#1
I was wondering how hard it would be to mod a blackheart amp to accept octal tubes? ax84 sells this amp kit that can accept both eight and nine pin output tubes and i looked at both schematics and they look very similar, so is it possible or would it be a lot more that just adding a octal tube socket?
Current Gear:
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#2
It's dead simple. IF you have a location near the nine pin socket where you can mount an octal socket, cut a hole, mount the socket, connect the wires. You can use a separate cathode resistor and bypass capacitor, so the idle current is optimized for the octal tube, when it's used instead of the EL84.
Meadows
Quote by Jackal58
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Quote by SK8RDUDE411
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#3
im not that great at electronics but if i make the output section of the blackheart...
http://www.rowbi.myzen.co.uk/Blackheart/Documents/BH5_schematic%20Rev%2020070611.pdf

look like the output section of the ax84 amp (pic below) is that all i need to do. can i wire them in parallel like they show in the pic and use one or the other? also if i wanted to use a kt66 or a kt77 would i need to do any other mods since they would increase the wattage of the amp?
Attachments:
ax84.png
Current Gear:
ESP LTD MH-250
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Blackheart little giant head
2 homemade 112 cabinets
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#4
Quote by shredzilla
im not that great at electronics but if i make the output section of the blackheart look like the output section of the ax84 amp (pic below) is that all i need to do. can i wire them in parallel like they show in the pic and use one or the other? also if i wanted to use a kt66 or a kt77 would i need to do any other mods since they would increase the wattage of the amp?
Yes, wiring them in parallel will work. But don't have both tubes in at the same time. You'll draw twice as much current from the B+ supply. That could destroy either your power transformer or your output transformer. Maybe both.

Using a KT66 or KT77 won't increase the output of this amp. At least not appreciably. That will be limited by the B+ voltage and the impedance of the OT. However, the idle current might be higher or lower than desired. In the case of the KT66 and KT77 both of them will idle with much more current than the EL84 with the same size cathode resistor. If that's the case you can use a higher value for the resistance of R10*. That's why I suggested using separate cathode resistors. The only popular octal tube that will have similar characteristics to the EL84 will be a 6V6.


Meadows
Quote by Jackal58
I release my inner liberal every morning when I take a shit.
Quote by SK8RDUDE411
I wont be like those jerks who dedicate their beliefs to logic and reaosn.
#5
so if i make the power section of the blackheart (below) look like that pic it would work. r10 is the cathode resistor correct and it should be higher resistance value for the kt66 couldn't i wire them in parallel and put that one resistor on a spdt switch if i decide to go between the two. the whole reason i would be doing this mod would be to get a different sound. i hope to make it sound more like an old marshall kind of like the vintage moderns.
Attachments:
blackheart.png
Current Gear:
ESP LTD MH-250
Ibanez rg470
Peavey ValveKing 112
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2 homemade 112 cabinets
Boss SD-1
#6
Of course, you could use a switch to select which cathode resistor.

Using a KT66 will make this amp sound different.
Like an old Marshall or a VM? I doubt it. This is still a single ended output.
Meadows
Quote by Jackal58
I release my inner liberal every morning when I take a shit.
Quote by SK8RDUDE411
I wont be like those jerks who dedicate their beliefs to logic and reaosn.
#7
^^Vintage Modern? That thing doesn't really sound like a vintage Marshall at all, to me. Very fizzy, IMO, in a bad way.

Not to threadjack here, but here's my threadjack - SYK, I have a similar mod schem for a Valve Junior. Would I encounter the same issue with the cathode resistor? I was thinking about using a KT88..
#8
how high would i need to raise the resistance value of the cathode resistor? would the pentode/triode switch have to be taken out if i do this mod? also are there mods i could do to the tone stack to make it sound more like a marshall. i know they have mods for the vj; would they be about the same for the blackheart considering they are very similar amps.
Current Gear:
ESP LTD MH-250
Ibanez rg470
Peavey ValveKing 112
Blackheart little giant head
2 homemade 112 cabinets
Boss SD-1
#10
i wish i had the money to buy a kit and make my own amp but i dont. the mods wouldn't be that expensive to do if all i have to do is buy a tube socket a replace a couple of components and mess around with the tonestack.
Current Gear:
ESP LTD MH-250
Ibanez rg470
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2 homemade 112 cabinets
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#12
Quote by forsaknazrael
Not to threadjack here, but here's my threadjack - SYK, I have a similar mod schem for a Valve Junior. Would I encounter the same issue with the cathode resistor? I was thinking about using a KT88..
Yes. The EL84 in the Blackheart, VJ, etc requires only a small difference in potential (voltage) from cathode to grid, to keep the current low. 6v6s (and 6K6s and a few other less common octals) are similar in this regard. Most of the common octal power tubes require a higher voltage.

With an EL84 the VJ will idle at 41mA and a voltage from grid to cathode of 9 volts, using a 220 ohm resistor.

A KT88 will idle at well over 100mA with 25 volts across that resistor. This won't be a problem for the tube. That's 30ish watts, if the power supply can could the voltage up there. The tube will be fine. The transformers will probably fail.

Quote by shredzilla
how high would i need to raise the resistance value of the cathode resistor?
I can't say for certain, but I'd estimate a 1K resistor with 40mA of current gives you 40Volts. That would be about right, assuming there is 300 volts on the plate. But I have no data for the Blackheart. I'm only guessing


Quote by shredzilla
would the pentode/triode switch have to be taken out if i do this mod?
No.

Quote by shredzilla
also are there mods i could do to the tone stack to make it sound more like a marshall.
The values used in the tonestack of the diagram you linked to are exactly the values and configuration of a Marshall.

Quote by shredzilla
i know they have mods for the vj; would they be about the same for the blackheart considering they are very similar amps.
Probably. Again, I don't have any data on power supply voltage or transformer impedance/DC resistance/turns ratio, etc. But they do look very similar.
Quote by Tackleberry
Sounds like you just need to break down and build your own amp or get a kit.
Sounds like you just learned that what's right for you isn't always what's best for somebody else.

Modding is fun and a person can learn a great deal by doing it. These amps are cheaper than most kits, and complete with cabinet. For some, modding is a good way to get started. For others, kits are king. Scratch-builts can be awesome. It all depends on the individual's goals and previous experience.
Meadows
Quote by Jackal58
I release my inner liberal every morning when I take a shit.
Quote by SK8RDUDE411
I wont be like those jerks who dedicate their beliefs to logic and reaosn.
#14
You can "match" a KT88 to just about any PT and OT combination, in terms of idle current. Just get the data from the transformer manufacturer and select a cathode resistor that will give you the desired current at the voltage you find on the graphs for that tube.

In terms of getting the maximum transfer out to the load (impedance matching), that's an entirely different story. I can't help there.
Meadows
Quote by Jackal58
I release my inner liberal every morning when I take a shit.
Quote by SK8RDUDE411
I wont be like those jerks who dedicate their beliefs to logic and reaosn.
#16
so i was looking at some of the valve junior mod posts here on ug and i found one that looked kind of interesting. it was actually in one of your threads SYK.
http://members.optusnet.com.au/eyes.only/vj_mods_3/vj.htm
I was wondering how i would go about doing that to my blackheart? I made some sketches of what i think i would need to do and if you could help me refine them i would appreciate it. I'll put the sketches up tomorrow once i scan them in.
Current Gear:
ESP LTD MH-250
Ibanez rg470
Peavey ValveKing 112
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2 homemade 112 cabinets
Boss SD-1
#17
so here's the stock version.

and here's what the mod would look like with the extra valve as far as what i came up with.

but im not sure if that would work since i just kind of took things from the vj mod page and threw them into the blackheart schematic. also, i don't know where pins 4, 5, and 9 of the second valve would go to in the blackheart schematic.
Current Gear:
ESP LTD MH-250
Ibanez rg470
Peavey ValveKing 112
Blackheart little giant head
2 homemade 112 cabinets
Boss SD-1
#18
Quote by shredzilla
but im not sure if that would work since i just kind of took things from the vj mod page and threw them into the blackheart schematic. also, i don't know where pins 4, 5, and 9 of the second valve would go to in the blackheart schematic.
tbh, I found it easier to just hack a drawing than to figure out what was happening with yours. 4, 5, and 9 are the heater connections. Just hook them up to the same supply as the original 12AX7.

*Click for Larger Version*


Adding two triode sections will put a little more load on the B+ supply. You should measure the voltage at the point indicated before making this mod. You might need to decrease the value of R16 to bring the voltage back up.
Meadows
Quote by Jackal58
I release my inner liberal every morning when I take a shit.
Quote by SK8RDUDE411
I wont be like those jerks who dedicate their beliefs to logic and reaosn.
#19
I just have a few questions and i put them on the pic.

stupid question but how do i check the voltage? i think i put the negative part of the multimeter on the ground and positive end on that point correct? Also if i wanted to throw a master volume and a standby where would those go?
Current Gear:
ESP LTD MH-250
Ibanez rg470
Peavey ValveKing 112
Blackheart little giant head
2 homemade 112 cabinets
Boss SD-1
Last edited by shredzilla at Apr 25, 2008,