Ampeg PF50T question

[mickebas]

Maybe I'm stupid but I just got my PF50T and there's nowhere a clear indication of how the impedance selector at the speaker input should be set for 4 / 8 ohm ?

 

[turcmic]

It is related to your cab impedance. Check the sticker on the back of your cab and match accordingly.

And no, you're not stupid That's a legitimate question

Have fun! Lots of people here say that's a great amp.

 

[mickebas]

It is related to your cab impedance. Check the sticker on the back of your cab and match accordingly.

And no, you're not stupid That's a legitimate question

Have fun! Lots of people here say that's a great amp.

It is related to your cab impedance. Check the sticker on the back of your cab and match accordingly.

My question was probably a bit unclear since I'm not english speaking. I know I want to use the 4 ohm option most of the time but to use the amp without a speaker, only taking the line signal for recording, it's supposed to be in the 8 ohm mode and what I don't really find anywhere in the manual is when is the little selector 4 ohms and when is it 8 ohms? There's really nothing anywhere that clearly indicates this. I guess I will eventually find out anyway but it would be nice to know from the start.

 

[two fingers]

It's actually a little switch. Take your fingernail and place it in the little slot in the switch. You should be able to move it back and forth with very little effort. Simply move the switch toward 4 ohm for 4 ohm operation, and toward 8 ohm for 8 ohm operation. The one in this pic is set to 8 ohms because the slot in the switch is set to the right. I couldn't find a clear image of the switch set to 4 ohms.

 

[mickebas]

Thanks for the photo! What I wasn't clear about was when the switch was in 4 or 8 ohm operation mode and I don't want to take any chances using it without speaker load
Now I get it. Great. Thanks

 

[two fingers]

Thanks for the photo! What I wasn't clear about was when the switch was in 4 or 8 ohm operation mode and I don't want to take any chances using it without speaker load
View attachment 3940758

That looks like a great little rig. Beautiful gear pic!

 

[JimmyM]

My question was probably a bit unclear since I'm not english speaking. I know I want to use the 4 ohm option most of the time but to use the amp without a speaker, only taking the line signal for recording, it's supposed to be in the 8 ohm mode and what I don't really find anywhere in the manual is when is the little selector 4 ohms and when is it 8 ohms? There's really nothing anywhere that clearly indicates this. I guess I will eventually find out anyway but it would be nice to know from the start.

That was a misprint in the original manual. You can use it without a cab regardless of whether it's set at 8 or 4 ohms.

 

[mickebas]

That was a misprint in the original manual. You can use it without a cab regardless of whether it's set at 8 or 4 ohms.

Aha. Great. Thanks JimmyM

 

[chupacerveza]

That was a misprint in the original manual. You can use it without a cab regardless of whether it's set at 8 or 4 ohms.

It's not just in the manual; it's in their videos as well.... Good to know and obviously better idiot-proofing.

 

[Wasnex]

That was a misprint in the original manual. You can use it without a cab regardless of whether it's set at 8 or 4 ohms.

I can't say this is not true as Ampeg may have decide the amp is safe with an impedance mismatch on the dummy load.

However the block diagram suggests the amp will see an impedance mismatch if the selector is in one position, and the impedance will be matched in the other position. Better to run with the impedance matched IMHO.

When a plug is inserted, the contact is lifted off the dummy load resistor. When the plug is pulled out, the contact drops back down on the dummy load resistor. AFAIK the dummy load is 8 ohms.

@hodgy indicated the switch should be in the 8 ohm position when no speaker is attached. See this specific post: HOT AMPEG NEWS! PF20T, PF50T, PF112HLF

The confusion was because the manual originally stated: "The 8 ohm position has internal load resistors, always ensure this position is set when no external load is attached." See this post from the same thread: HOT AMPEG NEWS! PF20T, PF50T, PF112HLF

The intent of the quoted statement is good, but it is not technically accurate because the dummy load connects to either the 4 ohm or the 8 ohm output, rather than just the 8 ohm output. I believe it is still good advice to say: Always ensure the 8 ohm position is set when no external load is attached. This will ensure the 8 ohm output is connected to the 8 ohm dummy load.

 

[Zbysek]

I can't say this is not true as Ampeg may have decide the amp is safe with an impedance mismatch on the dummy load.

However the block diagram suggests the amp will see an impedance mismatch if the selector is in one position, and the impedance will be matched in the other position. Better to run with the impedance matched IMHO.

View attachment 4118034

When a plug is inserted, the contact is lifted off the dummy load resistor. When the plug is pulled out, the contact drops back down on the dummy load resistor. AFAIK the dummy load is 8 ohms.

@hodgy indicated the switch should be in the 8 ohm position when no speaker is attached. See this specific post: HOT AMPEG NEWS! PF20T, PF50T, PF112HLF

The confusion was because the manual originally stated: "The 8 ohm position has internal load resistors, always ensure this position is set when no external load is attached." See this post from the same thread: HOT AMPEG NEWS! PF20T, PF50T, PF112HLF

The intent of the quoted statement is good, but it is not technically accurate because the dummy load connects to either the 4 ohm or the 8 ohm output, rather than just the 8 ohm output. I believe it is still good advice to say: Always ensure the 8 ohm position is set when no external load is attached. This will ensure the 8 ohm output is connected to the 8 ohm dummy load.

This is interesting. Could you please clarify what do you mean by "impedance matched"?

My understanding is that the whole discusson deals with situation where there is no speaker cab connected to the amp. In this scenario, how can be impedance "matched"?

 

[Wasnex]

This is interesting. Could you please clarify what do you mean by "impedance matched"?

My understanding is that the whole discusson deals with situation where there is no speaker cab connected to the amp. In this scenario, how can be impedance "matched"?

Everything I have read suggests the dummy load in this amp is 8 ohms.

Tube amps use an output transformer to match the high impedance of the tubes to the low impedance of the speakers. Because of this, a tube amp will make the same power at different impedances as long as it sees the expected load.

The output transformer in PF50T amp has two output taps. One tap expects a 4 ohm load and the other tap expects an 8 ohm load. Impedance is matched when amp sees the expected load. In other words, when you have the amp set up for 4 ohms and connect a 4 ohm speaker, the impedance is matched. Also when you have the amp set up for 8 ohms and connect an 8 ohm speaker, the impedance is matched.

If you have the amp set up for 4 ohms and connect an 8 ohm speaker, you have an impedance mismatch. In other words the amp does not see the expected load.

If you have the amp set up for 4 ohms and pull the speaker chord, the amp connects to the 8 ohm dummy load. This is an impedance mismatch, just like if you had the amp setup for 4 ohms and plugged in an 8 ohm speaker.

Some amps are safe with an impedance mismatch and some are not. Also some amps are safe with a 1:2 mismatch, but not a 2:1 mismatch; while other amps are safe with a 2:1 mismatch but not with a 1:2 mismatch. If the amp is not safe with a mismatch, the resultant damage can be extreme and expensive.

Even if the amp is safe with an impedance mismatch, this condition is harder on the tubes and reduces how much power the amp can make. The mismatch can also cause subtle changes to the way the amp sounds, and some players may find the difference pleasing. It's advisable to confirm the amp is safe with a particular type of impedance mismatch before exploring how the mismatch effects the amps performance.

I prefer to run my tube amps into the expected load, but I have run my Amp SVTs at 8 ohms in the past on a few occasions. The designer of the SVT says the amp can take it.

 

[Zbysek]

Everything I have read suggests the dummy load in this amp is 8 ohms.

Tube amps use an output transformer to match the high impedance of the tubes to the low impedance of the speakers. Because of this, a tube amp will make the same power at different impedances as long as it sees the expected load.

The output transformer in PF50T amp has two output taps. One tap expects a 4 ohm load and the other tap expects an 8 ohm load. Impedance is matched when amp sees the expected load. In other words, when you have the amp set up for 4 ohms and connect a 4 ohm speaker, the impedance is matched. Also when you have the amp set up for 8 ohms and connect an 8 ohm speaker, the impedance is matched.

If you have the amp set up for 4 ohms and connect an 8 ohm speaker, you have an impedance mismatch. In other words the amp does not see the expected load.

If you have the amp set up for 4 ohms and pull the speaker chord, the amp connects to the 8 ohm dummy load. This is an impedance mismatch, just like if you had the amp setup for 4 ohms and plugged in an 8 ohm speaker.

Some amps are safe with an impedance mismatch and some are not. Also some amps are safe with a 1:2 mismatch, but not a 2:1 mismatch; while other amps are safe with a 2:1 mismatch but not with a 1:2 mismatch. If the amp is not safe with a mismatch, the resultant damage can be extreme and expensive.

Even if the amp is safe with an impedance mismatch, this condition is harder on the tubes and reduces how much power the amp can make. The mismatch can also cause subtle changes to the way the amp sounds, and some players may find the difference pleasing. It's advisable to confirm the amp is safe with a particular type of impedance mismatch before exploring how the mismatch effects the amps performance.

I prefer to run my tube amps into the expected load, but I have run my Amp SVTs at 8 ohms in the past on a few occasions. The designer of the SVT says the amp can take it.

Thanks for the explanation