Can this be right?

It's just been pointed out to us that our Pultec MEQ-5 drops the signal level by almost 4 dB, which we've confirmed in our lab with a signal generator / analyser.

After changing ALL the valves the relative o/p level is now -3.5dB but I don't know if this particular unit is supposed to be "zero in, zero out" or if this signal drop is normal.

Any idea anyone?

Phil
Techie, Olympic Studios
Phil Plumpton, Olympic techy.
Compooter man, electronics noob.


CJ

Pultec MEQ-5 relative output level = -3.5dB even in bypass?
« Reply #1 on: June 11, 2007, 03:06:15 PM »
s/b zero I/O

losing gain from an aged tube perhaps?
original transformers?
check xfmr taps, on I/O.
cj
If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html

SSLtech

Pultec MEQ-5 relative output level = -3.5dB even in bypass?
« Reply #2 on: June 11, 2007, 03:09:53 PM »
Quote from: "CJ"
losing gain from an aged tube perhaps?

Well that's where he went wrong... he changed the valves, when he should have replaced the tubes!
 :green:

Keef
"A waist is a terrible thing to mind"
Quote from: PRR
Ah, but that was 1999; we don't party like that any more.

CJ

Pultec MEQ-5 relative output level = -3.5dB even in bypass?
« Reply #3 on: June 12, 2007, 01:02:29 PM »
He should have changed the pistons as well.
Chatter on, boy, a touch of the writers cramp will soon sort you out.
 :green:
If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html

Pultec MEQ-5 relative output level = -3.5dB even in bypass?
« Reply #4 on: June 13, 2007, 10:42:57 AM »
lol, it's comedy hour at Prodigy Pro :-D

Right - I think I've found the problem... there was a dry joint on an inductor.

Unfortunately, I found this when the inductor fell off the circuit and consequently I haven't got a clue where it was supposed to be soldered in.

Anyone either got a schematic I could have, or can tell me where the "U2M 5MMF +/- .5" inductor is supposed to go?  :oops:
Phil Plumpton, Olympic techy.
Compooter man, electronics noob.

gyraf

Pultec MEQ-5 relative output level = -3.5dB even in bypass?
« Reply #5 on: June 13, 2007, 10:45:27 AM »
Here's a redraw - not guaranteed to be correct: http://www.gyraf.dk/gy_pd/pultec/meq5.gif


Jakob E.
..note to self: don't let Harman run your company..

Pultec MEQ-5 relative output level = -3.5dB even in bypass?
« Reply #6 on: June 13, 2007, 10:49:22 AM »
Quote from: "gyraf"
Here's a redraw - not guaranteed to be correct: http://www.gyraf.dk/gy_pd/pultec/meq5.gif


Jakob E.


Thanks Jakob, I already saw that one on your website - it's just the EQ stage, not the output stage.

It says "see the PEQ-1" schematic for the rest of it, but on your site there's only the "EQ-1" not the "PEQ-1" and that "EQ-1" seems to be different - I can't see an inductor anywhere on it. :-?
Phil Plumpton, Olympic techy.
Compooter man, electronics noob.

CJ

Pultec MEQ-5 relative output level = -3.5dB even in bypass?
« Reply #7 on: June 13, 2007, 12:53:16 PM »
He might have used CAD symbols, look for resistor like denotes; tructures, boxes, etc.

Even in bypass mode, the loose inductor should be out of circuit.
So you might have other problems.
If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html

drpat

Pultec MEQ-5 relative output level = -3.5dB even in bypass? New
« Reply #8 on: June 13, 2007, 04:40:12 PM »
...
« Last Edit: September 17, 2010, 09:49:56 AM by drpat »
I use the LA2a on very hot signals like lead vocal.

CJ

Pultec MEQ-5 relative output level = -3.5dB even in bypass?
« Reply #9 on: June 13, 2007, 04:44:13 PM »
that might be an oscillation supressor cap, it won't affect anything in the audio band, unless the amp was oscillating in ultra sonic mode, chewing up watts to oscillate out of ear range.

the cap will probably not be on Jakes schemo, he uses a different line amp, right?

that cap should go from plate to grid if I remember right,
V1, 12AX7?  into a 6AQ5. Single ended? Cathode feedback?
If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html


drpat

Pultec MEQ-5 relative output level = -3.5dB even in bypass? New
« Reply #10 on: June 13, 2007, 07:52:29 PM »
...
« Last Edit: September 17, 2010, 09:50:30 AM by drpat »
I use the LA2a on very hot signals like lead vocal.

Pultec MEQ-5 relative output level = -3.5dB even in bypass?
« Reply #11 on: June 17, 2007, 09:36:20 AM »
Quote from: "drpat"
Quote from: "OlympicPhil"
Anyone either got a schematic I could have, or can tell me where the "U2M 5MMF +/- .5" inductor is supposed to go?  :oops:


That's not an inductor, it's a 5pF ceramic cap. It should live somewhere in the makeup amplifier, so it will affect the operation, even in bypass.

Is it a SS or tube unit?


Oh, really?

Damn that Wikipedia giving me false answers :evil:
Phil Plumpton, Olympic techy.
Compooter man, electronics noob.

Pultec MEQ-5 relative output level = -3.5dB even in bypass?
« Reply #12 on: June 17, 2007, 09:38:24 AM »
Quote from: "drpat"
OK, I see it's a valve unit :oops:  I'm sure Bomb Factory should have a schematic on their website :green:

Now that I know it's a tube unit, CJ is right, it goes from pin6 to pin7 on the 12AX7...


Oh, OK, thanks for that.

I've changed that now (it only measured about 1.5pF on my capacitance meter - too low for my regular DMM but the cap meter picked it up) and it's made no difference  :mad:

It's looking like it's a transformer problem I think.  I've tested/changed everything else :-?
Phil Plumpton, Olympic techy.
Compooter man, electronics noob.

jdr

Pultec MEQ-5 relative output level = -3.5dB even in bypass?
« Reply #13 on: June 17, 2007, 02:37:09 PM »
Here's a schematic for the MEQ-5 line amp, though I can't say for certain that it's accurate.

Have you verified the tube complement in your unit? Most especially that it is a 12ax7 in circuit? Someone may have previously substituted a 5751 or  12au7 or something by accident, or to purposely reduce the gain.

Hope the schematic helps,
Good luck!

John

Pultec MEQ-5 relative output level = -3.5dB even in bypass?
« Reply #14 on: June 19, 2007, 04:39:00 AM »
Quote from: "jdr"
Here's a schematic for the MEQ-5 line amp, though I can't say for certain that it's accurate.

Have you verified the tube complement in your unit? Most especially that it is a 12ax7 in circuit? Someone may have previously substituted a 5751 or  12au7 or something by accident, or to purposely reduce the gain.

Hope the schematic helps,
Good luck!

John


Oh my God, you're right, you know... someone before me had changed the 12AX7 to a 12AU7!  When I changed the valves, I used the valves that were in there when I got there as my guide for what to replace them with!

I'll keep you posted... changing it for a 12AX7 now...
Phil Plumpton, Olympic techy.
Compooter man, electronics noob.

Pultec MEQ-5 relative output level = -3.5dB even in bypass?
« Reply #15 on: June 19, 2007, 04:43:48 AM »
Yup, that's exactly what it was... with a 12AX7A, (is there a difference between a 12AX7A and a 12AX7 by the way?), it now gives a relative level of +0.25dBr.

Thanks all for your help, I feel like such a muppet now :-D

Phil
Phil Plumpton, Olympic techy.
Compooter man, electronics noob.

kazper

Pultec MEQ-5 relative output level = -3.5dB even in bypass?
« Reply #16 on: June 19, 2007, 05:27:33 AM »
Quote
I feel like such a muppet now


Which one?
Intentionally left blank

jdr

Pultec MEQ-5 relative output level = -3.5dB even in bypass?
« Reply #17 on: June 19, 2007, 12:28:50 PM »
Glad it worked out :thumb:

Quote
is there a difference between a 12AX7A and a 12AX7  by the way?


The "A" denotes a later revision of the original type. (I think) in this case 12AX7A is lower noise when powered by an AC heater supply because the heater filament is spiral wound (like a helix), and also the filament has a slower/controlled warm-up.

John

CJ

Pultec MEQ-5 relative output level = -3.5dB even in bypass?
« Reply #18 on: June 19, 2007, 06:44:03 PM »
don't use the 12ax7.
it will be very noisy, in my experences, at least.
but they are pretrty rare, so most likely, you will have an A in your stash.
If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html


 

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