Is feedback required for UL EL84PP?

leadbelly · 20th January 2006, 12:49 AM

I'm still debating whether to build my EL84 amp as UL or triode or with a switch for both. One consideration, do I need a negative feedback circuit with UL connection? The majority of vintage UL EL84 schematics I have seen do include a negative feedback circuit from the OPT secondary to the input stage, but then I came across this schematic which made me question this:
http://www.electra-print.com/images/PSA_EL84_reva.pdf
Comments? Suggestions?

Kent Smith · 20th January 2006, 03:00 AM

I recently built my first UL amp (after building a few triode amps) using 6AQ5's. I did not want to use global negative feedback if I could avoid it, but just hooking the screen grids to the UL taps was not enough. UL is a form of local negative feedback, so I tried inserting some series resistance to control it. It worked! I ended up with 300 Ohms in between the taps and screen grids, and it sounds great. I would try with no resistance first, then add resistors in steps until it sounds clean.

My circuit is similar to that one, except that I use a triode for the input stage and use an interstage transformer instead of the cap/choke combination. Either way works. I think you will find that it is easy to get working and sounds very good!

leadbelly · 20th January 2006, 01:41 PM

Thanks! When you say that your design was similar, did you also leave the cathode resistor of the EL84 unbypassed as shown in the above schematic? I guess that is also local feedback employed by that amp.

andyjevans · 20th January 2006, 02:34 PM

I have a Leak Stereo 20 which is much modified, but taking off the feedback loop in UL was a breakthrough in sound for me. Much more natural. I agree - try the 300 ohms resistor. Andy

Kent Smith · 20th January 2006, 03:32 PM

Yes, my understanding is that the bypass caps are not required in the push-pull circuit, so I only use caps on the input tube cathode.

Thanks Andy, for confirming the idea that global neg. feedback is best avoided!

kopite · 20th January 2006, 06:04 PM

My current amplifier is the circuit you have pointed out from Electraprint. My previous build is a se EL34 but this amp beats it easily for sound quality. The 6au6 in triode mode is a very underated tube. The phase splitting transformer works very well.
The circuit is easy and well thought out. Give it a try and report back your findings.
Phil.

Bandersnatch · 20th January 2006, 09:54 PM

Hey-Hey!!!,
If the speakers don't do anything screwey, a very high output Z amp can drive them. Think all-pentode. But if you need a bit more NFB and still don't want to do the nasty loop thing...try this one.

It works best with pentodes in the front stage.
cheers,
Douglas

gingertube · 23rd January 2006, 02:09 AM

Leadbelly,
See my post here for what I do:
Is this EL84M UL PP OK?
I use some shunt feedback (local feedback) and a small amout of global feedback.
Cheers,
Ian

andyjevans · 23rd January 2006, 10:23 AM

gingertube - you were threatening to post aschematic of your EL84 amp - would be ijterested to see it + the CCS in the output. Andy

gingertube · 25th January 2006, 12:02 AM

Andy,
Posted here:
EL84 Amp - Baby Huey

The CCS for the output tubes is on page 2. Note I use separate CCS for each tube to allow Class AB operation.

Cheers,
Ian

20th January 2006, 12:49 AM	#1
leadbelly diyAudio Member Join Date: Dec 2002 Location: Calgary, Alberta	Is feedback required for UL EL84PP? I'm still debating whether to build my EL84 amp as UL or triode or with a switch for both. One consideration, do I need a negative feedback circuit with UL connection? The majority of vintage UL EL84 schematics I have seen do include a negative feedback circuit from the OPT secondary to the input stage, but then I came across this schematic which made me question this: http://www.electra-print.com/images/PSA_EL84_reva.pdf Comments? Suggestions? __________________ Aerodynamics are for people who can't build engines. Enzo Ferrari

20th January 2006, 01:41 PM	#3
leadbelly diyAudio Member Join Date: Dec 2002 Location: Calgary, Alberta	Thanks! When you say that your design was similar, did you also leave the cathode resistor of the EL84 unbypassed as shown in the above schematic? I guess that is also local feedback employed by that amp. __________________ Aerodynamics are for people who can't build engines. Enzo Ferrari

23rd January 2006, 02:09 AM	#8
gingertube diyAudio Member Join Date: Oct 2004 Location: Adelaide South Oz	Yep - a bit is required Leadbelly, See my post here for what I do: Is this EL84M UL PP OK? I use some shunt feedback (local feedback) and a small amout of global feedback. Cheers, Ian

25th January 2006, 12:02 AM	#10
gingertube diyAudio Member Join Date: Oct 2004 Location: Adelaide South Oz	Reply to Andy Andy, Posted here: EL84 Amp - Baby Huey The CCS for the output tubes is on page 2. Note I use separate CCS for each tube to allow Class AB operation. Cheers, Ian

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20th January 2006, 03:00 AM	#2
Kent Smith diyAudio Member Join Date: Jun 2004 Location: Wylie, Texas	I recently built my first UL amp (after building a few triode amps) using 6AQ5's. I did not want to use global negative feedback if I could avoid it, but just hooking the screen grids to the UL taps was not enough. UL is a form of local negative feedback, so I tried inserting some series resistance to control it. It worked! I ended up with 300 Ohms in between the taps and screen grids, and it sounds great. I would try with no resistance first, then add resistors in steps until it sounds clean. My circuit is similar to that one, except that I use a triode for the input stage and use an interstage transformer instead of the cap/choke combination. Either way works. I think you will find that it is easy to get working and sounds very good!

20th January 2006, 02:34 PM	#4
andyjevans diyAudio Member Join Date: Sep 2003 Location: London	I have a Leak Stereo 20 which is much modified, but taking off the feedback loop in UL was a breakthrough in sound for me. Much more natural. I agree - try the 300 ohms resistor. Andy

20th January 2006, 03:32 PM	#5
Kent Smith diyAudio Member Join Date: Jun 2004 Location: Wylie, Texas	Yes, my understanding is that the bypass caps are not required in the push-pull circuit, so I only use caps on the input tube cathode. Thanks Andy, for confirming the idea that global neg. feedback is best avoided!

20th January 2006, 06:04 PM	#6
kopite diyAudio Member Join Date: May 2005 Location: NZ	My current amplifier is the circuit you have pointed out from Electraprint. My previous build is a se EL34 but this amp beats it easily for sound quality. The 6au6 in triode mode is a very underated tube. The phase splitting transformer works very well. The circuit is easy and well thought out. Give it a try and report back your findings. Phil.

23rd January 2006, 10:23 AM	#9
andyjevans diyAudio Member Join Date: Sep 2003 Location: London	gingertube - you were threatening to post aschematic of your EL84 amp - would be ijterested to see it + the CCS in the output. Andy

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