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Old 8th July 2009, 09:01 PM   #11
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Take a look at Bruce Rozenblit's web sit . www.transcendentsound.com
He sells very good sounding OTL kits. Much better way for some with little design experiance. He is in his second generation of the designs.
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Old 9th July 2009, 12:04 AM   #12
Brit01 is offline Brit01  United Kingdom
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Quote:
For this reason, unless you have cheap speakers that can be easily replaced, I would consider some form of fail-safe protection circuitry. Fusing the output at minimum. Or perhaps monitoring the current of each output tube (across a small resistor) with a comparator IC driving a transistor stage(s) to activate (or deactivate) a relay in series with the speaker output. Just food for thought.

Certainly good advice. Relays would be excellent, my Carver power amp has these as it is a bit of a beast pumping out 380 watts per channel at 8 ohms. These work very well.

Also fast blow fuses.

But if I was to build an OTL it would be a low powered unit to drive some home made line arrays for mid/upper range. Cones in the price range of 3-5 USD

I could use my SS Carver to drive the low frequencies from a valve preamp (it sounds magical with my current Aikido linestage) and the OTL to drive the line arrays.
I like this idea very much.

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Old 9th July 2009, 01:04 AM   #13
Brit01 is offline Brit01  United Kingdom
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When the data states that the Anode dissipation (on each triode), is 4 Watts, is this referring to the output power? Or is it the grid dissipation (0.3W)?

Looking at the 6N6P (which I happen to have 10 and sound great) have a very low resistance of 68 ohms.

Could a number of these be used in a small OTL design?
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Old 9th July 2009, 01:31 AM   #14
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Quote:
Originally posted by Brit01
When the data states that the Anode dissipation (on each triode), is 4 Watts, is this referring to the output power? Or is it the grid dissipation (0.3W)?
Anode dissipation refers to the DC input power supplied. Plate voltage x plate current = watts dissipated by the plate. Anode means plate, not grid. Sorry, I have no experience with Russian or Chinese tubes and don't know that number without looking it up.
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Old 9th July 2009, 01:54 AM   #15
Brit01 is offline Brit01  United Kingdom
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Thanks for the clarification.

Another good explanation:

Cathode voltage (cathode pin to ground) divided by cathode resistor value in ohms multiplied by HT voltage (anode pin to ground) minus cathode voltage.

A recent amp I just built with 6N6P has a anode voltage of 220V, cathode to ground voltage of 4V and cathode resistor of 200 ohms.

4/200* (200-4) = 3.92 watts.

This is quite a good number for a little triode with very low plate resistance.
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Old 9th July 2009, 09:21 AM   #16
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The 6N6P tube has a Rp of 1.8kohm, not 68 ohm, 68 ohm is the cathode resistor value for typical operation, see here http://www.mif.pg.gda.pl/homepages/f...113/6/6N6P.pdf

2 requirements that must be met by tubes in an OTL are low enough output impedance and to be able to handle high peak currents.

the output impedance for the inverted Futterman that is shown in the schematic has an output impedance which is Rp/(2(1+)) so with 6N6P we would get 1800/(2*(1+22)) which is 39 ohm, if we connect a few in parallel we could reduce this to a reasonable value that could be lowered even further by feedback.

The 2nd requirement is more difficult to fulfill, 6N6P have a max allowed anode current of 45mA and if we don't want to exceed that in operation it would give us very low output power.

For a push-pull OTL that works in class AB the average current of each tube is the peak current divided by PI so for 45mA average we would get a peak current of 141 mA, this would give an output power of ~80mW in 8 ohm. Doubling the number of output tubes would increase the utput power by 4 times and so on...

6N6P is therefore not a good choice for an OTL, 6080 variants are OK, 6C33C is better as with this tube you don't need more than 2 to get useable output power, (25W in 8 ohm).

Parallel connection of output tubes can give a lot of problems and should IMHO be avoided if possible, therefore the 6C33C is probably the best alternative for an OTL today
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Old 9th July 2009, 12:27 PM   #17
Brit01 is offline Brit01  United Kingdom
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tubetvr

Great info thks. Ah yes I had this data but wasn't sure if it was 68 or 1.8K. I understood Rp was the symbol for plate resistance.

There is not much choice of tubes with a low enough Rp and sufficient power output.

I'll have a look for some 6C33C.
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Old 9th July 2009, 07:48 PM   #18
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I do believe the 68 ohms is the subjested cathode resistor
and the plate resistance is 1.8K . By the way the circuite
you posted is not the Futterman it is the tecknics sp? variation
of the futterman. In the Futterman the top plate resistor
of the phase inveter goes to the top output tube and the
cathode resistor in the phase inverter goes to the botom
output tube. And as posted that circuite will have a verry
high output impedience.
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Old 10th July 2009, 05:30 AM   #19
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Quote:
By the way the circuite you posted is not the Futterman it is the tecknics sp? variation of the futterman
Yes as I wrote it is an inverted Futterman described in the schematic.

In the standard Futterman both tubes work as if they where cathode grounded and the output impedance is Rp/2

In the inverted version both tubes as if they where cathode followers and the output impedance and is Rp/(2*(1+))

Quote:
And as posted that circuit will have a very high output impedance.
No, this circuit gives lowest possible output impedance of any Futterman variation, however I would change the value of the cathode resistors that should be in the order of few ohms.
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Old 10th July 2009, 03:14 PM   #20
coresta is offline coresta  France
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You , as i did, can read here and there many horrible stories about dead loudspeakers by an OTLamp ... I have built my first one 22 years ago , i rebuild it 10 years ago , i built 2 monster (10 tubes each) monoblocks 2 years ago and so on .... only with 6080 :
- 24 hours preageing for NOS ones
- testing and metering NOS and used ones to get pairs
- never going higher than 140 volts B+ and B- rail
- securing the bias rails with dual grid leak and pot resistors

I have never met any "BUMP" or "CRACK", POP from my beloved Altec 515 8G
Pierre

PICS of the monsters are on the forum ...
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