I could use some help figuring this out. My first build is a Bell & Howell projector amp converted to a guitar amp. It's a 6V6 push-pull with a 5Y3 rectifier, 6SJ7 pentode first stage and a 6SL7 dual triode driver/PI stage. I was shooting for something like a Fender Deluxe and used the original Deluxe tone circuit. The fuse is a 2A 5mm x 20mm slo-bio. The schematic is attached. Also a table of the measured voltages.
The amp works pretty well for a first build. It's loud and delivers a nice Deluxe distortion at higher volume. Speaker = Jensen CQ-12", 8 ohms.
I have two issues with it. Maybe they're related--I'm still pretty new at this.
1. Compared to my Marshall JTM-30 combo, the sound is overly treble, almost harsh. Also very percussive, LOTS of pick attack in an unmusical way. I can't seem to get anything close to that "clean" Fender treble sound. If I look at the waveforms, I don't see any obvious clipping. Do my filter cap values look okay?
2. The amp blows its fuse instantly when I turn on the amp, but ONLY when plugged directly into a wall outlet or a power strip. If the amp is plugged in through my variac, the fuse doesn't blow. The variac is set to 117 VAC, same as my wall current.
If I run it through the variac and then though a current limiter with a 100 W bulb, the bulb doesn't flare up. It just glows steadily dim and the amp runs. I can play it at max volume without blowing the fuse. If I plug the CL into the wall and my amp into the CL, the fuse blows instantly even though the CL is barely glowing. But when the amp is plugged in through the variac, the fuse doesn't blow whether I also use the CL or not.
Why would having the variac in the loop keep the amp from blowing a fuse?
I'm hoping someone will have ideas about other things I can check, and/or maybe have some ideas about how to fix the trebly percussiveness of the sound?
Thanks in advance for any help.
The amp works pretty well for a first build. It's loud and delivers a nice Deluxe distortion at higher volume. Speaker = Jensen CQ-12", 8 ohms.
I have two issues with it. Maybe they're related--I'm still pretty new at this.
1. Compared to my Marshall JTM-30 combo, the sound is overly treble, almost harsh. Also very percussive, LOTS of pick attack in an unmusical way. I can't seem to get anything close to that "clean" Fender treble sound. If I look at the waveforms, I don't see any obvious clipping. Do my filter cap values look okay?
2. The amp blows its fuse instantly when I turn on the amp, but ONLY when plugged directly into a wall outlet or a power strip. If the amp is plugged in through my variac, the fuse doesn't blow. The variac is set to 117 VAC, same as my wall current.
If I run it through the variac and then though a current limiter with a 100 W bulb, the bulb doesn't flare up. It just glows steadily dim and the amp runs. I can play it at max volume without blowing the fuse. If I plug the CL into the wall and my amp into the CL, the fuse blows instantly even though the CL is barely glowing. But when the amp is plugged in through the variac, the fuse doesn't blow whether I also use the CL or not.
Why would having the variac in the loop keep the amp from blowing a fuse?
- I've searched forums and followed troubleshooting lists.
- I've swapped out the rectifier tube with a known good tube but it didn't work.
- I've tried a solid state rectifier tube replacement. Got lots more power but still blew the fuse.
- I've pulled tubes and added them back in one by one. I've swapped the two power 6V6's. Right now I'm out of fuses and waiting for more.
I'm hoping someone will have ideas about other things I can check, and/or maybe have some ideas about how to fix the trebly percussiveness of the sound?
Thanks in advance for any help.
Attachments
The fuse rating may be too small for the amplifier's surge current. Try a fuse of twice
the current rating, and/or a slow blow type instead of a fast blow.
The Variac may introduce enough series resistance to reduce the current surge and
to not blow the fuse you are using.
the current rating, and/or a slow blow type instead of a fast blow.
The Variac may introduce enough series resistance to reduce the current surge and
to not blow the fuse you are using.
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I se at least one error on the diagram, you have no CT on the PT secondary. And for that matter no heater wiring. But it leads me to wonder if your mains side wiring is accurate or if you just drew a generic mains circuit. perhaps your mains plug is mis=wired so the fuse winds up across the neutral and hot. On teh variac, the wiring may be reversed.
Thanks for the feedback.
@rayma. I know the variac must be adding in some "cushion" effect but you're suggestion the induction effect of the variac transformer is limiting the current inrush. Makes sense to me. I'll try some higher current fuses and see if I can home in on the right level.
@Enzo:Yes, it's a simplified schematic, sorry for the confusion I caused. There is a CT off the HT secondary and it's grounded to chassis with the mains ground/earth. However it does show the actual wiring: mains "Hot" through the fuse and then to the switch and then into the PT primary. I thought that was best practice. Is it actually the problem?
Thanks again.
@rayma. I know the variac must be adding in some "cushion" effect but you're suggestion the induction effect of the variac transformer is limiting the current inrush. Makes sense to me. I'll try some higher current fuses and see if I can home in on the right level.
@Enzo:Yes, it's a simplified schematic, sorry for the confusion I caused. There is a CT off the HT secondary and it's grounded to chassis with the mains ground/earth. However it does show the actual wiring: mains "Hot" through the fuse and then to the switch and then into the PT primary. I thought that was best practice. Is it actually the problem?
Thanks again.
Don't use a fuse larger than the Variac's current rating, unless you need a large paperweight.
They're not kidding about the winding's rating.
They're not kidding about the winding's rating.
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I don't much care - at this point - which order the switch and fuse come, they won't cause blows. I am concerned your amp and the variac one or both may have the neutral and hot reversed.
Also make sure your wall outlet is correctly wired in your shop. In other words does it act this way in another building? get one of those little outlet testers at the hardware store or Harbor Freight. The little plug with three lights on it, and which ones are light or dark tells you if the wiring is right.
Also make sure your wall outlet is correctly wired in your shop. In other words does it act this way in another building? get one of those little outlet testers at the hardware store or Harbor Freight. The little plug with three lights on it, and which ones are light or dark tells you if the wiring is right.
Enzo, The amp itself has an IEC socket with the L, N, and E stamped on it so I think I got that right. My shop outlets test good.
But you were right about the variac. It has a three-pronged wall plug but it's showing an open ground on the variac's output socket. I assume the cure is to make the incoming ground (green) wire from the wall outlet connect to the ground pin on the variac's outlet, and that the variac chassis should be grounded to earth as well? I'll have a look inside it and report back.
bansuri, thanks for the catch. I did get that wrong. It actually sounds sort of okay the way I did it, but I'm going to rewire it the right way.
But you were right about the variac. It has a three-pronged wall plug but it's showing an open ground on the variac's output socket. I assume the cure is to make the incoming ground (green) wire from the wall outlet connect to the ground pin on the variac's outlet, and that the variac chassis should be grounded to earth as well? I'll have a look inside it and report back.
bansuri, thanks for the catch. I did get that wrong. It actually sounds sort of okay the way I did it, but I'm going to rewire it the right way.
Thanks, PRR. On the theory that it never hurts to ask, why does it make more sense with a pentode first stage?
High output impedance. Turning pot load to zero does not imply higher THD or require an "infinite" coupling capacitor.
Okay. Inside the variac I don't fully understand what I'm looking at so I've attached some pictures--happy to take more if that will help.
Three wires come in from the variac power cord, go through a switch, and then into the variac main coil with a meter in series. There is also a thin black rectangular component you can see on the images--that's the circuit breaker/reset button assembly. The switch, meter, and reset are all the top of the front panel.
The variac output socket is at the bottom of the front panel as you look at it and there's a picture of it below also. I can see the ground wire attachment point but there is NO ground running to it from the incoming green wire from the mains earth. There is a green wire grounding the chassis but that's all I can see. Also the wires going to the circuit breaker are green but I don't think those wires are at ground--confusing.
What about my just adding in a connection from mains ground to the variac socket ground? It seems safer to me to have it but I don't want to defeat some design feature of the variac and make it LESS safe.
Thanks
Three wires come in from the variac power cord, go through a switch, and then into the variac main coil with a meter in series. There is also a thin black rectangular component you can see on the images--that's the circuit breaker/reset button assembly. The switch, meter, and reset are all the top of the front panel.
The variac output socket is at the bottom of the front panel as you look at it and there's a picture of it below also. I can see the ground wire attachment point but there is NO ground running to it from the incoming green wire from the mains earth. There is a green wire grounding the chassis but that's all I can see. Also the wires going to the circuit breaker are green but I don't think those wires are at ground--confusing.
What about my just adding in a connection from mains ground to the variac socket ground? It seems safer to me to have it but I don't want to defeat some design feature of the variac and make it LESS safe.
Thanks
I'd suggest inserting a 1N4007 in series with each anode of the 5Y3GT, as that rectifier valve may be arcing on power-up to cause sufficient primary current to exceed the fuse capability. The fuse blowing appears to be on the margin, and it may not take much additional primary winding resistance to lower the surge current profile sufficiently for the fuse to survive.
One thought I had, that may or not be relevant.
SAFETY FIRST!
DO THIS BEFORE ANYTHING ELSE!
The variac may have no direct earth connection (many I have seen are floated above earth) - doubtless some engineer thought about removing the safety earth connection through the variac, for an unknown reason.
I would check that the variac has earth continuity from input plug to amplifier chassis safety earth - with the variac disconnected from the mains!
IF there is no earth continuity, then you could have earth leakage, or an earth fault in the amplifier, which causes the fuse to blow when the amp is directly connected.
BEWARE!
If there is no continuity, then your amp chassis, will be potentially live!
SAFETY FIRST!
DO THIS BEFORE ANYTHING ELSE!
The variac may have no direct earth connection (many I have seen are floated above earth) - doubtless some engineer thought about removing the safety earth connection through the variac, for an unknown reason.
I would check that the variac has earth continuity from input plug to amplifier chassis safety earth - with the variac disconnected from the mains!
IF there is no earth continuity, then you could have earth leakage, or an earth fault in the amplifier, which causes the fuse to blow when the amp is directly connected.
BEWARE!
If there is no continuity, then your amp chassis, will be potentially live!
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Question answered.
mondogenerator is right--the variac has a chassis ground but no earth ground.
Since my last post I've talked to the engineer who sold me the variac. He confirms that the varied has an open ground by design. Apparently I mis-heard that it was also an isolating transformer. He walked me through some tests that confirmed to him that I have a ground fault somewhere in the amp. I'm having no luck in running that down yet but it's got to be something tied to ground that shouldn't be. What's irritating is that the amp itself is working pretty well for a Deluxe clone, loud and clear with little hum.
Thanks for all the help everyone!
mondogenerator is right--the variac has a chassis ground but no earth ground.
Since my last post I've talked to the engineer who sold me the variac. He confirms that the varied has an open ground by design. Apparently I mis-heard that it was also an isolating transformer. He walked me through some tests that confirmed to him that I have a ground fault somewhere in the amp. I'm having no luck in running that down yet but it's got to be something tied to ground that shouldn't be. What's irritating is that the amp itself is working pretty well for a Deluxe clone, loud and clear with little hum.
Thanks for all the help everyone!
Problem Solved
Sorry for not capping this thread off sooner folks. The problem was my mistake and it was a classic. The primary side of the PT had a third lead which in my ignorance I took to be one of the secondary center taps.
So I ran it to the chassis ground with the other center taps. Bad idea. Turns out it was a 95 VAC input primary tap that was used for the Japanese export market.
So I was running something not far below mains current straight into my chassis. Yeah, that will blow a 2A fuse. Every Time.
When I re-read the schematic and realized my blunder, I disconnected and wrapped off that lead. And thanked my dumb luck that I wasn't dead.
The amp now runs fine or as well as can be expected for a bastardized, first-build Deluxe-like clone. Thanks to everyone for all their advice. There's a lot of real world experience on this thread and I appreciate everyone who made suggestions.
Sorry for not capping this thread off sooner folks. The problem was my mistake and it was a classic. The primary side of the PT had a third lead which in my ignorance I took to be one of the secondary center taps.
So I ran it to the chassis ground with the other center taps. Bad idea. Turns out it was a 95 VAC input primary tap that was used for the Japanese export market.
So I was running something not far below mains current straight into my chassis. Yeah, that will blow a 2A fuse. Every Time.
When I re-read the schematic and realized my blunder, I disconnected and wrapped off that lead. And thanked my dumb luck that I wasn't dead.
The amp now runs fine or as well as can be expected for a bastardized, first-build Deluxe-like clone. Thanks to everyone for all their advice. There's a lot of real world experience on this thread and I appreciate everyone who made suggestions.