Mesa Boogie running 12AX7 overvoltage?

[Gareth Magennis]

I have a Mesa Boogie Dual Rectifier on my bench which runs 3 sections of
ECC83 at over 400v Anode voltage.
The spec says these have 300v max Anode voltage.

How can this be?


Here's a link to a different model's schematic to the one I have, that does
a similar thing:

http://www.schematicsunlimited.com/m/mesa-boogie/mesa-boogie-dual-rectifier-amplifier-schematic



Gareth.

[Gareth Magennis]

Oops, I'm an arse,

that would be Anode to Cathode PD, so probably in spec.


But anyway, this does seem rather a curious thing to do, most other guitar
amps I've ever seen don't have pre-amp valve Anodes anywhere near this, they
are always well downstream of the Power amp HT.



Gareth.








"Gareth Magennis" wrote in message news:0mJbx.752771$m15.3...@fx45.am4...

[Phil Allison]

** The problem I see there is the heater to cathode voltage is over the 180V limit for a 12AX7.

http://free-hosting.infodiv.com/audiotube/small_signal_triode/small_signal_triode0034/12AX7_Page_1.jpg

Same goes for all the Fender amps that use 12AT7s for the phase splitter, where the cathode voltage is around 140V when the spec says 90.

http://free-hosting.infodiv.com/audiotube/small_signal_triode/small_signal_triode0002/12AT7_Page_1.jpg

The reverb drive valve in most Fenders is also a 12AT7 where the anode voltage is over 400V ( driving a small tranny) while the spec is 300.

I bought a dozen JAN numbered 12AT7s once ( branded GE ) that lit up purple inside if you used them in the reverb position in Fenders, plus made a lot of noise. They were perfectly OK in the phase splitter position.

Amp makers often say to use only their brand name valves, sometimes for a good reason.


... Phil











.... Phil

[Gareth Magennis]

"Phil Allison" wrote in message
news:5d351c76-368f-4d54...@googlegroups.com...

Right, measured all the voltages today. It has 6 pre-amp valves.
One half of V2 runs at 343v Anode/Cathode, the Reverb valve is pretty much
as you described Fenders circuit, i.e. a mighty 398v on a ECC83, running
through a small tranny.
All other valves run below the 300v Anode max.

I also measured the Heater/Cathode voltages. One was 206v, another 205v.
All others well below the 180v max.


This made me laugh though - I was measuring the Heater/Cathode voltage using
pin 5 as reference, and measuring pins 3 and 8 with a multimeter.
I got the various voltages on V3 to V6, but V1 and V2 measured around -10v.
That's a NEGATIVE 10v.

I had to have a good look at the schematic to find out how this was
possible!



Thanks, this has been educational.



Gareth.

[Gareth Magennis]

"Gareth Magennis" wrote in message news:Na%bx.611578$JH2.2...@fx11.am4...

Just one update, one of the Heater/Cathode overvoltages I found looks like
it is due to a manufacturing error.
It's in the FX loop send.

The schematic shows an ECC83 with 425v on the Anode, a 10M resistor from
Anode to Grid, and a 3M3 resistor from Grid to ground.
The Cathode is grounded by a 100K resistor. Input Grid resistor is 220K.
The schematic shows the Cathode voltage to be 113v, where I measured the out
of spec 207v.

Turns out that in the amp I have, BOTH these resistors are 10M.
Maybe the production machinery was misloaded with the wrong value resistor.




Oh and if anyone's interested in how you can measure a negative 10v
Cathode/Heater voltage in this amp on V1 and V2:-

V3 to V6 and the output valves have their heaters driven by a conventional
6.3v AC winding, centre tapped to ground.
So, any of the 3 heater pins on the valve base will be the reference ground
point to measure the Heater/Cathode voltages.

V1 and V2 heaters, however, are driven from a 12v regulator, each valve
having the two 6.3v heaters in series instead of parallel.
One end is at 12v, the other is grounded.
I unknowingly used the pin at 12v as the ground reference. The real
Heater/Cathode voltage was 2v or so, giving the erroneous reading of minus
10v.



Cheers,


Gareth.

[Phil Allison]

Gareth Magennis wrote:


>
> Just one update, one of the Heater/Cathode overvoltages I found looks like
> it is due to a manufacturing error.
> It's in the FX loop send.
>
> The schematic shows an ECC83 with 425v on the Anode, a 10M resistor from
> Anode to Grid, and a 3M3 resistor from Grid to ground.
> The Cathode is grounded by a 100K resistor. Input Grid resistor is 220K.
> The schematic shows the Cathode voltage to be 113v, where I measured the out
> of spec 207v.

** What a crappy way get grid bias with 300V across a 10Mohm resistor = heaps of resistor voltage noise. The usual and better way is to add about 1.5kohms in series with the cathode resistor and then a 1Mohm back to the grid from point where they join. The 1Mohm then has no voltage across it and is "bootstrapped" by the cathode signal giving very high input impedance.

> Turns out that in the amp I have, BOTH these resistors are 10M.
> Maybe the production machinery was misloaded with the wrong value resistor.
>

** Boogie's PCBs all look hand loaded to me.


... Phil