Bias problem!

[Steve Conner]

No, I'm not confused but if you wired an amp like that, you probably are!

Where is the input jack? Is that it in the top right hand corner of the picture?

Where does the wire from the input jack run? Is it the thick blue wire that I see leaving the back of the jack and snaking in all directions through the chassis?

Is it shielded cable? If not, that's probably why the amp is oscillating.

Also, are the speakers connected correctly? If not, that might explain the sparking and quiet sound.

Yes, seeing a blue glow inside the electrodes while tubes are passing current seems to be normal. Clouds of glowing blue gas inside the tube are bad, though, and if anything else glows red except the heater, that's bad too.

[Chuck H]

Whoah... I don't want to upset anyone but...

Theres no possible way that amp could be stable. You really need to build a whole new board and wiring layout that has the circuit running in line with the tubes and ALOT less crossover in the stages. I didn't examine it long, but I can't imagine the grounding scheme is helping. And that can be critical in a high gainer.

Maybe you have to rebuild the board and rewire it, but you already have the chassis, pots, tubes and transformers in place. And you know what the likely culprit is, and it's fixable. But you absolutely can't cut any time corners when it comes to laying out a high gain amp.

I didn't even know if I should send this post. But Steve was very diplomatic about it, so... You really need to do a new layout.

no offence, Just want to help.

Chuck

[thomasdj]

Hi friends

Yep, I know and thanks for all of your help. This is my first build of a high gain amp..and PCB layout amp..

When I started this project I only had 4" x 6" PCB boards available which is why it was difficult to place components and so on..but now I got no limits! I got double side PCB's!

I know when making PCB amps the signal should be on one side of the board..and ground on the opposite side..I keep the power supply circuit as is..which is stargrounded to chassis underneath the PCB's (Left side in picture). But what about the preamp circuit? ..the board will be at least 16" long and 4" wide.. how should the different stages be grounded?? If this was a PTP build I would just go for the starground methode (near the OT). And what about the switching circuit?? where should it be placed?

Untill now I have only placed the components in my new PCB layout..from input signal to PI.

Are there anything else I should know when making PCB amps? If I have to route the signal on both sides of the PCB are there any limitations? like distance for instance??

Thank you so much!

Thomas

btw..sorry if my spelling is poor..I am from Denmark in Europe where we talk like #%¤&"""#### and grunts all the time

[Steve Conner]

The main problem in tube amp layout is capacitive coupling. If you put a wire connected to a later stage near a wire connected to an earlier stage, the stray capacitance between those wires will let signal feed back, and make the amp oscillate.

Even a tiny stray capacitance can ruin your day. As an example, the worst thing you could do in the world of layout would be to run the input jack wire near one of the power tube plate wires. A high gain amp could get unstable if the plate wire came within about, say, 6 inches of the input jack wire.

Generally, we avoid that kind of screw-up by laying out all the components in a line that matches the signal flow. So the signal travels from one end of the chassis to the other, getting bigger as it goes, and never loops back on itself. Wires associated with later stages never need to cross wires associated with earlier ones, and the wires can all be short.

As you get more experience, you'll know what you can get away with. For example, you can get wires from different stages as close together as you like, IF you shield them. You often have to cross wires if you want an arrangement of front panel pots that doesn't match the signal flow.

For your amp, I'd recommend replacing the wire from the input jack to the first stage grid with shielded cable. If that doesn't work, start replacing all the other long runs of signal wire one by one, starting with the ones associated with V1.

BTW, I have friends in Denmark, and they have better coffee than Scotland at least

[thomasdj]

The main problem in tube amp layout is capacitive coupling. If you put a wire connected to a later stage near a wire connected to an earlier stage, the stray capacitance between those wires will let signal feed back, and make the amp oscillate.

Even a tiny stray capacitance can ruin your day. As an example, the worst thing you could do in the world of layout would be to run the input jack wire near one of the power tube plate wires. A high gain amp could get unstable if the plate wire came within about, say, 6 inches of the input jack wire.

Generally, we avoid that kind of screw-up by laying out all the components in a line that matches the signal flow. So the signal travels from one end of the chassis to the other, getting bigger as it goes, and never loops back on itself. Wires associated with later stages never need to cross wires associated with earlier ones, and the wires can all be short.

As you get more experience, you'll know what you can get away with. For example, you can get wires from different stages as close together as you like, IF you shield them. You often have to cross wires if you want an arrangement of front panel pots that doesn't match the signal flow.

For your amp, I'd recommend replacing the wire from the input jack to the first stage grid with shielded cable. If that doesn't work, start replacing all the other long runs of signal wire one by one, starting with the ones associated with V1.

BTW, I have friends in Denmark, and they have better coffee than Scotland at least

Yep Steve...we have good coffee! I do use shielded cable at the input jack (it's the grey wire upper right on picture..that goes straight to V1).

But what about the switching circuit and the ground issue? could it be routed at the same side of the PCB..the one facing down? should I starground each stage or can I make one big ground plane of the entire preamp inclusive the switching circuit? How long shall the traces between components be..to avoid any possibility for hum, oscillation etc..?

Thanks

Thomas

btw..Ok, we have better coffee in Denmark ..however I would love to try some of your fishing lakes in Scotland! ..since the danish fishing environment is totaly dead

[Enzo]

Steve knows a lot more about board layouts than I do, but a guitar amp is not a critical application. You have to pay attention, but it is not a high speed digital circuit nor does it run at RF freqs. From what you have posted it sounds like you are concerned over ground planes on the pc borad and such. You don't have to worry that a couple traces become a transmission line reflecting the signal back at you. At least not as a major concern.

Plenty of amps built on single sided boards, and I would not be so sure I would even want a ground plane covering one side of a two sider. It becomes a giant capacitor.

You can easily star the grounds on a pcb, ground traces tunning all down to the one point where they join. I see amps that way all the time, as well as video monitor chassis, and so on.

Look closely at a commercial amp board. Two parallel traces can couple as Steve mentioned. But you can also run a third parallel trace between them and have it part of ground (just connected at one end) and it serves to shield them one from another, or at least decouple them. First time I saw traces to nowhere, I was confused.

In my head, ground issues are more about shared current paths than where they are physically. In other words, if you have a relay switching parts in the first stage, you don't want the coil current using the same ground return as the signal circuits, otherwise the coil supply ripple current will be added to your signal. Doesn't matter that the relay is right next to the amp stage, it matters that they share copper.

I hope I haven't missed your point entirely. While we want to keep RF noise out of our amos, we don't need to build then like they were radios. I see ground planes as more a high freq systems element, not a hum abatement technique.

[thomasdj]

Steve knows a lot more about board layouts than I do, but a guitar amp is not a critical application. You have to pay attention, but it is not a high speed digital circuit nor does it run at RF freqs. From what you have posted it sounds like you are concerned over ground planes on the pc borad and such. You don't have to worry that a couple traces become a transmission line reflecting the signal back at you. At least not as a major concern.

Plenty of amps built on single sided boards, and I would not be so sure I would even want a ground plane covering one side of a two sider. It becomes a giant capacitor.

You can easily star the grounds on a pcb, ground traces tunning all down to the one point where they join. I see amps that way all the time, as well as video monitor chassis, and so on.

Look closely at a commercial amp board. Two parallel traces can couple as Steve mentioned. But you can also run a third parallel trace between them and have it part of ground (just connected at one end) and it serves to shield them one from another, or at least decouple them. First time I saw traces to nowhere, I was confused.

In my head, ground issues are more about shared current paths than where they are physically. In other words, if you have a relay switching parts in the first stage, you don't want the coil current using the same ground return as the signal circuits, otherwise the coil supply ripple current will be added to your signal. Doesn't matter that the relay is right next to the amp stage, it matters that they share copper.

I hope I haven't missed your point entirely. While we want to keep RF noise out of our amos, we don't need to build then like they were radios. I see ground planes as more a high freq systems element, not a hum abatement technique.

Thanks Enzo!

A lot of things make sence for me now! ..especialy the thing with traces going nowhere! ..thanks for the tip with the switching circuit! I'll make ground points rather than one big ground plane! ..I am almost 100% sure that this issue has something to do with my current layout..that oscillate! ..I am making a new one now and post it when I am done..

Thanks

Thomas