8ohm speaker in 4ohm fender reverb amp? i know its overdone but lets hear it

[claytushaywood]

I have a fender silverface pro reverb and am looking to put some big old jbl's in it. either an e120 or k120. These speakers are rated at 8ohms but actually measure around 6.2ohms in general. I know its not that bad for the amp to use a little bit higher speaker impedance than called for, in these old fenders at least. and i know its harder on the power tubes, but some people actually really like the sound of the pushed tubes... most people that have tried this actually- from my experience. I know there are some posts online that adamantly disagree... saying transformers will blow out and only lower impedance is okay- that is regarding transistor based amps- not tube based- at least that's what many pro's have said on the subject- i think the crazy ass Lord Valve disagrees with me here too- but he is nuts- and i know- i used to live in denver i know

Anyways, about how much harder is this on the tubes? will it take away 10% of their life? 20%? anyone know? I bias around 60-65% so I'm not too hard already. Using one of these speakers would save my back and my need to get an external cab and turn my vintage fender into a head unit, save money etc. so replacing tubes 3 times a year instead of twice for a bit better tone and lighter weight is exactly what i'm looking for!

Thanks!

[printer2]

6.8 is the DC resistance, the impedance would be rated as 8 ohms but it varies with frequency. I would not worry about running it with 8 ohms, if your bias is not out of whack the plates actually will dissipate less and the screens a little more. Use a set of tubes and see how long they last. The tubes may not care that much.

[claytushaywood]

anyone ever tried this and had tubes just get burnt up? I'm thinking I'll give it a go

[kleuck]

Originally Posted by claytushaywood

I have a fender silverface pro reverb and am looking to put some big old jbl's in it. either an e120 or k120. These speakers are rated at 8ohms but actually measure around 6.2ohms in general. I know its not that bad for the amp to use a little bit higher speaker impedance than called for, in these old fenders at least. and i know its harder on the power tubes, but some people actually really like the sound of the pushed tubes... most people that have tried this actually- from my experience. I know there are some posts online that adamantly disagree... saying transformers will blow out and only lower impedance is okay- that is regarding transistor based amps- not tube based- at least that's what many pro's have said on the subject- i think the crazy ass Lord Valve disagrees with me here too- but he is nuts- and i know- i used to live in denver i know

On the contrary, with SS amps, any impedance above the rated one is fine, just drops power, but absolutely no danger for the amp itself.
With tubes, you should always match the impedance, and a higher one is what is dangerous.
Anyway, speaker impedance is only a rounded value at medium frequencies, and most speakers have a DC resistance lower than the actual impedance at say 1Khz.

[printer2]

A higher impedance is a danger for two reasons. You get greater screen current, really a concern if you do not have resistors on the screens but even back in the day when this amp was built they were included. The other thing is the transformer converts a magnetic flux into a voltage and corresponding current in the secondary circuit. Basically you get so many V (volts) and A's (amps) for the amount of magnetic flux in the transformer core. The lower the impedance of the speaker the lower the V but higher the A. The higher the speaker impedance, the lower the A but higher the V.

With no load on the transformer the voltage can go very high, enough to punch though the wire insulation which then causes a short. Doubling the rated speaker load on a Fender should not cause this to happen, the output transformers usually can take a 100% mismatch in the load.

[1955]

Don't do it. There was a recent thread about this very thing. It can mess things up, very enlightening posts there. I had no idea, but apparently it can do damage.

[limbe]

To put it simply,the resistance of a speaker is a part of the impedance so the impedance will always be a bit bigger than the resistance.

[big-daddy-59]

Originally Posted by claytushaywood

I have a fender silverface pro reverb and am looking to put some big old jbl's in it. either an e120 or k120. These speakers are rated at 8ohms but actually measure around 6.2ohms in general. I know its not that bad for the amp to use a little bit higher speaker impedance than called for, in these old fenders at least. and i know its harder on the power tubes, but some people actually really like the sound of the pushed tubes... most people that have tried this actually- from my experience. I know there are some posts online that adamantly disagree... saying transformers will blow out and only lower impedance is okay- that is regarding transistor based amps- not tube based- at least that's what many pro's have said on the subject- i think the crazy ass Lord Valve disagrees with me here too- but he is nuts- and i know- i used to live in denver i know

Anyways, about how much harder is this on the tubes? will it take away 10% of their life? 20%? anyone know? I bias around 60-65% so I'm not too hard already. Using one of these speakers would save my back and my need to get an external cab and turn my vintage fender into a head unit, save money etc. so replacing tubes 3 times a year instead of twice for a bit better tone and lighter weight is exactly what i'm looking for!

Thanks!

That model Fender uses TWO 12" 8Ohm speakers wired in parallel to give a 4 ohm load at the output transformer. What you're proposing won't change the the load the amp sees at all.

[printer2]

Originally Posted by big-daddy-59

That model Fender uses TWO 12" 8Ohm speakers wired in parallel to give a 4 ohm load at the output transformer. What you're proposing won't change the the load the amp sees at all.

I thought he wanted to run one 8 ohm speaker.

[jefrs]

Oh no not again

DC resistance of an 8ohm impedance speaker is 6ohm.

Problem is that the 8ohm is measured at 400Hz, this is roughly G 3rd fret 1st string, most of the guitar's notes are below that and the speaker's impedance rises to 50ohm at its resonance frequency around 80Hz, bottom E open 6th.

This means a guitar amp is always mismatched. A guitar amp is built tough and can handle this.
But you cannot go under the specified load with a valve amp.
A 50% overload is acceptable.
A 100% overload is the limit, and must not be cranked up.

This amp is designed for a 4ohm load., doubling its load will not do it any favours.

[claytushaywood]

for all the naysayers saying you absolutely cannot go up- SRV ran a 2ohm transformer with an 8 ohm speaker load in some of his amps. and he cranked them up. but of course diaz himself would change the tubes midshow with oven mitts.

and yes it's one 8 ohm speaker with a 4ohm transformer. sorry i should have been more clear

This is definitely a popular thing to do, on purpose, for a particular effect. and I am absolutely positive it wont kill MY amp... being a vintage fender. this is not a modern production crate or an amp that cant take some abuse. Tons of people do this, on purpose, for the effect.

I'm really not interested in the naysaying unless you actually have personal experience- not someone that brought you an amp that said this was broken and this is why.

I'm really looking for people that do this or have done this and know what the effect on tube life is, if any.

Thanks for the posts so far guys. and jefrs would 50% be a 4ohm tranny with a 6ohm load and 100% be a 4ohm 8ohm deal?

[kleuck]

First your load is NOT 6 ohms, but a normalized 8 one, and can be 25 omhs at 5 Khz !
Your tubes see about 440 volts static, about 600 dynamically ; if you double the load at the secondary, you double the load on the primary, quess what happens to the dynamic voltage ?
Cesar Diaz said he replaced the super reverb OT with Twin one's....but i've never read a single word from him about a 400% mismatch.
Anyway, you can probably play with a 100% mismatch with no risk, impedance ratios are not that precise, nor as said before the Speaker impedance, which varies with frequency, the OT's impedance ratio is choosen to give the amp the more power for a given B+ and that's all.
But you cannot say it's good practice or has no importance at all.

http://www.geofex.com/tubeampfaq/TUBEFAQ.htm#matchspkr

[printer2]

You could install catch diodes on the primary of the output transformer. Amp manufactures tend to use them now days. They clamp the inductive back EMF so that the voltage does not get over 2X the supply voltage protecting the transformer, sockets and tubes from breakdown. In the Hotrod it is CR4-5, R3000, Fender PN 029690001.

http://ampwares.com/schematics/hotrod_deluxe.pdf

[Wally]

Clay, first thought....matched speaker load/OT is prefereable and sounds better than either a 50% lower load or a 100% higher load. I am not goingt o get into what oen or the other of thesemismatches will/might do to the OT and circuit. I am going to say that Fender allowed for the 50% mismatch ....so they weren't afraid that their amps couldn't handle that. And...I know of 4 ohm OT's that have run an 8 ohm load...1X15" in a TR....for decadse. NOte: that player is a country player andnot a push it to the limit rocker.

Re: SRV and his 1X15" Vibroverbs that Diaz modded. It is true that Diaz installed Twin REverb, 4 ohm OT's in those amps for SRv. However, when running 2 X 6L6's through that 4 ohm OT wiht an 8 ohm load, the whole equation is actually a 'match'. REmember,when one pulls two 6L6's out of a TR to lower the output, one can maintain the match ofimpedance requirement v. load by discsonnecting one speaker to put an 8 ohm load on there...for a match.

[Wally]

ADN....Diaz installed that OT in order to increase power an stiffen up the output section...more punch, less saturation in comparison to that little Vibroverb OT. HE knew what he was doing in the 'matching impedances' department. HE coudl have put in a bAssman OT for the increase inpower and stiffness....but he would have had a mismatch, right? 4 ohm OT and 2 x 6L6's with an 8 ohm load. HE chose to mod to a matched situation. that right there is a lesson in matching and mismatching form someone whounderstaood the situation very well, I would think.

[claytushaywood]

I had read this... then I realized it was an article about SRV's amp... that was supposedly bought... but they failed to include pictures of it...

http://www.valcoamp.com/page/srvs-final-notes-a-harsch

you can skip down

[Wally]

Okay, Clay...I stand in error. My memory is not what it used to be. Here is the quote from the interview with Cesar Diaz in the August 2000 issue of teh Tone Quest/ Vol. 1 No. 10....page 5.

"We also chagned the output transformers on the Super REverbs to those from a TWin Reverb, and on Vibroverbs we used output transformers from a Bassman."

So, that seems to be a mismatch on the SR's whiel the Vibroverb is 'matched' with that 4 ohm OT and the 8 ohm load workign with two 6L6's, right?

Diaz goes on to say: "I'm not such a stickler on matching impedance becase Fenders are very tolerant amps. You just about feed them anything."

Hmmm.....I find the SR situation with the 2 ohm load on a 4 ohm OT with two 6L6's to be interesting. He matched 100% to the lower side, right? 2 ohms speaker load against what is in effect an OT/power tube combination that is looking for an 8 ohm load....in a technically ideal situation. That supporst his 'feed 'em anything' thought, right? That might also support jefrs contention that the fEnder OT's were actually built for half of the specified load?????? IF that is true, then running a 2 ohm speaker load against that 4 ohm OT with only 2 X 6L6's is a 50% mismatch.
IF this 'half of spec' thing is so, then that 8 ohm 1X15" in that TR that I know of has been running 200% moreload than the OT was spec'd for....and doing it long time, right? Which also supports Diaz's 'feed 'em anything'. ?????

Whatever floats the boat, imho...but I like a match. I bet that those amps got frequent routine general service, though....just to make sure everything was what it should be.

[mercy]

Originally Posted by claytushaywood

for all the naysayers saying you absolutely cannot go up- SRV ran a 2ohm transformer with an 8 ohm speaker load in some of his amps. and he cranked them up. but of course diaz himself would change the tubes midshow with oven mitts.

and yes it's one 8 ohm speaker with a 4ohm transformer. sorry i should have been more clear

This is definitely a popular thing to do, on purpose, for a particular effect. and I am absolutely positive it wont kill MY amp... being a vintage fender. this is not a modern production crate or an amp that cant take some abuse. Tons of people do this, on purpose, for the effect.

I'm really not interested in the naysaying unless you actually have personal experience- not someone that brought you an amp that said this was broken and this is why.

I'm really looking for people that do this or have done this and know what the effect on tube life is, if any.

Thanks for the posts so far guys. and jefrs would 50% be a 4ohm tranny with a 6ohm load and 100% be a 4ohm 8ohm deal?

I've been running my '74 Deluxe Reverb that has a 4-ohm OT into a single 8-ohm speaker for the last two months, playing lound (master at "10", gain at 7) for a couple hours at a time and the tubes (old GE's) sound good still (as of yesterday.)

[jefrs]

Originally Posted by claytushaywood

for all the naysayers saying you absolutely cannot go up- SRV ran a 2ohm transformer with an 8 ohm speaker load in some of his amps. and he cranked them up. but of course diaz himself would change the tubes midshow with oven mitts.

and yes it's one 8 ohm speaker with a 4ohm transformer. sorry i should have been more clear

This is definitely a popular thing to do, on purpose, for a particular effect. and I am absolutely positive it wont kill MY amp... being a vintage fender. this is not a modern production crate or an amp that cant take some abuse. Tons of people do this, on purpose, for the effect.

I'm really not interested in the naysaying unless you actually have personal experience- not someone that brought you an amp that said this was broken and this is why.

I'm really looking for people that do this or have done this and know what the effect on tube life is, if any.

Thanks for the posts so far guys. and jefrs would 50% be a 4ohm tranny with a 6ohm load and 100% be a 4ohm 8ohm deal?

A 6ohm impedance is nominally 3x8ohm speakers (2-series & 1-parallel to 5.333ohm) and is a +50% load

An 8ohm load on a 4ohm tap is a +100% load, which is fine if you don't mind risk of blowing the output valves and/or the OT cooking - the the OT will get hot enough to fry an egg with the correct load. Remember that 8ohm load is actually somewhere between 50ohm and 8ohm.

And as you have a vintage Fender, its OT is actually wanting a 2ohm load [ref Gerald Weber, Hip Vintage Valve Amps...], it can run 2ohm and 4ohm, and maybe a little over. With a 4ohm speaker it is already running at +100%, it is designed to drop to 2ohm when you add a 4ohm ext cab in parallel, this does provide a little more power.

If you want to do it properly then run two 8ohm speakers in parallel, which is a 4ohm load.

What happens when the load goes wrong? - I had an AC15 blow the speaker (G12-15) which shorted and caught fire, which caused a fly-back on the OT, I got an electric shock which knocked me on my back, meantime the main fuse blew and all the lights went out, the girls started screaming as the boys started a fight.

[printer2]

Originally Posted by jefrs

An 8ohm load on a 4ohm tap is a +100% load, which is fine if you don't mind risk of blowing the output valves and/or the OT cooking - the the OT will get hot enough to fry an egg with the correct load. Remember that 8ohm load is actually somewhere between 50ohm and 8ohm.

You know I hate questioning you on some of your posts but either I miss the logic or there is not enough info for me to process the information so I hope you do not mind my asking a few questions.

So I take it an 8 ohm load will cause a transformer to get hotter than if it is feeding a 4 ohm load that it was designed for. How does that happen? Let's take a case of a short circuit across the transformer output. Not much voltage but a lot of current. And since the only real resistance (although we are dealing with AC the correct term is impedance but since those less technically confident have an easier time with resistance we will use it) is due to the output winding, the voltage drop occurs across the winding and since the current is equal in the whole circuit the power is dissipated in the transformer (Power = Volts x Amps).

Now the other extreme is if the transformer is lightly loaded. we will get a higher voltage but a lower current in the secondary. Given that the current is equal in the secondary loop and that there is more of a voltage drop in the load rather than in the secondary winding I would think we would assume that most of the power will be dissipated outside the transformer. In my neck of the woods this would cause the transformer to run cooler.

Now if you chart the amount of power dissipated in a transformer, with a dead short as all the heat developing inside the transformer and with the transformer lightly loaded with most of the power turned to heat outside the transformer, you might take it with a leap of faith that driving a four ohm load will heat the transformer up more than driving an eight ohm load.

Or you might not. I am scratching my head on this one.

What happens when the load goes wrong? - I had an AC15 blow the speaker (G12-15) which shorted and caught fire, which caused a fly-back on the OT, I got an electric shock which knocked me on my back, meantime the main fuse blew and all the lights went out, the girls started screaming as the boys started a fight.

Actually I think the speaker shorting would load down the transformer. Maybe the speaker opened after the short and then the transformer ha no where to loose the energy resulting in the fly-back?