Mesa Boogie Dual Rect Rebuild

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CJ

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when you get stuck like that, you have to go back to basics, breadboard the tubes with an external pwr supply and do a plot of bias v vs plate current. and maybe play with the screen voltage,
 

abbey road d enfer

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one thing he did not do was change the bias voltage or lower the B+.
Yes, this guy is running around in circles, when the problem is staring at his nose.
he might have got 5K:8 or 5.2K:8 instead of 5.8K:8.
He could have looked at the datasheets and found that a good compromise between EL34 and 6L6 at 450V B+ is between 5k and 5.6k.
Precision doesn't matter much since the speaker's impedance may vary from 6 to 50 ohms across the frequency range.
 

Tubetec

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Ive seen bias runaway issues with JJ EL34's , its easy to fall into thinking its a circuit related issue but you'll only end up tearing out whats left of your hair trying to find an non existant needle in a haystack .
Id never figured out it was the tubes were the issue at the time , but when I went back to look at the amp again (from my own collection) a few years later one of said offending JJ's the vaccum had broken down completely and the getter had turned to snow . Seems like poor vaccum and gas build up within the tube caused it to draw heavy grid current , which pulled down the bias volts on all the rest of the power tubes .

Mil spec 5881's are my prefered choice for Fender and Boogie 6L6 based amps , one issue though is the short base found on the Russian version isnt held in place by the tube retainer ,thats only an issue with amps with the chassis at the top where the tubes go in base upwards . They are available with the regular style base also though for a bit more money than the Russian version .
 

Rybow

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I played a Mesa Rectoverb 50 series 2 combo for years. Loved the sound. Always thought Mesa was top of the line in terms of quality.
Now I find myself having an existential crisis as my whole world has gone up in flames 😢
 

Matador

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They are great, right up until the point that they aren't. 😁. The intentionally obfuscated/hidden schematics make them even harder to repair.

Say what you will about Peavey, whereas many of their amps are built in a similar fashion that make servicing them difficult (the Classic 30 has a 3-dimensional PCB mounted at right angles with zillions of tiny wire jumpers, and the whole mess has to be removed in order to service it), at least there were clear published schematics so you could figure out where to start.

To add insult to injury Mesa also epoxy potted a bunch of their "magical" EQ circuits, which tended to run around to other sections of the PCB. 🤬
 

CJ

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Vacuum Tube Valley

The EL34 Power Tube

'Originally designed for movie sound amplifiers.
Spec sheet initially had the Max Anode at 800 Volts!
Jim Marshall selected it for the JTM45 because it was cheaper than the US made 6L6.
Pronounced distortion characteristics. Distinctive sound."


 
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CJ

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columns are # of tubes used for test, max watts before clipping and percent distortion.
 

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Tubetec

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Never tried the 7581A , the current Tung sol version looks like a nicely made tube though , I did see a couple of older Ampegs over the years that used 7027A ,another solid performer .

Legend has it the very first Marshall Plexi 100W amps had well above 500 volts at the anode , Ive seen one , nearly bought one ,but never worked on one of that vintage .
 

CJ

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Radio Spares, Partridge, need to hack those OPT's.

i thought the 7591 would sound clean compared to the EL34's, but it looks like it has a higher distortion figure. Maybe Ampeg designed their amps to keep distortion down.
 

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CJ

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got a 10K bias pot in there, everything seems to be alright,

re-checked the tubes, installed, one was drawing too much grid current which was dragging the bias down on that side thru the 330 k resistor, changed the tube and biased cold, everything stable,

bias pot shown at blu arrow>
 

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CJ

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schematic for pot install,

this amp was missing one of the 3.3 K resistors marked "unit domestic" which might have been the reason for all the tube eating. leaving that resistor out would increase late current.

think we are running about minus 41 bias and 460 late V with solid rect.
 

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Tubetec

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Making the pot accessible without pulling the chassis is handy for the amp tech , but will the end user be able to resist the temptation to 'tweak' things is a possible downside too . Still if the amp is working and you have the trust of the customer shouldnt be an issue .
Nikoli Tesla must be turning in his grave at the shocking quality of the tubes they turn out under his name these days , poor ould divil got a raw deal .
 

CJ

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as luck would have it, the pot is tuned up full blast and we only have 60% plate disp, so the only thing the customer can do is make it run cooler.

i am not going to tell him it is there, and i put a blob of industrial strength hot glue on there,

i just watched Tesla today, he got a job offer with a 50,000 dollar signing bonus from Edison .
when he got over here Edison said 'April Fool's! that's just the way we do bidness in the US.'

so Tesla went over to Westinghouse who treated his employee's very well.
 

Matador

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I've had good luck installing heater elevation circuits (Mesa's included), since the screen supply is generally along the south edge of the PCB's so it can jog over to the output section. Two resistors and a 100uF/100V cap from the screen supply, sent over to the center tap of the heater secondary, lifting up (or cutting) the center tap and making the connection there. 50V seems to work well.

At least for Mesa's it brings the overall noise down close to what I've seen when I've experimented with 6.3V battery supplies when chasing down hum in high-gain guitar amps.

Anyone else does such retrofits in Mesa's (or otherwise)?
 

okgb

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I have a mesa 20/20 power amp [ el 84 ] I'll look for the schematics and see if you guys think it could use some precautionary work. I like it but the fan is pretty noisy. I hardly gig so it's not often ran hard for long.

Ok attached the schemo, I see no reason to parallel a 47k and 10k resister to ground off of the first grid, does that seem low anyway? makes me wonder about the rest of the schematic
 

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Tubetec

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Less than 10k ohms is low for a guitar amp , then again they probably expect the power amp to be driven with solid state gear ahead of it . You could change it up to 1M if you wanted to connect a normal anode load 12ax7 stage ahead of it , you will get a little extra hum from the AC heater supply into the grid also.
The 220k ohms in series with the grid of the power tubes looks a bit odd too .

There are a few old Fender cathode biased amps connect the heater centre tap to the power amp cathodes and across the cathode resistor to ground , not an option on a fixed bias amp like the 20/20 .
 

Tubetec

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Other interesting things in the 2020 also , different arrangement around the phase invertor , no usual LTP resistor , how cant the second grid where feedback is applied not have a dc connection to ground ?
 

okgb

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Less than 10k ohms is low for a guitar amp , then again they probably expect the power amp to be driven with solid state gear ahead of it . You could change it up to 1M if you wanted to connect a normal anode load 12ax7 stage ahead of it , you will get a little extra hum from the AC heater supply into the grid also.
The 220k ohms in series with the grid of the power tubes looks a bit odd too .

There are a few old Fender cathode biased amps connect the heater centre tap to the power amp cathodes and across the cathode resistor to ground , not an option on a fixed bias amp like the 20/20 .
I believe Mesa expected it to be driven with one of their tube pres like the formula, still no reason for two resisters, low to cut the signal?
 
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