Mesa Caliber 50+ Voltages

[SigiZ58]

Hello together,

I bought a used Mesa Caliber 50+. There are schematics for this model, where the voltages are given with

-47 V Bias
450 V Anode

I found some modifications in my amp
- The 100Ohm Heater resistances were replaced with a 220Ohm Trimmer that had smoked up meanwhile so There was a terrible hum before...
- Somebody had worked at the BIAS Circuit. It does not match with the schematics. The voltage has been -39 V instead of -47

The circuit in my amp

-      ----- 68k ---|
-T----|             |------ 33k------- From here it fits with the given schematics ......
-      ----- 82k ---|              -39V
-                   |------ 
-                 220u    |
-                   |    47k 
-                   |     |
-                   ------- 
-                       |
-                   Ground 

Compared to the two schematics available:

-                    ------ 68k -----
--T----- 47K -------|                |------.......
-                |   ------ 82k -----  -47V 
-                |
-              220u
-                |
-              Ground 

What I did was to cut the resistor parallel to the 220u, so I have now the correct BIAS Voltage 47V

But still the other Voltages are to low.
With standby on, there are 450V. But when I switch standby off (tubes powering on), The voltage drops to 435 V. The dependent voltages are dropping the same factor (405 instead of 417)

Any advice or hints from the cracks here ?

I am living in Europe with 220V

Thank you very much!

[soundguruman]

Hello together,

I bought a used Mesa Caliber 50+. There are schematics for this model, where the voltages are given with

-47 V Bias
450 V Anode

I found some modifications in my amp
- The 100Ohm Heater resistances were replaced with a 220Ohm Trimmer that had smoked up meanwhile so There was a terrible hum before...
- Somebody had worked at the BIAS Circuit. It does not match with the schematics. The voltage has been -39 V instead of -47

The circuit in my amp

-      ----- 68k ---|
-T----|             |------ 33k------- From here it fits with the given schematics ......
-      ----- 82k ---|              -39V
-                   |------ 
-                 220u    |
-                   |    47k 
-                   |     |
-                   ------- 
-                       |
-                   Ground 

Compared to the two schematics available:

-                    ------ 68k -----
--T----- 47K -------|                |------.......
-                |   ------ 82k -----  -47V 
-                |
-              220u
-                |
-              Ground 

What I did was to cut the resistor parallel to the 220u, so I have now the correct BIAS Voltage 47V

But still the other Voltages are to low.
With standby on, there are 450V. But when I switch standby off (tubes powering on), The voltage drops to 435 V. The dependent voltages are dropping the same factor (405 instead of 417)

Any advice or hints from the cracks here ?

I am living in Europe with 220V

Thank you very much!

Depends on "which" output tubes are being used. -47 may not really be correct.

There are numerous ways but the bias is affected by the phase inverter tube.
You almost have to select just the right tube for the phase inverter, current wise....
Boogie sells tubes that have been selected for the phase inverter. They pick those by hand...
You may find that the schematic does not match what is on the board.
and you may find that the voltages don't match the schematic either. That's common.

You need to come up with the correct bias for the output...that does not over current the phase inverter tube cathode.

As far as HUM in this amp goes...
The input jack grounds the output of the tone stack, when the guitar is unplugged.
That switching contact has to be working (or replaced), the switching contact goes bad quite often....

The amp has a pretty good HUM to it, even when it's working correctly.
The reason is the design of the grounding, mostly.

The power supply ground should be split from the signal ground.
Separating the grounds will reduce the noise quite a bit.

[SigiZ58]

Hello Soundguruman, thanks for your reply

Depends on "which" output tubes are being used. -47 may not really be correct.

There are numerous ways but the bias is affected by the phase inverter tube.
You almost have to select just the right tube for the phase inverter, current wise....
Boogie sells tubes that have been selected for the phase inverter. They pick those by hand...

You are right, of course. I should have mentioned that I only use regular MESA 6L6 and 12AX7 Tubes for this reason. So - if the schematics is right - the -47 Voltage should be right for this tubes.

The origin problem with this amp was that i got it and at once replaced the Sovtek Tubes with branded MESA Tubes. All was fine (except the strong hum) until I made a really "loud" test.
When I had Master at 7 and Gain at 8 or so - for my taste now the amp started to work really - after some minutes it made crickle crackle, the amp lost volume and I had a really loud hum.
Found that I had smoked up the Power-Tubes. When the amp was opened, I found replacing the tubes and put in the origin heater resistors instead of the last-user-builtin trimmer fixed the problem.

The amp does not hum very much anymore but the voltages seem a bit strange. As far as I know, a lower BIAS Voltage makes a greater current through the Tube running it hotter, so it makes sense they smoke up when becoming too hot. With the "regular" -47V and the original MESA Tubes at all places I should be on the safer side.

You may find that the schematic does not match what is on the board.
and you may find that the voltages don't match the schematic either. That's common.

To be honest, my problem is, I don't find any information about the transformer output voltages without power consumption. Or - asked different - If I have 450 V without power consumption (empty load is 450V/300k) Is then a drop of 15V in the first stage normal when power flows through the tubes ?
As I said, If I knew at least the no-load output AC Voltage of the transformer this would help analyzing.

You need to come up with the correct bias for the output...that does not over current the phase inverter tube cathode.

I hope this is fullfilled when I use "original" Mesa Tubes

As far as HUM in this amp goes...
The input jack grounds the output of the tone stack, when the guitar is unplugged.
That switching contact has to be working (or replaced), the switching contact goes bad quite often....

The amp has a pretty good HUM to it, even when it's working correctly.
The reason is the design of the grounding, mostly.

Thank you very much for this input, I will check this.

The power supply ground should be split from the signal ground.
Separating the grounds will reduce the noise quite a bit.

Do you have hints or examples how this can be realized here in this case ?

Thank you again very much.

[g1]

There are numerous ways but the bias is affected by the phase inverter tube.
You almost have to select just the right tube for the phase inverter, current wise....

You need to come up with the correct bias for the output...that does not over current the phase inverter tube cathode.

This is a very typical fixed bias type amp.
http://www.el34world.com/Forum/index...ch=24965;image

I don't see how the phase splitter affects the bias in any way.
Or how you could "over current" the phase inverter cathode.

Please explain.

[SigiZ58]

This is a very typical fixed bias type amp.
http://www.el34world.com/Forum/index...ch=24965;image

Thank you for the link to the schematics .... This is my amp and shows what I described.

The BIAS adjustment Voltage was in any case affected (dropped) by the 47k parallel resistance to the 220u that is not shown in this schematics.

[g1]

Perhaps someone may have modified the bias to run EL34 tubes?
It is normal for the voltage to drop a bit when you switch out of standby. I think you will be ok if it seems to be running proper and sounds good.
Do you have 220V coming from your outlet? Is the amp labelled for use with 220, 230, or 240V ?

[SigiZ58]

Perhaps someone may have modified the bias to run EL34 tubes?
It is normal for the voltage to drop a bit when you switch out of standby. I think you will be ok if it seems to be running proper and sounds good.
Do you have 220V coming from your outlet? Is the amp labelled for use with 220, 230, or 240V ?

It sounds good for me. The amp is labelled for 220V.
I made a test with all tubes away and the Voltage seemed normal then.

I will see when the amp gets his next real load test .... One Time blown away a Mesa was like a heart attack...... I really couldn$t believe it is possible to blow away an amp instead of the speaker. It should be impossible ........