Can't get sound out of my ODS-100-HRM

[Mike_J]

I recently completed a Dumble 100 watt HRM ODS.  The amp was based on ic-racers HRM-102 schematic on the Amp Garage forum.  I had completed it without the jfet input jack.

The amp had a nice Fender overdrive tone.  However, that is not what I was going after for this amp.  Therefore, I decided to change the overdrive tone stack from plate to cathode follower driven in hope of bringing more of a Marshall crunch to the overdrive channel.  In order to install eyelets for the mosfet I had to pretty much dismantle the amp and at the same time I added the jfet input circuitry to the amp.  I also changed the 12ax7 feeding the PI.

I turned the amp on and plugged in and had no volume.  I took preamp plate voltage readings.  The voltages to the plates for the PI measured 445V, 400V at V2 and 380V at V1.  When I measured the voltages on the PI I hear static through the speakers but do not hear anything when I measure the plate voltages for V1 or V2.

I discussed this with tubenit and he suggested changing back to having the plate feed the OD circuit again to see if the problem went away but it did not.  My first thought is that the voltage string resistors are not correct, which may be part of the problem, but that does not explain getting no sound out of the amp.

I tested the preamp tubes with my tube tester and they all tested positive for transconductance.  The heater voltages on each preamp tube test point is 3.35Vac.

Does anyone have any suggestions for troubleshooting this problem?  If I ever figure out how to shrink the size of the pictures I have taken inside the amp I will attach them.  They are over three megs each.

[HotBluePlates]

If I ever figure out how to shrink the size of the pictures I have taken inside the amp I will attach them.  They are over three megs each.

If you have a Windows machine, use MS Paint or Windows Picture Manager to resize the image, depending on the age of your operating system.

I recently completed a Dumble 100 watt HRM ODS.  The amp was based on ic-racers HRM-102 schematic on the Amp Garage forum.

Please post the schematic as well, as I don't know my HRM-102 from a hole in the ground. 

... I decided to change the overdrive tone stack from plate to cathode follower driven in hope of bringing more of a Marshall crunch to the overdrive channel.  In order to install eyelets for the mosfet I had to pretty much dismantle the amp and at the same time I added the jfet input circuitry to the amp. ...

I turned the amp on and plugged in and had no volume.  ...

Hopefully, you are exaggerating when you say "dismantle the amp." Obviously, if the amp worked before, whatever you tinkered is likely where the problem lies. If you tinkered fewer things, you'll have fewer places to look (which is 90% of why people say to do 1 modification at a time).

But since you seem to have d.c. voltage present at all preamp tube plates, make sure the same is true of all cathodes. A 0vdc reading is probably suspect in your amp.

If you have a steady hand, and a meter with alligator clips on the probes, try this:
Clip the black meter lead to ground and set the meter to measure d.c. volts. If your meter is not auto-ranging, set it to its highest scale. Carefully measure the voltage at the output tube grids (pin 5). You should hear a small pop when the meter probe touches the grid. Now move backwards to the phase inverter plate and measure d.c. volts. You should see a voltage and hear a pop about the same volume as the output tube grids.

Now move back to the phase inverter grid. When you touch the grid with the probe, the pop should be a little louder than it was at the plate (due to tube gain).

The measurements do 2 things: you're checking for voltages, but the "Pop!" tells you everything from that point to the speaker is passing audio (enough to get something out). When you work backward from output to input and find a plate or grid that makes no pop when the last item checked did, you have found a place where audio is not passing. Now you just need to figure out why audio isn't passing (tube not getting d.c., tube not passing current, bad ground, disconnected coupling cap, short-to-ground, etc.)

[tubenit]

You simply open the picture in PAINT and then hit either “resize” or “shrink” or “expand” and then resize/shrink/expand smaller.   Very easy to do.

You definitely have an odd problem with this.   IF you bypass the tone stack, will it work then?

I’d simply start jumpering and bypassing things until you find what does work.

With respect, Tubenit

[Mike_J]

I am using Windows 8 and a 16 mp camera that belongs to my daughter.  I wonder if I use my wife's 3.2 mp camera if my problem would go away as it would not capture as many pixels.  I must solve the picture problem before I solve the transfer of the schematic problem.  Does anyone think using a lower mp camera would help?  So far I have tried saving the picture in jpeg and shrinking the picture.  Shrinking the pictures removes too much content.

I was thinking about buying a phone with a 41 mp camera in it but that would seem to make the camera useless for purposes such as this because it captures too much information.

[eleventeen]

"Does anyone think using a lower mp camera would help?"

Yes.

[Mike_J]

Here is ic-racers hrm-102 schematic. 

[HotBluePlates]

Tell me what you get with the pop test. Also mention what setting the relays are in when you do the test.

[Mike_J]

Hopefully, you are exaggerating when you say "dismantle the amp." Obviously, if the amp worked before, whatever you tinkered is likely where the problem lies. If you tinkered fewer things, you'll have fewer places to look (which is 90% of why people say to do 1 modification at a time).

But since you seem to have d.c. voltage present at all preamp tube plates, make sure the same is true of all cathodes. A 0vdc reading is probably suspect in your amp.

If you have a steady hand, and a meter with alligator clips on the probes, try this:
Clip the black meter lead to ground and set the meter to measure d.c. volts. If your meter is not auto-ranging, set it to its highest scale. Carefully measure the voltage at the output tube grids (pin 5). You should hear a small pop when the meter probe touches the grid. Now move backwards to the phase inverter plate and measure d.c. volts. You should see a voltage and hear a pop about the same volume as the output tube grids.

Now move back to the phase inverter grid. When you touch the grid with the probe, the pop should be a little louder than it was at the plate (due to tube gain).

The measurements do 2 things: you're checking for voltages, but the "Pop!" tells you everything from that point to the speaker is passing audio (enough to get something out). When you work backward from output to input and find a plate or grid that makes no pop when the last item checked did, you have found a place where audio is not passing. Now you just need to figure out why audio isn't passing (tube not getting d.c., tube not passing current, bad ground, disconnected coupling cap, short-to-ground, etc.)

Unfortunately it only gets worse.  I also changed the power transformer.  I came to the same conclusion you did regarding doing more than one thing at a time within a split second of not hearing any volume out of the amp.  Oh well, hopefully live and learn.

Pin three of the power tubes reads 463 volts.  Pin five of the power tubes reads -43.6.  The plate of the PI tube pops.  The grid does not pop.  There is no voltage on the PI grids or the cathodes.

I made the observations above with the relays energized and not energized.  Didn't make a difference.

I also have pictures to post.  Using the camera with the smaller mp rating and shrinking them a little made the difference

EDIT: Fixed broken quote

[Mike_J]

Tell me what you get with the pop test. Also mention what setting the relays are in when you do the test.

I responded but apparently edited the author portion out of my post.  The PI grid and cathode voltages are zero.

[Mike_J]

One thing I failed to mention earlier.  The heaters for the power tubes glow brightly while there is no noticeable heater glow at all in the preamp tubes.  This is in spite of getting a reading of 3.35Vac for the heaters on all the tubes.  In the pictures above I use stranded 18 gauge for the power tube and 18 gauge single strand for the preamp tube heaters.  Is it possible that there is a break in the wire but voltage is still being recorded?

[HotBluePlates]

Tell me what you get with the pop test. Also mention what setting the relays are in when you do the test.

...  The PI grid and cathode voltages are zero.

Well, that's not good. 

Look at the connections from ground -> 390Ω feedback resistor -> 24kΩ tail resistor -> 820Ω cathode bias resistor (and 1MΩ grid leaks) -> phase inverter cathode (and grids). Then verify a large positive voltage at B+ #3 -> voltage at outside lugs of 10kΩ trimmer -> phase inverter plates.

You say you have plate voltage and bias for the output tubes, so presumably the PT wiring is not wrong.

[sluckey]

One thing I failed to mention earlier.  The heaters for the power tubes glow brightly while there is no noticeable heater glow at all in the preamp tubes.  This is in spite of getting a reading of 3.35Vac for the heaters on all the tubes.  In the pictures above I use stranded 18 gauge for the power tube and 18 gauge single strand for the preamp tube heaters.  Is it possible that there is a break in the wire but voltage is still being recorded?

A break is possible, maybe even likely due to how tightly the filament string is twisted. Measuring voltage from filament pin to chassis will not detect a broken wire unless both wires are broken. Don't measure filament voltage to chassis. Measure it ACROSS the pins of a tube socket, ie, for a 12AX7, one probe on pin 9 and the other probe on pins 4,5. Then you will know if voltage is really getting to the tube. Cold filaments will definitely cause no grid/cathode voltage on the PI tube.

[Mike_J]

Tell me what you get with the pop test. Also mention what setting the relays are in when you do the test.

...  The PI grid and cathode voltages are zero.

Well, that's not good. 

Look at the connections from ground -> 390Ω feedback resistor -> 24kΩ tail resistor -> 820Ω cathode bias resistor (and 1MΩ grid leaks) -> phase inverter cathode (and grids). Then verify a large positive voltage at B+ #3 -> voltage at outside lugs of 10kΩ trimmer -> phase inverter plates.

You say you have plate voltage and bias for the output tubes, so presumably the PT wiring is not wrong.

I think you probably solved the mystery.  There is no continuity to ground at that point so it probably broke off when I removed the circuit board.  Since the wire is under the board I did not see it.  I thought I checked all of the grounds for continuity but I must have missed that one.  Will follow up after I get a look under the board.

The plate voltages for the PI plates are around 435V.

[Mike_J]

Ground is okay.  I was testing the wrong point.  I tested my preamp tubes for transconductance and all tested okay.  Each heater point on the preamp tube sockets reads 3.35Vac.  However none of the preamp tube heaters appear to be lit.  What are the odds that the heater wires going from the V4 power tube to the PI tube has a break in it that shows voltage that is not really there?  I know it seems like a long shot but from what I am seeing there is plate voltage on the preamp tubes but they are not working.  It looks to me like the heaters aren't working and without them I don't see how the tubes could work.

I put a new power transformer in that has an 8A heater winding.  That should be more than enough for four EL34s and three 12ax7s.  Wouldn't the voltage drop if the power tubes were drawing too much current?

Otherwise the only thing I can think of is doing a complete continuity test again but I doubt that is the problem.  I think it is in the heaters for the preamp tubes.

[sluckey]

Each heater point on the preamp tube sockets reads 3.35Vac.

Measure the filament voltage ACROSS the filament pins, NOT between pin and chassis. What do you get for each of the tubes that are not lit up?

[HotBluePlates]

The biggest mistake to be made when troubleshooting is to flail about looking for problems in random places.

You know there is no current flowing through the chain from the phase inverter plate resistors all the way down to ground. No sound can get to your output tubes if the phase inverter is not working. So make the phase inverter work before dinking with anything else.

A systematic, step-by-step approach will save you lots of time.

[sluckey]

You know there is no current flowing through the chain from the phase inverter plate resistors all the way down to ground. No sound can get to your output tubes if the phase inverter is not working. So make the phase inverter work before dinking with anything else.

He did say there's no noticeable glow in the preamp tubes. No glow = no plate current flow. I'm trying to get him to verify that the tubes are lit. That needs to be on top of the list.

[Mike_J]

You know there is no current flowing through the chain from the phase inverter plate resistors all the way down to ground. No sound can get to your output tubes if the phase inverter is not working. So make the phase inverter work before dinking with anything else.

He did say there's no noticeable glow in the preamp tubes. No glow = no plate current flow. I'm trying to get him to verify that the tubes are lit. That needs to be on top of the list.

I measured the ac voltage across pins 1 and 4 & 5 of each preamp tube and get zero voltage.

[Mike_J]

I lightly wiggled the heater wires from V3 to V4.  The wire on Pin 7 of V4 (first power tube) was broken.  I will create a new twisted pair of wires to go from V3 (PI tube) to V4 (the first power tube).  My guess is getting heater voltage to the preamp tubes will help my amps current condition.

[sluckey]

I measured the ac voltage across pins 1 and 4 & 5 of each preamp tube and get zero voltage.

Pin 1 is a plate, not a filament.

Measure it ACROSS the pins of a tube socket, ie, for a 12AX7, one probe on pin 9 and the other probe on pins 4,5.

My guess is getting heater voltage to the preamp tubes will help my amps current condition.

Yes. May be other issues too, but you'll never know until you light 'em up.

[Mike_J]

I measured the ac voltage across pins 1 and 4 & 5 of each preamp tube and get zero voltage.

Pin 1 is a plate, not a filament.

Measure it ACROSS the pins of a tube socket, ie, for a 12AX7, one probe on pin 9 and the other probe on pins 4,5.

My guess is getting heater voltage to the preamp tubes will help my amps current condition.

Yes. May be other issues too, but you'll never know until you light 'em up.

Correction:  Pins 9 to 4,5 yielded zero voltage ac.

I agree there will be other problems.  I want to change the OD tone stack to a cathode follower.  Will need to set the voltage string.  Need to adjust the OD trimmers.  Need to adjust the PI trimmer.  Need to balance the FET circuit which is a real pain.  Every one of those things can cause problems but I will tackle them one at a time instead of using the shotgun approach like I did here which contributed to the problems I had here.  With the help of the people on this site I am confident this amp will be great.

I fixed the broken wire and will fire it up tomorrow and start on the other items I just mentioned (presuming it fixes the preamp problem.)

Thank you sluckey and HotBluePlates for helping me with this.