Hiss in Express Build

[Funkalicousgroove]

[MarshallPlexi]

Tomorrow I will be living in the amp. I'll let you know what I find.

Thanks for your kindness guys.

[MarshallPlexi]

Here are today's findings:

Shorten NFB Wire: NO EFFECT
Remove Tone Control: NO EFFECT
Add Dedicated PT Filter Cap: Slightly less hash.
Metal Film Resistors: There is less hash, but this could be because of the ground layout etc.
Star Ground: There is less hash and I can confirm that the ONLY ground on the entire amp is the PT bolt. The Safety ground is connected elsewhere.

I have found that the hum is coming from the shielded cables from the input jack to V1a grid and the Volume Pot to V1b grid. When I have the volume all the way down, the placement of the volume pot shielded cable controls the level hum, and when the volume is up, the placement of the input jack shielded cable controls the level of hum. I tried using a bare wire for the volume control wire to V1b grid and the hum was noticeable less, but the amp started to become unstable. A 100pF cap across the PI plates solved this though. The bare wire is very microphonic, so I took it it. It just seemed like bad design anyway.

I replaced the bare with shielded cable again and grounded both ends only to have MORE hum. I have tried grounding the shield at every location in the chassis to no avail. Any ideas why a shielded cable is noisier than a bare wire? I'm using George L's cable for the shielded cable.

Thanks!

[doctord02]

only ground your shielded cable at _ONE_ end, not both. Grounding both ends will create ground loops.

[ServiceTech]

MarshallPlexi;

I've been reading your post and thought I might contribute a couple of cents. The George L cable you used is a very noisy cable, low in capacitance for sure but a bit noisy when shaken or rattled. Besides, it's so damn thin that soldering it makes me a bit nervous. It sounds like this cable is a major contributor to the problem. Do you have some different shielded wire? Even wrapping a wire around a plain wire and grounding it at the input end it is worth a shot.

[dobbhill]

Please give us details about the tubes you are using: manufacture, type, etc. The only time I've had problems with hiss was cured by tube swapping and removing carbon comp resistors.
HTH
D

[MarshallPlexi]

MarshallPlexi;

I've been reading your post and thought I might contribute a couple of cents. The George L cable you used is a very noisy cable, low in capacitance for sure but a bit noisy when shaken or rattled. Besides, it's so damn thin that soldering it makes me a bit nervous. It sounds like this cable is a major contributor to the problem. Do you have some different shielded wire? Even wrapping a wire around a plain wire and grounding it at the input end it is worth a shot.

I have some Teflon shielded cable that I'll try. You're right the George L's does worry me. The center conductor is just too small.

Thanks!

[MarshallPlexi]

Please give us details about the tubes you are using: manufacture, type, etc. The only time I've had problems with hiss was cured by tube swapping and removing carbon comp resistors.
HTH
D

Success!! DC Heaters cured it! Now it's DEAD quiet with all the controls maxed! Sure there is some hiss left over, but nothing compared to where it was when I started. I'm going to hook up a permanent regulator and see what that does. For now I'm powering all the tubes off of a bench power supply and it's working great.

Thanks for the help guys!

I'm using all Svetlana tubes and I'm using Metal film plate resistors.

[Funkalicousgroove]

[MarshallPlexi]

I agree on the coax, try belden 8451, it's a foil shield type wire and works very well, grounding both ends is probably the root of your hum. there is a very low occurrence of "Microphonic hook-up wire" it is most probably the tube it's connected to. If you like I can send you a few feet of this wire, I buy it by the roll. You really should consider the grounding scheme I told you about a few posts ago in leiu of the single point star ground.

I did not ground both ends. I tried this, but quickly changed it. The DC solved the problems for me. Thanks for your offer to send the cable. It is greatly appreciated, but I will now require it for this project.

Thanks again!

[ServiceTech]

Wait a minute.................changing to DC filament power solved hiss??????? Hissssss not hummmmmm???

[MarshallPlexi]

Honestly, it did help with that RUSH sound, and the hum is 97% gone. The wire that is causing me grief is the volume pot lead to the grid. It's position seems to greatly effect the hum level.

[MarshallPlexi]

Okay, more news.

A 1k 25W resistor between the plate and screen supply makes the amp less noisy. At first I thought this was due to the magnetic field of the choke, so I moved the choke around and eventually made a switch that let me select between the choke and the 1k resistor which were externally mounted at this point. Well the resistor is more quiet.

Also, upgrading to 200uF for the plate supply does help a great deal with hum. I can't play test right now, but I'm hoping that 200uF does not make the amp too stiff. If it does, I may vary my 1k resistor to get the feel of the amp back.

Ideally I would use 100uF with a choke, but that doesn't seem to lend itself to the most quiet amp possible.

More reports later as events unfold.

[Funkalicousgroove]

[MarshallPlexi]

More lessons learned.

In this amp, the choke is noisier than the 1k resistor, but the choke has a better feel to the amp in that is it more firm. So I had the 1k resistor in and the amp was very quiet and I remembered the Torres Diode trick, so I tried it and it worked great!

Here the deal. After the screen filter cap, add a 1N4007 diode to the B+ power rail and when the amp is played, the preamp will never be starved for voltage when the current demands are high in the output section. This works because the diode will only allow current to flow in one direction and if the power amp tries to pull current the other way during periods of high current, the diode will stop this.

This really did help the amp, as the power section has a good feel to it and the preamp section maintains it's crisp sound. In my instance, this is the best of both worlds.

This might help you guys in your experiments.