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Old 22nd January 2006, 12:06 AM   #1
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Default Is this EL84M UL PP OK?

Well, after many changes to what I originally started to build, I think I have made up my mind on what to build. It's mostly straight out of Morgan Jones' book, the output stage from the Bevois Valley amp, the transistor CCS, and his simplified of the Maida regulator. I was hoping someone would be so kind as to briefly review for errors, sanity check, and simple enhancements I missed.
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Old 22nd January 2006, 12:07 AM   #2
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And the power supply:
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Old 22nd January 2006, 01:29 AM   #3
anatech is offline anatech  Canada
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Hi leadbelly,
I would run the B+ for the output stage straight from the filter, not regulated. I haven't checked, but if this is stereo, will a 6CA4 supply enough current?

A simple zener diode or string would be fine to run the voltage amp stage with. Or a simple two transistor regulator (error amp and pass).

-Chris
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Old 22nd January 2006, 07:37 AM   #4
Giaime is offline Giaime  Italy
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Hello Leadbelly,

that's very similar of what I have in mind for my next amp, only that it won't be UL but straight triode.

Why are you using 6922 input tube (thus necessing big voltage drop to power it - from 320 to 120V)? Won't a 12A*7 work there?

Like this (intended for the triode section of ECL86, similar to 12AX7 or, better, 5751):
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Old 22nd January 2006, 08:11 AM   #5
Danko is offline Danko  Hungary
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Please excuse me, for being offtopic....

But, why need that CCS (in the first post) 2 transistors? One transistor isn't enough? I've built a CCS with one transistor, and it worked fine for me.

Again, sorry for being offtopic
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Old 22nd January 2006, 02:36 PM   #6
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Quote:
Originally posted by anatech
I would run the B+ for the output stage straight from the filter, not regulated.
Well PSUD2 tells me there is 20V of ripple at that point; surely that would be audible?

Quote:
Originally posted by anatech
I haven't checked, but if this is stereo, will a 6CA4 supply enough current?
It's rated for 150mA.

Quote:
Originally posted by Giaime
Why are you using 6922 input tube (thus necessing big voltage drop to power it - from 320 to 120V)? Won't a 12A*7 work there?
I read a lot of good things about ECC88 and 6922, so I bought a bunch of them. I generally read bad things about 12AX7 I like the idea of another regulator to decouple the input stage PS from the output stage PS anyways; that it is a much bigger voltage drop is really unimportant to me, basically 1 resistor becomes much bigger.

Quote:
Originally posted by Danko
But, why need that CCS (in the first post) 2 transistors? One transistor isn't enough?
The 2 transistor version is higher performance. In Jones' book, he puts the single BC549 CCS at 92 kohms and the double BC549 CCS at 32 Mohms.
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Old 22nd January 2006, 03:54 PM   #7
anatech is offline anatech  Canada
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Hi leadbelly,
Your ripple (p-p value?) will be cancelled by the push pull stage. Look at the older amps built in the 50's and 60's.

As for the 6CA4, I like a little more headroom. A 5AR4 comes to mind.

The 12AX7 was designed for voltage amplification. It has a high plate resistance, but performs well when used correctly. Most new designers don't seem to have a clue on that one. The trick is to keep high frequency losses down (thanks to Miller). I do have amplifiers that use the 12AX7 and they sound great. You can use a 6922 or 6DJ8 also, your choice. They are a totally different animal.

SY and others can comment more on that topic.

-Chris
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Old 22nd January 2006, 04:28 PM   #8
SY is offline SY  United States
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1.5K plate load resistors are a problem. Why did you choose such a low value? You'd want something more like 20K. 6DJ8 is a reasonable input tube choice.

Cathode CCS is a good idea. I haven't checked your values yet.

With properly loaded anodes, a 6DJ8 diff amp should give you a gain of 12 or so on each side. You need 8V to drive the EL84 to clipping, so with a standard 2V input, you've got room for feedback to work.
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Old 22nd January 2006, 05:10 PM   #9
Giaime is offline Giaime  Italy
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Quote:
Originally posted by SY
1.5K plate load resistors are a problem. Why did you choose such a low value? You'd want something more like 20K.
Maybe a typo? I think it should be 15k.
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Old 22nd January 2006, 05:13 PM   #10
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Quote:
Originally posted by SY
1.5K plate load resistors are a problem. Why did you choose such a low value?
The input stage is basically this:
http://home.c2i.net/semfielec/tubepre.htm
except using single triodes. I have built that one with paralled triodes and like it fine.

Quote:
Originally posted by SY
With properly loaded anodes, a 6DJ8 diff amp should give you a gain of 12 or so on each side. You need 8V to drive the EL84 to clipping, so with a standard 2V input, you've got room for feedback to work.
I am trying to avoid using global negative feedback, hence the 330 ohm resistors on the UL taps as a starting point. Can I decrease the gain of the input stage to help stability?
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