• 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.

6N30P for high voltage

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Hi again,

I am stuck trying to choose between tubes driving my DC coupled OTL Stax Headphone amp. After evaluating various topologies over the past few years :(, the configuration will be p-channel cascoded input stage driving a stage with a parallell configuration. I am wondering which tube to use.

My hope was to use the 6N30P at about 10mA per side with 5 tubes in parallell, but I realize the voltages are a little high. Another alternative I have is to go with 6S4.

Any comments on tube choice? Stage shown is balanced long tailed pair with output taken at zero volts.

Petter
 

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Thanks Reihard

Since the datasheet does not cover high voltage, I am uncertain about whether to go for it or not. I probably will. Perhaps I will even do the full monkey (use each half of an H30 as cascode for a 2SK170 JFET) ...

Single stage JFET input OTL stage.

Do you have schematics for the EL34 Stax unit?

Petter
 
Petter,

I also have a STAX phone amp scheduled as a future project. Mine is at a very early stage of 'blue-skying' ideas at present.

The data I have indicates max DC plate voltage is 250V, with a 400Vh-k limit. But why would you want to use 5 6H30's in parallel? Especially at 10mA, where they're not even that linear or good sounding.

You don't need that much drive current and about 350Vp-p to produce very high levels. John Broskie of Tubecad uses 5965's as output in his, and he's no fool. Why not try them or a 6BX7, ECC99, 12B4A, trioded 7189A or if you must EL34?

The STAX schematic with 6CG7/6FQ7 can be found <a href="http://www.bonavolta.ch/hobby/en/audio/hdgsta.htm">here</a>

If you haven't already seen the Tubecad articles, they're well worth a read. Can be found in the 99 archives. There are also a couple of Kevin Gilmore designs at Headwize.

Cheers
Brett
 
Drive current

Thanks, Brett!

I am familiar with the sources you mentioned except the link to the Stax amp. I used to own a SRM-T1, and it was awesome, except that it was quite obvious that the tubes were working overtime.

I should have gone to a direct drive OTL 300B from the start, but now that the cabinet is built (with teflon sockets and all), it is kind of hard to go back for this version.

As far as output current goes, I have calculated that I need about 7.5mA current phase to phase. This means that I probably should have significantly more! So parallell small signal tubes (9 pin sockets) seems to be the order of the day.

So this is what I will likely do:

Option 1: Use a 12AX7 or more likely 6N30P as a folded cascode in stage 1, driving parallell 6S4 outputs (2 6S4 units per phase). This is DC coupled and if I use a 6N30P for the input, it can be run at say 30mA per phase in a balanced LTP configuration. Input stage voltage gain 30 times per phase, output stage with low gain (5-10 times per phase) and thus low output impedance.

Option 2: Use a p-JFET monkey input stage with cascode driving the same output

Option 3: Use a n-JFET monkey into the 6S4's and take the output at +300V mean.

I originally planned to do as per attached document (now somewhat out of date but still pretty cool as an X-Stax Pass type unit). (Sorry 100K limit causes this 350K zipped 8 page word design document to be put elsewhere, I don't know where yet)

Petter
 
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