Hi, Iam Daniel. This is my very first post here at DIY Audio.
Iam an Engineer, but I had focus in computers, not amps. So Iam a newbie in this land.
I have build a Tiny Terror (Valve Guitar Amp) clone. And Iam having some problems with it.
The amp works, and do what is supossed to do, but when you rise GAIN over than 4 the sound breaks and your ear something like a 'whistle' and you can't play nothing. (Sorry about the term, English is not my mother language).
This is my very first Tube Amp, so dissmiss the mess that you would see in the pic.
There are 3 shielded cables, the lowest one is the guitar input, the middle and upper ones are the GAIN pot conections to the first 12AX7 tube (PreAmp Stage). I had done this trying to minimize interference, but without success.
I hope anyone could give some hints where to start looking for this noise source.
There is an audio in the zip file so you can listen what the amp is doing. I believe this is a kind of oscillation, but Iam not sure.
Thanks in advance.
Daniel.
Iam an Engineer, but I had focus in computers, not amps. So Iam a newbie in this land.
I have build a Tiny Terror (Valve Guitar Amp) clone. And Iam having some problems with it.
The amp works, and do what is supossed to do, but when you rise GAIN over than 4 the sound breaks and your ear something like a 'whistle' and you can't play nothing. (Sorry about the term, English is not my mother language).
This is my very first Tube Amp, so dissmiss the mess that you would see in the pic.
There are 3 shielded cables, the lowest one is the guitar input, the middle and upper ones are the GAIN pot conections to the first 12AX7 tube (PreAmp Stage). I had done this trying to minimize interference, but without success.
I hope anyone could give some hints where to start looking for this noise source.
There is an audio in the zip file so you can listen what the amp is doing. I believe this is a kind of oscillation, but Iam not sure.
Thanks in advance.
Daniel.
Attachments
Relocated to the amps & instruments forum where the guitar amp gurus hang out.. 
Just a few ideas, I an no expert but I have built a few guitar amps.
Its more than likely a lead dress issue. You can try moving some of the wires around with the amp running. Set the gain control so you just start to hear the oscillation, use a NONE conductive object to move the wires around.
Check the wiring around v1 (Input valve).
The wiring around the phase inverter looks a little bunched up.
Try removing V1, If you still hear the oscillation the issue is after the input stages.
Are the coupling caps the correct size? A higher value can cause instability in some high gain amps.
Just as a side note your heater wiring looks very strange to me, normally with AC heaters one would tightly wind the wires and run them around the edge of the chassis away from the main board.
Hope this helps.
Its more than likely a lead dress issue. You can try moving some of the wires around with the amp running. Set the gain control so you just start to hear the oscillation, use a NONE conductive object to move the wires around.
Check the wiring around v1 (Input valve).
The wiring around the phase inverter looks a little bunched up.
Try removing V1, If you still hear the oscillation the issue is after the input stages.
Are the coupling caps the correct size? A higher value can cause instability in some high gain amps.
Just as a side note your heater wiring looks very strange to me, normally with AC heaters one would tightly wind the wires and run them around the edge of the chassis away from the main board.
Hope this helps.
> dissmiss the mess that you would see
No, the mess is probably part of the problem.
Amplify a small signal big. Let some of that big signal sneak back to the input. It is amplified again, and again, and again.... scream.
I can't work out your layout. But I bet later signals are sneaking into earlier stages. Remember that high frequencies couple through air. Not well, but a guitar amp has a LOT of gain.
I see three too-long wires on the 12AX7 nearest the input jack. Little antennas.
Find pictures of a happy Tiny Terror and try to emulate the wiring.
No, the mess is probably part of the problem.
Amplify a small signal big. Let some of that big signal sneak back to the input. It is amplified again, and again, and again.... scream.
I can't work out your layout. But I bet later signals are sneaking into earlier stages. Remember that high frequencies couple through air. Not well, but a guitar amp has a LOT of gain.
I see three too-long wires on the 12AX7 nearest the input jack. Little antennas.
Find pictures of a happy Tiny Terror and try to emulate the wiring.
I have to agree with the other contributors regarding the wiring layout of your amp. I recently built an ECL82 based guitar amplifier from scratch, just to see if I could do it. I used point to point wiring but kept it short and tidy and paid attention to its routing. To further protect against oscillation, I made sure the grid stopper resistors were wired hard up against the valve pins and also included anode load bypass capacitors. I was somewhat amazed, but very chuffed, when it worked first time!
Thank you all for your responses.
Galu, this is a very well known Amp. There are a lots of schematics (a lot of them coming from the same guy) on Internet. This is one: Prowess Amplifiers - Orange - Schematics - Tiny Terror Iam not an expert too, but there are grid resistors on it.
By the way, I found this in Aiken. very interesting.
Dazcwl, could you please explain me"lead dress issue"? I just learn English at University, sometimes I get lost in translation. But you are right, the shielded cables are the reason of what you said. I just twist and 'de-twist' the cables that goes from GAIN pot to grid of V1 & V2 (halves) because when I moved (with mi fingers) the scream changes.
Can I just remove V1, turn it on & see what happens? Iam gonna double check caps.
The heater wiring...well this is my first Amp. That was the last thing I had done in building process, the cables was a little short. I was a little anxious...
PRR I was afraid someone said that. Unfortunatelly (remember, Iam anxious) I found this after my building work. thats a tiny properly wired. Do you think my best shot is wiring it (properly) again?.
Again thank you all for your responses. I got a lot work.
Galu, this is a very well known Amp. There are a lots of schematics (a lot of them coming from the same guy) on Internet. This is one: Prowess Amplifiers - Orange - Schematics - Tiny Terror Iam not an expert too, but there are grid resistors on it.
By the way, I found this in Aiken. very interesting.
Dazcwl, could you please explain me"lead dress issue"? I just learn English at University, sometimes I get lost in translation. But you are right, the shielded cables are the reason of what you said. I just twist and 'de-twist' the cables that goes from GAIN pot to grid of V1 & V2 (halves) because when I moved (with mi fingers) the scream changes.
Can I just remove V1, turn it on & see what happens? Iam gonna double check caps.
The heater wiring...well this is my first Amp. That was the last thing I had done in building process, the cables was a little short. I was a little anxious...
PRR I was afraid someone said that. Unfortunatelly (remember, Iam anxious) I found this after my building work. thats a tiny properly wired. Do you think my best shot is wiring it (properly) again?.
Again thank you all for your responses. I got a lot work.
Use a wooden stick to move the wires and see if it changes the squeal. Maybe you can find a position where it works for you.
The real solution would be to rewire the thing. Heater cables twisted and near the chassis. Avoid parallel signal wires. Use shielded wire for long runs, like from the input jack to the first tube. Move the wiring near the chassis, so that the magnetic field is reduced by the grounded chassis.
The real solution would be to rewire the thing. Heater cables twisted and near the chassis. Avoid parallel signal wires. Use shielded wire for long runs, like from the input jack to the first tube. Move the wiring near the chassis, so that the magnetic field is reduced by the grounded chassis.
Wires that are too long, or too close to other wires, can cause unwanted positive feedback (as PRR explained.) Result: howling (oscillation.)
That's telling you that you have lead dress problems for sure.
Try to keep wires short, and away from each other. Use coaxial cable (shielded cable) for sensitive wires carrying small signals to grids of triodes.
After you fix the squeal, if you find you have too much hum, then you can worry about improving the heater wiring.
At the least, you do have to clean up the existing mess of long wires all jumbled together.
Unfortunately, analogue circuits do not have the same kind of immunity to noise and interference that digital circuits do!
-Gnobuddy
Finally I found some time to repair my amp.
I shorten all the wires from PCB to 12AX7 valves.
The Amp sound improves a little. Now I can use Master at 6, before only at 4.
Even this point is not the final station it just show me that this is the correct way.
I have to clean up all the mess.
Thank you all guys!
I shorten all the wires from PCB to 12AX7 valves.
The Amp sound improves a little. Now I can use Master at 6, before only at 4.
Even this point is not the final station it just show me that this is the correct way.
I have to clean up all the mess.
Thank you all guys!
I built a Tiny Terror Clone as well and had oscillation problems despite being very aware of "lead dress" during the build (it was about my 10th Guitar Amp build and I've done twice that many HiFi Amps).
I found I needed 10K grid stop resistors on both phase splitter triodes (V2) and also on the 2nd half of V1 on the triode input feed from the 1st gang of the gain control pot.
The bodies of the grid stop resistors need to be right against the tube socket pin, that is, not on the wiring board but right at the tube socket.
The 68K on the input triode should be mounted that way too.
Also check the final filter capacitor (the one providing the B+ to V1) for dry solder joins at either end. Bad connection would cause "motorboating" form of oscillation rather than the "parasitic" form of oscillation we have discussed above.
Unfortunately the fact that the oscillation frequency chnages as you push wires about does not tell you thaat the oscillation is one form (parasitic or motorboating) or the other.
Cheers,
Ian
I found I needed 10K grid stop resistors on both phase splitter triodes (V2) and also on the 2nd half of V1 on the triode input feed from the 1st gang of the gain control pot.
The bodies of the grid stop resistors need to be right against the tube socket pin, that is, not on the wiring board but right at the tube socket.
The 68K on the input triode should be mounted that way too.
Also check the final filter capacitor (the one providing the B+ to V1) for dry solder joins at either end. Bad connection would cause "motorboating" form of oscillation rather than the "parasitic" form of oscillation we have discussed above.
Unfortunately the fact that the oscillation frequency chnages as you push wires about does not tell you thaat the oscillation is one form (parasitic or motorboating) or the other.
Cheers,
Ian
Hi Ian!
Thanks for your valuable input, and your instructions on how to install grid stop resistors.
As you can see in my quote from post #3, I alerted Daniel to the subject of grid resistors early on in the thread.
He replied that grid resistors were fitted, but I couldn't see any resistors wired hard up against the valve pins in his photograph.
Daniel - perhaps you should look again at the topic of grid resistors over at aikenamps.com
Valve circuits are high impedance so any long leads will pick up interference and radio.
Especially inportant is the input to the amp, very short leads and screened where possible. Keep wires away from transformers and HVAC.
Grid stoppers are vital to stop RF etc getting in.
I have sometimes had to resort to RC input filter as grid stopper wasn't enough.
Especially inportant is the input to the amp, very short leads and screened where possible. Keep wires away from transformers and HVAC.
Grid stoppers are vital to stop RF etc getting in.
I have sometimes had to resort to RC input filter as grid stopper wasn't enough.
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