guys, i want some opinions. i've got an old blackface fender pro reverb. the speakers are waaayy tired (old mojotones). i horse traded for some new greenback celestions (16 ohms). i can wire 'em up to 8 ohms and stick 'em in the pro, but will that sound as good as a 4 ohm load? more importantly, will that harm my amp? pros are made to drive 4 ohm loads ideally. anybody have any experience with this? thanks, jon
Might sound a little different, the volume control will be up more [for same level] with speaker load of 8 instead of 4 ohms.
As long as you have continuity [good connections...I always throw that in] the amp should see less load.
If you go to 2ohms [not recommended] [some do tho] you will find the weak link in your amp maybe very much fast, faster at the very least.
I think it's tubes and transformers that get "hit' when you drop speaker load too low, boost the input, and crank the amp is recipe for certain disaster.
Yout amps rated at four and your only putting 8 on it... should be fine.
The volume control on the amp probably will get up enough to drive the speakers. Should be a touch cleaner and lower volume if anything.
Overloading the amp might damage it. With overload I mean a too low speaker impedance. Underloading it won't be much of a problem I think, just that the tone might be a bit different and that the amp won't go as loud.
Tube output stages are all about balance.
A 6L6 wants to see somewhere around 6K plate impedance when it's running in class AB at 400 volts. The output transformer "transforms" the 4 ohm speaker impedance into the 6K impedance that the tube wants. If you substitute 8 ohms for 4 ohms the tubes will now see twice the plate impedance or roughly 12K (transformers work on ratios). That's quite a jump (100%!) and now the bias voltage on the 6L6 grids may be out of range (on the cold side) but more importantly the increased plate resistance will cause a drop in the plate current and a corrosponding increase in the screen grid current. Translation = you may fry the tubes in the process. If that happens and the fuse doesn't blow, the fried tubes can then kill the OT.
Unless you know the amp is speced by the maker to work with a given impedance, using anything other than the spec can spell disaster very quickly. It's a risk I won't take without a meter and multiple fuses in the circuit (plate and screen). This goes double or triple for any old blackface.
Sorry to rain on the parade.
Take care,
-Peter
The Champ is big for this type of question. Putting a 10" speaker in is a popular mod. I've pointed out on several forums that it's a nice mod, but to make sure you find a 4ohm speaker. Then you get the standard reply: "Fenders are very tolorant of impendance mismatches" Right! Go ahead, it's not pretty if the fuse doesnt blow, and the rectifier goes.
you guys all pretty much confirm what i was thinking...BAD IDEA! i was just hopin' that i would hear some feel-good stories like...
"i did it and it sounded great! my amp never ran better!"
or " the impedance mix-match brought out a whole new side to my blackface pro and now it is shitting solid gold tones!"
i'm gonna put them into my 2x12 and use the old 8 ohm celestions in my pro. thanks for the help and sobering thoughts! jon
QuoteThat's quite a jump (100%!)
Yes, but I believe there are a lot of Fender amps that can handle this 100% jump.
I need to look it up in The Ultimate Tone, but it covers this and I recall that for certain amps, it's fine. Not saying it's fine in this case right now until I look it up.
Quote from: cajununicornguys, i want some opinions. i've got an old blackface fender pro reverb. the speakers are waaayy tired (old mojotones). i horse traded for some new greenback celestions (16 ohms). i can wire 'em up to 8 ohms and stick 'em in the pro, but will that sound as good as a 4 ohm load? more importantly, will that harm my amp? pros are made to drive 4 ohm loads ideally. anybody have any experience with this? thanks, jon
You can do this with no problems at all and lots of folks really like the difference in sound...you may experience a slight reduction in volume and a slightly softer sound, which for lots of fenders is welcomed anyways...
It's easier on your trannys, which is also welcomed in some of those amps...
I've been running my 63 blond Bassman head into 8 and 16 ohm Marshall cabs for years with no problems, and the mismatch actually sounds better to me than into the 4 ohm load...at 8 or 16 it's less muddy in the lows and less harsh in the highs...kind of a no-brainer...
As was said, going in the opposite direction can present problems (but doesn't always)...
Fender transformers and 6L6s are mildly tolerant to impedence mis-matching. The thing won't blow up if you did it but it may not sound right either. Only do this for a SHORT term solution (i.e. get you through a gig or two).
Some may say they like the sound of improperly matched speakers but I think thats just another gimick/trick that guitarist would like to brag about but don't know what they are doing kinda like the EVH/variac thing. I usually tell people that they are better off playing with their tubes then they are screwing with functional parameter bits of the amp like impedence.
To sum up its okay to do it SHORT term but DO NOT make it a perminant thing and/or run the amp hard. Get yourself a correct set of speakers. You amp specifies that impedence for a reason.
Andrew
i saw a thread very similar to this at the AX84 forum...
pretty much what was said was that going double or half the rated value is ususally ok...But then again these guys are talking about using new, sturdy transformers. I dunno if a '63 OT would be more sensative...
one thing i should add is that my transformer has long since been replaced...pro reverbs have these dinky little trannies that blow pretty easy. it's got a transformer from a fender super 60 in it now. i'll probably use my 8 ohm set of vintage 30s (4 ohm parallel) instead of the 16 ohm greenbacks. maybe the greenbacks will sound better with my hiwatt anyway. it's a luxury problem! jon
Quote from: The Tone GodFender transformers and 6L6s are mildly tolerant to impedence mis-matching. The thing won't blow up if you did it but it may not sound right either. Only do this for a SHORT term solution (i.e. get you through a gig or two).
Some may say they like the sound of improperly matched speakers but I think thats just another gimick/trick that guitarist would like to brag about but don't know what they are doing kinda like the EVH/variac thing. I usually tell people that they are better off playing with their tubes then they are screwing with functional parameter bits of the amp like impedence.
To sum up its okay to do it SHORT term but DO NOT make it a perminant thing and/or run the amp hard. Get yourself a correct set of speakers. You amp specifies that impedence for a reason.
Andrew
If you were referring to my post, I would just like to inform you that I've been a professional guitarist and tech for 28 years, I'm not bragging about anything and this is no trick... I think you need to educate yourself about this a bit more, because it really doesn't sound like you have alot of real experience with it...being so cautious or uptight about this just makes people scared to work on or mod their own amps...
The amp runs cooler when you run a 4 ohm load into 8 or 16, it's not running it harder at all, you're going to confuse people with this mis-information...and transformers are much more tolerant than you think, when you take into account the difference between windings of one tranny to another, and the differences in speaker impedances as well...with older amps you may have a gross mismatch even though you're on the right impedance tap...
I can't even tell you how many people who run two 8 ohm celestions in their twins at 16 ohms and love the sound...it's alot smoother, and the trannys don't get as hot...again, a no-brainer
The reason many pros I know prefer sound of the mismatch is because a 4 ohm load is the least musical sounding of the three choices...it's almost a dead short and you're pushing the amp very hard...you can do way more damage using any of the current attenuators than any mismatch would ever do...
Another instance would be the old Orange amps that had the same PT and OT for their 80 and 120 watt tops...the only difference was the extra two output tubes but still you had the same 4,8, and 16 ohm speaker taps, therefore in actuality being run on the "wrong" tap most of the time...these amps have lots higher plate voltage than any Fender amp and were more unstabe for various reasons but I never saw or heard of one of them blowing up since I found out about them in the mid-70's...
I don't think it's a good idea for people to post info that they "think" is correct, or guesses or hearsay for that matter...please post real information from real experience so not to confuse the ones that are trying to find the correct answer... :)
I'm not going to get dragged into the negativity that has seemed to be encompassing this board lately. I will say I do take offense to you questioning my qualifications as you do not know what my background is. I'm also not going to get into a pointless "who's qualification's are big" argument but I am qualified to know what I am talking about.
I was not taking a direct stance against what you said but other musicians who like to do funny/unique things in their quest for tone but don't realize that they are damaging their equipment in that quest. I think its more important for a working musician to have a working amp that sounds ok then a broken amp with a expensive repair bill which they can't play gigs with to pay the bills.
None of the information that I offered is incorrect and is backed by other posters in this thread and other experts. I did not dictate a doomsday situation but will not paint a perfect picture situation either for this case. The fact is mis-matching the impedence of the tranformer/tubes is bad and furthured emphisised the louder you play.
I stand by what I said, use the correct impedence speaker and don't screw with it. That is not mis-information no matter what you think/feel.
Andrew
Here's one way to see if this mismatch falls into safe territory....
Put a DMM set for DC current in the plate lead of one side (assuming that both sides are similar), and measure the plate current. Now shift the meter to the screen and measure the current through there.
The "acceptable" numbers will vary all over the place, but to use rough numbers I would avoid drawing more than 55mA on the plates with no signal, and less than 4mA on the screens without signal. Look at this page from RCA for more specs: http://www.triodeel.com/6l6gc_p2.gif (if you manually change the page number, this is page 2 out of a total of 9)
There are just too many variables to say "yes" or "no" to any random mismatch. Your meter will tell you what's happening, but you have to know what the tubes will handle to make those numbers meaningful.
The good news is that the natural shift the tubes take will tend to be toward colder operation thanks to the increased bias voltage relative to the plates. Just make sure that you don't exceed the screen ratings and it's going to be OK.
Take care,
-Peter
Quote from: The Tone GodI'm not going to get dragged into the negativity that has seemed to be encompassing this board lately. I will say I do take offense to you questioning my qualifications as you do not know what my background is. I'm also not going to get into a pointless "who's qualification's are big" argument but I am qualified to know what I am talking about.
I was not taking a direct stance against what you said but other musicians who like to do funny/unique things in their quest for tone but don't realize that they are damaging their equipment in that quest. I think its more important for a working musician to have a working amp that sounds ok then a broken amp with a expensive repair bill which they can't play gigs with to pay the bills.
None of the information that I offered is incorrect and is backed by other posters in this thread and other experts. I did not dictate a doomsday situation but will not paint a perfect picture situation either for this case. The fact is mis-matching the impedence of the tranformer/tubes is bad and furthured emphisised the louder you play.
I stand by what I said, use the correct impedence speaker and don't screw with it. That is not mis-information no matter what you think/feel.
Andrew
"I will say I do take offense to you questioning my qualifications as you do not know what my background is"
I'm sorry I thought that was what you were doing...if you weren't then OK no offense to you either...
my main point was that you don't always have to go by the book...it can be very limiting...sure, you need to learn the rules before you brake them, but when you get to that point that's usually where the fun begins IMO :)
PEACE
2 examples of mismatched impedances. One by design and the other anecdotal.
The one by design...
Matchless Spitfire.
An 8 ohm speaker load connected to an 8 ohm OT secondary stepped up to a 4k OT primary fed by 2 EL84's with an Raa of 8k (according to the specs).
The load on the tubes is mismatched...
in this case it would be -
(4Kohm/8ohm) * 8ohm = 4kohms
To get this setup to "match" Matchless would have had to tinker with the load impedance...the easiest way being to just up the speaker load to 16ohms giving -
(4kohm/8ohm) * 16ohm = 8kohm
A simple addition of a 16 ohm speaker load would have then matched the impedance that the tubes were looking for which would be...
(8ohm/4kohm) * 8kohm = 16ohms.
Well, of course we all know they didn't do this and the Spitfire is arguably one of the best sounding amps going with a so far so good reliability. From my own experience with these amps the mismatched load seems to open the amp up in a way that isn't there once you match the loads. The matching creates a better, more efficient power transfer but it seems like it comes with the downside of smothering the sound. There's a power loss with the mismatch but what a great tone.
The anecdotal...
A Stromberg Carlson PA was converted into 2 6L6 guitar amp for a guy around 1998. I think it was originally made sometime in the late 50's but I'm not sure. He came back to me recently to spruce it up because he was going to donate it to help raise money for a friend of ours who has developed a rare form of cancer.
It was set up to push an 8 ohm load and I noticed that he had 2 8ohm Webers wired in series. I asked him how long and how hard he had been running it that way and the short answer was basically..."since day one and wide open".
The amp sounds beautiful.
I don't think the impedance mismatches here are as critical as they might be in other electronics situations.
The real surprise for me was how much impact the mismatches have on the tone clarity.
Quote from: AmmscrayI'm sorry I thought that was what you were doing...if you weren't then OK no offense to you either...
No problem. Its all cool. :)
Quotemy main point was that you don't always have to go by the book...it can be very limiting...sure, you need to learn the rules before you brake them, but when you get to that point that's usually where the fun begins IMO :)
That I agree with. I belive that the people who are elbow deep in this stuff are the ones more willing to play with things.
One love and great tone to all.
Andrew
One more thing... a little off topic.
Peter,
You're running a "modern" 6L6 at 400 Va AND drawing 55ma of idle current? A 22 watt burn at idle? Man, I need to get a set of those tubes. Which ones are you using?
Quote from: csjPeter,
You're running a "modern" 6L6 at 400 Va AND drawing 55ma of idle current? A 22 watt burn at idle? Man, I need to get a set of those tubes. Which ones are you using?
When you're running in the low end of class AB
1, there isn't too much difference between no signal and full signal. I didn't want to cite figures that were too low since so many amps these days are stocked with 5881s (or Russian tubes that are similar to the 5881 in dissipation) and they're run a bit on the hot side. The figures were somewhat derived from the RCA 6L6 data sheets from 1960, and the stress they see in a Fender Super.
What am I using? These days I have some Sylvania STR367s in my Twin Reverb and Sovtek 5881s in another amp. No.... I'm not running them that hot. :) Tubes can sound amazing when doing the "swan song", but I want my tubes to last.
con0ATake care,
-Peter
Quote from: csjOne more thing... a little off topic.
Peter,
You're running a "modern" 6L6 at 400 Va AND drawing 55ma of idle current? A 22 watt burn at idle? Man, I need to get a set of those tubes. Which ones are you using?
I was wondering about that myself...in my experience anything over 40 or under 20 Ma's doesn't sound right and on the former will definitely diminish your tube life...I personally like to see 6L6's and 5881's at around 35-36...
BTW CSJ great info on the Matchless, I didn't know about that :)
QuoteI personally like to see 6L6's and 5881's at around 35-36...
That's where I like them too.
When I have time to look up TUT, I will post the findings but for myself I also agree with Ammscray in that I run my Bassman at 8 ohms instead of 4.
I remember asking if this was ok, and later confirming it by reading TUT and other books.
I did measure current draw as well many times and it was fine.
Finally, yes, the tone is better with the mismatch (to me).
I also like the high 30s for bias.... I tend to tune by ear and the lack of glowing plates.
I just listed those numbers as the maximum that a 6L6 would take before meltdown since that was the question at hand and my first answer was more conservative. :D
I also listed 4mA as a max on the screens, but I don't think you would ever want to run like that. Svetlana 6L6s (who brands that tube now?) could handle 10mA (5W) on the screens and/or 30W on the plates before meltdown. 2.5mA on the plates is more like what you should see in a typical amp.
As long as everything is balanced (esp. thermal issues), the numbers don't mean too much by themselves.
http://www.svetlana.com/graphics/products/pdf/6L6GC.pdf
Take care,
-Peter
Cajununicorn,
As you've probably seen before this topic is open to alot of debate. Here we have pretty much the same four responses.
1.) Don't do it at all...always match as closely as possible. Better safe than sorry.
2.) Mismatch up but maybe not down. (The Stromberg Carlson example - assuming that the original OT primary was built to match the Raa load specs of 2 6L6's...which I did assume at the time).
This might increase the flyback voltage spikes the plates suffer with the higher than expected impedance. If so, run a string of 1N4007s from the plate to ground (anode side to ground) to absorb the excess voltage. As far as I'm concerned this should be SOP anyway.
3.) Mismatch down but maybe not up. (The Matchless example). "May cause tube stress". This I wonder about. As far as I know the 6L6 won't really care...it'll keep putting out it's set amount of plate current into the lower load. More current isn't "drawn" out of it somehow. Of course, current can be increased by rebiasing the tube or simply just turning up your amp. This increase in current isn't related to the transformer though. The OT is only like a tennis player smacking back and forth whatever comes it's way. (I hope this metaphor doesn't get me in trouble too...wait, I think that was more like a simile...phew...)
4.) Mismatch and move on. (Myself, Ammscray and Aron). The heck with it...what's it sound like?
So what's the answer still?
I don't really know. I respect the other opinions here and I've seen where even the expert speaker builders, tranformer winders, power amp designers - though generally in favor of matching loads - waffle a little when it comes to saying exactly what the critical limits are. The debate gets heated, especially with audiophile types, when it comes to operating tube amps into what they call "grossly" improper impedance matches. Is 8 into 16 (or any of the other several guitar amp possible mismatches) grossly improper? I honestly really don't know.
As Peter points out, there are so many variables. If you've got a truly valuable vintage something or other amp and you want to keep it pristine...then, sure, I'd say "play it safe".
But, like Ammscray, I'd wonder just exactly how "matched" it is or even was in the first place. And sometimes I just can't help but wondering... "what would it sound like if I plugged, hmmm... THIS into THERE!" :wink:
This got way too long... sorry for the rehash of stuff I'm sure most of you already know.
csj, i appreciate all the info...very interesting which twist and turns the topic took. it's a vintage amp, but i've long since decided to use it as my main road/club amp. it handles the stress very well and gets used a lot. i'm pretty utilitarian when it comes to gear. i wasn't even looking for a better tone due to impedence matching, missmatching, ect... i just wanted something that worked and sounded right. i love using celestions in fender amps. BTW, my hiwatt sounds awesome with the greenbacks. thanks for all the input from everyone. jon
Quote from: aronQuoteI personally like to see 6L6's and 5881's at around 35-36...
That's where I like them too.
When I have time to look up TUT, I will post the findings but for myself I also agree with Ammscray in that I run my Bassman at 8 ohms instead of 4.
I remember asking if this was ok, and later confirming it by reading TUT and other books.
I did measure current draw as well many times and it was fine.
Finally, yes, the tone is better with the mismatch (to me).
Impedance matching in tube amp circuits is probably one of the most mis-understood subjects. This question frequently comes up along with "can I remove 2 power tubes from a 4 tube amp and how does it affect impedance?"
Kevin O'Connor has posted the some answers on his FAQ page for those interested. http://www.londonpower.com/faq.htm#JAN04
From a personal stand point I have operated all of my Fender amps with mismatches, both up and down, for over twenty years with no blown transformers or melted tubes. Some of these amps are operated 8 hours a day with no problems at all. While mis matched loads do change the tonal character, and you will lose some power, it is just another choice available to the musician. For years Fender produced amps with extension speaker outputs with no load requirement. Other manufacturers like MESA have included "half power" switching on thier amplifiers with no requirement to "match" the speaker load or adjust the bias. Fender actually suggests setting the impedance selector on the ToneMaster to whichever setting you prefer. I do not know of any guitarist that has damaged an amp due to mismatched loads. I have actually seen a fender twin,with a shorted voice coil in one speaker, provide enough power to heat the voice coil red hot and ingnite the speaker cone and grill cloth. After replacing the speaker the amp was fine 8) Mismatch and move on