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FDP Forum / Fender Amps: Vintage (before 1985) / 1968 Blackface Pro Reverb Speaker Help

peacock5150

usa/ca

Sep 20th, 2014 11:46 AM   Edit   Profile   Print Topic   Search Topic

First of all this sight is awesome: I have a quick question I have a 68 Fender Proreverb and I have a 4 OHM OT can I safely connect a 4 0HM cabinet in the Extension input and play both onboard and cabinet speakers at the sametime and not fry the OT .tHANKS FOR YOU HELP

Guitar Fool
Contributing Member
**********
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Sunshine State

Just a pawn in someone else's game
Sep 20th, 2014 12:00 PM   Edit   Profile   Print Topic   Search Topic

I can't answer your question...someone will soon though I'm sure...

but i did find a cool link to your amp...
the information may be of use to you

68 Pro Reverb specs

BbendFender
Contributing Member
*******

Country city guy

I like to bend a B.
Sep 21st, 2014 05:58 PM   Edit   Profile   Print Topic   Search Topic

You probably can but I would not do it myself. You might hook up an 8 ohm cabinet in the ext. slot but with a 2 ohm load on a 4 ohm OT, you might be pushing it.
Again, someone (tech) will give you better info.

Roger Ball
Contributing Member
**********
***

Canada

New guitars and old amps!
Sep 21st, 2014 07:37 PM   Edit   Profile   Print Topic   Search Topic

Any Fender amp from that era should be able to handle a speaker load from 1/2 to 2x the nominal impedance. So yes, a load of 2 ohms made up from the the internal speakers plus an external cab should be OK.

sunnydaze

Taipei, Taiwan

Sep 22nd, 2014 01:59 AM   Edit   Profile   Print Topic   Search Topic

The stock cab has two 12 inch 8 ohm speakers wired in parallel for a 4 ohm load. You could re-wire the stock cabinet from parallel to series (makes it a 16 ohm load) and then add the 4 ohm extension cab. That would change the overall load to 3.2 ohms - pretty close to the 4 ohms the OT is expecting in that circuit.

Can't tell from your message whether the extension cab is 4 ohms from one or two (or more speakers). Depending on how your extension cab is set up, may be able change the extension cab to series wiring (and increase the load) instead of changing the stock cab.

mike

(This message was last edited by sunnydaze at 04:03 AM, Sep 22nd, 2014)

6G6

Texas

Fender power to the people!
Sep 25th, 2014 01:21 PM   Edit   Profile   Print Topic   Search Topic

Rodger for the win.
Those old Fenders were pretty forgiving.

sunnydaze

Taipei, Taiwan

Sep 25th, 2014 10:55 PM   Edit   Profile   Print Topic   Search Topic

The OT can probably take the mismatch, although the stock Pro Reverb OT is not exactly a beast in terms of size. Going down to a 2ohm load, as opposed to going up to an 8 ohm load, is going to be tough on the output tubes.

That said, I've got a '68 Pro Reverb, and I mostly run it with the stock cab 2 x 12 at 4 ohms, but I don't give it much thought if I want to add a second cab. Sometimes I do that at home when I'm noodling around, or comparing amps/equipment. It's a silverface, but it's an early '68 and had the blackface circuit. But that's me, I've built several amps (too many if you ask my wife) and don't worry about stuff too much, am certain I can easily repair any damage I might cause.

Mike

Hammond101
Contributing Member
******

So. Cal. USA

Sep 26th, 2014 10:24 AM   Edit   Profile   Print Topic   Search Topic

"The OT can probably take the mismatch, although the stock Pro Reverb OT is not exactly a beast in terms of size. Going down to a 2ohm load, as opposed to going up to an 8 ohm load, is going to be tough on the output tubes."

That is exactly backwards of how it really work in a tube Fender. The OT will be much happier at 2 ohms that at 8 but it is still doable. A 100% mismatch is no issue for a Pro reverb.

sunnydaze

Taipei, Taiwan

Sep 28th, 2014 10:30 PM   Edit   Profile   Print Topic   Search Topic

"That is exactly backwards of how it really work in a tube Fender. The OT will be much happier at 2 ohms that at 8 but it is still doable. A 100% mismatch is no issue for a Pro reverb."

I didn't say it was tougher on the OT, said that 2 ohms is tougher on the tubes. Pretty sure that's true.

If you get bored, check this out. Jammin' John lays it out about as well as I've seen.

http://www.tdpri.com/forum/amp-central-station/197323-speaker-impedance-mismatch.html

Mike

(This message was last edited by sunnydaze at 02:53 AM, Sep 29th, 2014)

sunnydaze

Taipei, Taiwan

Sep 28th, 2014 11:10 PM   Edit   Profile   Print Topic   Search Topic

In the interest of full disclosure, I just came across this wisdom from the Valve Wizard (Merlin Blencowe:

http://www.valvewizard.co.uk/se.html

"Remember though, the load line drawn is only correct provided the right impedance speaker is plugged in. Connecting a higher impedance speaker will cause the load line to rotate anti-clockwise around the bias point, possibly causing screen-grid failure due to passing below the knee of the grid curves (although if you're lucky, the screen resistor will fail open first). It can also cause arcing in the transformer due to much higher anode voltages being developed when the valve is overdriven. Connecting a lower impedance speaker will have the opposite effect; the load line will become more steep, pushing the valve into cold Class A operation which may or may not cause over dissipation of the anode (thankfully it usually doesn't). It is therefore always safer to plug in a lower impedance speaker than a higher one, if you have to"

I have all of Merlin's books, but the above excerpt is not in his books, its only on his website. I knew that higher impedance mismatch can cause flyback voltages to damage the OT, but at the same time, have always heard lower impedance mismatches are harder on the tubes.

Merlin is saying the lower impedance mismatch is tough on the tube screens. That's the first time I've seen anyone make that statement. I'll take it as accurate - Merlin is truly a valve guitar amp expert, and I've learned a lot from his books.

Mike

(This message was last edited by sunnydaze at 06:28 AM, Sep 29th, 2014)

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

Merrill, Wisconsin

Age is just a number...mine is big
Oct 1st, 2014 03:53 PM   Edit   Profile   Print Topic   Search Topic

Your Fender will happily endure a 2 ohm load. You will lose a couple of inaudible watts (which will be made up for by the increased speaker area) and the power tubes will run a bit hotter, but no damage will result.

Plug in your cabinet and have fun. We've been running 4 ohm Fenders at 2 ohms for decades without fail due to the mismatch.

Don't do the same with a Marshall.

Boogies don't care about mismatches.

valdee

USA

Dec 22nd, 2014 08:00 PM   Edit   Profile   Print Topic   Search Topic

Yes Sir have been running ext cabs in Fender BF since the sixties and we didn't know 8ohm from 16ohms back then same with tubes we would go to Thrifty drug stores and just by tubes put them in and rock on never seemed to have issues,Oh yeah the good old Days where good!


Peegoo
Contributing Member
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The band is awful

and so are the tunes
Dec 23rd, 2014 03:04 PM   Edit   Profile   Print Topic   Search Topic

Steve's right.

If you want to be conservative though, don't play the amp at full whack on the volume knob. That makes it a whole lot easier on the power tubes and OT.

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

Merrill, Wisconsin

Age is just a number...mine is big
Dec 23rd, 2014 03:12 PM   Edit   Profile   Print Topic   Search Topic

I rewired one of my Bassman 100's to use 6550's. That and an increase of B+ capacitance made it into a giggable amp.

I ran two 4 ohm cabinets for a long time before I found out the 6550's changed the ideal speaker load from 4 ohms to 8. My 2 ohm load was a 200% mismatch, yet the amp ran fine.

I never went below 4 ohms after I found out my mistake. When I ran the two 4 ohm cabinets after that, I used a series box which resulted in the ideal 8 ohm load.

I detected no difference between the 2 ohm or 8 ohm load in tone or performance.

I don't recommend going further than a 100% mismatch.

valdee

USA

Sep 11th, 2017 08:49 AM   Edit   Profile   Print Topic   Search Topic

Thats cool

FDP Forum / Fender Amps: Vintage (before 1985) / 1968 Blackface Pro Reverb Speaker Help




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