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

[Anonymous]

msi, I have that schematic you're talking about. I figured it was wrong since it's missing a couple of parts and there are some weird values (presence cap is 3900pF, for example), but maybe it's more correct than I had thought. Anyone know where / how I can upload that?

If it's a paper schematic you can scan it onto a PC and save it as a jpeg, than upload it to a picture host like postimage.org (that's what I use) then post it using the 1st forum code.
Or if it's already a pic (jpeg.png...ect..) then you can just upload straight to postimage.org.

PDF, so not sure where I can host that.

 

[JCDenton6]

Ah ok, you can upload that to mediafire, then post the link here when it's done uploading.

 

[Yetti]

I gotta say this is some awesome knowledge to soak in.

However in regards to the boogie archives for hosting on a website. There is still free hosting sites out there that will allow you to just register and upload all the files and boom.
I did a quick Google search and www.freehostingcloud.com didn't look in depth but will certainly work.

Just like back in the early days (for me lol) tons of people and bands had websites on this hosting place called geocities.
You could even then register on another site for a redirection link so you could have a link that wasn't www.freeshitywebsite.com/user/boogiearchives/blah blah. And just make it www.boogiearchives.to
Or something of that sort. I'd even go crazy make a Facebook a YouTube and twitter and all kinds of ****.
Could come up with other names and crap!

If y'all want a site... zip those files up and send to me and I'll make it happen.

 

[JCDenton6]

msi, I have that schematic you're talking about. I figured it was wrong since it's missing a couple of parts and there are some weird values (presence cap is 3900pF, for example), but maybe it's more correct than I had thought. Anyone know where / how I can upload that?

I forgot to mention that you can use multiupload and dropbox as well.

 

[Anonymous]

Dual Rectifier Revision C Schematic

That's the one I have on file. Comments, corrections?

EDIT: If it's not big enough for you, just download it. Then you can zoom in much more than Google lets you.

 

[JCDenton6]

Ah you used google docs, forgot about that.
Thanks again for uploading this!

 

[r_cod_81]

Hi folks, just wondering if this mod would be relevant to the Recto recoding preamp? I could use some more gain on my red channel... I'd be leaving the clean channel alone.

 

[Third Age Amps]

I don't think this mod gives it more gain, it just refines the tone.

I would invest in different pickups or a boost pedal before trying to coax more gain out of the 5 gain stages it already has. It's already pretty high-gain. Any more and it might just propagate buzzy fizz or flubby low-end. Plus giving it more gain could destabilize the preamp adding more hum or creating oscillation.

 

[JCDenton6]

I went ahead and removed the resistors off of the cathodes that were added on in the later revisions (don't forget to add jumpers) and it seemed to open the amp up more.
I noticed the distortion sounded more aggressive and had a sharper attack, closer to a SLO. I feel like my Triple has gotten even closer to the tone of a Rev C, it's scary close at this point.

So far I'm thrilled with the progress thus far and will get some updated clips up soon. I feel that just changing the gain pots from 250k to 1meg and changing C55 from .10uf to .0047uf would've been enough, but I couldn't leave well enough alone.

 

[AdmiralB]

Years ago pre500 lore was kept solely to the transformers. This post is the first I've seen that challenges that, and I've been around for a while. Everything I've read in the past is that the transformers for the early Duals were from the MarkIII's used until that supply ran out.

I assume we're talking about output transformers, since the power transformers were never from earlier amps.

The early DRs did use non-Simulclass 100W Mark III outputs....easy way to tell, if you have a 16-ohm tap, it's not a Mark III OT. The Mark III-equipped amps have jacks marked '4 ohms' and jacks marked '8/16 ohms'. This was a bad move.

I've owned three pre-500s and two of them had fried PCBs at the power supply. I'm 99% certain it was due to the use of 16-ohm cabs.

 

[screamingdaisy]

I have a Recto with Mark III output and power transformers.

 

[AdmiralB]

I have a Recto with Mark III output and power transformers.

That's very interesting. How does a Mark III power transformer provide 5V for the rectifier tubes?

 

[YellowJacket]

Years ago pre500 lore was kept solely to the transformers. This post is the first I've seen that challenges that, and I've been around for a while. Everything I've read in the past is that the transformers for the early Duals were from the MarkIII's used until that supply ran out.

I assume we're talking about output transformers, since the power transformers were never from earlier amps.

The early DRs did use non-Simulclass 100W Mark III outputs....easy way to tell, if you have a 16-ohm tap, it's not a Mark III OT. The Mark III-equipped amps have jacks marked '4 ohms' and jacks marked '8/16 ohms'. This was a bad move.

I've owned three pre-500s and two of them had fried PCBs at the power supply. I'm 99% certain it was due to the use of 16-ohm cabs.

Mesa 'claims' that 8 ohm output with a 16 ohm cab is a correct mismatch. An 'incorrect' mismatch would be an 8 ohm output with a 4 ohm cab.

I went ahead and removed the resistors off of the cathodes that were added on in the later revisions (don't forget to add jumpers) and removed LDR 10 and it seemed to open the amp up more.
I noticed the distortion sounded more aggressive and had a sharper attack, closer to a SLO. I haven't tried rewiring the 1st gain stage yet, but I feel like my Triple has gotten even closer to the tone of a Rev C, it's scary close at this point.

So far I'm thrilled with the progress thus far and will get some updated clips up soon. I feel that just changing the gain pots from 250k to 1meg and changing C55 from .10uf to .0047uf would've been enough, but I couldn't leave well enough alone.

You and MagicEight are officially the Boogieboard mythbusters. It figures that the difference in tone is actually due to a couple of caps and resistors, not some magic output transformer. I'm sure the output tranny colours the tone somewhat but picking which frequencies to bleed from the gain pot will definitely have a far more drastic effect.

A video comparing a Rev F with a Rev G. The Rev G sounds more 'recto' since it is darker and creamier sounding.
http://www.youtube.com/watch?v=5FFB3JsYMiw

 

[AdmiralB]

Mesa 'claims' that 8 ohm output with a 16 ohm cab is a correct mismatch. An 'incorrect' mismatch would be an 8 ohm output with a 4 ohm cab.

Yes, they claim that. That doesn't make it true.

Generally speaking, one shouldn't mismatch. If you have to, going numerically lower is safer. So your second example is generally safe (it's how all those BF and SF Fender extension speaker jacks worked). Going the other way, not so much. Lots and lots of discussion on this out there, try RG Keen's tube amp FAQ.

The two cooked amps both experienced arcing between the plate and the heater contacts on one or more power tube sockets (pins two and three). That dumped the plate voltage into the heater circuit and burned the board. Oddly, Mesa used flameproof resistors everywhere EXCEPT the heater supplies, pretty much ensuring incendiary results.

The cause of the arcing was flyback, and that was almost certainly due to upward impedance mismatch.

I imagine they were just using up old inventory on the OTs, or maybe didn't see the need to design a new one. But when these amps came out, I'd think most of the target market who owned a 412 owned a Marshall cab, so they had to at least pay lip service to 16-ohm operation. And it'd probably have worked fine, if these were say jazz or surf amps.

Anyway, Mesa has a history of releasing products before they're really ready...just about everything they've ever made underwent numerous revisions, usually not readily apparent externally. The power supply on the early DRs is really poorly designed in general - for example, the switching system takes power from the bias circuit, and just knocks the voltage down with a giant resistor.

 

[JCDenton6]

You and MagicEight are officially the Boogieboard mythbusters. It figures that the difference in tone is actually due to a couple of caps and resistors, not some magic output transformer. I'm sure the output tranny colours the tone somewhat but picking which frequencies to bleed from the gain pot will definitely have a far more drastic effect.

Thanks, I guess we are. :lol:
I think after I figure out the process of rewiring the 1st gain stage and pull out a few more resistors (if they impact the gain signal at all) this will be a full on "pre 500" mod. (although it's really, really close at this point)

Just wish I had a full schematic of a Rev G, I snagged the one of the Rev C TheMagicEight posted, I think I am finally getting the hang of reading and understanding them.
Looks like we're close to figuring this out. Thanks for your input on the tranny issues AdmiralB, I didn't realize that Mesa did that.

 

[screamingdaisy]

That's very interesting. How does a Mark III power transformer provide 5V for the rectifier tubes?

No idea, but I'm pretty sure they figured something out.

 

[AdmiralB]

That's very interesting. How does a Mark III power transformer provide 5V for the rectifier tubes?

No idea, but I'm pretty sure they figured something out.

The answer is, it can't. No Mesa amp used rectifier tubes prior to the DRs, at least no 100W amp. Therefore, you don't have a Mark III power transformer.

 

[screamingdaisy]

That's very interesting. How does a Mark III power transformer provide 5V for the rectifier tubes?

No idea, but I'm pretty sure they figured something out.

The answer is, it can't. No Mesa amp used rectifier tubes prior to the DRs, at least no 100W amp. Therefore, you don't have a Mark III power transformer.

Do I need to take photos of the part number?

 

[AdmiralB]

I guess so.


EDIT - I should be more specific.

A Mark III PT is, by itself, incapable of powering 5U4 rectifier tubes. So there are two possibilities - either you don't have a Mark III PT, or, your amp has a secondary filament transformer for the 5V supply.

I can imagine that Mesa probably had a prototype or two set up that way, since one would want to prove the concept before specing and buying a load of iron. But I'd be really surprised...astounded, even by Mesa standards...if any were ever sold that way.

So yes, I'd love to see proof of a Mark III-powered amp with rectifier tubes.

 

[screamingdaisy]

Turns out you're right and I'm wrong. Did some reading up and the 561136 is the Mark IVA PT. Thought they were both from a Mark III.