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Old 3rd August 2014, 10:19 PM   #1
engels is offline engels  Israel
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Question Interstage transformer ratio, spacecharge tubes in PP

I'm fiddling with PP design with two 12K5 spacecharge tubes (those with low anode voltage, max 30V). As the voltages are relatively low and the iron is light I wanted to try an interstage transformer instead of a tube PI.

I've got some cheapo tiny 600:600CT lying around so I used one.

My OT is another cheapo-tiny 1.2KCT to 8ohms.

Everything works but fairly quiet, way more quiet than a SE 12K5 amp. Quiet, but despite the low output there's lots of distortion, sounds a lot like another spacecharge amplifiers with SE out maxed out. I've checked the circuit with a different OT, which is relatively big and has approximately the same ratio, and there was almost no change in the output.

Which leads me to the interstage transformer, which is probably the problem. Unfortunately I don't have any other suitable transformers to try in that spot, so instead of buying some for testing I'm here to ask the wise men. I happen to have no experience with transformer PI's , so sorry if the question is beyond stupid.

I'll post the schematics later, but tell me first, is it possible to use 600:600CT for PI? The output tubes work in grid bias mode, which is the way spacecharge tubes prefer. The transformer's secondary is center-grounded and the signals go through caps to the grids of output tubes, each with 1M ground reference. 12K5's cathodes are grounded.

My intuition tells me transformer's 300 ohms to the ground is way too low to put before a tube with 1M ground reference, so I need at least 2M CT secondary and probably something like 500K primary. Is that correct?
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Old 4th August 2014, 04:46 AM   #2
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You are confusing the matching ratio with winding resistance, transformers do not have an impedance as such, but they have an impedance ratio. So a 600:600CT transformer does not mean you have two 300 Ohms secondaries.
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Old 4th August 2014, 04:52 AM   #3
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try using a 70v 10w audio matching transformer to the speaker. I got mine for $5
the tiny 1.2k to 8 ohm may be so small as to attenuate everything below 250 hz.
worse yet the core may be well into saturation causing distortion.
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Old 4th August 2014, 10:50 AM   #4
engels is offline engels  Israel
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2 jazbo: you're right, my mistake. Actual measured resistance of the transformer is a bit lower, around 295Ohms per half. Which still acts like a 300 Ohm reference to the ground.

2stocktrader: that's what I did. I have a couple with multiple primaries and tested the circuit with it. It's quite an overkill for spacecharge purposes, but it worked fine. And the "tiny" one I've used originally puts out approximately the same volume and distortion, without overheating. That's why I'm saying it looks like the problem is in the PI.
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Old 4th August 2014, 11:17 AM   #5
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Quote:
Originally Posted by engels View Post
Actual measured resistance of the transformer is a bit lower, around 295Ohms per half. Which still acts like a 300 Ohm reference to the ground.
You are still confusing DC resistance with AC impedance... please post the schematic, may be we can spot something...
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Old 4th August 2014, 11:41 AM   #6
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A "600 ohm to 600 ohm CT" transformer was intended for operation in a low impedance circuit, probably solid state or telephone coupling. Attempting to use it in a high impedance tube circuit will result in poor performance even if the ratios are right because it doesn't have enough primary inductance.

Quote:
transformers do not have an impedance as such, but they have an impedance ratio.
The ratio is the main parameter, but you usually can't get away with using a transformer much beyond a 2 to 1 change from its intended use. A 600 ohm to 600 ohm transformer will probably work OK as a 300 ohm to 300 ohm transformer, but probably not as a 60 ohm to 60 ohm due to excess winding capacitance, nor as a 6000 ohm to 6000 ohm transformer due to lack of inductance (not enough turns of wire). You are asking it for quite a bit more.

There are exceptions of course. A higher quality transformer may be pushed a bit farther because it probably has more inductance and lower capacitance to begin with, and these guidelines don't apply if the frequency range of operation is changed or restricted.

You 600 ohm transformer probably looks like a dead short to the driver tube. You will probably find that the performance improves as the frequency is increased unless it was a telephone(or modem) coupling transformer, they aren't meant to do much above 3 KHz.
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Old 4th August 2014, 08:33 PM   #7
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Thank you for the answers,
I think tubelab more or less nails the problem, it sounds almost like a dead short.

The question then is - what transformer ratio may work in this application? I will not buy some Altec for this amp, it seems right to find something like Xicon -
Click the image to open in full size.

They have this 42L series, I may try to order something like 2K:10K to see if it works better. Maybe ordering several transformers and testing them may be the best solution - I may order several with different ratios for some $10-15.

http://www.mouser.com/ds/2/449/XC-600127-202003.pdf

This is not going to be a hi-fi amp, that's for sure. Awful distorted guitar amp with cheap parts, this is probably the correct description. By the way, spacecharge tubes sound awesome with guitars, but they're way too quiet. Even too quiet to put a tone stack in the chain. So this is an attempt to get some volume out of spacecharge tubes.
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Old 4th August 2014, 09:37 PM   #8
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Quote:
Originally Posted by engels View Post
I think tubelab more or less nails the problem, it sounds almost like a dead short.
The key words you left out are "to the driver tube", without knowing which tube you are using, it's hard to know which type of IST is suitable for the application. But IST with so called low resistance windings ("a dead short") are used regularly, such as the Lundahl 1660.
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Old 5th August 2014, 03:43 PM   #9
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Default scheme

Here's the schematics of the damn thing.

Click the image to open in full size.

The preamp tubes are wide variety of 7-pin triodes, s.a. 12AJ6, 12FM6, 12BF6, 12AT6 etc. They all work similar, with little differences in gain and frequency response.
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Old 5th August 2014, 03:48 PM   #10
engels is offline engels  Israel
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Quote:
Originally Posted by jazbo8 View Post
The key words you left out are "to the driver tube", without knowing which tube you are using, it's hard to know which type of IST is suitable for the application. But IST with so called low resistance windings ("a dead short") are used regularly, such as the Lundahl 1660.
You're correct as always, the tubes I'm using are 12AJ6 for preamp and 12K5 for output.
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