Mesa Boogie Dual Rectifier 2 channel - Distorted clean sound

[Danila-master]

My friend asked me to help with his Triple Rectifier 150W dual channel Amp.
The issue is with the clean channel.
It begins to distort even at 10% of gain with passive pickups.
When i'm playing slightly - it sounds like a regular clean channel, when i make hard picking or play chords- it sounds distorted.
Issue appears in preamp section. When I use an external preamp connected into a return jack - it sounds pretty good.

Channel Style select switch is in the middle position,
Orange channel gain is set to "Clean Rhytm"

All tubes have been changed to a new ones, i tried two different sets of 12AX7-EH tubes.
Heater voltages are OK - 6.2V

I tested all optocouplers and they are looking pretty good.
about 100 Ohm open ant more than 2Meg closed.
I applied 150mV peak-to-peak voltage at the amp input and checked voltages and gain stage-by stage.


With 15-20% of gain setting I can see clean sine wave at the V3b Cathode,
With 50% of gain i have distorted sound after the V3a stage.
Calculated and measured gains have really close values, excepting the V3a and V3b stages.

I desoldered V1 V2 V3 sockets and cleaned them in solvent.
Also I cleaned all tracers and solder joints with solvent.
After that I dried PCB and sockets for about 2 hours.
Then I soldered all sockets back without using of additional Flux - to prevent any additional conductance.

Also I found on some forums - that there are some issues with 12AX7EH tubes in V3 and V4 stages, because of Heater to cathode Voltage ratings.
I replaced the V3 tube with a new Ruby 12AX7-AC tube - no success.
Also i tried JJ, Groove Tubes and Sylvania tubes.

After that I excluded the V3a and V3b Stages from this amp. I soldered these stages on a separate tube socket and connected wires to a PCB.
(Ground, Power and Input)
This was made to exclude any impact from a PCB side.
Also I connected heater voltage from a separate power supply - to exclude Heater to Cathode voltage issue.
As a load for the Cathode follower I built a a separate tone stack.


I installed the LT-Spice software and modeled this preamp.
As I can see:

with 15% of Gain and 100mV input signal


(Green -V3a Anode; Blue - V3b Cathode)
voltage at the input ("X" point) is 6.35V
the anode AC Voltage on V3a is about 272V (Clean)
V3a cathode AC Voltage is 2.27V it swings from 0.6 to 2.8V


with 15% of Gain and 150mV input signal


(Blue -V3a Anode; Green - V3b Cathode)
voltage at the input ("X" point) is 9.94V
the anode AC Voltage on V3a is about 324V (distorted)
V3a cathode AC Voltage is 3.1V it varies from 0.0V to 3.1V

In real conditions I can see the following values:
Input - 100mV; Gain - 15%
Point "X" - 6V
V3a Cathode AC Voltage - 2.5V (slightly distorted)
V3a Anode AC Voltage - 225V (slightly distorted)

Input - 150mV; Gain - 15%
Point "X" - 10V
V3a Cathode AC Voltage - 2.75V (distorted)
V3a Anode AC Voltage - 230V (distorted)

All parts are new, Socket is new.
I tried about 6 different tubes with the same result.
I tried a new Ruby 12AX7-AC tube - no success.

So the V3a output is lower than the simulated one.
Real amp:
Plate DC voltage - 223V
Cathode - 1.7V
I got 230V p-p Max at the V3a Plate, but sym shows 320V p-p max. (distorted signal)
This is about 30% lower than the estimated value.
Such way it works with less headroom and begins to distort earlier.

I don't know why this happens(((((

[MarkusBass]

Can you post the simulation file?
It seems to me that you have two (most probably) independent problems: distored signal on CLEAN channel and differences between simulation and the real circuit. That's why I'd like to see the simulation. Have you taken into account audio tapper of some pots? It is hard to believe that you get 324V AC on V3a. Do you mean RMS voltage? What is the power supply voltage for the preamp?

BTW, on the schematic you posted has 4 gain stages (one of the stages is low gain). It seems to me that 150 mV pp signal on the input is much to much. Or, maybe the schematic you posted is not a schematic of the clean channel?

Mark

[warpedmusician]

Can you post the simulation file?
It seems to me that you have two (most probably) independent problems: distored signal on CLEAN channel and differences between simulation and the real circuit. That's why I'd like to see the simulation. Have you taken into account audio tapper of some pots? It is hard to believe that you get 324V AC on V3a. Do you mean RMS voltage? What is the power supply voltage for the preamp?

BTW, on the schematic you posted has 4 gain stages (one of the stages is low gain). It seems to me that 150 mV pp signal on the input is much to much. Or, maybe the schematic you posted is not a schematic of the clean channel?

Mark

The clean channel and the dirt channels of this model of Dual Rectifier share everything. The LDRs throw switches to give the desired response. The schematic has a listing of LDR switch states for each mode.

[MarkusBass]

The clean channel and the dirt channels of this model of Dual Rectifier share everything.

So you confirm that the clean channel has 4 gain stages.

The LDRs throw switches to give the desired response. The schematic has a listing of LDR switch states for each mode.

It's a pity that OP hasn't specified which LDRs are ON and which are OFF. After checking the schematic I see that for the clean channel all cathode capacitors are disconnected and LDR4 is off (which makes the R322 resistor active - it forms a voltage divider with the GAIN pot). But still there are 4 gain stages which is rather unusual for clean channels.
If you look at any demo of the amp (e.g. this one: https://www.youtube.com/watch?v=rWdphJMp0GA at 3:20), you will se that this is a high gain amp and the clean channel it played very quiet - just several Watts instead of 100W.
Also the listing of voltages on the schematics above does not look right. For example, voltage on cathode follower is higher than on the preceding stage (110V versus 100V). I think that it should be exactly the opposite.
Then the voltages listed on the simulations also do not seem correct. For example, on picture #4 the maximum voltage is 363V, the minimum is 90V so p-p voltage is 273V but you cannot say that this is AC voltage. The real AC voltage is: 273/2/square root of 2 = 96V.
If the owner of the amp says that previously the amp wasn't distorting that quick, I'd say that one of LDRs (for example LDR4) is not OFF as it should be. Have you verified that all LDRs are in state listed on the schematic?

Mark

[Danila-master]

Voltages are the following:
C - 408
D- 388
E- 387

In my simulation I used a resistor divider instead of a Gain potentiometer 850K / 150K
EQ section is being set at all about 12 o'clock, but IMHO it is not critical in this case.

The voltage in the point "C" should be about 422V DC/ The V3 stage is being connected to this point.

I mean 324V p-p (peak-to-peak)


This means (324V/2)*0.707 = 114.5 V RMS

150mV p-p is 53mV RMS - input test sine wave.

This is a 150W Dual Channel Rectifier.


So It has only two channels.

LDR's were the first thing I checked.
On the clean channel we have only 3 cathode LDR's LDR3a, LDR3b and LDR10.
They all are open, and all cathode bypass capacitors are disconnected.
I tried to short them and mentioned a gain boost in all of 3 LDR's.

[MarkusBass]

So you shouldn't say that AC voltage is 324V. AC voltages are usually specified as RMS. This was confusing but I see that you have peak-peak voltages in the cursor window.
You mention cathode LDRs but they are not that important as LDR4. Have you checked LDR4 too?
I was wrong that clean amps do not have 4 gain stages. I remember I was fixing Egnater Tourmaster and it has 4 gain stages in the clean channel. And the clean sound is powerful in the amp. But I remember that it has different voltage dividers - something like 1.5M to 220k, or 33k resistor in parallel to 1M GAIN pot. This could allow for much more headroom.
In case of Dual rectifier the first thing I would check is LDR4.
Can you post the simulation file? I could do it from scratch but I don't have that much time.

Mark

[Danila-master]

Sorry for confusing you with values.
Here is my sim file.
https://drive.google.com/file/d/0B7A...ew?usp=sharing
I didn't manage to upload it directly to this forum (

LDR4 is opened too. Accordingly to a switching table it should be open on a clean channel.

[MarkusBass]

There are several problems with the simulation. You used a library, which you did not included in the download (dmtriodep.inc). When I added some old version of the library that I have on my disk, LTSpice complains about mismatch of triode's pins. So I had to replace all triodes with general triode symbol and another model. You should always include models that are non-standard. And triode model in non-standard.
Other problems/mistakes in the simulation:
- the input resistor in your simulation has "1m" value. This is 1 milliohm. If you need 1 Mega Ohm, it should be marked as "1Meg". With 1m resistor I get 80A current through this resistor, which is obviously completely wrong.
- the bright capacitor (at least on the schematic I have) is 1nF and not 82pF as you have.
- you replaced the MID pot with one 12.5k resistor but there should be two resistors 12.5k (the pot is 25k).
- the BASS pot should be replaced with 500k resistor (if you want to have a half of 1Meg).
- it is very important to have pots with correct tapers (instead of two resistors). This is especially visible with GAIN pot, which has audio taper. You replaced it with 850k and 150k resistors and you say that that this is 15% of the pot. No, it's not. With audio taper this is almost 50% (pot is on 12). I suggest that you replace all resistors back with pots. I may try to do it tomorrow.

The reason why the cathode follower is clipping is still not clear for me - I have to read about it. I just know that cathode follower can be used to control clipping in the amp (contrary to emitter follower).
I suggest that you find another Dual Rectifier (or the amp's demo) and check it. Maybe this is by design. Remember that this is high gain amp and the clean channel in this amp is - nobody knows why. Most probably you can decrease gain and clipping but this will influence also the other channel (and this will be bad).
Anyway, you need to fix the problems with the simulation first. And the most important is audio GAIN pot.

Mark

[Danila-master]

- the input resistor in your simulation has "1m" value. This is 1 milliohm. If you need 1 Mega Ohm, it should be marked as "1Meg". With 1m resistor I get 80A current through this resistor, which is obviously completely wrong.
- the bright capacitor (at least on the schematic I have) is 1nF and not 82pF as you have.

You are completely right!!! It is my mistake....

- it is very important to have pots with correct tapers (instead of two resistors). This is especially visible with GAIN pot, which has audio taper. You replaced it with 850k and 150k resistors and you say that that this is 15% of the pot. No, it's not. With audio taper this is almost 50% (pot is on 12). I suggest that you replace all resistors back with pots. I may try to do it tomorrow.

I used regular resistors because I don't have a pot SPICE Model (

I measured a brand-new alpha A1M pot and got the following:
At 9 O'Clock - 20KOhm
At 12 O'Clock - 140KOhm

- the BASS pot should be replaced with 500k resistor (if you want to have a half of 1Meg).

Will set it to 140 KOhm to be at 50% + 12.5KOhm (a half of the MID Pot)

- you replaced the MID pot with one 12.5k resistor but there should be two resistors 12.5k (the pot is 25k).

This is like 50% of a linear pot.

I suggest that you find another Dual Rectifier (or the amp's demo) and check it.

There is one more rectifier in my town, but it is being used very often and it is hard to get a time window to bring it at home and to compare amps. 10 days earlier I discussed with its owner.
Maybe this Sunday I will get it.


Thank you so much for your help !

P.S.
I'm newbie with LTSpice, I prefer using MultiSim for transistor circuits, but is bad for tube simulation.
Tomorrow I will fix my simulation and will upload both schematic and SPICE Model files. There is no sense to waist your time. Thank you again!

[g1]

I have no idea about the simulations, but do have some practical questions that may get you some where.
Is this issue something that has changed? Was he able to get a good clean sound with this amp before?
A picture of the control settings would be helpful.
Is the channel master set to full? This is the most important thing for a clean sound, high master setting, lowest possible gain setting.

[MarkusBass]

Is this issue something that has changed? Was he able to get a good clean sound with this amp before?
.....
Is the channel master set to full? This is the most important thing for a clean sound, high master setting, lowest possible gain setting.

These are very important questions. If I were to guess, I'd say that your friend just bought the amp and he does not know how to use it for clean sound (but maybe I'm wrong). In the name of the amp there is "solo head", which means (at least for me) that this is high gain amp. To get clean sound from this head you need to set MASTER pot to 100% and GAIN pot to no more than 40-50%.

Regarding your simulation: there are still mistakes in it. In the power supply you have 100 Ohms and 2x22k resistors. But if you look at the schematic, you will find that there should be 2.7k, 22k and 15k resistors.
If you correct your simulation to have audio tapper GAIN pot, you will get the following results:

This is the cathode follower output signal with 75mV (amplitude) input signal and the GAIN pot set to 50%. You can see that this is not a perfect sinusoid but it is not distorted as in your simulation.

The reason why it is not a perfect sinusoid is related to 220k resistor in the preceding stage. I think that this is done on purpose so I wouldn't change it. But just for a test you could solder another 220k resistor in parallel to 220k resistor (to get 110k) as see what gain and distortion you get.
Here is what happens when you change gradually the resistor from 100k to 220k. With 100k you get a perfect signal (the green trace at the top). When you increase the resistor, the gain also increases and the distortion as well. In my opinion this is done on purpose (because this is high gain amp). In general, the stage preceding the cathode follower is designed to maximise the distortion and not to minimise it (as you friend would expect). And I'm not surprised with such a design.


PS: if you allow in your profile sending emails to you, I can send you the simulation (with all corrections). I did not change the tone stack (resistors to pots) because the problem appears on the cathode follower.
Maybe Merlin could chime in and explain the reason for the 220k resistor (better than I did).

EDIT: this should solve your problems with the simulation but maybe not with the real amp. If it distorts very early I would check the pot settings (GAIN 40% and MASTER 100%), test it with a guitar with singles and not humbackers, verify that the GAIN pot is original and it has audio tapper. Actually, if you think for a while, all the problems that you have could be a result of replacing the GAIN pot with linear pot. So verify whether it is original.

Mark

[Shemham]

For some reason 2-channel Rectifier schematics show 1Meg Gain pot value when the actual pot is ~250Kohm. This has been verified by users measuring the gain pot. This will also distort the simulation result.

[Danila-master]

For some reason 2-channel Rectifier schematics show 1Meg Gain pot value when the actual pot is ~250Kohm. This has been verified by users measuring the gain pot. This will also distort the simulation result.

You are right! I forgot to mention this fact.
I measured Orange Gain Pot and it was 250K, I thought that someone changed this pot and replaced it with a new Audio taper 1M Pot. but, even when i put a 200K resistor in parallel with a gain pot - it doesn't resolve this issue.
Anyway - I will put original pot back and will re-check AC values.

We tried to put gain at a minimal possible value and master at 100% and it still distorts peaks.

P.S. - Already updated my profile.


I will be at home after a 3-4 hours. Will put 250K Gain pot back and will re-check it.

Thank you all for your help! You are great!

[MarkusBass]

You are right! I forgot to mention this fact.
I measured Orange Gain Pot and it was 250K, I thought that someone changed this pot and replaced it with a new Audio taper 1M Pot. but, even when i put a 200K resistor in parallel with a gain pot - it doesn't resolve this issue.

I wonder how could you forget about such important (for the gain of the channel) change. This changes a lot.
Here is how the signal on the cathode follower looks like with 250k pot, pot set to 50% and 75 mV amplitude input signal:



My suggestion is to change back the pot and verify signal levels in the amp with the simulation. If they don't match, find out why.

Mark

[Danila-master]

Sorry for that fact.
This amp was repaired some years ago by other technician from other country.
There are some potentiometers which have been changed (with correct values).
I saw a 1M gain pot in schematic and thought that the 250K was a workaround to reduce the preamp gain.
This is why I replaced this pot (I did'n knew that the schematic contains errors).
Indeed I will replace it.

I got good news - tomorrow morning I will have other rectifier. 100% working.

So I will be able to check preamps side by side.