The Canadian Guitar Forum banner

1 - 20 of 22 Posts

·
Premium Member
Joined
·
190 Posts
Discussion Starter #1
Can I plug a head into a Silverface Deluxe Reverb as long as impedance matches? By plug in, I mean just the speaker into the head. Thanks.
 

·
Premium Member
Joined
·
17,325 Posts
In short, yes.
Some will say that 100% impedance mismatch is OK.
I would add that if you choose to mismatch play at low volumes for short durations.
 

·
Premium Member
Joined
·
11,846 Posts
In short, yes.
Some will say that 100% impedance mismatch is OK.
I would add that if you choose to mismatch play at low volumes for short durations.
I agree - a X2 mismatch (16:8 or 8:4) shouldn't be a problem, unless you are pegging the amp for a couple hours. Or unless it's a Mesa or Traynor - usually their iron is big enough to take the extra heat.

If it's a vintage head, I'd be extra cautious.
 

·
Premium Member
Joined
·
11,846 Posts
He said the impedance matches.
I saw that - and it is the best situation. But just in case he wants to try other combinations, a mismatch isn't the end of the world. Some people seem to think you introduce a tiny mismatch and ......

upload_2017-5-29_11-13-47.jpeg


I remember a time when transistors were the new, fragile technology and everyone was afraid of toasting them. We treated tube equipment like crap back then, and it just kept working anyways. I keep hearing comments like "I turned my amp on without a load for 2 seconds. Is it ruined now? Should I just buy a new one?" If it is ruined, just as well, because it was obviously a POS.
 

·
Registered
Joined
·
4,835 Posts
The other concern is power handling. You wouldn't want to crank a 100W or even 50W head into that (if it's stock).
 

·
Premium Member
Joined
·
2,078 Posts
In most cases, a mismatch either direction won't cause you too much grief....however, it can be a risky venture at higher volumes. Personally, I certainly wouldn't do a 100% higher impedance mismatch with anything from Dagnall. I've replaced several output transformers in Marshalls and many I suspect, were due to blasting with mismatched impedance.....just sayin'.
 

·
Registered
Joined
·
5,492 Posts
I saw that - and it is the best situation. But just in case he wants to try other combinations, a mismatch isn't the end of the world. Some people seem to think you introduce a tiny mismatch and ......

View attachment 90489

I remember a time when transistors were the new, fragile technology and everyone was afraid of toasting them. We treated tube equipment like crap back then, and it just kept working anyways. I keep hearing comments like "I turned my amp on without a load for 2 seconds. Is it ruined now? Should I just buy a new one?" If it is ruined, just as well, because it was obviously a POS.
No kidding. I know a certain guitarist unnamed, who played a 3 hour gig with a Super Reverb (2 ohm) into an 8 ohm speaker.
 

·
Registered
Joined
·
3,567 Posts
I think going from 2 or 4 to 8 ohm is probably ok, you'll just lose some volume?

it's going from 16 or 8 to a 4 ohm or 2 ohm load that can cause problems, with tube gear ( I believe? )
 

·
Premium Member
Joined
·
2,078 Posts
I think going from 2 or 4 to 8 ohm is probably ok, you'll just lose some volume?

it's going from 16 or 8 to a 4 ohm or 2 ohm load that can cause problems, with tube gear ( I believe? )
Not really. In a solid state amp, the lower you go impedance wise the more current the transistors will deliver to the point where in theory, they blow. Fortunately, most newer amps have a thermal cutout circuit which prevents this from happening. An increase in impedance draws less current from the transistors to the point when the impedance is high enough (no load at all) that they turn off.
In a tube amp, the lower you go from rated impedance, the more current the tubes have to deliver to the load. Basically, you wear the tubes out faster as they are delivering more than they are supposed to based on the mismatch plate impedance. Here the important part....when the load is too high the plate impedance is reflected back to the tubes via the output transformer and creates a situation where if the volume is loud enough the primary side of the transformer experiences flyback voltages that can be 1000's of volts due to the impedance being so high, much of the voltage basically cannot transfer to the secondary and the swing voltages increase. Eventually in extreme cases, the winding fails and opens up or shorts out.
The extreme scenario is blasting a tube amp with no load....that's a sure fire recipe for assured destruction eventually.
 

·
Premium Member
Joined
·
4,486 Posts
This is from Hughs and Kettner

 

·
Registered
Joined
·
3,567 Posts
Not really. In a solid state amp, the lower you go impedance wise the more current the transistors will deliver to the point where in theory, they blow. Fortunately, most newer amps have a thermal cutout circuit which prevents this from happening. An increase in impedance draws less current from the transistors to the point when the impedance is high enough (no load at all) that they turn off.
In a tube amp, the lower you go from rated impedance, the more current the tubes have to deliver to the load. Basically, you wear the tubes out faster as they are delivering more than they are supposed to based on the mismatch plate impedance. Here the important part....when the load is too high the plate impedance is reflected back to the tubes via the output transformer and creates a situation where if the volume is loud enough the primary side of the transformer experiences flyback voltages that can be 1000's of volts due to the impedance being so high, much of the voltage basically cannot transfer to the secondary and the swing voltages increase. Eventually in extreme cases, the winding fails and opens up or shorts out.
The extreme scenario is blasting a tube amp with no load....that's a sure fire recipe for assured destruction eventually.
so you can damage a fender super reverb ( designed for a 2 ohm load ) by playing it into a 16ohm cab?

I thought that scenario was fine...it was only dropping from 16 ohms down to 2 ohm load that would be sketchy
 

·
Premium Member
Joined
·
2,078 Posts
so you can damage a fender super reverb ( designed for a 2 ohm load ) by playing it into a 16ohm cab?

I thought that scenario was fine...it was only dropping from 16 ohms down to 2 ohm load that would be sketchy
Fender amps have good transformers in them and will take mismatches well....believe me, I've seen just about every combination imaginable. Having said that, there is always a risk and with certain amps is can be potentially lethal. One Fender amp which comes to mind is the old tweed or brown Super amps OT 45216 it was very prone to failure. It's one that did not take large mismatched loads well. As I stated earlier, neither do Marshall's.
Generally speaking, many output transformers will handle 100% mismatch however, caution is required.
 

·
Registered
Joined
·
5,492 Posts
so you can damage a fender super reverb ( designed for a 2 ohm load ) by playing it into a 16ohm cab?

I thought that scenario was fine...it was only dropping from 16 ohms down to 2 ohm load that would be sketchy
A super at 16 ohms gets very attenuated, with the output transformer burning off 30W or so as heat.
 

·
Premium Member
Joined
·
2,078 Posts
A super at 16 ohms gets very attenuated, with the output transformer burning off 30W or so as heat.
Not really cboutilier. It's actually the relationship between resistance,voltage and current via the transformer in a tube amp. The current would be higher if the load was say, 1 ohm in a Super Reverb in the example. Then the transformer would get warm. The opposite occurs with very high impedances on the secondary. No current flow due to the high impedance so the voltage rises. If the reflected impedance is high (let's say open) the voltage can rise into the 1000's of volts and as stated, will arc over from winding to winding effectively frying the transformer.
 

·
Registered
Joined
·
5,492 Posts
Not really cboutitier. It's actually the relationship between resistance,voltage and current via the transformer in a tube amp. The current would be higher if the load was say, 1 ohm in a Super Reverb in the example. Then the transformer would get warm. The opposite occurs with very high impedances on the secondary. No current flow due to the high impedance so the voltage rises. If the reflected impedance is high (let's say open) the voltage can rise into the 1000's of volts and as stated, will arc over from winding to winding effectively frying the transformer.
I can assure you that the OT gets hot. Ive run my Super Reverb into a 16 ohm Lady Luck
 

·
Premium Member
Joined
·
17,325 Posts
Here the important part....when the load is too high the plate impedance is reflected back to the tubes via the output transformer and creates a situation where if the volume is loud enough the primary side of the transformer experiences flyback voltages that can be 1000's of volts due to the impedance being so high, much of the voltage basically cannot transfer to the secondary and the swing voltages increase. Eventually in extreme cases, the winding fails and opens up or shorts out.
As simple as they may appear, I find transformers difficult to totally understand. I have read about them through time and I am sure that I am missing some very basic concepts.

"plates" and "laminations" are the same thing...correct?

"flyback" and "reflected" voltages are the same thing...correct?

What is a basic explanation of "swing voltages"? I went down a neverending rabbit hole trying to read about this.

@nonreverb does a fine job of explaining this and I have learned much from his responses regarding electronics theory on several occasions. Many years ago, he walked me through the steps of soldering a tinsel wire at the middle point. I was successful with this repair and was totally amazed that I was capable of such a task. Thanks again for all of your teachings in this forum.

Cheers

Dave
 
1 - 20 of 22 Posts
Top