What went wrong? AA864 / AB165 on toggle switch

[fevzay]

Last year, I built a single channel AA864. Worked out the few bugs. Has been a great amp, but I wanted to try the AB165 front end (extra gain stage). Last week, I tried to get it done.

Took my time, but I could not figure out why I had a big volume drop on the AA864 side. Used a mini Carling SPST switch. The AB165 side even uses all of the AA864 side, so I was stumped.

I had a wire going from the junction of V3 B's plate and plate resistor going TO the switch, then one side going to the AA864 sides 220k mixing resistor. The other out went to the AB165 circuit. That side worked fine. The AA864 was giving sound, but at a very poor volume. Tried reflowing and redoing connections, but it didn't make a change.

Any ideas? At first, I thought it was a poor connection but I had no luck after reflowing the associated connections (and even redoing them entirely).

Should I have placed the AA864 sides 220k resistor BEFORE the switch? Would that make a difference?

This seemed like a cool idea, but I probably don't have the electronic chops to understand 100% how things work. Maybe this wasn't ever going to work given some shared circuitry.

[fevzay]

Here is a (poor quality) picture.

I've since taken the switch out, but I'd like to know where I went wrong.

[fevzay]

Here is the AB165 front end hardwired in.

[pompeiisneaks]

Do you have a schematic showing how it is built? It isn't easy to 'see' what's wrong if we don't know what's expected to be right? losing signal sounds like you're accidentally offering a lower resistance path to earth for the signal somewhere, if it doesn't do it with the switch in the other position. Either that or just too much resistance into the next stage causing current and signal loss/throttling.

~Phil

[fevzay]

I built this essentially off of a bone stock Bassman AA864 schematic and layout. No changes. I just skipped the Bass channel as I didn't want to complicate things for my first build. I used a 1971 Silverface Bassman as the donor for the transformers, etc. The chassis is just a Mojotone Bassman chassis (when they offered them).

The switch is as described above. Connecting between V3b's plate and the plate resistor is the IN (to switch). One side of the SPST goes straight to the AA864 220k mixing resistor (to the PI). The other side goes to the 220k resistor leading into the V2 gain stage circuit. The circuit follows back to the buss wire next to the AA864 sides 220k (leading into the 500pf cap).

Your response is very appreciated! Can you explain what might cause what you described? Sorry...I'm here to learn, and I fully admit to being a little wet behind the ears.

Side note: My voltages were all spot on to the AA864 schematic. Can post a picture of the voltages this afternoon. Nothing looked off (to me).

[fevzay]

The AA864 side is so simple that I don't understand what could have gone wrong. From V3 Pin 6, usually it joins the 100k plate resistor and jumps straight to the 220k mixing resistor. The only thing added was a wire going to the switch and from the switch to the 220k resistor. Surely something between the two sides isn't jiving well together. Its just funny that the more complicated side worked flawlessly (even using all but those two connections FROM V3's plate to the switch AND the wire TO the PI.

I realize the AB165 gain stage layout looks a little funky, but I made sure it all was correct. As I said before, its funny that THAT is the side that works flawlessly (not the simple run from the V3b junction to the switch and back to the 220k resistor).

My only guess is that the AB165 side (V2) is somehow interfering with the AA864 side when not in use. If its drawing current away from the AA864 side. Really though.....not a clue.

I'm probably going to keep the AB165 front end hardwired into the amp (I like the sound better). Just seemed like a cool idea to switch between the two of them. I suppose I could build that out into the Bass channel (separating the two channels entirely).

[pompeiisneaks]

I may be misunderstanding the actual circuit you've built. I've looked at the normal channel on both amps and they seem to me to be 100% identical with the exception of a 500pF cap on the 100k anode resistor on the ab165. This, to me, seems like something that could just be switched in/out with the right rated dc switch(it would need to handle the 340VDC or more I'm sure.) Is there something else you're doing differently on the normal channel that I'm missing? You mentioned the 220k mixing resistors being in place for both channels, (my thoughts are basically that from that 220k mixing network back, you replaced the bass instrument channel on the aa864 with the normal of the ab165?

or am I missing what you're doing. Again, a schematic makes this a lot easier than just guessing. If you're adding it down below by the 220k, then you need another 220k mixing resistor for the new channel, to ensure that it doesn't bleed back signal into the other side. (this may explain the gain loss). Also, normally if you pull a channel from the amp to get the 'exact' behavior you'd need the 220k mixing resistor from the other channel to ground to simulate that channel still being 'there'.

Since I don't 100% fully understand what you're doing, I could be way off.

~Phil

[sluckey]

Wrong value plate resistor. See pic...

[fevzay]

Great eye!

Yes....like a doofus, I forgot to mention that I changed that already. Of course, after firing the amp up last week....it went straight down (because of the 100 ohm resistor). Worried I did some real damage, but all I had to change was the capacitor in the bias circuit (and, of course, the resistor to 100k). I think the second chassis picture shows the change already.

[fevzay]

I may be misunderstanding the actual circuit you've built. I've looked at the normal channel on both amps and they seem to me to be 100% identical with the exception of a 500pF cap on the 100k anode resistor on the ab165. This, to me, seems like something that could just be switched in/out with the right rated dc switch(it would need to handle the 340VDC or more I'm sure.) Is there something else you're doing differently on the normal channel that I'm missing? You mentioned the 220k mixing resistors being in place for both channels, (my thoughts are basically that from that 220k mixing network back, you replaced the bass instrument channel on the aa864 with the normal of the ab165?

or am I missing what you're doing. Again, a schematic makes this a lot easier than just guessing. If you're adding it down below by the 220k, then you need another 220k mixing resistor for the new channel, to ensure that it doesn't bleed back signal into the other side. (this may explain the gain loss). Also, normally if you pull a channel from the amp to get the 'exact' behavior you'd need the 220k mixing resistor from the other channel to ground to simulate that channel still being 'there'.

Since I don't 100% fully understand what you're doing, I could be way off.

~Phil

First, you can ignore the second picture. That is of the amps current wiring (AB165 hardwired in). I did in fact leave a 220k resistor in the area dropping into the PI's 500pf cap, but it isn't connected to anything.

Here are the two Fender layouts and schematics.

When building the amp, I built a single channel AA864 Normal channel. It worked perfectly. Had to change one of the dropping resistors from 1k to 4.7k since I didn't have a V1 or V2 wired up (and pulling current).

Recently, I had the idea to add the extra gain stage from the AB165 Normal channel via a switch. I wanted to toggle between the AA864 Normal and the AB165 Normal (extra gain stage). The two schematics show the simple addition of the gain stage after the 220k resistor leaving the normal channel. The AA864 and AB165 Normal channels are identical from input jack until leaving V3 (normal channels preamp).

I appreciate your thoughts on this. I feel like you're getting closer to the answer than I could on my own (obviously!).

Is my mini Carling toggle SPST switch not rated high enough for this kind of thing? https://www.tubesandmore.com/products/s ... 2-position

Definitely wondering if not going from the 220k mixing resistor TO the switch's IN/middle lug was a mistake. Instead the resistor was placed after the switch going into the PI (ie. 500pf cap).

[pompeiisneaks]

again, the explanation doesn't seem to fit what I'm understanding.

You say 'extra gain stage' I may still not be understanding what you're talking about, because you then mention 'toggle between two different normal inputs'

First off for it to be an extra gain stage, it has to be in between the first stage, and the second, but your wording seems to imply choosing A input or B input.

So I'm just trying to fully understand, without a schematic, which you mean? It sounds like you're explaining two separate things that are completely different, but that you imply are the same.

Let me try to explain them just as 'input A and input B' instead of specific fender bassman schematics.

Are you wanting input A to be switchable so that it goes from input A to the half triode 7025 before the 12AT7 PI to a separate B stage FIRST as in cascaded through two gain stages (extra gain stage) or are you meaning you want to toggle between input A or input B but not both at the same time? (this is party of why I keep hinting that having a schematic makes this understandable in a few seconds instead of back and forth, as visualizing your idea is much trickier since it involves language and we all tend to not communicate as well as we think we do).

Depending on which you want changes things significantly. Adding an extra gain stage means the first explanation, but a lot of your explanation leads me to believe you're doing the latter.

~Phil

[pompeiisneaks]

oh and I also can't tell from the picture how that's wired because:

a. all wires are the same color
b. they're underneath the switch so I can't tell which lead is which
c. they go under board, so I don't know where they connect in after that.

again why I'd love a schematic

Sorry not trying to be too much of a PITA, just trying to understand what I can't see or fully understand yet.

~Phil

[fevzay]

I'll try to draw up a schematic for where I broke into the circuit, but it would be better to think of the toggle switch as an ON/OFF for V2.

[fevzay]

Not a schematic, but maybe it'll help.

[pompeiisneaks]

Okay now I see the problem. You have a buss connection instead of another 220k mixer resistor where the extra gain stage meets the other side of that mixer. this is causing one side of that switch to be able to send signal back the other way and bleed off some of that signal, thus why it's quieter. On the other hand, though, this doesn't seem to conform to the AA864 or AB165 as there seems to be 1/2 of a 7025 missing in your layout. there's supposed to be a gain stage, then a tone stack, then a recovery stage, then on to another gain stage before the PI. In your case you're somewhat restoring that as 'optional' it seems here with that switch. If I'm reading it right anyway.
Either way, I'd guess you want to just swap out that 'buss' jumper with another 220k mixer to ensure you don't lose signal off to the other side of that Y.

~Phil