Matchless Lightning inspired build

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#82
I was expecting this to be closer to the results we were looking for a gain of 40 volts. Also, note how your cathode voltage went up by about 1.6 volts due to the increased cathode current? That is what should have happened since the tubes are conducting harder. Raising screen voltage on pentodes has much greater influence than the same increase in plate voltage. Just a thought, you could make this switchable putting a mini toggle out of the way somewhere on the back once you get the chassis into the cab.
 
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#83
Take 10.6 v divided by 150 ohms(cathode volts and ohms), you get 70.6mA total cathode current. Divide that by two and you get 35.3mA per tube assuming a close match. This should be fine. The tube data sheets gives 65mA as max.full load for one tube, although I wouldn't push it to that extreme. The screen current at idle should be small enough to mostly ignore, maybe 4-7 mA but only at idle. Remember, cathode current is plate plus screen current, so as the tubes conduct harder, screen current becomes important. But for quiescent(zero signal) conditions, you should be fine. From what I have learned so far, it's the current that matters and the voltages determine how much current flows.
The next thing that would be useful is to get accustomed to data sheets and plate characteristic curves and knowing how to plot load lines for certain screen and plate voltages and how to find bias points. I know I need to get better at that as well!
 
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#84
Thanks boobtube. Makes sense.

I went through all the math as best I understand it and came up with the following:

Actual screen voltage ..... 266.40
Actual plate voltage ....... 340.90
Cathode current .............. 35.33
Screen current ................. 3.13
Plate current .................. 32.20
Screen dissipation ............ 0.83
Plate dissipation ............. 10.98
Watts output RMS .......... 15.53
 
#85
I just happened to think we have forgotten the most important questions of all: Can you hear a difference now that the screens are up and if so, do you like the sound? There is still a large difference in screen and plate voltages. Usually there is only a few volts difference, because the plate and screen connections are on both ends of the choke with no dropping resistor, but why do that if you like it how it is, right? Also, this way there is less stress on the power tubes and they should last longer.
 
#86
Yea, I really like it now. It's subtle but there's a bit more midrange grit to it which is very pleasant. I'm happy with the dissipation levels too; I think it's safe but not too conservative. I may try a lower value screen supply resistor at some point, but I'm in no hurry.

I also replaced the input resistors with some fancy audiophile 47k metal films I had lying around. It has killed the hiss and opened up some harmonic content on the sustain. It's stupidly quiet now - I keep wondering if I've accidentally turned it off!
 
#87
You have discovered one of the most common and most overlooked sources of noise in amps. This resistor obviously carries the signal that gets amplified the most, so if it generates noise, it is amplified the most too.Carbon comp in this position is usually a mistake and I wanted to say something at the beginning when I saw that you used CC resistors, but they don't ALWAYS contribute noise, so I didn't say anything. Another solution is to reduce the value to 15K or so. It still accomplishes what it should do as the function of grid stopper. So many amps still have the 68K as a carry-over from the early Fender days.
Good to hear that you like the difference without pushing those EL84s to the edge.
Remember also to keep tabs on those preamp voltages at the sockets as the amp settles in.
 
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#88
Indeed. Makes total sense.

I figure - as there's two input resistors effectively in parallel - I'm now running a "23.5k" input resistor, which by my calculations acts as a 44kHz low pass filter. A lower value is probably just asking for RF interference.
 
#90
I re-visited that section in Merlin B's book on preamps and because of Miller capacitance, the average input capacitance of a 12ax7 gain stage is roughly 100pF if the gain of the stage is 60. You multiply the 1.6pF given in the data sheet and multiply it by the gain of the stage +1. If you use that as the value for C in the formula, a 10K resistor will have a cutoff frequency of 159kHz. Radio interference below this isn't very common. Just curious what capacitance value did you use?
The input capacitance will be different in different situations because of different amounts of gain but regardless, you solved the problem of the most common source of hiss in an amp.
 
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#95
Audiofreak88,

I have a new power transformer for a Peavey Classic 100, which used 8 EL84, if you're interested and if I can find it after moving.

I have a new 65 watt 5K Edcor output transformer that was going to be overspec for a project with 4 EL84, if you're interested and if I can find it after moving.
 
#96
cyclecamper said:
Audiofreak88,

I have a new power transformer for a Peavey Classic 100, which used 8 EL84, if you're interested and if I can find it after moving.

I have a new 65 watt 5K Edcor output transformer that was going to be overspec for a project with 4 EL84, if you're interested and if I can find it after moving.
Click to expand...

Haven't tried any of the Edcor units wonder if it sounds better than this Magnetic Components 3.6k 50w Marshall Iron haha,
 
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#97
You can easily tell had it running Stone Cold with Bias/Current draw on the 6550s haha,
weird but I like that with them 6550s...that sluggish/slight delay/crossover more harmonic distortion haha, does present a lot of Non-Linearity kinda everywhere especially way up ton 3rd order junk there in the frequencies/transfer, they are Linear as hell anyways though... for them "Hairy" Lead runs/solos its too much fun that added phase/timing skewed response, throw some pitch shifter/modulator effects then its pretty Outer Space like haha
 
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#99
For sure though dude, try to get a hold of them old RCA/GE 6BQ5s only reason would want EL84/6BQ5 amp, great clean/chimey/snappy character....current production go JJ...Russian/Chinese are asking for trouble especially if you are going to try throw some pretty high Voltages on them...
 
#100
Forgot to update this thread as I've been enjoying playing the amp too much!

I designed a head and cab for it and had a guy build it from walnut. Really really happy with how it turned out. I'm currently running a Celestion Alnico Blue and it sounds fantastic.

Pic of it in use attached. I got so many positive comments that night about the looks and the sound! ... now onto the next project 😀
 

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