I am a noob so if I say something really stupid you will know why. Look at V3. I have 37vdc on the Pin2 Grid and I have 36vdc on the Pin 7 Grid. that can't be right. Seems like I have a short right?
I have looked an prodded and checked and resoldered and can't make it go away. Any ideas?
Thanks...Dave
No, that grid voltage is fine. The cathodes are positive and coupled to the grid through the 1M grid reference. That puts a voltage on the grids.
As for this being correct or not for a TrainWreck? I don't know this amp well enough. Sorry.
Thanks for the info. I retook the V3 grid voltages using the 1M ground reference and they were fine. Guess I need to learn more about how the Phase Inverter works
So, I am back to square one. I am at the point where I am going to have to just start replacing random parts.
dab,
FWIW, when you post the voltages it's really helpful if you also post the supply voltages. Just knowing the plate voltage often isn't enough...it's the plate voltage relative to the supply voltage that is sometimes what is needed. For example, a plate voltage of 230vdc is fine if the supply is ~330vdc but not good if the supply is 230vdc (there is no current flowing in this second case...but then your cathode voltage would be zero). Ohm's Law...V=I*R or V/R=I...so the voltage drop across your plate load resistor (i.e. the supply voltage minus the plate voltage) divided by the plate load resistor value gives you the plate current. It doesn't look like your voltages are too out of whack (not that familiar with Liverpulls).
Read the articles on the Aiken site on Long-tail pair phase inverters.
Here's my quick description. First you need to know these two little factoids:
A signal input at the grid is amplified out-of-phase at the plate.
A signal input at the cathode is amplified in-phase at the plate.
Think of the first, main input triode as a regular gain stage with a plate load resistor (the 82k) and a cathode resistor (the 470 ohm). The signal input at that grid is amplified out-of-phase at that plate. But notice that the cathodes are tied together and the cathode resistor is not bypassed. If a cathode resistor is not bypassed with a cap then a small signal exists on that cathode (the bypass cap just shunts the AC signal to ground just like a filter cap...but we have no cap). And this signal is in-phase with the input signal at the grid. So we have an in-phase signal on our PI cathode and that signal is the input for the bottom (i.e. second) triode. And since signals input at the cathode are amplified in-phase at the plate we get a signal on the second PI plate that is in-phase with the main input. So there's your two signals of opposite phase. Read the Aiken articles for the rest.
EDIT: you might try measuring the AC voltage at your PI plates. You'll need to input some sort of signal into the amp...it should measure in the 100mV range...have a friend just constantly pluck an A string softly if you have no other options. Measure the AC voltage on the PI plates...if it's equal or close to it then you likely don't have a balance problem or anything. When the amp is cranked that signal on the PI plates might be in the 60vac range when measured with your DVM (don't quote me on that...that's a VERY ballpark figure just to give you an order of magnitude).
Awesome. thanks for taking the time to post this. It was very helpful and i appreciate the opportunity to learn from you. i will try what you suggest and get a supply voltage.
Dave
Fischerman wrote:dab,
FWIW, when you post the voltages it's really helpful if you also post the supply voltages. Just knowing the plate voltage often isn't enough...it's the plate voltage relative to the supply voltage that is sometimes what is needed. For example, a plate voltage of 230vdc is fine if the supply is ~330vdc but not good if the supply is 230vdc (there is no current flowing in this second case...but then your cathode voltage would be zero). Ohm's Law...V=I*R or V/R=I...so the voltage drop across your plate load resistor (i.e. the supply voltage minus the plate voltage) divided by the plate load resistor value gives you the plate current. It doesn't look like your voltages are too out of whack (not that familiar with Liverpulls).
Read the articles on the Aiken site on Long-tail pair phase inverters.
Here's my quick description. First you need to know these two little factoids:
A signal input at the grid is amplified out-of-phase at the plate.
A signal input at the cathode is amplified in-phase at the plate.
Think of the first, main input triode as a regular gain stage with a plate load resistor (the 82k) and a cathode resistor (the 470 ohm). The signal input at that grid is amplified out-of-phase at that plate. But notice that the cathodes are tied together and the cathode resistor is not bypassed. If a cathode resistor is not bypassed with a cap then a small signal exists on that cathode (the bypass cap just shunts the AC signal to ground just like a filter cap...but we have no cap). And this signal is in-phase with the input signal at the grid. So we have an in-phase signal on our PI cathode and that signal is the input for the bottom (i.e. second) triode. And since signals input at the cathode are amplified in-phase at the plate we get a signal on the second PI plate that is in-phase with the main input. So there's your two signals of opposite phase. Read the Aiken articles for the rest.
EDIT: you might try measuring the AC voltage at your PI plates. You'll need to input some sort of signal into the amp...it should measure in the 100mV range...have a friend just constantly pluck an A string softly if you have no other options. Measure the AC voltage on the PI plates...if it's equal or close to it then you likely don't have a balance problem or anything. When the amp is cranked that signal on the PI plates might be in the 60vac range when measured with your DVM (don't quote me on that...that's a VERY ballpark figure just to give you an order of magnitude).
Sorry, got real busy at work and had to stop having fun for a while.
Don't you just hate it when some guy (me) starts a thread about some problem and then when you get to the end of the thread you never find out what was wrong with the amp?
In this case it was one or more bad filter caps. I replaced just about everything else on the amp, rewired, reflowed, adjusted lead dress etc.
Nothing helped, some things made it worse. I changed out all of the 20uF filter caps and VOILA!!! Problem gone.
Thanks to everyone for your assistance, I learned a lot during this exercise and having a successful conclusion made all of the work worthwhile and fun.
That's great dab! What brand of filter caps were in there before and what did you switch to?
Can you use your DMM to measure the caps you pulled out? Measure for capacitance and resistance and since they're e-lytic it will matter which way you connect them. Just curious if there is anything easily measurable here.
Fischerman wrote:That's great dab! What brand of filter caps were in there before and what did you switch to?
Can you use your DMM to measure the caps you pulled out? Measure for capacitance and resistance and since they're e-lytic it will matter which way you connect them. Just curious if there is anything easily measurable here.
The original caps came with the Ceriatone Liverpool kit. they were orange and were branded "Holy Grail". The replacements were Sprague ATOMs.
I measured the caps I took out and they all were pretty close to advertised value for capacitance. I measured for AC leakage on them and it didn't show any. Don't know exactly why switching them helped. I don't believe it was solder joints or connections as I went over those about 3 times to make sure earlier in the process.
The original caps came with the Ceriatone Liverpool kit. they were orange and were branded "Holy Grail". The replacements were Sprague ATOMs.
I measured the caps I took out and they all were pretty close to advertised value for capacitance. I measured for AC leakage on them and it didn't show any. Don't know exactly why switching them helped. I don't believe it was solder joints or connections as I went over those about 3 times to make sure earlier in the process.
That's very interesting. It's not the first time I've read that a person swapped out the Ceriatone caps for Sprague Atoms and the amp sounded better. My Express was a Ceriatone kit and it always sounded pretty crappy...even though I changed the preamp several times (it's now a Concorde preamp)...the same ratty quality to the distortion was/is always there ...but I never went back and swapped out those caps. The amp was/is very quiet so I assumed they were fine. I just happen to have some extra Atoms for the Rocket build I'm doing. I'm gonna have to try them (and it's times like this I'm soooooo glad I didn't do the pile-o-caps with glue and instead made a little circuit board for the filter caps).
keithrick wrote:Glad you got it sorted out Dave! Hows it sound now? I have all the parts here for my build just need some time!
Keith
Sounds really good! My only complaint is that I wish it had a little more headroom. It's breaking up pretty good at about 9 o'clock on the volume.
Use the guitar v knob. Set the amp on 1:00PM and it should clean up nice when you back off the guitar volume. If not, somethin' ain't right.
Dan
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