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Old 7th August 2008, 07:44 PM   #51
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You can run grid stop resistors that are as little as 600 ohms with no worries. One watt is a good value, as a shorted tube can fry a 1/2W device.

Cathode resistors are another concern. The higher the mu the lower the value can be, but 5W devices are recommended. An arc-over can take out a 3W device easy enough.

6AS7Gs are probably the most economical tube you can use. They hold up well, have good linearity and can handle an surprising amount of grid current, although the schematics shown thus far won't support the grid current operation.

Even in a lower-powered amp its a good idea to keep the B+ up. You can't just go for more current- the tube will saturate. 140V is ideal when dealing with the tubes mentioned so far. The right voltages are often hard to find in off-the-shelf transformers, so bucking the primary with a smaller transformer is one way to get to where you want. Of course the B+ transformer should have enough current reserve to operate easily without saturation at the higher voltage.
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Old 7th August 2008, 09:24 PM   #52
Fuling is offline Fuling  Sweden
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All 16 33k 0,6W gridstoppers are already soldered in position and Im not planning to replace them unless I really have to.
33k should be high enough to protect the tubes from both oscillations and grid meltdowns and still low enough not to cause an early HF rollof.

A few more volts on the plates would be nice, buying new transformers and PSU caps would not be that nice...
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Old 8th August 2008, 04:41 AM   #53
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More voltage will not make things better. It is still lack of current and high Zout that is the problem for an OTL.

Example: 1pr 6AS7 at max 13W with 140V/90mA gives more ClassB power but less than half ClassA power and double Zout. If you multiple the tubes the realtionship will remain.

Quantity or quality?
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Old 8th August 2008, 08:43 AM   #54
Fuling is offline Fuling  Sweden
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Quote:
Quantity or quality?
Both, thanks!
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Old 8th August 2008, 05:59 PM   #55
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I refered to Class-A quality and Class-B quantity. So in this case I think the only option is more parallelled tubes!

Still waiting for an interesting thread at hififorum, not so many nowadays!
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Old 8th August 2008, 06:21 PM   #56
Fuling is offline Fuling  Sweden
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Just kidding, quality wins over quantity every time.
The expected 3-4W is perfectly normal in my world, everything above 8W is considered "high power"

Quote:
Still waiting for an interesting thread at hififorum, not so many nowadays!
I agree. I started a thread about my experimental OBs today, but its really not that interesting. (Though it might become very interesting if I get those four 15" drivers that Guru mentions in the thread...)
Im working tonight, if nothing unexpected happens I might have time to start a new thread about circlotron over there.
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Old 8th August 2008, 06:36 PM   #57
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Quote:
Originally posted by Fuling
J
The expected 3-4W is perfectly normal in my world, everything above 8W is considered "high power"
HAHAHA In my world, anything less then 200watts is low power!!!
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Old 8th August 2008, 06:50 PM   #58
Fuling is offline Fuling  Sweden
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Quote:
anything less then 200watts is low power!!!
Welcome to the tube forum

Below 1W is low power, though Ive built a few (even good sounding) amps in that power range.
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Old 12th November 2008, 06:47 PM   #59
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Hi Roky !
Did you have any success with the amp ? A smoke column ?

Gaston
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Old 12th November 2008, 10:07 PM   #60
Fuling is offline Fuling  Sweden
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Hi!

I actually spended a couple of hours on these monoblocks a couple of days ago hardwiring some resistors in the input and driver stages.
I moved to a 110 years old house in August so there hasnt been much time...
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