Can I use a dummy load in place of a speaker to avoid an impedance mismatch?

[mattysaurus]

Hello all!

A month or so ago I stumbled upon a Blues Deville 410 for free at a yard sale, and of course swooped on it. Got it home, noticed it was missing power tubes, half its speakers, and the standby switch. No big deal, easy fix. Got new power tubes and standby switch and she comes back to life. More recently, I noticed one of the speakers was starting to go bad. Annoying, but since I have so little invested in I decided to get some speakers to fill out the cabinet.

Took the amp out of the cabinet, put in the new speakers, and.... it doesn't fit. One of the transformers on the back side of the amp must be aftermarket, and it's making it impossible to put a speaker in the top right position. Obviously this is an issue.

I currently have the 2 8ohm speakers wired in series plugged into the internal jack, and another 8ohm speaker plugged into the external jack. Normally the amp expects 8ohms, but when something is plugged into the external jack it switches to the 4 ohm winding, and I'm currently sitting at 5.33, which isn't ideal, but not even a 100% mismatch. I am a little concerned about blowing out the single 8ohm speaker, though by the math it's only getting about 27 Watts and it's a 35 Watt speaker (Jensen Mod 10-35).

I noticed that parts express sells this product:
https://www.parts-express.com/8-ohm-...#lblProductQ&A

Could I wire this into the other 3 speakers series parallel to fix the impedance mismatch and balance the speakers? I only play this amp at low levels so I'm not worried about heat, but I'm not sure if this would adversely affect tone or just be a bad idea overall. If no, do you have any other suggestions? Should I just run it as a 2x10 combo with a single speaker in each jack? Thanks!

[nevetslab]

Hello all!

A month or so ago I stumbled upon a Blues Deville 410 for free at a yard sale, and of course swooped on it. Got it home, noticed it was missing power tubes, half its speakers, and the standby switch. No big deal, easy fix. Got new power tubes and standby switch and she comes back to life. More recently, I noticed one of the speakers was starting to go bad. Annoying, but since I have so little invested in I decided to get some speakers to fill out the cabinet.

Took the amp out of the cabinet, put in the new speakers, and.... it doesn't fit. One of the transformers on the back side of the amp must be aftermarket, and it's making it impossible to put a speaker in the top right position. Obviously this is an issue.

I currently have the 2 8ohm speakers wired in series plugged into the internal jack, and another 8ohm speaker plugged into the external jack. Normally the amp expects 8ohms, but when something is plugged into the external jack it switches to the 4 ohm winding, and I'm currently sitting at 5.33, which isn't ideal, but not even a 100% mismatch. I am a little concerned about blowing out the single 8ohm speaker, though by the math it's only getting about 27 Watts and it's a 35 Watt speaker (Jensen Mod 10-35).

I noticed that parts express sells this product:
https://www.parts-express.com/8-ohm-...#lblProductQ&A

Could I wire this into the other 3 speakers series parallel to fix the impedance mismatch and balance the speakers? I only play this amp at low levels so I'm not worried about heat, but I'm not sure if this would adversely affect tone or just be a bad idea overall. If no, do you have any other suggestions? Should I just run it as a 2x10 combo with a single speaker in each jack? Thanks!

The speakers you got in this cabinet...are they Eminence V1030's? That's what Fender uses in this amp, and their P/N on the speaker is # 048832 and should be somewhere on the magnet or frame. The stamped metal frame has four spokes, and the upper speakers have to be oriented so the spokes don't interfere with the xfmrs of the chassis (which occupy the space between the spokes. Other replacement speakers having more spokes or speaker frame won't fit due to that. The xfmr that's causing the interference fit is stock, not an aftermarket part as you thought.

I just looked at a picture of the Jensen Mod 10-35, and, that's the problem.....it's spkr frame occupies the necessary void, and, as you found, won't fit.

You could just wire the third upper speaker that fits in parallel with the two series-connected speakers, and connect to the INT Spkr jack, so you're still on the 8 ohm tap. The upper speaker will see more current from the amp than the lower pair, so there will be a touch more output, though it won't hurt anything. Doubtful you'd really hear the difference.

The other speaker that was in the cabinet that wasn't going bad.........still working? You could try putting it into the remaining space, and might even connect it in series with the other upper spkr, then wire those two pairs in parallel. Don't know the sensitivity differences.....but, even if not connected, it will occupy the space, and serve as a passive radiator on bass notes.

[mattysaurus]

The speakers you got in this cabinet...are they Eminence V1030's? That's what Fender uses in this amp, and their P/N on the speaker is # 048832 and should be somewhere on the magnet or frame. The stamped metal frame has four spokes, and the upper speakers have to be oriented so the spokes don't interfere with the xfmrs of the chassis (which occupy the space between the spokes. Other replacement speakers having more spokes or speaker frame won't fit due to that. The xfmr that's causing the interference fit is stock, not an aftermarket part as you thought.

I just looked at a picture of the Jensen Mod 10-35, and, that's the problem.....it's spkr frame occupies the necessary void, and, as you found, won't fit.

You could just wire the third upper speaker that fits in parallel with the two series-connected speakers, and connect to the INT Spkr jack, so you're still on the 8 ohm tap. The upper speaker will see more current from the amp than the lower pair, so there will be a touch more output, though it won't hurt anything. Doubtful you'd really hear the difference.

The other speaker that was in the cabinet that wasn't going bad.........still working? You could try putting it into the remaining space, and might even connect it in series with the other upper spkr, then wire those two pairs in parallel. Don't know the sensitivity differences.....but, even if not connected, it will occupy the space, and serve as a passive radiator on bass notes.

That's correct, the two original speakers were the v1030's, and it's definitely an aftermarket transformer, as I tried to put a v1030 there and it still hit the speaker. This thing is twice as big as the other transformer on there (not the choke). The speaker that's going bad buzzes really bad on certain notes, it's a no go for actual use. How would I wire the 3 speakers in series parallel? I assumed there'd still be a speaker mismatch. Wouldn't the single speaker be overloaded power wise as well? One of the benefits of running the external jack is that at 4 ohms it's only running around 41 is watts versus the 60 watts when just using the internal jack.

[nevetslab]

That's correct, the two original speakers were the v1030's, and it's definitely an aftermarket transformer, as I tried to put a v1030 there and it still hit the speaker. This thing is twice as big as the other transformer on there (not the choke). The speaker that's going bad buzzes really bad on certain notes, it's a no go for actual use. How would I wire the 3 speakers in series parallel? I assumed there'd still be a speaker mismatch. Wouldn't the single speaker be overloaded power wise as well? One of the benefits of running the external jack is that at 4 ohms it's only running around 41 is watts versus the 60 watts when just using the internal jack.

On each speaker, there are two blades per terminal (+), (-). You've already wired one pair in series going from the (-) on one to the (+) to the other, and taking the remaining connections of each back to the amp. Instead of using those connections to the amp, connect the third speaker to those terminals, (+) to the one series spkr, and the (-) to the other series speaker to they're properly phased. Now, connect your amp phone plug cable to the remaining (+) & (-) terminals of the single speaker. It's in parallel with the series-connected speakers, and you're now connected to the 8 ohm tap using the INT Spkr Jack. If you wanted to use this series-parallel spkr wiring with the 4-ohm tap, move the plug to that EXT SPKR jack, and insert an spare open circuit plug into the Int Spkr jack, since that jack has a switching contact. Without doing that, it shorts the output of the amp. While not an ideal impedance match, it won't hurt anything.

[mattysaurus]

On each speaker, there are two blades per terminal (+), (-). You've already wired one pair in series going from the (-) on one to the (+) to the other, and taking the remaining connections of each back to the amp. Instead of using those connections to the amp, connect the third speaker to those terminals, (+) to the one series spkr, and the (-) to the other series speaker to they're properly phased. Now, connect your amp phone plug cable to the remaining (+) & (-) terminals of the single speaker. It's in parallel with the series-connected speakers, and you're now connected to the 8 ohm tap using the INT Spkr Jack. If you wanted to use this series-parallel spkr wiring with the 4-ohm tap, move the plug to that EXT SPKR jack, and insert an spare open circuit plug into the Int Spkr jack, since that jack has a switching contact. Without doing that, it shorts the output of the amp. While not an ideal impedance match, it won't hurt anything.

But aren’t I still at a impedance imbalance? If, just using the internal 8ohm tap, I run two speakers in series for 16 ohm, and then one speaker parallel at 8ohm, it’s still going to be 5.33 on a 8ohm tap, which is probably a little worse than 5.33 on a 4 ohm tap. Alternative, if I run the 2 speakers in parallel And wire the third to series, I’m looking at 2.7ohm on an 8 ohm tap. Please correct my math if I’m wrong.

[Randall]

"While not an ideal impedance match, it won't hurt anything."

This. ^^^ Either tap will be fine. Example, many Fender amps have both internal and external speaker jacks with no means of switching taps, because there were no extra taps. You had a 8 ohm speaker and an 8 ohm transformer, and when you added an additional 8 ohm ext. cab the 8 ohm transformer drove a 4 ohm load. We did it all the time, nothing ever blew up from it.

[nevetslab]

But aren’t I still at a impedance imbalance? If, just using the internal 8ohm tap, I run two speakers in series for 16 ohm, and then one speaker parallel at 8ohm, it’s still going to be 5.33 on a 8ohm tap, which is probably a little worse than 5.33 on a 4 ohm tap. Alternative, if I run the 2 speakers in parallel And wire the third to series, I’m looking at 2.7ohm on an 8 ohm tap. Please correct my math if I’m wrong.

Only way you'd get 2.7 ohms is with all three 8 ohm speakers in parallel. In that mode, each speaker is then seeing the same voltage, instead of the series-connected 8 ohm pair, where each is seeing half the voltage (and half the power). In this mode, then your best match is the 4 ohm tap....using two spkr cable plugs, with the one pair in parallel (wired (+) to (+), (-) to (-)) There you WOULD be using two plug to get the amp to switch to the 4 ohm tap.

Fender's Vibro-King amp using three 8 ohm 10" spkrs in parallel. I think the Output Xfmr on it is a 2 ohm coil, but nevertheless, this will work. The 4 ohm tap will give you the best power transfer and equal power on all three speakers. You could try both and see if one wiring scheme sounds better than the other.

[Helmholtz]

Yes, you can use an 8 Ohm (ca.20W) resistor in place of a missing speaker with the original speaker wiring and the main speaker output. The resistor will reduce available sound output by 25%.

Can you post a picture of the replacement transformer?

[mattysaurus]

Yes, you can use an 8 Ohm (ca.20W) resistor in place of a missing speaker with the original speaker wiring and the main speaker output. The resistor will reduce available sound output by 25%.

Can you post a picture of the replacement transformer?

If I ever pull it out again I will. It didn’t have any identifying label as far as I could tell.