Mesa Boogie Dual Rectifier 2 channel - Distorted clean sound

[MarkusBass]

You should have the corrected simulation in your mailbox.

Mark

[Danila-master]

You should have the corrected simulation in your mailbox.

Mark

Got It ! Thank you so much for your time!


Hello All !

Thank you very match for your assistance!

I went wrong when I replaced a 250K with a 1MOhm potentiometer.
I referred to the schematic and thought that previous technician changed it for some reason.
Then I broke my mind with lots of measurements, checking and trying to get a clean sound....

Today i got another full-working dual rectifier without any signs of the soldering iron.
I checked it with the same test conditions and got the following results:

Green lines are for the working amp.
As you can see - DC voltages are about the same.

AC measurements:


There are two differences between simulation and real amps:
1 - first stage gain is lower in real amp.
2 - V3a gain is about twice higher (but it seems to me that this is being caused by a SPICE model. In my simulation file it shows 84V p-p vs 41.5V in Marcusbass simulation)
Anyway.... it doesn't matter.
In my measurements I got V3a Plate voltages less that V3a Cathode voltages.
But I think that this is being caused by my scope's input impedance (1M) which is less than tube input impedance.
(Unfortunately I don't have a probe with a divider )

There is a difference in Gain potentiometers between these two amps.
I need to set a little bit more gain on a "Bad Amp" to get the same output as on the working amp.

Rest signals look fine.

With all EQ buttons at 12 o'clock Orange Master 100% and Send Level 100%
I checked signal at the Send Jack. Maximum clean level is 2.25V p-p on both amps.
When I add some more gain it begins to distort (When the Orange Gain is slightly above 50%).

I tested both amps side by side with my guitar and other 50 watt amp (Guitar -> Mesa Amp -> Send ->50Watt's return) and both sounds the same.

So the issue is in design but not in the failed components.
I really spent a lot of time trying to figure out a cause of the issue which doesn't exist.......

One last thing is a strange piece of wire:

It is being placed between the C22 and R241:

One side of this wire is not connected and placed between plate bypass cap and a grid resistor.
Other side is being connected to the pin 9 (heater) on this tube.
Very strange thing. It is something like a shield....
But why? Grid signal is In-Phase and Plate is in counter-phase.
So there is no positive feedback, only negative feedback.
Does it really help?

Previous technician moved this wire:

And I didn't knew what is this for and completely removed it...

P.S.

I would like to thank you all for your help. I posted this issue on some other forums and got help only at the The Boogie Board and they advised me to post this topic at your form (many thanks to afu)
With your help I managed to find answers on my questions and to understand that there are no issues with this damn amp.

[g1]

Those wires are "gimmick" capacitors, and boogie uses them quite a bit. Sometimes they are actually built with traces that go nowhere.
Think of a small trace that ends with something that looks like @ or similar.

As far as the original issue of getting a good clean sound, I think the user will have to reduce the guitar volume control a bit. You did say it only happened under hard picking.

[MarkusBass]

The differences between the simulation and the real amp may be a result of some resistors incorrectly specified on the schematic. For example, if you change the first anode resistor from 220k to 100k, you get 4Vpp signal on cathode. Can you check what are anode resistors in real amps? Mesa used 100k, or 150k in other amps so 220k seems to me a little bit high.

Mark

[Enzo]

And I didn't knew what is this for and completely removed it...

Always dangerous to find something you don't understand, and just lop it off and throw it away.


G1 mentioned gimmick capacitors, a term meaning a capacitor made with wire or similar, rather than an actual component. Sometimes they are just a couple parallel traces, though sometimes traces that do not go to anything are also acting as a local shield. Look at the Peavey Classic 30 schematic, page 2 has the layout. This layout shows the traces. Upper right corner, next to V1, see that snaky looking tracery? That is a capacitor.

[Danila-master]

The differences between the simulation and the real amp may be a result of some resistors incorrectly specified on the schematic. For example, if you change the first anode resistor from 220k to 100k, you get 4Vpp signal on cathode. Can you check what are anode resistors in real amps? Mesa used 100k, or 150k in other amps so 220k seems to me a little bit high.

Mark

Of course I will re-check all values in the V1 stage to see if there are differences between schematic and a real amps.

Always dangerous to find something you don't understand, and just lop it off and throw it away.

As you can see on the last photo - it looked as a useless piece of wire. It was not on it's initial position.
Considering that this amp has been repaired - I thought that this wire was added during repair process.
I desoldered all sockets, cleaned them and then soldered them back. When I desoldered the V2 socket - this wire has been removed.


Anyway - I will put there another wire exact the same way as in other amp.

[Danila-master]

A short update:
1) V1 stage has correct values. It was a measurement error. Signal was slightly less than 150mV
2) "Capacitor Wire" Is in its place now.
3) I measured a Red Gain pot ant it is 500K instead of 1M. It is not original - replaced it with a 1M pot.

Tomorrow will test it

[Shemham]

3) I measured a Red Gain pot ant it is 500K instead of 1M. It is not original - replaced it with a 1M pot.

Stock Red channel gain pot should also be 250k in all the Recto models and revisions.

[MarkusBass]

A short update:
1) V1 stage has correct values. It was a measurement error. Signal was slightly less than 150mV

That's what I thought. If the voltages in second and third stage match, it almost not possible that there is 50% difference in the first stage. And what about other voltages (cathode follower stage)?

Mark

[Danila-master]

That's what I thought. If the voltages in second and third stage match, it almost not possible that there is 50% difference in the first stage. And what about other voltages (cathode follower stage)?

Mark

You can find both AC and DC measurements in my previous posts.
There are two tables with values.

[Danila-master]

Stock Red channel gain pot should also be 250k in all the Recto models and revisions.

Hmmm, I see more posts regarding this fact.
So I will Keep Old 500K instead of an original 250K, considering that my friend already accustomed to this setting.

[Danila-master]

Hate this amp ))))
I assembled it back, plugged output tubes, mounted it in a cabinet and now it has 100Hz hum
need to re-check it again.
master volumes affect level of this noise, when the loop is on (without a patch cable) there is no hum.
So this hum goes from the preamp section.
I didn't had enough time to check it this morning. Will check today evening when will be at home.

[Gregg]

Stock Red channel gain pot should also be 250k in all the Recto models and revisions.

It's also a 5% log pot (12-16k in mid position) hence the specific response it produces. There are many sites that stock original Mesa pots so maybe it will be a good idea to check some. Using a 1M is well known mod for this preamp. It changes the pre gain filter frequency and will result in "too much gain" response (compared to the 250k pot) especially if it's not a 5% log pot.

[Danila-master]

It's also a 5% log pot (12-16k in mid position) hence the specific response it produces. There are many sites that stock original Mesa pots so maybe it will be a good idea to check some. Using a 1M is well known mod for this preamp. It changes the pre gain filter frequency and will result in "too much gain" response (compared to the 250k pot) especially if it's not a 5% log pot.

Finally i finished this repair.
Hum has been caused by a missing contact with ground rail on one of the master potentiometers.
Gain pot on red channel is 500K. I left it in place to do not confuse amp's owner with different amount of gain.
Initial issue seems to be a EH tube in the V3 stage. When we tested it on the full volume - there were slightly distortions on a clean channel, but without hard fuzzy sounds.
All my actions:
1 - cleared and resoldered all tube sockets in the preamp section.
2 - replaced 3 cathode capacitors with new ones.
3 - Replaced V3 tube with a Ruby 12AX7
4 - Clean Gain replaced to it's original 250K Pot.

I think that actions 3 and 4 resolved this issue, but who knows.... )

I would like to thank all of you guys for your assistance and patience.
Great forum and great people!

[Gregg]

Initial issue seems to be a EH tube in the V3 stage.

V3 is the CF tube and depending on the batch EHs are known to die sometimes immediately in that position especially in Mesa DR where they are fed 400+ Volts.
Some years ago after I had several EHs die on me in a short period of time I emailed EH's tech to inquire about this and they said the maximum cathode to heater voltage of their 12AX7 tubes was +/-100V. There's a special thread somewhere in the forum about this.