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Help with SLO-100 debugging needed

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Offline KoumparosJ

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Help with SLO-100 debugging needed
« 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

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Offline _peter

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Re: Help with SLO-100 debugging needed
« Antwort #1 am: 2.09.2016 00:53 »
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
vintagevalveamps
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Offline KoumparosJ

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Re: Help with SLO-100 debugging needed
« Antwort #2 am: 2.09.2016 02:01 »
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

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Offline Olaf

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Re: Help with SLO-100 debugging needed
« Antwort #3 am: 2.09.2016 11:29 »
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.

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Stone

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Re: Help with SLO-100 debugging needed
« Antwort #4 am: 2.09.2016 11:55 »
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

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Offline KoumparosJ

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Re: Help with SLO-100 debugging needed
« Antwort #5 am: 2.09.2016 14:11 »
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?

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Offline Olaf

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Re: Help with SLO-100 debugging needed
« Antwort #6 am: 2.09.2016 16:49 »
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.

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Offline KoumparosJ

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Re: Help with SLO-100 debugging needed
« Antwort #7 am: 2.09.2016 17:15 »
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. 


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Offline Laurent

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Re: Help with SLO-100 debugging needed
« Antwort #8 am: 2.09.2016 18:19 »
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

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Stone

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Re: Help with SLO-100 debugging needed
« Antwort #9 am: 2.09.2016 18:41 »
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

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Offline darkbluemurder

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Re: Help with SLO-100 debugging needed
« Antwort #10 am: 5.09.2016 09:53 »
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
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Offline KoumparosJ

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Re: Help with SLO-100 debugging needed
« Antwort #11 am: 5.09.2016 12:40 »
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.

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Offline _peter

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Re: Help with SLO-100 debugging needed
« Antwort #12 am: 5.09.2016 13:12 »
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
vintagevalveamps
Der Imperativ von "messen" lautet: miss!

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Offline Laurent

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Re: Help with SLO-100 debugging needed
« Antwort #13 am: 5.09.2016 14:18 »
Hi,

Yes,  Peter is right.  Check R25.

Cheers,
Laurent

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Stone

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Re: Help with SLO-100 debugging needed
« Antwort #14 am: 5.09.2016 14:53 »
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