6EM7/13Em7 SE amp

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6EM7/13Em7 SE amp

Postby burnedfingers » Sat Dec 05, 2015 5:50 am

I will post a rough schematic of the 6Em7/13EM7 stage of the amp followed by the linestage and power supply.
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Re: 6EM7/13Em7 SE amp

Postby burnedfingers » Sat Dec 05, 2015 5:56 am

Power supply
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Re: 6EM7/13Em7 SE amp

Postby burnedfingers » Sat Dec 05, 2015 5:59 am

switched heater
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Re: 6EM7/13Em7 SE amp

Postby burnedfingers » Sat Dec 05, 2015 6:06 am

I'll probably do an SRPP in 6sn7/12sn7 or in 12SL7/6SL7
Last edited by burnedfingers on Sat Dec 05, 2015 7:18 am, edited 1 time in total.
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Re: 6EM7/13Em7 SE amp

Postby burnedfingers » Sat Dec 05, 2015 6:13 am

They claim about 2.5 watts per channel but upon testing my friends I came up with 1 watt on the onset of clipping. He had the EM7 kit from Roger's site and had purchased it used. It needs work as someone went into it and it has non-matching plate resistor values as well as non-matching cathode resistors. If anyone has the correct values I would appreciate a PM so that I can fix my friends amp. Unfortunately after several emails to Roger has hasn't answered them and this is from a guy that claims to answer his email.

Since I have two non-used Hammond output transformers It is my hope to use them instead of purchasing the Edcor units that most people use when putting this little amp together.

I would appreciate any advice and or comment pro or con on what I have posted.
Thanks
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Re: 6EM7/13Em7 SE amp

Postby burnedfingers » Sat Dec 05, 2015 6:19 am

I have pairs of these transformers if someone thinks I should employ them in the design. Hammond 157G 30H @ 40mA
Edcor XSM 10K/600
EDCOR WSM 10K/600
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Re: 6EM7/13Em7 SE amp

Postby Geek » Sat Dec 05, 2015 12:44 pm

-= Gregg =-
Fine wine comes in glass bottles, not plastic sacks. Therefore the finer electrons are also found in glass bottles.
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Re: 6EM7/13Em7 SE amp

Postby burnedfingers » Sat Dec 05, 2015 5:38 pm

I'd like to try your amp but I don't have the tubes. I'm not well versed in CCS's can I get by with a pair of the small transistors or do I need a MJE350 in Q2's spot?

Looks like you have some feedback in yours. Should I do some with the 6EM7/13Em7?
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Re: 6EM7/13Em7 SE amp

Postby Geek » Sat Dec 05, 2015 10:50 pm

I actually developed it with the *EM7 :))

The CCS won't work with those values with the *EM7, as it's like a section of 12AU7 rather than 12AX7, like the *FD7.

If you have Circuit Maker Student, you can get ballpark CCS values with the 12AU7 model.

Cheers!
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Re: 6EM7/13Em7 SE amp

Postby burnedfingers » Sun Dec 06, 2015 6:16 am

Actually its probably closer to the 6Sl7 than the 12au7.

Running windows 10 and tried to download their free program and it doesn't work.

I would think that in obtaining the correct R1, R2, and R3 values for the CCS would be simple as one should be able to compute the current draw and go from there but then again I'm guessing. I have some old diyaudio boards I got from SY years ago that share the same circuit that you use on your board.

I'm assuming that in order to use the CCS in the 6EM7 stage similar to the 6SL7 one would take the rail voltage and consider the current for the LED string. Given for example a B+ of say 330VDC one would simply take 330/5mA and come up with a value of 66K. Power dissipated is about P=(330V)(5mA)=1.65W so a 3 watt resistor should be used.

I would think the emitter resistors can be calculated with a 1.7 volt reference and a .07 volt drop. I would assume that with approximately 1 volt across them and a current of 2mA then R=1V/2mA would equal about 500R. So a combination of a R2 of around 450ohms and a trim pot of 100 ohms should make this workable.

Am I calculating this correctly? It is my assumption that a CCS would work in the 6SL7 similar section of the 13EM7/6EM7 tube.

If I am missing something please to correct me.
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Re: 6EM7/13Em7 SE amp

Postby burnedfingers » Sun Dec 06, 2015 11:02 am

Big transformer one from the cheap iron thread gives me 234VAC. Took this to a SS bridge and then a damper diode and a 47/450 cap and arrived at 323VDC
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Re: 6EM7/13Em7 SE amp

Postby Geek » Sun Dec 06, 2015 2:04 pm

Here's what I'd do then:

1) Hookup a 500R trimmer in the emitter of the CCS.
2) Turn until proper voltage point on the anode of the stage is obtained.
3) Measure the trimmer and substitute a fixed resistor.

Enjoy the music [:)
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Re: 6EM7/13Em7 SE amp

Postby burnedfingers » Tue Dec 08, 2015 4:43 am

I thought you meant it wouldn't work at all. So changing the R1 and R2 values would be the ticket? So, the formula as listed in the 2006 diyaudio constant current source board info is incorrect?
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Re: 6EM7/13Em7 SE amp

Postby Geek » Tue Dec 08, 2015 12:28 pm

Yeah, within this or that... transistor forward voltages vary from lot to lot, multiplied by the cascode gain can make it a pretty large error, but will get you in the ballpark.
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Re: 6EM7/13Em7 SE amp

Postby Gingertube » Tue Dec 08, 2015 9:41 pm

The high mu section of the 6EM7 is VERY 6SL7'ish with respect to Mu, gm and biasing. Run it at about 1.5 to 2 mA which will require a cathode voltage of approx. 2.5V.

For "stability" of the current in current sources or or current mirrors, it is recommended that you run 450mV minimum across the current sense resistor in the emitter of the control transistor. This minimises the effect of changes (with temperature in particular) of the transistor itself. (one of those SS designers rulz of thumb)
So using a typical Green LED as the reference (Vf typically 1.8V) in the top transistor of the CCS (Geeks Circuit) would mean a "smidge" over 1V across the current sense resistor - plenty. Single 560 Ohm resistor is what I would go for to set just less than 2mA. The triode will require approx. 1K5 cathode bias resistor. Leave the 220R and increase the 820 to 1K2 or 1K5
Cheers,
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