• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Tube vs. Jfet Preamp for Dynaco ST-35 tube power amp

rcollege

Member
2010-12-02 5:06 pm
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.
 

45

Member
2008-12-18 2:29 am
UK
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
 
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.
 

45

Member
2008-12-18 2:29 am
UK
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.
 
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rcollege

Member
2010-12-02 5:06 pm
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.
 

kevinkr

Administrator
Paid Member
:cop:
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.
 

rcollege

Member
2010-12-02 5:06 pm
:cop:


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.:eek:

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.
 
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john65b

Member
Paid Member
2005-01-09 2:32 am
Chicago
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...
 

45

Member
2008-12-18 2:29 am
UK
:cop:


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.

Actually there should be vacuum tube rectifiers which can work without transformer. There were made for TV's. I cannot remember the name now....
Am I wrong?
 
:cop: It's not a matter of the rectifier, it's a matter of basic safety. It is FAR too easy for any exposed bits of metal (knobs, connectors, cables) to carry a DEADLY voltage when no isolation is used. And it is just as likely to kill an innocent person as it is the idiot who "knew better" than everyone else.

Drop that line of inquiry immediately.
 

45

Member
2008-12-18 2:29 am
UK
:cop: It's not a matter of the rectifier, it's a matter of basic safety. It is FAR too easy for any exposed bits of metal (knobs, connectors, cables) to carry a DEADLY voltage when no isolation is used. And it is just as likely to kill an innocent person as it is the idiot who "knew better" than everyone else.

Drop that line of inquiry immediately.

OK I drop it, although I do not completely agree with you. It's dangerous anyway and you have to know what you are doing. Ignorance and slapdash attitude are not excuses for mistakes!
 
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rcollege

Member
2010-12-02 5:06 pm
Thank you for all your replies.

I built a few pre-amp circuits this weekend. These ran to the ST-35.


All had the same simple circuit with differences in Resistor and capacitor values.
Remember, I only need a gain of -1 so I arranged the circuit to give a gain of 2.

1- single Jfet per channel. J111 fet
2- single 6FQ7 running both channels
3- Single 7N7 running both channels.

Reference was the preamp stage of a Marantz 2226B

My Wife (who would rather stay with the Marantz) , My 8 yr. old son and myself all agree that the tube sound better...hands down...it didn't even take a full verse of the music.
So far the 7N7 sounds the best.

Now here's the catch every circuit I tried used 24VDC. I used a voltage splitting circuit consisting of 3 resistors to get the 24V down to 6.3V while letting 600ma draw from the power supply. Yes...it was hot- 2 60ohm supply resistors in parallel...one 1200ohm to ground.

I used a Switching power supply (heresy I know) and the 6FQ7 is dead quiet...the 7N7 is nearly dead quiet.

Tubes definitely came out on top.
Now for a real power supply...but will more supply voltage make a difference with this level of gain?
 
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Rewind

Member
2009-01-18 7:42 pm
Oslo
Rcollege, wonderful experiment. Just what I was looking for.

I was going through the same dilemma when picking mic preamp for my music studio. Of course everyone want a tube preamp because they think this will give them that nice 60's tube vibe. But to use tubes to their full potential everything should be tube. The preamp tubes will need a rectifier tube. The system you are listening to could have tube SET amps, using high sensitivity speakers, like horn speakers. And the recording medium should not be a digital sound interface with opamps, but directly to tape. That would mean great sound. Neil Young was experimenting with tape vs digital medium. Apparently the sound loses its soul when recorded to digital medium, even if being recorded on tape first. The sound regains the soul only when recorded back to tape. Weird.

If you pick a 12au7 mic preamp with a wall socket, like the Electroharmonix EH 12AY7, you will just be ******* in the wind, listening to dry and boring tube saturation that says "fail" all over it. This is the time-waster: Electro-Harmonix 12AY7 Tube Microphone Preamp | Musician's Friend I hope this is not what you are doing still, with your switching supplies, Rcollege! ;)

In the end I chose a non tube Neve replica mic amp from Golden Age audio. Golden Age Project Pre-73 Preamp MKII | Musician's Friend

It will be replaced with a real Neve, if I ever become really serious with audio recording. The Neve sound just screams professional quality because it will give me a similar sound as all the pros. And it is much easier to get it right. Whatever Beatles used at the peak of their career, was probably maintained by an army of BBC engineers. My sound interface, being very digital and full of opamps, seem to prefer the non tube neve mic preamp.

So should I check if I can use a Neve-like preamp for my tube amps in my hifi system? Maybe I should abandon the 12au7 and the 6SN7 tubes from the line level signal path all together? Naah, they do a great job and share something special with the rest of the power tubes and the rectifier tubes - probably at a quantum level that only human ears can detect.
 

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Rewind

Member
2009-01-18 7:42 pm
Oslo
I read your question about using too low voltage preamp tubes. I have made a guitar overdrive preamp using a 12au7 with a 24V wall socket. It sounds like this: Matsumin's Valvecaster - YouTube

Amazing for bass guitar. Hifi? Not so... I think you should stick to the specs of the tube if you don't want an dirty effects overdrive pedal.

The sound will always be a little harder on a non tube rectified, even if you use that method to soften up the bridge diode by putting several caps in between. I have several tube amps and I notice it a lot, and will need a tube rectifier on channels above 300Hz in my multiamped horn system.
 
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