# Will Blues Jr PT work in a PR?



## StevieMac (Mar 4, 2006)

Trying to get a PR chassis working again (no PT or OT) and someone has generously offered trannies from a Blues Jr. III. The obvious question then: As the PR has tube rectification and the BJ is SS, can I use that PT as a donor? I don't know the specs on the BJ III unit, other than this:

Fender Original Blues Junior Power Transformer. 15 watts. Equivalent to #049967.

Specifications
 Audio
15 W (RMS)
Primary
117 V, 60 Hz
Secondary
330 VDC, 68 mA after rectification
Filament Winding
6.6 VAC, 2.6 A
Mounting Centers
2" x 2.5"
Mounting Type
Horizontal
Weight
2.6 lbs

 

Steve


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## jb welder (Sep 14, 2010)

Quick answer, if you want to keep the tube rectifier the BJ PT will not work. Tube rectifiers require a dedicated winding on the PT for their 5volt heater.


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## StevieMac (Mar 4, 2006)

jb welder said:


> Quick answer, if you want to keep the tube rectifier the BJ PT will not work. Tube rectifiers require a dedicated winding on the PT for their 5volt heater.


Thanks. Since I wasn't necessarily concerned with the circuit being stock, and was considering "hot rodding" with a bigger OT for 6L6s, maybe that PT and SS rectification could still work? 

Steve


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## Wild Bill (May 3, 2006)

StevieMac said:


> Thanks. Since I wasn't necessarily concerned with the circuit being stock, and was considering "hot rodding" with a bigger OT for 6L6s, maybe that PT and SS rectification could still work?
> 
> Steve


Sure it would work! You just have to make a few allowances. First, as already mentioned, you need 5volts at a couple of amps from somewhere for a tube rectifier. If the PT can't supply this, then if you want to still use a tube rectifier then you need a small extra transformer. Hammond has some good choices and there should be a source somewhere near Kingston.

If you don't want to use a tube rectifier then keep in mind that a tube rectifier has an appreciable voltage drop that increases as the current draw increases. This is what gives a bluesy "sag" or compression to the tone when you crank it up. As the power demanded goes up the supplied voltage will be pulled down to the plates of the output tubes.

A solid state rectifier has no sag and thus sounds "tighter". 

A tube rectifer will drop at least 20 volts of supplied DC. For a pair of 6L6s you should use a 5AR4 or a 5U4. The 5U4 might lose 50 volts or more when you are wailing the amp loud! So for a High Voltage secondary of whatever voltage you will get that much more DC supplied voltage when you are using silicon diodes. In other words, if the trannie put 400 vdc on the output tube plates with a tube rectifier you can expect to see up to 450 vdc or more with no tube rectifier.

Naturally, your filter caps might have to have a higher rating and the tubes would be run much hotter. You will need to pay attention to the specs. 6L6s probably would still be within specs and work fine but 450vdc to 6V6s is a bit much and might hurt their lifetimes.

Wild Bill/ Busen Amps


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## StevieMac (Mar 4, 2006)

Thanks so much for your thoughtful response Bill. I knew I was missing something and of course you picked it up immediately...a small auxillary transformer could indeed be used to maintain tube rectification. That would also allow either a stock set up or the use of 6L6's, which draw more current than 6V6s I believe (not sure about draw from an EL84, which the BJ uses). 

There's new hope!

Steve


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## jb welder (Sep 14, 2010)

Hang on, the tube rectifier heater (5V) uses a different winding than the other tube heaters (6.3V). So using an aux. transformer to run the rectifier heater will not affect how many tubes the regular heater winding can run. You are still limited to the 2.6Amps total for the pre and power tubes. Unless you want to run a second aux. transformer lol.


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## greco (Jul 15, 2007)

jb welder said:


> Unless you want to run a second aux. transformer lol.


Why keep adding transformers...why not just go for one big one ?!

Cheers

Dave


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## WCGill (Mar 27, 2009)

6V6's would be ok in that amp (socket change of course), but 6L6's would be a waste as the transformer doesn't have enough power to realize the full wattage of the big bottles.


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## jb welder (Sep 14, 2010)

As WCGILL mentioned, your output power will be limited by the power transformer. Doesn't matter how big of an output transformer or power tubes you use. The most power you can get out with that PT is around 15 watts (maybe 20 with SS rectifier). "was considering "hot rodding" with a bigger OT for 6L6s". If you were wanting more power, you need to decide how much you want and choose the appropriate power transformer right from the start.


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## StevieMac (Mar 4, 2006)

jb welder said:


> Hang on, the tube rectifier heater (5V) uses a different winding than the other tube heaters (6.3V). So using an aux. transformer to run the rectifier heater will not affect how many tubes the regular heater winding can run. You are still limited to the 2.6Amps total for the pre and power tubes. Unless you want to run a second aux. transformer lol.


Yes, of course, I was obviously confused so my bad. There should be ample current for the PR's 6V6s however as they actually draw less than EL84s the BJ's PT is designed for.


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## Adicted to Tubes (Mar 5, 2006)

Come on guys! A blues jr PT cannot handle the heater current of a pair of 6L6's!!!!It's not rated for that.A pair of 6v6's will work just fine however.I have converted a Blues Jr into a princeton reverb circuit and it has a lowly 350v on the plates of the 6V6's and it sounded quite good,but definitely had lower output power.But that being said,it sounded sweet as can be.
Total waste to put 6L6's in the amp with a tiny little PT and OT from a blues jr.
Suck it up,save some coin and get some Hammond iron for your Princeton and do it right.


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## StevieMac (Mar 4, 2006)

So...what do you _really_ think? Don't hold back, as I'd like to hear your true opinion on the matter. *;^)

*To be clear though, I'm actually undecided which direction to take the PR project in. On one hand, I'm tempted to see just what can be accomplished with the BJ donor trannies, recognizing that final output would indeed be reduced. Hopefully, that would be offset with killer tone, making a sweet practice/recording amp...as you experienced. On the other hand, I already have a 40 watt OT and would only need a suitable PT and 12" loaded cab to turn the PR into a gig-worthy amp...with plenty of volume on tap. At this point then, things could still go either way....


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## nonreverb (Sep 19, 2006)

I'm surprised that little BJ power tranny can handle the extra filament load of the PR circuit...must get kinda hot...


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## StevieMac (Mar 4, 2006)

nonreverb said:


> I'm surprised that little BJ power tranny can handle the extra filament load of the PR circuit...must get kinda hot...


What extra filament load? I mentioned using an auxillary transformer to supply the rectifier tube. The BJ has 2xEL84 (700mA each) & 3 preamps (300mA each) or ~ 2.3 A, while PR has 2x6V6 (450mA) & 4 preamps or ~ 2.1 mA. Wouldn't it be expected to run cooler?


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## nonreverb (Sep 19, 2006)

Indeed! For some reason, I was thinking 6L6 after all this talk about them as an option...you are correct sir!



StevieMac said:


> What extra filament load? I mentioned using an auxillary transformer to supply the rectifier tube. The BJ has 2xEL84 (700mA each) & 3 preamps (300mA each) or ~ 2.3 A, while PR has 2x6V6 (450mA) & 4 preamps or ~ 2.1 mA. Wouldn't it be expected to run cooler?


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## StevieMac (Mar 4, 2006)

nonreverb said:


> Indeed! For some reason, I was thinking 6L6 after all this talk about them as an option...you are correct sir!


Could you please repeat that last part...ha ha! You're right about the BJ's PT being totally inadequate for the filament load using 6L6's. Think I'm gonna look into a higher spec PT...


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## nonreverb (Sep 19, 2006)

lol...always willing to admit a mistake! :food-smiley-004:



StevieMac said:


> Could you please repeat that last part...ha ha! You're right about the BJ's PT being totally inadequate for the filament load using 6L6's. Think I'm gonna look into a higher spec PT...


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## StevieMac (Mar 4, 2006)

Alright...decided to "suck it up" and get a proper set of trannies. As luck would have it, a deal too good to resist presented itself here on the forum: http://www.guitarscanada.com/amps-c...-champ-chassis-classic-tone-transformers.html . Should make the project quite a bit simpler. 

Steve


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## jb welder (Sep 14, 2010)

Proper set of trannies for what model? You understand that the OT is single ended and the PT is for a 15W tops amp? The PT filament winding is actually less than the BJ PT, 2A instead of 2.6A This transformer set is for a single ended Champ or Princeton. The princeton reverb is not single ended.


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## StevieMac (Mar 4, 2006)

Correct! I should have looked more closely at the OT specs as it clearly states "single ended". The description mentioned it being OK for 2x6V6 in _parallel_...which obviously threw me off. Oh well, perhaps the OT can be used on a later project. With regard to the PT, the description actually lists the PR among the amps it's intended for....despite the 2A total filament current. 

Worst case scenario, I sell off the "Champ" pair and return to the donor trannies. The OT is still fine for 15 watts. The PT will also still work. While I couldn't find a 5V filament tranny, I have a 6.3V 4A tranny that I think I can wire a 10W .68 Ohm wirewound resistor to in series with the 5AR4 rectifier filament...to reduce it to 5V.


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## jb welder (Sep 14, 2010)

StevieMac said:


> With regard to the PT, the description actually lists the PR among the amps it's intended for....despite the 2A total filament current.


 Sorry, my error, forgot that PR is only 15W amp. Maybe you should try it wired up with the 2 6V6's parallel, not too many like that around, you might like it!


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## StevieMac (Mar 4, 2006)

jb welder said:


> Maybe you should try it wired up with the 2 6V6's parallel, not too many like that around, you might like it!


Funny you mention that because now I'm SERIOUSLY considering that option. From what I've read, the parallel design works really well for harp, which is probably what I'd be using this smaller amp for anyway. Hmmm....time to hit the books!


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