I do not doubt that you do a good job of servicing the amps.
But this is a question of design engineering (the thing I do a good job of); in this case, we ALWAYS need to do the calculation.
Oh, I just reinstalled the 8.2K missing resistors and removed the ugly Master Volume, and voilà ! If it works, don't fix it... 😉
The master volume mod - in lieu of one of the input jack :
The modified circuit :
The restored circuit to its original condition :
T
Sorry, I do not do much complicated calculations when I design and build mine...
https://guilhemamplification.jimdofree.com/
T
I'm talking about the input to the amp. And what is this term called PI. Is that some sort of short name for 'phase inverter'
Yes, exactly.
The R value is used to tailor the sound, as the amp hits clipping. With many players, this happens all the time.
Many early guitar amps were never intended to be driven to 11....or even 9. But in the bigger and louder world of late 50's and on music they were often abused.
Somewhere around 1960 my parents gave me an electric guitar for Christmas, but without an amp because the music shop guy told my mom that an unamplified electric would be quieter than an acoustic (true). Little did any of the people involved know what they had started. At first I used old radios for amps, then it was a Magnavox HiFi, but somewhere around age 10 I convinced a friend to take apart his Fender Champ so that I could trace out its schematic.
It took a while but I constructed my first true DIY guitar amp from that traced Fender 5C1 Champ. It worked pretty well considering that all of the parts came from dead TVs, radios and other discards. A year or two later I took apart a Vox Tone bender germanium fuzz box and traced its circuit. After building a copy with two transistors from a dead "2 transistor radio" that didn't survive a trip into the lake, I discovered something that I didn't understand.
When I "set the controls for the heart of the sun" (everything turned up full), one blast from the guitar would cause my DIY amp to mute (be unresponsive to any input) for a second or more. I got the same response from my friends real Champ, only his was worse. Something was wrong with my DIY fuzz box, except that it worked fine on other people's amps, and it worked fine on the Magnavox HiFi amp. By this time the old Maggie had become my guitar amp since it was much louder and had bigger speakers.
It would be over a year before I found out what was going on and how to fix the Champ. Look at the schematic for the 5C1 Champ. It was from a time before "fuzz boxes" or other such musical weaponry existed. The input pentode is "self biased" with a 5 MEG resistor! Yes, the 12 year old "dumm blonde one" had discovered blocking distortion on the INPUT stage of a guitar amp. Once discovered a 5 minute hack job turned that self bias stage into a cathode bias stage. A bit more experimentation revealed that the 2 meg screen resistor is also a source of long time constant recovery from overload. Neat trick from the dark ages. stick a 6SK7 tube into that 5C1 instead of the 6SJ7. It still sounds the same until you hammer it, at which point the distorted sound is not quite as nasty.
"Farting out" is a guitar player term that applies to the effects heard from the bleed off of charge from the coupling cap in the grid circuit of a tube due to grid current caused by overdrive. It is heard during the recovery of that charge which is usually restored through the grid to ground, or grid to bias circuit resistor.
Most modern "high gain" amps have the gain staging set so that the grids of the output tubes do not get hammered hard enough to cause extreme cases.
No, again - sorry, Guys : the small capacitors (OEM) values have been chosen to cut bass enough, in order to avoid muddy overdriven tone, notably on pick attacks. Keep the same 8.2K resistor, replace the 10nF it with 0.1µF and listen to the overdiven tone now...
Do you know how much clean power output the Marshall 1974X is able to deliver before clipping ?
T
... But you don't know how to calculate the value of these resistors, as you said yourself.
In any case, configuring the EQ is better done in the preamp section.
In any case, configuring the EQ is better done in the preamp section.
If the designer's intention was to carefully configure the overdrive sound through the time constants AND add bass rolloff AND save BoM cost with a smaller capacitor - all achieved in one capacitor and a carefully chosen set of resistor values...
... Those designers are smarter than you think!
... Those designers are smarter than you think!
The good designers are more in the "cut and try" method than you think, even by today, where everything seems to be predictable and simulable. There's not zillions of designs for good tube guitar amp : more genius ideas, more copying those, than calculations or simulations.
T
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Thank you sir. About three and decades later, I was stumped there for a second like I overlooked it in class back then.
I know you've been here for a while. Big fan. Do you show them our formulas at times for calculating things in the tube circuit?
Or does this site have a sticky somewhere. I'm just curious because of the responses.
Sure you have cut & try for voicing the amps; but Marshall and a few others designed amps 50-60 years ago that were tough and reliable, as well as well-voiced. Reliable designs require at least some understanding of real design work.
You only have to look at some of the disaster-amps of the 1990s to see what happens when basic design skills are lacking:
Ashdown Peacemaker. Overheated resistors, burned PCB, bulging cap, valves run WAY out of spec
Some players like the sound of them, but they are unreliable - No thanks!
Marshall were one of the first to embrace software modelling creating some of the first tube models for spice.
Now, away from the diversion into guitar amps - Back to the question of grid stoppers.
The famous GEC book "An approach to Audio Frequency Amplifier Design", written by staffers of the GEC and Marconi Osram Research Laboratories shows a number of worked out amplifier designs.
All those with RC coupled output valve grids have large-value grid resistors (usually 10k).
But check this one without RC coupling - no series resistor, no stopper:
It's hard to imagine anyone knowing better than these folks whether a stopper is needed - and here it is not.
OTOH, the RC-coupled amps are all at risk of blocking distortion, without the series resistor of 10-15k.
The famous GEC book "An approach to Audio Frequency Amplifier Design", written by staffers of the GEC and Marconi Osram Research Laboratories shows a number of worked out amplifier designs.
All those with RC coupled output valve grids have large-value grid resistors (usually 10k).
But check this one without RC coupling - no series resistor, no stopper:
It's hard to imagine anyone knowing better than these folks whether a stopper is needed - and here it is not.
OTOH, the RC-coupled amps are all at risk of blocking distortion, without the series resistor of 10-15k.