5W EL84 drive EL84 SE

[nesha]

OK, a newbee question for Tube master: I like your new schematics, but somthing about pentode mode - What about rising pentode response in high frequences range, since there is no NF of some sort...And what electolitics do you racomend? How this amp compares with your dirrectcoupled el84 design,soundwise? Thanks

[Shoog]

Rising response in a pentode driver stage shouldn't be an issue. This is because it is working into a purely resistive load (the RC network of the driven tube).
The reason you get a rising response in an ouput stage is because of the high rp of the pentode in relation to the falling reactance of the output transformer.

My understanding is that pentodes make excellent driver stages. I will be using pentode drivers in my next power amp.

Shoog

[nesha]

Thanks Shoog, my question was about output pentode, but answer lies in that 2K resistor before output primary,wright?

[Shoog]

The 2K resistor is simply there to prevent excessive screen current on signal peaks, it will have no effect on the rising response.
I personally think that the pentode mode of this implementation will exhibit a rising response. I would implement a plate to plate feedback arrangement to tame this, and this could easily be switchable in and out using a two gang switch for the pentode mode and feedback (search on RH84 if you are interested) If you were doing this I would make the driver EL84 pentode (possible switchable again).

Shoog

[nesha]

This beginers darkest fears come true! Kidding aside, what about this idea: use wide range (not fullrange!) driver like supravox 285-2000 (101 db,45-7KHz ) ? Pentode output amp would extand (hopefully) response to 8-9 KHz (allmost fullrange) , and then dial in super tweeter of 90db sensitivity? It seems to me it is possible to get reasonable flat response from loudspeaker, and keep SE pentode amp with 0 nfb. I know feedback is not that bad thing,and great sounding amps of this kind already exist, but I just want to try something different,I know this amp wouldn t work with other speakers, but there is pentode/triode switch for them... A?

[Ty_Bower]

Originally posted by Shoog: My understanding is that pentodes make excellent driver stages.

Interesting. Would you care to comment further? I was under the impression that people generally tended to criticize well-known amps such as the Dynaco Stereo 70 and the Mark III for their use of a pentode driver (7199, 6AN8). The theory was that a pentode first stage was used only in the interest of keeping the cost low. It's about the only way you can make a full front end (AF amp plus phase inverter) with a single tube and still get the required gain. People argue that nearly any other topology would be an improvement.

[Audio_idiot]

Hi Nasha,
I have the same thought about a good wide range + tweeter combo. but did not have the skill to design a suitable x-over...

Did consider an active x-over + bi-ampi for that. BUT that stayed as a plan still.

Have you any action plan for that?

Cheers

[Shoog]

Interesting. Would you care to comment further? I was under the impression that people generally tended to criticize well-known amps such as the Dynaco Stereo 70 and the Mark III for their use of a pentode driver (7199, 6AN8). The theory was that a pentode first stage was used only in the interest of keeping the cost low. It's about the only way you can make a full front end (AF amp plus phase inverter) with a single tube and still get the required gain. People argue that nearly any other topology would be an improvement.

Just because some designs have used them that way, and for that reason, doesn't make the statement generally applicable.
Try looking at this link for a respected designers opinion on the use of a pentode driver.

http://www.pmillett.com/813_se_triode_amps.htm

My intended application is in a lose clone of Gary Pimms Tabor amp. Here a 6AU6 is used with plate to plate feedback to the output pentode. The high output impedance of the driver is just what is needed in this application.

So take your choice, believe a general audiophile myth or take the word of two excellent electrical engineers. I know what I will choose to believe. Every device and every circuit has an applicable application in the hands of an intelligent designer.

Shoog

[nesha]

Hi Audio Idiot, I would keep it simple, like just cap on tweeter and fullrange without any crossover, or find an old equilizer and try it in front of powerful amp in biamped configuration.I consider open baffle with Audio Nirvana super 10 fullrange, and Eminence alfa 15 for bass. AN s 10 without any crossover (open baffle provides natural rollof - hi pass) and would be driven by Decware Zen or Tube Master design el84 or transconductance el84 . In my dream system 15" woofer would be powered by F1 ( by Nelson Pass) , a transconductance amp to inject more power in that rising impedance of the bass driver - kind of equalization by itself. I don t know if it is agood idea but Ill try...

[tubemaster]

Originally posted by nesha
OK, a newbee question for Tube master: I like your new schematics, but somthing about pentode mode - What about rising pentode response in high frequences range, since there is no NF of some sort...And what electolitics do you racomend? How this amp compares with your dirrectcoupled el84 design,soundwise? Thanks

Hi Nesha,

you are quite right. The behaviour of the circuit depends a bit on the output transformer. The result in pentode mode is not always linear on the scope but still can be very pleasing.

I don't recommend any specific electrolytics, use what feels good to you. It gives each amp an individual note since component choice can be heard in such amps. The coupling cap can be a nice type.

That's why it's hard to compare the amps soundwise, but they do have something in common...

Best regards
Michael