Pigbenis27
Member
- Joined
- Sep 26, 2017
- Messages
- 16
Hello There everybody!
My buddy has got a problem with his Dr. Prescription RX Head that's throwing us both for quite a loop! I actually Dr. Z the following letter to hear what their thought are on the problem but while i'm awaiting a response I know there is plenty of smart people on these forums that might be kind enough to take the time to read through what I got and give me some advice. Thank you in advance for any knowledge or help you can afford!
The gear in question is a 2004 Prescription head; Never had problems. This is a boutique amp so keep in mind there are no schematics available. The preamp tubes are 3x 12AX7s and the Output Tubes are 4x EL84s. it has three differnet outputs: 4ohm, 8ohm and 16ohm. So Supposedly this friend was in the middle playing a set and his guitar sound quickly faded out and then simply never came back and it has stayed that way for about a week now. It powers on and the tubes all light up and look good but there is zero signal between any of the three outputs.
I've gone through all of the basic tests that I can:
1. Fuses are all fine and not blowing when replaced. The front Light also comes on when powered up. The cables him and I are both using are good to go as are the speakers both him and I have been using to test it.
2. All tubes are lighting up and testing fine all the way down the line and so are replacement tubes I have on hand for testing purposes. I'm using my orange valve tester to check them in addition for common sense visual signs of which there is none: No Cherry Plating, Arcing, Rattling inside the tubes, Microphonic indicators or broken getters. This includes the Rectifier Tube (a 5AR4) so there doesn't seem to be any power circuitry problems.
3. None of the Filter Caps are leaking or bulging and visually look top notch.
4. No Loose wires anywhere; no burnt resistors, no popped or leaking electrolytic caps; in fact, no components of any kind look even remotely suspicious.
5. Both Input Jacks are sturdy, and the entire inside is actually quite clean for how much use it has seen.
6. All of the Tube Sockets are also sturdy. None of the soldering tabs slide out when I give a quick tug on them and the brackets inside are nice and tight and seem to be gripping the tube pins just fine.
7. So I tested the continuity between the input jacks and the volume knob and it tests fine. Then I Tested continuity between the preamp tube socket connectors and the other pots/switches. All fine. So I Test continuity between all four of the output EL84s. All Fine. Finally I test continuity between the Tip of the Input jacks and the Tip of all 3 output jacks. All fine!!!!!
8. So Finally I ran a 1Khz tone through the unit with a dummy load connected. I have a handy dandy AC tester that I connect to a small speaker that allows me to hear the test tone when I hold it against the lead of a component, jack, connection etc. It appears as though I'm losing the signal at Pre Amp Valve 2 pin 2. Meaning I can hear with my device where the audio hits the tube socket but I can't find it coming off of the tube socket anywhere. I'm also noticing that none of the potentiometers other than the volume pot are having an affect on the signal anywhere in the circuit; but again, there is no audio in the circuit after valve 2.
Now I didn't actually go to school for any of this and as such I'm currently in the process of improving on my tube knowledge. I'm not entirely sure which pin is supposed to be the tube's output. I'm using the power of deduction to assume that pin 2 is the input of these particular pre-amp tubes (maybe all pre-amp tubes or at least 12AX7s?) since I can clearly hear the signal on these pins and they seem to be coming from logical inputs. However; I don't know which pin is supposed to be the output of a pre-amp tube. I can Tell you that the sound I get from my audio tester device off of pins 9 and 5 on all of the pre-amp tubes is a really really nasty low end hum. Like more of an angry bear growl than a hum. I noticed these pins are all daisy chained to the corresponding pins on the next pre-amp tube then daisy chained to pins 4 and 5 on the output tubes and then finally fed into one of the three transformers. Is this nasy noise normal? Or does it indicate a problem witht the transformer they are all connected to?
My only idea is possibly Pin 2 on valve 2 is broken. Perhaps it's still stuck inside of the socket so it doesn't pop out when I tug it and the brackets that clench the pin are fine but the metal tab is disconnected from the bracket inside of the socket? Or since I get continuity through the whole circuit and; please correct me if i'm wrong, resistors can still pass continuity even if they're shot then I thought perhaps it is a resistor somewhere in the circuit between Valve 2 and 3? Or finally perhaps It is a Transformer Thing? Unfortunately I know even less about Transformer than I do Tubes. Again; I'm building on that as well just one step at a time.
Anyways; I apologize for the long email I'm sure it's kind of a pain in the ass to read through that much but I'm trying to expand my knowledge on amps, tubes, audio circuits, etc. and I figured there would be no one better to help out with this particular problem than you! So any help and time you can afford to give me would be very greatly and humbly appreciated. Have a great rest of your day and thank you for reading my email and hopefully I'll hear from you in the future!
My buddy has got a problem with his Dr. Prescription RX Head that's throwing us both for quite a loop! I actually Dr. Z the following letter to hear what their thought are on the problem but while i'm awaiting a response I know there is plenty of smart people on these forums that might be kind enough to take the time to read through what I got and give me some advice. Thank you in advance for any knowledge or help you can afford!
The gear in question is a 2004 Prescription head; Never had problems. This is a boutique amp so keep in mind there are no schematics available. The preamp tubes are 3x 12AX7s and the Output Tubes are 4x EL84s. it has three differnet outputs: 4ohm, 8ohm and 16ohm. So Supposedly this friend was in the middle playing a set and his guitar sound quickly faded out and then simply never came back and it has stayed that way for about a week now. It powers on and the tubes all light up and look good but there is zero signal between any of the three outputs.
I've gone through all of the basic tests that I can:
1. Fuses are all fine and not blowing when replaced. The front Light also comes on when powered up. The cables him and I are both using are good to go as are the speakers both him and I have been using to test it.
2. All tubes are lighting up and testing fine all the way down the line and so are replacement tubes I have on hand for testing purposes. I'm using my orange valve tester to check them in addition for common sense visual signs of which there is none: No Cherry Plating, Arcing, Rattling inside the tubes, Microphonic indicators or broken getters. This includes the Rectifier Tube (a 5AR4) so there doesn't seem to be any power circuitry problems.
3. None of the Filter Caps are leaking or bulging and visually look top notch.
4. No Loose wires anywhere; no burnt resistors, no popped or leaking electrolytic caps; in fact, no components of any kind look even remotely suspicious.
5. Both Input Jacks are sturdy, and the entire inside is actually quite clean for how much use it has seen.
6. All of the Tube Sockets are also sturdy. None of the soldering tabs slide out when I give a quick tug on them and the brackets inside are nice and tight and seem to be gripping the tube pins just fine.
7. So I tested the continuity between the input jacks and the volume knob and it tests fine. Then I Tested continuity between the preamp tube socket connectors and the other pots/switches. All fine. So I Test continuity between all four of the output EL84s. All Fine. Finally I test continuity between the Tip of the Input jacks and the Tip of all 3 output jacks. All fine!!!!!
8. So Finally I ran a 1Khz tone through the unit with a dummy load connected. I have a handy dandy AC tester that I connect to a small speaker that allows me to hear the test tone when I hold it against the lead of a component, jack, connection etc. It appears as though I'm losing the signal at Pre Amp Valve 2 pin 2. Meaning I can hear with my device where the audio hits the tube socket but I can't find it coming off of the tube socket anywhere. I'm also noticing that none of the potentiometers other than the volume pot are having an affect on the signal anywhere in the circuit; but again, there is no audio in the circuit after valve 2.
Now I didn't actually go to school for any of this and as such I'm currently in the process of improving on my tube knowledge. I'm not entirely sure which pin is supposed to be the tube's output. I'm using the power of deduction to assume that pin 2 is the input of these particular pre-amp tubes (maybe all pre-amp tubes or at least 12AX7s?) since I can clearly hear the signal on these pins and they seem to be coming from logical inputs. However; I don't know which pin is supposed to be the output of a pre-amp tube. I can Tell you that the sound I get from my audio tester device off of pins 9 and 5 on all of the pre-amp tubes is a really really nasty low end hum. Like more of an angry bear growl than a hum. I noticed these pins are all daisy chained to the corresponding pins on the next pre-amp tube then daisy chained to pins 4 and 5 on the output tubes and then finally fed into one of the three transformers. Is this nasy noise normal? Or does it indicate a problem witht the transformer they are all connected to?
My only idea is possibly Pin 2 on valve 2 is broken. Perhaps it's still stuck inside of the socket so it doesn't pop out when I tug it and the brackets that clench the pin are fine but the metal tab is disconnected from the bracket inside of the socket? Or since I get continuity through the whole circuit and; please correct me if i'm wrong, resistors can still pass continuity even if they're shot then I thought perhaps it is a resistor somewhere in the circuit between Valve 2 and 3? Or finally perhaps It is a Transformer Thing? Unfortunately I know even less about Transformer than I do Tubes. Again; I'm building on that as well just one step at a time.
Anyways; I apologize for the long email I'm sure it's kind of a pain in the ass to read through that much but I'm trying to expand my knowledge on amps, tubes, audio circuits, etc. and I figured there would be no one better to help out with this particular problem than you! So any help and time you can afford to give me would be very greatly and humbly appreciated. Have a great rest of your day and thank you for reading my email and hopefully I'll hear from you in the future!