4 Ohm speaker tap question...

[dodger916]

Anybody know what would happen if the speaker (say 8 ohm, but not relevant) was accidentally plugged into the 4 ohm tap instead of the 8 ohm tap? Would it produce any sound without the other 4 ohm tap in use? (I'm guessing yes as they are parallel) And if so, what would the impedance be? (I'm guessing 8 ohm) This question pertains to a Mesa head with the 8-4-4 output taps.

Thanks.

 

[igfraso]

Nothing wrong happens.
In fact, it makes the amp sound sweeter and that is the trick recommended by Mike Bendinelli to make your 100/60 watter mimic a Simul-Class amp.

Run your amp on 100 W and connect your 8 ohm speaker on the 4 ohm tap for that sound.

 

[dodger916]

Nothing wrong happens.
In fact, it makes the amp sound sweeter and that is the trick recommended by Mike Bendinelli to make your 100/60 watter mimic a Simul-Class amp.

Run your amp on 100 W and connect your 8 ohm speaker on the 4 ohm tap for that sound.

Thanks, but I'm wondering what the impedance is of the single 4 ohm tap. I don't have a 60/100 amp; the amp I plan to use is a DC-3. I'm also aware of safe mismatches.

 

[thunda1216]

the 4 ohm tap is 4 ohms, whether you use one or both. There are two outlets to allow the use of two 8 ohm cabs or one 4 ohm cab.

 

[dodger916]

the 4 ohm tap is 4 ohms, whether you use one or both. There are two outlets to allow the use of two 8 ohm cabs or one 4 ohm cab.

Thanks.

 

[Don]

From the manual-

"One 8 ohm and two 4 ohm speaker jacks are provided. The DC-3 is not overly sensitive to speaker mismatches and will not be
damaged by them, except that very low ohmage loads will cause the power tubes to wear faster."

"Feel free to experiment with the speaker jacks, trying different loads. Many great sounds and feels may be achieved by experimenting with different combinations of speaker impedance. Keep in mind the greatest clean headroom is always achieved with proper impedance loads...i.e. an 8 ohm speaker connected to an 8 ohm jack."

 

[dodger916]

Thanks, Don. I read that, but I've always been confused about the 4 ohm taps! I know that those taps are in parallel, and that running two 8 ohm loads in parallel = 4 ohms, so I assumed each of those taps was really 8 ohms, resulting in 4 ohms when two 8 ohm loads speakers were plugged in. I think I did a good job of confusing myself! :?

 

[stokes]

The labels 4ohm and 8ohm really has more to do with the turns ratio whithin the OT,than any dc resistance you can measure.When you use an 8ohm load in the 8ohm tap with a pair of 6L6 the reflected impedance to the primary will be roughly 4200 ohms primary plate to plate impedance,which is the "ideal" reflected impedance.It is "ideal" by the tubes designers,not necessarilly what will sound best.Now if yoiu plug that same 8ohm load into the 4ohm tap,you change the reflected primary impedance to roughly 8400ohms.Now,while that number is not what is considered the design "ideal",it will make the tubes run a bit cooler,the higher reflected impedance acts as a kind of current limiter.You thought you were confused before,Frank?It is perfectly safe to do,I've been running my C+ this way forever.I find it takes some of the edge off the output as it will reduce the output wattage slightly.The thing to remember is that these numbers,8ohm,4200ohm,are impedances and are frequency dependant,that is why you get a reading lower than 8ohms when you measure an 8ohm speaker with an ohmmeter.So mismatching,within limits,merely shifts the response of the output section.Just FYI,the physical layout of the OT secondary in a Mesa is such that the 8ohm tap is at the end of the winding while the 4ohm tap is somewhere between the begining and the end of the winding.So naturally the turns ratio between the primary winding and the 8ohm tap is roughly twice the ratio at the 4ohm tap.

 

[Don]

I think I did a good job of confusing myself! :?

It's a confusing subject!

 

[dodger916]

The labels 4ohm and 8ohm really has more to do with the turns ratio whithin the OT,than any dc resistance you can measure.When you use an 8ohm load in the 8ohm tap with a pair of 6L6 the reflected impedance to the primary will be roughly 4200 ohms primary plate to plate impedance,which is the "ideal" reflected impedance.It is "ideal" by the tubes designers,not necessarilly what will sound best.Now if yoiu plug that same 8ohm load into the 4ohm tap,you change the reflected primary impedance to roughly 2100ohms.Now,while that number is not what is considered the design "ideal",it will make the tubes run a bit cooler since there wont be as much current in the primary of the OT.You thought you were confused before,Frank?It is perfectly safe to do,I've been running my C+ this way forever.I find it takes some of the edge off the output as it will reduce the output wattage slightly.The thing to remember is that these numbers,8ohm,4200ohm,are impedances and are frequency dependant,that is why you get a reading lower than 8ohms when you measure an 8ohm speaker with an ohmmeter.So mismatching,within limits,merely shifts the response of the output section.Just FYI,the physical layout of the OT secondary in a Mesa is such that the 8ohm tap is at the end of the winding while the 4ohm tap is somewhere between the begining and the end of the wiring.So naturally the turns ratio between the primary winding and the 8ohm tap is roughly twice the ratio at the 4ohm tap.

Thanks, Carl.

What do you think about this: At gigs, I'd like to have my DC-3 head (4 x EL84) close to me rather than on the speaker cabs. While I could run two speaker cables (one for each 8 ohm cab) from the 4 ohm taps, I'm thinking of running a single speaker cable from the 4 ohm tap into a serial/parallel box that has two 8 ohm loads in parallel, resulting in a 4 ohm load. From what you're saying this should be kosher, and a "match".

And from what you're saying, running an 8 ohm load off the 4 ohm tap with a DC will help with the high % of MPD these amps see, and preserve the EL84s.

And when you say "mismatching,within limits,merely shifts the response of the output section.", how does the response shift? What are the sonic implications?

I'm going to try my C+ today with the 4 ohm tap...

Thanks again!

 

[dodger916]

I think I did a good job of confusing myself! :?

It's a confusing subject!

Yea. As with most internet learning, it will probably get more confusing before it becomes clear!

 

[gts]

What do you think about this: At gigs, I'd like to have my DC-3 head (4 x EL84) close to me rather than on the speaker cabs. While I could run two speaker cables (one for each 8 ohm cab) from the 4 ohm taps, I'm thinking of running a single speaker cable from the 4 ohm tap into a serial/parallel box that has two 8 ohm loads in parallel, resulting in a 4 ohm load. From what you're saying this should be kosher, and a "match".

Frank do either of the speaker cabs have a parallel out?
If so it could have you the use of the serial/parallel box.

But to answer your question:
Two 8 ohm speakers wired in parallel = a 4 ohm load. So yes this would be "kosher and a 'match"

 

[dodger916]

What do you think about this: At gigs, I'd like to have my DC-3 head (4 x EL84) close to me rather than on the speaker cabs. While I could run two speaker cables (one for each 8 ohm cab) from the 4 ohm taps, I'm thinking of running a single speaker cable from the 4 ohm tap into a serial/parallel box that has two 8 ohm loads in parallel, resulting in a 4 ohm load. From what you're saying this should be kosher, and a "match".

Frank do either of the speaker cabs have a parallel out?
If so it could have you the use of the serial/parallel box.

But to answer your question:
Two 8 ohm speakers wired in parallel = a 4 ohm load. So yes this would be "kosher and a 'match"

Cabs are Mesa single 1x12, George (1/2 back and Thiele). No parallel outs. I made the box to accommodate different speaker configs for my Laney Cub 12R and Orange Dual Terror. Now it will also enable setting up speaker cabs on both sides of the drummer with minimal wiring.

 

[stokes]

Frank,when I said "mismatching,within limits,merely shifts the response of the output section."I shouldnt have used the word "merely".What I am getting at there is that since the "impedances" in the primary,secondary windings and the speaker are all frequency dependent,they react differently at different frequency ranges.When you use the 4ohm tap with an 8ohm load you are shifting the reflected impedance in the OT,thereby shifting freq.responses.I cant tell you numbers or formulas for what these ranges are,as I'm not an engineer,I can tell you my experience is that I see a slight decrease in output power,subtle volume drop,hardly noticeable.But more noticeable is it takes the harsh edge off the top,and smooths and brings out the mids a bit.I have the EV speaker in my C+ which is a very efficient speaker that makes these changes subtler than tests I've done with less efficient speakers.And as always,tone is purely subjective so how good or bad it works for you may be different than it is for me.

 

[igfraso]

Since we are all immersed in this confusing subject (me included), let me share something else that Mike Bendinelli said while talking about mimicking a Simul-Class tone with a 100/60 watter.

If I got it correctly, he mentioned that running the amp on 100 W with an 8 ohm speaker on an 8 ohm tap is similar to running it on 60 W with an 8 ohm speaker on a 4 ohm tap.

So he was suggesting that running the 100/60 amp on 100 W and with an 8 ohm speaker connected to a 4 ohm tap was approximating that "Simulclass tone".

 

[gts]

Daniel while it's a related subject, it's veering a bit off/ away from the focus of what Frank was asking about.

But Carl's response(s) sure do help bring what you posted to the table for further discussion!
Maybe we should start another thread. "How to mimic a Simul C+'s tone using a 60/100 C+"

 

[igfraso]

gts, dodger916:
It wasn't my intention to hijack the original thread.

I thought that, since we were talking about turns whithin the OT where the 8ohm tap is at the end of the winding (4200 ohm) while the 4ohm tap is somewhere between the begining and the end of the wiring (2100 ohm), maybe the relationship between power and impedance was relevant for the tone you get on each combination.

But now that I reread the original message and realized that Frank was talking about a DC-3, I quickly shut up and offer my apologies.

Sorry,
Daniel

 

[dodger916]

No worries, Daniel! It's still valuable info that merits re-mentioning, especially for new members who may not have read that before or old members (like me!) who have forgotten it.

gts, dodger916:
It wasn't my intention to hijack the original thread.

I thought that, since we were talking about turns whithin the OT where the 8ohm tap is at the end of the winding (4200 ohm) while the 4ohm tap is somewhere between the begining and the end of the wiring (2100 ohm), maybe the relationship between power and impedance was relevant for the tone you get on each combination.

But now that I reread the original message and realized that Frank was talking about a DC-3, I quickly shut up and offer my apologies.

Sorry,
Daniel

 

[stokes]

"And from what you're saying, running an 8 ohm load off the 4 ohm tap with a DC will help with the high % of MPD these amps see, and preserve the EL84s."
Sorry Frank,I didnt address this in the first response.It wont help with actual plate dissipation.Your bias and plate current remain the same,when you use the 8ohm load on the 4ohm tap you get a higher reflected impedance to the tubes by way of the primary.This creates a kind of current limiting effect,tubes will operate a bit cooler,EL84's run very hot,temp wise,at normal bias and current values so this effect is also subtle.Dont want you to think you will be able to handle the tubes,it doesnt make them that much cooler.As I write this,I get the feeling I may have some numbers reversed in my last reply,gonna go check and fix,if I did.

 

[dodger916]

"And from what you're saying, running an 8 ohm load off the 4 ohm tap with a DC will help with the high % of MPD these amps see, and preserve the EL84s."
Sorry Frank,I didnt address this in the first response.It wont help with actual plate dissipation.Your bias and plate current remain the same,when you use the 8ohm load on the 4ohm tap you get a higher reflected impedance to the tubes by way of the primary.This creates a kind of current limiting effect,tubes will operate a bit cooler,EL84's run very hot,temp wise,at normal bias and current values so this effect is also subtle.Dont want you to think you will be able to handle the tubes,it doesnt make them that much cooler.As I write this,I get the feeling I may have some numbers reversed in my last reply,gonna go check and fix,if I did.

Thanks, Carl. I didn't think the effect would be very great, but any little bit helps with DC's