Parallel PP EL84 and ouput transformer question - diyAudio
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Old 4th January 2003, 07:24 PM   #1
corbato is offline corbato  India
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Question Parallel PP EL84 and ouput transformer question

This is my first post at the forums having been a lurker for many months now.

I intend to embark on my first Tube amp project and have even located a plans for it at http://www.geocities.com/tjacodesign.../valveamp.html which is a Push-Pull parallel EL84 design with self biasing.
I could have looked for something simpler, but then I needed the higher output of the parallel EL84s. Till now I have made, among others, assorted SS power amps including Elliot DOZ and the JLH Class A 15 watts, several Discreet Phonostages and Speakers.

Since locating the above plan I have bought 8 nos of EL84 (BEL India) and the ECC81 (Mullard NOS). I ordered for the output Transformers from a supplier in New Delhi. Allthough they look quite sturdy and heavy I was quite disappointed with the freq range of 70Hz to 12KHz as labeled on the Transformer. I have no choice as getting these items in India is difficult and sourcing from EU or US vendors terribly expensive.

As such I request feedback from member on the following:

A) Is the schematic as given in the above URL any good?
B) Is the there any way the freq curve of my Transformer can be made better.

I HAVE to use the already purchased Valves and Transformers. So any suggestion must keep this in view. Thanks in advance.
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Old 4th January 2003, 07:32 PM   #2
Colt45 is offline Colt45  Serbia
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schematic looks fine.

personally I'd drop that current supply BS and just put in a cathode resistor... call me old fashioned
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Old 4th January 2003, 09:08 PM   #3
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Default EL84

Hi,

Quote:
A) Is the schematic as given in the above URL any good?
This circuit should work fine.

Quote:
B) Is the there any way the freq curve of my Transformer can be made better.
The spec is given under what circumstances?

You can't improve on it really.
The question is how it will perform once it is included in the NFB loop.

I suspect that if you don't need too much power it could still work acceptably well.

Cheers,
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Old 4th January 2003, 10:31 PM   #4
SY is offline SY  United States
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Quote:
Originally posted by Colt45
schematic looks fine.

personally I'd drop that current supply BS and just put in a cathode resistor... call me old fashioned
It's just another case of doing something complicated for the sake of doing something complicated. From a purely engineering point of view (best amplifier performance), a bypassed cathode resistor (this IS an AB amp!) is a better, more reliable, less expensive choice. Not as cool, though.

See, for example, Dalzell's article on Low Loading in the December '59 issue of Audio for some real-world ways to properly cathode bias EL84s.
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Old 6th January 2003, 12:53 PM   #5
corbato is offline corbato  India
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Quote:
Originally posted by Colt45
schematic looks fine.

personally I'd drop that current supply BS and just put in a cathode resistor... call me old fashioned
yes..even i thought so...

Quote:
Originally posted by fdegrove
The spec is given under what circumstances?
Specs: 70 to 12000 cps 3db. Suitable for 4 x EL34 in Parallel Push Pull mode Class AB1. Primary Imp 4000 ohm. Secondary Imp 4-6-8-32 Ohm. 150 D.C. mA

Quote:
Originally posted by SY
See, for example, Dalzell's article on Low Loading in the December '59 issue of Audio for some real-world ways to properly cathode bias EL84s.
could you kindly provide a web link, if available, for the same ?

I am also attaching a pic of the Transformer.
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Old 6th January 2003, 12:56 PM   #6
corbato is offline corbato  India
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oops...somehow the pic does'nt show up...how can it be done?
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Old 6th January 2003, 01:02 PM   #7
SY is offline SY  United States
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I don't think there's a web link, but this article should be available in most University engineering libraries. It may have also been reprinted in one of Audio Amateur's "Audio Anthology" compilations.

Basically, what Dalzell points out is that you can bias for much less idle current using a cathode resistor than the callouts in the tube manuals. On short bursts, the amp will act like a fixed bias unit, i.e., will put out more power than the usual cathode bias arrangement. The tradeoff is that the amp will not perform as well on bench tests requiring full power out for more than a second. In his suggested low-loading biasing of EL84s, you aim for the cathode current to be equal to that of the fixed bias arrangement.
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Old 6th January 2003, 01:26 PM   #8
corbato is offline corbato  India
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thanks..i'd suspect this arrangment will sound musically more pleasing.

i have also found this page quite usefull. visit their archives for some good info.

cheers and thnx again from a somewhat cold and rainy india.
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