boogie mark iii gain stage.

noworrybeefcurry · Post by **noworrybeefcurry** » Fri Feb 18, 2011 1:20 am

i recently played through a friends boogie mark III coliseum head and the drive on it blew me away. so i got ahold of some mark III schematics and began analyzing the gain stages. there was an area that had me a little confused.

after the first lead gain stage, after the coupling cap and grid stopper, directly conected to the next stages grid is a resistor and capacitor in parallel with eachother to ground. what is happening there? is this simply a bleeder circuit?

any insight would be much appreciated!

darkbluemurder · Post by **darkbluemurder** » Fri Feb 18, 2011 11:55 am

noworrybeefcurry wrote: is this simply a bleeder circuit?

Sort of, yes. The grid load resistor is bypassed with a cap. The result is a dampening of highs, i.e. some highs are bled off before reaching the tube grid. This may either have been the tone Mesa Boogie was shooting for or a necessity to prevent the amp from oscillating, given the tightly packed layout.

Cheers Stephan

jerrydyer · Post by **jerrydyer** » Wed Feb 23, 2011 3:53 pm

anyone got a CLEAR mark III schem? The one I had is really hard to read.
thanks
jerry

noworrybeefcurry · Post by **noworrybeefcurry** » Thu Feb 24, 2011 11:35 pm

most all early boogie schematics are like that, believe me ive searched. The ones that ive found are the clearest to read are over at hoffmanamps.com under his tube amp library of information.

soundguruman · Post by **soundguruman** » Wed Mar 02, 2011 2:28 pm

If you are looking for that sound with less noise, etc...look at British Vox schematics instead.

noworrybeefcurry · Post by **noworrybeefcurry** » Wed Mar 02, 2011 9:33 pm

a vox for that high gain boogie thing?

joey · Post by **joey** » Thu Mar 03, 2011 2:27 pm

noworrybeefcurry wrote:a vox for that high gain boogie thing?

Not even close, those MK IIC+, MKIII's, and MKiV's have a lot more saturation than any vox will give ya.... I mean I don't think you will be able to play convincing John Sykes, or master of puppets out of any vox amp I know of. Most of the schemes on line are slightly wrong as well. a friend of mine, and I reversed a C+ as a group effort, and while most of those schemes are mostly correct, there are just some slight things that may affect their tone somewhat.

noworrybeefcurry · Post by **noworrybeefcurry** » Thu Mar 03, 2011 5:12 pm

what were some of the errors that you guys found with the original schematic?

soundguruman · Post by **soundguruman** » Fri Mar 04, 2011 1:33 am

how much of that gain is really usable? you turn it past 3 and it starts to mud out. and the noise from so many stages put together...

joey · Post by **joey** » Fri Mar 04, 2011 1:24 pm

noworrybeefcurry wrote:what were some of the errors that you guys found with the original schematic?

just small petty stuff mostly, a couple of wrong cathode bypass caps, and some other odds and ends, but nothing major... and the hardest things to find if you wanted to build one are the inductors for the Active EQ.

The biggest thing though is the OT, and simulclass output. You really need that to get it to sound correct, but the kicker is that the simulclass OT is a hoax, it's not really tapped. and it also specs out to be closer to a Marshall OT, than a Fender.

rockstah · Post by **rockstah** » Fri Mar 04, 2011 1:26 pm

joey wrote:
noworrybeefcurry wrote:what were some of the errors that you guys found with the original schematic?
just small petty stuff mostly, a couple of wrong cathode bypass caps, and some other odds and ends, but nothing major... and the hardest things to find if you wanted to build one are the inductors for the Active EQ.

The biggest thing though is the OT, and simulclass output. You really need that to get it to sound correct, but the kicker is that the simulclass OT is a hoax, it's not really tapped. and it also specs out to be closer to a Marshall OT, than a Fender.

how so is it a hoax? and so what happens when you flip that SC switch?

noworrybeefcurry · Post by **noworrybeefcurry** » Fri Mar 04, 2011 6:51 pm

joey wrote:
noworrybeefcurry wrote:what were some of the errors that you guys found with the original schematic?
just small petty stuff mostly, a couple of wrong cathode bypass caps, and some other odds and ends, but nothing major... and the hardest things to find if you wanted to build one are the inductors for the Active EQ.

The biggest thing though is the OT, and simulclass output. You really need that to get it to sound correct, but the kicker is that the simulclass OT is a hoax, it's not really tapped. and it also specs out to be closer to a Marshall OT, than a Fender.

what were the correct values of those bypass caps? im just curious because im working on a build that has a boogie based gain stage.

-Thanks

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