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IT (interstage Transformer) Driver
IT (interstage Transformer) Driver
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Old 20th July 2018, 02:05 PM   #1
mashaffer is offline mashaffer  United States
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Default IT (interstage Transformer) Driver

An idea for an IT driver. I have been briefly looking at the problem of adequately driving an interstage transformer which would ease the requirements on the trannie's construction. Initially I was looking at driving it in the cathode of a follower stage but that did not look promising.

The power triode in the 6EM7 has very low Rp and the tube is not horribly expensive or hard to find. By using the included small signal triode as a VAS to feed the power triode we can add a bit of FB to further reduce the driving impedance.

Of course this doesn't allow the eliminating of all coupling caps and plate resistors but it would (if used to drive the output tube(s)) allow eliminating them at a particularly advantageous position.

Maybe???
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File Type: png 6EM7_IT_Driver.png (97.1 KB, 221 views)
File Type: png 6EM7_IT_DriverResp.png (27.3 KB, 220 views)
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Old 20th July 2018, 02:34 PM   #2
45 is offline 45  Italy
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Depends on what you want to achieve. For low frequency high performance you might need at least 40H @50mA DC to ensure low distortion already at 20H and very low from 30Hz and above.
On the other end you need a transformer that suits your 750R source impedance without peaking....

This should be nice:
Sowter type 9525
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Old 21st July 2018, 02:29 AM   #3
PRR is offline PRR  United States
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With FB connected, what are R1 R2 doing for you?

In general, the fat side of the EM bottle is a BIG DC current which severely affects your choice of transformer.
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Old 21st July 2018, 04:17 AM   #4
mashaffer is offline mashaffer  United States
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Oops. R1 and R2 and the associated cap are leftovers from earlier circuit that I forgot to delete. :}

The idea of the power triode was to take advantage of the low Rp. FB was to get a bit more bang for the bottle. As drawn it pulls about 20mA. I could try biasing a bit cooler and see what the resulting higher Rp does. Could also try other lower current tube types but it is hard to find 1k or lower Rp under 20mA.

More investigation in order. I appreciate the input very much.
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Old 21st July 2018, 04:32 AM   #5
PRR is offline PRR  United States
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> hard to find 1k or lower Rp under 20mA

Plot it. I get near 1,250 Ohms rp for 6EM7 at 150V and 1K bias.
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File Type: gif 6EM7-150V-1k.gif (43.4 KB, 155 views)
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Old 21st July 2018, 04:35 AM   #6
TonyTecson is offline TonyTecson  Philippines
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IT (interstage Transformer) Driver
with 1.25k rp, does that mean a primary impedance of 3.3k or more?
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Old 21st July 2018, 05:07 AM   #7
mashaffer is offline mashaffer  United States
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Putting in the specs for the humble Hammond 124A using the 6N1P at only a few mA this topology looks pretty good.
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File Type: png 6N1P_IT_Driver.png (82.4 KB, 161 views)
File Type: png 6N1P_IT_Driver_out.png (29.4 KB, 158 views)
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Old 21st July 2018, 05:11 AM   #8
mashaffer is offline mashaffer  United States
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Quote:
Originally Posted by PRR View Post
> hard to find 1k or lower Rp under 20mA

Plot it. I get near 1,250 Ohms rp for 6EM7 at 150V and 1K bias.
You are right I did not take into account the amount of Rp variation with bias point.
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Old 21st July 2018, 12:09 PM   #9
45 is offline 45  Italy
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Quote:
Originally Posted by mashaffer View Post
Putting in the specs for the humble Hammond 124A using the 6N1P at only a few mA this topology looks pretty good.
Bad idea, IMHO. The 124A is not a good interstage transformer. It's usable at the first stage and already is quite compromised. 110 mH of leakage inductance cannot really be considered HiFi....good for telephony, guitar amps or similar applications. It cannot take DC, is 1:3 step-up and has really poor high frequency response. The effective capacitance might become just too much for the driver, especially if running at few mA. The miller capacitance only as seen by the driver will be 9 times higher in comparison to DC coupling, LC coupling, 1:1 IT or RC coupling. Just a guideline, assuming 5-7K plate resistance for a triode like the 6N1P running at 5-8 mA (the lowe current the hight the plate resistance of course) then f3 will be around 15-20KHz, -1 dB at 7.5-10 KHz and here very likely will already have A LOT of distortion when swinging several volts..... There is no shortcut. If you want to use IT coupling you need to invest some money. At least a transformer doesn't wear off and its resale value will be as good as its quality.

Last edited by 45; 21st July 2018 at 12:29 PM.
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Old 21st July 2018, 05:03 PM   #10
mashaffer is offline mashaffer  United States
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Any of the 124 series acceptable?
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