A good sounding EL34 PP sch please - Page 6

woodturner-fran · 17th February 2012, 12:41 PM

He says that it is pretty high gain without feedback - did you notice that?

Good luck with the phonostage, I did the valve itch a while back, excellent sound and well worth it.

Fran

cjkpkg · 17th February 2012, 01:14 PM

yes gain was higher but from memory it wasnt that much...

woodturner-fran · 19th February 2012, 06:11 PM

Progress so far: heaters wired, ps in place (crclc), iron all mounted. Ready to start the circuit next.

woodturner-fran · 20th February 2012, 12:09 PM

Cjkpkg: did you use one ccs for each output pair (bias set to ~80-100ma) with a balance pot or a single ccs per tube?

Fran

cjkpkg · 20th February 2012, 02:54 PM

Yes, I used the LM317HV which is good to 57V - I think the cathodes come in around 40V which was pretty close to the limit for a standard LM317. One CCS per EL34 Pair with a balance pot. I think I had some 50ohm pots kicking around from another project.

89mA if I remember correctly...

woodturner-fran · 20th February 2012, 10:36 PM

Aaargh, I ordered up some LM317HV last night,..... added to the order just to bring it up to the minimum order level. What did I find in my parts box tonight? Grrrr....

Anyway, I've powered it up and it works. I've a big ole buzz going on though. I need to troubleshoot it a bit, but on first run all the voltages are there or thereabouts. Time to go tweak the grounding!!! Might need to shield the feedback and input wiring too.

Fran

cjkpkg · 20th February 2012, 11:04 PM

I will have to check out my grounding scheme...Mine is very quiet...

however IIRC...
IEC ground to star ground
PSU ground to star ground
PSU ground to amp PCB ground
amp PCB grounds tied together
RCA ground to PCB ground
Feedback ground to star ground

CT of EL34 heater xformer to B+/4 tap on PSU board which is also the H- for the DC heaters on the pre/driver tubes

woodturner-fran · 20th February 2012, 11:22 PM

Need to go over it again. I think though that a ct heater for the EL34s doesn't go to 1/4 B+ normally though? Doesn't it just go to ground? I know I made another booboo - I wired the AC EL34 heaters out of phase.... should have been in phase so that it would be cancelled in the output Tx.

I'm using a buss ground with the low current grounds at one end and the high current grounds at the other, this high current end is tied back to main reservoir cap ground. Worked really well before for me...

So there's a few things to sort...

Fran

woodturner-fran · 21st February 2012, 10:31 PM

The hum is largely under control. Turns out the EI transformer I was using for the 6SN7 heaters was the culprit. No matter how I tried to position it, the hum was there. I subbed in a 12V toroidal and the hum is much reduced. Probably don't have it to horn levels yet, but its very close to being right.

Gonna be a few days until I get back to it unfortunately!!!

Fran

woodturner-fran · 25th February 2012, 11:26 PM

Hi,

I have a new issue - one that I haven't come across before. I have quite a bit of hiss in one channel. I've built the amp as per the schematic with 2 changes: A pair of red LEDs on the cathode of the input 6SN7 (giving me 3.6V) and a simple LM317 ccs on the cathodes of the EL34s (set to 83mA for the pair). Voltages check out across the amp within a few volts of predicted.

Through trial and error, I have found out that the hiss completely disappears if I touch my DMM probe off the back of the LM317HV. Take the probe off, and gradually the hiss comes back up (over 5 secs say). The LM317s are just sitting in mid air inside the chassis, with some short wires from the valve socket to the 20R pot and then to the LM317. Both channels are done this way, but only 1 hisses. Both are fairchild LM317HV.

Any ideas?

Fran

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17th February 2012, 12:41 PM	#51
woodturner-fran diyAudio Member Join Date: Jan 2006 Location: Ireland	He says that it is pretty high gain without feedback - did you notice that? Good luck with the phonostage, I did the valve itch a while back, excellent sound and well worth it. Fran

17th February 2012, 01:14 PM	#52
cjkpkg diyAudio Member Join Date: Aug 2008 Location: Roswell GA	yes gain was higher but from memory it wasnt that much...

19th February 2012, 06:11 PM	#53
woodturner-fran diyAudio Member Join Date: Jan 2006 Location: Ireland	Progress so far: heaters wired, ps in place (crclc), iron all mounted. Ready to start the circuit next.

20th February 2012, 12:09 PM	#54
woodturner-fran diyAudio Member Join Date: Jan 2006 Location: Ireland	Cjkpkg: did you use one ccs for each output pair (bias set to ~80-100ma) with a balance pot or a single ccs per tube? Fran

20th February 2012, 02:54 PM	#55
cjkpkg diyAudio Member Join Date: Aug 2008 Location: Roswell GA	Yes, I used the LM317HV which is good to 57V - I think the cathodes come in around 40V which was pretty close to the limit for a standard LM317. One CCS per EL34 Pair with a balance pot. I think I had some 50ohm pots kicking around from another project. 89mA if I remember correctly...

20th February 2012, 10:36 PM	#56
woodturner-fran diyAudio Member Join Date: Jan 2006 Location: Ireland	Aaargh, I ordered up some LM317HV last night,..... added to the order just to bring it up to the minimum order level. What did I find in my parts box tonight? Grrrr.... Anyway, I've powered it up and it works. I've a big ole buzz going on though. I need to troubleshoot it a bit, but on first run all the voltages are there or thereabouts. Time to go tweak the grounding!!! Might need to shield the feedback and input wiring too. Fran

20th February 2012, 11:04 PM	#57
cjkpkg diyAudio Member Join Date: Aug 2008 Location: Roswell GA	I will have to check out my grounding scheme...Mine is very quiet... however IIRC... IEC ground to star ground PSU ground to star ground PSU ground to amp PCB ground amp PCB grounds tied together RCA ground to PCB ground Feedback ground to star ground CT of EL34 heater xformer to B+/4 tap on PSU board which is also the H- for the DC heaters on the pre/driver tubes

20th February 2012, 11:22 PM	#58
woodturner-fran diyAudio Member Join Date: Jan 2006 Location: Ireland	Need to go over it again. I think though that a ct heater for the EL34s doesn't go to 1/4 B+ normally though? Doesn't it just go to ground? I know I made another booboo - I wired the AC EL34 heaters out of phase.... should have been in phase so that it would be cancelled in the output Tx. I'm using a buss ground with the low current grounds at one end and the high current grounds at the other, this high current end is tied back to main reservoir cap ground. Worked really well before for me... So there's a few things to sort... Fran

21st February 2012, 10:31 PM	#59
woodturner-fran diyAudio Member Join Date: Jan 2006 Location: Ireland	The hum is largely under control. Turns out the EI transformer I was using for the 6SN7 heaters was the culprit. No matter how I tried to position it, the hum was there. I subbed in a 12V toroidal and the hum is much reduced. Probably don't have it to horn levels yet, but its very close to being right. Gonna be a few days until I get back to it unfortunately!!! Fran

25th February 2012, 11:26 PM	#60
woodturner-fran diyAudio Member Join Date: Jan 2006 Location: Ireland	Hi, I have a new issue - one that I haven't come across before. I have quite a bit of hiss in one channel. I've built the amp as per the schematic with 2 changes: A pair of red LEDs on the cathode of the input 6SN7 (giving me 3.6V) and a simple LM317 ccs on the cathodes of the EL34s (set to 83mA for the pair). Voltages check out across the amp within a few volts of predicted. Through trial and error, I have found out that the hiss completely disappears if I touch my DMM probe off the back of the LM317HV. Take the probe off, and gradually the hiss comes back up (over 5 secs say). The LM317s are just sitting in mid air inside the chassis, with some short wires from the valve socket to the 20R pot and then to the LM317. Both channels are done this way, but only 1 hisses. Both are fairchild LM317HV. Any ideas? Fran