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Valve junior quiet signal troubleshooting
Valve junior quiet signal troubleshooting
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Old 26th November 2019, 08:33 PM   #1
d4v3 is offline d4v3  Estonia
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Default Valve junior quiet signal troubleshooting

Hi,

I recently made some mods to my Harley Benton GA5 5W practise amp and everything seemed to work quite well. I changed some resistor values, removed the tone circuit, added master volume and changed the tubes to soviet equivalents 6p2p-ev(modded pin+jumper to accept 12ax7 layout) and 6p14p-ev.

Until one day I tested one old prototype fuzz face pedal, that was wired backwards and required to change adapter pin polarity to work. Unfortunately by the point I realized, it was too late and the 12v 500ma adapter was with the wrong polarity and messed something up in the amp.

For testing purposes I have put back the tubes, that the amp came with, the chinese 12ax7 and sovtek el84.

Currently the amp will produce a very weak signal, when the Volume and Master are turned to max setting. Seems, that the loudness is not affected by the Volume knob, so I'm guessing, that the issue is related to the preamp tube not working properly, as the signal only becomes audible, when maxing out the volume on Master. Although the hiss will become louder while turning up the Volume, as it would usually, when cranking an amp.

I have checked the voltages on both tubes:
12ax7
1)199
2)0
3)1
4)6
5)6
6)204
7)0
8)1
9)0

el84
1)0
2)0
3)9
4)0
5)6
6)0
7)323
8)0
9)317

Here is the schematic:
Click the image to open in full size.
Master vol in more detail:
Click the image to open in full size.

Any help on what to troubleshoot next, or how to go about this, would be much appreciated.
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Old 26th November 2019, 11:15 PM   #2
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by d4v3 View Post
Unfortunately by the point I realized, it was too late and the 12v 500ma adapter was with the wrong polarity and messed something up in the amp.
Can you clarify what the 12V 500 mA adapter was connected to? You mentioned it was powering a Fuzz Face, but that cannot damage your tube amp. Were you also using the same 12V 500 mA adaptor for some other purpose in your tube amp?
Quote:
Originally Posted by d4v3 View Post
I'm guessing, that the issue is related to the preamp tube not working properly
All your DC voltages (for the preamp tube) seem normal. If there is a problem there, it is an AC-only problem, such as a bad capacitor, bad pot, or missing wire.

My suggestion is that you get an audio signal or tone generator of some sort (in a pinch, you can even use an old MP3 player, with a 0.01uF, 630 volt capacitor connected in series with the output.) Apply the test tone (or even music) to the control grid (G1) of the EL84: do you hear it from the speaker?

If no, the problem is with the EL84 stage. Fault-find and fix.

If yes, set the master volume to half-way, and move the test probe back to the wiper of the master volume pot. Do you hear the music/tone? If no, the wire from master vol to EL84 is disconnected or not soldered at one end.

If yes, apply the test tone to the "hot" end of the master volume pot. Turn the pot to full volume. Do you hear the signal? No? The pot is bad, or mis-wired.

Yes? Apply the test tone to the grid of V1b. If you hear the tone (much louder now), V1b is working okay. If not, fault-find V1b and associated components; in particular test/replace the 22uF and 0.022uF caps.

Continue in the same way, moving back one stage at a time, until you arrive at the input jack.

Somewhere along the way, you'll arrive at a point where the signal suddenly stops, or becomes much weaker. The fault in the circuit is therefore between this point, and previous point you tested.

Divide and conquer - it worked for the ancient Roman empire, and works for electronics trouble-shooting. Keep narrowing down the location of the problem, one step at a time, until you have identified exactly which component is at fault. At that point, the fix is easy and obvious.

Good luck!


-Gnobuddy
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Old 3rd December 2019, 12:29 PM   #3
d4v3 is offline d4v3  Estonia
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Quote:
Originally Posted by Gnobuddy View Post
Can you clarify what the 12V 500 mA adapter was connected to? You mentioned it was powering a Fuzz Face, but that cannot damage your tube amp. Were you also using the same 12V 500 mA adaptor for some other purpose in your tube amp?
Hi,

Thanks for all the suggestions.

It was connected to the fuzz face pedal. It has one of these tips, that you can flip in order to change polarity. I plugged in the pedal, and it did not work, so I flipped the adapter end, while the amp was working. I dont know, if this itself can harm the amp or could it have been a spark, that made its way to the circuit.

Luckily dad has a tone generator lying in the basement unused. So I asked to borrow it and tested preamp and power tubes. It seemed to amplify the signal from tube pins without issues (with plenty of hum especially on the preamp side). Will start to narrow it down a bit more.

I have replaced the first 22uF electrolyte cap in the circuit. Currently waiting for some spare parts to arrive. Most likely will try to replace the 0.022uF caps with Vintage K75-12 PIO caps and go from there.
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Old 4th December 2019, 12:27 AM   #4
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by d4v3 View Post
...one of these tips, that you can flip in order to change polarity. I plugged in the pedal, and it did not work, so I flipped the adapter end, while the amp was working.
I don't think you could hurt your tube guitar amp this way. You could certainly damage the Fuzz-Face, though.
Quote:
Originally Posted by d4v3 View Post
Luckily dad has a tone generator lying in the basement unused.
Perfect! Exactly what you need right now.
Quote:
Originally Posted by d4v3 View Post
It seemed to amplify the signal from tube pins without issues (with plenty of hum especially on the preamp side).
Just to double check: 1) Is the signal-generator grounded to your tube amp input ground? 2) What kind of cable are you using from the signal generator to the guitar amp?

If the signal generator ground is connected to the guitar amp input ground, and you're using a shielded cable (just like a guitar cable, with a central conductor and an outer braid), you shouldn't have much hum. You will need just a few inches of unshielded wire at the end of the cable that connects to the guitar amp, so that you can clip it to the point you're testing.
Quote:
Originally Posted by d4v3 View Post
Will start to narrow it down a bit more.
Are you still using the Fuzz-Face? It's probably dead, and if you're still using it, that may be what's killing the guitar signal before it ever reaches the amplifier input.

What happens when you plug the guitar directly into the guitar amp, without using the Fuzz-Face?
Quote:
Originally Posted by d4v3 View Post
I have replaced the first 22uF electrolyte cap in the circuit.
Remember, only replace if there is a problem - if that stage isn't amplifying the signal as it should. Otherwise you'll be spending money replacing parts that are already good.
Quote:
Originally Posted by d4v3 View Post
Most likely will try to replace the 0.022uF caps with Vintage K75-12 PIO caps and go from there.
PIO caps are very popular with guitarists who don't know any better, but paper and oil are poor-quality materials to use in a capacitor. There will be no improvement in sound, and these low-quality caps are expensive.

My suggestion is to use inexpensive film caps, or even ceramic caps. Just make sure they have a high enough voltage rating. Coupling caps between tube stages should have a voltage rating that's at least a bit more than the B+ voltage. I suggest using 630V rated caps, or something close to that, so you have a safety margin.


-Gnobuddy
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Old 4th December 2019, 08:47 AM   #5
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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"PIO caps are very popular with guitarists who don't know any better, but paper and oil are poor-quality materials to use in a capacitor. There will be no improvement in sound, and these low-quality caps are expensive.

My suggestion is to use inexpensive film caps, or even ceramic caps. Just make sure they have a high enough voltage rating. Coupling caps between tube stages should have a voltage rating that's at least a bit more than the B+ voltage. I suggest using 630V rated caps, or something close to that, so you have a safety margin."


May I suggest that PIO capacitors are indeed outdated and of low quality compared to newer capacitors but I would not recommend Ceramic capacitors unless you want to turn your amplifier into a microphone like Fender did with the front end on some of their models, by mistake!
Ceramics can become very microphonic as they age.
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Old 4th December 2019, 06:17 PM   #6
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by JonSnell Electronic View Post
...I would not recommend Ceramic capacitors unless you want to turn your amplifier into a microphone...
I have indeed encountered microphonic ceramic caps, but the last time was maybe thirty-five years ago, in the front end of a cassette tape player, where the signal level was less than a millivolt, and there was an enormous amount of voltage gain following that capacitor.

Guitar signals are maybe a hundred times bigger than the signal from a cassette playback head, burying microphonic effects at least 40 dB lower. On top of that, the guitar amp discussed in this thread is not a high-gain death metal design, and crucially, ceramic cap technology is not the same as it used to be.

I'm confident there will be no issue with microphonics if ceramic caps are used in this amp - I've been using them, problem-free, for years, in similar designs of my own.

Aside from the normal evolution of electronics technology during the last four decades, there was a world-wide tantalum shortage for a while, and ceramic cap technology evolved rapidly during that period so that ceramic caps could take over where tantalum caps used to be used. IIRC this happened somewhere in the early 1990s, more than 25 years ago now.

Now you can buy inexpensive MLCC (multi layer ceramic chip) caps up to 22uF, well into the territory once occupied only by electrolytic caps. These make nice cathode bypass caps, much smaller and less expensive than same-capacitance film caps, and more durable than same-capacitance electrolytic caps.


-Gnobuddy
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Old 11th December 2019, 01:44 PM   #7
d4v3 is offline d4v3  Estonia
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Good news everyone, the amp is alive and working well again. I am a bit confused, what was the initial issue. The steps I took after checking with tone generator and confirming, that the signal was passing to the preamp tube and fine after the preamp tube.
I resoldered all my solder joints just to be sure.
Replaced all the 1/4watt resistors I had used for the mods
Did replace the 0.022 caps with the PIO ones, as these are the only ones I could source locally in a short timeframe. 50 euro cents each, they did not cost too much either. May be replacing these with Orange caps in the future, which are on the way.

Most likely initially I made the issue worse by replacing components, as I left one resistor place empty and had to add a jumper, which I almost forgot. Would have taken a lot longer, if I hadnt checked the schematic point to point. Lessons learned.

And the fuzz is still working as well, so no harm done I guess.

I even went a step further and converted the Valve junior schematic to be closer to Tweed champ. With the mods, it sounds very good. Still missing a couple of components, but overall I am very pleased with it.

Thanks to all for useful tips.
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Old 12th December 2019, 12:23 AM   #8
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by d4v3 View Post
May be replacing these with Orange caps in the future, which are on the way.
FYI - Orange Drop caps are "film caps", i.e. they use a thin plastic film as the dielectric between the two capacitor plates.

There is nothing wrong with Orange Drops, but they cost more than other brands of film caps, which perform identically for our purposes (guitar amp.)
Quote:
Originally Posted by d4v3 View Post
Most likely initially I made the issue worse by replacing components
I've suspected this ever since your first post - that's why I suggested you systematically look for the fault(s) using a signal generator.

Congratulations on a happy outcome!


-Gnobuddy
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