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Old 14th February 2013, 08:24 AM   #1
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Default Tube vs. Jfet Preamp for Dynaco ST-35 tube power amp

Hi all,

I have a Dynaco ST-35 17wpc tube amp. A Marantz 2226b is acting as the preamp. It uses el84's for its output stage. 1V of input max's out the amp at 17.8wpc...and everything I have puts out close to 2V rms.

I did a very simple 1 JFET per channel per circuit that opened the sound up and added clarity.
I did a single mosfet design as well as an opamp...I also did a passive preamp...that was ok.
The active pre amps sounds better and fuller with this unit...IMHO

I bought a 6fq7 tube and tube holder to do a low 24V tube preamp...but it seems that high voltage is preferred...so I may be barking up the wrong tree.

My question is...will a low gain tube pre amp sound significantly different from a Jfet class A circuit? Is it wise to run tubes at low voltages? How low voltage is recommended?
I can get some 7n7's for under $7 each...I heard good reviews...though I also read they are more microphonic.

Too bad I can't plug in a cord from the wall to a rectifier...I have 2 huge chokes and good size capacitors...if were only that simple

Any suggestions would be appreciated.
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Old 14th February 2013, 08:49 AM   #2
45 is offline 45  Italy
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I prefer solid state preamps, both phono and line.
If you want to run tubes at low voltage you should at least use those that work well like ECC86 or ECC88. Among the others there also are tubes which were not born for such application but will work fine at 24V plate voltage like the 6V6!
Within the 6SN7 family there is the 12SX7 which was particularly made/selected to work with batteries at 25-30V plate voltage (which means the supply has to higher anyway).

45
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Old 14th February 2013, 08:58 AM   #3
SY is offline SY  United States
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Quote:
Originally Posted by rcollege View Post
I bought a 6fq7 tube and tube holder to do a low 24V tube preamp...but it seems that high voltage is preferred...so I may be barking up the wrong tree.

My question is...will a low gain tube pre amp sound significantly different from a Jfet class A circuit? Is it wise to run tubes at low voltages? How low voltage is recommended?
It depends on your goal. If you want the preamp to have a "sound" (i.e., be an effects box), then this is one way to do it. If you want your preamp to merely make a small signal bigger without otherwise significantly affecting it, then this is not a good approach.
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Old 14th February 2013, 09:50 AM   #4
45 is offline 45  Italy
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I forgot to mention another good device for this application: the 6H30. This is expensive but can work well because it is quite linear even at low voltage/low current. It could be run at 24V/3mA (about -1.5V bias). This would leave about 1V for input swing before grid current starts. At this low level the valve still has about 5 mA/V mutual conductance and less than 3K plate resistance. The supply voltage has be to higher than 24V of course (unless one uses a really good OPT with just few volts drop). For a simple common-cathode amp with anode resistor from 27K to 33K, the supply has to be 107V (24.5 anode voltage +1.5 bias +81 drop across Ra) to 125V. Gain will be around 21-22 dB.

Last edited by 45; 14th February 2013 at 10:01 AM.
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Old 14th February 2013, 10:32 AM   #5
Loren42 is offline Loren42  United States
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Quote:
Originally Posted by rcollege View Post
Hi all,

My question is...will a low gain tube pre amp sound significantly different from a Jfet class A circuit?
Well, any poorly designed and executed preamp or amp will add coloration (distortion) to the signal chain.

Was there a particular type of distortion you were looking for?
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Old 14th February 2013, 03:27 PM   #6
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Thank you for your replies.

As far as what type of distortion...a softer sound- rolling off the highs a tad.

It is hard to say...as my Marantz has THD of .1% but the ST-35 has a THD of 1%...and sounds clearer and cleaner. So as the purist in me says not to have distortion...the music lover realizes that the type of distortion is more important than THD.
This is why I was looking into tubes for the pre amp. I have heard good things about the SE solid state though.

I'm not sure any of this is helpful...I'm hoping it is.
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Old 14th February 2013, 03:29 PM   #7
SY is offline SY  United States
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You can get plenty of distortion either way.

The other thing to consider is a tone control. I know, it's not fashionable, but if you want to soften the sound, it's a way to go. My favorite are "tilt" types- the old Quad and Crown preamps did it that way and it was remarkably efficacious.
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Old 14th February 2013, 04:37 PM   #8
kevinkr is offline kevinkr  United States
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Quote:
Too bad I can't plug in a cord from the wall to a rectifier...I have 2 huge chokes and good size capacitors...if were only that simple.
I hope you actually understand just how dangerous this is, and also that such remarks may be construed as a violation of forum rules.

For your own sake please review the safety thread, the forum rules, and should you not understand the danger inherent in this do not hesitate to ask for an explanation here.
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Old 14th February 2013, 05:08 PM   #9
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Quote:
Originally Posted by kevinkr View Post



I hope you actually understand just how dangerous this is, and also that such remarks may be construed as a violation of forum rules.

For your own sake please review the safety thread, the forum rules, and should you not understand the danger inherent in this do not hesitate to ask for an explanation here.
My most extreme apologies.

If this thread needs to be deleted due to safety, I completely understand and take no offense.
Again my apologies.

Or if you think it would be better, you could explain how dangerous almost unlimited amperage with high voltage can be.

Last edited by rcollege; 14th February 2013 at 05:20 PM.
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Old 14th February 2013, 05:41 PM   #10
john65b is offline john65b  United States
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I prefer a tubed front end on a SS amp (as I need the power on my ESLs)

I was a bit unimpressed with a 6H30 for preamp duty, and preferred the 6922.

I have a simple JFET BOZ that I cobbled together with a 100k Stepped attenuator. It was great (very quiet, but a bit too much gain). Then I built a 6SN7 / 6SN7 Aikido which was (to me) much better than the JFET BOZ on my Krell KSA-50 Clone. Then I tried a 6922 / 12BH7 Aikido and was even happier (I think the 6H30 would be an excellent replacement for the 12BH7 on the back end of this Aikido).

I know some would disagree, but that's just me.

Incidentally, I have the 6SN7 / 6SN7 Aikido populated PCB board and tubes for sale on the swap meet...
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