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Discussion in 'Amp Tech Center' started by Twanginator, Sep 25, 2013.
I got this one... Before the output transistors are changed they sound poopy?
You're not the amp tech in question are you?
iv been reading up on t his, and i too came back to best reason to upgrade is because they will last longer and theorfor in theory save you money and possible damage? anyhow, i picked up a orange micro tiny terror head/speaker. thought about changing the tube in it. just because i dunno how long the chinese one will last. and i got a good deal on a NOS tube. however, i am confused about one thing.
what do they mean when they say, long plates, short plates? does this require a special socket? that has stop be from buying a couple of tubes as i did not know what it ment. thanks.
hey munchx hope all is well. im doing alright after my brain surgery. vision is slowly getting not so blurry. hope my vision comes back to 100% soon. this blurry vision/cant tell shades of color. either black or white to me.. could not pick out a hot pink bikini if my life depended on it. laughs.
In my limited experince playing with tubes, I have noticed that to my ears, any amp with more than about 6 pots involved in the circuit will demonstrate little variation with tube swapping. When I was tube rolling my Epi vavle Junior, I spent hours playing. Every time I'd find a great preamp tube I'd start swapping the EL84, and then do the preamp tubes again, and there were lots of variations in tone to be found. (I ended up with an RCA 12AY7 and a TungSol 6BQ5 from the 60s...awesome) My Excelsior, my 18-watters, all similar experiences. My Rivera, not so much. It did sound better with a new V1&V2 and power tubes, but the differences were much less pronunced, and I didn't know how old the tubes I replaced were.
And if you're playing metal with the gain at 13 and the MV at 1.3, then I wouldn't bother shopping for NOS tubes. Any reliable NP should be fine.
No, you are reducing the signal into the pre-amp, they are not clipping or clipping less. The power valves have a very high input signal level, it is difficult to make them clip, everything has to be at 11.
What you can get with high power on a loud note or power chord is sag where the rectifier cannot provide enough current and so drops the supply rail HT.
Interestingly perhaps, the British supply is usually 10-15VAC higher than the amp design (it measures as almost 250VAC), and so we don't get the same sag. Amps tend to be spec'd at 230-240VAC.
The guitar vol knob is a very good, excellent way to control the amp, you can spool up to hold and sustain a note and make it sing.
There is a lot of variation in the EL84 between makes and models, you have noticed that.
With the pre-amp bottles the variation is more subtle. There are bad bottles, some very bad as to useless.
You can get pre-amp bottles more suited at particular positions. A low-noise for V1. A high-gain there may also work, but note a high-gain on test doesn't always produce a good clear musical note. There are other better ways to produce an over-driven sound,, or to clean it up, such as playing with the bias or cathode-capacitor on the pre-valve.
Dropping from a 12AX7 to a 12AY7 always sounds totally bland to me, no sparkle, like a beer that has gone flat and stale.
That is truly the case, because speakers have thin paper cones which become 'excited' by the signal driving them... a bit like a kazoo does. This causes the cone to generate harmonics which DO NOT come from the amp electronics.
There are defects deliberately designed into guitar speakers, which Celestion's Research Director, Ian White, calls "Beneficial Defects"
NO amount of tube/valve swaps can come close to these effects!
This is common knowledge in the professional guitar amp design world.
Not really, this is because transistors don't suffer the same 'mechanical' hang-ups as valves. That is not to say they are bad, that is not true. Many have a bad image of transistors and that's largely based on false claims and a total lack of understanding. Transistors need to be biased... they run at up to 120°... and they need negative feedback. They're not that much different to valves broadly speaking.
Valves 'mainly' act as simple EQs on the amp's tone - transistor don't. But this can be replicated in solid state with ease by adding caps to ground in the signal path or with negative feedback; but most manufacturers don't think of it - as they design SS amps mainly for young kids who want a shredder!
There is one UK maker of SS 'old school' toned amps... but I can't talk about them.
Sorry, what? You mean someone actually designs those SS kiddies bedroom blasters?
I'd like to meet whoever does that to gently wring their neck (metaphorically speaking) because that is one of the things that put my nephew off learning to play guitar. They bought the poor lad one of those wretched beginners kits without asking me, when I could have and would have sourced a reasonably decent guitar and amp for less dosh.
In fact I fixed up that Aria 'strat' for him but by then he'd packed up.
Negative feedback sounds right, so bad he didn't want to play it
As to the OP, I have a 5E3 clone I built and a friend gave me a new set of GT 6v6's.
I put them in a couple of nights ago and had the worst sound ever. Put in a set of
RCA's I had laying around and the sound was fantastic. Lesson learned. Never use
GT's in a 5E3.
Gave our rhythm guitarist one of these many years ago going into a pair of Celestion 12's. Sound was aggressive in a good way. Can't remember what we used for a preamp. Our other guitarist was jealous of his tone, had a Twin Reverb that he could never get loud enough to get into the sweet spot.
No global NFB. I attribute that partly to why it sounded good. Regularly had to replace blown output transistors though.
My tech says the same thing. He strongly advised me to swap out the 6V6s in my DR to 6L6s with the solid state rectifier and whole shebang.
I was apprehensive initially, but he told me that it would be way more economical and sound no different. I was still hesitant but in the end he told me that he'd do it for the same price as a standard re-valve job and and if I didn't like it, he would reverse it and buy the 6L6s off me. This guy is very well respected, he's the best around and does work for all the local retailers as well as a lot of pro bands. However, he is not a musician. He's just an engineer and he measures an amp's performance with an oscilloscope rather than his ears.
Long story short, the 6L6s are still in there and I haven't had it revalved in about a year and counting. I believe him.
different tubes make it react differently IMHOE,
which may or may not make an audiable distinguishable difference
( I can hear subtle differences FWIW )
The tone stack needs to be modified to change the tone
( read:sound characteristics) with just a tube change
You cant make a twin sound like a mahshall
Speakers make the most immediately noticeable change
in my humble opinionated experience, but again
you wont get a marshall from a Twin
And he's correct. That speaker is the "voice" of the amp not the tubes. They simply amplify the signal from the pickups which are the "voice" of the guitar.
IMHO the type of electronic components used send the signal along and amplify it but do little if anything to change it's basic tonality. If found that when I replaced the stock 12ax7 in my Bugera v5 with a Tung-Sol the amp ran quieter but the sound was the same. But when I swapped the stock speaker out for an Eminence 820H it became a whole new amp. Bigger sounding, less boxy and more open and a bottom end that simply didn't exist with the stock speaker.
This is an interesting thread. What's most interesting is how the divergent opinions about exactly what has the greatest impact on tone seems to have obscured the fact that most of us are tone junkies. If anything (speaker swap, tubes, or circuit mods) alters our tone, we'll probably (eventually) be chasing it.
I know that in pro audio, we spend ungodly amounts in order to squeeze out that final .01% that separates one piece of gear from another. In some cases, I wonder if even dogs can hear the difference. Still, we do it. It's no different here.
On a guitar amp, when I swap a tube, a speaker, or do a circuit mod, that difference is definitely noticeable. It may be subtle, it may not, but I can hear it. Ultimately, it's a question of what I like. If I do, I keep it, even if it's just a smidge. Smidges add up.
This forum provides a great deal of specific, useful info. What a resource!
If he has been doing this for decade consider the fact he may have done tube swaps for dozens of customers wanting to change their tone. I'm betting almost all of them come back some time later still not happy with their tone.
No emitter resistors whatsoever on the output devices, and no adjustability on the bias voltage to them either.
Somehow I'm not surprised those unfortunate output transistors fried frequently. Thermal runaway looks like an ever-present danger with this amp. I'm guessing it was designed in the early days of solid-state design, before someone invented the Vbe multiplier bias circuit that got rid of most of the difficulty in biasing output transistors in a class B push-pull complimentary output stage.
Actually there is plenty of global negative feedback in that circuit, by the way. Look at R2 and R1 - they feed back a portion of the output signal to the base of Q1, which is the input for the amp. In other words, this is the classic op-amp inverting amplifier layout, and gain with feedback is nominally (-R2/R1) or -10. Providing the preamp has very low output resistance, that is.
A good hefty dose of global negative feedback is a wonderful thing in a Hi-Fi amp, well proven as the best way to get almost perfect, distortion-free performance as long as you start with a decent circuit design. Trouble is, things go sour very rapidly if you ever overdrive an amp with a lot of global NFB - and that's the way many electric guitar amps routinely get used, so lots of NFB is not such a great idea for those sorts of guitar amp.
Unless it's an acoustic guitar amp, that is: I have no doubt whatsoever there is a ton of global negative feedback in my Acoustic AG-30 acoustic guitar amp, which is essentially a little powered wedge monitor speaker with a few onboard audio FX.
The trajectories of individual electrons don't matter, for the same reason they don't matter inside a copper wire - there are so many of them! Remember, it takes of order a thousand trillion electrons per second to make up even a tiny current of just one milliampere!
Tubes do affect audio "tone", because the transfer function from grid to plate is non-linear. Put an audio signal through a non-linear device and you generate harmonic and intermodulation distortion - the output contains frequencies that were not present in the input signal. Depending on the amount and exact nature of the nonlinearity, your ears will detect anything from the very slight "warming" or "shimmer" of an input triode to the harsh, buzzy distortion of a heavily overdriven amp.
So yes, tubes do affect the tone of sound coming through, to some degree. Otherwise we'd all chuck those expensive, hot, fragile glass bottles in the recycling bin and start using MOSFET's instead!
The issue here is a subtler one - does the transfer function of one brand of 12AX7, say, differ from the transfer function of another?
If the cathode and anode ("plate" in American) were infinitely large parallel metal plates, the answer would be "No!" All triodes would sound the same.
In practice the plates are neither parallel nor infinitely large. That means the exact shape of the electric field between the plates can vary a bit from one brand to another. That in turn means the transfer function from grid to plate can vary a little, particularly in the case of those valves that have vastly different electrode geometries. (Look at the photo of a bunch of supposedly identical tubes someone posted.)
How much does it matter? I would listen to the voices of reason and experience on this thread, i.e. muchxs and Bill M.
There are three primary parameters for a triode, mu, Rp, and gm. All are affected by the distances between the electrodes, and to a lesser extent, by the shape of the electrodes.
It all comes down to Maxwell's equations, which tell you what fields and potentials and currents you'd get, given the electrode shapes and positions and voltages. These days with the right software it would be quite easy to use your PC to calculate the exact electrical behaviour of any tube, given accurate measurements of all its electrode shapes and positions.
The amazing part is that most if not all of the tubes we guitar players use were designed and built before the days of electronic computers. Some very smart people working with some very crude methods still managed to come up with all the electrode designs we've come to know and love, even the remarkably sophisticated designs for beam power tetrodes such as the 6L6 and 6V6.
There were even more sophisticated beam tetrode designs later, used mainly in high power RF, and mostly unknown to us guitar players. And after that, apparently a final generation of extraordinarily good, very high power triodes - also used in high power RF transmitters. Triodes so good that these final versions of the technology, derived from the earliest and most primitive type amplifying tubes, ended up outperforming the best beam tetrodes that preceded them (the triodes had much lower distortion).
From what I've read, those final generations of tubes were in fact designed using computers - many of them also tube-based, though some semiconductor-based computers may have been used for the last few iterations of the last few tube designs.
Tubes have minor variations due to manufacturing tolerances. If you swap one 12AX7 for another, you may have ended up replacing a tube with a mu of 110 with another tube with a mu of only 90.
So you'll end up with a few decibels less voltage gain, a little less harmonic and intermodulation distortion, and maybe a slightly different frequency response and/or output resistance.
Some of these things may be enough to be audible. But, as Bill M. points out, they don't change the sound any differently than a tiny adjustment to the gain, volume, or tone controls would have.
Bill M. is perhaps too much of a gentleman to point out the following, so I will: using a guitar cable that is a few feet longer, moving your guitar amp a foot closer to a wall, walking two feet to one side or the other, or turning down the volume or tone on your guitar a smidge will ALL change the sound of the amp as much or more than that expensive NOS tube swap, or zero-cost twitch to the control knobs.
So IMO it would be completely misguided to draw conclusions from any such change in tone such as "Tung Sol's are more musical". If anything, what you've found is merely "this particular Tun Sol has 2 dB less voltage gain in this circuit than my previous tube had, and I like that."
It may be true that some NOS tubes are more durable - I don't have the experience to know for myself. So those who are swapping to NOS for durability's sake may have a point.
For the rest of us, it's probably mainly a little bit of completely irrational fun with no logical basis whatsoever, but hey, life is full of those!
Actually thermionic emission - the process where some electrons in the cathode gain enough thermal energy to escape from the cathode material and fly into the space near the electrode - is, from a physics point of view, very much like boiling, in which some molecules gain enough thermal energy to break out of the liquid they're in and fly into the air above the liquid.
In both cases there is an energy barrier to be overcome in order for the electron or molecule to escape, and in both cases the energy necessary to overcome the barrier comes from heat. The essential physics - and the math describing both phenomena - is very similar.
So you often find that particular sentence - "electrons boiling off the cathode" - or something very similar, in books on vacuum tube theory.