200W bass amp noise help

[jonasnoble]

Good morning all, I have several preamp projects in the works, but while I'm waiting on transformers and chassis, I figured I would set to work on rebuilding this amp I built ~5 years ago.

It's a 4xKT88 amp with similar preamp/phase splitter topology to the older Mesa tube bass amps. I had noise issues I thought were caused by the transformers. So I bought new, bigger ones from Hammond, and still have issues. I don't think it's magnetic couple between PT and OT. Now I'm in the process of putting it in a new chassis (I powder coated the last one, and though I ground off all the grounding points, I'm concerned that's where my noise is coming from.) I see 60Hz, 120Hz, and harmonics, so I can't tell if it's line, rectified, or both. What I did notice, when manipulating the Master Volume (pre phase splitter) all the way up or all the way down, the noise disappeared. Anywhere in the middle and it's unusable, for recording at least.

I'm not done with the rebuild, but should be within the next couple days. I'll post pictures and audio, when I can. My current interest, does anybody have any thoughts on wire type, and what it may do to noise? I'm wondering if I used cheap wire, or bigger than necessary gauge may induce hum issues.

This was/is a prototype, so it has some unplanned, last minute elements I'm trying to sort out during the rebuild.

Thanks in advance for any help you can offer.

 

[Khron]

What I did notice, when manipulating the Master Volume (pre phase splitter) all the way up or all the way down, the noise disappeared. Anywhere in the middle and it's unusable, for recording at least.

That sounds like there's a (too) high output impedance of the stage that's just before the volume potentiometer. The source impedance that the (in this case) phase splitter sees is the two halves of the volume pot in parallel with each other, and that happens to be minimum at the ends of travel.

Is there some way you could add a cathode follower triode just before the volume pot?

 

[ccaudle]

I powder coated the last one, and though I ground off all the grounding points, I'm concerned that's where my noise is coming from

Do you use chassis for any of the current return paths? Or just for shielding? Chassis metal is not particularly low resistance compared to copper, so if for example you had power supply return currents and signal return running through the chassis you could get increased noise.

does anybody have any thoughts on wire type, and what it may do to noise?

The only effect wire has on noise is by how it is twisted or not. Magnetic pickup can be reduced by having the signal and return paths (or power and return paths) twisted together so that there is minimum distance between the two wires.

 

[Philbo King]

If using a linear supply you might check for 60/120 Hz AC noise on the power supply output. Ideally with an oscilloscope, though a DMM on ACV range would do on a pinch.

 

[Matador]

I powder coated the last one, and though I ground off all the grounding points, I'm concerned that's where my noise is coming from

Ideally there is only 1 single chassis connection to the circuit 0V: right at the input jack.

I had a similar issue with a Reverend amp that I couldn't find a satisfactory explanation for: here was the punch-line:

So although I understand what was causing the problem, the underlying root cause still makes no sense to me. Why would the input impedance to the PI cause a static DC imbalance between the output tubes? I tried 5 or 6 different PI tubes, and all did exactly the same thing, and the circuit is a pretty standard LTP that you can find in a million other amps. The plate resistors, coupling caps, grid resistors, and bias resistors were all removed and tested and all were normal.

I would check to see if your amp has the same root-cause: my buzz was caused by a static imbalance in DC currents through the OPT, which only manifested itself if the impedance driving the PI has 'high' (say several hundred kohms): if you have 1 ohm cathode resistors, you can confirm this easily, just by calculating how much current is flowing through each half of the OPT.

Do you have a schematic?

 

[jonasnoble]

It's mostly this Mesa 400 schematic, I put different transformers, running the KT88's at 725V.

It is a Cathode follower into the Master Volume -> grid of the PI. I don't have any of the effects loop components in it, so the 33k cathode resistor, 0.1uF cap, and 15k resistor feed directly to the master volume.

I can't wait to get this thing put together, hopefully this weekend, so I can start trying some things and learning.

 

[Khron]

What about a lower-value volume pot? Or what value do you currently have installed?

 

[jonasnoble]

What about a lower-value volume pot? Or what value do you currently have installed?

I believe I have the 250k in there, but I'll check when I get home. (I barely remember building this thing.) Now that you guys are talking about the impedance, I have a place to look. I can try lowering that volume pot, for sure.

 

[jonasnoble]

Chassis is punched, transformers are mounted. Still have some drilling to do for the controls, but it's coming along.

One thing I've noticed since you guys directed me to the PI input: the master volume wiper lead runs directly over the output transformer primary center tap. I've built a hundred amps with this layout, in a Fender black face chassis, but never one running this high of voltage. There's always a little ripple on the transformer primary, as it's tapped off the first stage filter. This could be as simple as lead dress.

I'm going to try removing the master volume to see if that's it. If it is, I'll experiment with a lower value pot, and running shielded cable. This should've been an easy catch for me, but I got stuck in convention and couldn't see outside the box.

I'll update you guys tomorrow evening I hope. Thanks for all the input.

 

[jonasnoble]

Nothing near as good as some of the work I'm seeing on here, but it's finally coming back together. I got sidetracked with another project this week, and just put this one back on the bench.

 

[Matador]

There's always a little ripple on the transformer primary, as it's tapped off the first stage filter. This could be as simple as lead dress.

Ripple on the center tap of the primary isn't so much of a concern: it will be cancelled by the two opposite phases of the pull-pull primary.

 

[jonasnoble]

Understood. I was mostly concerned about the proximity of the master volume tap (feed to the phase inverter grid) running right over the top of that center tap. I'm pretty sure I was injecting that ripple right into the phase inverter, driver stage. I'll know more tomorrow night I think.

 

[jonasnoble]

Okay, we were right. It was the input to the phase inverter. I've finally gotten it assembled and running good. Quiet as can be, even when I got it up to 112Wrms before clipping. Respectable volume for a tube bass amp that doesn't weight 112 pounds.

There are probable a couple things I could play with to get more clean headroom. Anybody have any experience with ultra linear taps? I've hooked them up before and didn't hear much of a difference then, but I'll try again over the next couple days.

Anyways, you guys are kings. Thanks for the help.

 

[user 137017]

I have a cheap KLD kit amp. These are noted for a lack of head room (full crunch at gain setting of 3) i replaced the first tube a 12AX7 with a tamer tube ECC81 full crunch now at gain setting of 8 on the dial. Yes i could have just altered the cathode resistor or added a potential devider along the input stage, but i didnt fancy altering a good amp that i had rewired a little already

 

[user 137017]

Other tubes worth looking at if you are going own that road ECC81 ECC82 ECC84. The 12AX7 is ECC83

 

[jonasnoble]

That's a great idea, I hadn't thought of. Most of my distortion is happening in the early stages. I'll try subbing some in tonight and see how it goes.

 

[Matador]

Okay, we were right. It was the input to the phase inverter.

What did you end up changing/fixing, exactly?

 

[jonasnoble]

It was way more simple than I thought. There were probably several things going on, but the main factor was the presence of HT ripple near the master volume tap- feed to the phase inverter.

I just didn't catch it when I was installing the bigger transformers previously, the output primary center-tap was under the eyelet board, I didn't realize the signal line was laying right on top of it.

When I drilled and punched the new chassis, I made sure to run all the leads better for noise and clean up all the grounds. It's super quiet now.

And now the fun part begins. I bought some new test equipment that should be here soon. I want to start measuring and tweaking these to get the good good.

 

[Cqwet Dbdfte]

725V on those KT88's?
Their life would be fast and furious.
I have a similar multitube PA for my living room enjoyment modded from old ALTEC 1569A quad EL34, now running 6550's. Each tube has separate bias control, as tube specs are soft and ageing makes no improvements.
The screen grids are very well regulated at 380V with super low noise and very gradual startup, like a minute, to match heaters. Bias current about 25mA per tube measured over 5 Ohm cathode resistors.
Plate supply is unregulated 485V with 1500uF storage. (Keeps playing at low levels for a few minutes after turning off).
Output levels would normally be only a couple of W's.

Tube output transformers are not great at low frequencies, I wish the guys running around in their cars with massive bass would use them.

110W from six tubes seems overkill, a single pair of KT88 should be able to make 100W at 560V according to the data sheet.

What is the idea of using tubes for bass? Class-D amp would be cheaper , more power, lower cost, and distortion effects could be arranged for the input signal. If only bass, a 60Hz isolation transformer could be used on output

 

[Newmarket]

Do you use chassis for any of the current return paths? Or just for shielding? Chassis metal is not particularly low resistance compared to copper, so if for example you had power supply return currents and signal return running through the chassis you could get increased noise.

Mmmm...OTOH there is usually such a mass of "Chassis Metal" that it's not an issue. Whether chassis is ferrous or not makes a difference wrt H field immunity.

The only effect wire has on noise is by how it is twisted or not. Magnetic pickup can be reduced by having the signal and return paths (or power and return paths) twisted together so that there is minimum distance between the two wires.

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