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Old 11th June 2013, 08:30 PM   #1
milosus is offline milosus  Serbia
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Default OTL 6AS7

hello,
I have one question, I decide to build this amp
http://www.solaris.no/electronics/po...terman_OTL.gif
Question is, is it ok to put on output capacitor 4700mkF like speaker protection (degradation in sound from this capacitor is not so important -i will put good one, more important is to save speaker)
And what is bias voltage?

Thank you very much
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Old 12th June 2013, 11:44 AM   #2
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4700mkF is not a value, so I'm going to assume it's 4700mF you are referring to.

For this to work, you first of all need a bi-polar cap. Foil caps are bipolar but not available in the ranges you would need. Back to back electrolytics are an option, but not suited IMO for series DC protection. There are better options available for speaker DC protection. Just search this forum for a wealth of info.

Bias is the negative potential the grid is at relative to the cathode. The grid only works in the negative voltage range, so the offset is necessary to let it amplify the whole input signal (class A). It also controls the amount of quiescent current (when no actual signal is applied) flowing through the tube. In the given push-pull configuration (class B or AB), it too sets the point where the upper tube 'takes over' from the lower and vice versa.
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Old 12th June 2013, 03:58 PM   #3
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Quote:
Originally Posted by milosus View Post
hello,
I have one question, I decide to build this amp
http://www.solaris.no/electronics/po...terman_OTL.gif
Question is, is it ok to put on output capacitor 4700mkF like speaker protection (degradation in sound from this capacitor is not so important -i will put good one, more important is to save speaker)
And what is bias voltage?

Thank you very much
No, and never / my advice/....You will lost amazing sound and amplifier will sound for a level low. You can use in output fast fuss about 0.5 - 1 A / You can try using 2/3 from full output power/. I,m using OTL and OCL 6AS7 that sound better my SEs 300B and 2A3.
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Old 12th June 2013, 04:25 PM   #4
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It's not really a Futterman design at all.

It's closer to Onder.

The danger in an OTL is a shorted tube placing full voltage at the outputs. Imo, you can use a cap there, but ought to be sure to as you said use a "good one".

The other alternative is to put a good crowbar type DC offset protection circuit on the output. That might consist of a Triac or pair of SCRs (or a "solid state relay") that upon detection of more than a few volts of offset would be triggered to SHORT the output to ground or short the rails (after the fuse). You would then rely upon fuses to blow in the rails or on each tube.

However, fuses are somewhat non-linear elements, and that may be a factor.

The other thing that would be required is to put in a servo type bias circuit that will try to keep the output centered on zero volts DC.

The addition of a comparator that "looks" at the bias voltage to see if the bias is sliding "too far" and if it does, trips a relay at the secondary or primary of the B+/- supplies would provide safety in the event that ur tubes start to go bad.

Individual pots for each tubes "sub-bias" setting would be a good idea too... I'm not sure that the bias ought to go back through the -60v rail or have the bias pot as part of the cathode circuit of the input. Maybe this is a trick to keep hum down or something like that?

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Last edited by bear; 12th June 2013 at 04:28 PM.
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Old 12th June 2013, 05:52 PM   #5
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Sensitive speakers can be stressed at start up even with an output cap. Shorting the output to ground until the amp is fully stable is a good idea.

Shoog
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Old 12th June 2013, 07:23 PM   #6
milosus is offline milosus  Serbia
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Ok, I got it, Thank you for your replays
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