Third gain stage (I think) hum and white noise

[ab165]

Hello Everyone at The Amp Garage. I've been reading posts and trying to fool with amps on and off for a couple of years now.

I just picked up a project amp (AB165) and it's time to try and fool with amps again...Which leads me to my current problem...

The amp is a '66 or '67 Blackface AB165 in need of some TLC (maybe lots).
The amp sounded kind of crappy tonewise, plus it had some added some intermittent noises. I'll try to keep the story short.

What ive done up to this point:
* Changed all filter caps (TAD),bias cap (mojotone 100uf@100v), and bypass caps (Nichicon 22uf@50V).

*Changed the bias set-up to the typical set-up as found on AB763. (Biased to 40 mA and 41 mA@ 461V)
* Changed the phase inverter section to the typical set-up from an AB763.
*Changed the NFB loop to be like an AB763 (or AA864) and swaped the black and green leads to the output jacks.

* I removed the 470K feedback resistor on the third gain stage going from the plate to the grid. (raised gain, noise and hum).

*Third gain stage...I also removed the cathode bypass cap and changed the grid to ground resistor from 470K to 220k (sort of like the AA864) trying to reduce some gain...still noisy.

The hum and white noise goes away when I pull v2 (3rd gain stage).

I experimented by bypassing the 3rd gain stage (pulled V2, and jumped a wire from pin 7 grid to the input cap of the phase inverter.

The hum and noise went away, and the guitar signal was OK (a little weird sounding). Also, without v2 I had to turn up the volume knob a few notches to get an equal volume.

I don't THINK that I screwed up the wiring...Where should I be looking?

Thanks for any advise,
Chris.

[Stevem]

Your next step is to replace all the preamp plate load resistors ( 100k 1/2w) with metal film type 1 waters, do the same with all the cathode resistors, but they can stay 1/2 watt.
It would not hurt to change out the 68k resistors on the input jacks also.

You need to do a good cleaning job on all the preamp tube sockets also as on the tube pins themselves as this can make for added noise also.
sometimes just popping a tube in and out of a socket with cleaner on its pins will do the job, but when a socket is both clean and good you should be able to rock that tube around a bunch and not hear a peep!
Sometimes on older tubes that have been in the amp along time or on N.O.S stuff I need to take may dermal tool with a small round wire wheel in it to get the tube pins themselves nice and shinny and then a wipe a coat of WD40 on the pins from a wet Q-tip.

[ab165]

Hi Stevem,

I changed most of the preamp plate resistors in the amp (with new 1/2 watt carbon comp resistors) The only ones that I left original, are the two plate resistors on the Normal Channel (I figured that I'll leave that channel stock, and mess around with the bass channel. For the bass channel I changed the bass and mids tone stack caps to .022 and .047).

The noise seems isolated to V2 (3rd gain stage) because when I did my "V2 bypass experiment" most of the noise was gone. Do I have too much gain at V2?
* 220K input mix resistors to new .02 coupling cap
* 220K grid to ground resistor
* new 100K plate R
* existing 1.5K (measures at 1.8K) cathode R

Cleaning the tube sockets seems like a good idea which I should have done first. (Actually, I should have checked for the oversized 25A fuse which was in the amp before I ever plugged the amp into the wall)

I'll go see if Radio Shack sells contact cleaner anymore

Thanks again,
Chris.

[Firestorm]

When you removed the feedback resistor, you turned V2 from a low-gain mixing stage to a high-gain (almost 30x without the cathode bypass cap) stage. So as Steve says, you need the quietest components possible in front of it and good clean connections. You will also need to fix the lead dress, which probably stinks given the era. Even then, you probably have too much gain to be useable. You could attenuate the signal AA864-style and/or fiddle with the gain of the stage itself by reducing the plate resistor and increasing the cathode resistor. This stage will clip, but you can control how it clips this way.

[ab165]

Hi Firestorm,
I think you're right about the gain being too high.

I was trying to attenuate the signal AA864 style by swapping out the grid to ground resistor to 220K as in the AA864. I figure that I have the same series resistance entering the 3rd stage with the two 220K mix resistors..

The schem forAA864 the shows a 220K series resistor between the grid and the input coupling cap, while the AB165 has the two 220K series resistors from both channels feeding into the coupling cap, and then into the grid.

Do series resistors and caps function differently depending on which postion they are in ? (Res>Cap>Grid -vs- Cap>Res>Grid?).

Back to the noise issue I got home late, but I shot in some contact cleaner in the sockets and replaced and removed the tubes several times...

Since it was late, and I wasn't going to plug in a guitar, I removed the normal channel and bass channel tubes, leaving only V2, V4 (PI), and the power tubes.

Still got the noise and hum when I turned the amp on, the noise went away when I pulled V2. I wondered about the new coupling cap being leaky, so I cliped the end entering the grid...Still noisy (V2 back in the socket)

Tomorrow when I get home from work I'll try another filter cap on the B+ supply.

Here's a question...Could the old power supply series resistors be making noise? When I ordered parts, some stuff came to me wrong so I left the existing resistors in place.

That's the story so far, Thanks.

[Firestorm]

AA864 mixes the channels right before the PI, so there's a 220k/330K divider attenuating signal into the power amp by about 40%. And there's a 220K/220K divider in front of the Bass Channel's third stage. AB165 mixes in front of the (shared) third stage. As designed with the 470K feedback resistor, signal into the PI is only about 2X the signal entering the third stage. Without feedback, it's almost 30X hotter, with no channel mix attenuation.

Old dropping resistors can indeed be noisy. But I think with your present gain structure, you will be amplifying every random noise in the signal path as well.

[ab165]

In all honesty I didn't consider the signal going into the phase inverter. My intention was to have a gain structure more or less somewhat equal to an AA864 of an AB763 without trem or reverb on both channnels (3 stages before the PI)

At this point, I've added what I think is a voltage divider in front of the input cap to the phase inverter made-up of a 220K resistor in series before the PI input cap (.001) and another 220K resistor to ground from the junction the of the series resistor and cap. So now I have:
- the two 220k mix resistors> to the 3rd stage input cap (.01)
- another 220K resistor from grid to ground
- 1.5K cathode resistor (no bypass cap)
- from the 3rd stage plate, a 220K resistor> to another220K resistor to ground> to a .001 PI input cap.

Did I get this right, or do I still have way more gain than an AA864?

It did bring the noise level down somewhat. (still too much though)

I think a suprising situation...The bass channel sounds noisier than the normal channel even though it has new plate resistors, and gets it's B+ from the next stage of filtering (27K dropping resistor and a 16uf filter cap)

Funny, I thought this was going to be a walk in the park with new electrilytics , some plate resitors, and biasing the tubes (is was set at 22mA and 35mA)

[Firestorm]

Can you make a little drawing of that section? I can't quite visualize how you've got it now.
Theoretically, the channels should nearly the same now (lead dress aside). There are some small frequency differences in the original schematic (plate bypass caps). Are those still stock?

[ab165]

Hi Firestorm,

I cant make a drawing now (or more honestly, I dont know how to put it in computerland)

The phase inverter section right now is pretty much exactly like an AB763 inculing the two 22oK mix resistors into the .001 PI input cap. The one resistor gets the signal from the 3rd gain stage (V2), and the 220K resistor which does not have an input from (what would be in an AB763) the "other channel"...That resristor I have going to ground.

As far as the pre amp tubes (1 and 3), I removed any/all caps on plates that wouldnt be on a typical "cool Blackface" Fender. The Normal Channel I left normal, and the Bass Channel I left the 330pf treble cap, I left the 8.2K mid resistor, and I changed the mis and bass caps to .022 and .047.

I did this mostly to try to learn about the different sounds with different tone stack values.

If I can get the amp working properly, I'd like to be able to compare the different tone stacks directly "on the fly".

At this point in the game, I dont want to make much more gain than a more typical Fender with the "extra" gain stage after the pre-amp tube. I hope I'm on the right track.

Thanks for all of your help.

Chris.

[Firestorm]

What have you got on the input side of V2?

[ab165]

On the input to V2, I have the two 220K mix resistors going to a .01 input cap (or maybe .02, I forget now)...After that I have a 220K grid to ground resistor just befor the grid to V2. Cathode Restistor (1.5K) is not bypassed.

Thanks again

[Firestorm]

Two 220Ks in series ground right off the plate? That would pull your plate supply voltage down some. Or do you have a coupling cap before the divider. In the AB165, the 220K mix resistors go to an AC ground (through the other channel's 100K), not to a DC ground. Compare the AA864. Sounds like you also have an extra 220K in there after the cap. The cap should come first, right off the V1 plate, then your divider with the grid at the junction.
Where is the actual channel mix? Maybe I'm not following.

[ab165]

Errr...I'm a dope.

Yeah, I have the the two 220K resistors going INTO the PI cap, so yes I have the resitor to ground coming off of the V2 plate through the other 220K resistor.

I guess that because I had the resistors placed in a V-shape on the board, the signal would know which way I wanted it to go.

So I guess I accomplished nothing in reducing the noise once I fix my mistake.

I dont think that I screwed up the channel mix (YET). That is pretty much stock AB165 with each channel having a 220K resistor exiting the channel and forming a V where they intersect witch the input cap to the 3rd stage. The other thing that I did at the 3rd stage was to replace the 470K grid load resitor to 220Kas an effort to compensate for the removal of the feedback resistor on V2.

I'll be getting home late so I probably wont do anything with it tonight.

[ab165]

Hi, I'm back...

I returned the third stage to stock with the 470K feedback resistor, 47oK grid load resistor, and a removed my screw-up ( the two 220K resistors befor the PI input cap).

Basically I have the stock (almost) AB165 pre amp (all caps across plate resistors removed and the tone stack mods to the bass channel) going into a stock AB763 PI.

So now I'm hoping to be at a better starting point to work on hum and noise.

The Hum is an octave below A# on the guitar, so I think that makes it 60 cycle hum.
*Hum gets louder and quieter with rotation of the volume knobs, but is always present with or without a guitar plugged in.
* Hum is in both channels, but seems louder on the bass channel.
* Hum is less when the two preamp channels are removed.
*The hum dissapears when the 3rd stage tube is removed.

The White Noise/hiss is louder on the bass channel even though I changed all of the board mounted caps and resistors for the bass channel except for the existing 330pf ceramic treble cap.

I lifted the cold side of the 330pf cap and measured something like .05 vdc.

Another experimant that i did was to lift the 'hot side' of the mix resistors entering the 3rd stage and a lot of the noise seemed to go away.

That's all my new so far. I have to go to work.
Thanks,
Chris.

[Firestorm]

The hiss is going to be preamp tubes and/or resistances. The earlier it gets in, the louder it becomes. Carbon comp resistors are noisiest, so you want metal film or at least carbon film for things like the 1M resistors to ground on the inputs, plate resistors, grid loads, the feedback resistor on V2, channel coupling resistors and so on. The plate resistor bypass caps you removed look like they were there to tame noise. If you can make your plate resistors 1 or 2 watts, that will alleviate one type of resistor noise.

Pots are resistors, too, but you probably don't want to change them out for non-carbon types, so make sure they are clean, especially volume and treble. A bad solder joint is a resistance, so reflow anything suspect. The dropping resistors in the power supply are an often-overlooked source of noise.

60Hz hum that varies with the volume pot setting is either a ground/ground loop problem or filament hum. Make sure all of the grounds are solid. In Fenders, connections to the brass grounding plate sometimes look good but aren't really soldered. You need a largeish soldering iron to touch those up.

If the filament winding is center-tapped, make sure that ground is solid. If there is no center tap, you'll need the artificial one with two well-matched 100Rs. Sometimes, Fender PTs die and get replaced with later versions, which often don't have center taps.

Finally, be sure that none of the grid leads are close to the filament wires.