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$m15.315085@fx45.am4...

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.

 

[Phil Allison]

Gareth Magennis wrote:

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


** 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-b830-2a8ac0e3ee21@googlegroups.com...

Gareth Magennis wrote:

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


** 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




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$JH2.267660@fx11.am4...



"Phil Allison" wrote in message
news:5d351c76-368f-4d54-b830-2a8ac0e3ee21@googlegroups.com...

Gareth Magennis wrote:

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


** 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




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.





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