Another tube amp question in the Stomp Box Forum...

[BrianJ]

I've been posting at the tube amp section of the "music electronics forum".  Replies are few and far between so I figured, given the superior activity of this site, perhaps I'd get some advice here instead.

I'm buying a Blackheart Little Giant amp head (if you haven't heard of them, check out the Valve jr's worst nightmare...).  I want to replace the OT with something of higher quality and I started shopping around.  The most expensive option was a Mercury Magnetics OT for $160.  The question actually doesn't involve price/performance at all.

I emailed them for a recommendation and the model they suggested has a 9K primary (they actually had a $325 "Mod" kit w/ OT, PT and choke - no thanks).  I was under the impression that 5w class A EL84 amps usually employed a 5k primary.  What impact do you guys think may result?  I am all for trying and seeing for myself but I am concerned about running my EL84 through an unhealthy load.  Wouldn't a 2X primary impedance have a similar effect as running a 2X speaker?  Of course, they claim that many people have installed this unit and love it.

Any thoughts?

P.S.- Anyone thinking of adding some value to their VJ cheap should consider throwing in a Hammond 125 SE OT.  I used the 10 watt model and the improvement in quality was far superior to the $35 price tag and single drilled hole.

[Jered]

  Have you posted this question at  http://www.ax84.com/bbs/index.php  ?  These guys can get into detail about what your options are,...and what not to do.
  Jered

[Hanglow]

Can you not just use a hammond 125ese?  The blackheart is essentially a modded VJ.... You will probably need to alter some resistors for voltage and maybe the tonestack values after putting it in

Go to  sewatt.com to find out

[petemoore]

  Used HiFi stuff...what I used anyway...
  Works great and extra cool mojo IME, lots less than a MM Tranny.
  But I'd guess a MM Tranny would be all mojo too.
  Wouldn't a 2X primary impedance have a similar effect as running a 2X speaker?
  Letsee the math goes....been a while... 
  Damify can remember the equation for sure.
  There's a square root in there too though...
  Here: I remembered it just fine...
  Get a low voltage AC transformer (about 10V or less) and place it across the speaker taps of the output transformer (OT).
Next, measure the voltage across the speaker taps with a multimeter.
Now, measure the voltage across the primary side of the OT (plate-to-plate).
Divide the measured primary (plate) voltage by the secondary (speaker) voltage. This number is the turns ratio.
To determine the load resistance for the desired given speaker load, square the turns ratio you calculated and multiply that number by the desired speaker load (i.e. 4, 8, 16 ohms).


 

[Zben3129]

Well lets see, single el84 S/E output stage.

At 250 volts on the plate in SE, you will get an output impedance of 5200 ohms. So, once you adjust that for your plate voltage, you can determine roughly what the primary should be. A hammond 125ese is going to be your best bet.

Also, MM mod kits/transformers are generally frowned upon by the amp community, because

a. They are way overpriced.
b. They Can be bad for your amp.
c. They make you buy unnecesary parts

a. Think abnout the price of the mod kit.
b. Valve Jr mod kit, for example, runs the tubes WAY to hot with the resistor substitutions they give you. And possibly the BH5 OT, haven't seen it yet
c. Choke for a VJ? Comeon...




Zach

[BrianJ]

The Mod Kit was always out of the question - I'll be damned if I'm going to buy a boutique PT and choke - that's why I emailed them to give them a chance at selling something reasonable.  The MM "Value" is questionable indeed but certainly comes second to functionality.  So my question has been answered:

A 9k primary, even at 350V, is excessively high for a SE EL84 amp.

I can testify that the Hammond 125 series will make a world of difference.  I was just going to give the bigger is bester approach one shot in my years of DIY.  Maybe heyboer has a reasonably priced compatible OT...

Whatever I select it will be a 5k primary.  Thank you to everyone who replied.

[m-theory]

From what I've read, both the BH and the v3 Vj's have fairly decent OT's.  You'd probably hear a difference with an ESE, and if you want to mod for octals, that's the one to go with, but a DSE probably won't yield much, if any difference, and a CSE would actually be a step down. 

I've heard clips of both, and I actually think the v3 Vj sounds better than the BH, although I've not heard either up close and personal.  I've got a Marshall-modded Vjr that I absolutely love.  I really wouldn't have a use for the 5 watt BH, although I'm in for one of their 1/2 watters when they're finally released. 

[BrianJ]

The DSE is the model I put into my VJ.

The Primary Z is 5k (The "improved" value that the Version three units have and mine lacked), and it is nearly twice the size of the VJ V2 OT.  I was doing the swap as an experiment to see how much of a difference would be made and I was shocked. 

I actually bought the transformer for a different project but now it will need to be replaced.  I haven't been more certain that something was worth the time/money in a long time.


I broke down and ordered a Mercury Mag OT (a 5k primary model).  I can report back on the value if anyone is interested.