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Messages - Amp1

#1
Sorry guys for late reply, strong cold hit me down...

Finally found the fault, it was one socket of power tube.
Sockets are old but looks like new.
After I checked all mentioned and unmentioned possible sources I look at the sockets with magnifier and penlight and there were anode pin and bias damaged. Bias pin was stretched and anode slightly burned, black.
After I changed sockets, output signal is symetric and sound is amazing.

Thanks for suggestion!
#2
I have problem with tube amp Orange OR120.
Got it in non working condition, power section is refubished, all ok.
There are only 2 tubes on output, two of them are pulled off, impedance is set appropriately, one level lower.
Amp starts hard clipping (neg.side only) at low input.
Inverter (12ax7) is concertina/cathodyne, known as may be problematic.
Schematic is from early 70', as is seen on the photo.

Anyone with idea where to start?
It looks like too low bias in inverter?

Resistors and caps around inverter and power tubes are new.
Output tubes are new Tad EL34, running at 500vdc at plates, bias is set to 27ma.

Here is scope screen and schematics:

#3
I agree, the source of problem is not found, it's still there.
That 47pF cap is just a pill, pushing the pain back to acceptable level...
Also agree, with loss of knowledge&practice it's time consuming procedure - to find a reason.
Such taking pills I found as common practice of repairman... 8)

Interesting/similar case with Fender Proreverb (all tube amp):
The same problem occured on friends Proreverb (I think 74').
Amp put out farting noise (audible, but not as hard as from Roc pro), after some level of volume.
I noticed same oscillation scope form on bias rail.
When I took bias voltage from my clean bench source, problem dissapear...
There were several revisions of Proreverb schematics (slightly changed over time).
Comparing all those, the amp I had on the bench was without small ceramic caps from bias (tube pin) to ground.
After I added those caps (according to newer revisions of schematics), oscillations gone, were not visible and audible anymore.


#4
Thank you Roly!
Well, it was done with a little help of my "ssguitar" new friends ;)

You said "If nothing else is wrong"...
...yep, something for sure is wrong if I need to add a cap to stop oscillating.
I'm thinking about several reasons, but I'd let specialists to begin with this topic if they wish.
Power supply is stable, DC voltage drop is minimal on full power.

Probably it would be better (and faster) to change all elements in power amp.
This way you dissmount and assemble everything once only.
I hate soldering/desoldering old PCB, it's always dangerous to destroy cooper traces.
#5
I've been busy with other things, now I'm back.
Thank you for all your sugestions and opinions, always welcome!

I think I found a bug...pretty fast this morning.
Since preamp is fine and everything checked I fosuced into power stage including driver&Return input.
When I was measuring voltages (during hard clipping, with speaker connected) around driver chip I heard,
that clipping/oscillating stops if I touch specific points.
These points include pin6 of MC1436, both bases of TIP142/147, around all biasing diodes.
Feeding sin signal into return I got clipped (oscillation) positive top of wave - see photos.
It begins at specific (low) level of input signal, no matter of frequency.
Photos: upper signal is pin 6 of MC1436 and lower signal is from speaker.
One photo is made before adding ceramic cap and one (no clipping) after.

So I soldered a cer cap 47pF across base -collector of TIP142.
Oscillating is not present anymore, I got clean 22,5Vac on 8ohm load, as it should be.
I added trimpot 250kohm instead of resistors R14 and R15, to be able to further regulate/decrease level of signal through preamp.
Input was very sensitive, too much.
Guy using this amp is folk guitarist with Framus AZ10 and low output Zoller pickups, he needs clean only.
Now we are both satisfied  :), sound is really nice, Fender (SS) clean and usable power.

Regarding sound from clips.
I recorded it with iPad, close to speaker, so I put a towel accros it.
It was too loud to be recorded usable and I was just tried to present that clipping sound...
Speaker is 8ohm EVM12L in HQ 18mm birch box, made excatly following EV plan TL806, very good experience!
Although these speakers sounded very good on cleans, they're not as good for overdriven signals, my opinion.

As test signal sin&sq I use LAG26 (0,1% dist) ton generator and HP8903 distortion measurement system (sin only)
For  RMS AC signals there is Fluke 189, Marconi TF2337A (RMS 1kHz&400Hz and also AC part of HP8903.





#6
Thank you guys!

@Roly
Yes, I also think it's power amp, I can't believe it's "normal" or input is too sensitive.
Thanks for tip on scope ground, video is recorded this way now.
Idle Bias is set via diodes I thought, yes.
Wouldn't be better to set it a bit colder? Any sugestions on best and simplest way?

@g1
For sure I will try your tip, but I have to put chasis back to combo.

@J M Fahey
I don't know if it's normal, usable volume on clean channel is up to 2 of 10. (Guitar vol max)
If I set guitar vol 1/2 and amp vol 1/2 is almost good enough, but it's not 65W then, much less.
Yes, photos of scope traces are made with dummy load hooked on output.
Your explanation of plain SS amps scary me that this one it's like it should be :(
I didn't have much SS amps on bench, but this one surprised me (comparing to tube).

I recorder a clip, it's on youtube, hopefully you can see/hear what I'm talking about.
Amp is sitting on bench, chasis is out of combo, speaker is EVM12L (covered with towel).
Begining: amp1/guitar10 - then amp5/guitar5 - then amp10/guitar3...
https://www.youtube.com/watch?v=9q7VtuyCtzA



#7
Hi guys (and girls?), new member here.
I found this forum very helpfull with polite debate and full of usefull information.
Therefore I'm kindly asking for some help with SS guitar amp.
I try to repair tube amps from time to time, ... this time I have SS amp on the bench.

It is about Fender Roc Pro 700.
Problem is, amp is clipping/cutting off, or farting, on stronger attack.
So, at low volume it sounds nice as it should, but if you turn volume a bit higher and hit the chord it sounds like blocking distortion.
It seems it goes to full conduction (short DC pulses) at peaks only.
It is related to clean and drive channel, but in clean is clearly noticeable.
If I hook guitar (vintage strat) into Return of FXloop I get same problem, less noticeable as this input is less sensitive.
Guitar output is max around 100mVAC (volume and tone stack full open).
If you hit the chord hard, I may turn up volume in clean channel to max 2-3 without hard clipping on peaks.
Measuring signal (scope) at Send output, signal is not clipped.
Also output of driver chip MC1436 (pin 7) is clean, problem is in power amp (TIP142/147), I think.

Testing with tone generator&scope clean/normal channel:
Input singen: 40mVac (volume max)
Output dummy load 8,2ohm: 22,7Vac before clipping
Input singen: 190mVac (volume half)
Output dummy load 8,2ohm: 22,8Vac before clipping

Testing with tone generator&scope on Return input:
Input singen: 140mVac (att.=-7dB)
Output dummy load 8,2ohm: 22,5Vac before clipping

I did change electrolityc caps, all of them.
All solder joints were inspected and mostly re-soldered and clean.
Heatsink and transistors (TIP142/147) were cleaned and fastened with thermal paste.
Power supply DC voltages are all OK, as per schematic
(+42,6Vdc/-42,2Vdc; -17,60Vdc/+17,36Vdc)
DC on output (+/-), no loudspeaker, is around 2mV.
Output transistors are getting warm with no signal on input, but not hot, after 15min.
Voltage across emitter resistors 0,47ohm/5W = around 48mVdc each (100mA), no signal, no load
Bias, emitter-base TIP142=1085mV; TIP147=1113mV; no load, no input
-------------------------------------
Any thoughts regarding biasing output transistors?
Probably, if you look at scope photo, it is kind of oscillation problem?
Photos:
*sin wave after begining of clipping
*square wave before clipping

Also, when you hook a scope to such output – where to put ground of scope?
Minus of speaker is not at the same potential as ground!
(Photos are made without ground of scope connected, otherwise amp oscillate)