Blackface AA864 Bassman Bass Channel

[rhaigh]

I have a blackface bassman AA864 that I may be having trouble with. I haven't played through a blackface bassman in many years and I wanted to make sure what I am hearing through the bass channel in correct. Two questions actually...

1. The normal channel is perfect. The bass channel sounds muffled which i'm assuming is because of the filtering for bass. The weird thing is that even though the volume and bass controls work fine, the treble seems to control volume as well. If the treble is on 0, then I can crank the volume and there is hardly any volume. If the treble is on 10, then the volume control turns up as it should. Loud. Is this the way that this channel works in these amps? Is it because the treble control is set up for a bass instrument ans filters out most of a guitar signal as it is turned down? Is it normally muffled like this?

2. Hum issue... The normal channel gets no hum whatsoever when the bass channels tubes are pulled. Sounds great. When the 2 bass channel preamp tubes are put back in, the amp hums. Not crazy loud but enough to be annoying. This hum occurs as soon as the amp is taken off of standby. It hums with no guitar plugged in. Volume controls do not change the hum. The filter caps we're replaced not too long ago. I tried about 6 fairly new 12ax7's in the normal channel and they all sound good with no hum. When I try them in the bass channel, the amp hums with all of them. So it's probably not the tubes. The cathode 25uF caps have been replaced. The other caps in that channel are original (the blue ones). The grounding scheme is original as well as the tube sockets (they are in good shape). The only thing replace in this amp is the filter caps and cathode 25uF caps and the pots. Brand new 6L6's too. Any ideas on what this could be?

Thanks.

[rhaigh]

I should have mentioned that the amp has a grounded 3 prong plug installed and the ground switch/cap disconnected.

[Chuck H]

It sounds like you may have a ground fault in the bass channel somewhere beyond the tone stack/volume control circuits. Try narrowing the problem a little better by pulling just the first tube in the bass channel only. Hum still there? If yes then the hum is beyond that tubes circuits but before the PI since that circuit is shared by both channels.

It's considered "good form" here to post a schematic of the circuit in question when one is available.

[Randall]

The treble/volume issue sounds like a leaky or shorted tone cap to me. Probably the .1uF between the treble and bass controls. At any rate all three of those .1 caps should have ~200v on their common side and next to nothing on the other. If you see anything more than say 1v, you have one or more bad caps. Fix that first, then see if you still have a hum issue.

[rhaigh]

There was still hum there after pulling the first tube. Thanks for adding the schematic.

[Chuck H]

There was still hum there after pulling the first tube. Thanks for adding the schematic.

Well that limits the hum issue to the single triode third stage. But there's still this"

The weird thing is that even though the volume and bass controls work fine, the treble seems to control volume as well. If the treble is on 0, then I can crank the volume and there is hardly any volume. If the treble is on 10, then the volume control turns up as it should. Loud.

This IS abnormal behavior. First, can I confirm that the amp was working fine the last time you used it and that there have been no attempts to modify it? It's hard to imagine that the two issues are unrelated since they popped up at the same time, but right now that's the way it seems. Are there any scratchy noises when rotating pots? I think you need to open the amp and get some voltage readings on the tube pins.

[rhaigh]

I can't remember if there we're issues or not years ago when I used it last (~20 years). The only things I changed we're to add shielded wires to the input jacks and deep/bright switches, 3 prong power cord, replaced pots, disconnected gnd switch and cap, 25uF cathode caps replaced, filter caps replaced, and new tubes. I checked the voltages when I put new tubes in and they looked about right but i'll check again when I get home tonight. Thanks.

[Chuck H]

The filter caps we're replaced not too long ago.

I can't remember if there we're issues or not years ago when I used it last (~20 years).

Are the filter caps ~20 years old and sitting unused this entire time then? Filter caps are affected more with time than use. In fact it's worse if they're NOT used. That is, new caps 20 years ago with no mileage on them are likely WORSE than 20yo caps in a well used amp. Electrolytic cap manufacturers use to post an expiration date on their products (I seem to recall 5 years being typical) and recommend "rotating stock" (moving old stuff to the front of the rack so it moves before the new stuff). I don't know if any still do this, but it supports my point. To complicate matters, caps don't test well with typical bench gear. Even if you can measure the uF with a meter it still won't tell you if the cap is going to perform with 400VDC+ on it. But since the normal channel isn't humming perhaps we can ignore this for now. Still, it would be good to cook those caps for awhile to see if they hold up and to get any electrolyte that can still liquify to do so. Since I don't have a Variac (recommended) I've used a basic "light bulb current limiter" for this. If you don't have one it's easy to look up and cheap build. Basically you would plug the amp into the limiter, turn it on and set all the knobs to zero, take it out of standby and and just let it sit like that for maybe six hours. Don't leave the house and stay in close proximity so that if a capacitor fails you can shut down the operation.

If those caps are indeed twenty years without use I, personally, would just replace them again. But not everyone here would agree with that. And, you didn't mention the bias supply caps. It's important to change these too whenever a cap job is done because they are also electrolytic caps prone to the same failure mode of time and a failure in this circuit can cause major damage to the amp.

[rhaigh]

I forgot to mention that the bias cap was replaced too. All these changes we're recent ones (not 20 years ago).

[Jazz P Bass]

I would lift one end of the treble capacitor (250pf) and see if the hum disappears.
It sounds like that cap is shorted.

[Chuck H]

I would lift one end of the treble capacitor (250pf) and see if the hum disappears.
It sounds like that cap is shorted.

Except that pulling that circuits tube doesn't stop the hum.?. Even if that cap were shorted it shouldn't act like a volume control either. Not AFAICT looking at the schematic.

[rhaigh]

Looks like I might have found a solution. I removed the 0.001uF cap on the 220k voltage divider and everything fell into place. I hear no hum, and I get much, much better tone controls. The amp really brightens ups. Sounds great. Thanks for all the help. I was jamming with the amp cranked up, having a blast, and then the fuse blew. Hmmmm. So i popped in a new fuse, turned on the amp to find the tubes beginning to red plate. Didn't make sense. I popped out the tubes and measured the voltages to find that the bias voltage was gone. The wire that leads to the center of the bias pot cracked off. lol. Oh well. Thanks again.

[Chuck H]

Why in the world that .001 cap would cause hum in any failure mode is beyond me. But I've seen stranger things on those old, carbon impregnated paper boards. And yes, that cap IS one reason the "bass" channel sounds muffled. Sounds like you're on track with the repair as it stands now.