Hi, everyone.
My son and I just completed our first tube amplifier. With some assistance from my father; an old EE and HP alum. It's been a blast and I've really got the fever.
We decided to first build Poindexter's Musical Machine. I had to dust off some old skills, tap into my son's (he's pursuing an EE degree) and check in with my dad. I'm in IT but have some electronics background. So two channels with a 5965 preamp and two 6v6 tubes (x2)
We had a few obvious problems the first time around with design and implementation but overcame them. Had to rebuild the power sections once or twice but we finally have something.
The voltages check out pretty well, and it produces sound, but very very quiet. On the plus side, the sound is awesome free of hiss and hum. power tubes biased correctly. I've tested every component that I know how to with no issues. I've poked every solder joint and looked the wiring
we like to do our own work, but we're stumped. I will get an oscilloscope and call in my dad for some advice, but my question is this: If you had an amp that worked fine with the exception of low volume across both channels, what would you check *first*. I mention both channels because I guess that eliminates issues with bad tubes.
The plate load resistor values on the preamp are about 13% higher than they should be; that's getting fixed mid-week. Besides I see higher resistance values produce more gain rather than less. Counterintuitive.
Anyway, any advice would be appreciated. Obviously too many variables to mention but maybe someone has had the same issue..
Thanks
My son and I just completed our first tube amplifier. With some assistance from my father; an old EE and HP alum. It's been a blast and I've really got the fever.
We decided to first build Poindexter's Musical Machine. I had to dust off some old skills, tap into my son's (he's pursuing an EE degree) and check in with my dad. I'm in IT but have some electronics background. So two channels with a 5965 preamp and two 6v6 tubes (x2)
We had a few obvious problems the first time around with design and implementation but overcame them. Had to rebuild the power sections once or twice but we finally have something.
The voltages check out pretty well, and it produces sound, but very very quiet. On the plus side, the sound is awesome free of hiss and hum. power tubes biased correctly. I've tested every component that I know how to with no issues. I've poked every solder joint and looked the wiring
we like to do our own work, but we're stumped. I will get an oscilloscope and call in my dad for some advice, but my question is this: If you had an amp that worked fine with the exception of low volume across both channels, what would you check *first*. I mention both channels because I guess that eliminates issues with bad tubes.
The plate load resistor values on the preamp are about 13% higher than they should be; that's getting fixed mid-week. Besides I see higher resistance values produce more gain rather than less. Counterintuitive.
Anyway, any advice would be appreciated. Obviously too many variables to mention but maybe someone has had the same issue..
Thanks
I have a slightly different schematic (the 06 version) but it's the same basically. At first glance nothing is different. Size of mine is too big to post. I even have the 120/130 in parallel to make the 62.5 on the preamp.
I am using some higher quality NOS resistors for those, but they test higher than they're rated for. Replacements on the way. I'm also using NOS 5965s but new 6v6's. High quality coupling Caps. everything in here has been probed so I've avoided most of the Noob errors, I think. I even have multiple sets of eyeballs.
Hammond 125d output transformer.
I am using some higher quality NOS resistors for those, but they test higher than they're rated for. Replacements on the way. I'm also using NOS 5965s but new 6v6's. High quality coupling Caps. everything in here has been probed so I've avoided most of the Noob errors, I think. I even have multiple sets of eyeballs.
Hammond 125d output transformer.
It's likely to be a design or build error. Make sure all the screen grid connections are right,
and that the screen resistors are 100R, not 100k etc.
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@fleapower
Assuming all of your voltages are in-line, there are two basic strategies for this sort of troubleshooting - signal tracing and signal injection. They may be used separately, or simultaneously.
So if the music doesn't come out the other end, the ideas are simple:
For tracing - you apply a musical (or other) signal and then check for the presence and level of that signal everywhere it should appear using a signal tracer. The tracer is really just a separate, high-gain amplifier that allows your to 'listen in' on any area of the circuit. You work from input to output.
For injection - you apply a (usually) non-musical signal, like a sine wave, at various points in the circuit, and see where it is that it gets cut off. The injector might be a simple oscillator or function generator. Find the stage that doesn't have the correct output for the input you give. You work from output to input.
The classic Eico 147A Signal Tracer:
Here's a cute little battery-power transistor signal injector (I have a similar one, called a "Metrex Genie", just a simple sawtooth osc.):
Assuming all of your voltages are in-line, there are two basic strategies for this sort of troubleshooting - signal tracing and signal injection. They may be used separately, or simultaneously.
So if the music doesn't come out the other end, the ideas are simple:
For tracing - you apply a musical (or other) signal and then check for the presence and level of that signal everywhere it should appear using a signal tracer. The tracer is really just a separate, high-gain amplifier that allows your to 'listen in' on any area of the circuit. You work from input to output.
For injection - you apply a (usually) non-musical signal, like a sine wave, at various points in the circuit, and see where it is that it gets cut off. The injector might be a simple oscillator or function generator. Find the stage that doesn't have the correct output for the input you give. You work from output to input.
The classic Eico 147A Signal Tracer:
Here's a cute little battery-power transistor signal injector (I have a similar one, called a "Metrex Genie", just a simple sawtooth osc.):
An externally hosted image should be here but it was not working when we last tested it.
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pix
Not sure what's apparent from the pix. Remember... my first attempt
No Hum. No other audio issues that I can discern. In fact, sticking my head between the 8 ohm speakers make me drool!
Output is from an iPhone at full volume. known-good cables and I've swapped a few in and out.
Legendre ... that's the part my dad's the absolute master of. My son will get a scope from school and with his help we'll get a lot further. I will read your post for comprehension in a bit.
The 100 vs 100K ... I checked it but I will again
Not sure what's apparent from the pix. Remember... my first attempt
No Hum. No other audio issues that I can discern. In fact, sticking my head between the 8 ohm speakers make me drool!
Output is from an iPhone at full volume. known-good cables and I've swapped a few in and out.
Legendre ... that's the part my dad's the absolute master of. My son will get a scope from school and with his help we'll get a lot further. I will read your post for comprehension in a bit.
The 100 vs 100K ... I checked it but I will again
Attachments
What Legendre said..
Also...
To check for basic wiring mistakes, I take a clean copy of the schematic and mark every connection with a highlighter as I find it in the amp.
Doublechecking pin numbers and connections on the tubes has caught a few mistakes, also.
Wiring around the inputs and volume pots would be one place I'd look.
Chassis ground connections would be another.
The output transformer secondary is connected to ground?
Also...
To check for basic wiring mistakes, I take a clean copy of the schematic and mark every connection with a highlighter as I find it in the amp.
Doublechecking pin numbers and connections on the tubes has caught a few mistakes, also.
Wiring around the inputs and volume pots would be one place I'd look.
Chassis ground connections would be another.
The output transformer secondary is connected to ground?
Isn't the output voltage pretty low, around 1.0 volt or so. Tried any other sources, like a CD player etc?
jeff
Good point.
Also, what speakers are connected?
That said, my musical machine is pretty loud - I definitely wouldn't want to put my head next to the speaker when it was 'wide open'....not that it gets to 'wide open' often (ever?)...
Good. The scope falls on the signal /tracer/ side, as you might realize.. You can use an iPod (I sometimes use a $0.99 Chi-Com iPod Shuffle copy) for the injector.
Find yourself an .mp3 file that's a 400Hz-1KHz sine wave, and put it on repeat.. use that to supply signal and use the scope to see where it does and doesn't appear. There are also Signal and Function Generator apps for the iPods / Android, but I'd be careful about connecting that sort of sensitive gear (phones) to a tube amp.. it's possible to wreck it. This is why I use a dirt-cheap player if that's the way I want to go.
Yeah it's pretty freaking bad. But for my part, it's not the +worst+ I have ever seen.. and by a fair distance. You should see what someone once did to "improve" a $15K hi-end preamp, but I don't know if I still have the photos.
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Do you mean: "Since the heater wires aren't twisted, I hope they are DC, not AC heaters." ?
That was one of my thoughts as well.
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