Transformerupgrade for Blues Junior to 30w

[Oddvar]

Does anyone know what transformerupgrade to use for Blues Junior to 30w?  With 4/8/16 ohm output?

[sluckey]

Does anyone know what transformerupgrade to use for Blues Junior to 30w?  With 4/8/16 ohm output?

You will need more than just a transformer to get 30 watts.

[tubenit]

Blues Junior might get 18w with the two EL84 tubes.  It won't get 30w with two EL84's.

with respect, Tubenit

[Oddvar]

So, is there anyway to rebuild it to 25-30 watts?  Other tubes etc?

[sluckey]

Sure. But you will need a different PT, OT, and output tubes. 6L6s or EL34s are common and cheap. But this is a lot of work if you plan to use the BJ chassis and may be more trouble than it's worth. IMO you would be better off to start with a 2 x 6L6 amp or just do a total scratch build.

Why are you focused on 25-30 watts? That 18 watt BJ should be loud enough to make your neighbors mad.

[Oddvar]

Trying out possibilities of making it loud enough for a band situation.  I see that BillM has done som work to make it more towards 30W, but the transformer he has made only take 120volt.  So I thought maybe there is a transformer ot and pt that could drive 2 or maybe 4 EL's, or maybe som other valuetubes?

[Ed_Chambley]

You can make a Blues Jr take 4 EL84's, but if you are playing in a band today where your stage volume is too loud using 15 watts you have other problems.  Best thing you can do is get you a Eminence Wizard which has a sensitivity of 102 db.  Just the speaker over the stock one in a Blues Jr will increase the amps volume a lot.  Probably more than adding different power tubes.

[HotBluePlates]

Trying out possibilities of making it loud enough for a band situation.  I see that BillM has done som work to make it more towards 30W, but the transformer he has made only take 120volt.  So I thought maybe there is a transformer ot and pt that could drive 2 or maybe 4 EL's ...

The next step up in power would be either 2x 6L6's or 4x EL84's, each of which will use essentially the same output transformer.

But the bigger output tubes (or double-tubes of what you have now) means more heater current required. And since your overall high-voltage (B+) supply voltage won't be doubled, you need double-current for that winding as well.

I'm losing track among the several threads you have going on the Blues Jr... Are you ditching the circuit board and/or changing the overall amp, or just wanting to make it louder?

I ask because the power transformers for the Blues Deluxe & Blues Deville (the natural next-step up) have supply voltages as much as 150v higher at some points. Ordinarily, I would have recommended using one of them, because they also have the lower-voltage winding needed for the solid-state used in those Fender models (including yours).

But they output a B+ as much as 150v higher than your Blues Jr, which means you need to also rework the power supply (filter cap values/arrangement) to make them work. That will be at least a hassle on a p.c. board.

Which then makes me think the shortest (and probably cheapest) path is to sell the Blues Jr & buy a Blues Deluxe or Blues Deville. Once you buy 2 transformers, 2 more tube sockets, a UniBit or punch to make the holes (will they fit?), and filter caps plus resistors... Well, you will likely exceed the difference between the price you could sell a Blues Jr and the price of a used Deluxe/Deville.

[DummyLoad]

use the allen amps TP25 for 2 x EL34 or 2 x 5881. use the TO26 output transformer with same tubes.

http://www.allenamps.com/parts.php#transformers

the PT is a drop-in replacement: the OT will require a refit. CHECK SPEAKER CLEARANCE! these are taller and wider than the stock parts.

--pete

[Shack]

wow, we all looking to redo the blues jr.... I understand what he says, cause I keep building the 15-20 watt amps and with my band, I cant hear them....I want this tone as loud as my HRD. I suspect Oddvar is in my situation too

[Oddvar]

Hi and thanks for great answers.  I am redoing the whole Blues Junior, as I have redone my two Pro juniors.  I have just built a Bassman 59 and redone Epiphone Valve and a VHT. Also working on the Hoffman 18w Marshall board.  Working my way throught the ampunderstanding buisness and I am now at the power stage.  I don't quite understand what makes the wattage work and I am curious to know if I could put in other tubes and a new transformer to make the Blues Jr a bit louder.  (As a hobby). A red that Billms PT TP24 with 2 6L6GC  = 30W.  They don't make the PT to work with 240v, so, is there an option elsewhere?  He doesn't seem to alter the board in any radical way? This is purely from a hobbyist point of view, working with layouts as guide.

[HotBluePlates]

... This is purely from a hobbyist point of view, working with layouts as guide.

I too started understanding amps with layouts. If you want to get somewhere, you'll have to go way beyond that, and learn some basic theory of how electricity & electronics work. The first 4 modules of NEETS will get you started (I'm sure you'll wander to a couple of the other modules).

I'm at a point where I don't like working from layouts; they just show me "this connects to that." Schematics show you how a circuit is arranged, without the issues of physical spacing, tube pinouts, etc.

...  I don't quite understand what makes the wattage work and I am curious to know if I could put in other tubes and a new transformer to make the Blues Jr a bit louder.

Power = Voltage * Current.

That equation can be manipulated with algebra to arrange it in different forms. We might combine it with Ohm's Law, Voltage = Current * Resistance; might might use algebra to rearrange either or both equations to solve for what we need to know.

If Power = Voltage * Current, and we need double-power but Voltage stays unchanged (perhaps because we're already using a voltage as high as practical or which the tubes will allow, which is typical in many guitar amps), then it should be obvious Current must double.

2*Power = Voltage * (2*Current)

How do you get the "2*Current" to happen? Let's rearrange the formula for Ohm's Law noted earlier. Current = Voltage/Resistance. To get "2*Current" to happen, we need half-Resistance:

2*Current = Voltage/(0.5*Resistance)

The PT determines the voltage available, but the high-voltage winding has a specific rated current capability. So for the 2*Current to occur, the new PT needs same-voltage and double-current of the original.

But what's the "Resistance"? It's the OT primary impedance. If supply voltage stays the same, the OT primary impedance must drop by half, or no additional current can flow.

And the output tubes are the medium by which we draw current from the PT and control its flow through the OT primary impedance. So the output tubes need to be capable of the new 2*Current as well. Amp manufacturers don't typically leave a lot of unused capability, in the output tubes, so we probably will need either twice as many of the same tube-type, or same-number of a bigger type which can pass 2*Current.

Prove the above by plugging in some numbers. Let's say your Blues Jr has 43mA RMS through the entire OT primary of 8kΩ in its original configuration.

Power = Voltage * Current, and Voltage = Current * Resistance, so Power = Current²*Resistance = (0.043A)²*8kΩ = ~15w.

If we halve OT primary impedance and double current, we get Power = Current²*Resistance = (0.086A)²*4kΩ = ~30w.

So the answer is you need a PT with same-voltage and double the current capacity, an OT with half the primary impedance, and twice as many (EL84 or 6V6) output tubes or same-number of a bigger tube type (like 2x EL34's or 2x 6L6's).

[jjasilli]

This is a recurrent topic, focused on the Blues Jr.  The problem as stated is volume.  But if you get the volume, the problem may become clean vs. overdrive tone.  I.e., if you have sufficient clean volume, the amp may be too loud when overdriven. 

Meanwhile, per Ed Chambley's Reply re doubling the amp's watts:  Let's assume you can get the Blues Jr. to 30 watts = 2X 15W the stock power of the BJr.  Ea doubling of amp power produces 3dB more of SPL, which is the volume level of sound being heard.  A 3dB difference is barely noticeable and may not be enough to cure your problem.

The stock speaker already has a high SPL of ~100. http://billmaudio.com/wp/?page_id=295   http://www.eminence.com/pdf/Legend_1258.pdf   A replacement -- chosen for volume, not tone -- might get you to 102.  Let's be optimistic and pretend that's 103.  This is another 3 dB increase caused by speaker efficiency.  Effectively, that makes a 30W amp into the new speaker as loud as a 60W amp into the stock speaker.

The stock speaker is rated for 75W.  So you could try an experiment.  Play a 60W amp into your BJr's speaker and see if you have enough volume in your band situation.  (Be lenient with overdive for the test as that will exceed the stock speaker's rating. 

[Shack]

Great explanations ....I think I learn more here than the books ive read on this.....what is explained here makes the OConner, Torres and Gerald Weber books more clear

[Oddvar]

So the answer is you need a PT with same-voltage and double the current capacity, an OT with half the primary impedance, and twice as many (EL84 or 6V6) output tubes or same-number of a bigger tube type (like 2x EL34's or 2x 6L6's).
[/quote]

Great answer for me, thank you.