Output Problems with a Dr. Z Prescription RX amp head
« on: March 16, 2019, 07:52:43 PM »
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!


scott2000

Re: Output Problems with a Dr. Z Prescription RX amp head
« Reply #1 on: March 17, 2019, 09:26:33 AM »


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.


Looks mean nothing . Are you saying caps are not leaking DC? Or just not leaking fluid/ooze? Have they been tested? Resistors tested?

Not sure what noise you are describing in regards to the growl but, pin 5 and 9 of a 12ax7 are the heater and it's center tap.

Tube's outputs depend on the circuit setup but, usually the plate or cathode...... Definitely brush up  to familiarize yourself with how a tube works.... you really should know this kind of stuff before messing around.....




Deadly voltages in there......... You need to familiarize yourself with B+ voltages.......

Once you are able to give some real measurements, troubleshooting will be much easier......

Checking resistors should be easy enough.........

Good luck


abbey road d enfer

Re: Output Problems with a Dr. Z Prescription RX amp head
« Reply #2 on: March 17, 2019, 01:31:00 PM »
This is a boutique amp so keep in mind there are no schematics available.
You should trace it yourself, it's not so hard.

Quote
It appears as though I'm losing the signal at Pre Amp Valve 2 pin 2.
It surely means the point where you get signal is the grid 1 (12AX7 being a dual triode, there are no grid 2 or 3 anyway), at pin 2 or 7. This stage is the likely culprit. have you measured the DC voltages on the cathode (pin 3 or 8) and anodes (pin 1 or 6)?. That's troubleshooting 101.

Quote
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.
This information is available easily, just google 12ax7. Anyway, in 99% of cases, the input is on the grid and output on teh anode in 90%.

Quote
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.
That's the heaters! 6.3Vac.
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?

Quote
My only idea is possibly Pin 2 on valve 2 is broken.
Have you tried a new one? The tube may be duff... It happens.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

pucho812

Re: Output Problems with a Dr. Z Prescription RX amp head
« Reply #3 on: March 21, 2019, 04:39:18 PM »
careful with that ax eugene.

not much  to add here as  it has been covered but I will stress safety.  If you are not sure what you are doing with tubes, you can hurt yourself so do some reading before you go in.  I would try replacing the tubes, it's simple, easy and you have minimal chance of injury.   It's quite common to not see an issue with a tube but have it be spent(finished, needs replacement).  on rare occasions you will see white power inside  the tube which indicates  the vacuum has broken or worse the tube glass cracked or some other signs but usually  a tube will be done and there are no signs other then no signal.  depending on how they wired the el84's you may be able to test them in pairs to see if you have a bad power amp tube but being how you appear green on this, I would  not bother. If you had access to a tube tester you could at least confirm a faulty tube.
Best of luck on this and take extreme caution as to avoid any injury.
You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is.

shabtek

Re: Output Problems with a Dr. Z Prescription RX amp head
« Reply #4 on: March 22, 2019, 08:24:32 PM »
Keep 1 hand in your pocket when poking around, so that if you get some electricity on you  it does not go across your heart.
"really fine players do not use stomp boxes or master volume, they match the amp to the room and turn it up to 11.  Stevie Ray, BB King, Albert King, Duane Allman, Dicky Betts, Louis Armstrong"
   -CJ

Re: Output Problems with a Dr. Z Prescription RX amp head
« Reply #5 on: March 23, 2019, 03:29:35 AM »
I was going to nip in and mention the one handed probing technique ,Shabs beat me to it .

A croc clip on your ground lead and plastic insulator over all but the very tip of the + probe is also a good plan .

The pre-amp tubes have of course two heaters , check carefully that both sections light up ,
A bad resistor or cap will very often make your amp sound bad , but wont often cause no sound at all , check for bent pins or damaged tube sockets also .

andyfromdenver

Re: Output Problems with a Dr. Z Prescription RX amp head
« Reply #6 on: March 24, 2019, 09:39:46 AM »
Hi!  if you can follow the one hand behind back approach,
put your meter on DC and ground the black to the chassis.  Take a DC voltage reading on the preamp tube pins 1,6,3,8
report those voltages.
a sudden turn off sounds like the HT fuse blew (assuming there is one), but you said you checked that.  Checking voltages will give the team valuable info.
get the pinouts for all of the tubes, check the plate voltages and cathode voltages for the tubes and report back with clear info.

I am not responsible for any damage to you or your amp for the above input :)

Best wishes,
Andy
2018 current fav movie: Black Panther
2018 current fav book: Foundation ( I. Asimov)
1997-2018 fav artist: Erik Satie

Gus

Re: Output Problems with a Dr. Z Prescription RX amp head
« Reply #7 on: March 24, 2019, 10:33:16 AM »
a google search found this it looks like you have to join to see attachments.
https://ampgarage.com/forum/viewtopic.php?f=6&t=11494&sid=ac6e26c1e85971221e3e7fd31bdecc94&start=15

 Your questions read  like you should not be working on the amp without getting further training

« Last Edit: March 24, 2019, 10:42:39 AM by Gus »

CJ

Re: Output Problems with a Dr. Z Prescription RX amp head
« Reply #8 on: March 24, 2019, 11:49:46 PM »
that is gonna be dirty jacks on the effects loop jacks (sen/return) or (preamp/pwr amp)

and nobody dies working on tube amps,  (see "shock log" in the Brewery by cj)

bacon will kill you quicker than a bullet,

If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html

Re: Output Problems with a Dr. Z Prescription RX amp head
« Reply #9 on: March 25, 2019, 04:26:21 PM »
Hey Everybody!

So hopefully you're all still there! Sorry it took so long to get back to this thread but I took a few notes from Scott 2000 and Abbey Road and spent this last week doing some reading on tubes, tube pinouts and such. I appreciate you guys sending me in that direction! Thank you for Andy from Denver too! I took your cue and after reading up on testing plate voltages and pinouts I have some new info for ya that can hopefully shed a little more light on this problem! I actually didn't see the rest of the responses until i got back on here but thanks for everyone shooting me what you know! So here's what I got for ya now:

So I'm still getting the same effect here. I can get an audio signal to Pre-amp tube 1 and out again but as soon as it hits the Grid (thanks again Abbey Road!) It stops right there. This time around I also noticed that the first pre amp tube isn't amplifying the audio signal at all in fact the signal is actually loosing volume at the first tube and then goes out completely on the second pre-amp tube.

Now I tested the resistors across the tube sockets and all of the ones leading up to the pre-amp sockets and they are all testing good (to whoever mentioned the resistors I did this before I saw your message. lol.)  Same with all of the potentiometers sending signal to the tubes. I also tested continuity from one side of each pin in the socket to the otherside of the pin and all of the pins are passing continuity in all three pre-amp tube sockets.

Now I know 100% these tubes are good; I put all three 12ax7s and the EL84s through a valve tester which says they are fine and swapped the 12ax7s into another amp in which case they worked! I've also replaced the tubes in this amp with ones I know are working and the amp still did not make any sound. I forgot who mentioned it; but none of the getters are turning white either and I also checked to make sure both heaters in each 12ax7 are lighting up and indeed they are.

So; after reading up on how to test plate, cathod and grid voltages and printing off specs for these specific 12ax7s (thanks again andy from denver!!) I took some voltage readings and this is what I got (the results are nearly Identical for all three preamp tubes by the way):

Grid 1 Voltage = -.7v on pin 2 (should be -2v in a class a amplifier according to the spec sheet)
Cathod voltage = Pure ass zero volts. nothing. doesn't matter what DCV setting my meter is on and that's on pins 3 & 8.
Plate Voltage = 1v (Should be 250v but both pins 1 & 6 read 1v)

So can any of you guys tell me what this means? is this a problem with the transformer connected to the pre-amp tubes? or is this perhaps something wrong with the power circuit? again even after all of these tests the fuses (included the HT fuse; sorry forgot again you mentioned that) are still perfectly fine! The rectifier tube is also lighting up just fine too?

So this is where I am at now. If anybody has any idea what these readings mean I'd grealty appreciate the insight! Thanks again for all the responses! Have a great day you guys and thanks ahead of time for any help you can afford!



pucho812

Re: Output Problems with a Dr. Z Prescription RX amp head
« Reply #10 on: March 25, 2019, 05:16:08 PM »
that is gonna be dirty jacks on the effects loop jacks (sen/return) or (preamp/pwr amp)

and nobody dies working on tube amps,  (see "shock log" in the Brewery by cj)

bacon will kill you quicker than a bullet,

no but the  electrical bite is none to fun either.
You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is.

abbey road d enfer

Re: Output Problems with a Dr. Z Prescription RX amp head
« Reply #11 on: March 25, 2019, 06:03:39 PM »
Grid 1 Voltage = -.7v on pin 2 (should be -2v in a class a amplifier according to the spec sheet)
Cathod voltage = Pure ass zero volts. nothing. doesn't matter what DCV setting my meter is on and that's on pins 3 & 8.
Plate Voltage = 1v (Should be 250v but both pins 1 & 6 read 1v)
What are you measuring the voltages with?
You need to check the voltages delevered by the power supply. Check the other side of the plate resistor; you should see about 200-300V.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

Re: Output Problems with a Dr. Z Prescription RX amp head
« Reply #12 on: March 26, 2019, 03:48:22 PM »
Hey There Abbey Roads,

Thanks for responding back.

So I'm using a commercial electric DMM set to measure DCV. I have the power and standby switches on; giving the tubes about 10 minutes to warm up; all knobs turned to zero.  Black lead grounded at the chassis and red lead on the tube socket pin.

I'm measuring plate voltage from pins 1 & 6, Grid voltage from pins 2 & 7 and Cathode voltage from pins 3 & 8 on the 12AX7s

I went back and measured the DCV on the EL84 output tubes as well; Plate voltage on pin 7, Grid Voltage on pin 2 and Cathode voltage on pin 3.

When my meter is set to higher DCV settings I get no readings at all. When it's set to a lower setting (2 volts) I get these readings:

V1 (12AX7) plate: -.84, Cathode: .02, Grid: -.41

V2 (12AX7) Plate: -.86, Cathode: .07, Grid: -.52

V3 (12AX7) Plate: -.83, Cathode: .003, Grid: -.65

V4 (EL84) Plate: -.91, Cathode: 0, Grid: -.67

V5 (EL84) Plate: -.96, Cathode: 0, Grid: -.66

V6 (EL84) Plate: -.94, Cathode: 0, Grid: -.71

V7 (EL84) Plate: -.93, Cathode: 0, Grid: -.71

Disclaimer here: I got the DMM leads in the right connections on the DMM. I also just recently measured the plate voltage on a quad of 6L6s in my Fender this last weekend and got accruate readings for those tubes. Same system I'm using here: black to ground, Red to plate pin, meter set to measure 600 DCV (the highest setting on my meter).  I've rechecked and rechecked the fuses and they are still not blown. I've pulled out the plate resistors and they are measuring fine, I also pulled out the tubes and tested them once again and they are testing fine in a valve tester. The 12AX7s are working in my fender.  If I'm doing something wrong here please let me know.

I also took the liberty of drawing up the schematic like you said. It looks like sh*t though so I'm gonna re-draw it and if it helps I can also post that here. Thanks again for the help and the advice.

abbey road d enfer

Re: Output Problems with a Dr. Z Prescription RX amp head
« Reply #13 on: March 26, 2019, 06:13:59 PM »
You need to check the PSU. For some reason it just doesn't deliver the B+ voltage. It could be many things, including open secondary, blown rectifiers, blown resistors. You need to check voltages at the secondary (AC volts), after the rectifier (about 300Vdc) and follow through the resistors that distribute the voltage to the anode resistors.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

fazer

Re: Output Problems with a Dr. Z Prescription RX amp head
« Reply #14 on: March 26, 2019, 07:28:27 PM »
I use a chart I use  . I run pins of tube across and tubes down .  I take a dc meter to ground and + lead to PIN number.  Then take ac reading for heaters .  I also measure the ac voltage across the transformer and rectified dc voltage at the powersuppy cap nodes.

fazer

Re: Output Problems with a Dr. Z Prescription RX amp head
« Reply #15 on: March 26, 2019, 07:36:18 PM »
bad pdf try this and see if it works for a voltage grid on tube sockets.

PRR

Re: Output Problems with a Dr. Z Prescription RX amp head
« Reply #16 on: March 26, 2019, 08:04:23 PM »
> Plate Voltage = 1v

You are a farmer. You get water from a lake on the hill. You want 400mm of water in your cattle-trough, you got it, the cows are happy. BUT you got 1mm of water in the dog-trough and the dogs are too dry to bark!

"Obviously" you walk the ditch from cow-trough to dog-trough and see why water is not getting through.

Since a good meter can "find" 1V on tube pins just from heater effect, no B+, I suspect you have a break in the B+ supply filter chain.

fazer

Re: Output Problems with a Dr. Z Prescription RX amp head
« Reply #17 on: March 26, 2019, 10:27:46 PM »
Quote
I suspect you have a break in the B+ supply filter chain.
. Or break in cathode resistor.  Or cold solder pin

abbey road d enfer

Re: Output Problems with a Dr. Z Prescription RX amp head
« Reply #18 on: March 27, 2019, 03:17:50 AM »
. Or break in cathode resistor. 
No; there would be high voltage on the anode.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

JMFahey

Re: Output Problems with a Dr. Z Prescription RX amp head
« Reply #19 on: March 27, 2019, 04:37:30 AM »
Agree.
Plate resistor may be open or it´s not getting voltage from the supply.

Measure voltage at both ends of plate resistor; then pull tube and measure again, post all 4 values.

Dr Z amplifiers are very simple, minimalistic, and there lies their charm, so drawing its circuit is very easy.

Design - Make - Service Audio Equipment since 1969.


 

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