It is one thing to exceed dissipation and current parameters in a class AB or B OTL as it doesn't happen very often, (only at music peaks) but in this case the circuit draw constant high current all the time.

If it was safe to run any tube continously at 2.5 rated current or 3.5 times rated anode dissipation I believe more would do so gladly.

That the circuit was developed in 1952 doesn't say anything about its performance or reliability, that OTL's haven't been built comercially according to the schematic says more. You can find quite many of similar overoptimistic circuits on the net. That someone unknown on the net claims to have been testing the circuit doesn't give much of credit either.

My own interest in this is not to promote class A OTLs but I want others to realise that it is possible to build a very reliable OTL with reasonable output power but then it must be working in class AB, real class A or single ended OTL is a moot idea except for anyone accepting very low output power.

Regards Hans
 
poynton

poynton

Member
Joined 2005
tubetvr said:


If it was safe to run any tube continously at 2.5 rated current or 3.5 times rated anode dissipation I believe more would do so gladly.
Click to expand...

Obviously, the tubes would not be run at 2 - 3 times rated dissipation !! I do not think the 2A on the drawing refers to average current but to peak or PSU capacity.


My main concern is :-

Will it work with a CCS instead of choke ?

All other things can be changed.

Andy
 
tubetvr

tubetvr

Member
Joined 2003
qtron.se
I do not think the 2A on the drawing refers to average current but to peak or PSU capacity.
Click to expand...

It is very unusual to write values of peak currents in a schematic, it is common practise to give the values for average current or in some cases idle current. In this case the 2 values are the same as it is a class A amp.

2A idle current would give you an output power of 32W in 16ohm but with safe idle current of 800mA that would be reduced to 5.1W.

In addition to the shaky cirtcuit itself there are no method included to avoid current hogging between the tubes, 6080/6AS7 are known to have differences between the 2 triodes even in the same envelope and there are differences between individual tubes also, therefore you need a method to ensure that none of the tubes draw too much current this is even more important when rtunning close to max. The easiest way is what is recommended in the datasheet for 6080/6AS7, namely to use small individual cathode resistors for each triode section.

If you can exchange the choke for a CCS I don't think is not important given all the other issues but yes, I don't see any big problem with that, that is the smallest of the problems.

It never stops amazing me this quest for a class A preferably single ended OTL, tubes even the best ones are not suited to this purpose and for myself I don't see the benefits of this topology compared to class AB with 20-30db lower distortion.

Regards Hans
 
kevinkr

kevinkr

Administrator
Joined 2004
kta-hifi.net
Paid Member
I had a couple of admittedly odd thoughts about this design:

Call me cynical - but I have never seen 2000uF electrolytics made in this era, they would have been as big as a house.. :xeye:

Secondly what would you use to get 165V @ 2A in 1952? (A few or more 5U4 in parallel perhaps?) Silicon diodes were still a few years away.. Selenium or copper oxide rectifiers would have been huge to handle these currents and would be very lossy as well.

I have a little trouble believing anyone designed such an amp in 1952. Could this thing be a hoax or nothing more than a design proposal?

I have heard an amplifier based on this approach using a couple of 6AS7G in parallel - a paltry few watts of output power. Some people use the 6C33-c-b for this kind of application with cathode chokes.
I have not heard one with a ccs..
 
lineup

lineup

Member
Joined 2005
E88CC 6DJ8 6922 + 6AS7 6080 OTL headphone amplifier

.

Speaking of OTL SE
maybe this SE OTL Headphone Amplifier
might be of interest to someone

E88CC ( 6DJ8, 6922 ) + 6AS7 ( 6080 ) OTL output

More specific he uses Svetlana SV6AS7, as output follower.
Datasheet: http://www.tkhifi.com/datablade/6AS7_SVETLANA.pdf
But also he built a version with 6080.
See pictures.

An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.


Project page:
http://members.chello.se/illuwatar/Tube/6AS7 Headphone/headphone.html
Schematic, using E88CC + SV6AS7:
http://members.chello.se/illuwatar/Tube/6AS7 Headphone/images/amp_schematics.gif


lineup
 
lineup

lineup

Member
Joined 2005
kambule said:
wonder wot is the purpose of the heatsink behind the tube. Is it for the rectifier or helping the tube to cool down.
Click to expand...

Svetlana SV6AS7G ...
See datasheet: http://www.tkhifi.com/datablade/6AS7_SVETLANA.pdf
... is a high power and Big Dual Triode. E88CC looks very small beside it.

:cool: So this DUAL TRIODE tube can run at like 80-100 mA cathode current. :cool:
In this headphone amplifier he runs at like 50 mA.

And
Heater for SV6AS7 uses 6.3 Volt and 2.5 Ampere!
He uses a 5 A regulator IC for this heater:
For the heater, I used a LM323 - a 5A/5V regulator.
To get the required 6.3V for the heaters,
I used two diodes of type 1N4148
to raise the reference for the regulator with 1.3V.
Click to expand...

This regulator needs a heatsink. Because of this High Current
Before asking more, Here is the link again.
So you can read yourself, instead of I read the stuff and tell it to you ;)

Svetlana SV6AS7G, SE OTL Headphone Amplifier
Schematics for both amplifier and POWER SUPPLY is available.

lineup
 
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