'68 Silverface Bassman AC568

[pchilson]

Hi,

I have this amp with this circuit. There is not a whole lot of info out there on this circuit except to say it sucks. I've been trying to find some specifics on what to change and the reasons why but posts I've read always seem to fall short of the whys and or hows.

Perhaps this thread could be a starting point for the hows and whys for mods on this circuit maybe even along with some pics if available.

My particular amp, in my opinion, sounds good but has a high amount of noise. All preamp tubes are new Tung-sols and the power tubes are not new but strong Tung-sol 5881s. The bias balance does nothing for this noise and both tubes are well matched. Bias readings are around 33mA. Power filter caps have been changed along with the bias cap and most of the bypass caps but not all of them. There appears to be a replacement OT, it is a Fender OT but it doesn't appear to be stock. Could the leads be reversed? Any help appreciated.

Thanks

[RWood]

Could you give more description about the nature of the noise? Is it continuous or intermittant? High, medium or low frequency? Affected or not by a guitar being plugged in and/or volume being raised? Stuff like that.

When you replaced the filter caps did you also replace the dropping resistors and the plate load resistors? Any coupling caps? Did you measure pin voltages?

If your noise is an oscillation then yes, switching your OT leads could help - easy to find out. If the oscillation stops when you disconnect the negative feedback line, then yes, swap the blue for the brown.

Give us a little more info and you'll get some good replies.
RWood

[pchilson]

I never know if its hiss or hum. The 'noise' increases/decreases with the volume controls. The same on both channels. I don't know what frequencies but the noise is frequency different on each channel. Also the noise changes with the bright and deep switches. Pluging in a guitar with single coils significantly increases the noise, humbuckers somewhat less so but the noise still increases.

There is no oscillating noise.

Any cap changes took place before I came into posession of the amp. All of the filter caps and looks like one of dropping resistors was changed. Bias cap has been changed. All of the coupling caps are original. I don't know about the plate load resistors. I have not measured pin voltages.

So if its not oscillating its not reversed leads on the OT?

[Wakculloch]

All those carbon comp plate resistors are usually the source of excess noise. Don't take any notice of folks saying certain models are rubbish, what they mean is it doesn't suit them personally, then the internet sheep follow.

[beakos]

Was the amp inside the head box when you heard the hum? I'm not too impressed with the installation of the bypass caps!

[pchilson]

Was the amp inside the head box when you heard the hum? I'm not too impressed with the installation of the bypass caps!

Yes, the chassis was installed in the cab. I'm not all that impressed with that job either, looks just lazy to me.

[pchilson]

All those carbon comp plate resistors are usually the source of excess noise. Don't take any notice of folks saying certain models are rubbish, what they mean is it doesn't suit them personally, then the internet sheep follow.

I'm not worried about what other folks say. I am looking for info on design choices that could be contributing to the noise or just general bad design. I have changed out all the plate resistors now and one of the dropping resistors that I had on hand for metal film types. Noise level changed not one bit. I am now going to call this a "hum". It gets louder with the volume control and is not there when the volume is zeroed.

I am looking for possible causes for the hum. Could it be pots? How would you check other than swapping them out? Any bones you can throw me?


Anyone have a layout for the AC568 Bassman? I've never run across one.

Thanks.

[Enzo]

Hiss, hum, they're easy. Both words are onomotopoeia - words that sound like what they describe. Hiss - ssssss sound like a TV channel off the air. Hum - mmmm sound like you are...humming. Hiss is air escaping a tire, hum is that sound when you touch the tip of a guitar cord.

Oscillation means the amp is making a sound or signal all on its own. it is actually a form of feedback at some point in the circuit. It can be anywhere from very slow - we call that motorboating, and it comes out like whup whup whup like a tremolo or something. Or it can be a tone of some sort - wooooo, wheeeee, etc. High pitched shriek or low like a boat horn or in between. And it can do it at frequencies too high for human hearing. A steady 100 watts of something you cannot hear at all.

reversed OT leads can cause oscillation, but so can a million other things. Having them correct does not prevent all the other ways it happens.

Hum and hiss and noises are not generic, any more than noise in your car is generic. Just about any part or any connection in an amp can cause some sort of noise. Every source of noise has its own cure. The cure for one nooise probably will have no effect at all on the other sources of noise. Your car could have noises from: a loose tail pipe banging, a stone in the hub cap, something loose in the trunk, wind whistling around loose weatherstripping, loose fan belt, hole in muffler, beer can under the seat, wife in the passenger seat. And the cure for each of those would not help with any of the others. Although the duct tape I use to secure the muffler might help in the case of the wife. You have to find out where each noise source is, and then fix it. If you start throwing parts at the amp, you may or may not solve your noises. And replacing a lot of parts that don;t need it is just a whole lot of extra opportunities to cause more problems in the amp.

[Gibsonman63]

Here's a couple of things to check:

The input jacks are shorting jacks. Check them to make sure they are really shorting out when you don't have an instrument cable plugged in.

Check the heater wire polarity on the power tubes. The heater wires should not be crossed, twisted yes, but back to the sam terminal on the next tube socket. It won't be a huge difference in hum, but everything helps and Fender didn't pay attention to this.

Does the amp hum with no instrument plugged into it?

Also try repositioning the amp and keep it away for any flourescent or neon lights.

[pchilson]

Here's a couple of things to check:

The input jacks are shorting jacks. Check them to make sure they are really shorting out when you don't have an instrument cable plugged in.

Check the heater wire polarity on the power tubes. The heater wires should not be crossed, twisted yes, but back to the sam terminal on the next tube socket. It won't be a huge difference in hum, but everything helps and Fender didn't pay attention to this.

Does the amp hum with no instrument plugged into it?

Also try repositioning the amp and keep it away for any flourescent or neon lights.

I will check the jacks and heater wiring. Thanks for the tip.

The amp hums with no instrument plugged in and it increases when you plug in an instrument (without touching the volume knob). Goes up again if single coils are used but thats normal I guess.

[JoeM]

Loosen the jacks, and and then re-tighten. I've read somewhere here they loose their 'bite' into the brass plate.

[beakos]

Try pulling one preamp tube at a time and seeing if the hum goes away. This can help isolate which area of the amp has the problem.
Don't forget to reinsert the tube before pulling the next one.

[Rhodesplyr]

I'm interested by the fact that you say the noise increases with the volume controls on BOTH channels and that the noise is different on each channel.

That would mean that either you have separate problems in each input stages or that there's a problem with the leg of the power supply that they share. I don't know the layout well enough to tell by looking, but were the first stage bypass capacitors of both channels replaced? (I've seen those white-jacketed Mallory capacitors fail in several Rhodes Suitcase Pianos.) Or, as Joe suggests, the two input stages share a ground, and grounds that rely on jacks benefit from periodic loosening, (cleaning, if possible), and retightening.

I like the reissue Tung-Sol 12AX7s, but some are noisier than others. Try swapping them between channels to see if the character of the noise moves.

Also, you said that the output tubes are 5881s? All Bassman schematics I've seen show 6L6GCs. I know that old-stock Tung-Sol 5881s are not rated for 6L6GC circuits. I'm not sure about the Tung-Sol reissues, but I would check. 5881s are rated for 23W on the plate; 6L6GCs are rated 30W.

[pchilson]

Try pulling one preamp tube at a time and seeing if the hum goes away. This can help isolate which area of the amp has the problem.
Don't forget to reinsert the tube before pulling the next one.

Looking at the back of the amp, right to left.
Preamp tube 1 pulled, nothing on bass channel, normal channel unchanged.
Preamp tube 2 pulled, hum on bass channel, nothing on normal channel.
Preamp tube 3 pulled, hum on bass channel, nothing on normal channel.
PI pulled, nothing on either channel.

Oddity when tube 3 pulled? Don't both channels use V3? Why does the normal channel go silent and the bass not?

[pchilson]

I'm interested by the fact that you say the noise increases with the volume controls on BOTH channels and that the noise is different on each channel.

That would mean that either you have separate problems in each input stages or that there's a problem with the leg of the power supply that they share. I don't know the layout well enough to tell by looking, but were the first stage bypass capacitors of both channels replaced? (I've seen those white-jacketed Mallory capacitors fail in several Rhodes Suitcase Pianos.) Or, as Joe suggests, the two input stages share a ground, and grounds that rely on jacks benefit from periodic loosening, (cleaning, if possible), and retightening.

I like the reissue Tung-Sol 12AX7s, but some are noisier than others. Try swapping them between channels to see if the character of the noise moves.

Also, you said that the output tubes are 5881s? All Bassman schematics I've seen show 6L6GCs. I know that old-stock Tung-Sol 5881s are not rated for 6L6GC circuits. I'm not sure about the Tung-Sol reissues, but I would check. 5881s are rated for 23W on the plate; 6L6GCs are rated 30W.

As far as the 5881s go, I don't think they are the problem cause I tried a pair of 6L6GCs in it and the hum problem was unchanged.

The Tung-Sol 12AX7s are all new, I know that doesn't rule them out but I had them in a Bandmaster and they were fine there.

All the bypass caps have been changed by a previous owner. Maybe perhaps there is an issue with one of them?

I need to open it up and check the heater wiring and loosen and retighten the jacks.