Voltage sag problems in tube amp (SS rectifier) - Page 1
Forum section: electronics / projects

author: Deactivated-1
author_rank: Frequent Contributor
timestamp: June 03, 2020, 03:59:47 pm
content: I have a terrible sag in volume after I play something through the amp and it "flubs out" basically, I measured the voltages and found that:the bias supply is okall the B+ supplies are sagging a LOT and even before the choke.so could the 4uF capacitor before the choke be of too small value? I don't have an LCR meter and I've heard that if you don't know the value of the choke it should better be left alone (the capacitors left alone)so should I increase the value? and to what value?here's the schematic of the power supply[ Attachment Invalid Or Does Not Exist ]maybe the rectifier diodes are dead? I used the only ones I could find and they were some Russian 1N*something* and put 4 in series. since I couldn't stand to wait shippingand also it will decrease the pretty high B+. I could use a tube rectifier like a 5Z4S, but I've heard that for 2 6L6's or 6P3S's in this case, it's underpowered and will steal a lot of output power. and also I just don't really want a tube rectifier, I don't really see the point. plus my power transformer got uncomfortably warm when I had the tube rectifier instead of the SS one.

author: greenpossum
author_rank: Frequent Contributor
timestamp: June 03, 2020, 04:20:14 pm
content: I wonder if your diodes can really handle the current without causing a lot of drop. Do they get warm? Is the 4uF capacitor the original value? It seems a bit low. Pity you don't have a scope to look at the waveform after the diodes. If the diodes or capacitors are inadequate there will be a lot of ripple.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 03, 2020, 04:30:18 pm
content: What is the choke value, what is the nominal minimum and maximum load current, and what is the design B+ voltage?Tim

author: duak
author_rank: Super Contributor
timestamp: June 03, 2020, 04:36:33 pm
content: Please also provide the DC resistance of the transformer and choke windings.

author: greenpossum
author_rank: Frequent Contributor
timestamp: June 03, 2020, 04:46:40 pm
content: Not 1.6. Should be sqrt(2) =~ 1.4.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 03, 2020, 04:51:05 pm
content: Welllll... pop the choke out of circuit and measure it with a signal generator and voltmeter?For example 1H will resonate at with an impedance of 1kohm; if DCR is dominant, then it'll have a Q about 7, and a series resistance of R1 = 10kohms can be used to set up a bandpass filter to measure the resonance.https://www.seventransistorlabs.com/Calc/RLC.html#frq(in this case, R2 will be the equivalent parallel resistance, a combination of core loss and a transformation of DCR.)What about the output transformer (impedance, ratio and taps), and output tubes? Bias resistors? We can check those, and design backwards to find what it should operate at.Tim

author: TimFox
author_rank: Super Contributor
timestamp: June 03, 2020, 08:07:04 pm
content: When putting lower-voltage rectifiers in series for a higher reverse-voltage rating, it is mandatory to add equalizing components to avoid bad sharing of the reverse voltage due to unequal reverse-bias leakage currents in the individual diodes. Back in the day, we used between 100k and 1 M with a 0.01uF disc capacitor at each diode.Also, the resonant frequency of 8.4 H with the series combination of 4 and 20 uF (3.3 uF) is about 30 Hz, which may be too high (I assume you are in the 50 Hz part of the world).There will be a lot of ripple across the 4 uF capacitor: in 10 msec (100 Hz ripple), 100 mA will pull down the voltage by V = IT/C = (0.1) x (0.01) / (4 x 10-6) = 250 V pk-pk, neglecting the load through the choke. Your current is probably higher than that.If we neglect the 4 uF, this could be treated as achoke-inputfilter, which extracts theaveragevoltage, rather than thepeakvoltage, from the rectifier.Once you know the parameters of the transformer and the choke, a good piece of freeware to calculate the results of the rectifier and filter is "PSUD2" from Duncan's Amp Pages:http://www.duncanamps.com/psud2/download.htmlwhich shows the buildup of voltages, input surge current, and response to a step in load current. The user interface is somewhat idiosyncratic, but once you figure it out is very powerful.

author: mikerj
author_rank: Super Contributor
timestamp: June 03, 2020, 08:25:23 pm
content: 2*Pi*F where Pi=3.1415 and F=frequency

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 03, 2020, 08:44:07 pm
content: That's 24:1 for total primary? And, 8 ohm load, so that'll be 4.6kohm a-a. In class AB, each tube sees 2.3kohm for the initial class A range, decreasing to 1.15k as the opposing tube goes fully into cutoff.Seems pretty low, for two tubes and such a stupendously high supply voltage...Yes, a 16 ohm load would probably be better suited, of course that will likely sacrifice some low-frequency response. Unless the transformer is in fact designed for 9.2k to 16 ohms, in which case there you go, your load is wrong and that's that.6P3S is rated 20W or so, so a 15W idle is 28mA at 530V, or 44mA at 340V. In class AB, the peak current will be maybe 3-4 times higher, or 176mA tops (assuming enough screen voltage to draw that current at Vg1 = 0). 176mA into 1.15 to 2.3k is 200 to 400V. Maybe 300V mean, since we're going between the conditions in class AB.530V seems to be an incorrect supply voltage.8.4H and Q 19.1 is very believable for a choke of that type, that's good.8.4H resonates with 4uF at 27Hz, so it's not a resonant type filter.Likely, if the transformer and choke were original and as intended, it was supposed to be a choke-input filter (which gives 0.9 * Vrms or 340V DC).8.4H at 50Hz mains, full wave rectified, has a critical load of 7.56kohms, or 45mA. That's one tube at idle, kind of marginal but should be okay. Load shouldn't be much less than the class A setting so this should be okay.The supply voltage will swell very high (530V) until the tubes come up.So yeah, remove the 4uF, and increase the other cap if ripple is a bit irritating.Tim

author: TimFox
author_rank: Super Contributor
timestamp: June 03, 2020, 09:05:38 pm
content: Quote from: ELS122 on June 03, 2020, 08:17:23 pmAs noted, that is the angular frequency for a sine wave. That equation is for the inductor L that will resonate with that capacitor C at frequency f, not what you want in a rectifier circuit.Note that a choke-input filter for a rectifier is a different concept than a resonant LC circuit. With a true choke-input filter, the choke tries to keep the current through it constant, and the voltage swing at the node between rectifier and choke is rather large, with a reasonable ripple voltage after the choke. Again, I highly recommend PSUD2 software, but you need to look at the textbook treatment of rectifiers and filters. Putting a small capacitor before the choke is a somewhat recent idea; back in the day, only the choke and output filter capacitor were used, sometimes followed by another L-C stage.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 03, 2020, 11:59:06 pm
content: Doesn't mean it was designed properly, or at all. Audio is full of "it makes sound, ship it!"Clearly either the supply voltage or the output transformer is grossly mismatched to the tubes in use. It would be marginal to OK for 6L6GCs, but not 6L6GB/6P3Ss. Or at 16 ohm load if the transformer losses and frequency response are adequate.Tim

author: greenpossum
author_rank: Frequent Contributor
timestamp: June 04, 2020, 12:19:30 am
content: You say the transformer ran hot when you had a tube rectifier. Maybe this was due to the extra heater draw and the transformer is deliberately underrated because they didn't expect it to be used continuously at full volume hence the droop. Maybe that's just the way it is.

author: TimFox
author_rank: Super Contributor
timestamp: June 04, 2020, 03:04:17 am
content: If you do an actual analysis, you will find a huge difference between having 100uF or 4uF ahead of the choke. In the heyday of tube amps, one either used a capacitor-input filter or a choke-input filter. The choke-input filter was easier on the rectifier (peak current), but gave less voltage output, with reasonable efficiency and less droop under load.

author: trobbins
author_rank: Frequent Contributor
timestamp: June 04, 2020, 03:57:52 am
content: You have the output stage screens at B+ (ie. circa 500V). I'd suggest the B+ was purposefully designed to sag so as not to damage the screens - that was common for guitar and PA amps. If you want to stiffen the B+ then you need to manage the screens.Check your schematic against the amp around the input/PI stage cathodes.You will need to measure the power transformer primary winding resistance, and use the calculator in PSUD2 to get the equivalent secondary winding resistance. Prepare your PSUD2 simulation, including the choke resistance, and effective idle current from the output stage as load, and check that the simulated voltage is close to the measured voltage - and preferably post here as you may make some newb mistakes. Then you can explore what the power supply provides when loading is increased, and confirm that is close to your measured sagged level. Then you can work through any options you may have - and given you probably aren't aiming to replace the power transformer or choke, then that leaves the filter capacitance values, and the repercussions of changing them.

author: TimFox
author_rank: Super Contributor
timestamp: June 04, 2020, 02:49:09 pm
content: One detail about PSUD: the transformer parameters used are the no-load voltage and secondary resistance value, although manufacturers normally specify the voltage at full load. If you know the efficiency, or measure it directly, you can estimate the no-load voltage for the computation. Note that if you use a proper choke without a capacitor before it, the voltage will be lower, but it will not fall so fast with load current. There is a minimum value of current for this effect, for which I refer you to the textbooks (see "critical inductance").Also, with a vacuum rectifier such as the 5Z4, there is a limit to the capacitance used in a capacitor-input filter, due to the stress limit on the peak cathode current. The 1939 RCA data sheet recommends 8 uF max (unless you have a large source impedance), or 5 H minimum for a choke-input filter.

author: floobydust
author_rank: Super Contributor
timestamp: June 04, 2020, 05:28:55 pm
content: OP you may not have a power supply problem, you might just be wasting the available current.Example is if you push low bass through a tube amplifier, the output transformer goes very low impedance at low frequencies and you'll draw lots of B+ current and roast output tubes- but the speaker doesn't see it. 47nF/240k is around 14Hz? which I think is way low. You don' t want too much chug.Another example is if AC balance is terrible, it's not operating efficiently in push-pull and any common-mode going to the output transformer is also wasted as heat in the output tubes.Your circuit seems imbalanced from the unequal plate resistors 150k/160k, 91k/160k and the entire 6N2 stage seems just wrong to me. You'd be better off adding a tube and using a Fender or Marshall phase-inverter. All that trouble to set DC balance but you want AC imbalance for distortion? It's not done in the power amplifier section, instead always upstream of it.To prove what I'm saying, need to scope both O/P tube cathodes (resistors). I think 1R is very low, I would use at least 10R which keeps distortion low in the output stage and affects sound character. If you scope the sum between the cathode waveforms, or make a summer with two 1k resistors coming off each cathode to a common point, then to your scope, you can see when symmetry falls apart. Normally (at the sum) there is DC bias with no AC as each O/P tube is anti-phase. You can consider adding an AC balance trimmer to hear what that sounds like.

author: TimFox
author_rank: Super Contributor
timestamp: June 04, 2020, 07:12:46 pm
content: A regulated power supply for a class-AB or class-B amplifier feeds a pulsating load, and the stabilizer circuit needs to be at least as good as the amplifier. Why not try a conventional choke-input rectifier circuit? The reduced output voltage is not due to bad efficiency. Remember my earlier example calculation of the ripple voltage across the first capacitor: it charges up to the peak rectifier output (1.4 x rms input), then droops down by a large pk-pk voltage. The mean voltage at that point has been reduced from the peak voltage by one-half the ripple waveform, whose magnitude is directly proportional to the load current. Further filtration will reduce the AC magnitude of the ripple, but even without loss the output of the filter will be the mean voltage across the input to the further filtration. In an ideal choke-input circuit, the input to the choke is the rectified sine wave, whose average value is approximately 0.9 x (rms input). With no loss in. the choke (and sufficient current through the choke, see "critical inductance"), that mean voltage is conserved. Designed properly, the result has good "load regulation" (although none of these circuits has "line" regulation by itself).

author: floobydust
author_rank: Super Contributor
timestamp: June 04, 2020, 11:35:42 pm
content: Some guitar amps have mushy power on purpose, as part of their overload sound.Choke-input I find regulation is good but very hard on rectifier diodes, they must be ultra-fast recovery such asUF4007and the big choke needs a snubber across it or you will get kV switching hash with the leakage inductance ringing. Not a problem with most tube rectifiers they don't have trr unless mercury vapor lolI don't know what the circuit looks like now, post #1 has no cathode follower and a capacitance multiplier.OP you can try just add a zener at one of the taps or for the cap multiplier, get some say 91V 5W parts in a string to add regulation.I think PSUD2 is missing audio frequency loads and that the load currents are not static or always in-phase. YI would use 10-22nF to the O/P tube grids to limit the low frequencies. Just Spice sim the whole amplifier and see what the freq. response is like.You might just run the O/P tubes (and maybe screens) right after the rectifier, leaving the choke only for filtering the small stages. You'd see 15-22uF compared to your 2.2-5uF and large value resistors, so the caps could be larger unless they are film caps you are using.

author: trobbins
author_rank: Frequent Contributor
timestamp: June 05, 2020, 01:17:04 am
content: Can you confirm that you have measured the unloaded secondary HT voltage (eg. is it 380-0-380), and the choke series resistance (shown as 105 ohm)?Can you show the simulation results section as well, as that shows other details, and helps interpret your plot (as the plot doesn't show parameters), and allows others to quickly cross-check your sim?How did you identify the sim cap ESR ? Eg. the first filter cap is shown with an ESR of 16 ohm.Can you confirm the recommended anode+screen dissipation of the 6P3S is circa (0.5*480V*70mA)/0.7 = 24W?What is your application for the amp? The initial post just mentioned 'play something through the amp'.Can you post a schematic of the whole amp, along with idle operating voltages? The initial comment of 'flubs out' may be related to distortion from earlier stages.

author: pardo-bsso
author_rank: Frequent Contributor
timestamp: June 05, 2020, 01:32:24 am
content: Besides what everyone else has said, can you scope the current on the rail?Perhaps you need to add a bit more of idle load so that the choke current is continuous instead of dropping to zero on every other line cycle.

author: TimFox
author_rank: Super Contributor
timestamp: June 05, 2020, 03:01:17 am
content: Although PSUD does not have sinusoidal load current, it does have load current steps to check for unwanted ringing in the filter, etc. Fast rectifiers are good. Note that the choke input filter reduces peak rectifier current. For snubbing, I refer the reader to the textbooks. Choke input was almost required with mercury vapor rectifiers, since they were very unhappy with excessive peak currents.

author: trobbins
author_rank: Frequent Contributor
timestamp: June 05, 2020, 05:28:18 am
content: Ta. Note that electrolytic ESR specs are a maximum spec value at 20C, and typically will be much lower in value - but no issue. Also the choke inductance will be different as it will have a different ACV and DC current than your test setup.You measured the idle DC current through the choke by adding an extra 100R in series and that indicated 47mA and 150mA at 'full volume'. That aligns with a simpler PSUD2 sim where everything after C2 is replaced by either a 10k load for about 47mA output, or a 2.55k load with 150mA output.Can you describe your intended application, as full volume for a hi-fi amp is the onset of sinewave clipping, whereas a guitarist pretty much wants a square wave output from a sinewave input.Using PSUD2 with a stepped load current change shows no ringing for that setup (with 100R test resistor moved), just some under or over shoot. You should find similar operation when increasing filter caps as long as you don't do anything too drastic (ie. keep output capacitance sufficiently larger than input cap). That should get B+ sag down to circa 40V.Whether you need to reduce sag any more perhaps again depends on your application. For example low distortion doesn't necessarily require zero sag - there are so many other issues that would dominate that aspect.

author: trobbins
author_rank: Frequent Contributor
timestamp: June 05, 2020, 11:19:35 pm
content: I suggest you confirm correct operation of the PI/feedback cathode circuitry - it presently shows the switched feedback connected to one side of the PI as providing the dc idle bias of the PI, as well as a feedback signal path to one side of the PI. If you don't have the same output stage gain or output transformer turns ratio then you could have an unstable amplifier. If you put a link over the 100 ohm, then that will effectively remove the feedback signal, and allow the level of feedback to be determined and allow you to confirm the DC biasing of the PI stage is ok.Perhaps if you move the screens to a resistor dropped node as per the linked schematic, as well as add say 100 ohm screen stoppers, and use ss rectifier (2x UF4007 in series should provide sufficient PIV margin, and use diodes from the same batch to avoid any need for balancing parts) with those filter cap values and see how it sounds. If you still think sag is an issue, then I think you need to verify that all other aspects of your diy amp are operating correctly, and then experiment with stiffening up the B+ and screen capacitors. Just replacing the PT is imho a knee jerk reaction, as you lose any experience with what the present setup can provide.Also perhaps add a grid stopper to the 4th 12AX7 triode as that could easily be grossly over-driven. The 100k grid-stopper for the 1st stage may be more about rolling off some HF response as the amp could be too bright - but that would depend on if you are using 12AX7 and your guitar.

author: trobbins
author_rank: Frequent Contributor
timestamp: June 06, 2020, 03:30:19 am
content: Yes the 100R is the AC feedback signal being applied to half the PI stage, and so adjusting the 100R, or the feedback resistance, or the OT tap, will adjust the total feedback level. Just by checking the speaker output voltage both with and without feedback (for a lowish signal level), you can calculate the -dB feedback level that is inherent - I'd suggest you not exceed 6dB feedback, and even then without suitable measurement tools you could still end up with quirks that you may incorrectly attribute to other aspects of amp operation.I suggest you confirm the common cathode bias voltage and the idle anode voltages for that PI, and confirm that both plates generally hit the rails and cut-off symmetrically (as an initial setup) - although without a scope of some kind (even a soundcard with scope software and a bit of input signal management) you are starting to get in to deep water for your capabilities to modify circuitry or use proven circuitry but with different key parts. Unless you can confirm all sections of the amp are operating ok, your decisions to modify any part of the amp could easily be misled by your assumptions.I also see that you have modified the input circuit - you may want to check that 470pF, as it could make some significant change to your guitar pickup/pot setting response.