Доброго времени суток. Тут в интернет-пространстве напоролся на пару сайтов с темами про переборку схемы моего комбика, только вот в английском не силён и обозначения все эти не ясны. Вот тут есть два сайта:http://www.guitarnuts.com/amps/vc508/index.phphttp://www.thefret.net/showthread.php?t=9379
На первом более устаревшая модель этого комбика, но во втором помойму с современным проблемы совпадают. Конкретно волнует фраза op amp, not full tube. Кто нибудь сможет донести суть и может быть чего нибудь посоветовать.
Вот тут для тех, кому проблемно зайти на сайты непосредственно:
The Crate VC-508 guitar amplifier is a small "inexpensive" single-ended 5-Watt tube amplifier. It uses a single EL84 and a single 12AX7. It is not an "all-tube" amplifier, there is a TL072 op amp up front to provide the initial gain stages. This solid-state front end and the spatter-painted instead of Tolexed cab are obvious attempts to keep manufacturing costs to a minimum. However, that doesn't necessarily make them bad. The painted cab looks okay and is easy to touch up and the solid-state front end is actually a very good idea that I wish we'd see more of. Op-amps make sense in the first couple of gain stages where their low noise, freedom from filament hum, and low output impedance make them ideal for initially boosting the signal well above the noise floor and for driving lossy tone stacks.
That said, I frankly wasn't expecting much from the VC-508 because it seems very much to have been designed by bean counters. Why did I even buy one, you ask? Because I felt that the amp might be a good starting point for making a much better sounding tube amplifier at reasonable cost. If you look about, you can pick up a used VC-508 for less than the cost of parts to build an AX84 P1, for example. Recently, I've been experimenting with tube preamp circuits using a Spice simulator and those experiments seemed to indicate that almost all production amps these days are using preamp circuits that are actually biased for 12AT7 tubes, not 12AX7 tubes. I figured that a used VC-508 would be a cheap way to test my theory.
While I wasn't expecting much from the VC-508, I have to say that the reality was far worse than anything I'd imagined possible. I can honestly report that I have never heard a worse-sounding tube amp. The only way I can think to describe the overdrive is that it resembled the sound a table saw makes when it hits a nail. In fact, my Danelectro Nifty-Fifty™ solid state amp breaks up more smoothly than my VC-508 did. I imagine that it's possible that I got a bad sample, especialy since this amp was used (though like new, not a nick and still had the hang tags on it). However, I doubt it, because when I went through the amp with scope and voltmeter all of the readings were very close to the annotations on the factory schematic.
The oscilloscope quickly revealed the problem. Actually, to be honest the Spice simulations I ran before I even received the amp revealed the problem, the 'scope just confirmed it. The first tube stage was being driven into very harsh clipping well before the power tube was even getting close to compressing. Almost all tube amps using 12AX7s in the preamp are actually set up for 12AT7s. When you put a 12AX7 in the circuit the stage will overdrive at the top of the output waveform with very harsh clipping. Put a 12AT7 in that same circuit and the voltage level on the plate changes and nine times out of ten the overdrive will begin at the bottom of the waveform and be smoothly rounded instead of harshly clipped at the top of the waveform.
As I said, that's common to almost all 12AX7 preamps and one of the reasons that preamp overdrive has such a bad reputation. Take a circuit where no matter what combination of gain and volume you use the preamp is clipping before the power section starts overdriving and you might as well have a solid-state amp - and a bad one at that. That was exactly the case in the VC-508 I was working with.
The first thing I did was replace the 12AX7 with a 12AT7 and then test the amp again. (By-the-way, the tubes on this amp are hidden behind the chassis, between the chassis and the speaker baffle, you pretty much have to remove the chassis from the cab to change a tube!) Just this simple change made a drastic improvement in the sound (that's not unusual, many amps benefit from having one or more preamp tubes replaced with a 12AT7). Most of the painful "sawing-nails" sound was gone, but with the lower gain preamp tube it was now impossible to overdrive the output tube well because of the strange design of the "volume" control.
This amp has a line out, which is cleverly wired using a TRS jack such that if you use a 1/4" mono plug it is a line out and the power amp is muted, but you can wire a stereo plug for a complete effects loop, sending on the tip and receiving on the ring. Somebody made two fairly poor decisions regarding this jack, though. First, they decided that the volume control must affect the line out (instead of being content with the gain and tone controls), and second, they decided that the line out should be limited to an extremely weak signal, well below what is typical for a "line" level signal.
As a consequence of those design decisions, the "volume" pot is located between the two tube preamp stages, instead of between the last preamp stage and the power tube. This means that the relationship between the second tube stage of the preamp and the power amp can't be changed, severly limiting the tonal usefulness. Also, the "line out" really can't be used well with devices, such as a Digitech Studio S200, that expect a true line-level input. Fortunately, both problems are fairly easy to fix.
My second modification, which in all honesty I'd planned before I even received the amp, was to move the volume control from its oddball location to a more useful position between the last preamp stage and the power tube. Unfortunately, this amp uses those silly little plastic PCB mount pots and I couldn't find a 250k pot that would fit in place of the 10k pot. What I ended up doing was adding a full-sized pot to the bottom of the chassis, so it must be adjusted from the back of the amp. That's a minor inconvenience and it may have been for the best, as if I'd used the original pot location I'd have had to string wires carrying fairly high level signals over to the preamp section.
I found that I was able to make the modification without cutting any traces on the PCB. First, I lifted the ground leg of the existing volume pot and put a 56k resistor between it and ground. That made the existing volume pot pretty useless, but raised the available drive to the second tube stage of the preamp. Now, the second tube stage of the preamp will overdrive just slightly before the first tube stage, and they will both overdrive smoothly at the bottom of the output waveform instead of clipping harshly at the top (because we're using a 12AT7 tube).
Of course, this makes it absolutely essential to attenuate the signal between the last preamp stage and the power amp or we will be driving the power amp tube ridiculously hard. Again, by lifting a leg of the coupling capacitor and removing and relocating the power tube grid resistor I was able to wire in the new master volume pot without cutting any circuit traces.
The total parts list for this modification? A 12AT7 tube, a 56K 1/2W resistor, a 2.2K 1/2W resistor, a .022u PIO capacitor (because I had one I wanted to try, so I replaced the coupling cap, you may re-use the existing .047u poly cap), and a 250k audio taper potentiometer.
I can't honestly say that it's now the best sounding amp I've ever heard. However, the modified amp not only sounds better, it honestly sounds about as good as I think an amp can be made to sound through an inexpensive 8-inch speaker in a particle-board open-back cab (an extension cab with a better speaker is planned next). One can now get anything from clean through light crunch through heavy distortion out of either the preamp, or the power amp, or both. Even at much higher preamp distortion levels than I am inclined to use the tone remains quite warm. As presently modified it's a keeper, with a decent AlNico speaker I think it could become a "go to" amp.
location of new volume control
Location of New Full-Sized "Master Volume" Potentiometer
view of modified guts
A View of the Modified Guts.
another view of modified guts
Another View of the Modified Guts.
The large (1.4Mb) PDF schematic of the modification is available here. Be sure you read the large number of notes and warnings on the schematic!
Update - 4/15/2004
I ordered a Signature 8 ("Sig8") alnico speaker from Weber VST and installed it. This was the alnico speaker recommended by Ted Weber after I explained to him the sound I was looking for. I had been leaning towards the more expensive Vintage 8, but it turns out it has a much larger magnet and will not clear the tubes in the VC-508. The difference with the Weber speaker was noticeable with the master volume all the way up and the amp cranked to about 4 and better on the gain. The Weber was smoother while maintaining good definition and a tight (for an 8" speaker) bottom end. This is not to say that the Celestion ceramic speaker installed by the factory is not a good speaker. In fact, at very low ("baby sleeping in next room") settings of the master volume the difference between the speakers was subtle enough that I had to listen closely to an "AB" tape to hear the difference.
The Weber alnico speaker also offers another advantage – the magnet is narrow enough that it should be possible to mount the speaker to the back of the speaker baffle instead of the front (I didn't notice this until I had the speaker in, I'm probably going to change it soon). I've never cared much for speakers mounted to the front of a baffle with no cushioning between the steel basket and the baffle, as is done with the VC-508, because it's just asking for rattles to develop if the amp is played at high volumes a lot. Also, there is less support for the speaker cone rim than with the speaker "properly" mounted behind the baffle.
I also added some cooling holes to the rear cover. Almost all combos tend to trap heat and with its unusual placement of the tubes the VC-508 seems worse than most in that regard. Obviously, if you do this you need to make sure not to leave the amp unattended around small children who are given to inserting paper clips or what have you into the holes. While the amp still gets quite warm, as all combos do, it runs considerably cooler which could significantly prolong its life.
location of new volume control
A few minutes with the drill press gives cooler operation.
(Obviously, remove the cover before drilling it!)
By-the-way, if you need touch-up paint Krylon interior/exterior "semi-flat" black paint is a near-perfect match.
My next modification is going to be to reduce hum. This amp hums fairly loudly, even compared to other tube amps. To be fair, it's not very noticeable when you're playing. However, it becomes very noticeable as soon as you close mic it. SE amps are sometimes more prone to hum than PP amps because there is no way to cancel the filament hum in the output stage. However, this amp has more hum coming from the preamp stage and that is most likely because the heater filaments are not referenced to anything. Unfortunately, it won't be possible to reference the filament voltage to the output cathode because the filament supply also drives a pair of voltage doublers to develop the +/- seven volts needed for the op-amp. I'll have to give it some thought but if all else fails I'll build a small DC filament supply and mount it in the bottom of the cab – the amp just has too much potential to let a little thing like excessive hum ruin it.
Update - 4/17/2004
Well, I burned a little midnight oil last night and built a DC filament supply into the VC-508. The results were somewhat disappointing. There was a reduction in hum, but the amp still retains enough hum to make close miking and recording while playing at low amp volumes a no-go. There is simply too much AC wiring run all over the amp, and it's pretty obvious that space and cost savings were the primary concerns of the circuit board design. Don't get me wrong, it's still a great little amp, you just aren't going to be making a lot of high-quality recordings with it. Of course, I guess you can't really expect a "studio amp" for the price this thing sells for...
"Later that day..." – In reviewing the pictures for this article I noticed that many of the AC carrying wire pairs were not twisted very tightly. I went back in and twisted them and moved them around a bit and got rid of more of the hum. Most of the remaining hum, which is pretty faint and present regardless of gain and volume positions, is probably from coupling between the transformers and ripple on the HV supply. There isn't much to be done about the coupling on such a tiny chassis, and I'm not sure I can find room for additional filter caps. Besides, the hum is now reduced to the point that hiss has become the more apparent nuisance. I put a scope on it and most of this "white noise" appears to be coming from the solid-state preamp. I think I'm going to try replacing the op-amp chip with a lower-noise version and replacing the capacitors in this section with better quality low-leakage caps. The factory caps look like tantalum caps and if they are that is about the worst possible choice for a low-level, high-gain audio circuit. The hiss is only noticeable when the gain and volume controls are both turned up, but any noise is too much noise...
I also remounted the Weber speaker so that it is mounted to the back of the baffle in the traditional manner. There is just enough room to do so...
new transformer and speaker
Things are getting tight...
This morning I was playing and realized that a resonance that had been present around E to F# (on the D string) is gone. This resonance was just enough to make the tone sound just a little "off" when played as part of a double-stop. In fact, it was subtle enough that I'm not sure whether the resonance disappeared immediately after I first installed the Weber speaker or only after I remounted it to the back of the baffle. In any case, the F# sure sounds better now.
The following are some more pictures of the DC filament modification, though I'm not really recommending it – it's an awful lot of work for only a little improvement.
The new 12.6VCT transformer for the DC supply.
Upon further reflection, orienting the new transformer with the existing power transformer probably wouldn't make much difference anyway. There appears to be little, if any, magnetic coupling between the PT and OT, I'm pretty sure most of the hum is due to the AC wiring being run willy-nilly through the amp.
The half-wave rectifier and filter.
This new filament supply uses a 12.6VCT 1.2A transformer, three 3 amp diodes, and two 4700uf 35V capacitors. It is a basic full-wave rectifier, the third diode is used to reduce the loaded output from about 6.9V to 6.2V – this amp already runs quite hot and I figured I'd rather have the filaments on the cool side of acceptable than on the hot side. There is about 1V pk-pk ripple on the supply, which is okay for a filament supply.
circuit board changes
Only two changes (one a cut trace) needed on the PCB.
и второй текст, тут попроще:
I bought one of these after I sold my perfectly modified Epiphone Valve Junior with the help of 18watt members and 300+ pages of chit-chat. I get bored rather easy, and was going to go with the Valve Junior again (I sold it to see if I could get the combo and do similar things, but the price increase told me that there were greener pastures) as a platform, but decided to try out the Crate V5 since it was a combo not much bigger than the VJ. I like 10 inch speakers, I liked the looks of the amp, and thought that even if I have to replace everything and even make up a tagboard for it, it would still be cool. I also was hearing what others have been saying about the Crate and when I got it, surprise! I agreed that there was something amiss and decided that to just accept it and give it back would be like throwing in the towel. I knew that it could be better and I systematically worked on each section to see how my $100 initial investment could be improved on.
Speaker was pulled first. Weighing about a pound and with a tiny magnet, tiny voice coil, stiff suspension and a plastic (!?) cone, I could tell just by tapping on it that this was a tone sucker and big part of the bass weakness. I figured that it could be used in an amplifier to take to the beach or other dirty, wet place. The chassis looked awfully close to the speaker magnet, and I didn't think I could get anything in there that would be decent.I measured the depth of other speakers that I had and found that I was wrong. There WAS room for a better speaker. I popped in a 50 watt Eminence 10 inch and it cleared the chassis. Hooked it up and fired it up- better tone and bass, but the amp was still shrill and unmusical.
Looking inside the amp on the circuitboard I saw of all horrors, an op amp!. OK, I know, you can get good sound out of op amps, but looking at the circuit with the volume control in the feedback loop in the second stage, was didn't sit right with me. After the op amp, the tube circuit was fairly conventional other than a quasi- parametric tone circuit between two tube sections of the 12AX7. But looking at the output of the second triode section, I could see that 90% of the signal was dumped just before it got to the grid of the power tube. I guess that is why they needed the op amp in there- to make up for lost gain in the output tube's voltage divider at the grid. (Why?)
So what I did is to cut some traces and add some jumpers to make it an all tube amp. After the input resistors (1.5Kohm and 1 Meg) I cut that trace going to the input of the op amp and sent it straight to the first tube grid and cut away any other parts that were there that might have influence. I decided that the tone control was not a bad thing, just different- and I left it as is. Since I now had a 250Kohm volume pot out of the circuit, I put that between the tone circuits and the grid to the second triode in place of the attenuation scheme that was there and to maintain grid to ground loading and control. I cut the traces around the volume pot and ran jumpers to the appropriate places (the pot connects to the output of the tone circuit, the two lower resistors were cut out, and the other end of the pot was run to ground. The wiper fed the second triode's grid.
I checked for stability (good) output power-6.1 watts at clipping; up to 9 watts fully overdriven. and residual noise 7mV rms (from the power supply). The tone control can be more easily understood when you use a fuzz pedal or other distortion box into the input. It changes the midrange tone in weird ways, but it is better than just a treble cut, since your guitar already has that. Put your metal pedal in front of it and see how much range of tones you can get from it.
Clean on this amp is very good. You can have your amp set at 12 noon, and your guitar all the way up and it will be clean until you start hitting notes hard (or if you have hotter pickups, perhaps a bit sooner. Alternately, you can turn the amp all the way up and still get clean tones with the guitar just cracked open and swell into the realm of output tube distortion. It is very easy on the ears and very pedal friendly. Using external EQ helps you get the tone that you need/want from various axes.
I had ordered a new transformer for this amp (EDCOR 15 watt- the same one I used in my Valve Junior with great success- $20.64 plus $6.37 shipping), but I am happy enough with it the way it is (the output transformer in the Crate is much bigger than the Valve Juniors'- 10 watts instead of 5). I get deep tone from only a 10 inch speaker and I have to be careful not to use the neck pickup too much- it rattles the pictures and things on the wall in the living room. That transformer is going to have to wait for my 6550 single-ended project. One big output tube- should be cool and about 10 watts!
If you just want to add lots of gain easily and just get your feet wet in trying things, pull out resistor R27 (10K) and short out R 15 (100K) and that will get all the lost voltage (the sound) to the output tube and not lost in those 2 resistors. That way you still have the 2-stage op amp pre driver and it might be more of what you want (if more is better- like metal tone). I wanted pure tube and I got it with only an Exacto knife, 3 pieces of short wire, and some cutters and soldering rig, I didn't need to replace the output transformer, or add any parts in any location. Other than replacing the speaker, that was my only expense. I got the 50 watt Eminence for $12.00 each when I bought 4 from a music outlet store on line. I have used those speakers in amps from this 5 watt one to a stereo 15 watt rig and for a 40 watt combo as well. Tubes are all stock. Surely a better transformer and/or tubes will make a difference, but I got night and day difference with these simple mods. Sounds great for little $$$. If you want, and you can pull the board and replace it yourself, I can mod it for anyone for $40.00 plus shipping (which would be nominal in a Priority mail flat rate envelope ($5.00). There is hope for Crate V5 to become the 'next big thing in small amps.
Deafelectromark (alias manoteal)
Благодарю за внимание.