NOVICE TUBE AMP QUESTION

[Bacchus]

I just got my Fender Champion 600 back from the shop (long story, involving shocking customer service which convinced my not to give them my business again over online retailers).

Basically, I want to know if I can plug it into my eight ohm Marshall cab using the four ohm output.

This should be fine, right?

[Doog]

Yup- the speaker cab being a higher impedance is fine, the other way round is not.

[Bacchus]

That's what I thought. Just thought I'd make sure, so that if it blows up then someone on here is liable for the costs.

You okay with that?

[Doog]

I'll have a certain someone delete the facts. But go ahead.

[Justin J]

from what i understand, a mis-matched load on the output transformer will result in different sounds than an optimum load as more or less voltage will run through the tubes.
i think a greater impedance isn't really a problem like doog said, but will probably wear out the power tube faster than usual.

[Mike]

Yes, the sound will differ and output will not be the maximum as the energy transfer is not as efficient. This mismatch will generally be ok because "most" transformers are built to withstand things, however Matching is always best and I probably wouldn't be all that keen to do it on such a small amp with such a small output tranny, but that's just me. That's what annoys me about the Champion - they scrimped on the transformer - give at least a 4 and 8 ohm tap ffs. Annnyway.

Here is a relevant Q and A:

Q: Can I plug my 8 or 16 ohm Marshall as an extension cabinet into my Fender Twin Reverb? (Don B.)

Most Fender Twin Reverb amps, including the Blackface reissue, operate only on a 4 ohm load and have no means of selecting any other impedance. Exceptions to this are the last original silverface issues that are rated at 135 watts, and later channel-switching models. Operating both together brings the speaker load to 2.7 ohms which is not a problem at all for the Twin Reverb to handle. If, however, you use only the 8 ohm Marshall cab and unplug the Fender speakers, then the amp could be at risk if you play at loud volume.

Whenever you mismatch a tube amp with a much higher load (speaker) impedance, a higher signal voltage is produced across the primary of the output transformer. One loud popping note on the high E string and you could arc-over the output tube sockets, fry the tubes, or zap the output transformer. This is why you cannot run a tube amp without a speaker connected to it. If you must mismatch the load impedance use a lower than rated one, as you are doing now by connecting both cabinets.

Having said this, and making it very clear that I do not advise anyone to connect a higher than rated impendence to any tube amp, generally there is no problem when operating the amp at 8 ohms if your amp is well designed and in good working condition. Connecting a 16 ohm load to a 4 ohm amp is, of course, more risky.

http://www.rickonslow.com/FAQ-LoudSpeak ... 9.html#q14


This is a pretty awesome resource:
http://www.geofex.com/tubeampfaq/TUBEFAQ.htm

Q: Will it hurt my amp/output transformer/tubes to use a mismatched speaker load?


Simple A: Within reason, no.
Say for example you have two eight ohm speakers, and you want to hook them up to an amp with 4, 8, and 16 ohm taps. How do you hook them up?

For most power out, put them in series and tie them to the 16 ohm tap, or parallel them and tie the pair to the 4 ohm load.

For tone? Try it several different ways and see which you like best. "Tone" is not a single valued quantity, either, and in fact depends hugely on the person listening. That variation in impedance versus frequency and the variation in output power versus impedance and the variation in impedance with loading conspire to make the audio response curves a broad hump with ragged, humped ends, and those humps and dips are what makes for the "tone" you hear and interpret. Will you hurt the transformer if you parallel them to four ohms and hook them to the 8 ohm tap? Almost certainly not. If you parallel them and hook them to the 16 ohm tap? Extremely unlikely. In fact, you probably won't hurt the transformer if you short the outputs. If you series them and hook them to the 8 ohm or 4 ohm tap? Unlikely - however... the thing you CAN do to hurt a tube output transformer is to put too high an ohmage load on it. If you open the outputs, the energy that gets stored in the magnetic core has nowhere to go if there is a sudden discontinuity in the drive, and acts like a discharging inductor. This can generate voltage spikes that can punch through the insulation inside the transformer and short the windings. I would not go above double the rated load on any tap. And NEVER open circuit the output of a tube amp - it can fry the transformer in a couple of ways.

Extended A: It's almost never low impedance that kills an OT, it's too high an impedance.

The power tubes simply refuse to put out all that much more current with a lower-impedance load, so death by overheating with a too-low load is all but impossible - not totally out of the question but extremely unlikely. The power tubes simply get into a loading range where their output power goes down from the mismatched load. At 2:1 lower-than-matched load is not unreasonable at all.

If you do too high a load, the power tubes still limit what they put out, but a second order effect becomes important.

There is magnetic leakage from primary to secondary and between both half-primaries to each other. When the current in the primary is driven to be discontinuous, you get inductive kickback from the leakage inductances in the form of a voltage spike.

This voltage spike can punch through insulation or flash over sockets, and the spike is sitting on top of B+, so it's got a head start for a flashover to ground. If the punchthrough was one time, it wouldn't be a problem, but the burning residues inside the transformer make punchthrough easier at the same point on the next cycle, and eventually erode the insulation to make a conductive path between layers. The sound goes south, and with an intermittent short you can get a permanent short, or the wire can burn though to give you an open there, and now you have a dead transformer.

So how much loading is too high? For a well designed (equals interleaved, tightly coupled, low leakage inductances, like a fine, high quality hifi) OT, you can easily withstand a 2:1 mismatch high.

For a poorly designed (high leakage, poor coupling, not well insulated or potted) transformer, 2:1 may well be marginal. Worse, if you have an intermittent contact in the path to the speaker, you will introduce transients that are sharper and hence cause higher voltages. In that light, the speaker impedance selector switch could kill OT's if two ways - if it's a break befor make, the transients cause punch through; if it's a make before break, the OT is intermittently shorted and the higher currents cause burns on the switch that eventually make it into a break before make. Turning the speaker impedance selector with an amp running is something I would not chance, not once.

For why Marshalls are extra sensitive, could be the transformer design, could be that selector switch. I personally would not worry too much about a 2:1 mismatch too low, but I might not do a mismatch high on Marshalls with the observed data that they are not all that sturdy under that load. In that light, pulling two tubes and leaving the impedance switch alone might not be too bad, as the remaining tubes are running into a too-low rather than too-high load.

Basically I don't do it, especially not with small amps. And if you going to do it - don't crank it.

[Bacchus]

Shit.

I've done it and I've been cranking it. If it didn't do damage the first time, is that any guarantee that it won't do damage the next time? I'm guessing not.

I always assumed that too low an impedance would cause problems, and that too high an impedance will just not sound great.

(By the way, it sounded fucking great)

[Mike]

Like the article says, if the transformer is good quality you should be fine with 2:1 up. I guess just do what you're comfortable with - it's not a rare or pricey amp so..

I'd be more inclined to find a community of Champion users to ask their experiences, and/or email Fender - they might actually pull their finger out of their arse, but they'll probably just say "blah blah recommended 4 ohm only".

4 ohms is so useless to everyone concerned.

[Bacchus]

Like the article says, if the transformer is good quality you should be fine with 2:1 up. I guess just do what you're comfortable with - it's not a rare or pricey amp so..

I'd be more inclined to find a community of Champion users to ask their experiences, and/or email Fender - they might actually pull their finger out of their arse, but they'll probably just say "blah blah recommended 4 ohm only".

4 ohms is so useless to everyone concerned.

It's not at all pricey, but I've already probably blown up one amp, and I don't want to blow another.

A Champion 600 forum might be a good idea.

Just so I know:

If there was a problem, it would be a complete failure, right? The sound isn't going to slowly ebb away or give me any warning about there being a problem?

[Doog]

Remember that post someone made recently, about some open mic dude who slowly fried his Mesa/Boogie by using the wrong impedance? I guess it can take a while, but if memory serves, he was running an 8ohm load off the 16 ohm output I think.

[Mike]

Like the article says, if the transformer is good quality you should be fine with 2:1 up. I guess just do what you're comfortable with - it's not a rare or pricey amp so..

I'd be more inclined to find a community of Champion users to ask their experiences, and/or email Fender - they might actually pull their finger out of their arse, but they'll probably just say "blah blah recommended 4 ohm only".

4 ohms is so useless to everyone concerned.

It's not at all pricey, but I've already probably blown up one amp, and I don't want to blow another.

A Champion 600 forum might be a good idea.

Just so I know:

If there was a problem, it would be a complete failure, right? The sound isn't going to slowly ebb away or give me any warning about there being a problem?

Nope, you'd just be fucked, that's all you'd know about it.

[NickS]

Remember that post someone made recently, about some open mic dude who slowly fried his Mesa/Boogie by using the wrong impedance? I guess it can take a while, but if memory serves, he was running an 8ohm load off the 16 ohm output I think.

Yep, it took him 2 years but he got there in the end. 2x8 ohms with 8 ohm output.
http://www.shortscale.org/forum/viewtopic.php?t=13332
Here's an example of a Bogner Shiva killed by higher impedance, from RMMG:

>> Well, I blew an OT in the Shiva last week (long story) and I was
>> able to get my 100 JCM 2000 that I traded for the Shiva to do last
>> night's gig. That amp sounded great. The Bogner has a lot of mids
>> (lower mids) you dial it out, it's just there. Maybe that's why you
>> didn't like it. That amp is ALways just pfat. It can't thin out.
>
> That sucks about the Bogner...you try to play it without a speaker
> hooked up or something?

I was trying to see if an amp tech could modify the clean channel for me
(TOO clean)... He couldn't (ha also uses and gigs and older Shiva head and
cab, but mine is NEW)... Left the impedence at 4 ohms and my cab is 8 and
was AT 8 when I dropped it off. I never checked it after I picked it up
(dumbass me), played some gigs with it on 4 with my 8 ohm cab. Stuck it in
my 16 ohm Marshall cab and it took a few minutes and whammo... Bye Bye PT.

Jeff