So What now

duderduderini · 17th March 2008, 12:04 PM

Hi Guys
Ok I have spent the last 6 months building differing types of preamps.
I have 2 power amp options the 1st being an ultralinear pp 6550 amp and the second being a chinese built but modded (read made safe) 12at7(srpp) ---> 6ca7 ----> 300B set.
Of all the preamp configs i have tried the 6sn7 in srpp sounds most pleasing to me with the PP amp. With the SET amp it doesnt sound as good.
I have just built another type of preamp using a number 30 triode with an IXYS CCS but whilst it is more dynamic and detailed than the 6sn7, alas it is entirely fatiguing. This is true of it connected to both power amps. (i know the set amp is not strictly a power amp).
The 300b set on its own sounds pretty good but of course doesnt have a phono stage (which the preamp(s) I have built do)
I am thinking.. do I ditch the 12at7 srpp "line stage" section of the set amp and connect my preamp out into the 6ca7? or is it a feature of my psychoacoustics that I am relegated to a pp amp if i want a phono section. Obviously the pp amp has feedback which really throws me as I still maintain that the best sounding system i have is the set amp with some bookshelf speakers I built ( which are different to the ones in the living room and in a smaller different shaped room)) but If I move these speakers into the living room then It loses its magical sound due to room acoustics, I assume.
Any Ideas anyone?.
Thanks
Nick

kevinkr · 17th March 2008, 01:51 PM

Can't answer all of your questions, but I use a SET power amplifier (300B) with my 26 dht transformer coupled line stage and my d3a/5842 phono stage. I think you need to revisit the operating point you have chosen for the 30 triode. The input impedance of the amplifier you are driving should have an input impedance at least 10X greater than the rp of the 30 to avoid loading it down - given its rp I would use short low capacitance interconnects.

You need the gain from the 12AT7 in your SET amplifier unless the line stage you proposed to replace it with can swing sufficient voltage to drive the amplifier to full output. I suspect with a resistive load on the plate of the EL34 that you are talking in the order of 60Vpp so I believe you need to keep that stage. Many people here love the 12AT7A, I think it is fine in PP amplifiers, but in an SE amplifier I would be inclined to try something else - maybe the 5687 or 5842 - either of which would provide enough gain in this application and have much lower rp. (better hf performance and less thd at large signal swings.)

duderduderini · 18th March 2008, 10:37 AM

Hey there Kevin.
In my preamp i run the #30 at 2mA cathode resistor is 1.5k
b+ into the IXYS is 240V dc on pin 2
Pin 3 has a variable resistor on it.
I adjust that till i get 2mA across the cathose resistor
The b+ on the plate of thr triode is 100Vdc
Am i adjusting this IXYS in the wrong manner?
I have a good mind to use either a 280F or e180F to drive the 6ca7 on the set amp.
What operating point should i run on the #30?
I am running a 470k resistor to ground on the output of the #30.
Grid leak is the volume pot at 100k

kevinkr · 18th March 2008, 01:27 PM

Hey Nick,
First thing I would do is add a cathode bypass capacitor across that 1.5K resistor, or perhaps better still replace with a couple of LEDs, this should lower the rp back to the nominal value for fixed bias and result in better tolerance of external loads.

I'm not sure about the operating point - I'll think about that a bit more and post later.

Kevin

17th March 2008, 12:04 PM	#1
duderduderini diyAudio Member Join Date: Mar 2004 Location: Albury NSW Australia	So What now Hi Guys Ok I have spent the last 6 months building differing types of preamps. I have 2 power amp options the 1st being an ultralinear pp 6550 amp and the second being a chinese built but modded (read made safe) 12at7(srpp) ---> 6ca7 ----> 300B set. Of all the preamp configs i have tried the 6sn7 in srpp sounds most pleasing to me with the PP amp. With the SET amp it doesnt sound as good. I have just built another type of preamp using a number 30 triode with an IXYS CCS but whilst it is more dynamic and detailed than the 6sn7, alas it is entirely fatiguing. This is true of it connected to both power amps. (i know the set amp is not strictly a power amp). The 300b set on its own sounds pretty good but of course doesnt have a phono stage (which the preamp(s) I have built do) I am thinking.. do I ditch the 12at7 srpp "line stage" section of the set amp and connect my preamp out into the 6ca7? or is it a feature of my psychoacoustics that I am relegated to a pp amp if i want a phono section. Obviously the pp amp has feedback which really throws me as I still maintain that the best sounding system i have is the set amp with some bookshelf speakers I built ( which are different to the ones in the living room and in a smaller different shaped room)) but If I move these speakers into the living room then It loses its magical sound due to room acoustics, I assume. Any Ideas anyone?. Thanks Nick __________________ "Better to say nothing and keep them guessing than to speak and remove all doubt."

17th March 2008, 01:51 PM	#2
kevinkr diyAudio Moderator Join Date: Sep 2004 Location: Boston, Massachusetts Blog Entries: 6	Can't answer all of your questions, but I use a SET power amplifier (300B) with my 26 dht transformer coupled line stage and my d3a/5842 phono stage. I think you need to revisit the operating point you have chosen for the 30 triode. The input impedance of the amplifier you are driving should have an input impedance at least 10X greater than the rp of the 30 to avoid loading it down - given its rp I would use short low capacitance interconnects. You need the gain from the 12AT7 in your SET amplifier unless the line stage you proposed to replace it with can swing sufficient voltage to drive the amplifier to full output. I suspect with a resistive load on the plate of the EL34 that you are talking in the order of 60Vpp so I believe you need to keep that stage. Many people here love the 12AT7A, I think it is fine in PP amplifiers, but in an SE amplifier I would be inclined to try something else - maybe the 5687 or 5842 - either of which would provide enough gain in this application and have much lower rp. (better hf performance and less thd at large signal swings.) __________________ www.kta-hifi.net

18th March 2008, 01:27 PM	#4
kevinkr diyAudio Moderator Join Date: Sep 2004 Location: Boston, Massachusetts Blog Entries: 6	Hey Nick, First thing I would do is add a cathode bypass capacitor across that 1.5K resistor, or perhaps better still replace with a couple of LEDs, this should lower the rp back to the nominal value for fixed bias and result in better tolerance of external loads. I'm not sure about the operating point - I'll think about that a bit more and post later. Kevin __________________ www.kta-hifi.net

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18th March 2008, 10:37 AM	#3
duderduderini diyAudio Member Join Date: Mar 2004 Location: Albury NSW Australia	Hi Kevin Hey there Kevin. In my preamp i run the #30 at 2mA cathode resistor is 1.5k b+ into the IXYS is 240V dc on pin 2 Pin 3 has a variable resistor on it. I adjust that till i get 2mA across the cathose resistor The b+ on the plate of thr triode is 100Vdc Am i adjusting this IXYS in the wrong manner? I have a good mind to use either a 280F or e180F to drive the 6ca7 on the set amp. What operating point should i run on the #30? I am running a 470k resistor to ground on the output of the #30. Grid leak is the volume pot at 100k __________________ "Better to say nothing and keep them guessing than to speak and remove all doubt."

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