Question about Pro Reverb OT impedance

[Regis]

I have a 68 Pro Reverb that I had Ted Weber recone the original Oxford speakers in because one was blown and the other was fuzzing out. I've been playing it for a while.

This amp had a weird circuit in it, with a bias balance pot and 150 ohm cathode resistors on the power tubes, as well as the snubber caps.
When I first got this amp I couldn't find a schematic that matched, but just found that it is probably an AB668.

According to the schematic it has an 8 ohm output transformer, and when I had Ted recone the speakers I assumed it was a typical Fender with a 4 ohm output tranny and had him rebuild them as 8 ohm speakers for a 4 ohm load. Now it seems I'm running them on an 8 ohm transformer.
Fender was capable of anything back then, so who knows what I've got here.

Question is, how do I know for sure? Do I do that test where you put an AC voltage through the transformer and use calculations to figure the output impedance? Or is there a simpler way to tell?

If it's an 8 ohm tranny I'll have to either change the transformer, recone the speakers yet again, or buy new speakers.

[PRNDL]

is there a simpler way to tell?

Are there any numbers stamped on the transformer?

Also, you have to rewire the speaker coil to change the impedance, which is a lot more work and expense than a recone. I'd recommend checking the speaker impedance with an ohmmeter.

Fenders are able to handle the mis-match of 4 instead of 8 ohms. The different impedance reflected to the output tubes will, however, change the way they sound, especially on breakup.

If you're really concerned, a simple solution would be to buy a new output transformer. They're not that expensive and are fairly easy to install.

[Bruce / Mission Amps]

Question is, how do I know for sure? Do I do that test where you put an AC voltage through the transformer and use calculations to figure the output impedance? Or is there a simpler way to tell?

If it's an 8 ohm tranny I'll have to either change the transformer, recone the speakers yet again, or buy new speakers.

Yes you should just do it and cut your losses.
Sell those speakers on Ebay (they will sell) and buy a new pair of 16 ohms speakers.
A good set of speakers in this amp are the newish, Eminence;
Red white and Blues or the Texas Heat.

http://www.partsexpress.com/pe/showd...number=290-818

http://www.partsexpress.com/pe/showd...number=290-822

[Regis]

Thanks for the replies, guys. I'll check the numbers on the transformer when I get home tonight.

Well, we have one vote for new speakers, and one vote for a new transformer. Anyone want to break the tie?

I looked transformers over, they range from Brand X at about $35.00 to $100+ for MM. Some of them have dual 4 and 8 ohm taps.

People say changing the OT will give more bottom end and headroom but I don't really want more headroom, I like the amp breaking up early the way it does now. Would I lose that if I got a fancy new OT??

Maybe I'll get luck and it will turn out to be a 4 ohm OT after all, I'll report.

Thanks,

Regis

[Jag]

Except for the fact that I don't really like Oxfords, I'd say change the OT.

Since the fact is I don't really like Oxfords, I'd say change the speakers.

How's that for stiiting on the fence?

Actually, to me, that really would be a real coin toss. I'd probably go with the speaker change.

[52 Bill]

According to the schematic, the output transformer is a 125A6A. This is the standard Pro Reverb, Bandmaster, etc. transformer rated at 4 ohms.

I don't know why the schematic shows 2-16 ohm speakers.

Check the transformer number first to see if it's a 125A6A/022848.

[Regis]

According to the schematic, the output transformer is a 125A6A. This is the standard Pro Reverb, Bandmaster, etc. transformer rated at 4 ohms.

I don't know why the schematic shows 2-16 ohm speakers.

Check the transformer number first to see if it's a 125A6A/022848.

I checked the amp and the transformer reads 022848 606820, so that means it is a 4 ohm OT from 1968?

Goddam Fender anyway, getting me all shook up like this. Maybe they actually put a few 8 ohm OT's in some PR models, god knows why.

About my question earlier regarding what the difference would be if I put a beefier OT in this amp, would I lose my early breakup? It would be nice to get some better bottom but I don't want to lose the early breakup I get now.

Thanks all,

Regis

[stokes]

I would check the OT like you said,putting a .5vac signal on the secondary and measure the voltage on the primary etc.... If it turns out to be the 8ohm tranny like you suspect,wire the speakers in series for a 16ohm load,its true that the tranny can stand the mismatch,but it will be better for your tubes,they will run a little cooler with the 16ohm load on the 8ohm out,and you just might like the sound and it wont cost you anything.If it is the 4ohm tranny,then you are good to go as is.Fender did a lot of things that dont always follow the playbook.

[mbratch]

The AB668 uses the same OT as the blackface amp. I think Fender marked the schematic incorrectly.

That being said, you can be off by 1/2 or x2 with the impedance and not hurt anything.

[Dave Curtis, dB AudioTech]

I would check the OT like you said,putting a .5vac signal on the secondary and measure the voltage on the primary etc.... If it turns out to be the 8ohm tranny like you suspect,wire the speakers in series for a 16ohm load,its true that the tranny can stand the mismatch,but it will be better for your tubes,they will run a little cooler with the 16ohm load on the 8ohm out,and you just might like the sound and it wont cost you anything.If it is the 4ohm tranny,then you are good to go as is.Fender did a lot of things that dont always follow the playbook.

A lower impedance mismatch is generally accepted as better than an upward mismatch on a tube amp. That's why Fender uses a shorting jack for the main speaker and parallels the extention speaker jack. Marshalls tend to be less forgiving of upward mismatches than Fenders (flyback/arcing).

What you described is true for solid state amps, where higher impedances (to infinity) are easily handled, and lower impedances are destructive.

[Fretts]

The AB668 uses the same OT as the blackface amp. I think Fender marked the schematic incorrectly.

That being said, you can be off by 1/2 or x2 with the impedance and not hurt anything.

Well, you're better off loading the OT x2 than by 1/2. It's easier on the tubes and OT.
I agree, I think the 16 Ohms was a mistake. Or maybe it was a plan that never happened, for whatever reason.

Better tranny ought to give you bigger low end punch and possibly more open high end,... your crunch is more a tube thing than a tranny thing for the most part.

[pdf64]

'Well, you're better off loading the OT x2 than by 1/2. It's easier on the tubes and OT.'
What's your rationale for this assertion?

[Fretts]

I inquired at Magnetic Components about connecting different loads to my 4 ohm output transformer and got a reply from VP Of Engineering Mark Sacketti. In a nutshell, going lower will increase the current flowing and place a burden on both the OT and the power tubes; going higher won't hurt anything, you just won't get all of the power. Here's an edit of what he wrote me:

"...Putting a 2 Ohm load on a 4 Ohm secondary also halves the primary impedance. This will increase the currents in both windings by over 40%. That will double the copper losses in each winding, resulting in a large increase in the temperature rise. Yes it will work. How hot it will get depends on the power level and the duration you play. You stand the risk of burning out the output transformer and the tubes. The same risk potential exists for the power transformer also, since you are increasing its burden as well. This is why there are different impedance taps and secondaries. .."


And going the other way, asking about a 2 ohm OT and connecting a higher 4 ohm load to it...

"...Putting a 4 Ohm load on this transformer will not cause any problems whatsoever, except you will not be able to get full rated power out of the amp because of the mismatch...."

This post by RG looks at what can make an OT die, and it ties in with what Mark has advised...
http://music-electronics-forum.com/t19729/#post162637

[Bruce / Mission Amps]

Mark and Joe from Mag Com have worked with me on many transformers and they are rather conservative.
And I suspect that 40% increase was if the amp could deliver the same actual output power with that load ratio.
I'd rather go one step low on high tap myself but I've never had an OT failure for doing either. No load or real high load on a low tap, yes, I think that can cause damage.

[Fretts]

@ Bruce...
I haven't put this to any real rigorous tests other than hooking just about anything handy to a Fender amp at one time or another. I always sort of crossed my fingers and hoped for the best, and in fact nothing untoward ever actually happened to me. But if we were to dispense advice on what's advisable and what isn't, don't you think Mark's opinion is reasonable and electronically sound? We may not always encounter amps with conservatively built transformers that can accept intentional abuse, wouldn't you say?