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FDP Forum / Amp Mods, Repairs, and Projects / PRRI speaker load question.

Tygr1

Texas

Jul 5th, 2016 04:44 AM   Edit   Profile   Print Topic   Search Topic

Can a stock '65 Princeton Reverb Reissue handle a 4 ohm load?

pdf64

UK

Jul 5th, 2016 07:20 AM   Edit   Profile   Print Topic   Search Topic

The user manual advises to avoid a mismatch; so doing that may void the warranty, if it caused a problem.

In reality it would probably be fine unless you overdrive the power amp heavily.
At higher signal levels, the power tubes will be under more stress and their operational life may be reduced, compared to if they had their intended load.

(This message was last edited by pdf64 at 09:21 AM, Jul 5th, 2016)

Peegoo
Contributing Member
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Eat. Sleep. Guitar.

Repeat
Jul 5th, 2016 08:57 AM   Edit   Profile   Print Topic   Search Topic

Yeah, most Fender tube amps can handle a 100% Ohms mismatch...but not for continuous playing at high volumes.

The output transformer can quickly overheat, and if it shorts out it can take other expensive components with it.

Tygr1

Texas

Jul 5th, 2016 10:22 AM   Edit   Profile   Print Topic   Search Topic

Thanks kids!

ejm

usa

Jul 6th, 2016 06:52 AM   Edit   Profile   Print Topic   Search Topic

Why do you ask?
What are you thinking of doing?


Tygr1

Texas

Jul 6th, 2016 08:55 AM   Edit   Profile   Print Topic   Search Topic

Thinking of occasionally running it through my Fender 4x12 cab. Only at home.

Steve Dallman
Contributing Member
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Merrill, Wisconsin

Age is just a number...mine is big
Jul 6th, 2016 09:12 AM   Edit   Profile   Print Topic   Search Topic

Interesting that the manual states not to use anything but an 8 ohm load. I run my actual 65 PR (not a reissue) at 4 ohms all the time and have for years.

Traditionally, Fenders, blackface and silverface can handle a 100% mismatch without problems. I suspect the warning in the manual for the RI is due to EU electrical standards.

I would not worry as long as you are between 4 and 16 ohms. If you run a mismatch, lower is slightly better for the amp than higher but either is fine.

Fender has sold amps in the past decade that can handle mismatches but would not officially say so due to regulatory concerns. On FDP a long discussion went on years ago concerning the Pro Jr.

Viera
Contributing Member
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Santee CA

Poser extraordinaire
Jul 6th, 2016 03:08 PM   Edit   Profile   Print Topic   Search Topic

From the 65' Princeton Reverb Manual:

"N. EXTERNAL SPEAKER - Plug-in connection for an external speaker. This jack is wired in parallel with the INTERNAL SPEAKER JACK (M) and affects the speaker impedance load. Use 8 ohm minimum total. To use the external speaker output, first disconnect the internal speaker. Then connect a 16 ohm speaker load (minimum) to the internal speaker jack and another 16 ohm speaker load (minimum) to the external speaker jack."

Could someone please explain how to go about doing this?

I've been running my PRRI using an external cab with an 8 ohm speaker together with the 8 ohm internal speaker. Now I'm wondering if I'm screwing up. Should I be using 2 16 ohm speakers instead? I'm confused.

FWIW, I did read the "Ohms??" Notice/thread in the Amp Mods forum.

pdf64

UK

Jul 7th, 2016 03:40 AM   Edit   Profile   Print Topic   Search Topic

What impedance is your Fender 4x12?

Tygr1

Texas

Jul 7th, 2016 04:22 AM   Edit   Profile   Print Topic   Search Topic

The Fender cab is 4 ohms.

Peegoo
Contributing Member
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Eat. Sleep. Guitar.

Repeat
Jul 7th, 2016 05:02 AM   Edit   Profile   Print Topic   Search Topic

Viera

"Could someone please explain how to go about doing this?"

Speakers in parallel (generally) present an impedance load of the Ohms of one of the speakers (16 Ohms in this example) divided by the number of speakers--two in this case (16 รท 2 = 8 Ohms).

The instructions in the manual you cite require two external 16 Ohm speaker cabinets in order to achieve an 8 Ohm load. You cannot use the internal 8 Ohm speaker.

The way you're doing it with two 8 Ohm speakers in parallel presents a 4 Ohm load to the amp. As Steve states above, four Ohms will work without damaging the amp, but it's not optimal.

Running an amp with a speaker impedance mismatch is a lot like driving your car in first gear all the time: it makes the engine work harder than it needs to, it creates excessive heat, and it wears out parts faster.

If you always run your PRRI with an external speaker, consider using two 16 Ohm speakers.

(This message was last edited by Peegoo at 07:05 AM, Jul 7th, 2016)

pdf64

UK

Jul 7th, 2016 05:03 AM   Edit   Profile   Print Topic   Search Topic

It could be re-wired to 16 ohms.
You've got an 8 ohm speaker cab also?

But really, as long as you aren't cranking it, things should be fine if you carry on as you are.

Viera
Contributing Member
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Santee CA

Poser extraordinaire
Jul 7th, 2016 11:03 AM   Edit   Profile   Print Topic   Search Topic

Peegoo & pdf64

Thank you. I have one more dumb question if I may:

I actually have been running the amp at around 8 or 9 on the volume knob but with the guitar's volume at a low number with great sonic results (in a fairly small room at home).

Is the load on the amp determined only by the amp's volume level or does the overall volume (amp volume + guitar volume setting) make a difference on the load that the amp is carrying? I suspect the amp volume only, but I'm a dunce when it comes to the inner workings of amps.

Thanks again.



pdf64

UK

Jul 11th, 2016 12:15 PM   Edit   Profile   Print Topic   Search Topic

The audio power that the amp is putting out is dependent on the signal level at the power tube grids.
Every control in the signal chain, from pickup strength to power tube bias (and before it eg picking action) that affects that level therefore affects the power output.
So the amp may be putting out full power when set to '2' on the volume, or not making full power with every control up full, depending on the signal level it's being fed.
The particular number that anything is set to is pretty much irrelevant / does not correlate to a particular power level.

guitarcapo

U.S.A.

Jul 11th, 2016 12:20 PM   Edit   Profile   Print Topic   Search Topic

The external speaker jack is wired in parallel to the internal jack, I always assumed this to mean that you could plug an additional 8 ohm speaker to the jack. The amp would then see 4 ohms.

It never crossed my mind that the manual wanted the user to disconnect the internal jack and use two 16 ohm speakers so that the amp would still see 8 ohms.

What if you don't have two 16 ohm speakers?

Steve Dallman
Contributing Member
*********

Merrill, Wisconsin

Age is just a number...mine is big
Jul 11th, 2016 12:25 PM   Edit   Profile   Print Topic   Search Topic

I started playing guitar in 1967 and over the years ran many Fenders with extension cabinets at half the optimum impedance as did every other guitar player and band I knew, without damage to the amps.

As an amp tech (for Fender and a dozen other companies) since 1980, I've never repaired an amp that went down due to running an extension cabinet.

My personal amp is a 65 PR (actual 65 not RI) which I use with an ext cabinet and before I got this 10 years ago, I used my blackface 67 DR which I've had since the early 80's. My 65 blackface Bassman pushed two 4 ohm cabinets since 1971. And for bass, it was pushed hard.

I have repaired Marshalls that went down when pushed into mismatched loads, but never a Fender.

Viera
Contributing Member
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Santee CA

Poser extraordinaire
Jul 11th, 2016 01:20 PM   Edit   Profile   Print Topic   Search Topic

Excellent posts Steve and pdf64. I won't be rushing out and buying 2 16 ohm speakers. :-)

Thanks a bunch.

FDP Forum / Amp Mods, Repairs, and Projects / PRRI speaker load question.




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