Operating points can shift radically in tube amps, especially in simple ones, and excess distortion often results. Triodes are more susceptible than pentodes, but pentode screen voltage stability is crucial. If it was practical to make tube amps as complex as many SS amps, then most of these unwanted effects could be eliminated. In my own work, I find SS devices extremely useful in a supporting role, to optimize tube circuit performance without losing the sonic delicacy of tube amplification. Constant current sources, for instance, can improve power supply rejection ratio, increase gain, and reduce distortion all at once. You can make CCSs with tubes, but what if you need six of 'em in a chassis that's already crowded, along with extra heater current and an extra DC power supply rail?
SS amps behave that way because all contemporary SS amps include large amounts of negative feedback - which keeps the amp's performance (including voltage gain) almost constant as long as there is adequate loop gain, even if the actual open-loop gain of the circuit does fall with supply voltage.
Most tube guitar amps are really, really simple designs. Lowering the B+ voltage will lower anode voltages, screen voltages (for pentodes and beam tetrodes), and cathode currents. Usually all those things contribute to reduced voltage gain.
Very simple solid-state circuits behave that way, too: the voltage gain of a single common-emitter transistor stage is almost directly proportional to collector current - which will usually track supply voltage if a simple resistive biasing scheme is used.
As far as the OPs question, I'm with Shoog and Bob Richards. I have a Fender Princeton Reverb guitar amp with a valve (tube) rectifier - because it's a reissue model, and Fender tried to satisfy their vintage-loving customers. But I would never go out of my way to use a tube rectifier in anything, any more than I would be desperate to put spoked wooden cart-wheels on my car.
The unspoken bit: valves, used for amplification, tend to have substantial technical imperfections, which often turn out to be beneficial to electric guitar sound for many genres of music.
The rectifier? It just turns AC into DC. Tube rectifiers have substantial technical imperfections too, but the only one which matters to *some* guitarists is their excessive resistance: easily replaced by a cheaper, cooler, more reliable power resistor, as Bob Richards explained.
-Gnobuddy
I am actually glad that many people think valve rectifiers are obsolete. At least their price will remain low. Thanks!😀
Who cares? It's not the point. Hifi is not guitar amps and using SS to filter with RC networks will waste power anyway. Depends on the application. Size matters!😀
Silicone tubing is pretty cheap. I bought 6' of 3/8"id tubing to use in my brewery and it was less than $15 including shipping.😀
So I see tube amps driven to max can become compressors. Any time I've used compressors the attack and decay times have a major affect (as do threshold and hold time)on the outcome. And setting them wrong can ruin the music. What are these in a tube amp? I doubt you guys know. And the Chances are that even one of these is randomly set correctly are very slim. Again people take a real phenomenon and barely understanding it, then try to use it to convince people that there is an improvement. Bad compression is worse then no compression.
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A tube amp is more like a soft clipper than a compressor.
I s'pose a valve-rectified B+ getting hauled down can in theory change the gain of a tube amp with no NFB, providing a bit of subtle gain adjustment, but I wouldn't call an overdriven tube amp a compressor by definition.
I agree with you entirely in principle, but you're forgetting one important thing: back when these amps were new, musicians created new music using these amps.
Good musicians always find ways to use the weaknesses and defects in their equipment to their advantage. Any good guitarist with an amp with built in compression/expansion will tinker with it, altering her playing technique, until sooner or later she finds something that sounds good, and takes advantage of the flaws in the amp. And that's what will end up on the record.
Years later, other young people are still listening to those records, and want to play the same music. And they find that using the same sort of faulty amp allows them to re-create the original sounds.
So what happens is that these sorts of saggy/squeezy amps become well suited to one particular genre, or one style of music. Often, they become much worse choices for some other types of music.
And that's exactly what I notice: blues players, playing long sustained notes, and striving to put expressiveness into those notes, may like saggy valve amps that compress their guitar signal. But you won't find a lot of very technical players using these sorts of amps; shred and prog rock players want an amp that responds to their lightning-fast picking, not an amp that is slow and saggy and takes a couple of hundred milliseconds to recover from the previous note.
There is plenty of that phenomenon going on, no doubt about it. The world of electric guitar is as full of utter nonsense as the world of audiophiles.
In this particular case, though, the evidence is that there is some truth to the claim. But it's only applicable to some players and some genres of music; having a guitar amp with a soggy, squeezy response is not every guitarists cup of tea, it's not a universal improvement, it's just a niche thing that suits some players and some types of music.
-Gnobuddy
and filter caps as well, recycling is the way to save mucho dinero....
just test them with an esr meter....
I've got a not-so-big honkin' 5V4 on my RH84 (I'm not going any further than that) I threw together about 6 years ago. I haven't swapped it for a SS rectifier because (1) I'm too lazy, (B) I'm trying to get my workbench sorted out after a major move, and (III) it looks so cool fulfilling its space-heating duties next to those smaller honkin' EL84s and the one wee 12AT7. Truth be told, I'm like AJT; I smoke 'em if I've got 'em, but I don't wring my hands with worry about which is "better".
Otherwise, I began reserving the 'R' word for my doctor after turning 45 (hey, we all knew it was coming sooner or later 😀). Now where did I put my popcorn...
Otherwise, I began reserving the 'R' word for my doctor after turning 45 (hey, we all knew it was coming sooner or later 😀). Now where did I put my popcorn...
With the RH84 being single ended, and thus class A, rectifier resistance is moot.
I virtually never see vintage high-voltage equipment around here, so no chance of salvaging old high-voltage caps from them.
But a few years ago I discovered that some surplus electronics vendors now sell electrolytic capacitors taken from (used) disposable cameras. These are caps that served as the energy storage in the camera flash circuitry.
Because of their original application, voltage ratings are reasonably high, and ESR is low. Also capacitance is really large - more than ten times larger - compared to the stuff vintage guitar amp designs use. For example, 270 uF, 330 V caps.
I now use these in my valve guitar amps (with solid-state rectifiers, or suitable series resistors to limit peak currents if necessary).
A single cap of this size can do a surprisingly good job of filtering the B+ rail for a preamp, for example. The reactance of a 270uF cap is only about 10 ohms at 60 Hz, so if you have a 10k series resistor in the B+ line followed by one of these caps, you can expect 60 dB (one thousand times) ripple attenuation at 60 Hz, 66 dB at 120 Hz, 70 dB at 180 Hz!
If you had a whole volt of 60Hz ripple before the 10k resistor, it will be reduced to a millivolt across the 270uF cap!
The 330 volts rating may not be enough for every valve application, but there's a work-around there, too: use a lower voltage transformer, and a voltage doubler circuit that only puts half the B+ on each filter cap. Now those 330V caps are good enough for anything up to 660 volt power supplies - terrifyingly high, higher than any sane amateur should ever try to work with.
-Gnobuddy
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