Practical application of noise reduction

[Gibsonman63]

Plate Resistor Change

I changed R14 from 100K to 220K, original value was 150 ohms. It got rid of the excessive treble on the front of the note. With an SG plugged directly into the amp, it sounds like an 80's Marshall should, thick and crunchy with rich harmonics.

Next week, I will try a few experiments to reduce the hum.

[pdf64]

The 2 triode sections within a dual triode are independant, one my be fine, the other may have any number of problems.
There used to be a method on geofex of determining whether dc heaters would help. I can't find it now, but basically it's to sub a 6V battery for the heater supply.

[Gibsonman63]

Plan B... for now

I currently have JJ preamp tubes in the amp and Mullard power tubes. I have read mixed reviews on this forum regarding JJ. I ordered a couple of Mullard and a couple of Sovtech tubes to experiment a little with finding a quieter first preamp tube.

I'll post again after more experiments and do some research in the meantime about DC heater element supplies.

[wizard333]

I've had a 4210 (and a 2205 and a 2210) for years, done various mods over the years.

I recently changed the clean channel to a Fender clean channel (no small feat) and now the two channels do not share the first stage at all. Gain channel has significantly more gain. I always liked the versatile gain channel on these (100k midrange pot has a lot to do with that) but HATED the clean channel. The clean channel now sounds a lot like a Fender Pro.

I decided to try to reduce the noise related to that new extra gain. I did ALL 2W metal film on the gain channel. Maybe a tad less noisy but nothing to write home about. You also need to be careful trying to put "large" resistors in there; you only have so much room between holes on that pcb and if you try to put something physically too big in there, its going to be messy and maybe too close to other components.

Lead dress helped at least as much:

1) Twist the heaters tight.
2) Twist the supply wires for the switching system that come off the heater pins of V2.
3) Run your signal wires as close to the chassis as possible after they come off the board, on the chassis if you can.
4) Shorten any overly-long signal wires; there were several in mine.

I also changed the bias filter caps from 2x10uf, which left about 40mvac of ripple there, to 2x100uf, which left about .3mvac of ripple. Theoretically most of that get cancelled in the push/pull power section, but nothing is perfect. Less ripple on the bias voltage seemed to reduce the overall hum a bit.

The PT has a CT for the heaters; thats my next project is a DC bias on the heaters. Check out the theory and design forum for a discussion on how to do that without mucking up the power supply for the switching system (and in my case, the power supply for my solo boost mod).

The bottom line on these things is high gain amps are going to make some noise with the gain cranked. The best you can do is try to minimize it.

There are other things this amp needs, the BIGGEST one is that it has no RF rejection resistor at the input at all, no ferrite bead, nothin. These amps are notorious for producing Spinal Tap moments where the amp blasts the local radio station out of the gain channel. Remove the cap at the input, its a .047 as I recall right after the input before the signal goes to the grid of V1a, and replace with a 33k resistor. Could try a ferrite bead there to if you wanted and were concerned about losing gain, but I noticed no overall gain reduction, just no more "Nigel at the Airforce base" moments.

[Robert M. Martinelli]

There are other things this amp needs, the BIGGEST one is that it has no RF rejection resistor at the input at all, no ferrite bead, nothin. These amps are notorious for producing Spinal Tap moments where the amp blasts the local radio station out of the gain channel. Remove the cap at the input, its a .047 as I recall right after the input before the signal goes to the grid of V1a, and replace with a 33k resistor. Could try a ferrite bead there to if you wanted and were concerned about losing gain, but I noticed no overall gain reduction, just no more "Nigel at the Airforce base" moments.

Very good point!
Grid stopper resistors should always be present in a properly designed amp.

As a foot note, "RF rejection resistors", better known as "grid stoppers", don't cause any signal loss, provided they're placed AFTER the grid leak resistor. Under normal conditions the control grid does not draw any current, so there can't be any voltage (signal) drop across the grid stopper resistor. Placing them BEFORE the grid leak resistor forms a voltage divider, so the signal indeed suffers an attenuation.

Hope this helps

Best regards

Bob

[wizard333]

OH YES and on that point, that reminds me:

When you replace that input cap on the Marshall, you cant just remove the cap, stick the resistor there, and call it a day. Else you get the voltage divider issue that Bob brought up.

Due to the physical layout, what I did was this:
1) remove both the input cap and the grid leak resistor (thats the 1M to ground at the input, physically its immediately to the left of that little red input cap

2) put your grid stopper ( I used the term RF rejection to clarify its purpose here; "grid stopper" may not mean jack to the person that started the thread but anyone who's had one of these amps will know about radio frequency interference lol) where the input cap used to be,

3) Under the board, you need to solder the grid leak between the jack side of your grid stopper and the ground leg of that input jack. You have to do this under the board because the physical space on top wont allow for it. For my mod purposes that was good since I don't have the channels sharing V1 anymore I could connect the input of V2 in the hole where the tube side of the grid leak used to be. I run the inputs of both channels in parallel.

4) Dont freak about removing the board; just remove your knobs and the nuts behind them and it will come right out and flip over. Be careful not to yank any wires and make DAMN SURE your caps are drained before you do it.

[Gibsonman63]

Another Approach

I ordered the following tubes to try in the first position, which is the first stage for both channels:

Mullard 12AX7/ECC83 marked 09 09 on the bottom
Tung-Sol 12AX7 marked 08 08 on the bottom
Sovtek 12AX7LPS marked 09 04 on the bottom
Sovtek 12AX7WB Marked 08 08 on the bottom

The tube I had in it was a Tung-Sol 12AX7 marked 08 10 on the bottom

The Sovtek 12AX7WB was the best sounding tube in my opinion as well as the quietest. Out of all the tubes I tried, this one made the most dramatic difference. This made more of a difference than the Metal Film resistors.

Can anyone tell me what the numbers at the bottom mean?

Also on the subject of noise reduction; I bought Kevin O'Connor's book, "The Ultimate Tone" and am working my way through it. The second chapter discusses shielded heater wiring. Has anyone tried this? Worthwhile?

[wizard333]

The Sovtek 12Ax7WB is a particularly low noise tube; thats why some manufacturers spec it for low noise aps. VHT Valvulator leaps to mind.

I don't usually care for it in amps as a tone tube.

I'm curious where you sourced it? I see VHT Valvulators with dead tubes every so often and I have a hard time finding the WB, the WA seems to be all any of my suppliers carry.

[Gibsonman63]

Source

I ordered from thetubestore.com Audio and vacuum tubes for your amplifier.

You are correct about tone. It sounds nice on the crunch channel, but so-so on the clean, but I generally stay on the crunch channel and just roll my guitar volume up and down. I would say that would be a good argument for your mod of using dedicated pre-amp tubes for the clean and crunch channels.

[Gibsonman63]

Quieter isn't always better

Well, I played my weekly session last night at the local watering hole. Very little hum on the crunch channel of my amp, but it just didn't cut through the mix like it should and felt overcompressed. In a three piece band, the guitar should really be able to cut through! Using the TS-9 just killed the dynamics. Fortunately, I brought extra tubes. I put the Mullard in. The hum was back, but the amp sounded so much better.

I guess it is hard for me to judge how my amp sounds until I put it next to bass and drums.