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BlackHeart "Little Giant"

CB has forgotten more about tube amps than most of the rest of us know....

Got my pile of tubes in to start experimenting with in the Blackheart tomorrow, vintage Fender labeled GE 7025 & 12AT7 and a Gold series Groove Tubes ECC83 for V1 stage and a silver series GT EL84 for V2.

CB - The main appeal of the Mojo mod kit is to go to a tube rectifer, does that really make that much difference sound-wise?
 
I'm not familiar with the Mojo-mod...

But tube rectifiers, again vs silicon, are different.  Again, non-linear, and also the impedance is much higher.

Silicon rectifiers (diodes) act quickly and efficiently.  Their impedance is extremely low. They will deliver linear output on demand, until they reach the point of <poof>.  Tubes, on the other hand... are not linear at all.  Their impedance increases as the current increases. 

This means, on a tube rectified amp, its a bit like inserting a variable resistor into the power supply, whose resistance is non-linearly proportional to the current being used.

However, if you don't get that rectifier close to the bleeding edge, you wont see the non-linear characteristics of it.  Using a rectifier capable of 200ma or 225ma on an amp that only pulls 70ma, is about the same as having a solid state rectifier. 

The net result of the tube rectifier (and also the power transformer, and also to a lesser degree the first stage filtration) being sized "just so", is to introduce the all important "sag" to the power supply.  What that means is big current draws show a decrease in voltage that is disproportionate to their current increase.  Sag.  Some folks call it "envelope", but thats more a term used for describing the acoustic properties of the output, rather than the electrical properties of the power supply.

To players, this is how a note blossoms when you dig in, how the tone (dynamics really) becomes compressed and squished at volume.  How you can play clean with the volume on the guitar turned down, then flick your pinkie on the volume pot, and get a totally different beast.  Its the "touch" of the amp, its feel and dynamics.

So, cant say on that kit.  Link me?  Maybe I can give it some looksee for ya.
 
The mojomusicalsupply.com website appears to be down right now, a link to the funky Google cached version is below; not a lot of details, would make an educated guess that they're using an EZ81 rectifier or equivalent; the Blackheart  5W head is specifically designed for modding: http://www.blackhearteng.com/mods.html


http://64.233.167.104/search?q=cache:gAmFFPXdPOkJ:www.mojomusicalsupply.com/item.asp%3Fpid%3D577299%26pg%3D44991%26id%3D5550100+mojo+blackheart+mod+kit&hl=en&ct=clnk&cd=1&gl=us&client=firefox-a
 
Not too much on the cached page.  Maybe the mojo link will work again later.

Here's what - it can be doing much more than about 60ma output.  Five watts... The Champ does about 50ma on the 6V6 and uses a 70ma transformer.  Preamp tube uses "maybe" 5ma (more like 3).  Some leakage in the power supply.. a few 1/1000ths of an ampere.  So that is about 55, maybe between 55 and 60 milliamps.  They used a 5y3 rectifier which can deliver 125milliamps.  It doesn't say a whole lot.  If you got a rectifier that was rated for say... only 70ma, then you'd have that thing flat out wailin' when you dimed it.  Totally change its flavor from Champ or more like a tweed tone (lot o' sag).

So... this depends on what they got in the mojo kit.

Modding a small amp to tube rectifier is more a function of knowing what to cut, what to splice, how to mount the tube, and where the 5v filament (high current) is going to come from.  My guess: They add a socket, add a small filament transformer.  Wire the rest in, as it needs to be.
 
The mod kit consists of a turrent board and components to replace the stock wave-soldered PCB with text you saw on the cached paged about tube rectifier upgrade, weird thing is I don't see a punch out in the existing chassis for the rectifier socket...

Mercury Magnetics has tranformer upgrades and a "cannibal" kit to mod it to a dual 6V6 13 watter...

http://www.mercurymagnetics.com/pages/mainframe.htm
 
They stole the cannibal kit from my Champ to HOT-ROD Champ design, one of which Vic is playing.  Bastids.

At any rate, I still like Mercury... cant quibble over intellectual property when there's tone involved... and its good tone at that (pats hisself on de back...)

Replacing the entire board is something that can be done.  I hate turrets, prefer eyelets.  If you have a schematic for it, the layout for eyelet population is probaby rather straight forward.

I happen to have about 25 linear feet of eyelet board here, and couple thousand eyelets or turrets (have both).... in case you want to dabble in a bit of intellectual property "borrowing".  Seems to be all the rage.

 
I thought with a single-ended Class A amplifier current draw is constant. How would a tube rectifier v. silicon matter? How would you get any "sag"?
 
neilium said:
I thought with a single-ended Class A amplifier current draw is constant. How would a tube rectifier v. silicon matter? How would you get any "sag"?
Click to expand...

I think (and I'm sure C.B. will be able to correct if necessary) that class A amplifiers are always on, the valve is biased such that it is always conducting, but the valves don't draw full power in their quiescent state.

Depending on the configuration, Class A amplifiers have a maximum theoretical efficiency of 50%, although, in practice, most are less than half as efficient.

So most Class A amplifiers will need at least a 20W power supply to drive the speaker with 5W.
It's been a while since I did electronics at university, so any mistakes may be blamed on the intervening, alcohol infested, years.  :occasion14:

Cheers - Rab.
 
If anyone's looking for a "fabulous" deal on one, here's one on ebay.

Only $179.99 plus $23.80 shipping.   And!!!!!  They have 7 available.   

http://cgi.ebay.com/CRATE-BLACKHEART-LITTLE-GIANT-BH5H-GUITAR-AMP-HEAD_W0QQitemZ290202744931QQcmdZViewItem?hash=item290202744931&_trksid=p3285.c54.m20.l1116

If someone is actually buying these, I need to quit my job and start re-selling stuff on ebay.
 
Let me see if I can answer the Class A questions.

You're not gonna like my answer.... but.. textbook, yes, the Class A design does not vary in its power needs, whereby quiescent power consumption is the same as full output power consumption.  By the book.

In the real world... you'll see that your typical Fender Champ goes from about 45ma quiescent to about 52-55ma at full bore.  Not much but just enough to get it to sag a bit.

The Tweed Deluxe is also Class A - cathode biased..... NOT!  Its an attempt at A/B cathode biased, which is a guarantee for sag. 

You must remember that even AB amps are running A until they reach a point where the bottom of the wave starts to go by-by... being filled in by the other half of the Push/Push pull circuit.   

Ideally, if you drive a Class A amp hard, you'll get equal clipping on top and bottom.  In practice, the tops always get clipped a bit more.  Thats ok, the asymmetrical waveform actually sounds pretty nice.
 
jackthehack said:
Got another note from musiciansfriend.com, now the 1x12 cabs are backordered to 11 April....
Click to expand...

look for a local store that carries it, the store i go to had three
 
The mojo site's back up:

http://www.mojomusicalsupply.com/item.asp?pid=577300&pg=44991&id=5550101

Looks like you drill a hole near the front of the chassis for the rectifier socket
 
jackthehack said:
The mojo site's back up:

http://www.mojomusicalsupply.com/item.asp?pid=577300&pg=44991&id=5550101

Looks like you drill a hole near the front of the chassis for the rectifier socket
Click to expand...

I think with a good set of instructions (step by step with check boxes and testing procedures and safety safety safety spelled out) this would be a good first project for someone to get a taste of diy amp-building/modding. Do a little metalwork, do a little soldering without the fear of getting in over your head or permanently destroying a priceless amp.

However, the instructions I downloaded from that page strike me as cursory and targeted at someone with more experience. I think a smart person with no experience and a fair amount of patience and some research skills could figure it out.
 
They also ditch the triode pentode switch....

all for a tube rectifier

personally... use the amp as it is... its ok, its fine, its toneful

WHEN the board craps out - and it will eventually -

Then do a total rebuild from scratch.
 
I can't live without the triode/pentode switch, spend more time in triode mode so far...

Brain Salad Surgery results with new tubes in the house. DEFINITELY improves the voicing of the amp, may be the only mod you need to do to this puppy. First impressions playing my Korina/Wenge VIP through just a CryBaby into the amp; need to do a lot more comparison with different pickup combos.

All tests with Groove Tubes silver series EL84 in V2; makes no difference in volume, but definitely doesn't break up as much dimed as stock no name Chinese EL84. Makes pentode mode much less harsh and more usable than with stock tube for my tastes

Groove Tubes GT-ECC83 in V1 - My favorite so far; sounds a bit cleaner further up the volume dial, but still saturates well to completely above 7.5-8 on the dial; "sings" well at that setting on the edge.
Vintage Fender-labeled 7025 in V1 - Very interesting, good alternate sound. Maybe a hair less volume output, but less treblely at volume, a bit warmer/darker sound.
Groove Tubes GT-12AX7-R Seems to saturate the EL84 quicker than the other preamp tubes, not my first choice, but if you're looking for a more distorted, overdriven sound you;d probably like this one.

More later; will probably wait till I get the cab before recording clips.
 
Look for EI Serbia 12AX7 Jack.    Might want to try an Sovtek 12AX7WB  the WB is a killer "dark" tube... very creamy thick mids.  The EI is a really REALLY nice tube - but make sure you get a quiet one, as they are a bit prone to microphonics.
 
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