modify parallel push-pull EL84 to single push-pull

chungtat · 28th October 2005, 09:10 AM

Recently I bought a integrated amp. The power output stage is using parallel push-pull EL84. Total 8 tubes.

The output transformer is 4K pri. impedance.

I don't need that much power and considering using only 4 tubes. That is changing from parallel push-pull to single push-pull.

What modifications do I need to do without changing the output transformer? Bigger cathode resistor? Or no need to modify anything?

The B+ is kind of high, 450 VDC.

Thanks for any help.

ray_moth · 28th October 2005, 10:22 AM

You will need a higher load impedance. I doubt if you can get away without changing the OP tranny, unless you can use different secondary taps for the speakers, e.g. connect 8 ohm speakers to the 4 ohm tap.

chungtat · 28th October 2005, 10:35 AM

I have forgotten to mention that the output transformer is ultralinear.

Tks.

chungtat · 28th October 2005, 10:37 AM

It does has 4 ohm taps.

One thing I dont understand is that even the ratio is correct, the load impedance still doesn't match.

ray_moth · 28th October 2005, 03:17 PM

Quote:

. . . even the ratio is correct, the load impedance still doesn't match

How do you know it doesn't match? The normal p-p load I've seen for a pair of EL84 in UL PP is 8k. Connecting 8 ohm spkr to 4 ohm tap on a 4k p-p OPT should give you 8k. Just be careful if the paralleled EL84s share a cathode resistor. You will need to double the value to get the same bias voltage, since you will be halving the current.

rdf · 28th October 2005, 04:03 PM

If you don't need that much power why not try strapping them triode? Assuming the driver cleanly handles the increased grid capacitance.

chungtat · 31st October 2005, 02:40 AM

Thanks a lot! I will try that.

It is using parallel PP and I thought that make it single PP may have a positive effect on the sonic quality.

The cathode resistor is 10R for each tube. So I will double it for the 4 tubes.

What is good for cathode resistor for output stage? WW or carbon composite?

Thanks for your help.

ray_moth · 31st October 2005, 05:14 AM

Quote:

The cathode resistor is 10R for each tube. So I will double it for the 4 tubes.

Hold on! I don't follow your logic. If there is really a separate 10R resistor at each EL84 cathode, then there is no need to double anything. Also, with only 10R present, it suggests that the amp is using grid bias, not cathode bias, which would probably require no adjustment to change from parallel to single PP.

Also, I don't believe changing from parallell to single PP will make any audible improvement to the sonics. Indeed, the opposite is more likely to be true. If you want better sonics, then it might be better to try rdf's suggestion.

chungtat · 31st October 2005, 05:35 AM

Here is the schematic of the output.

analog_sa · 31st October 2005, 07:02 AM

Quote:

Also, I don't believe changing from parallell to single PP will make any audible improvement to the sonics.

It very probably will. I have never heard parallel tubes sound better. For reasons i don't understand parallel EL84 sound particularly bad.

28th October 2005, 09:10 AM	#1
chungtat diyAudio Member Join Date: Mar 2005 Location: HK	modify parallel push-pull EL84 to single push-pull Recently I bought a integrated amp. The power output stage is using parallel push-pull EL84. Total 8 tubes. The output transformer is 4K pri. impedance. I don't need that much power and considering using only 4 tubes. That is changing from parallel push-pull to single push-pull. What modifications do I need to do without changing the output transformer? Bigger cathode resistor? Or no need to modify anything? The B+ is kind of high, 450 VDC. Thanks for any help.

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28th October 2005, 10:22 AM	#2
ray_moth diyAudio Moderator Emeritus Join Date: Jan 2004 Location: Jakarta	You will need a higher load impedance. I doubt if you can get away without changing the OP tranny, unless you can use different secondary taps for the speakers, e.g. connect 8 ohm speakers to the 4 ohm tap.

28th October 2005, 10:35 AM	#3
chungtat diyAudio Member Join Date: Mar 2005 Location: HK	I have forgotten to mention that the output transformer is ultralinear. Tks.

28th October 2005, 10:37 AM	#4
chungtat diyAudio Member Join Date: Mar 2005 Location: HK	It does has 4 ohm taps. One thing I dont understand is that even the ratio is correct, the load impedance still doesn't match.

28th October 2005, 04:03 PM	#6
rdf diyAudio Member Join Date: Jun 2004 Location: big smoke	If you don't need that much power why not try strapping them triode? Assuming the driver cleanly handles the increased grid capacitance.

31st October 2005, 02:40 AM	#7
chungtat diyAudio Member Join Date: Mar 2005 Location: HK	Thanks a lot! I will try that. It is using parallel PP and I thought that make it single PP may have a positive effect on the sonic quality. The cathode resistor is 10R for each tube. So I will double it for the 4 tubes. What is good for cathode resistor for output stage? WW or carbon composite? Thanks for your help.

31st October 2005, 05:35 AM	#9
chungtat diyAudio Member Join Date: Mar 2005 Location: HK	Here is the schematic of the output.