Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: kagliostro on June 30, 2011, 02:17:22 pm

Title: A strange PP schematic - Mesa Boogie Mark V - How it works ??
Post by: kagliostro on June 30, 2011, 02:17:22 pm: Few time ago I find the schematic of the Mesa Boogie Mark V

(I posted a link in the schematic section)

yesterday eveneeng I was looking to the Power section of that schematic and I find that I can't understand it

the schematic seems to use an Ultra Linear OT, but sometime ago I've read of a guy who disassembled a M.B. MarkIV (if I'm not wrong) and that wasn't an UL OT, so I don't know for sure about the OT

as I can understand there is a switch for pentode/triode, but there are also some switchable zeners that I don't understand

other thing the tube in the bottom of the schematic is allways cathode connected to ground, his PP correspondent not

other problem are the resistors connected in grid pin5 ???

also the switch between the tubes in the middle of the schematic is confusing me

WHAT I CAN SAY ................ I'M LOST

Please need your help to understand that strange thing

THANKS

Kagliostro
Title: Re: A strange PP schematic - Mesa Boogie Mark V - How it works ??
Post by: VMS on June 30, 2011, 02:57:56 pm: The user manual explains the three modes of the Power Amp:

http://www.mesaboogie.com/manuals/user_manuals.html#gpm1_2 (http://www.mesaboogie.com/manuals/user_manuals.html#gpm1_2)
Title: Re: A strange PP schematic - Mesa Boogie Mark V - How it works ??
Post by: kagliostro on July 01, 2011, 06:44:39 am: The user manual explains the three modes of the Power Amp

Yes, OK, the function are clear

but how the circuit works ?

as example, which is the function of that zener diodes ?

I'm not able at all to understand it

Thanks

Kagliostro
Title: Re: A strange PP schematic - Mesa Boogie Mark V - How it works ??
Post by: FYL on July 01, 2011, 08:46:06 am: Quote
Few time ago I find the schematic of the Mesa Boogie Mark V

Most Mesa schemos are false.

Quote
as I can understand there is a switch for pentode/triode, but there are also some switchable zeners that I don't understand

Lower screen voltages = lower power and a different grain.
Title: Re: A strange PP schematic - Mesa Boogie Mark V - How it works ??
Post by: VMS on July 01, 2011, 09:14:55 am: I also thought that there were errors but then I noticed that the wires from PI to PA are flipped.

RYSE1, RYSE2, RYSE3 and RYSE4 are for 10Watt SE mode. This mode is cathode biased with those 360 ohm resistors and the 220k grid-leak resistor is connected to ground. Screen voltages are dropped with zener diodes and 1k6/25W plate stop resistors are added.

RY100 is the simul-class switch that switches between 90Watt and 45Watt mode.

This is my interpretation but I don't guarantee it to be right.
Title: Re: A strange PP schematic - Mesa Boogie Mark V - How it works ??
Post by: kagliostro on July 01, 2011, 01:31:46 pm: Thanks FYL & VMS

Quote
Most Mesa schemos are false

don't know, but may be, often I've read about mistakes on the Mesa schematics

(the 1960 Davoli Krundaal italian amp schematics are full of mistakes)

OK about the purpose of those zeners

Quote
RYSE1, RYSE2, RYSE3 and RYSE4 are for 10Watt SE mode.

RYSE1 ... ??

Quote
This mode is cathode biased with those 360 ohm resistors and the 220k grid-leak resistor is connected to ground.

220k grid-leak resistor ... do you mean 2M2 ?

what about RYTP1 & RYTP2, they act a triode/pentode mode, do you think they act simultaneously with other switch ?

Thanks

Kagliostro
Title: Re: A strange PP schematic - Mesa Boogie Mark V - How it works ??
Post by: VMS on July 01, 2011, 01:43:50 pm: Quote
RYSE1 ... ??

Page 7/8 on the schematic.

Quote
220k grid-leak resistor ... do you mean 2M2 ?

No.

Quote
what about RYTP1 & RYTP2, they act a triode/pentode mode, do you think they act simultaneously with other switch ?

User manual page 42 shows that channel 3 has a triode/pentode switch.
Title: Re: A strange PP schematic - Mesa Boogie Mark V - How it works ??
Post by: kagliostro on July 01, 2011, 01:58:10 pm: Hi VMS

Thanks for the quick replay

Kagliostro
Title: Re: A strange PP schematic - Mesa Boogie Mark V - How it works ??
Post by: kagliostro on July 02, 2011, 02:59:03 am: Some days ago we were talking about morphing between PP and SE

http://www.el34world.com/Forum/index.php?topic=11667.0 (http://www.el34world.com/Forum/index.php?topic=11667.0)

now there is this Mesa schematic with their PP to SE solution

I was looking to that schematic and a question comes

if after the PI we have the split of the signal how can the SE amp work ?

isn't halved the signal if we take it only from one side of the PI ?

very confused about :dontknow: :help:

THANKS

Kagliostro
Title: Re: A strange PP schematic - Mesa Boogie Mark V - How it works ??
Post by: VMS on July 02, 2011, 04:24:09 am: Quote
isn't halved the signal if we take it only from one side of the PI ?

No, PI outputs have the same signal but so that one is inverted and the other one is non-inverted.

Here is some more info about the SE/PP switch:

http://ampbooks.com/home/amp-technology/amp-patent-7173488/ (http://ampbooks.com/home/amp-technology/amp-patent-7173488/)
Title: Re: A strange PP schematic - Mesa Boogie Mark V - How it works ??
Post by: kagliostro on July 02, 2011, 04:38:25 am: Quote
No, PI outputs have the same signal but so that one is inverted and the other one is non-inverted.

Many thanks

I don't know well the matter and often I'm confused, sometimes I know something but didn't remember (I think that I'm too stressed)

Thanks again

Kagliostro
Title: Re: A strange PP schematic - Mesa Boogie Mark V - How it works ??
Post by: VMS on July 02, 2011, 05:54:19 am: Hey, you're not alone, this circuit has confused me also.

I thought that if you take the push or pull tube out of push-pull you'll lose half of the waveform, but I guess it doesn't happen if the SE mode is in Class A. :dontknow:

VMS