G9 mod with Pultec-style filter section - question

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Val_r

Well-known member
Joined
Jun 5, 2004
Messages
306
Location
Naples, Italy.
Hi,

I have built the G9 dual pre (only the first Channel). I was thinking on modifying by adding the Pultec-style clone PEQ1 between the first and the second stage (V1 and V2). Without building the 2 channels.

Since the pultec stage requires a low driving impedance, I mounted an OEP A262 A3E (wired as 6k25 i/p and 600 ohms o/p) between V1 o/p and pultec i/p, and wiring directly the pultec o/p to second stage i/p.

It works smoothly, but obviously an extra gain from the second stage is needed.
Any suggestions?

(I was thinking on changing R30 and R29, or changing V2 from the actual ECC82 to an ECC81).

What's the formula to determine the input impedance of a S.R.P.P. stage?

All for now.

Thanks,

Val

:?
 
Hi Val,

What you want won't be possible, I think. Not enough gain no matter how you do it. You'll need at least one extra gain stage.

Here's a very good resource on calculating SRPP stages:

http://www.tubecad.com/may2000/

Jakob E.
 
G9 first stage have high output impedance, your load
must be not more than 50k.
It is possible to replace v1b with ecc88 SRPP and use
pentode in triode connection instead of v1a; ef86/e804f or 6au6
have good linearity in triode mode and very low noise too.
Your output impedance will be low enough to drive
passive circuit. According to Pultec schem, you don't need to install
620 Ohm input resistot (there's no trafo to terminate it).
After eq, you go to unmodyfied g9's srpp.
In case of huge interest, I can post schem of this mod,
tested and working.
Cheers,
Igor.
 
Hi Yakob!
What about this input stage mod for G9? :)
http://img.photobucket.com/albums/v436/IJR/G9MOD.jpg
 
Hi Igor,

Yes, that would surely be a nice alternative. But it would also complicate the design quite a bit, as ECC88 and EF86 would need 6.3V heating.

How much gain can be obtained from a triode-coupled EF86 in a circuit like that?

Jakob E.
 
hi,

one of my next diy projects should be a passive pultec type equalizer, but without any gainstage, just the passive circuit...

for gain makeup I want to run a preamp after the passive eq cirucit...
(tube (G9), transistor (n72)... whatever i want)

can anybody tell me exactly, what type of trafos I need at the input and at the output of the eq, to match the impedance of the passive circuit...??

thanks
mat
 
You won't need any transformers for the filter circuit to work.

If you need balanced in, use a 600:600 Ohms transformer - or possibly a 600:150 Ohms if you're not sure of the previous stage's driving capability.

This because the input impedance of the filter can go as low as 75 Ohms at max.hi.boost.

Jakob E.
 
But it would also complicate the design quite a bit, as ECC88 and EF86 would need 6.3V heating.
12.6 v and stereo working fine for me :)
(heaters of '86's in series):parallel to:(heaters of '88's in series)
 
thank you jakob... so if I would use an unbal. input at the pultec filter I could plug it into the insert jack on the G9 for gain makeup?? would that work??


mat
 
Hi,

The following mod seems to work better.
out of V1 (first channel) to primary of OEP tranny (6.25k), out of oep (600 ohms) to i/p of Pultec stage, out of pultec stage directly to the i/p of V1 (second channel); and its output directly into V2 (of first channel), wired with the 2:1 o/p transformer.
From a Mic signal, it is possible to achieve 3.78 Vrms (that's morethan 12Vpp!!), anyway respectable.

Any advice?

Thanks,
Val

:cool:
 
I don't think the first gain stage has low enough output impedance to drive decent low-frequency through the transformer - not even at 6.3:1.

But try it - and let us know how it goes..

Jakob E.
 

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