IT (interstage Transformer) Driver

[50AE]

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Grazie for this information. The wire I intend to use is 0.2mm net diameter + 0.027um insulation of modified polyesterimide + polyamide imide, this info claimed by the manufacturer. So, the overall distance between two wires will be 0.054um of this kind of insulation, assuming perfect conditions and no insulation flaws, such as pinholes.

But the best thing to do would be, to wind a transformer and test it for dielectric breakdown.

 

[mashaffer]

IME it's not a good idea to attempt a general solution. The interstage transformer can work really well but best practice is usually to design the circuit around it. So first decide what you want to do and fix the requirements for the IT and the best driver for the actual IT you are going to use. An IT that is excellent for one application can easily be the contrary for another one.

The 6LU8 is a tube I like a lot but its triode is not really great for IT coupling because its plate resistance is too high. For example, you might use the two triodes of the LU8's for the input stage/phase inverter and then pick a separate driver for pentodes. This way you could drive the transformer in PP-to-PP which already makes life a lot easier. ITs that are also splitters are generally those with poorer high frequency performance.

I hadn't thought of the P-P to P-P option. No DC current makes the tranny easier just like in OPTs. That might even make line transformers like Edcor and Cinemag make a possibility would it not? A LTP or buffered Cathodyne could use an ungapped tranny.

 

[45]

Grazie for this information. The wire I intend to use is 0.2mm net diameter + 0.027um insulation of modified polyesterimide + polyamide imide, this info claimed by the manufacturer. So, the overall distance between two wires will be 0.054um of this kind of insulation, assuming perfect conditions and no insulation flaws, such as pinholes.

But the best thing to do would be, to wind a transformer and test it for dielectric breakdown.

Yes, real stress test is the definitive answer. Those few times I have made a transformer for 500V or more anode voltage I have always have it tested.

By the way the standard double insulation magnet wire I use with 0.2 mm net copper diameter is 0.245 mm diameter in total. So it's 45 microns separation between two adjacent wires. That's not suitable for high DC voltage differential. Unless the insulating material can make such a huge difference I don't thins 9 additional microns in separation can make a big difference.

A LTP or buffered Cathodyne could use an ungapped tranny.

Yes. Then need to check what's the best source impedance for that specific transformer (LTP is more flexible in this respect), if it has enough inductance to avoid low frequency distortion and it has enough headroom for driving the power stage. Despite PP-to-PP drive it should be able to handle at least 1-2 mA DC current (both for some little unbalance of the driver and little grid current of the power tubes). This is usually no a problem for Si-Steel EI and C cores. It might be a problem for more exotic materials....More DC current handling if you want to go into positive grid drive. I am currently playing with the LL1635/5mA. Check this, it's cheaper than the 1692a.

 

[mashaffer]

Data sheets can be confusing. Take this one



Is the primary inductance for each phase or for the two in series or the two in parallel? So if we used this PP on a LTP how much inductance will each driver see (I presume pins 4 and 11 are connected to B+ and 1 and 8 to the driver plates)?

Since the DCR of the primary specifies each can we assume that the secondary DCR which doesn't specify is for both in series thus each phase has 145 ohms DRC?

 

[THD+N]

I did some testing on a Hammond 126B. Despite Hammonds reputation, the results were excellent. I am driving a 211 with the circuit and it is great.

Look up THD+N and search for the post.

Nick

 

[mashaffer]

Thnks Nick.

 

[45]

Primary inductance always means total value with all primaries in series (unless specified) for both PP and SE (with right phases of course).

The version you show is for 30 mA DC current. For PP operation you can ask (free of charge) a different air-gap for lower DC current or PP version (which allows about 2-3 mA DC typically) so that you get a lot more inductance.

 

[andyjevans]

I did some testing on a Hammond 126B. Despite Hammonds reputation, the results were excellent. I am driving a 211 with the circuit and it is great. Nick

The 126 series are good sounding bifilar wound interstages. But the real trick here is to use the 126C at 15mA, and to use two in series just as plate chokes with a teflon output cap. This is a bulky option needing space, but it's very good sounding indeed. You get 212H inductance and the sound is slightly better than a Lundahl LL1660/18mA used just as a plate choke. This suits a driver operating at 15mA and the best I've found after many years building with DHTs is the 10Y. Since that's expensive the second best option, which is only a tiny touch less good sounding but hugely cheaper is the Russian 2P29L wired as a triode. The 4P1L is happier at 20mA and over (more like 30mA) so that means the 126B or two of those in series as plate chokes, but even then the 2x126C with the 2P29L is better. All these would smoke a 6EM7. For 2 stages use a 01A with a Bartola Valves Gyrator into the 2P29L.