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Old 23rd April 2012, 01:46 AM   #1
Fenris is offline Fenris  United States
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Default New amp: 13DE7 SE hybrid

The design for this amp started as a scrapbox amp. A lot of ideas and sources came together in its design. The primary factor was actually two torroid transformers that didn't have a use. They were 120:14v with single primaries and dual secondaries, which are for the most part useless for tubes. I used RightMark audio analyzer to do a frequency response and they were totally linear through the audio band, at least at the low power my soundcard put out. As a comparison, Hammond 125 transformers showed noticeable roll-off at high and low frequencies, so I'm reasonably confident that theoretically these are OK.

Next up, how to design the output stage. The reflected impedance with an 8 ohm load would be about 480 ohms, too low for most any tube, at least the ones I could afford. So I decided a MOSFET would be put into service as a follower with a decoupling capacitor to keep DC out of the transformer. The follower topology keeps Miller capacitance low. I found cheap W7NB80 high voltage MOSFETS I found for $1 each with heatsink and thermal pad at Goldmine Electronics. I also found 100 1.8uF 400v capacitors for $10 as well. I decided on a B+ of about 200v to stay within the transformer's spec. Honestly, I could have gone with twice the B+ but I didn't want to have to use more expensive higher voltage parts. I used a CCS on the source set to 120ma. The blocking cap is made up of 14 1.8uF caps to give me about 25uF for a cutoff frequency of about 10hz.

For the gain stage(s) I needed a gain of about 100 to get full swing (realistically there's about 10 volts at each end that are lost due to the MOSFET and CCS). I found the neat little 13DE7 (a 6DE7 or 10DE7 could be used with different heater voltages) that has two dissimilar triodes in it. The first is lower power triode with a gain of about 17 and the second is a higher power triode with gain of 6. I used a CCS on both with LED bias, one cheap red LED on the first gain stage and seven in series on the second. This gives about 1.9v and 13v on the biases. The two stages are capacitor coupled, with the MOSFET directly coupled with the anode of the second stage. A linear 100K pot with a 10K bypass gives a pseudo-log volume control. All together, very simple and straight-forward single ended design.

The PSU is my old standby. I had a dozen PCBs made up and use them with all my amps. It's a simple MOSFET follower with a zener string used as a voltage reference. I used cheap photoflash capacitors and the same W7NB80 MOSFET. The PSU also gives me a 50v tap for heater lift.

The whole thing was done point to point with co-ax for the input signal wiring and a tight twist on the heater AC wires. It fired up first try with no problems. It puts out probably about 5-8 watts, about the same as the T-amp it will replace.
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Old 23rd April 2012, 04:55 PM   #2
Fuling is offline Fuling  Sweden
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Splendid!
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Old 24th April 2012, 01:39 AM   #3
Junm is offline Junm  Philippines
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do you have the complete schematics including the ccs you use?... i want to try this one since i have some those parts lying around...
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Old 24th April 2012, 02:57 AM   #4
Fenris is offline Fenris  United States
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The 120ma CCS is the standard LM317 and 10M45s cascode (Fig. 13C) used by Walt Jung in his two part articles:

http://waltjung.org/PDFs/Sources_101_P1.pdf
http://waltjung.org/PDFs/Sources_101_P2.pdf

The 3ma and 4ma are the same thing except with a LM334 in place of the LM317, except with an appropriately changed set resistor and the pins swapped (since the pinout is 180 degrees from the 317). You could use any high voltage CCS, even a simple 10M45s with two resistors. I just had three dozen CCS PCBs made up along with a dozen power supply PCBs, so it's very easy for me to build one up quickly.

The power supply toroid is an Antek 100VA 175v model. Inexpensive and high quality.

Also, I left out a resistor in the schematic. An additional 300K resistor needs to be tied from the junction of the 0.1uF cap and the 300 ohm resistor to ground in order to reference the grid of the second stage to ground.
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Old 24th April 2012, 03:25 AM   #5
Junm is offline Junm  Philippines
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Thanks for the PDF and i have my idea on what to do....by the way how about using a triode-pentode tube? say 6bm8 or russian 6f3p or 6f5p...will it be possible to drive the mosfet with a pentode?
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Old 24th April 2012, 03:03 PM   #6
Fenris is offline Fenris  United States
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The pentode would need to be triode strapped or else you would need some global feedback to linearize the pentode. The reason I chose the tube I did was because the first stage with a gain of 17 would take the approximately .7v input signal and amplify it to about 12 volts. The second stage has a bias of 13 volts from the LED string, so it can handle up to 13 volts of input and the gain of 6 will drive it to +/- 78v maximum. I also chose the tube because the tube would bias easily at 2v with a LED.

I'm not sure what the characteristics of the pentode stage will be if they are strapped into triode operation. The first stage mu of those tubes is pretty high, about 70. You might need to drop some amplification with voltage divider, or use global feedback.

The MOSFET can easily be driven by a tube at audio frequencies. If you're worried about gate capacitance, use a MOSFET like the IRF820 that has lower capacitance and use a lower value of gate stopper resistor, say 1K instead of 4.7K (I used 4.7K because I have a large collection of old-school carbon resistors from 40 years ago, quite useful as grid stoppers).
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Old 24th April 2012, 03:54 PM   #7
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Like it - hows it sound ?

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Old 27th April 2012, 08:02 PM   #8
Fenris is offline Fenris  United States
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I haven't had a chance to do any critical listening, too much life is interfering with my hobbies. Hopefully in the next day or two I'll get a chance to attach it to some real speakers.
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Old 27th April 2012, 11:15 PM   #9
AJT is offline AJT  Philippines
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hi,

did you set the 120mA current sink at the mosfet output at half the B+?

you got your divider resistors here mislabled:
Click the image to open in full size.
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Old 28th April 2012, 01:06 PM   #10
Fenris is offline Fenris  United States
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Nope, divider resistors are not mislabled. They are not used for the CCS, they are used as a voltage lift for the heaters, about 50v above ground. The CCS doesn't have a voltage reference - it doesn't need one since it only cares about current. The MOSFET gets its DC voltage from the anode of the second tube stage, which based on the datasheet should be about half B+ with 4ma and -14v on the grid (it's about 90 volts in real life).
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