High Voltage Tube Driven Overdrive Pedal Builderino

[kleydejong]

Basically yes! Modern pedal power supplys do not use mains Safety Earth and which have multiple 9V supplys they are isolated each other. Then there is only one Signal Reference in signal chain!

Because metal box must be connect to SE it should be isolated from 0V = SR as much as possible. I have used picture kind circuit in my few amp builds. Obviously in mains powered effect that 10 ohm resistor could be much higher because effect fuse current is low. Zeners should make "connection" much softer/looser as well but they should be connect series and opposite direction because zener voltage is reverse way and thru normal dirention there is normal diode voltage.

When there is instrument cable from effect output to guitar amp input there come solid / hard wired connection to SE.

I recently found this little guy while studying the circuit and layout of a custom build I bought some years back, before I took the plunge on building some myself. I couldn’t see the values on the diodes, but the builder had the whole of the ground bus coming to this point.

View attachment 1312055View attachment 1312056

I applaud your pluck. Tube based boost pedals are the road less travelled.

But so far, you're basically building a class A amplifier with a couple of gain stages and a basic tone stack in the middle. Nothing wrong with that. But, since you are driving a regular guitar amp, do you really need TWO more gain stages and a tone stack? Your typical guitar amp already has three and its own tone stack, right?

I was going to say why not select a tube that has some diodes in it while you are at it and use them for diode clippers or a kewel tube compressor? But since you've gone this far, maybe that horse is already out of the barn. So maybe ignore my idea? But I was going to suggest a different tube - a ddt, a triode with two diodes in it like the 12AT6 for example. About like a 1/2 of a 12AY7, but with two diodes built into it. Perfectly happy with about 120-160v on the plates. Cheap and easily available too.

Thus the idea of a tube diode based overdrive/distortion pedal. or a tube overdrive with compressor.

Anyway...

Whatever you do, have you thought about using TWO cheap doorbell transformer for your primary transformer? Get a 120/16V and a 120/12V doorbell or similar and wire them secondary to secondary. 120V primary from wall current. Secondary is 16V - Tap, rectify/regulate that 16V down to 12VDC with a 7812 regulator for your tube's DC heaters from there - and also send the unrectified 16V straight into 12V secondary of xformer #2. That turns things around and makes transformer #2's output of was the the "primary" into a secondary. It should make about 160VAC...which you then rectify and filter. That's going to be plenty of current for the plates on a lot of ddt tubes. A 12AT6 only needs 2.5ma to work just fine at those voltages, and should swing about 60 volts-ish.

or... have you thought about using something like a 12AJ6, a high mu triode and double diode that runs on very low plate voltage? They were used in automobile tube radios back in the day. plate voltage is 12vdc. Then a simple 120 to 12V transformer gets you all you need.

But again, you're pretty far down the road as is. Sorry for the flight of fancy.

I ditto what others have said about the grounding scheme, and the ground lifter trick. You're going to need that.

Regarding the ground lift function, it seems like a problem that my input and output jacks are chassis grounded. Would I need to isolate them into the 'Circuit Ground' as well for the lift to work properly?

 

[Crowe Baaah]

Regarding the ground lift function, it seems like a problem that my input and output jacks are chassis grounded. Would I need to isolate them into the 'Circuit Ground' as well for the lift to work properly?

Above my pay-grade to know whether it is required or not, but I can tell you that in this amp the input and the output jacks are isolated and grounded via a jumper wire from the ground tabs to the modified star scheme.

 

[2L man]

Regarding the ground lift function, it seems like a problem that my input and output jacks are chassis grounded. Would I need to isolate them into the 'Circuit Ground' as well for the lift to work properly?

They should be isolated! If not and you use it with amplifier there come one Signal Reference thru the cable and another thru the mains SE.

 

[kleydejong]

I took it back apart and tried applying some labels and applying a clear coat over top the black. I am using simple Ace Hardware branded spray paint. For some reason I don't understand the clear coat didn't interact well with the black. I am going to move forward. I honestly don't care to spend any more time fiddling with how this one looks.

Reassembled the hardware. Then I started on some wiring. I'm using two nylon spacers to elevate the turret board. I color coded my wires for the pots to keep some organization. They're going to be trapped under the turret board and hard / impossible to access.

 

[kleydejong]

Updated my layout per the changes to the schematic.

 

[2L man]

Now when you use mains Safety Earth and normal signal cable which has the shield connect to the amplifier there come parallel ground loop because jacks have connection with box/SE.

 

[King Fan]

Kley, you're getting lots of good advice. If I'm seeing right, it looks like you've now isolated your jacks?

(Edited as I look a your pics) -- Let me ask you about the IEC connector and wire colors. The inside/outside and which-way-up issues make IEC wiring hard to discuss; you'll do 10x better with a continuity tester and your own eyes than I can with pictures.

I can't tell how your actual power wiring works here; I'm just looking at your layout diagram -- which may be different?

If I'm right, I thought an IEC entry has hot on the left *as seen from inside with neutral down.* If so, your drawing is maybe switching the neutral? or you have black and white wire colors reversed? As you'll know, switching the neutral is less safe in certain fault conditions. If it's just a wire color issue, of course, it only risks confusion, and you probably won't be confused. And if the real orientation and wiring is different, I'm down the wrong rabbit hole altogether.

As for fuse v. switch first, folks here tend to like fuse first, but as you'll also know it's complicated -- maybe dealer's choice. As a minor detail, I now draw my own fuses in DIYLC so I can show hot to tip -- *that* detail might just matter.

 

[kleydejong]

Kley, you're getting lots of good advice. If I'm seeing right, it looks like you've now isolated your jacks?

(Edited as I look a your pics) -- Let me ask you about the IEC connector and wire colors. The inside/outside and which-way-up issues make IEC wiring hard to discuss; you'll do 10x better with a continuity tester and your own eyes than I can with pictures.

View attachment 1314573

I can't tell how your actual power wiring works here; I'm just looking at your layout diagram -- which may be different?

View attachment 1314556

If I'm right, I thought an IEC entry has hot on the left *as seen from inside with neutral down.* If so, your drawing is maybe switching the neutral? or you have black and white wire colors reversed? As you'll know, switching the neutral is less safe in certain fault conditions. If it's just a wire color issue, of course, it only risks confusion, and you probably won't be confused. And if the real orientation and wiring is different, I'm down the wrong rabbit hole altogether.

As for fuse v. switch first, folks here tend to like fuse first, but as you'll also know it's complicated -- maybe dealer's choice. As a minor detail, I now draw my own fuses in DIYLC so I can show hot to tip -- *that* detail might just matter.

View attachment 1314563

Good feedback. The layout has the IEC jack upside down compared to the pedal. I did a review it with a power cord plugged in and confirm I'm switching the hot and the transformer is producing proper voltages. Thanks for the input though. Good to check.

I also took a good read through that fuse thread you linked and decided to swap the leads.

 

[kleydejong]

I'm wrapping my mind around this ground loop issue.

The is ground loop occurring because my IEC power connector has the middle 'green' wire bolted to the enclosure? That feeds 120vac from the IEC cable. The IEC cable then grounds the chassis to Earth through the wall.

Then there is a second path to Earth via the output jack. The Sleeve connection of the output jack runs through the cable into the amplifier. The sleeve of the input jack in the amplifier would also have a ground connection through THAT amplifier's safety earth 'green' wire grounding that chassis. That amplifier's power cord grounds to Earth through the wall.

Two paths to ground = ground loop = hum. Right?

So the solution is to:

1) Isolate the input and output jacks on my pedal. I found some isolation washers:

Isolation Washers for 1/4" Input Jack - Flat, Shoulder

Lower your noise floor and improve your star ground with these washers. This is the perfect item for isolating the "sleeve" connection of a Switchcraft style input jack from the grounded chassis. Proper isolation requires both a flat fiber washer and a shoulder fiber washer. Additionally, the input

www.tubedepot.com

OR

I could use plastic cliff jacks which do not ground to the chassis when they're installed.

Pardon Our Interruption...

2) Create a ground scheme that has the bolted safety earth connection from the IEC, and then use a star grounding scheme which all feeds into a ground lift circuit:

Do I need to do BOTH 1 and 2?

Merlin calls this a 'hum block network' - which I'm interpreting to mean the ground loop exists, but you negate it with the circuit.

Could I also negate ground loop hum by isolating the jacks and then having my star ground bolt to the chassis? Then its ground path would be through the IEC cable?

 

[2L man]

I'm wrapping my mind around this ground loop issue.

The is ground loop occurring because my IEC power connector has the middle 'green' wire bolted to the enclosure? That feeds 120vac from the IEC cable. The IEC cable then grounds the chassis to Earth through the wall.

Then there is a second path to Earth via the output jack. The Sleeve connection of the output jack runs through the cable into the amplifier. The sleeve of the input jack in the amplifier would also have a ground connection through THAT amplifier's safety earth 'green' wire grounding that chassis. That amplifier's power cord grounds to Earth through the wall.

Two paths to ground = ground loop = hum. Right?

So the solution is to:

1) Isolate the input and output jacks on my pedal. I found some isolation washers:

Isolation Washers for 1/4" Input Jack - Flat, Shoulder

Lower your noise floor and improve your star ground with these washers. This is the perfect item for isolating the "sleeve" connection of a Switchcraft style input jack from the grounded chassis. Proper isolation requires both a flat fiber washer and a shoulder fiber washer. Additionally, the input

www.tubedepot.com

OR

I could use plastic cliff jacks which do not ground to the chassis when they're installed.

Pardon Our Interruption...

2) Create a ground scheme that has the bolted safety earth connection from the IEC, and then use a star grounding scheme which all feeds into a ground lift circuit:

View attachment 1315486

Do I need to do BOTH 1 and 2?

Merlin calls this a 'hum block network' - which I'm interpreting to mean the ground loop exists, but you negate it with the circuit.

Could I also negate ground loop hum by isolating the jacks and then having my star ground bolt to the chassis? Then its ground path would be through the IEC cable?

Yes thats how the ground loop would come!

Because there is High Voltage it should never have possibility to come to instrument cable. To avoid ground loop the 0V can not be connect direct to Chassis/SE. If you would always first connect the effect to amplifier first it would get the SE connection there.

If there come severe HV leak that Merlin circuit is designed to burn HV fuse but there must be the HV fuse!

 

[kleydejong]

Tabling the ground loop thing for now. I had some time to build last night and made a LOT of progress.

The build was pretty tight. I'm glad I did color coded wires because I don't have great access to them once its together. I ended up ignoring the layout and building off the schematic. Old habits. I don't really trust the layout. It was a helpful exercise though, especially to maximize my turret layout. It ended up being pretty perfect.

 

[kleydejong]

I cruised through startup procedures. I had an issue with the 120vac pilot light. It needs to be connected to the two black wires going into the primary of the PT. I was also missing a jumper on my ground bus. Pretty easy fixes though. Fired her up and it makes NOISE!

Voltages:

The Red secondary wires on the PT are at 72 vac.
After the rectifier diodes it sits at 192 vdc.
After the filter caps and dropping resistor it sits at 189 vdc.
The plate voltage is at 129 vdc.
Grid voltage is in the millivolt range and jumps around a bit.
The cathode is at 1.3 vdc.

I put the values into a load line calculator. It gave me this. I'd like to explore this more later. Based on this result I changed my cathode bias resistor from 1.5k to 2.2k.

 

[kleydejong]

And my first tone clips. You can hear the ground loop buzz pretty bad in the room and on the cell phone. But into a loud clean Super Reverb it really doesn't show up that bad on the mic.



First impressions are pretty killer. I will be making quite a few adjustments.

Observations:

The ground situation is interesting. I first tested the pedal off camera into my combo amp. Very little buzz. Then testing it into a different power strip and into my Super Reverb it buzzes like crazy. Definitely need to make work of isolating the ground.

The pedal is EXTREMELY touch sensitive. I'm really pleased with the medium gain tones that clean up with a lighter pick attack. I have no desire to make this higher gain. It sits nicely.

The tone control and bass switch are a little disappointing. I suspect that because I'm usually cranking the gain control to push the second gain stage into distortion, any treble rolloff between the two stages gets renewed. Probably need to relocate the Tone control to the end near the Level control.

Forgot about the effect on/off light. Need to add that.

I checked and I have a 2 amp fuse installed. Needs to be more like 1 amp or even 1/2 amp.

I really want to spend some time rolling different 12AX7's. I'm using a spare old Marshall branded 12AX7. I'd be very curious to see how other tubes would sound.

 

[kleydejong]

I made a few updates tonight.

I added the hum buster network. I used a 50 ohm 10w resistor because it was the closest I had on hand. I used four 1N4007 diodes. I added this all to a new terminal strip bolted to the chassis.

I removed the sizzle control. So V1A runs into the de-coupling cap, gain pot, then 470k grid blocking resistor.

I added a simple treble control after the V1B de-coupling cap. I think this location will give better high end control. I suspect that putting it between the two gain stages meant that even if I'm rolling some high end to ground, because most of the distortion is generated by V1A overdriving V1B - then there is still some harmonic distortion re-energizing the high end on the output of the circuit.

Lastly I changed the bass cap from .68uf to .1uf. Should cut even more bass for a more noticeable effect.

 

[2L man]

I can't see where in a circuit these 1n4007 and 50R are but obviously not between 0V=Signal Reference and Box=Safety Earth? There are no SR wires for input and output jacks so they seem still to use metal box for it?

Did it lower the buzz?

That circuit must not be between mains SE and metal box because mains fuse is very strong and 1N4007 is only 1A.

SE must always be wired direct to metal box!

 

[Lynxtrap]

Is the "hum buster" the same as Merlin Blencow's ground lift circuit?
I've been curious to try it.

 

[kleydejong]

I can't see where in a circuit these 1n4007 and 50R are but obviously not between 0V=Signal Reference and Box=Safety Earth? There are no SR wires for input and output jacks so they seem still to use metal box for it?

Did it lower the buzz?

That circuit must not be between mains SE and metal box because mains fuse is very strong and 1N4007 is only 1A.

SE must always be wired direct to metal box!

I haven't actually had a chance to play it yet. My kids were in bed last night when I finished. So we'll see how it affects buzz today.

I did a once over here this morning. I currently have the following components connected to the chassis:

1. The middle 'green' wire on the IEC power plug.
2. The 6.3vac heater wires run to a terminal strip. Then two 100 ohm resistors tie to the middle lug of the terminal strip which is bolted to the chassis.
3. The input and output jacks still have their sleeve connection grounded to chassis. I have some plastic cliff jacks coming in the mail.
4. All other circuit grounds are linked together to one final green ground wire. That ground wire is tied into the bottom / lower lug of the new terminal strip for the 'hum block network'. The other two lugs are tied together and bolted to the chassis. I have a 50 ohm 10 watt resistor, a .1uf cap, and the diode network bridging between the circuit ground and the chassis ground.

I plan to add the input and output jacks to the hum block network when the new jacks arrive. I'll have to rethink how I do the heater wires if I were to add those to the hum block network.

 

[kleydejong]

Is the "hum buster" the same as Merlin Blencow's ground lift circuit?
I've been curious to try it.


View attachment 1316223

Yes that's what I'm using. I used a 50r 10w resistor because I had one on hand. I used 1N4007 diodes as shown. I don't have a lift switch. The bottom of that hum-loop block network is the middle / upper lug of my terminal strip which is bolted to the chassis.

 

[kleydejong]

I had a chance to test it a fair bit and conclude that the hum buster network does NOT fix my buzz. I still have the non-isolated jacks. I've got some plastic cliff jacks in the mail to test that.

 

[kleydejong]

Next I did some tweaking of the components controlling the bass cut and the tone stack.

Bass switch is 10uf for full bass and then I ended up liking .1uf for the cut position. .68uf wasn't super noticeable.

For the tone control I was experiencing some weird stuff with a 1M pot + 4.4nf cap. Not a lot of control until 6 on the pot. Then lots of treble and a big volume boost. I changed it to a 100k pot. I experimented with cap values including 2.7nf, 10nf, and then settled on 4.7nf.

With the bass cut off and the tone up I can get a more scooped tone. Trimming bass and turning the treble down is a nice midrange pushed tone. Worked well with both my strat and 335. I spent about an hour testing components, then playing a bit, then trying something different, play some more. It was a whole lot of fun for a Saturday night!

With these tone control tweaks the pedal is really starting to settle into a spot that I'm very happy with. I'm optimistic isolating the jacks will cure my noise issue and I'll be pretty much done.