OK-- fired it up today and found out that a 1A fuse is too small. Quickly replaced with a 2A and all the inrush limiters I could lay my hands on.
Here' the deal-- it makes sound so, that's the good news. The bad news is that the B+ is a little too high for what I planned (about 425V vs. 370V), but the really bad bit is that when music is playing it sounds like someone is turning the volume up and down while it's playing. Definitely trippy, but not what I was looking for,
When it first starts , things are good, but once it has a few seconds to warm up the wah-wah up-and-down volume thing starts. With no source signal, it sounds like a rhythmic thumping in the speaker and when I unplug it the thumping goes away as it powers down.
Could be a capacitor not quite up to the task maybe?
Thoughts?
Kofi
Here' the deal-- it makes sound so, that's the good news. The bad news is that the B+ is a little too high for what I planned (about 425V vs. 370V), but the really bad bit is that when music is playing it sounds like someone is turning the volume up and down while it's playing. Definitely trippy, but not what I was looking for,
When it first starts , things are good, but once it has a few seconds to warm up the wah-wah up-and-down volume thing starts. With no source signal, it sounds like a rhythmic thumping in the speaker and when I unplug it the thumping goes away as it powers down.
Could be a capacitor not quite up to the task maybe?
Thoughts?
Kofi
Sounds like could be positive feedback in there
I googled "motorboating (electronics)" ---check any nfb and in fact all connections.
http://en.m.wikipedia.org/wiki/Motorboating_(electronics)
Cheers
Terry
I googled "motorboating (electronics)" ---check any nfb and in fact all connections.
http://en.m.wikipedia.org/wiki/Motorboating_(electronics)
Cheers
Terry
OK-- still scratching my head on this one. All the screen / plate wiring looks correct so far. The amp sounds fine for about 20 seconds or so, but when the B+ settles in it begins the wah-wah behavior.
The voltage through the cathodes of the KT88s and the second 6SN7 seems unstable (bounces around a lot), however the first 6SN7 is able to hold a stable cathode voltage.
I swapped 6SN7s and the issue did not follow the tube, so we can eliminate the 6SN7s as the issue, I believe. I'll swap out the KT88s to see if that makes a difference.
I'll keep you updated. Any other ideas on how to diagnose this?
Kofi
The voltage through the cathodes of the KT88s and the second 6SN7 seems unstable (bounces around a lot), however the first 6SN7 is able to hold a stable cathode voltage.
I swapped 6SN7s and the issue did not follow the tube, so we can eliminate the 6SN7s as the issue, I believe. I'll swap out the KT88s to see if that makes a difference.
I'll keep you updated. Any other ideas on how to diagnose this?
Kofi
Alright, here's the latest...
I swapped out the rectifier tube and noticed a nice big arc on startup. I checked the PSU wiring and noticed that the two leads for the choke were slightly exposed before the PCB screw terminal and very near each other. I separated them and buried the exposed wire in the screw terminal.
After turning it back on, the b+ dropped from 425V to about 325V, so, I'd say the choke was shorted. I still have the wah-wah sound, so I wonder if I blew the filter caps and I'm just feeding the choke directly into the OPT?
I need to walk away for a bit. Any thoughts you could share would be appreciated.
Kofi
I swapped out the rectifier tube and noticed a nice big arc on startup. I checked the PSU wiring and noticed that the two leads for the choke were slightly exposed before the PCB screw terminal and very near each other. I separated them and buried the exposed wire in the screw terminal.
After turning it back on, the b+ dropped from 425V to about 325V, so, I'd say the choke was shorted. I still have the wah-wah sound, so I wonder if I blew the filter caps and I'm just feeding the choke directly into the OPT?
I need to walk away for a bit. Any thoughts you could share would be appreciated.
Kofi
So, I'm still in the process of tacking this down and I noticed that the input to the grid of the second KT88 starts squeaking / oscillating like mad when I attach a test probe to it while the amp is running. Definitely some instability here that could be due to oscillation from earlier on.
Still looking.
Pascal-- I may reach out to Pete in a bit, but this looks like a garden variety wiring screw up by me. I'll need to keep tracing the circuit for now.
More updates to follow.
Kofi
Still looking.
Pascal-- I may reach out to Pete in a bit, but this looks like a garden variety wiring screw up by me. I'll need to keep tracing the circuit for now.
More updates to follow.
Kofi
OK-- so I set out yesterday to find the source of the oscillation / motorboating. I checked every connection, measured every resistance, scoured the chassis for accidental shorts and... nothing.
Figuring that the power supply caps might be intermittently shorting when the HT is applied, I spent most of the day de-soldering the supply caps (including the KT88 cathode bypass caps) and re-soldering the set from the other channel.
I remounted the board and... no change. Awesome day.
Then, I ran across this post in another forum. In short, 'boba1000' is saying that the 1K resistor that's shared by the second stage of 6SN7s (R9, in Pete's schematic) is too low. He changed the 1K to a 3.3K and killed the oscillation.
I'll give that a whirl this morning...
Grrr...
Kofi
Figuring that the power supply caps might be intermittently shorting when the HT is applied, I spent most of the day de-soldering the supply caps (including the KT88 cathode bypass caps) and re-soldering the set from the other channel.
I remounted the board and... no change. Awesome day.
Then, I ran across this post in another forum. In short, 'boba1000' is saying that the 1K resistor that's shared by the second stage of 6SN7s (R9, in Pete's schematic) is too low. He changed the 1K to a 3.3K and killed the oscillation.
I'll give that a whirl this morning...
Grrr...
Kofi
OK-- it took me almost an hour to get R9 de-soldered and the 3.3K job in there as a replacement. Turned it on and...
THAT WAS IT!!
Looks like the 1K resistor was just pushing the 6SN7 too much. It's got no problem pushing an 88dB sealed three-way, so I assume my 96dB jobs in the living room would kill with this thing.
Too bad it's for a friend. Unless...
Kofi
THAT WAS IT!!
Looks like the 1K resistor was just pushing the 6SN7 too much. It's got no problem pushing an 88dB sealed three-way, so I assume my 96dB jobs in the living room would kill with this thing.
Too bad it's for a friend. Unless...
Kofi
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I've only completed one channel, but so far it sounds really great. I'm really impressed with how powerful it is, but then I've been a single-ended guy for a while...
I'll build the other one, then test them both to see wzup, but I expect I'll ultimately leave them just as they are once I'm done because lazy.
More to come...
Kofi
OK-- headed out to the shed to do some testing. I haven't fired up the o'scope in a long time, but I'm thinking I'll try the old 10kHz square wave test to see how it performs.
I'll post progress as I go...
By the way, Pascal, I think you DO need to jumper R7 (unless you are using a balanced input). If you leave it open, the phase splitter transformer primary will have no reference to ground.
Take a closer look at Pete's schematic and I think you'll agree.
More to come.
Kofi
I'll post progress as I go...
By the way, Pascal, I think you DO need to jumper R7 (unless you are using a balanced input). If you leave it open, the phase splitter transformer primary will have no reference to ground.
Take a closer look at Pete's schematic and I think you'll agree.
More to come.
Kofi
OK--- I pushed a little more voltage through it and the square waves are looking better. Check out the MULTIMEDIA ACTION HERE:
CLICK ME!!!!
Kofi!
CLICK ME!!!!
Kofi!
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