OTL by Tim Mellow

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Mike H
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#31

Post by Mike H »

Ditto!
 
"No matter how fast light travels it finds that the darkness has always got there first, and is waiting for it."
palme30
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#32 Re: OTL by Tim Mellow

Post by palme30 »

Hello David,

I am looking forward to repeat your otl version with 6c41. Could you please specify resistor and caps values in your attachments. What power trany exactly do you use, where it can be obtained?

Thanks

Arvydas

Hi All,
I know we've looked at this one before, but I think it was on an 'un-related' page (?) Just that I'm looking to build one and thought a simplified sketch of the circuit might be usefull (?) If someone with the necassary CGI skills could do it properly.......... using the original for part no's and values etc.

Hope it attaches ok,

ATB,

David.[/quote]
ACPEN
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#33

Post by ACPEN »

Hello,
All component values are the same as for the original 6C33C version. Notice that although using the same base, the 6C33 & 6C41 use different pin connections. The transformer is a 240v 500w 'Bench isolation type' as used in servicing workshops when working on live equipment. (When radios's & t.v's etc were worth repairing!) If you can find my write up on building the 6C41 version (this site or DIY audio under Tim Mellor threads)(I think!) I mentioned the need to uprate one of the psu caps (voltage wise) and my method of wiring the heater transformer to the secondary of the main isolation transformer. You can contact me via email if you get stuck. Otherwise, Good Luck with the build! You won't regret it! I've tried before to encourage someone on this site to build one!

Regards,

David.
Firebottle
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#34 Mellow OTL

Post by Firebottle »

Hi David,
I am building one of these OTLs but using different valves.
However I have found there is a flaw to the configuration of the driver stage when you turn the wick up. It's fine at lower volume but the driver stage is dynamically unbalanced at higher levels.
The bottom driver anode load (R13 100K) connection is the culprit. A great idea to give feedback for the bottom output valve but it causes distortion on negative going peaks.

The driver stage is a balanced LTP with a fixed supply voltage on the top driver anode load R12 but the 'equivalent' supply voltage for the bottom valve is anything but fixed, as it is connected to the output.
On the negative half of the signal, as the bottom driver anode is going more positive to cause the output to go negative, the net voltage change across R13 is double what is needed to keep the driver stage in balance. Due to the action of the LTP configuration the top driver is cut off more, giving a double whammy with distorted peaks.

I have overcome this by connecting the top of R13 to ground and utilising the usual bootsrap technique from output to (now tapped) R12.
The tapping point will be dependent on the mu of the output valves used.

:occasion5: Alan
Firebottle
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#35 OTL complete

Post by Firebottle »

As per the title the OTL is now complete and working.

I've used alternative valves to the original circuit and made a couple of modifications to improve the performance.
It started out as a mono test bed on an old piece of copper clad board to prove the circuit and optimise the operating conditions:

Image

I also wanted to find how much power I could achieve with different numbers of output valves. One gives 10W, two gives 26W and four gives 60W :shock: , all rms into 8 ohms.
Here is the stereo version on test on the workbench:

Image

There were a couple of hiccups, too low an ht on the driver stage due to me forgetting that the current draw would be double for the stereo version :?
Also there was a hum but only on one channel. Turned out that channel was oscillating big time, a couple of grid stoppers on the input stage cured that.
Here's the general view with the safety cage on:

Image

Rear view showing the bias test points above the speaker outlets:

Image

I rather like the enclosure but it is on the compact side. The side of the cage gets rather hot when in use :!:
Here is the internal view:

Image

Some more testing and listening to do to burn in the valves a little, here is the OuTLaw in action:

Image

I will be doing comparisons to my low power OTL in the next few weeks which should be interesting.
:occasion5: Alan
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#36

Post by Cressy Snr »

Great work. :D

I can't build OTLs so I'm always interested when somebody gets one up and running.
Sgt. Baker started talkin’ with a Bullhorn in his hand.
Firebottle
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#37

Post by Firebottle »

Thanks Steve. I can equate to your signature, I get a great buzz from valve circuit design.

With OTLs in particular it is always a tense time, even though you may have triple checked everything, when you switch on for the first time and hope you have the bias set so the output valves are off :confused2:

Here is the low power jobbie using 6080s I built a while back:

Image

:occasion5: Alan
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Dave the bass
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#38

Post by Dave the bass »

Its a caged beast!

Like it :-)

DTB
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steve s
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#39

Post by steve s »

SteveTheShadow wrote:Great work. :D

I can't build OTLs so I'm always interested when somebody gets one up and running.
You may not be able to build them steve but what you made sounded very good...
The tube manual is quite like a telephone book. The number of it perfect. It is useful to make it possible to speak with a girl. But we can't see her beautiful face from the telephone number
dearangelo
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#40 Re:

Post by dearangelo »

ACPEN wrote: Sat Jan 08, 2011 10:57 am Well, Nearly a month has gone by - and it's still working! It seems that around a weeks normal use was needed to settle in the 6C41C's - after that, there was no bias drift (can be a problem with OTL's) and the balance has remained virtually stable. So, so far so good! I decided to give it a 'bench test' of it's freq. response. It performed well, being flat between 20Hz - 50kHz using squarewaves (and up to 100kHz sinewave) Of course OTL's do perform exceptionally well in this respect, as there is no nasty transformer in the signal path! Think of what you get with an SE Amp. up to the driver stage output - thats what you get at the speakers - no droopy L.F or rounded H.F waveforms! That said, I,m more inclined to let 'listening' be the arbiter these days rather than test results.
I hope to hear some thoughts on this amp. when I manage to encourage someone else to build one! (I think Philip is wavering? - maybe in time for Februarys Audio jumble?)

ATB,

David.
Hello,
I know this is a very old post but since I am in a search for realizing Mellow's design, for my first OTL amp, I would like to hear how much lower in output would it be using 6c41s instead of 6c33, as you did ten years ago.
I hope this reply finds the target!
regards
Angelo
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Ray P
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#41 Re: Re:

Post by Ray P »

dearangelo wrote: Wed Nov 25, 2020 5:48 pm Hello,
I know this is a very old post but since I am in a search for realizing Mellow's design, for my first OTL amp, I would like to hear how much lower in output would it be using 6c41s instead of 6c33, as you did ten years ago.
I hope this reply finds the target!
regards
Angelo
You'll get about 6W using 6C41s. Dave posted about his builds on DIY Audio under the username 'nafunga' if you would care to search - this post might be useful as a starting point.

https://www.diyaudio.com/forums/tubes-v ... ost2407347
Sorry, I couldn't resist!
dearangelo
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#42 Re: Re:

Post by dearangelo »

Ray P wrote: Wed Nov 25, 2020 6:28 pm
dearangelo wrote: Wed Nov 25, 2020 5:48 pm Hello,
I know this is a very old post but since I am in a search for realizing Mellow's design, for my first OTL amp, I would like to hear how much lower in output would it be using 6c41s instead of 6c33, as you did ten years ago.
I hope this reply finds the target!
regards
Angelo
You'll get about 6W using 6C41s. Dave posted about his builds on DIY Audio under the username 'nafunga' if you would care to search - this post might be useful as a starting point.

https://www.diyaudio.com/forums/tubes-v ... ost2407347
Thank yoy Ray!
I 'll get there
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