Hiss in first preamp stage

[HobbswheresCalvin]

i am sure this is fairly common but i do not see a recent post on it.
so heres the issue i am having, i pull the first tube, no hiss, amp is dead silent. i change the tube.. it doesnt matter the hiss comes back the same,

i have redressed leads and tediously cleaned up my wiring to more match Francesca pictures, not much change. i also sheilded the input grid wire from the volume put into the second stage of the first preamp tube, not a whole lot changed.

ive been seeing conversations about grid stopper resistors might help,

not sure what to do here. i am turning this amp up to about 3 oclock or "7" to compete with my drummer and ge tthe amount of distortion necessary for what we are playing, the hiss isnt ....that bad... but it is annoying and could be much better i am sure. as this amp does hiss a little bit when the volume is at zero.

i am pretty sure it is early in the signal,
when i turn all the knobs to zero except the volume, the hiss is still there, if i then turn the volume up to 10 the hiss goes away, then if i add any eq, either the treble mid bass or presence, the respective part of the hiss becomes more pronounced

[El_Martin]

Hi!

That's a common problem...

TWs: for gigging OK, for living room you'll be rewarded with more or less annoying hiss.

It does react on tubes in V1 a bit on my liverpool 15 W.

I know, they're going to shoot me...the dedicated followers of Ken.

In other amps there's a 68k Gridstopper for 1st stage. This forms a filter w grid/cathode capacitance (around 3pF iirc, see technical data to be sure).

RC time would be proportional to: 68k*3pF

I'd suggest to try a lil grid stopper (1k) and compensate for it by adding a cap 68 times bigger than Cgc. Something around 115pF. Try 120pF soldered to the tube socket. I've not tested a bit larger Cs without any grid stopper...might be worth trying.

There are some other measures, the TW police won't find legal.



Peace!
Martin

[HobbswheresCalvin]

Man, i really appreciate the help, especially since your risking it, letting this info out in the wild amongst the traditional police.

my only issue is i dont quite understand what a grid stopper is let alone the math do do this yet. im an instrument tech who is dabbling in amp territory. if you could put it into basic terms with mayb a website or pictures of how i can physically do this i would greatly appreciate it

[HobbswheresCalvin]

ok so i tried a small resistor on the second grid of the amp, the one that comes out of the volume control. it got rid of the attenuated hiss i was getting but left a slightly louder hiss on that did not increase when i turned up the volume knob, obviously must be coming from after that section, when i unhook the plate lead of the second stage the amp is dead silent, its definitely happening somewhere in the second stage.

[martin manning]

You may have to sort through some tubes to find a quiet one, so try that.

A grid stopper is just a series resistor, preferably placed right on the grid pin. It makes a low-pass filter with the tube's internal capacitances to its plate and cathode, which are both at AC ground. That will help exclude HF noise from the signal path. The value of the total input capacitance is around 100pF for a typical 12AX7 stage. There is basic capacitance of a few pF from the grid to the plate (Cg-a) and the cathode Cg-k), but the Cg-a is multiplied by the stage voltage gain, which is called Miller effect. Fender commonly used 68k grid stoppers, which makes a knee at ~23.4kHz- out of hearing range for most people, so there is not much loss of audible signal. Others have use 22k, or 33k. If you increase the value from 68k to 150k say, the knee is brought down to 10.6kHz, which would reduce more of the hiss.

[geetarpicker]

Trainwrecks were designed to be played at gig level volumes and though you will hear noise at low levels it should actually be minimal compared to how loud it is. For playing around the house probably the best way to minimize the hiss is with an attenuator. IMHO wrecks actually have quite decent signal to noise ratios and are probably better than for example old 4 input Marshalls in this regard. You might check out my latest Youtube video with my '85 & '89 Express where the noise is barely audible. GK

[Markusv]

Again- Controversial and the "police" may not like it.

I have built a few Liverpools and/ or Express circuits now. I tend to parallel the 1st 2 triodes and use half value on the Ra and Rk respectively
I realize it changes the amps character somewhat. IMHO these amps have so much they throw at you that I don't miss whatever is "lost" in the process.
I also run a 5751 in V1 which seems to be a little more quiet.

I have built 3 of these amps for friends and other musicians and all agree it's the best they've played. Not my brilliance- that's all KF's doing but I mean even with the above tweaks the amps remain very special.

I want to state though that I'm not a great player and that undoubtedly folks like Glen K. would immediately feel the difference. They may / may not like it.
For a weekend player like myself though this setup proves just great and the noise is not imperceptible but I can easily live with it. I have a rocket clone that hisses more though it's a different hiss. Not as intense (frenetic) as the Liverpool for instance. I will probably never get to the point where I feel the tweaks limit my ability to express myself musically

Markus

[El_Martin]

Hi!

Yeah, Glen - your right. Not more hiss/noise than a Plexi. But: does anyone play a Plexi w/o mastervolume in the same room with his ears?

My low wattage build are waaay better as regards signal-to-noise ratio. The lil ECC82 PP (around 2 W) is dead silent. It's 3 gain stages, TW style.
I've not tried the attenuater thingy yet. Should work fine for me, too.

Most people are playing mastervolume amps these days. They tend to underestimate the noise of their amps. Give that MV pot a spin and listen to the noise THEN.
Another way to get rid of the noise is to cut down frequency within the amp. Most industry amps do not have the blistering treble and lack dynamics. Currently I'm more into a singing tone w. lesser attack than my TWs. In know, it's just a phase. Dabbling w. a hotrodded Plexi w. 2 EL84 for fun.

Anyhoo...like your playing and your tone, Glen. BTW: have you ever been tempted to play an other amp? Like the D*mble style amps? D. must have at least one Plexi for studio rental purposes, they say.

Lots of amp flavors-lots of players out there.

Ciao
Martin

[fishy]

It seems many try and improve the hiss levels. Grids oppress, change plate resistors, swap tubes lead dress, bright switch wiring......

I knew about the hiss going in and still messed with it but never managed to make any real change without destroying the beauty of the amp.

I have a 5751 in V1 which I like but it has little effect on the hiss to my ear.
If the hiss is still an annoyance when you are playing with a drummer, it might be something a little more than the normal hiss....

[HobbswheresCalvin]

the hiss is bearable, and yes i am technically at full bore with no master volume. but even without it, i typically turn the amp up to about 3 oclock to get the amount of overdrive and volume needed to play with our drummer, he hits rather hard. breaks sticks and cymbals constantly.
i am going to try a few grid stoppers and see if that doesnt fix some of it, other then that, when i install the PPIMV next week i should be able to control it better for lower volumes.

i did also have trouble with a radio signal come thru during practice last week. so hopefully the grid stoppers will help with that as well.

[MCK]

Do you have a bottom cover on your chassis? That made a huge difference for me.

[Markusv]

Amen

That is key-the bottom cover

[HobbswheresCalvin]

so here is my findings, we added a 100k 1watt resistor before the input of the second gain stage and the problem was mostly solved, the hiss does not increase with volume now, which is alot nicer, there is some small amount of hiss on the amp but it does not increase and that is what i like, and i am sure with the added PPIMV i wont have any problem weeding this noise out while practicing at lower volumes.

yes i had a cover on it when we tested it. it makes a whole lot more noise without it.

i am now loving this amp and cant believe all that ive learned so far from building it, reading about it and your guys help. i appreciate it, and cant wait to build another