Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Apexelectric on September 27, 2017, 07:10:31 am
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Having an issue with a new Revibe build. I used Sluckeys layout on this one and have everything working except for the reverb. Here's what has transpired so far in my troubleshooting since finishing build.
Found wrong value plate resistor for V1 - Replaced resistor and burned out V1
Found missing ground jumper on Mix pot
All worked except reverb
Triple checked ground continuity and component values on the rest of the amp
Checked all tubes
Checked coupling capacitors
Checked plate resistors and cathode bias resistors.
Checked solder connections
Checked under board jumpers
Checked voltages on all tube plates
Checked reverb cabled
Swapped out reverb tank
Checked isolated output transformer primary and secondary continuity/resistance
Checked reverb leads for proper direction
Used AC MV setting on DMM to check signal at grids
Questions:
Should I be able to see a varying AC MV reading on the output tube grid, if so I don't get one. Stays constant at around 100MV
Should I be getting DC voltage at the output tube grid? Reading 40VDC
How much AC voltage should I get on the secondary of the OT?
Any ideas on what else I might need to check for?
As always, thanks in advance.
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Don't worry about any AC signal voltages. All the important voltages are on my schematic. Measure voltages for ALL tube pins (including filament pins) for V1, V2, and V3. Post those voltages here.
Divide and conquer... Connect an 8Ω speaker to the reverb send jack (OK to use gator clip test leads to make the connection). Connect your guitar to the input jack. You should be able to hear a guitar in the test speaker and the dwell pot should act as a volume control. If so, then the problem lies with the recovery amp (V1A). If not, the problem lies with the reverb driver circuit (V2 and V3).
Divide and conquer... Rather than the above test, you could connect your guitar to the reverb return jack. (OK to use gator clip test leads to make the connection if you don't have the proper adapter.) Connect the output jack to the input of an amplifier. The tone and mix pots will affect the guitar signal. You should be able to hear the guitar in your amplifier. If so, the problem lies with the reverb driver circuit (V2 and V3). If not, the problem lies with the reverb recovery circuit (V1A).
Should I be getting DC voltage at the output tube grid? Reading 40VDC
NO! Look at the schematic for correct voltages. 40VDC indicates a problem such as failed coupling cap, bad tube, or wiring error.
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I suspected and did pull the coupling capacitor before the output tube grid and it tested fine. Maybe I'll swap it out after trying the tests you suggested.
All voltages tested within 5-10% of your posted voltages. Nothing suggesting any errors there.
Seems like I should be getting at least a couple volts AC on the OT secondary with a signal on it but am not getting anything.
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Are you sure you have 40VDC on pin 5 of that 6V6? Could it really be 40mV (millivolts) Look closely at your meter.
Measure the voltages I asked for. Do the tests I suggested. Report back.
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Will do. Thanks!
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I suspected and did pull the coupling capacitor before the output tube grid and it tested fine.
NB: the proper way to test a coupling cap for DC leakage is described in Fig.4-4 (P57) of the attached Jack Darr Book Ch 4
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In checking the voltages, I did confirm the presence of 44VDC on the output tube grid. This seems to indicate an issue with try coupling capacitor, which did not read any type of resistance across it and had proper capacitance. I will however replace both it and the output tube later today or tomorrow to see if that can be the issue. I would have expected noise in the circuit with D.C. In the output tube grid though?
When I found the mistake with the plate resistor on V1 it had a 1.5k installed instead of the 100K. I can see how this would have ruined the preamp tube but would it have caused problems further down the line at the output tube and its coupling capacitor?
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Pull V3. If you still have 44v on the grid then you have a wiring error.
Are you ever gonna do the tests I asked you to do?
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Did them. Just didn't have the time to post the results til now. Been at work.
V1A V1B V2A V2B V3 V4A V4B V5A V5B
Plate. 175 205 138 138 341 259 261 223 175-190
Cathode 1.4 1.62 1.78 1.95 0 2.3 2.3 1.76 1.69
Screen 341
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Did them. Just didn't have the time to post the results til now. Been at work.
V1A V1B V2A V2B V3 V4A V4B V5A V5B
Plate. 175 205 138 138 341 259 261 223 175-190
Cathode 1.4 1.62 1.78 1.95 0 2.3 2.3 1.76 1.69
Screen 341
What is there no cathode voltage on V3? (I thought the reverb driver tube was supposed to be cathode biased?)
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Yes. There is definitely an issue there.
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If there is no tube current in V3, then either the cathode resistor is not connected to ground, or the V3 cathode isn't connected to the cathode resistor, or the heaters in V3 aren't working, or maybe one of the tube socket pin clamps is not clamping.
Have you got 6.3VAC between the heater pins on V3?
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Yes. There is definitely an issue there.
Measure resistance from V3 pin 8 to chassis. Should be 1KΩ. Recheck wiring for V3. Should be easy to find the problem now.
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I have the proper resistance from the tube cathode pin to ground rail. 1K
I pulled the coupling capacitor and the 44VDC on the grid remains so it seems to be leaking from the tube.
Swapped the tube and same result.
Something seems to be preventing current flow through the output tube.
Do I have a bad OT?
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Tubeswell
Heaters are working properly
I'll check the pin clamping.
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My sincerest apologies for wasting your time. I found my mistake which was right in front of me the whole time. I can't believe that after staring at this thing for hours and hours I missed such a simple thing.
Crossed the cathode and grid wires on the tube socket. Lesson learned. Look for the obvious stuff before delving too deep in the more complex issues.
Once again, sorry for the trouble
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I have the proper resistance from the tube cathode pin to ground rail. 1K
That's impossible since you had the cathode and grid wiring crossed. If you had correctly measured resistance from pin 8 to chassis you would have seen that.
Time was not wasted. That's what we do here. Most all of us have made wiring errors at some time. We learn from those mistakes.
Hoffman's first law... If it was wired correctly it would be working.
Wiring error is the number one cause for a new build to not work.
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I have the proper resistance from the tube cathode pin to ground rail. 1K
That's impossible since you had the cathode and grid wiring crossed. If you had correctly measured resistance from pin 8 to chassis you would have seen that.
Time was not wasted. That's what we do here. Most all of us have made wiring errors at some time. We learn from those mistakes.
Hoffman's first law... If it was wired correctly it would be working.
Wiring error is the number one cause for a new build to not work.
I have to say something here because it needs to be said.
In a country and world that has really lost so much respect for others in how they speak and help each other (anybody see the news or go on face book lately? :w2: ), this, right here, is how you do it boys and girls! This is a great example of how to lead, teach and help each other as I see it. Passing on of wisdom, knowledge and understanding with respect, dignity and encouragement, it's all there! And we've all seen it, experienced it here time and time again.
Steve is not the only one, there are many others here who do the same, to many to list. I believe this is a big part of why this forum is the success it is and is why I'm glad to know all of you and to be a part of it! :icon_biggrin:
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this, right here, is how you do it boys and girls!
+1
There is a dichotomy between daily life, real life, and the virtual life, virtual life has devolved to a point I cannot even participate anymore, except here in Hoffmans world, where unique species can interact with respect as we were always meant to.
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+1 Steve and many others in here exhibit civility, care and concern for others on a daily basis in this forum. Sometimes even when I have nothing to ask or share I come in here to bask in the glow and remember my youth when folks were kind to each other. :worthy1: I fear our democracy is in the final phases of the Tytler cycle.
So, Apexelectric......how does it sound? :icon_biggrin:
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I don't post often but I've been a member since 2006 or so. Most of my current knowledge, along with a couple books I peruse, comes from this site and forum. In fact, I don't even frequent any other forums. Great place.
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Thanks for the encouragement and the understanding. It was downright embarrassing after realizing the error. There's a good lesson to be learned here.
I was was hung up with identifying my plate, cathode, and grid by the color coded wiring I used, that I never considered that I miswired the tube socket. So when asked to measure from V3 pin 8 to ground, in my head I heard, "measure from cathode to ground" and went to the color designated for the cathode rather than pin 8. It was only after I read your instructions again that I realized I wasn't on pin 8 for my measurements.
I will now be more vigilant in recognizing what PIN number, for a particular tube type, is wired to what section of the tube, rather than relying on the color coding scheme as a means of identifying that section during a troubleshooting session.
Lesson learned!
For the record, this forum has been a great tool for learning the ins and outs of amp building and repair. I have come a long way in the last 8 months since my first build and a lot of it has to do with all of you that are willing to help. Hopefully one day I can do the same for others who might choose to go down the same path as I.
Thanks again, Steve. I do always try to use the forum as a last resort and only when I think I have exhausted my troubleshooting abilities. This one unfortunately was well within my skill set.
On to the next one.
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Bnwitt,
It sounds great! Cant wait to give it a proper test. Life is crazy busy so time is limited but someday soon I'll have an opportunity to give it a proper workout.
Mike
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> downright embarrassing
Nevermind the embarrassment; get over it; we all done similar one time or another.
It was EDUCATIONAL.
Color codes are good, except when they trick your mind.
I recall being hung-up for months on a transistor console. Power supply died so I took it out and fixed it. When I put it back in, no work. I tested the supply some more. Still no-go. After far too long, I realized that the Red and Black DC wires were not + and -. It was wired for an older convention when most transistor gear was +positive+ ground. The Red was Negative Hot. I'm only glad the circuit was more robust than my brain and didn't burn-up from repeated reverse voltage.
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PRR, Good story. Thanks for that.
I guess admitting the mistake to all involved was my way of getting over it. Trying to put the ego aside.
In my day job, I'm used to being the man in charge and the one that everyone comes to for the answers so this exercise is one of humility. Definitely a good thing.
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When I did that console, I was the go-to, and fixed many problems in my sleep.
This butt-kicker embarrassed me at the time. Decades later I see it as a step in my education.