Can someone help me with this Bassman modification?

[Duncan]

Greetings!

I've got a Bassman AB165 that I'm modifying (Bass channel, the Normal channel is remaining stock). I found a Soldano schematic of the modification that I've implemented to the Bass channel, but I'm struggling to figure out where in the Phase Inverter to insert it. I've tried several configurations, but the best I've been able to get is very faint guitar sounds out of the modded channel (normal still operating normally, with expected volume).

I've uploaded both the schematic to the modification and an edited AB165 schematic.

Should I be replacing the circled 0.1uF capacitor with the 0.001uF at the end of the Soldano schematic? Or is there a different spot this channel should be inserted into the PI?

Any insight would be greatly appreciated.

[stratomaster]

The AB165 LTPI was likely not the intended target of that Soldano mod as it deviates from the typical Fender topology a good bit. 

You'd likely be better served by setting up mix resistors at the outputs of both channels where the "output" of the normal channel is after the triode with the local negative feedback.  Start with 220k and don't be surprised if it works better with 470k.

Alternatively, I don't see any reason why you couldn't lift the 2nd 0.1μF cap from the ground connection and tie in there as is.  The series sum is such that you can leave the original cap in without an audible effect.

[Duncan]

Alternatively, I don't see any reason why you couldn't lift the 2nd 0.1μF cap from the ground connection and tie in there as is.  The series sum is such that you can leave the original cap in without an audible effect.

Ala this?

[stratomaster]

I was thinking to tie into the lifted end, but that works too. 

[Duncan]

I was thinking to tie into the lifted end, but that works too.

Beautiful. I'll give it a go after work and report back.

Thanks!!

[stratomaster]

If you're going to tie in as drawn just take that 0.1μF cap out, or lift the other end if it.  As drawn with the original cap there's basically an input floating in space.  Could cause unwanted noise.  That's why I recommended tying into the lifted end.

[Duncan]

I have a feeling my v1 socket is having some issues. I bypassed the MV completely and inserted the channel into the same point that the normal channel goes.

When I do the “pop” test on the grids, every one makes the expected response except pin 2. I replaced the input jack, the 1M grid leak and the 68k grid stopper with no change.

[stratomaster]

Post voltages for that stage.

[AlNewman]

Which input are you plugging into?  From your schematic, if you were to plug into input 2, your signal wouldn't have a reference to ground.

[Duncan]

Which input are you plugging into?  From your schematic, if you were to plug into input 2, your signal wouldn't have a reference to ground.

Sorry, I should have mentioned that I took input 2 out of the mix and just went from the 220k grid stopper to Volume 2.

I’ll grab voltages after work. When I checked them yesterday, nothing really jumped out at me as off (v2 was a bit high on the plates, but nothing that would raise a flag).

[pdf64]

What V DC do you measure at V1 and V2 pins?

The AB165 power amp is probably not going to work nicely here, due to its virtual earth at the LTP input (inverting negative feedback amplifier).
I suggest to reconfigure it to pretty much any previous bassman version.
But maybe get the preamp sorted first :)

[Duncan]

Here are the voltages:

V1:
1 - 257v
2 - 0
3 - 2v
6 - 255v
7 - 0
8 - 2

V2:
1 - 385v
2 - 248v
3 - 250v
6 - 248v
7 - 0
8 - 2v

Looking at it now, V2A is looking v weird.

[pdf64]

Well, V2 includes a DCCF, so those elevated voltages kinda expected.
 
What I'm not keen on is that the DCCF is the 1,2,3 triode, with the preceding common cathode stage being the 6,7,8 triode.
It's almost always best for the layout to follow signal flow, especially for higher gain stuff.

So it may be worth reviewing that.

Also the voltages seem rather high; could the HT dropper values be tweaked to get the DCCF voltages a bit more reasonable?

[Duncan]

Well, V2 includes a DCCF, so those elevated voltages kinda expected.
 
What I'm not keen on is that the DCCF is the 1,2,3 triode, with the preceding common cathode stage being the 6,7,8 triode.
It's almost always best for the layout to follow signal flow, especially for higher gain stuff.

So it may be worth reviewing that.

Also the voltages seem rather high; could the HT dropper values be tweaked to get the DCCF voltages a bit more reasonable?

Good call. I’ll adjust the droppers a smidge. Thank you!

[Duncan]

Got it sorted out! V1’s socket was having so super iffy connections and I didn’t account for a jumper under the board. All is well!

Thank you everyone!!

[Willabe]

So it was a jumper under the board that was missing? Or the jumper had a bad solder connection?

[Duncan]

So it was a jumper under the board that was missing? Or the jumper had a bad solder connection?

No it was a jumper under the board that I forgot was there, which was sending B* to both sides of the 220k resistor connected to the plate of v1a

[UncleRuckus]

I'm new here, but it's possible that the two channels are out of phase (one inverted, the other not) causing a weak signal?

[Duncan]

I'm new here, but it's possible that the two channels are out of phase (one inverted, the other not) causing a weak signal?

Interesting! I haven’t tried running both channels at the same time yet, but I’ll definitely listen for that if/when I do.

[Willabe]

I'm new here, but it's possible that the two channels are out of phase (one inverted, the other not) causing a weak signal?

Interesting! I haven’t tried running both channels at the same time yet, but I’ll definitely listen for that if/when I do.

No, he was thinking that might be the problem.

He thought you might have been going through both channels and that they were out of phase and that was causing the low power.   

[Duncan]

I'm new here, but it's possible that the two channels are out of phase (one inverted, the other not) causing a weak signal?

Interesting! I haven’t tried running both channels at the same time yet, but I’ll definitely listen for that if/when I do.

No, he was thinking that might be the problem.

He thought you might have been going through both channels and that they were out of phase and that was causing the low power.

Oh I gotcha. Yeah, it was a combination of the v1 socket and the B+ going to both sides of a resistor.

All is well.

[pdf64]

Based on the latest schematic (posted elsewhere), I think UncleRuckus's suspicion is correct, the channel outputs are inverted with respect to each other.
Provided you don't want to use both channels at once (?), it's not a problem.

[Duncan]

Based on the latest schematic (posted elsewhere), I think UncleRuckus's suspicion is correct, the channel outputs are inverted with respect to each other.
Provided you don't want to use both channels at once (?), it's not a problem.

Yeah, I don't know that I would ever use both at the same time. Especially considering the gain disparity.

Also, I ended up inserting the modded channel into the same spot as the normal channel (updated schem attached).

[Willabe]

Yeah, I don't know that I would ever use both at the same time. Especially considering the gain disparity.

No, when 2 channels are out of phase, you can't use both at the same time, because of the gain/volume loss. Their not saying you should.

On some 2 channel amps, where the 2 channels are in phase with each other, some guys use both channels at once to get extra gain.

On certain Marshall's, they use both channels because 1 channel is too bright and the other is too muddy, both together works out great.

[Willabe]

Also, I ended up inserting the modded channel into the same spot as the normal channel (updated schem attached).

You might need another 220K R on the other channel. They use those R's as 'mixing R's to help isolate the 2 channels from each other. If your going to use channel 1 most of the time, you probably won't need the 2nd 220K R.

And it might be better to move the 2nd channels before the MV?

And you might want to use a different MV? Guys here will know which MV to try.

[Duncan]

Also, I ended up inserting the modded channel into the same spot as the normal channel (updated schem attached).

You might need another 220K R on the other channel. They use those R's as 'mixing R's to help isolate the 2 channels from each other. If your going to use channel 1 most of the time, you probably won't need the 2nd 220K R.

And it might be better to move the 2nd channels before the MV?

And you might want to use a different MV? Guys here will know which MV to try.

That’s another thing I did. I just did a global MV (type 3 I think)

[SEL49]

I would put a coupling cap somewhere on that dotted line.

[Willabe]

I would put a coupling cap somewhere on that dotted line.

I missed that. You now have dcv on the MV and TS pots.

[Duncan]

I would put a coupling cap somewhere on that dotted line.

I missed that. You now have dcv on the MV and TS pots.

Good flag.

I should note that I went with a global Type 3 MV, so it'll just be the Treble that would have DC on it. I'll get a coupling cap on there tomorrow (I was thinking 0.1uF).