Multi Speaker Wiring Question?.

saltyseadog

I have 2 x 8" 4ohm speakers wired in series at 8ohms plus 1 x 12" 8ohm speaker. My amp is running at 4 ohms. How do I wire these three speakers together to out put 4ohms?.

PhilKing

Wire the 8 ohm in parallel with the 2 series wired 4 ohm.  That will give you 4 ohm.

ICBM

PhilKing said:

Wire the 8 ohm in parallel with the 2 series wired 4 ohm.  That will give you 4 ohm.

Yes, like this.

Kalimna

How is the speaker power rating spread across such a network?

Cheers,

Adam

ICBM

Kalimna said:

How is the speaker power rating spread across such a network?

Half to the 8-ohm speaker, a quarter each to the 4-ohms.

Kalimna

So, if the amp was putting out 50W max, then each 'arm' of the parallel wiring would receive 25W? Therefore the 8Ohm speaker would need to be at least 25W rated, and each 4Ohm at least 12.5W? I realise there are caveats there too.

Cheers,

Adam

ICBM

Kalimna said:

So, if the amp was putting out 50W max, then each 'arm' of the parallel wiring would receive 25W? Therefore the 8Ohm speaker would need to be at least 25W rated, and each 4Ohm at least 12.5W? I realise there are caveats there too.

Yes, exactly.

Kalimna

Thank you - you really should publish a book one day on amps and suchlike!

ICBM

Kalimna said:

Thank you - you really should publish a book one day on amps and suchlike!

I've been told that many times, but it's not as easy as it sounds - getting it all organised in a logical way is tricky, and you can guarantee someone will always want the answer to a slightly different question...

saltyseadog

The 2 x 8" speakers are both Eminence Patriot 820H speakers each rated at 20W while the 1 x 12" is the original Eminence Special design speaker which came with my Fender BJr' in 1997 when I bought it. Last year I built a Fender Bandmaster/Bassman sized tweed covered finger jointed pine cabinet for the BJr'. Due to the amp being 8ohm I wired the two 8" 4ohm  820H speakers in series on one jack and the original 12" 8ohm speaker on another jack and used them seperately. Over time I realised that the 2 x 8" speakers were my favourite and although I liked the sound of the 12" I preferred the deeper tone of the 2 x 8".

                                    As I only play at home nowadays at so called bedroom volumes and after asking advice at a couple of guitar forums I decided to buy a Y cable with 2 x female 1/4" sockets and hear what the mixed sound of all three speakers sounded like and I really liked it. During this time a poster told me that Hammond Transformers made a drop in replacement output transformer for the BJr' with 4 - 8 - 16ohm options. I checked that out and bought the 1760F version. I also bought the recommended 3-way switch but was unsure how to wire it in so just installed the OT using the 4ohm wire and taped off the other two. Only problem I had was what was called phase oscilation feedback due to the blu and brown wires being different in the new OT. Once I swapped them over it was fine. My original post here was just to be certain I was wiring the speakers correctly.

            After finishing installing the new OT I found that I was getting power to the LED but not to the valves so I also put in a new valve PCB made by MOX at Black Magic Amplifiers as the original on my BJr' had been repaired three times over the last 20+yrs always on the V4 and V5 power valves due to heat and apparently sub standard materials. Finally finished bolting it all back together this morning and spent a few hours noodling.

                                                                                               Initially I find that it doesn't seem to be as loud as before. Previously at home I would have the Master at 12 and the vol/gain at 2 which was okay without annoying the neighbours. Today I had the vol/gain at 4 to get there. The 25db midboost knob on my Strat is very noisy but only when rotating it and the 3-way switch on my ES335 makes a loud pop while chaning pickups and the neck volume pot is noisy while rotating it as well. Previously there was a small crackle from the strat mid boost which I meant to use some switch cleaner on next time change strings. I only rewired the ES335 to 50's wiring a few weeks back using brand new Bourne pots and it was as quiet as a mouse previously so not sure what to make of that.

                                                                             Also found that playing through my pedal board which was also no problem before every time I used a footswitch I got the same loud pop when pressing the button. Otherwise it seems to be okay. Would the fact that both amp and speakers are now at 4ohms be causing these problems?.

            

ICBM

It’s not the speaker impedance - you have a DC leak from the amp via the input jack - that causes switch pop and guitar pots to be scratchy. Try replacing the V1 valve. It’s also possible you’ve disturbed a ground connection somewhere relating to it. The loss of gain is probably related to that as well.

saltyseadog

Thanks for the reply ICBM, I was thinking it must be a valve problem. When you say V1, do you mean the first 12AX7 of the three pre amp valves. In the BJr' the Pre amp valves are V1-2-3 with the power valves being V4 and V5. I did notice when I put the valves in that V5 was very bright at top of the valve where the pins insert into the PCB.

ICBM

Yes, V1 is the one at the end under the input jack.

If it’s not the valve itself it will be something like a bad connection in the cathode path to ground for the first valve stage. Most valve amps have the first gain stage directly connected to the input jack via a resistor, with no capacitor to decouple any DC voltage - even a tiny leak will make switches pop and pots scratchy. You can check for certain whether it’s this if you have a buffered pedal like a Boss - put that last before the amp and the problems (apart from poor gain in the amp) should stop.

saltyseadog

Again thanks for the reply ICBM, I changed the valves this morning and the popping/ crackling is still going on. Would the green wires going to the HTR on the PCB cause this if they were in the wrong holes. I ask as the holes in the old board were up and down while the holes in the new board are side by side. Although the instructions that came from Black Magic Amps defined the + and - for the LED there was nothing about the HTR wires so I soldered them in place as I thought they should go. I took some pics of the boards as I was going along and you can see what I mean.

https://i.imgur.com/sNLmaQv.jpg 

https://i.imgur.com/eaUZRLt.jpg


https://i.imgur.com/KzOAAwu.jpg


 

saltyseadog

Try again,

saltyseadog

Unsure how to post pics on these forums, the usual way doesn't seem to work?.

ICBM

saltyseadog said:

Again thanks for the reply ICBM, I changed the valves this morning and the popping/ crackling is still going on. Would the green wires going to the HTR on the PCB cause this if they were in the wrong holes. I ask as the holes in the old board were up and down while the holes in the new board are side by side. Although the instructions that came from Black Magic Amps defined the + and - for the LED there was nothing about the HTR wires so I soldered them in place as I thought they should go. I took some pics of the boards as I was going along and you can see what I mean.

No, it won't be a filament supply issue - it's AC, so the connection polarity isn't important. Even if the 'virtual centre tap' resistors are missing or not grounded, that shouldn't cause a DC leak on V1, it will just cause noise.

If it's not a valve issue I suspect something in the ground connections for V1.

saltyseadog

I am unsure how to check for that and cannot take it to the local tech as he has shut up shop due to Covid 19. As the amp is playing are there any dangers to using it until I can get it to a tech?.

ICBM

saltyseadog said:

I am unsure how to check for that and cannot take it to the local tech as he has shut up shop due to Covid 19. As the amp is playing are there any dangers to using it until I can get it to a tech?.

It depends on how bad the voltage leak is - it's probably not very high, but in the worst case it could damage something connected to it, maybe the preamp in the active Strat. Do you have a buffered pedal at all? If so, that will not only cure the problem - or at least the symptom - it will protect anything further up the chain as well.

I think the most likely culprit is a solder joint or a broken core in the ribbon cable to V1, by the way - this is quite a common problem with all this series of amps - the problem is that to get at it, you need to take the boards out again which further increases the risk of breaking the connections... it's really not a brilliant design.

saltyseadog

I tested the ribbon cable on V1 for continuity and it tested good on all 6 wires. Although the solder looked okay I decided to  resolder all the pins on V1. Plugged a guitar in but still get mid boost and volume knobs crackling on the strat and the same lack of loud volume. The only Boss pedal I have is the Boss FBM1 which I use for crunch so I took that off the pedal board and put it between the guitar and amp, crackling and popping gone like you said but the clean bypass channel is distorted now as well as the drive channel on both Bright and Normal inputs and the drive sounds thin. Although the solder is nice and shiny on all pins should I try and resolder V2 & 3 and check the other ribbon cables or is the problem confined to V1?. Thanks so much for taking the time

ICBM

saltyseadog said:

I tested the ribbon cable on V1 for continuity and it tested good on all 6 wires. Although the solder looked okay I decided to  resolder all the pins on V1. Plugged a guitar in but still get mid boost and volume knobs crackling on the strat and the same lack of loud volume. The only Boss pedal I have is the Boss FBM1 which I use for crunch so I took that off the pedal board and put it between the guitar and amp, crackling and popping gone like you said but the clean bypass channel is distorted now as well as the drive channel on both Bright and Normal inputs and the drive sounds thin. Although the solder is nice and shiny on all pins should I try and resolder V2 & 3 and check the other ribbon cables or is the problem confined to V1?. Thanks so much for taking the time

The problem is definitely in V1 if it's doing that - that confirms it's a DC leak issue, but it may be quite bad if it makes the Boss bypass distort. (There could be other problems too, but there must be one there.) Are you quite certain it isn't the valve?

Do you have a voltage meter? You can measure the voltages on the pins of V1 and see if there's anything odd. The pin numbers go anticlockwise from the gap so the ones you need to check are all on the side of the socket nearest the end of the board. Pin 6 should have a high DC voltage on it, around 150V I think. Pin 7 is the grid connection to the jack and should be - but won't be now - 0V, and pin 8 is the cathode which should be about 1.3V (all to chassis ground).