Which tubes are lowest noise?

[Geek]

Kevin or Geek --

How do you find an operating point and load line for a trioded pentode? Take the 6688 for example. thx.

Not familiar with that one, sorry

Cheers!

 

[moonbird]

Intersted in 6ej7 and 6eh7?? too

Thanks for responding Geek -- please explain how to set operating conditions for the 6ej7. Have you every used the 6eh7 in this purpose? thx.

 

[Geek]

Hi,

I "work backwards" to set my OP for the 6EJ7 in pentode. I begin with a 220K anode resistor and a 1K cathode resistor (bypassed), then chose a screen resistor to give me B+/2 on the anode (it will end up between 470K and 2.2Meg). Sound wise that way, it'll sound the same at any B+

Cathode resistor can be moved up to 3.3K before it sounds "objectionable" (thin), but I wouldn't go below 1K.

Here's a guitar amp input that used the "HiFi" selections for the input tube, because I wanted no OD from it:
25W head in the works

Tubes4e4 here has some triode data:
EF184 / 6EJ7 pentode triode strapped data

I did some 6EH7 work, but found the gain much lower and the microphonics of the ones I had terrible, so I never pursued it further.

Cheers!

 

[moonbird]

Wow thanks --

Now I remember looking at this schematic on your site - very cool. Many thanks. If I recall - you were considering making a "jazz" amp from this same topogy. Anything every come from that??

That 6ej7 triode curve chart is way cool. Guess they call you guys gurus for a reason - sheesh thx.

 

[Wavebourn]

I have some 11SJ7, but I don't use them because they are out of current production.

 

[Geek]

Hi,

Now I remember looking at this schematic on your site - very cool. Many thanks. If I recall - you were considering making a "jazz" amp from this same topogy. Anything every come from that??

R&D funds got tossed into HiFi.

It's basically that amp with the OD stage pulled. Reverb an option

That 6ej7 triode curve chart is way cool. Guess they call you guys gurus for a reason - sheesh thx.

Ah good, we have you fooled then

Cheers!

 

[moonbird]

All coming back now

Oh yeah and this amp has that way cool parametric eq too -- If I write you again on your site about it -- will you respond this time???. That strange OT you had in that schematic turned my lil brain in knots -- see yah on tone geeks --

 

[Geek]

LOL!

I just replied. It's not in my Firefox "speed dial", so I always forget to go there

Cheers!

 

[Garyepe]

Do you know the specs of the MIC?

Which dual triodes, Octal or 9 pin, are considered lowest noise and microphonics? I know that noise and microphonics varies with individual tubes of the same type, so my question is about what's typical to various types. The tubes are to be used in MC phono stage, MM phono stage and pre-amp.

Do you have the specs of the MIC? if you find your audio output have noise I think you have to check the sensitivity of the ECM , to check the sensitivity of the ECM is very higher and the ECM has inner capacitors or not.

Hope this can help you

Gary

 

[kevinkr]

Kevin or Geek --

How do you find an operating point and load line for a trioded pentode? Take the 6688 for example. thx.

It is actually quite similar to the way that you would make the determination for an SE output stage. You need to determine the maximum dissipation and mark the characteristic curve with the dissipation limit, then you make a choice of operating point based on the available supply voltages and currents that do not violate the dissipation limits. I usually choose a load line that allows me to run the tube at currents were the transconductance is high. (In the case of resistive loads I usually somewhat arbitrarily make them 4 - 5X the rp at the quiescent point if supply voltage permits.) I haven't talked about the differences between resistive and choke based loads, and a CCS load is still another case. I'd need to think about it before posting further comments as I tend to fudge a bit on this design aspect usually - however RD-4 (download from Pete Millett's site) or Morgan Jone's Valve Amplifiers 3rd edition should fill in the gaps.