'65 Pro Reverb project - part 3 - Speakers

[modusmongo]

I now have my amp running, and I need to bias the tubes, but that has to wait until the 2W 1Ω resistors arrive. In the mean time, I am building a 2 x 12" cabinet. I'll probably just use heavy plywood unless someone tells me that it's absolutely nessecary to use a hard wood.

My question is: can I use 2 8Ω Fender speakers - a 4Ω load? I will be ordering speakers and will go with whatever gives me the tone I want (a little dirty) without emptying my pockets (a little shallow). If the 4Ω load is only going to make tube life shorter, I don't care as I will be re-tubeing the amp, but if there are other dangers, please warn me! Since I have the 2 8Ω speakers on hand, I'd like to try them and see what it sounds like. I'd also be happy to get advice on what speakers to buy for the finished project (the Fender speakers need to go back into my blues jr and blues deluxe). I had a pair of 1970 JBL D120F's which I loved, but that is WAY out of my price range at the moment. I heard that the Emminence Texas series was good sound vs cost. Any suggestions? Thanks!

[riz]

Hey that's awesome you got it running! A quick google check reveals that you should have a 4-ohm transformer, so all good there. I've read that Weber makes a JBL clone and that folks like it, can't comment on their stuff myself.

[modusmongo]

Hi ho there. It's funny that you respond just at this minute. I was looking at the schematic for the amp. I'm sure I looked like HJS looking at the nuclear power plant manual - eyes all squinty, drooling a bit.

I couldn' find any mention of the ohm load in the schematic or on the back of the amp, but I was only looking at the speaker connection. Are you saying that the PT defines the minimum load?

So it's 4 ohms, and I can therefore wire in series in phase with 2 8 ohm speakers. Right?

I remember that you said you'd swap out the PT if you lived here (remember I'm using a step transformer), and I think that's what I want to do. Do you have any idea where I look for that? UK, I'd imagine, but I don't know what I'm looking for OR where to find it. What do you suggest?

Thanks

[modusmongo]

Hunh. I found this.

Is it the right one for my AA165?
Can I find it in Europe/UK?

Any other suggestions?

[riz]

That looks pretty nice, with the dual voltage primary--then you could use it in more of the civilized world! Others may have better suggestions, Mercury Mags stuff is nice but expensive. Never used their products, but if you want an original OEM vibe they're probably the best of money is not a huge concern. As for your speakers, you would wire two 8-ohm speakers in parallel, for a 4-ohm load, if your amp has the original output transformer, which, according to a quick google search of "fender pro reverb output" was a 4-ohm trans. Do your own double-check. Learn how to check your OT for reflected impedance, it's not hard, and it's educational. On a budget, you can do it with your computer as a voltage source (signal generator, described earlier) and a multimeter--I've done it. I've heard it said that Fender OTs are tolerant of a mismatch, but I don't see any GOOD reason to do that. If you can't figure it out I'll dig up some bookmarked links.

[modusmongo]

Thanks. I'll look into the "OT reflected impedance". I'm not concerned with the OEM look. I just want it to sound right and, preferably, have a 120/240 switch for when I travel.

[txstrat]

Your link leads to a PT (power transformer) but we're talking about a OT (output transformer), which sets the output impedance by it's winding ratio from primary to secondary winding.
If you have an output transformer with a 4 ohms tap you can wire two 8 ohms speakers in parallel to get those 4 ohms.

[modusmongo]

Thanks txstrat. We are kind of talking about both ;-) I need to be sure that the 4ohm speaker load is okay, and for that, if I understand, I need to test the OT. I also would also like to have a reliable 240v PT instead of using (and hauling around) the step down transformer.

[txstrat]

For a PT you could look here:
http://www.tube-town.net/ttstore/pro...er-Reverb.html
Although it's only 240v

[modusmongo]

Cool! Thanks. The 240v is the most important. Heck, if I take the amp back to the states, I'll put the 120 back in ;-)

Okay, I'm off to the hardware store to buy 20mm plywood for the cabinet. Found a good design here.

[riz]

Well, what PART NUMBER is on your OT? It should be stamped on it. That can tell you what it is, too. I've just been giving you ideas to help with the learning curve on this stuff, just in case you were interested... I'm not ALL THAT far ahead of you, but I have gotten REALLY into this amp stuff lately, and I'm really starting to put the puzzle pieces together. If you just want to fix this amp, and don't care much for getting into the tech stuff, the part number should suffice, if it's an original Fender OT. You would need to do the check if it's an unknown trans.

[riz]

Hey, I read the link wrong, and mistakenly thought that was a Mercury Mags trans. My bad! I have only bought trannies from a local shop, so I'm not the best guy to comment on where/who to order from.

[modusmongo]

If the OT is the bigger of the two trannies between the filter cap box and the PT, then mine is a 125A7A. I saw replacements for the 125A6A, and A7A on several sites (all referencing the black face pro reverb). According to this site, the 125A7A is 4 ohms. Can anyone confirm this?

Also, it seems that most pro reverbs had the 125A6A. Mine is a very early one (SN A00167). Maybe the original OT failed and was replaced? Maybe Leo threw in whatever he had in his garage?

The PT is a 125P5D which corresponds to the part number I found and also to that which txstrat found for sale here in Europe. Cool. I'll be buying one soon. Anyone want to take my 125P5D off my hands? SN 606523 "CSA-TEST" (whatever that means).

Thanks a lot for your help. And yes, I am interested in the big picture. It's a bit difficult to be so helpless ;-) Hopefully I'll get up to speed after a while.

[txstrat]

According to this site, the 125A7A is 4 ohms.

According to the same document the 125A7A is also 8 ohms (look for Vibrolux 6G11 or Vibroverb 6G16).

Mercury Magnetics claims an output impedance of 8 ohms for their replacement OTs.

See:Mercury Magnetics - Fender amp transformers

Both of them OTs should have the same (or almost the same) primary impedance of 4000 ohms and you can calculate the unknown impedance. Look here:

Calculating output transformer impedance | Tube amps DIY

If you plan just to use your two 8 ohms speakers in parallel (at 4 ohms then) you could get away with it like that, even if the secondary is 8 ohms.
If you plan to use a second cab with two 8 ohms speakers in parallel (at 4 ohms then) you would end up at 2 ohms and that would be BAAAAD for the OT.

[riz]

I found some charts online that had the 125a7a trans listed for the early Pro Reverbs. It was also used in some older amps, and looks to be something of a valuable piece because of that. 8 ohms. I think you should keep your original PT, but I'll happily take it off your hands!