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Old 8th August 2009, 02:57 AM   #1
gary h is offline gary h  United States
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Default 6sn7 alternative

Hi all,

I've been enjoying my Aikido line stage for about a year now. I'm using 4 6sn7s with a B+ of about 330 VDC.

The problem I am having is many of these tubes seem to become microphonic within a few months. I've tried the new Tung Sol and NOS Sylvania (Admiral, Philco.) So far I've gone through 4 that start to whine. I've placed my transformers on a 3/8" each of cork and rubber mat. This has helped slightly. I've also suspended the the PCB from the chassis using the same material as buffering pads.

The preamp is very sensitive, touching the tubes or tapping the chassis can be easily heard through the speakers.

At this point I am looking for alternative tubes. Given my relatively high plate voltage, octal pin scheme and 6.3V heaters, it seems that the options I have are the 6BL7 and 6BX7. I am willing to sacrifice gain, I rarely listen to my system past 10 o'clock but would rather not spend too much or totally sacrifice sound. (I presume the ECC33 is out.) The other octal twin triodes I've found have plate voltage limitations.

Any ideas? Bomb-proof tubes with low microphonics or another fix, perhaps restraining one or both of the stages?

Thanks,

gary
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Old 8th August 2009, 03:18 AM   #2
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Old 8th August 2009, 03:24 AM   #3
kevinkr is offline kevinkr  United States
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You might want to look at what you are doing with the filament supply, are you floating the filament supplies on a dc voltage or are they grounded?

I had some similar problems in circuits with 6SN7 and several 12a...7 types where the filament was significantly more negative than the cathode - the symptoms were singing, and severe microphonics.

I don't purport to understand the mechanism but it was pretty repeatable and was eliminated when I made sure that the filaments were elevated above the cathode voltage by about 20V or so.
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Old 8th August 2009, 03:24 AM   #4
gary h is offline gary h  United States
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I think this is a 9-pin tube with a max plate voltage of 250. I'm looking for a tube that will tolerate 330VDC.

g
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Old 8th August 2009, 03:28 AM   #5
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Quote:
Originally posted by gary h
I think this is a 9-pin tube with a max plate voltage of 250. I'm looking for a tube that will tolerate 330VDC.

g
Is it 330V that powers the Aikido, or 330V per tube? In such case your B+ should be 660V?
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Old 8th August 2009, 03:33 AM   #6
gary h is offline gary h  United States
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Hi Kevin,

I employed the floating heater supply that JB recommends in the instruction booklet; the DC reg. boards I am using for the heaters are referenced to B+/4 by way of a 0.1 uF cap and 100k resistor in parallel placed between B+ and ground.

What alternative do you recommend specifically, I'll try it.

gary
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Old 8th August 2009, 03:35 AM   #7
kevinkr is offline kevinkr  United States
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You're unlikely to find a good sounding sub for the 6SN7 IMO, take a look at my previous post as it hopefully (maybe not) might be relevant.

I am assuming that 330V is what is powering the Aikido, note that the voltage across each section is roughly half that, and further that imo the 6SN7 is not tolerant of voltages much in excess of 400V no matter how they are connected. (SRPP, WCF, cascode, etc.)

Edit: I would recommend something like 100V, however this may not be enough to solve the issue.
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Old 8th August 2009, 03:50 AM   #8
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6CG/FQ7 is almost electrically identical.

IMO, sounds as similar.

Cheers!
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Old 8th August 2009, 03:52 AM   #9
gary h is offline gary h  United States
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Wavebourn,

you are right it is 300 per tube. I was under the impression that because one triode sits on top of the other, both plates will get the 300V.

Kevin,

if I understand you correctly I should be using a different resistor value on the heater bias circuit to bring it down from 70V, which is about where it's at now. If I were to try a compromise, let's say 40V, what resistor values would I use instead of the 300k and 100k shown in the schematic?

g
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Old 8th August 2009, 04:04 AM   #10
gary h is offline gary h  United States
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Kevin,

I think you and Wavebourn are right, I misunderstand how the anodes work in this circuit. I've attached a schematic of my Aikido PS which includes the heater bias. The audio circuit is JBs Aikido using 4 6sn7s in series. How would you lower the bias closer to the cathode voltage?

thanks

gary
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