adjustment to my OTL head amp for ecc83 instead of ecc82 ?

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PeteC
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adjustment to my OTL head amp for ecc83 instead of ecc82 ?

Post by PeteC » Mon Jan 04, 2010 4:22 pm

Hi

I built this OTL head amp from Headwise a few years ago and enjoy it a lot.



However, 2 "matched" ECC82 which arrived at the weekend turn out to be completely mismatched ( £12 down the drain..) and having loads of Ecc83/12Ax7s lying around I wondered what mod i could do to this circuit to use a couple of ECC83s instead - that way I can have a matched pair and good stereo image.

How would I do this ?

Alternatively I wondered whether fitting some form of basic CCS on both channels would "match" the troublesome ECC82s I just bought ?

Any brilliant ideas ?

Thanks
Pete
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Post by Andrew » Mon Jan 04, 2010 4:36 pm

I would get some some decent 82's very different valve to an 83.

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Post by PeteC » Mon Jan 04, 2010 4:45 pm

they were brand new JJ ECC82's.

any good sources for nice 82's ?

Rgds
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Mike H
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Post by Mike H » Mon Jan 04, 2010 4:46 pm

Unfortunately ECC83 is a very different animal to ECC82. Presumably the circuit uses a fair amount of current hence the '82's. The '83's are just plain not a direct replacement in any conceivable way (despite identical pin numbering) and I can't think how to make them do anything like without a complete redesign of the circuit. Even then I have my doubts.

ECC82 = low gain, high current, high grid bias.

ECC83 = high gain, low current, low grid bias.

Not at all identical.

In what way are the 'new' '82's mismatched?

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Post by Mike H » Mon Jan 04, 2010 4:57 pm

It's this version is it?
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Post by PeteC » Mon Jan 04, 2010 4:59 pm

Thanks

when i install the new 82's there is hardly any volume in one channel compared to the other
- when I put back the Jan 5814's ( detsined for a pre amp build ) then the stereo image is near perfect.

I get that teh 12ax7 is not a possible replacement -

is it worth trying to match the curent with some form of CCS ?

Rgds
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Post by Toppsy » Mon Jan 04, 2010 5:09 pm

Watford Valves is a place I have used in the past for matched tubes with no issues.

They have quite a selection depending what you want to pay.

http://www.watfordvalves.com/products.asp?search=ecc82

I'd give them a call first and explain you need matched tubes. I don't think they charge any extra for this service. But could be wrong.

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Post by Nick » Mon Jan 04, 2010 5:28 pm

I think its more likely that one of the new valves is duff, they should not need matching in that circuit.

I would work out which valve causes the low gain by a process of elimination and just get a replacement for that.

You would expect small variations between valves, but not as much as you seem to be seeing.

You could put the good valves back in, measure the voltages on the pins, then put the new ones in measure again, and see what is changed.
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Post by Mike H » Mon Jan 04, 2010 6:29 pm

Deffo seems like the poor one is not at all well.

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Post by Andrew » Mon Jan 04, 2010 6:42 pm

PeteC wrote:they were brand new JJ ECC82's.

any good sources for nice 82's ?

Rgds
Pete
Send the dead one back.

Steve S used to have some nice NOS MAZDA (I think) 82's for sale, ask him, he might still have some.

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Post by PeteC » Mon Jan 04, 2010 7:29 pm

Its a duffer !

I measured it in circuit just now -

good valve measures ( triodes are paralleled )
Plate 332v
Grid ~90v
Cathode ~190v

Bad 'un measures

Plate 341v
Grid 15v
Cathode 17v

Its back in a little brown box going back to Philip at Bluebell - he's been really helpful to me in past , and has offered advice freely too - so hopefully he will replace it for me.

The Jan 5814's are going back into the head amp for now.

Thanks all

Rgds
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Post by pre65 » Mon Jan 04, 2010 7:32 pm

PeteC wrote:
Its back in a little brown box going back to Philip at Bluebell - he's been really helpful to me in past , and has offered advice freely too - so hopefully he will replace it for me.


Pete
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G-Popz THE easy listening connoisseur. (Philip)

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Post by PeteC » Mon Jan 04, 2010 7:36 pm

PS - yes Mike its the same circuit - higher plate voltages and some slight changes from Kurt's original design.

see here -
http://home.online.no/~wsaa/otl/otl.html

apart from very very slight hum ( 0.3mv ) - I love it !

Rgds
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Post by Mike H » Mon Jan 04, 2010 8:53 pm

Ah, this'un.


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Post by PeteC » Mon Jan 04, 2010 10:10 pm

yup ! thats the fella .....

why was my uploaded diagram so small i wonder ?

anyway, been swapping valves tonight and have now got 5814s with 6N1P's and it sounds rather good IMHO.

wil try ECC82s again when I get another good 'un sorted.

Cheers
Pete
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