A few questions...

[Laimbrane]

First off, I apologize to anyone in advance for wasting your time if I am doing so.

A short background... I'm a high-school teacher running a Tech Project class where students take on a difficult and time-consuming task and build something complicated. One of my students has taken on the task of building the 100W HRM Dumble ODS described at http://pdfelectronics.com/dumble-ods-100w-hrm/. I spent a year in college in EE, so this project is causing me to blow the dust off information I haven't dealt with in 15 years, but I have most of the basics and can pass that info onto him.

Yes, I know... he is (we are) very likely biting off more than can be chewed. However, he can afford (financially) to screw up, and I want to encourage ambition and enthusiasm however I can, so long as it's safe. He has all of the parts ordered (with the exception of a couple, as I'll get to), and the idea of doing this himself has gotten him (and me) to learn a great deal of information about circuitry.

So I had a bunch of questions from you experts, and I'm hoping that, as a group, you can answer them. Mostly they're just verification of what I believe we've already learned, but there are some stumpers, given that we've had to piece together what we could while lacking a wiring diagram.

For reference, here is the schematic.

1) (Most importantly) This is a high-voltage, dangerous device, so I want to make sure that nobody gets hurt; safety is a much higher priority than getting this thing working. My main question is: what parts of this are not safe to touch while in operation? I don't see any resistance (and therefore no current) across the outside of the case, but the screws holding many of the parts in place look like they connect electrically to the board. Do we need caps on those for safety purposes?

2) Regarding the B+ nodes (there are five): based on the pictures and a little deduction, I believe these are effectively "air-fives" across the diagram, but I want to verify that they do not represent something tangible. If they are some sort of tangible object, what does that B+ represent?

3) In the schematic, there are little cylindrical/bullet-shaped objects on both sides of the 250k Drive pot and the 1M OD Vol pot, one between the normal switch and the 22k resistor (top left), and one connected to the 1M Preamp Volume and Bright Switch (Mid-left, top). What are these things, and what is their function?

4) Is there any reason that the trimpots on the tonestack can't be replaced with a panel mount pot? The student made a mistake in ordering - he has a 250k trim and a 25k trim, but he's short the 1M trim (he has a 500k trim instead). He does, however, have an extra 1M pot. Is there any non-aesthetic reason that this can't be affixed to the inside of the case (with a non-conductive board in-between, obviously) and replace the trim? I know, it's a little unorthodox, but I'm okay with that so long as he's learning.

5) The four orange and one black thing that surround V4-V7 sockets (best seen in these two images) appear to be Bias Test sockets, am I correct? If this is the case, where do they connect, what is their function, and how "necessary" are they?

I probably have other questions, but we'll start with those. Thanks in advance, guys (and ladies) for any help you can give.[/url]

[NickC]

re: #2

The power supply circuitry is at the bottom of the schematic. There are five nodes supplying different voltages to different circuits. Those node voltages are not listed on the schematic at those junctures, but that information should be obtained.

To simplify the schematic, the power supply nodes are marked B+ 1, B+ 2, B+ 3, B+ 4, B+ 5. Those nodes match the points in the preamp, PI, power amp supply points, etc. The wires connecting those points is omitted from the schematic for the sake of clarity.

Seeking out a layout diagram for the amp is a good idea. It will show the location of components. The position of components in that build is critically important. I've never attempted to build one. It's a very complex and ambitious build, requiring great attention to detail.


re: #3

I believe that depicts use of shielded cable.


I'm sure more knowledgeable folks than I will jump in here, with answers to your other questions (and perhaps corrections for my reply).

[xtian]

Wow, wow, wow. (Sorry for sounding like a dog.) The ODS is at the top of the complexity chain. New builders usually start with something manageable like a Fender Champ! Not to say success is impossible, but it's a long shot. However, there is a great repository of detailed info available here.

Also, you bring up a great point about safety. Are you legally liable? Because I would get you both some real safety training before even starting. Because death is an actual possibility.

So, welcome ampgarage, and read on...

1) (Most importantly) This is a high-voltage, dangerous device, so I want to make sure that nobody gets hurt; safety is a much higher priority than getting this thing working. My main question is: what parts of this are not safe to touch while in operation? I don't see any resistance (and therefore no current) across the outside of the case, but the screws holding many of the parts in place look like they connect electrically to the board. Do we need caps on those for safety purposes?

The steel or aluminum chassis must be connected securely to earth, via the green wire in your three-wire power cord. Thus, any contact with the chassis will be safe. Any screws used to attach the circuit board to the chassis should be well clear of other components. And of couse any screws that touch the chassis will be safe because they're also connected to earth.

IMPORTANT! Learn about the power filtering capacitors (the big ones)! They can hold onto 500+ volts even after you turn the amp off, EVEN after you unplug the power cord from the wall. Learn how to safely discharge them, learn the role of the standby switch and that it can "hide" stored energy in isloated B+ sections, learn to observe the B+ with a multimeter, and make discharging the caps every time after use a habit.

2) Regarding the B+ nodes (there are five): based on the pictures and a little deduction, I believe these are effectively "air-fives" across the diagram, but I want to verify that they do not represent something tangible. If they are some sort of tangible object, what does that B+ represent?

Sorry, does not compute.

3) In the schematic, there are little cylindrical/bullet-shaped objects on both sides of the 250k Drive pot and the 1M OD Vol pot, one between the normal switch and the 22k resistor (top left), and one connected to the 1M Preamp Volume and Bright Switch (Mid-left, top). What are these things, and what is their function?

Those represent co-axial wire. The outside is the ground, and the inside is the signal wire.

4) Is there any reason that the trimpots on the tonestack can't be replaced with a panel mount pot? The student made a mistake in ordering - he has a 250k trim and a 25k trim, but he's short the 1M trim (he has a 500k trim instead). He does, however, have an extra 1M pot. Is there any non-aesthetic reason that this can't be affixed to the inside of the case (with a non-conductive board in-between, obviously) and replace the trim? I know, it's a little unorthodox, but I'm okay with that so long as he's learning.

The ODS is already crammed with components. Adding more full-sized pots will be troublesome. And the trim pots are typically set-and-forget.

5) The four orange and one black thing that surround V4-V7 sockets (best seen in these two images) appear to be Bias Test sockets, am I correct? If this is the case, where do they connect, what is their function, and how "necessary" are they?

I can't see the images you pointed to. Let's assume we're talking about bias test points. You want to be able to place one multimeter probe on pin 8 (cathode) of each power tube, and the other probe on ground. Thus you are measuring the voltage across the 1R resistors, and you can infer the current thru each power tube plate. Bias points are a convenience.

Best luck!

[Laimbrane]

First off, thank you for your answers...

re: #2

The power supply circuitry is at the bottom of the schematic. There are five nodes supplying different voltages to different circuits. Those node voltages are not listed on the schematic at those junctures, but that information should be obtained.

To simplify the schematic, the power supply nodes are marked B+ 1, B+ 2, B+ 3, B+ 4, B+ 5. Those nodes match the points in the preamp, PI, power amp supply points, etc. The wires connecting those points is omitted from the schematic for the sake of clarity.

That's what I figured, but your explanation is a really good one. Thank you.

Seeking out a layout diagram for the amp is a good idea. It will show the location of components. The position of components in that build is critically important. I've never attempted to build one. It's a very complex and ambitious build, requiring great attention to detail.

Ah, to dream. He has a layout diagram for a previous version, but it's pretty drastically different. He's been trying to obtain that diagram, but it hasn't been easy.

Again, thank you for your help.

[Leo_Gnardo]

If this is his first build, that's just crazy. Best to work up the experience needed to deal with it. That's sort of like a beginner machine shop student setting out to build a perfect copy of say, a Vincent Black Shadow motorcycle. "High hopes, he's got high apple pie in the sky hopes..."

If the student does have experience building fairly complex amps, then my hat's off to him for getting an early start, and good luck on the ODS.

[NickC]

.....

Again, thank you for your help.

You are welcome.

But xtian is right. This is among the most complex and difficult amp-builds there is, even for an experienced builder. Extreme caution advised.

[Laimbrane]

Also, you bring up a great point about safety. Are you legally liable? Because I would get you both some real safety training before even starting. Because death is an actual possibility...

...IMPORTANT! Learn about the power filtering capacitors (the big ones)! They can hold onto 500+ volts even after you turn the amp off, EVEN after you unplug the power cord from the wall. Learn how to safely discharge them, learn the role of the standby switch and that it can "hide" stored energy in isloated B+ sections, learn to observe the B+ with a multimeter, and make discharging the caps every time after use a habit.

I'm making sure to read-up about all the safety stuff I can find (discharging with high-wattage resistors, etc.). Thanks for the tips here, though.

The steel or aluminum chassis must be connected securely to earth, via the green wire in your three-wire power cord. Thus, any contact with the chassis will be safe. Any screws used to attach the circuit board to the chassis should be well clear of other components. And of couse any screws that touch the chassis will be safe because they're also connected to earth.

This is because people are "high-resistance" objects while the case is low-resistance, correct? Again, so long as it's plugged in and grounded, which is a really important consideration relevant to the safety issue.

2) Regarding the B+ nodes (there are five): based on the pictures and a little deduction, I believe these are effectively "air-fives" across the diagram, but I want to verify that they do not represent something tangible. If they are some sort of tangible object, what does that B+ represent?

Sorry, does not compute.

I think NickC answered it... the B+ symbols appear to be mostly the way for the diagram to shorthand connections across the board as well as to simplify the process of finding voltages across each of the parts individual parts.

Those represent co-axial wire. The outside is the ground, and the inside is the signal wire.

Excellent. So I can assume that the line going to the middle of the "bullet" is the signal and the line going to the side of the bullet is the ground?

The ODS is already crammed with components. Adding more full-sized pots will be troublesome. And the trim pots are typically set-and-forget.

Yeah, I figured. On the bright side, he only needs one of the full-sized ones, but thanks for your advice.

Thanks for everything, man... you guys have been super helpful.

[ToneMerc]

My attorney would most likely say given the lack of background information, there's too much liabilty involved and not to answer any of your questions.

From an instructor's perspective I say if you have to seek advice from random strangers on an internet forum to determine what not to touch inside of a student's chosen live high electrical voltage (470-500V) class project, that I would have opened myself up to a certain degree of personal liabilty and maybe even the school administration as well.

Like most things; all's well that ends well, until the unthinkable happens.

I will commend you though for willing to assist in such a complex project of this nature with your student, but I have a feeling that if something was to happen, you would get tossed under the bus.

"Mr Laimbrane can you tell me what makes you an expert of this particular project, how many of these have you personally built and prior to the incident, how did you determine this project was safe for your student?"

You have admitted this is a "dangerous device", so have you signed off on this project with your administrator? Is there any type of electrical safety training involved?

Here's my 2 cents to make sure you cover your backside.

TM

[JazzGuitarGimp]

Wow, that is adventurous!

#1 If you build the amp per the schematic, all of the B+ nodes have bleeder resistors. Here is what I do when I need to put my hands inside an amp after it has been powered up:

Make sure the standby switch is closed (in the operate or play position). Turn the power switch off and unplug the amp. Wait two minutes, then CAREFULLY connect the minus lead of your DMM, set to DCV, to the chassis. Then CAREFULLY connect the plus lead of the meter to either terminal on the standby switch. I use clip leads for this. Now watch the meter until the voltage decreases to a safe level. I shoot for <10V (yes, I'm a lightweight). Now unclip the positive probe and use it, without the clip lead, to probe each of the other B+ nodes to make sure they are also under 10V (they usually are, but it's better to be safe…)

#2 Any point in the schematic that says B+1 needs to connect to the B+1 node of the supply. Same goes for the other four nodes.

#3 Cylindrical thingies indicate shielded cable. Connect shield at one end only as it is used only as a screen. Grounding in any tube amp is critical. Grounding in a high gain amp (like this one) is tantamount to success. Ground loops can cause huge amounts of hum. There is a "Grounding Map" inset in the schematic. Follow it like the bible. There is a preferred type of coax amongst the rumble crowd, though I don't know what it is. Maybe someone here with chime-in with the part number. Generally speaking, the lower the capacitance, the better (I think).

#4 This will complicate the build, but if were me doing the build, I'd put the HRM pots on the front panel. The only pot (in the signal chain) I'd leave as a trimmer is the "OD Trimmer" at the entrance of the OD preamp.

#5 Every time the output tubes are replaced, the amp should be re-biased. The bias test points are chassis-mount connectors that accept the probe-tip of a DMM. They run at fairly low voltage (<60V) If the amp is wired without error. These are optional, but handy in that you don't have to poke around in a hot chassis to bias the amp. I would also suggest a panel-mount short-shaft pot with a lock-down nut for the bias pot. That way, you don't have to stick a screwdriver into a hot chassis to bias the amp. Lock the shaft down after the amp is properly biased so that it isn't accidentally changed.

Good luck!
Lou

[Laimbrane]

My attorney would most likely say given the lack of background information, there's too much liabilty involved and not to answer any of your questions.

From an instructor's perspective I say if you have to seek advice from random strangers on an internet forum to determine what not to touch inside of a student's chosen live high electrical voltage (470-500V) class project, that I would have opened myself up to a certain degree of personal liabilty and maybe even the school administration as well.

Like most things; all's well that ends well, until the unthinkable happens.

I will commend you though for willing to assist in such a complex project of this nature with your student, but I have a feeling that if something was to happen, you would get tossed under the bus.

"Mr Laimbrane can you tell me what makes you an expert of this particular project, how many of these have you personally built and prior to the incident, how did you determine this project was safe for your student?"

You have admitted this is a "dangerous device", so have you signed off on this project with your administrator? Is there any type of electrical safety training involved?

Here's my 2 cents to make sure you cover your backside.

TM

You're freaking me out, man...

I appreciate the words of wisdom. If it makes you feel better, his father is an electrical engineer, and I suspect we won't be done with the amp by the time the class is finished (Friday). I'll have his dad take a look at it and help him with the (probably nearly) finished product at the end of the week.

[NickC]

My attorney would most likely say given the lack of background information, there's too much liabilty involved and not to answer any of your questions.

From an instructor's perspective I say if you have to seek advice from random strangers on an internet forum to determine what not to touch inside of a student's chosen live high electrical voltage (470-500V) class project, that I would have opened myself up to a certain degree of personal liabilty and maybe even the school administration as well.

Like most things; all's well that ends well, until the unthinkable happens.

I will commend you though for willing to assist in such a complex project of this nature with your student, but I have a feeling that if something was to happen, you would get tossed under the bus.

"Mr Laimbrane can you tell me what makes you an expert of this particular project, how many of these have you personally built and prior to the incident, how did you determine this project was safe for your student?"

You have admitted this is a "dangerous device", so have you signed off on this project with your administrator? Is there any type of electrical safety training involved?

Here's my 2 cents to make sure you cover your backside.

TM

You're freaking me out, man...

I appreciate the words of wisdom. If it makes you feel better, his father is an electrical engineer, and I suspect we won't be done with the amp by the time the class is finished (Friday). I'll have his dad take a look at it and help him with the (probably nearly) finished product at the end of the week.

ToneMerc words are gospel truth.

My father-in-law is a retired EE Professor. He would know enough to be cautious, but no way could he build a Dumble.

A close friend is also a PhD EE Professor. His background is defense contracting, specifically RADAR and SONAR. He teaches electronics at a local college. He also would not be able to build a Dumble without a LOT of expert help. I've tried to enlist him in several projects. He looks at the schems and laughs and admits it ain't his forte.

I'm not trying to poo-poo this project. But ToneMerc's points are valid and a it's an excellent idea to engage maximum CYA in this enterprise. It really IS that dangerous.

[Firestorm]

Dumble himself couldn't finish that amp by Friday.

[ToneMerc]

My attorney would most likely say given the lack of background information, there's too much liabilty involved and not to answer any of your questions.

From an instructor's perspective I say if you have to seek advice from random strangers on an internet forum to determine what not to touch inside of a student's chosen live high electrical voltage (470-500V) class project, that I would have opened myself up to a certain degree of personal liabilty and maybe even the school administration as well.

Like most things; all's well that ends well, until the unthinkable happens.

I will commend you though for willing to assist in such a complex project of this nature with your student, but I have a feeling that if something was to happen, you would get tossed under the bus.

"Mr Laimbrane can you tell me what makes you an expert of this particular project, how many of these have you personally built and prior to the incident, how did you determine this project was safe for your student?"

You have admitted this is a "dangerous device", so have you signed off on this project with your administrator? Is there any type of electrical safety training involved?

Here's my 2 cents to make sure you cover your backside.

TM

You're freaking me out, man...

I appreciate the words of wisdom. If it makes you feel better, his father is an electrical engineer, and I suspect we won't be done with the amp by the time the class is finished (Friday). I'll have his dad take a look at it and help him with the (probably nearly) finished product at the end of the week.

I don't have a dog in this is fight, so no worries here. However, does his EE father know that the shown schematic doesn't follow modern electrical safety practices regarding how the power transformer primary should be switched and fused?


TM

[Laimbrane]

My attorney would most likely say given the lack of background information, there's too much liabilty involved and not to answer any of your questions.

From an instructor's perspective I say if you have to seek advice from random strangers on an internet forum to determine what not to touch inside of a student's chosen live high electrical voltage (470-500V) class project, that I would have opened myself up to a certain degree of personal liabilty and maybe even the school administration as well.

Like most things; all's well that ends well, until the unthinkable happens.

I will commend you though for willing to assist in such a complex project of this nature with your student, but I have a feeling that if something was to happen, you would get tossed under the bus.

"Mr Laimbrane can you tell me what makes you an expert of this particular project, how many of these have you personally built and prior to the incident, how did you determine this project was safe for your student?"

You have admitted this is a "dangerous device", so have you signed off on this project with your administrator? Is there any type of electrical safety training involved?

Here's my 2 cents to make sure you cover your backside.

TM

You're freaking me out, man...

I appreciate the words of wisdom. If it makes you feel better, his father is an electrical engineer, and I suspect we won't be done with the amp by the time the class is finished (Friday). I'll have his dad take a look at it and help him with the (probably nearly) finished product at the end of the week.

I don't have a dog in this is fight, so no worries here. However, does his EE father know that the shown schematic doesn't follow modern electrical safety practices regarding how the power transformer primary should be switched and fused?


TM

Um... maybe? How should the transformer primary be switched and fused differently from what's in the schematic?

[Structo]

To the OP, there are many types of Electrical Engineers.
One may be able to design electronic circuits, another may be
able to design traffic lights.
So to simply claim a person is an EE doesn't really mean anything
when talking about amplifiers, particularly vacuum tube amplifiers.

I think there are several things in play here.

The student wants to build a Dumble amp and not just any Dumble amp but a HRM ODS amp.
That is the later version of this amplifier and most here have not built one.

This is not a project for the uninitiated.

Also, with schematics, there is a level of understanding that the technician building this amp is
familiar with vacuum tubes and their operation.

For instance, the heater (filament) wiring is not generally shown
because a tech that is familiar with vacuum tube circuits will know or assume
that part of it without much explanations.

The other concern I have is that the OP does not seem to understand general nomenclature
pertaining to wiring or lead dress of a vacuum tube amplifier.

I can understand this because I was enrolled in Electronic Engineering in 1976 and very little was taught about vacuum tubes.
In fact, my instructor told us that he would spend one day on vacuum tubes because the transistor was to replace them all.....

When I joined this forum I probably didn't ask a question until I
had been a member for six months, which I spent reading info here as well as online information about vacuum tube amplifiers.

I understood basic electronic theory but vacuum tubes are a whole different subject when it comes to operation of these circuits.

There is a site (forget now) where college students built a Fender Deluxe amp as a project.
It was interesting how many mistakes and errors they made before the projects completion that eventually produced a working amplifier.

As many have said, he couldn't have chosen a more advanced circuit for his project.
You must be well versed in vacuum tube amplifier design before you attempt this, period.

I highly recommend you revisit this and choose a less complex amplifier.
He can always build several more amps before trying the Dumble circuit which is my recommendation.

I don't mean to be overly critical, but one must be aware of their limitations, and I think that this project is well beyond those limitations.

But I welcome you to prove us wrong.