Question about valve amp impedance outputs

[Osiris]

[size=4][font=arial,helvetica,sans-serif]I'm pondering the idea of getting an all valve amp, the only one I've ever had was when I started playing about 30 years ago, it was an old Park guitar amp and I really didn't know anything about them at that time, and if I'm honest, I still don't, having played through solid state and Class D ever since. But, happy as I am with Class D, this is an itch that I think it's about time to scratch….

My question is regarding the output from all valve (tubes as the cool kids and our brethren out in the colonies call them) amp outputs. Whereas the SS/Class D's that I've used seem to work on a minimum load i.e. 500 watts into 4 ohms, 300 watts into 8 ohms etc, it looks to me that some valve amps have dedicated impedance outputs i.e. an output for 8 ohms and a separate one for 4 ohms.

Do the same rules apply to the output i.e. if you have a 100 watt all valve amp and run it with an 8 ohm cab, are you only getting half (or thereabouts) of the power from the amp as you would with a Class D/SS amp? Would you get the full output running valve amps at 4 ohms? [/font][/size]

[bertbass]

Valve amps have an output transformer with various impedance taps, usually 16, 8 and 4 ohms. You set the impedance to match the cab(s). The full output is available in all impedances.

[Beer of the Bass]

[quote name='bertbass' timestamp='1488558129' post='3250048']
Valve amps have an output transformer with various impedance taps, usually 16, 8 and 4 ohms. You set the impedance to match the cab(s). The full output is available in all impedances.
[/quote]

+1
Just to add to that for the OP, the impedance marked on the outputs or selector is the optimum load impedance for that tap, not a minimum as is common on solid state amps. It's generally best to stick to that as not all amps tolerate a mismatch equally well.

Edited March 3, 2017 by Beer of the Bass

[Bill Fitzmaurice]

[quote name='Beer of the Bass' timestamp='1488559026' post='3250060']
the impedance marked on the outputs or selector is the optimum load impedance for that tap, not a minimum as is common on solid state amps.
[/quote]In fact, it's the maximum. The rules with valves are the opposite of SS. Valves aren't even bothered by a short circuit, but they can be toasted when there's no load.

[Chienmortbb]

If you put an output transformer on an SS amp it would behave in the same way as the thermionic valve /vacuum tube amp. However why put a huge lump of metal in there is you don't need it?

[Bill Fitzmaurice]

[quote name='Chienmortbb' timestamp='1488565142' post='3250134']
If you put an output transformer on an SS amp it would behave in the same way as the thermionic valve /vacuum tube amp. However why put a huge lump of metal in there is you don't need it?
[/quote]That's why SS amps stopped using output transformers in the 1960s when direct coupling technology was invented.

[Beer of the Bass]

[quote name='Bill Fitzmaurice' timestamp='1488562864' post='3250113']
In fact, it's the maximum. The rules with valves are the opposite of SS. Valves aren't even bothered by a short circuit, but they can be toasted when there's no load.
[/quote]

That's a generalisation that often holds up well enough to use it as a rule of thumb, but doesn't really describe how the amp operates. Output transformer failure is more likely with no load or a higher than intended impedance, but a lower impedance or a short will still run the valves outside of the operating point the designer intended and cause them to limit the current passed. On most amps this won't cause catastrophic failure but it will reduce the power available and change the sound of the amp. Depending on the operating point of the amp it may also increase dissipation, running the valves harder than necessary. The valves in amps which use a triode/pentode "half power" switch can be badly damaged by lowered speaker impedance when running in triode mode, and while that's something of a special case this feature is more common than it used to be.
Since valves are considered a consumable and output transformers are expensive to replace, too low a speaker impedance is clearly a safer bet than too high. The extent to which this will reduce the power available depends on how the amp is designed, so without extensive prior knowledge of a particular amp (drawing load lines etc) or enough anecdotal reports of a successful result with that amp, using the intended tap for the nominal impedance of your speakers is the best idea.
So "optimum" is a good word, I'd say.

[Osiris]

Thanks for the help everyone.

I have 2 different cabs, one is a 600 watt, 4 ohm, 2x12 and the other is a 300 watt, 8 ohm 1x12.
And I only ever use one or the other - never both together.

So using the scenario in my original post, if was to use either cab with a 100 watt all valve amp (using the appropriate output from the amp) then regardless of the cabs nominal impedance, I'd still be getting the full 100 watts through each cab?

[Beer of the Bass]

[quote name='Osiris' timestamp='1488568893' post='3250197']
Thanks for the help everyone.

I have 2 different cabs, one is a 600 watt, 4 ohm, 2x12 and the other is a 300 watt, 8 ohm 1x12.
And I only ever use one or the other - never both together.

So using the scenario in my original post, if was to use either cab with a 100 watt all valve amp (using the appropriate output from the amp) then regardless of the cabs nominal impedance, I'd still be getting the full 100 watts through each cab?
[/quote]

Yes, that's right.

[Osiris]

[quote name='Beer of the Bass' timestamp='1488569002' post='3250199']
Yes, that's right.
[/quote]

Brilliant, thank you [size=4] [/size]

I don't claim to understand any of the technical talk above.
What I do appreciate, however, is that you need to know what you are doing with a valve amp as it seems that you can easily kill it if you don't. And I'd rather be safe than sorry before diving in head first.

[Bill Fitzmaurice]

[quote name='Beer of the Bass' timestamp='1488568206' post='3250189']
a lower impedance or a short will still run the valves outside of the operating point the designer intended and cause them to limit the current passed.
[/quote]It's of far less concern than no load. That's why Fender amps used a switched main speaker jack to short the output transformer secondary when there was no speaker plugged into it. As for low impedance loads, when we were young and stupid we'd run Fenders with 1 ohm loads that never bothered them a bit.

[Beer of the Bass]

[quote name='Bill Fitzmaurice' timestamp='1488572275' post='3250238']
It's of far less concern than no load. That's why Fender amps used a switched main speaker jack to short the output transformer secondary when there was no speaker plugged into it. As for low impedance loads, when we were young and stupid we'd run Fenders with 1 ohm loads that never bothered them a bit.
[/quote]

Yes, the 60s/70s Fender amps (and similar designs) are quite robust in that respect. Not knowing what the OP's amp is, or the amp of anyone else reading this, it is sensible to urge some caution. Not all amps use the same topology as Fender, in terms of plate voltage, primary impedance and biasing. Just as an example, I have a Dynacord Eminent II which runs a pair of EL34 at unusually high plate voltage, and I have heard of more than one case of the increased dissipation from a short or lowered impedance causing the valves to fail and take out components around them when they do so.