Bad Brain
Well-known member
Do they disconnect tubes or change from pentode to triode operation?
edit: NM I see the kid's response. Didn't know some amps do this.
Last edited:
edit: NM I see the kid's response. Didn't know some amps do this.
311boogieman
Well-known member
I've ran my Quick Rod at half power with 2 tubes pulled and ohms cut in half for like 10 years. The only time I change power tubes is when I want to experiment with a different brand/flavor. It is back to full power now because I'm trying to get it to compete with my Dual Rec 
So yeah
So yeah
The~Kid
Well-known member
Yeah I know but IMO it's just more prudent and wise to use a pedal as a Faux amp/attenuator or an actual attenuator or similar that's meant for this on any given amp nvm ones that cost north of 1000$-1500$........
How much money does one save when the amps transformer or something else blows up and the whole amp is cooked?!?!?
Thats not pretty and well one can risk it and hope they make it out of it in the end all well and good......
Me I rather play it on the safe side and just not put any of my amps under such conditions even with "safe mismatches" or "safe matches" doing this stuff in either case.
Last edited:
glpg80
Well-known member
I wrote this years ago. I’m pasting it here. Demystifying the magic of transformers one post at a time.
And before the trolls begin trolling I do have an MSEE so be quick to think before you hit quote and reply to rebuttal any of this. It was my area of specialty in graduate school.
———————
The transformer doesn't have an impedance of its own; it only reflects a voltage (or impedance - just the square the turns ratios) from the primary winding to the secondary windings through a ratio of the number of turns of the secondary compared to the primary. Magnetic flux linkage in a ferromagnetic material due to Hopkinson's Law is the theoretical baseline as to why transformers work the way that they do.
The amount of magnetic flux in the ferromagnetic core is dependent upon current flow within the primary coil of wire, the number of turns on the primary, and the reluctance (magnetic resistance) of the ferrite core of the transformer. The current flow generated within the coil generates a magnetic field, which causes magnetic flux to follow the path of least reluctance. The magnetic flux then traverses through the ferrite material to the secondary windings; generating a current within the coils of the secondary through flux linkage. Voltage is then produced across the secondary terminals due to Faraday’s law of Induction.
In summary, transformers are passive lossy devices. Power in is power out minus losses. The impedance you’re changing on the secondary are strictly for maximum power transfer, stepping the primary tube impedances down to speaker load impedances. In essence, the transformer takes high voltage swings of transient plate currents from the tubes and transforms (hint, transformer) them into low voltage high current sources for the speakers.
I also want to note that there is a lot of magnetic flux stored in a transformer core - which can cause havoc to circuits when impedances are mismatched (primary OR secondary). This means mismatching secondary speakers to secondary windings reflecting an intended impedance from the plates, which may or may not be correctly matched with tube swaps to a particular transformer’s intended primary impedance, can cause transient responses which over frequency can be damaging at high volumes. Just keep that in mind.
—————
I’m in the camp that it’s perfectly fine to pull two tubes to double the primary impedance and then counteract the higher induced primary impedance by halving the secondary impedance. Just note that it’s harder on tubes with higher wattage speakers since they can inductively reflect a larger impedance swing back onto the plates of the tubes and short them out due to overvoltage. Those voltage limits in datasheets are stated as limits for a reason and the stored magnetic flux in an output transformer is no joke.
For simplicity sake it’s best not to pull tubes and half the secondary impedance when you have:
1) Old gassy tubes
2) Amps that have really high B+
3) Amps that are triode strapping the screen grids
4) When using dumb high efficient speakers with the capability of large inductive kickback.
Or any combination of the above. It’s a good way to short a tube out and blow a fuse.
And before the trolls begin trolling I do have an MSEE so be quick to think before you hit quote and reply to rebuttal any of this. It was my area of specialty in graduate school.
———————
The transformer doesn't have an impedance of its own; it only reflects a voltage (or impedance - just the square the turns ratios) from the primary winding to the secondary windings through a ratio of the number of turns of the secondary compared to the primary. Magnetic flux linkage in a ferromagnetic material due to Hopkinson's Law is the theoretical baseline as to why transformers work the way that they do.
The amount of magnetic flux in the ferromagnetic core is dependent upon current flow within the primary coil of wire, the number of turns on the primary, and the reluctance (magnetic resistance) of the ferrite core of the transformer. The current flow generated within the coil generates a magnetic field, which causes magnetic flux to follow the path of least reluctance. The magnetic flux then traverses through the ferrite material to the secondary windings; generating a current within the coils of the secondary through flux linkage. Voltage is then produced across the secondary terminals due to Faraday’s law of Induction.
In summary, transformers are passive lossy devices. Power in is power out minus losses. The impedance you’re changing on the secondary are strictly for maximum power transfer, stepping the primary tube impedances down to speaker load impedances. In essence, the transformer takes high voltage swings of transient plate currents from the tubes and transforms (hint, transformer) them into low voltage high current sources for the speakers.
I also want to note that there is a lot of magnetic flux stored in a transformer core - which can cause havoc to circuits when impedances are mismatched (primary OR secondary). This means mismatching secondary speakers to secondary windings reflecting an intended impedance from the plates, which may or may not be correctly matched with tube swaps to a particular transformer’s intended primary impedance, can cause transient responses which over frequency can be damaging at high volumes. Just keep that in mind.
—————
I’m in the camp that it’s perfectly fine to pull two tubes to double the primary impedance and then counteract the higher induced primary impedance by halving the secondary impedance. Just note that it’s harder on tubes with higher wattage speakers since they can inductively reflect a larger impedance swing back onto the plates of the tubes and short them out due to overvoltage. Those voltage limits in datasheets are stated as limits for a reason and the stored magnetic flux in an output transformer is no joke.
For simplicity sake it’s best not to pull tubes and half the secondary impedance when you have:
1) Old gassy tubes
2) Amps that have really high B+
3) Amps that are triode strapping the screen grids
4) When using dumb high efficient speakers with the capability of large inductive kickback.
Or any combination of the above. It’s a good way to short a tube out and blow a fuse.
Last edited:
MadAsAHatter
Well-known member
Sir, I reject your wealth of knowledge and substitute my own internet opinions. LoL.
The~Kid
Well-known member
If your James Brown (5150 JB, not the other one)......... or an amp tech or something and feel safe doing it, doing what your doing go ahead and do whatever......
But for me and well everyone else I really suggest you don't Mickey Mouse your amps and BS. And well you can take the risk and see if it works or not...... me Im going to play it safe and not do any such Mickey Mousing or "safe mismatching" whatever or pull tubes or whatever......
That shit is too cool for me and I'm just not that cool
But for me and well everyone else I really suggest you don't Mickey Mouse your amps and BS. And well you can take the risk and see if it works or not...... me Im going to play it safe and not do any such Mickey Mousing or "safe mismatching" whatever or pull tubes or whatever......
That shit is too cool for me and I'm just not that cool
Kapo_Polenton
Well-known member
I will say in my case that for my Randall Lynchbox, I have been running only 2 tubes in it for years now and it works without issues. The only time I have blown tubes is when i got crazy and decided i am going to crank this thing which was pointless because the Randall power amp is fairly clean and that feel is supposed to come in part, from the Vol side of in the preamp module. Once it was a blown tube, another time i blew an internal fuse so nothing would bias and all the tube safety lights were lit up. Now my 84 JCM800 100 watter, I am just leaving as is running 4 output tubes. It's also a case of knowing the amps in your amp locker. Some of my 800's mush out at too high a volume, others stay tighter. Same can be said about how vol affects your tubes lives.
R
rowboat
New member
I have an Acoustic 164 combo: 4 x 6L6GC @ 100 watts, or I & IV can be switched out of circuit via front panel rocker for 60 watts RMS output.
Excuse my duhh, but I'm stumped. Tech who worked on it strongly cautioned against using the half power option "without adjusting impedance".
Impedance where? The internal EVM is an 8ohm driver. There's a rear panel rocker for 8 or 4ohms, jack sockets for 8 or 4ohms..
When running the amp at 60 watts (tubes I & IV out of circuit), apparently that doubles the load on the OPT. Am I supposed to run two 8ohm speakers in parallel for a 4ohm load, and select "4ohms" on the rocker?
Excuse my duhh, but I'm stumped. Tech who worked on it strongly cautioned against using the half power option "without adjusting impedance".
Impedance where? The internal EVM is an 8ohm driver. There's a rear panel rocker for 8 or 4ohms, jack sockets for 8 or 4ohms..
When running the amp at 60 watts (tubes I & IV out of circuit), apparently that doubles the load on the OPT. Am I supposed to run two 8ohm speakers in parallel for a 4ohm load, and select "4ohms" on the rocker?
Attachments
Hiwatts-n-Gibsons
Well-known member
I know because THD Hotplates and Marshall Power Brakes were rated at 100 watts most assume that it's fine. I also know there are more than a few people out there who toasted good amps doing so. Personally I prefer being cautious when it comes to my 4 figure amps, and choose to go over-rated. My Mesa Powerhouse is rated at 250 watts iirc and my old Weber Mass 200 was rated at 200 watts obviously.
I had a couple Captor X's and they were constantly in the red whether running my Custom 100, VT22, or GT150, and tbt I felt the sound was neutered and suffered with any of my big amps when I opened them up and pushed the master up past halfway. I never felt comfortable cranking the master all the way. With my Rt 66 and Magnatone Panoramic they were great. Never had a 50 watt amp to try through them, but they should be able to handle that.
Hiwatts-n-Gibsons
Well-known member
I would look up the manual and see what exactly the rocker switch does before doing anything. In any event a total 8 ohm load whether or not it's one or multiple speakers would be run off a 4 ohm speaker tap. How exactly your amp would do that I'm not sure.
Is the EV hard wired or does it have a 1/4" plug wired to the end?
