A tube guy asks questions about a SS guitar amp - Page 1
Forum section: electronics / beginners

author: WimWalther
author_rank: Regular Contributor
timestamp: August 12, 2023, 02:44:18 am
content: As the subject states, 90% of my experience is in tubes, mostly hi-end & hi-fi audio, plus radios, test equipment etc. So please bear this in mind as I ask questions about the PA unit from a Polytone Taurus Elite music / guitar amp.Nameplate rating for this PA is 125W. I bought the unit used, so it's new to me. I did a power test and found that the unit makes 135W / 4R at hard clipping, but only about 80W / 4R with a visibly clean sine. This seems like a problem to me.The input power rating is 165W. To make 125W out it must be very nearly a class-B design as these figures are very close to the theoretical max of about 76% efficient.The PSU rests at +/-35Vdc. My first thought is the bias is off.. but I don't know what it should be. I measured ~40mA through the 0R075 emitter resistors R17/R18 , but this figure wanders around. The bias pot R14 is very near full CCW.Oh, and what are the 10W 0R3 parts R20/R21 used for?Thoughts & advises?

author: floobydust
author_rank: Super Contributor
timestamp: August 12, 2023, 03:29:36 am
content: What is the loudspeaker impedance, I think the original is 3Ω ? Which would explain your slightly lower power measurement. Every watt is not important nor attainable- mains voltage moves around and 125W of guitar is LOUD lol. Nameplates back then are approximate.The amplifier uses both voltage feedback and current feedback from the loudspeaker.R20, R21 are current-sense resistors that sample loudspeaker current and then divided down by R22, R23 and fed back. Note PA version can switch this off, and PA-60, PA-120 appear to not use it at all with R22, R23 deleted. Seems to be for long-term overload limiting (verses short term for speech clipping in PA use) or it sounds terrible in PA use. I would say the amp would not like a lot of overload without damaging something if the current-sense was disabled or the wrong (low) amount due to speaker impedance lower than the original design.The small electrolytics are very important and I would replace C2, C5, C6, C9 sight unseen given the age of the amp.The bias does affect the sound and can be set to a personal preference. 40mA is reasonable and a bit high I would see 20mA as a guess. More important is that the bias stays in the ballpark when the amp is hot, instead of going too low or too high as in thermal runaway. Full CCW on R14 is... max resistance for max bias I think, for a fatter warmer sound.

author: donlisms
author_rank: Frequent Contributor
timestamp: August 12, 2023, 06:22:12 am
content: Ithinkwhat you're saying is you're looking for an explanation of why you're only getting 80 watts where you expect 125. I do have some thoughts; I don't know if you'll like them, and I don't wanna be insulting or anything! For sure.Judging the distortion in a sine wave is very difficult if the distortion is reasonably low. You can see when it starts obviously clipping, but it's pretty hard to see when it's... well, a bit distorted, but otherwise kinda round and siny. To make the. measurements you're mentioning, I think you'd really want a way to measure the distortion, and then make a graph that shows power vs. distortion, and get a true picture of what's going on. If you can find specs, you can compare to them, but if not, make the assumption that the specs on the back panel came from the marketing department, not the engineering department.Then... the difference between 80 watts and 125 watts is less than 2dB. You can hear a difference like that in a reasonably steady sound source under great conditions, but if this is electric guitar... uh uh. The level of the guitar is all over the place anyway, usually well beyond any sort of stability that would let you determine what the level actually is, let alone whether it's within 2dB of what you expect! If you play with musical transient peaks of 10dB, for example, which would be pretty typical, then the... "non-peak", normal level at 10dB lower than that would be about 8 or 12 watts. The only time you have a chance to hear the difference is in those transient peaks. I'm assuming a non-totally-distorted instrument, here -- if the instrument is totally distorted, then its level will indeed be stabilized, but still... hearing 2dB? I doubt it, assuming a real-world musical context of almost any kind.The only difference between the two power ratings is if the amplifier is producing very near its maximum output. If you're somewhere between 0 and 75 watts (which I would wager a chicken sandwich is actually the case), then there is no difference; it's the same power in both cases.So the general point I'm trying to get at is that you might closer to 125 watts than you think, if the distortion is what the maker wants to provide for you. Even so, the level difference you're talking about in, the real musical world, is pretty much inconsequential. In other other words, you would be well within your rights to ignore this minor discrepancy, if you're looking for a practical conclusion. If you want a non-practical conclusion, where you've analyzed in depth, that has to start with good, high precision distortion and power measurements. Only then does it make sense to figure out how that distortion is getting there.

author: porter
author_rank: Regular Contributor
timestamp: August 12, 2023, 07:00:09 am
content: I wonder if the "visibly clean sine" will generate more loss in the power transistors.Quote

author: WimWalther
author_rank: Regular Contributor
timestamp: August 12, 2023, 08:11:19 am
content: Couple things.. these Polytone amps are targeted at jazz guitar, bass, keyboard and accordion. This model, the (3 channel) Taurus Elite is designed for a trio of players. I'm into synths, fwiw.Did you notice that the only apparent difference between this "120W" model and the 55W/80W models is the value of the PSU caps? That's kind of interesting..I'm used to measuring power mostly 'by eye'. That is, true RMS meter + oscope + HP204C into a 4R 200W NI load resistor. For my purposes, the useful Pmax is determined by the onset of clipping - as that's where the amp exhibits an abrupt, harsh shift in character / tone.So does anyone care to suggest a bias setting / procedure for this amp? What exactly is the bias control doing - varying bias current in the driver, which is tracked by the DC coupled output section?Are the 0R075 5W resistors the proper place to meter bias? What are the 0R3 10W parts doing?

author: WimWalther
author_rank: Regular Contributor
timestamp: August 12, 2023, 10:36:03 am
content: Quote from: floobydust on August 12, 2023, 03:29:36 amFull CCW on R14 is minimum bias, I believe. It does produce the smallest voltage drop across the 0R075 emitter resistors.The present driver is 4R. It may not be original, though neither of my schematics reliably state the correct impedance. The attached 3-piece schematic doesn't even show the PA for some silly reasonSo November of 1997 is now "back then"? Maybe so, but this amp doesn't show much use. I'd be surprised to find ailing electrolytic caps.

author: MrAl
author_rank: Super Contributor
timestamp: August 12, 2023, 10:36:34 am
content: Quote from: WimWalther on August 12, 2023, 08:11:19 amHello,The bias adjustment is used to adjust the zero signal current through the output transistors. The transistors have to always have at least a little current flowing through them C to E or else you see crossover distortion which can sound nasty. You usually want the current to be as low as possible because the higher it is the more power is wasted and the output transistors will have to dissipate this power even with no signal input.I assume you mean the potentiometer R14.

author: magic
author_rank: Super Contributor
timestamp: August 12, 2023, 01:44:40 pm
content: If you think the amp ought to be able to deliver more power before it clips, check for PSU rail voltage under maximum load.Even better, try go get a scope and see how much ripple there is.Bias is unlikely to affect max output much. If anything, it could perhaps decrease it by some fraction of a percent by increasing power consumption and pulling the rails silghtly down.

author: floobydust
author_rank: Super Contributor
timestamp: August 12, 2023, 05:27:50 pm
content: Did you check freq. response? Not at full power of course, but it also reveals low value capacitors and I'm finding small 100uF and less are drying out after 30-40 years, less for crappy Philips blue axials and the Spragues. The bungs dry out and shrink/crack. It's not hard to lift a leg and measure value and ESR or see if it shorted.TO-202's in a 1997 product?- when I see that, I assume it's an oldie from the late 1970's, you can see many revisions- the original model power amp is a much older circuit.Interestingly, a switch selecting high or low current-feedback is called "Wet/Dry" on the 100,102,103 (55W) models. So changing loudspeaker impedance 4/3 would affect the sound character of one of these unless that change is accounted for.The lower power models used a smaller power transformer, it's not just the filter caps that are smaller.I can't see your bias trimpot R14, used this pic but wasn't clear of the front.

author: Buriedcode
author_rank: Super Contributor
timestamp: August 12, 2023, 05:45:04 pm
content: I'm not sure if its been mentioned in this thread so far - haven't seen it explicitly stated - but guitar amps have fairly high distortion by design - even the "ultra clean" amps like the JC-120 will have some colour. That amp will clip approaching full volume/power (likely hard clipping, since it isn't by design), where-as PA/house amps are designed to be much cleaner since they're meant for pure amplification rather than adding tone.Most guitar amps will have some audiable distortion - as in grit - with their volume cranked up. The ones where this happens at almost full power are the ones designed for acoustic guitar - but they aren't really guitar amps, they're clean/synth amps with added extras for acoustic pickups.With that said, my limited experience of tube and SS guitar amps is, solid state tends to remain cleaner at high volumes then descend into distortion pretty rapidly, where-as tube amps its a gradual thing throughout the volume range, meaning that a tube amp is rarely "purely" clean.

author: floobydust
author_rank: Super Contributor
timestamp: August 12, 2023, 06:33:46 pm
content: Shure Vocal Master PA is another current-feedback solid-state amplifier i.e. VA300The advantage to this is the loudspeakers' (inductance) normally causes a high-freq rolloff due to the rising impedance. But this is compensated for with a current-feedback amplifier, which ends up giving you a HF boost essentially.So people complain about lack of high end with the Vocal Master's column speakers when they use them with an ordinary voltage-feedback power amp. The 6 drivers are in series and you get impedance rising quite high so the high mids and up aren't there.

author: WimWalther
author_rank: Regular Contributor
timestamp: August 13, 2023, 01:16:38 am
content: Quote from: MrAl on August 12, 2023, 10:36:34 amSure, these concepts of (idle) bias, class, operating points etc. are universal to both SS & tube designs. What I'm not used to are SS amp topologies including modes of feedback, direct-coupling strategies, those sorts of differences from tube gear.With tubes, we tend to see lots of global voltage feedback (hifi) and/or local, stage-wise degenerative current feedback (hi-end).

author: WimWalther
author_rank: Regular Contributor
timestamp: August 13, 2023, 01:27:51 am
content: These Polytone amps were suggested to me as being good for synth / kb use as they tend to be fairly clean, with good bass response. They're apparently appreciated by jazz guitarists, bassists (including amped upright), pedal & lap steel players, accordion, and others. The guy who started & ran the company was a famous accordion player who wanted electronics that made his boxes sound how he liked.I so happened to find this TE model locally, in apparently un-gigged condition suffering little wear & tear. It needed some basic repair work and the price was very fair imo. So I just took a leap and bought it.You'll notice that the model under discussion has no wet / distortion channels - just three identical clean channels each with hi/lo gain inputs. It does have the Polytone "modulator" effect, which is a phase / Doppler shift module that simulates a Leslie.Point being, I don't get the sense that these amps are intended to impart much "character" distortion.

author: WimWalther
author_rank: Regular Contributor
timestamp: August 13, 2023, 03:42:23 am
content: I set my dummy load up for 3R ((8R||8R)||8R+4R) and read 93W at onset of clipping. During test, the PSU fell from +/-35V to +/-27V.. bit of a dip.ETA: I still need to measure ripple under load..

author: WimWalther
author_rank: Regular Contributor
timestamp: August 13, 2023, 04:50:25 am
content: Quote from: Buriedcode on August 12, 2023, 05:45:04 pmRight or wrong, this is my perception as well. Tube power tends to ease more gently into breakup distortion vs. SS which seems to hit the wall at once.

author: Buriedcode
author_rank: Super Contributor
timestamp: August 13, 2023, 05:37:18 pm
content: Thats also why I believe people prefer the "sound" of tube amps for HiFi/Music - because they actually have a "sound" where-as solid state aims for true pure amplification with the minimum amount of distortion - something tube amps can't really achieve. A raw guiatr signal accurately amplifieed sounds pretty stale because pretty much every guitar sound ever has been through an amp with some kind of distortion - even the clean tones.I'd say getting to 80W before visibly noticable distortion (which is probably more audiable, since we arne't that great at seeing slight distortions on sine waves) is pretty good.

author: WimWalther
author_rank: Regular Contributor
timestamp: August 14, 2023, 02:34:25 am
content: Quote from: Buriedcode on August 13, 2023, 05:37:18 pmI've concluded there are a variety of reasons for preferring tubes to SS, and a tendency toward the colored / euphonic character of tubes is definitely among them.Other factors might include the fact that the high-impedance nature of tubes keeps current densities and cap values low, leading to reduced passive component artifact. Then there's the tendency toward simpler circuits with lower parts count.. and the avoidance of heavy op-amp & other IC use - many of which just don't sound very "natural" despite flawless specs.Now it's also important to realize that people may prefer tubes to SS for music listening vs instrument amplification for different reasons.. as a guitar amp is as much a part of the complete instrument as is the axe itself. So this belief that guitar+tubes=better can color one's opinion regarding music listening (therefore tube hifi= also better).And of course, psychology.. but I see no point in trying to unwrap that one!

author: WimWalther
author_rank: Regular Contributor
timestamp: August 14, 2023, 05:33:26 am
content: Question: Is it typical for a guitar PA design to lack both DC and AC balance controls? I was sort of surprised to find only a single bias pot.Doesn't this require matched sets of transistors?

author: magic
author_rank: Super Contributor
timestamp: August 14, 2023, 01:00:40 pm
content: C6 prevents multiplication of input offset voltage by closed loop gain of the amp.I guess that offset ends up being in low tens of mV here, not a dealbreaker.At this level of accuracy simply grabbing a pair of transistors from one production lot is good enough.Careful matching (and better circuit design in the first place) would be required if aiming for sub-10mV.

author: WimWalther
author_rank: Regular Contributor
timestamp: August 16, 2023, 04:38:59 am
content: Quote from: magic on August 14, 2023, 01:00:40 pmIn this case, I'm referring specifically to the T-03 output devices. They are complementary NPN/PNP pairs, so the paired devices must be manufactured in different lots..

author: magic
author_rank: Super Contributor
timestamp: August 16, 2023, 12:29:26 pm
content: It is typical for transistor amplifiers to have no output stage tweaking pots besides bias, and the one you posted is a fairly conventional circuit.Quiescent output voltage is set automatically by feedback. The amp is DC coupled between stages and has DC feedback through R8, so the input pair Q1 works to keep output DC equal to input DC. There is enough DC voltage gain in stages Q1 and Q2 that quiescent Q2 collector voltage has little effect on Q2 base voltage and even less effect on Q1A base voltage - hence DC offset from Q2 collector to the output is irrelevant. DC on the output comes from input stage mismatch/imbalance and base currents flowing through resistors (R2, R8 here). This is the usual case with transistor amplifiers.Zero output voltage automatically means that quiescent currents of upper and lower output stage halves are equal. Problems begin under load: the two halves may have different current gain and different voltage gain, requiring distorted Q2 collector waveform for clean final output if operated class B. Unfortunately, the nonlinear Vbe-Ic characteristic means that there is no easy way to "adjust" β or Vbe with trimpots. Symmetry can be improved by matching complementary pairs, but in class B even ideal symmetry doesn't remove crossover nonlinearities anyway.Somebody obsessed with symmetry could try things like trimming R15/R16 to equalize Q3/Q4 currents despite different Vbe in Q5/Q6 or inserting other trims to compensate for (a priori unkown) parasitic resistances inside transistors. I don't know if there is much to be gained in that, but it looks like no one ever bothers in practice. Evolution of transistor amplifiers went in the direction of reaping low-hanging and mass-manufactureable fruit and throwing feedback at remaining problems.

author: CaptDon
author_rank: Super Contributor
timestamp: August 16, 2023, 02:11:19 pm
content: The same old power rating trick. The amp may do 125w RMS briefly until the filter capacitors start to lose voltage. Your 80w vs. published 125w is just about equal to RMS vs. PEAK or the capacitor bank charged to no-output peak vs. their lower voltage (and power transformer resistance) at steady state sinewave output. You may like the sound of the Peavey KB-300 but it may require you to use a multiple input line level mixer with tone shaping controls for each input channel from your multiple keyboards/synths. The KB-300 and similar are 15" loaded with a horn and can generate a monsterous stage volume when needed. They also have line-outs for your sound man if he chooses D.I. over mic'ing.

author: WimWalther
author_rank: Regular Contributor
timestamp: August 17, 2023, 02:32:42 am
content: Quote from: CaptDon on August 16, 2023, 02:11:19 pmOk, sure. I get the "ratings game" stuff, though we tend to see much less fast-n-loose flimflam with hiend gear. The place I worked for made 30, 55/60, 150 and 200W monoblocks. They would make clean, rated power continuously into dummy load so long as the AC line were kept to 120V at the IEC.It sounds like some of the instrument amp outfits (like Polytone) used a rating system that you'd see in HAM radio gear - "PEP", or peak envelope power. I believe this equates to peak power measured over the course of a single 360' excursion. Don't quote me.Btw, I should point out that I'm a non-musician.. no chops here to speak of. What I do have is a kid's soul for the strange & weird and a lot of nostalgia for crazy 1970s synthesizer freak-outs and lush, fat 80s new wave leads. This drives me to screw around with synths and effects purely for my own amusement .

author: EPAIII
author_rank: Super Contributor
timestamp: August 17, 2023, 06:04:02 am
content: So your 125W amplifier only makes 125W when hard clipping is involved? And you are surprised. Welcome to thebigswhere people play HARD BALL. That nameplate did not say anything about the conditions where it was capable of 125W output, did it? NO, it didn't.Here in the US we have a popular size of "dimensional" lumber called a "2 by 4". Those numbers refer to the cross sectional size in inches: 2 inches by 4 inches. When I was a lad, back in the 1950s I noticed that those 2x4s did not measure a full 2 inches or 4 inches. In fact they were around 3/8" smaller in both dimensions. When I asked about that, the salesmen at the lumber yard laughed and explained that 2x4 referred to the rough cut size of the lumber, before it was dressed. Some time later, perhaps in the 1960s they decided to further "standardize" that to 1 1/2" x 3 1/2" for some reason that I never quite heard. I suspect the real reason both before and after that standardization was just to get more pieces of lumber from each tree. And I will bet you dollars to donuts that they lost less than 1/8" from the log to the dressed lumber. In other words it was $s $s $s $s $s and nothing else.Needless to say, I was not shocked when I started to learn about the specifications that electronic manufacturers claimed for their products. Things like a frequency response of +/-1db from 20 Hz to 20KHz for an audio amplifier. But only at a power level of 10% of the unit's rated power or less. Likewise for things like distortion, etc.It's calledsalesmanship.You gotta read the fine print. And your amplifier's measured performance does not surprise me. Not in the least.Those are my thoughts. My advise is to buy solid state audio equipment: it is far better than tube type equipment ever was. But I doubt you will take it.Quote from: WimWalther on August 12, 2023, 02:44:18 am

author: CaptDon
author_rank: Super Contributor
timestamp: August 17, 2023, 01:45:41 pm
content: The tube type gear still has its place in the world of music. I dearly wish I still owned my Earth tube amplifier head with 4 X 6L6 outputs. It had a slide bias circuit that would cut off the finals if there was no input signal for around 30 seconds. I was recently given a Peavey Classic twin twelve also using 4 X 6L6 finals. I am going to clean it up and gig with it for sure. Class B push-pull tube finals tend to add even harmonics even during mild overdrive conditions which are musically pleasing. Solid State, yes it has its place also. I have about a dozen Carver Clair Brothers CBA-1000 amplifiers that can do over 1000 watts RMS forever and weigh only around 12 pounds each. That would never happen with tubes!! Tube gear requires maintaining and tubes are becoming harder to find and hugely expensive but I will always have some tube gear in my stable. My current gig amps are a SuperNova 2 X 12" solid state very highly modified with twin solid state 'STK' style power bricks with an added 5 watt high distortion discrete solid state amplifier to drive my talk-box speaker. (SuperNova made by Tel-Ray the makers of Fender amps) My little gig amp and most often used is a highly modified solid state Rocktron R-20 with an added power supply and 50 watt STK power brick driving a full range 8" speaker. It gets hellish loud but lacks 'low-end' on stage due to speaker size. I added a balanced XLR line level D.I. output and when run through monitors and front of house the low end is rock solid!!! Solid State = low maintanence + good sound, Tube gear = high maintanence + incredibly good sound!!

author: TimFox
author_rank: Super Contributor
timestamp: August 17, 2023, 05:35:23 pm
content: Power measurement:Again, this is one of my pet peeves.During the 1960s, hi-fi manufacturers engaged in what I consider fraudulent practices.Starting with a possibly defensible practice of "IHF" power, where an external power supply was used to maintain full DC voltage during continuous sine-wave testing (since in musical use the actual power requirement was not continuous due to high peak-to-average demand of real musical programs), or power was measured during a short tone burst to avoid the DC droop.The fraudulent specifications went further to "+/- 1 dB" ratings, which allowed an actual power to be increased by 1 dB, or an extra 26%, to inflate 50 W to over 60 W.Even with legitimate measurements, if you measure the true RMS voltage across an 8\$\Omega\$ load and computeVrms2/R, that is themeanpower, not the RMS power.By elementary mathematics, the RMS power of a sine-wave voltage is an extra 22.5% higher than the mean power.However, "RMS power" is not a useful value, even though it is mathematically well-defined.

author: CaptDon
author_rank: Super Contributor
timestamp: August 17, 2023, 11:39:08 pm
content: I see that perhaps we in the audio production and sound re-enforcement business have been perhaps using the 'RMS' term loosely??? A common value for small amplifiers is 28.2 vrms across an 8 ohm resistive load and we always called that 100 watts RMS. I suppose from your statement we have an amplifier producing 100 'mean' watts. What I do know is that it generates 100 watts of true BTU heat in the resistor and can be verified by temperature rise of one liter of oil surrounding the power resistor. B.T.W., my CBA-1000 amps will do 90vrms across an 8 ohm dummy load all day long producing 1KW of 'mean' power and 70vrms all day long across 4 ohms producing 1.2KW of mean power. Those values are just shy of clipping onset. Of course if you allow for 10Db of 'headroom' for musical peak vs. average power we have only a 100w or a 120w amplifier! I laugh at these 'D.J.' grade amplifiers boasting as much as 5KW of power running from 120vac with a 5 amp line fuse!!! With + and - 80vdc internal power rails and that is before the 'droop'. I calculate a max of about 2.5KW in bridged mono mode at 2 ohms just before the line fuse pops (unless the TO-220 mosfets pop first!!!)

author: WimWalther
author_rank: Regular Contributor
timestamp: August 18, 2023, 12:14:42 am
content: Quote from: EPAIII on August 17, 2023, 06:04:02 amIn fact, it makes 135W or better in a hard clip, so why not just call it "135W" on the placard - working under your reasoning?QuoteHaving grown up and lived my entire life in the US, I'm thoroughly familiar with dimensional lumber. I also happen to own a home built in the early 1870s, so I'm likewise familiar with "full-dimensional" lumber which is sometimes sized by quarter inches.Again, in another example, I'm aware of the reasoning behind the sudden drop in horsepower ratings of car engines which occurred sometime around 1973.Anyway.. So guitar amp mfr's play fast & loose with power specs. I get it.

author: WimWalther
author_rank: Regular Contributor
timestamp: August 18, 2023, 12:31:51 am
content: Quote from: EPAIII on August 17, 2023, 06:04:02 amThis deserves its own reply.What do you think gives you the right to draw any conclusions about my personal views on this subject?I come here for friendly & helpful technical discussion, to fill in areas where my own knowledge is sorely lacking. Or at least this is what I try to offer, and what I hope / expect to receive in return. This said, I have no interest in the unsolicited opinions of random would-be mind readers. Nor am I even remotely looking to fight some pointless tube vs. solid-state battle.This would also be the perfect time to thank ALL of the helpful folks who have so far lent their time & knowledge to this thread. I'm slowly getting my bearings on this SS stuff - your efforts to that end are much appreciated!

author: TimFox
author_rank: Super Contributor
timestamp: August 18, 2023, 02:46:20 am
content: Quote from: CaptDon on August 17, 2023, 11:39:08 pmYes: your example of 28.2 V RMS across 8 ohms is a mean power of 100 W, and would produce the same heat in a water load as 28.2 V DC in an 8 ohm resistor, or 120 V RMS AC into a 100 W incandescent bulb submerged in water.RMS means "root mean square". If you square it, you get "mean square" that leads the reader to mean power.

author: WimWalther
author_rank: Regular Contributor
timestamp: August 31, 2023, 01:44:38 pm
content: Still plugging away here, came across a couple questions..On the first circuit, with the amp off, I'm measuring 10K from point 6 to ground. With it on, I measure 14.3V to gnd, with a B+ of 15.6V. Neither of these seem right to me. Shouldnt resistance be 147K or possibly 147K || 10K = 9K3? This is part of the "modulator" circuit, which is not working.What is the point of this second circuit. The Master volume output passes through it, via the LDR in an optocoupler / vactrol. There's a transistor, but it has no (extetnal) control signal..Is this a timed muting circuit? Like to prevent startup pop or thump? Thanks!

author: Deactivated-1
author_rank: Frequent Contributor
timestamp: August 31, 2023, 02:52:49 pm
content: Quote from: WimWalther on August 12, 2023, 02:44:18 amGuitar amps are typically power rated at10% THDIdk why everyone keeps mentioning how the speaker resistance is 3 ohms... speakers are very not-resistors. they can easily have 100x more impedance at higher frequencies.The amplifier is rated for whatever the speaker rated impedance is. So if it's built for a 4 ohm speaker, power is measured into a 4 ohm (dummy) load, not a speaker. If the speaker messures 3 ohms on a DC ohm meter, it doesn't matter, it's still gonna be rated for whatever the rated speaker impedance is.With clipping you would get more than 76% efficiency since the output transistors start to saturate, with "100%" clipping you could reach ~90% efficiency.

author: TimFox
author_rank: Super Contributor
timestamp: August 31, 2023, 04:28:11 pm
content: There was an old rule of thumb, used for biasing amplifiers, that 5% THD was visible on a simple oscilloscope.Just like flutes and violins, electric instrument amplifiers make harmonics as part of their application.

author: WimWalther
author_rank: Regular Contributor
timestamp: September 01, 2023, 05:40:29 am
content: The more I look at circuit #2, the more certain I am that it's an anti-pop device. On power-up, C46 charges through R68 & 69 gradually increasing the base voltage to Q5 until Vbe reaches about 5.8V. At that point, CR12 zeners and base current begins to flow into Q5, which begins conducting CE current through the opto OP4's LED.This causes the LDR of OP4 to begin passing signal, which continues until C46 is fully charged and the opto is fully on.One time constant = about 2.7s, so the whole operation takes just a couple seconds.Right? Right?? ;-)

author: magic
author_rank: Super Contributor
timestamp: September 01, 2023, 06:41:57 am
content: Seems reasonable.Regarding #1, it's possible that you are measuring R115 because +V is loaded with enough capacitance or other circuitry to look like a short in series with 10kΩ.In powered up state, 14.3V is about the expected open circuit voltage at point 6. Remember that R117 is shunted by the BE diode of Q10, so it's 0.7V there.editQuote from: WimWalther on August 31, 2023, 01:44:38 pmI see your point now. Yes, it should be less than 10k, unless apparent resistance between +V and GND is exactly the missing 700Ω.At this point obviously continuity of tracks and solder joints near all those resistors should be checked.

author: WimWalther
author_rank: Regular Contributor
timestamp: September 02, 2023, 09:52:37 pm
content: Right, I need to get the board out and onto the bench to proceed any further. Trouble is.. this thing was not designed to be worked on.It's like all three preamp boards + modulator board + psu were all wired up on an external jig and then transplanted onto the amp chassis front panel en masse.Really makes me wonder what the designer intended.

author: WimWalther
author_rank: Regular Contributor
timestamp: September 04, 2023, 02:15:46 pm
content: The fun never ends with this thing. There is a DIP-16 IC on the board, marked "PT2201" on the schematic. I have signals entering on pins 1 & 4, but nothing leaving it.And they've scrubbed the markings off! Dandy.So the NAND gate flip-flop is running, but PT2201 doesn't seem to care. Really wish I knew how this circuit is supposed to work. It's allegedly a Leslie simulator, but I'm not seeing any inductors or cap strings to accomplish phase shifts.Is one of the op-amps being used to emulate an inductor?

author: WimWalther
author_rank: Regular Contributor
timestamp: September 12, 2023, 02:00:19 am
content: Turns out that PT1022 is a TDA2201 BBD (bucket brigade delay) IC. Kind of makes sense, as one of the Leslie speaker's characteristics is a chorus effect.