Tube-Town Forum
Technik => Tech-Talk Soldano => Thema gestartet von: KoumparosJ am 1.09.2016 23:00
-
Greetings everyone,
I have recently completed an SLO 100 clone build, but i encountered some issues that i have not been able to solve, so i ask for your help.
First things first the components i used are:
Self made turret boards
inMadOut transformers
Sovtek 5881 power tubes
Sovtek 12ax7wc preamp tubes
F&T Electrolytic capacitors
Vishay Roederstein signal capacitors
1W carbon film resistors
5W wirewound resistors for the power supply
Alpha pots
The problems i have are two: the most severe is that when i power up the amp, the sound that i get is awful. It is crackling and breaking into some harsh distorted sound when the guitar is strum hard, even on the clean channel. The overdrive channel has the same effect on top of the overdriven tone. If i strum the guitar lightly there is almost no sound. It seems like a switching effect going from cutoff to clipping directly. The strange fact is that after ~20-30 minutes powered on, this effect almost disappears, and the sound is normal. The second issue that might be related to the first is that the clean channel is much less loud than the overdrive channel, so low that even at max volume it is less than a 5W amp that i have, comfortable for bedroom. This second effect remains even after the first issue solves itself after ~20 minutes.
I attach here a schematic of the amplifier with the measured voltages across the circuit where the nominal values are stated. I have generally a bit lower voltages (probably because the transformer is for 230 volts supply and i have ~225 at the outlet) but i get stupidly low voltages compared to the nominal at the plates of all preamp plates especially the third and fourth gain stage ones.
Thank you for taking the time to look into my problem and for the assistance
-
Hi,
just three quick remarks before bedtime:
- Is C12 really rated at 100V only? That would be much to less.
- Is R18 connected to ground?
- do you have a scope to trace the signal?
:gutenacht:
Peter
-
Hi Peter, thanks for taking some time to look into this.
Regarding your questions:
-The schematic is wrong at this point, the capacitor C12 is rated 600V
-R18 IS connected to ground as shown in the schematic
-Unfortunately i don't have a scope
-
Hi KoumparosJ,
The caps C27 to C32 you use in your Power supply are only rated with 300V? This is far too low. I would recommend at least 450V. Than you forget the bleeder resitor on C31 and C32, like the R54 and R55. Maybee this burnes already your caps and therefore you habe here a higher leagage current.
Remove all the tubes and test the resitor on their right values. There must be a mistake, especially in the last driver stage.
Best regards Olaf E.
-
Hi
Among the others I came across voltages at point C: 362 volts and next anode follower has 90 volts at anode (after 220k) and also the cathode follower has only 104 volts at cathode - I would recommend to check if the voltages remain just from the beginning of powering on the amp and if connections are right (soldering, value of resistor etc.).
The anode follower will have a current of around 1,2 mA which seems quite normal, but 90 volts is too low from my point of view.
The overall symptom you are describing is known to me from a Fender and a Marshall amp in the past which had broken anode and B+ resistors (both) - value increased with operating temperature and both started with a distorted sound.
Regards, Stone
-
Thank you very much for the replies everyone, i really appreciate it.
Dear Olaf, The actual filter capacitors C27 to C32 I use are rated at 350 Volts. Since they are in series they should be more than enough. All schematics i have found have 300Volt capacitors, and i already went a bit up on that matter. The capacitors C31+C32 do not have bleeder resistors in any of the schematics that i found ( 3 different sources i think). I think you are right in thinking that the error should be in the last two gain stages, i need to check this.
Stone, I don't have tha amp currently with me, but when i tested the 220k resistors going to the 90Volts are measured, i noticed that their value was not stable when i turned the amp off, but it changed with time (i don't remember now if it was increasing or decreasing). At the time i thought that this might be normal, thinking that some capacitor might be discharging, or my not so good multimeter is doing funny things, but now that you mention this i should focus more on that. Should the resistor measure 220k regardless if the amp is on or off?
On a side note, do you guys think it is worth reducing the value of the R56 resistor in order o get higher voltages down the chain, closer to the nominal values?
-
Hello KoumparosJ,
each electrolytic capacitor has a very huge derating over the applied voltage. If you really use an electrolytic capacitor close to its rating, its lifetime will drops in the region of 10 years. This is what I found in the data-sheet: Voltage derating is expressed as the percentage that the applied voltage is less than rated voltage, e.g., a 450 V capacitor operating at 400 V would have 11% voltage derating. In applications operating at less than 45 ºC no derating is needed, and with up to 75 ºC, 10% is sufficient. For higher temperatures and with high ripple current, 15% or 20% is appropriate. Since operating life continues to increase for further derating, military and space applications use 50% voltage derating. Means if the capacitor is rated for 450V 300V would be a good figure for the very high temperatures, we have in tube amplifier.
Because the capacitances are connected in series, the current passing through the capacitors must be equal, so each capacitor receives an identical charge. If their capacitances are equal, then the voltage across each one of them must be equal.
Unfortunately, even if the capacitances are equal, the leakage currents in each individual capacitor will not be equal. To equalise the voltage, and prevent one capacitor from exceeding its rated voltage, each capacitor should be bypassed by a resistor so that the resulting potendial divider chain forces the voltage to be equal. Means you choose a value for the resitors, where its quiescent current is 10 times higher than the quiescent current of the capacitors. 220k Ohm is a good choice. If you want to test your amplifier without tubes, your votage will easely increase to 550V. 2 X 300V would than be really critical. 350V you mentioned goes in the right direction. Even the company weber uses in their soldano kits 450V caps. This was my first SLO100 I build.
Best regards
Olaf E.
-
Thank you for the detailed explanation Olaf, i knew that going higher in rating is always good, but didn't know the numbers and the technical details that you provided. Since you also say that 350V rating is good enough ( not ideal), I will keep that in mind for my next projects.
-
Hi,
The cathode circuitry of your phase inverter seems to have an issue. You are driving the stage quite hot compared to original values. Maybe a wrong resistor in place ?
Cheers,
Laurent
-
Hi
I am not quite sure, but I think you will only get a real measurement of a value of a resistor if all capacitors have been discharged; so this implies the amp must be switched off and unplugged, capacitors (the B+ ones) need to be discharged safely.
Placing the leads near the resistor will do the thing IMHO.
Maybe Laurent has trapped the problem.
I have to say I had no time until now to have a closer look to the scheme.
Maybe this evening.
Regards, Stone
-
Stone, I don't have tha amp currently with me, but when i tested the 220k resistors going to the 90Volts are measured, i noticed that their value was not stable when i turned the amp off, but it changed with time (i don't remember now if it was increasing or decreasing). At the time i thought that this might be normal, thinking that some capacitor might be discharging, or my not so good multimeter is doing funny things, but now that you mention this i should focus more on that. Should the resistor measure 220k regardless if the amp is on or off?
I am not Stone but the resistor R22 should measure 220k in any case. Since the supply voltage seems to be in the expected range but the voltage at the plate is much too low, I suspect that R22 is defective.
Good luck,
Stephan
-
Hello again,
Sorry for not replying earlier, but i was away for the weekend.
I checked the layout and it seems correct but i will check again more carefully for any errors as Laurent suggested. Unfortunately i don't have the amp with me now to check if replacing R22 will solve the problem, but i will keep it in my mind definitely. Thank you again guys and if you have any more ideas please let me know.
-
Hi,
or maybe the cathode follower is drawing heavy grid current for some reason and therefore tearing down
the voltage at the feeding anode. Please upload some pics of your build.
Chears, Peter
-
Hi,
Yes, Peter is right. Check R25.
Cheers,
Laurent
-
Hi
Don't worry about the delays :) It happens to everyone of us not having time to answer all the time ;)
As Stephan suggested R22 may be defective or as Peter mentioned there is something wrong with the catode follower - even if the tubes are brand new there might be one broken (I have got this in the past two times).
At the moment you need to check and measure everything and all twice - but I am pretty sure we will analyse the problem and get the amp sounding.
Regards, Stone
-
Hello,
Here are a few pictures i took with my mobile phone, so the quality is not perfect. I did not yet put tire-ups to make it look pretty and tidy, as i wanted to make it work properly first, so it looks kinda ugly right now.
-
Continued (max data size restriction)
-
Last two:
-
The blue capacitor is a temporary replacement, as the original F&T was marginally within voltage rating and i wanted to make sure it didn't burn
-
Hi
Maybe I got it, but that would be a shot in the dark: referring to the first picture taken there is a red wire at the edge of the board which seems to be B+ - it looks like it is touched by the screw holding the board. Even it is not touched by the screw you need a minimum spacing of around 2,5 mm in order to make sure voltage will not "jump over".
I had a pretty similar failure a while ago within an amp - as you switched it on it was quite for a few seconds, than it started to "pop" and "crackle" and after a tubes had been warmed up it started to make "shooting noise" because of the crossover of the B+ voltage.
Same seem to apply to screw near 1k5 catode resistor in the top left corner. If it shortens the resistor to ground you have the so called "mu"-Amp which might overload the next stages beside ground hum (because the catode will be referenced to another point than all the other).
Fuse must not blow necessarily but voltage drop may occur.
Regards, Stone
-
The screws definitely don't touch the respective turrets, although i didn't know this about the 2.5 mm spacing. However the situation is the same even without the nuts, in which case the spacing from the screws is around 4-5mm. On the other hand, there might be a similar issue at the last large filter capacitors ( left top corner capacitor at the second picture, power supply board), where the ground turret ( the one with the black cable leaving, is ~1mm from the B+ coming from the choke and going to the preamp board ( purple cable). This one is really close, bad design by me i guess....
-
Hi
I am not sure about the spacing but if I refer to PCBs then it should be 2,5 mm or even more ... 1 mm might be too close - as of the spacing itself there is not much current flowing, but voltage is going across it if it is high enough.
Maybe another one can give a second opinion concerning the spacing to prove if I am wrong or right.
Is there any chance to disable the ground turret for testing purposes?
Regards, Stone
-
Hey!
I know the thread grew old already, but did you try to go out of the effect send into a different amp to locate where the problem is actually happening? Did you also try to give the whole chassis some hits, or parts of the turret board etc. to hear if there's some mechanical problem (could solder joint)?
The turret board layout looks really nice by the way :)
Best regards
Mathias
-
Hello everyone! I wish a happy new year to you and your beloved ones.
After quite some time now, i finally finished moving back to my homecountry, and back to building, so i need to solve these issues with my sloclone, with your kind help.
Following Stone's suggestion, i made sure all the B+ voltages are well separated from neighbouring ground turrets, and the situation is improved by a lot.
However i have the following issue. The overdrive channel is leaking into the Normal channel in some way. When the amp is on the normal channel and the gain of the overdrive channel is crancked above 4-5, the "clean" sound also has some distorted signal into it ( the more the overdrive gain the more the distorted signal). I suspected this might be because of the non-infinite resistance of the LDR2 when off, but even if i completely disconnect it from the board, the leaking is there at the same level as before. Can someone please give his/her insight on this?
-
Hi
Good to hear that the amp became more stable by separating B+ and ground lugs.
The effect you are noticing appeared two or three times within my 19" preamps - as I am using B+ filtering and PCB from the Anvil preamp kit and the Anvil itself is pretty quiet I assume some other failure, but I never could figure out what it caused.
I shorted the gain wipers of the gain potientiometers of the distortion channels to ground and that fixed the issue (but did not solve it in a a way). Also I could only hear distorted sound at very low clean gain settings.
Could you prove if you are hearing a distorted sound while shortening overdrive gain wiper to ground?
And the Vactec LDRs often are leaking or are broken - this can be proved by various problems within Mesa amps, especially channel switching is not working as expected (and clean channel does have a certain amount of distortion).
Regards, Stone
-
Greetings,
Well, Stone, when the gain wiper of the overdrive channel is shorted to ground, there is no overdriven sound leaking, but this is the same as having that pot to zero. What really baffles me is that with LDR2 completely disconnected from the circuit, not only there is overdriven sound in the normal channel (depended on the gain seting of the OD channel), but also the overdrive channel works pretty much as it should. Since the LDR2 is the only gateway for the overdriven signal, i really can't understand how this is possible. I am checking everything again, but please let me know if something comes to your mind. Thanks for your time everyone.
-
Hm,
Also the normal channel plays, at low volume, with LDR5 disconnected....strange
-
Hi
Can you point me to the schematic you are referring to? I am reading Joachims SLO 0.7 schem available here, but there is no LDR5 (or I am missing that totally).
If LDR2 ("behind" V2B , C10 and R20) and you still got "normal" overdriven sound at normal volume level there must be a bridge or something like that.
I would assume as long as that problem exists, you will have overdriven sound while being in normal channel as LDR1 will not necessarily shorten grid of V2B to ground (there will be a small resistance which might be sufficiant causing the problem).
Regards, Stone
-
Hi Stone,
I am using the schematic from slocloneforums.com. LDR5 is an addition to the original, placed after the (r17//c8) combination and before the r21 resistor, essentially switching the feeding of the signal of the clean channel into the grid of V3B. This is a common mod to stop the clean signal which is added to the overdriven in the original soldano, when in the overdrive channel.
But as i said with both LDR2 and LDR5 disconnected from the board ( in practice both paths of the normal and overdriven signal are cut), the amp still works, at somewhat lower volume. If I ground R21, there still exists some sound, but lower volume. Only when i ground the grid of V3B directly, there is no sound at all, which is strange. I have checked for bad connections/bridges, but i didn't find anything obvious...
-
Hi
Ok, I got the scheme and had a look to it.
If you connect R21 to ground on its "left" side (opposite side to grid of V3b) you still have a small grid resistor - if you 've still got sound then I would assume there is a leakage on "B+" (connection points A and B).
But - as you said - it sounds more like a bridge somewhere before V3b as with leakage on B+ the amp should run at very low volume level (as of my experience).
At the moment I do have no idea ... if you remove LDR2 and LDR5 there should be no signal - also (more or less) even very little signal avoided by stray or something like that but no "useable" volume level at all.
What about disconnecting V2a grid?
Regards, Stone
-
Hi,
well as signal from both channels is going through to the V3B, even with the LDR2 and LDR5 completely disconnected, i suspect it shouldn't be some kind of bridge, but rather B+ leaking. As i am not that experienced with this kind of stuff, could you please give me a few details on what this is, how should i check if this is the case and what might cause this? (faulty components? wirind errors and where) Thanks a lot once again
-
Hi
Normally B+ should be "free" of any signal, but due to wiring error or something like that there may some signal leaking from one stage to the other ...
There are several ways to check this.
First of all you can check the wiring and the placement of components and their values - do have the filter caps the right capacity, do they have the correct DC voltage rating, are they connected the right way and so on. And if they are connected in the same way as written in the scheme (it is important that you have the same series as in the scheme).
But I do have it only once or twice since I have started working on tube amps.
Maybe it can also be caused by a faulty filter cap or if the resistors value is too low, e.g. you did not use 15k / 10µF, but 1k5 / 10µF which will raise filter frequency from 1 Hz to 10 ... imagine using a 150 or 15 Ohm resistor by accident. But this should also be indicated by to high anode voltages.
My suggestions / thoughts ...
If LDR2 is removed, but there is still a lot of distortion sound in OD channel at a reasonable level of volume it might be that R20 is connected in a wrong way and does have a connection from C10 to R21 ...
If LDR5 is removed but there is still a good amount of clean sound in NR channel at a reasonable level of volume it might be that there is a bridge from R17 to R21.
I agree that it does not make any sense still having sound while having LDR2 and LDR5 removed.
But I think with B+ leakage you will not have a pretty loud clean or overdrive sound. Running grid wires in parallel to each other gives us various effects as well.
Regards, Stone
-
Hi, I had a very similar issue in an amp where I used these small switches for channel switching. It was just crosstalk because the two signals were too close together. Using a bigger switch solved the problem.
Cheers Axel
-
Hello,
Stone,
Thank you for your suggestions, i double checked everything and i can't find anything wrong...I will post some closeup photos tomorrow, maybe some fresh eyes can catch something that i missed
Axel, Thank you for your advice, but in this specific amp, the actual channel switching is performed by the LED/LDRs, so the small switches do not carry any signal. The clean/crunch switch does carry signal but this one performs perfectly.
The major issue right now is that i get sound, especially on the OD channel (but at low volume) even when the signal chain is broken. Could it be that because i am using 5W wirewound resistors for the power supply resistors R62, R63 and R64, they act somehow as transformers inducing signal from B+4(A) and B+3 (B), to the B+2 (C) supply of V3, V4 and V5??? (notation according to Joachims (sloclone) schematics respectively)
-
Hi
At the moment I am running out of ideas ... can you point me to the discussion about the wire resistors? I have no experience with them as I was always using metal and carbon types, but in general (as far as I remember from my apprenticeship) copper cores wired in circles are kind of choke, but I think with a very, very small effect.
But as I said ... I do not remember exactly and maybe Joachim has a totally different experience especially in B+ wiring / setup.
Regards, Stone
-
Hello,
well i am running out of ideas too... my thought about the wirewound resistors was exactly as you said, that maybe because of the winding of the wire in the resistor it acts somehow like an inductor... But still the large capacitors should filter any signal at that point to ground...I don't know, just guessing at the moment.
-
Hi
I was thinking about the effect you described, but at the moment I am just catching at some straws ... normally it should something like interspersal, but I do not know where - I will have a closer look to your pictures again later on.
Maybe - just another straw - it is some crosstalk between R20 and R21 if they are close together; that came into my mind as Axel mentioned the switch problem.
Regards, Stone
-
I tried removing some components, as suggested by a member of slocloneforums, mainly coupling capacitors to try and locate the problem, with the following results:
Removing C10, still sound at the same level (left out)
Removing C7 , still sound at the same level (left out)
Removing wire between R29 and R30, NO sound (reconnected afterwards)
Removing the wire from fx return to V4A pin 7, still sound at lower level (left out)
Removing R26, NO sound (reconnected afterwards)
Removing C6, still sound at same level (with all the above disconnected
Removing C2, NO sound (reconnected afterwards)
Removing V3, still sound at much lower level
Removing V4, No sound
The whole time LDR2 and LDR5 are removed. As a reminder i get sound on the OD channel and on the normal channel only if OD gain is high (same sound in both channels)
Any thoughts?
-
Greetings everyone!
Today is a good day, as i managed to fix the amp.
I had tried replacing the filtering capacitors C34 and C35 just to make sure they weren't faulty, but it hadn't made a change. Today I took a long shot, and replaced them not only with new, but with larger capacitors of 47uF and the amp works fine!!! What a monster amp and what a relief!!!!
I would like to thank all the members of the forum for maintaining this helpful community, and especially Stone who devoted a lot of time to look into my problems and help me fix them. Thanks guys, your help is much appreciated!!!
-
Hello
Happy to read about your success.
Maybe for some reason the value of the resistors is much lower than expected and stated and therefore using 10 µF caps did not work. I would not deny that it may also happen that the 10 µF caps were labeled wrong, this may also apply to the replacement ones if they are from the same manufacturer and out of the same series.
I will keep this information in mind - maybe it does make sense to increase filter cap value to 47 µF on SLO style amps in general.
Also thanks for your kind words as I believe a lot of people have read about your problem and started analyzing it by heart.
Regards, Stone