Want Pre500 tone? Here's how to get it!

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JCDenton6
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Re: Want Pre500 tone? Here's how to get it!

Post by JCDenton6 » Thu May 28, 2015 12:10 am

Nice!
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afu
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Re: Want Pre500 tone? Here's how to get it!

Post by afu » Tue Jun 02, 2015 4:27 pm

wolf wrote:hey guys...I just recorded another short clip of my modded recto...

https://soundcloud.com/wolfernus/sd-liv ... recto-test
That's SICK!!!! Nice.
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HughMan
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Re: Want Pre500 tone? Here's how to get it!

Post by HughMan » Thu Mar 29, 2018 7:59 am

Hi everyone,

my apologies for resurrecting an old thread, but could anyone please help me in obtaining the Rev C schematic mentioned in:

http://forum.grailtone.com/viewtopic.ph ... &start=120

The link mentioned on that page appears to be no longer available (https://docs.google.com/open?id=0B3ypbi ... 2VRWHdMek0).
I was wondering if anyone still has it and could PM me the details to obtain it?

Many Thanks in advance.

Cheers,
Hugh.

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Re: Want Pre500 tone? Here's how to get it!

Post by JCDenton6 » Sat Mar 31, 2018 4:14 pm

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HughMan
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Re: Want Pre500 tone? Here's how to get it!

Post by HughMan » Sat Mar 31, 2018 7:53 pm

Fantastic! Thank you very much for this.
Very interesting to see that the V1 tube is completely switched out of the circuit (via LDRs) for the Orange clean setting, in this revision.

I have a Rev D which is obviously much closer to the Rev C than the Rev F (for which the most common schematic is floating on the net).
My guess is that the changes from Rev C to Rev D are minimal.

Cheers,
Hugh.

Shemham
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Re: Want Pre500 tone? Here's how to get it!

Post by Shemham » Mon Apr 02, 2018 4:10 am

HughMan wrote: I have a Rev D which is obviously much closer to the Rev C than the Rev F (for which the most common schematic is floating on the net).
My guess is that the changes from Rev C to Rev D are minimal.

Cheers,
Hugh.
I think you're right that the Rev C and D circuits are nearly identical. Many have reported that Rev D is slightly darker compared to Rev C. You could measure the resistance of the presence pot on the Red channel to see if it's 220k or 100k ohms. This could be one possible difference between the two. Another forum member also claimed that the loop on Rev C does not function properly as Mesa had accidentally drawn the PCB with reverse polarity for the loop bypass relay coils.

HughMan
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Re: Want Pre500 tone? Here's how to get it!

Post by HughMan » Sat Apr 07, 2018 10:39 pm

Shemham wrote:I think you're right that the Rev C and D circuits are nearly identical. Many have reported that Rev D is slightly darker compared to Rev C. You could measure the resistance of the presence pot on the Red channel to see if it's 220k or 100k ohms. This could be one possible difference between the two. Another forum member also claimed that the loop on Rev C does not function properly as Mesa had accidentally drawn the PCB with reverse polarity for the loop bypass relay coils.
I just took a look inside the chassis and upon measurement, I can confirm that the presence pot values on a Rev D Dual Recto are:
Orange Channel: 22K
Red Channel: 220K

i.e., as per the RevC schematic supplied.

However, my curiosity got the better of me this time and since I was always puzzled as to what a certain bit of "goop" was for, I removed said "goop" and saw the following:

Image
https://ibb.co/kjhxEx

My apologies if this has been discussed before and is common knowledge, but what on earth is going on here? There's a bit of wire (thick, single conductor) from pin 9 of the V2 preamp tube (heater centre tap), that is just floating (i.e., not soldered anywhere else) and I'm not seeing this anywhere else in the preamp section. It is close to a resistor (the 475K V2A input grid resistor on the LDR side not the tube side), but not touching it, and a bit of "goop" was used to secure it there and I gather to prevent it from touching the resistor.

Does anyone know why this is there? It is rather strange.

Thank you .

Cheers,
Hugh.

APEMAN
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Re: Want Pre500 tone? Here's how to get it!

Post by APEMAN » Mon Apr 09, 2018 2:17 am

HughMan wrote:Does anyone know why this is there? It is rather strange.
... yes it is, could be a previous owner cut out a cap???

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Re: Want Pre500 tone? Here's how to get it!

Post by JCDenton6 » Mon Apr 09, 2018 5:05 am

I could've sworn that wire was there to reduce microphonics and other noise from V3 in the earlier Rectos IIRC :|
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HughMan
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Re: Want Pre500 tone? Here's how to get it!

Post by HughMan » Mon Apr 09, 2018 5:35 am

APEMAN wrote:
HughMan wrote:Does anyone know why this is there? It is rather strange.
... yes it is, could be a previous owner cut out a cap???
Yeah, my initial reaction was also that someone had soldered a wire in the wrong place (this is the centre tap of the heater after all) and then just cut it. But, (a) they could have cut it shorter and (b) the non-soldered end was fixed in place with neutral cure (I hope) silicone rubber (you can still see traces of that, I haven't removed all of it).

So it looks kinda intentional, but the fact that one end is floating has me puzzled.

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Re: Want Pre500 tone? Here's how to get it!

Post by HughMan » Mon Apr 09, 2018 6:07 am

JCDenton6 wrote:I could've sworn that wire was there to reduce microphonics and other noise from V3 in the earlier Rectos IIRC :|
Really? This one is on V2. I'm wondering how this could work? Parasitic/coupling inductance etc?
Puzzling indeed.

Cheers,
Hugh.

Shemham
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Re: Want Pre500 tone? Here's how to get it!

Post by Shemham » Mon Apr 09, 2018 12:04 pm

HughMan wrote: Really? This one is on V2. I'm wondering how this could work? Parasitic/coupling inductance etc?
Puzzling indeed.

Cheers,
Hugh.
This conductor is there on all 2 channel Rectos to my knowledge. There's a discussion about it in the thread below. Apparently, it seems to be there for hum reduction purposes:

http://music-electronics-forum.com/t20724-2/

Most it's importance to the circuit must've been very minor and likely mitigated some issue from the PCB layout.

And thanks for confirming the presence pot value.

HughMan
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Re: Want Pre500 tone? Here's how to get it!

Post by HughMan » Tue Apr 10, 2018 6:49 am

Shemham wrote:
HughMan wrote: Really? This one is on V2. I'm wondering how this could work? Parasitic/coupling inductance etc?
Puzzling indeed.

Cheers,
Hugh.
This conductor is there on all 2 channel Rectos to my knowledge. There's a discussion about it in the thread below. Apparently, it seems to be there for hum reduction purposes:

http://music-electronics-forum.com/t20724-2/

Most it's importance to the circuit must've been very minor and likely mitigated some issue from the PCB layout.

And thanks for confirming the presence pot value.
Thank you for the link to discussion on this.

Ok then, so it would appear that for hum reduction this setup provides negative feedback.

Cheers,
Hugh.

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