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In Reply to: RE: Are my ITs ok for this schematic? posted by DAK on August 21, 2014 at 10:35:44
When you say the transformers measure 1.5K/1.5K, do you mean you used an impedance bridge or something similar to make a measurement? Or is this the Electraprint rating? In either case, you can't hang 1.5K (or a transformer designed for that Z) on a 6SL7.
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Buy Chinese. Bury freedom.
Follow Ups:
Do you know if the IT is gapped or ungapped?
I guess the good news is that if the 6SL7 ends up sucking for you (which is very likely will, 130H in the 1:1 configuration seems low, albeit I haven't modeled the circuit and the local feedback will help), you can run the interstage in parallel feed to kick up the inductance, or try a 6SN7 instead.
This, of course, assumes that you have the Lundahl. Is there anything written on the transformers you have?
I can find no info regarding the specs of my particular ITs. How does one determine the suitability of the IT for a circuit or do you work at it from the other way, i.e., you draw up the circuit and then work out the suitability of the IT?? In the case of this 6SL7 - KT88 circuit, how does one determine the optimal values for the IT?
Perhaps I will get a pair custom made that would more optimally match the circuit than the "off the shelf" Lundahl 1660. I really enjoy the 6SL7 tube type, especially, the milspec 5691, 6188, and 6su7. I think those are excellent sounding tubes. cheers, Dak
A 1:1 interstage transformer will have a reflected impedance impressed on the primary that is the resistance (or impedance) connected to it's secondary but that's not the end of the story.Even if the transformer is stable without a "load" on it's secondary (most transformers have resonance problems if not loaded) there needs to be enough inductance to "support" the rated impedance of the primary.
(Transformers don't really have impedance's. They just have winding ratios, inductance and winding capacitance. Transformers have impedance's that they are meant to work with.
I believe the impedance ratings on transformers tell us about the inductance of the primary and the impedance ratio between primary and secondary.)
80Hy has a reactance of 10k ohms at 20Hz.A 6sl7, with it's plate in parallel, has a plate resistance of about 22k ohms.
To keep harmonic distortion low a triode needs to see a load impedance of at least 3 times it's own plate resistance.
A 5687 has a plate resistance of about 2.5k ohms.
I believe the transformer you have described should work if driven by a 5687.
For a 6sl7 to be "happy" with the load, I believe the primary inductance of the IT should be at least 400Hy.
I don't think a 6sl7 is well suited for driving IT's.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 08/21/14 08/21/14
Well, is there a way to measure the inductance of the ones I have? They are big suckers. 3" x 3 1/2" x 3 3/4". Since my LCR gives me such variable readings I don't even know if its close. Thank you for the info regarding the IT requirements as I welcome anything on this since there seems to be scant info available.
If I cannot use a 6SL7 input I would welcome the 6sn7 as another preference. I do have a schematic for that but it isn't an IT coupled, but the usual cap coupled. Thank you so much for your input. cheers, Dak
What Tre said! They're not the easiest thing to test, especially considering the DC issue. Whichever way you go on the transformers, 6SN7s will definitely be easier to work with. If you need more gain, maybe a cascade circuit with the 'SN7 would work for you. No idea how it sounds though. My advice is to take your time with this and get all your ducks in a row before making a decision. Assuming you can't identify the transformers you have, you might even want to attach one to a 'SN7 and see how it performs.
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Buy Chinese. Bury freedom.
Take pictures and send them to Jack at Electra Print.
He'll know his own stuff.
Once you know what they are then you can build something suitable with them.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
TK, that is the DCR measurement. I checked the Lundahl 1660 specs and I believe they are saying that the dcr is about 700 ohms on the primary and slightly less on the secondary. I am not familiar with ITs and wanted to hear what an amp with an IT sounds like. So, I guess that if I want to use this circuit I will need to get a pair of 1660s??
The Lundahls are specified for 25 Hz to 40 KHz if gapped for 10 mA and driven by a 14K impedance. Since two sections of 6SL7 in parallel will be 22K, the low end (and likely high end) will be reduced. Not as bad as I would have anticipated, but 6SN7 would be a better choice.
Are your transformers gapped? If not, they may be only useful for push-pull or parafeed use. I could probably measure one with DC bias and see what they'll do... I have access to the necessary instrument, though it's currently out-of-house for calibration.
My ITs are air gapped. Using 6sn7 in the front end would be okay but would they work with my ITs? I have 5687, 7119 tubes and could go that way but would prefer to stay with octal tubes.
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