Xits X4

Nice amp on the bench ! A kind of Vox AC4 with excellent components (Mercury Magnetics transformers, SoZo capacitors ….)




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Hiwatt DR103 : crunch mode mod !

Here is an Hiwatt DR103 a customer wanted to be modded « a la Jimmy Page ».

I proposed a slightly more easy and reversible mod … So, here we go !

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Marshall JCM900 2100SLX less bright mod

As many Marshall, the JCM900 2100SLX sounds VERY bright, outch !

So, I removed the 1nF bright cap (called …R22 !!!) and that is how it sounds now :

Easy mod, you can even cut one leg of R22 !

JCM9002100SLXPreamp(Ugly / poor readable schematic)

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Carslbro 50 TOP

I got this Carslbro 50 TOP to fix (burnt OT).


OT : Drake 784-213 (4.4k Zaa)


Schematic (thanks Chambonino !)


The point is : on « my » 50 Top, power supply is quite different : PT has a central tap and no bias tap.


On this 50 Top, power supply conforms to the schematic


The 50 Top I have to fix …


So, I retraced its schematic :


I replaced electrolytic capacitors, OT (Hammond 1750N, 3.2k Zaa for JMP50) and 47k bias replaced by a 100k variable resistor


It plays loud with great crunch. Nice vintage amp !

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Stramp 2100A

As used by Rory Gallagher and Leslie West …

Actually, even if Rory played on Stramp amps, I’m not sure he used this model …


Huge capacitors and transformers …


‘70s PCB, that can be accessed from inside and outside the chassis …



Stramp schematic



Did you notice how the standby works ? High voltages are always there when the amp is ON. When on standby, the Phase Inverter input goes to the ground.


OK, guyz, this amp sounds loud !

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vampwire2a vampwire2b vampwire2c


(thanks to http://www.chambonino.com)


See http://www.vampower.com for more information


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Miles Platting amp

I got this amp for a complete check-up. 50W, 3 x 12AX7, 2xEL34, 1 GZ34

End of ‘60s, beginning of ‘70s I think …




MilesPlattingWilsic Miles Platting amps were made by Wilsic


Here is a Wilsic 50W (picture from the very nice http://www.chambonino.com site, thanks to him !) :



Now, let’s compare under the hood :


Wilsic 50W (picture from Chambonino, thanks again)



The Miles Platting : a capacitor is missing. Is it « by design » or a previous mod on this amp ?


Anyway, a good sounding vintage british amp !

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Fender Bassman Silverface circa 1967

A nice Bassman to check :


From top to bottom : Bassman ’67,Bandmaster ’68, Bassman 100 ‘75



bassman_ab165_schemP1050891 P1050892 P1050893

Earth OK, no Deathcap : good !   AB165 schematic


DripEdge, blacklines, Schumacher (606) transformers and choke from 1966 and 1967 :


transfos1 transfos2 transfos3


tubesOld preamp tubes still well and alive !

Bassman recap’ed, bias checked with new power tubes (there is not bias pot on this model), hum balance set, let’s rock !




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Beer + OD

Just for fun …

IT-11_Audio_OD1 IT-11_Audio_OD2

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Blackstar HT5 bad sound

I got this nice little amp to fix. It fizzed. I checked the tube : OK … then I saw R39 resistor was burnt.

I spiced the FET PI, replaced the burnt resistor and compare voltage : good !



(I used IRF150 model instead of IRF830 …)


A few days later, my customer came back : his amp had the same problem, again !

R39 burnt !


I found this topic on the Blackstar forum (very helpful, thanks a lot, guyz !) : http://www.blackstaramps.com/forum/viewtopic.php?f=32&t=2135&sid=6267625e63d87d9553d419c17f908d83

BadTraceTrace broken, to rebuild …

Once TR3 is removed, I could check it (see : http://www.decibelcar.com/menuelectro/93-fettest.html , very helpful, thanks !) : TR3 OK

R39 and C46 removed, let’s check if there is a short somewhere :

ShortWe should get an infinite resistor between Source and Drain holes … and we read 50kΩ !

It seemed this amp had a bad PCB, so I drilled hole (0.7mm) to isolate TR3 pins, as suggested in the Blackstar forum :



and checked :

checkTR3pinsPerfect !


I removed R38, TR2 and C45 : no short there !

Soldered new 22k resistor, TR3, C46 … amp to the test !

Bias : 

Tips : Measure voltage across 4,7ohm resistor on cathode (=R33, common 12BH7 cathode resistor). Adjust to mV reading of approximately 46mV. Adjust balance pot for least amount of hum from speaker

12BH7 voltages :

  • Anode : 470V (!)
  • Grid 1 : -29V
  • Cathode : 45mV


Hope this helps


Update 2018

I got another Blackstar HT5 to check. In this one, there is a new designed PCB (I didn’t notice the “R39 bug”) … but how to bias this new version ?

Here is how I did (I’m very sorry, the photos are of poor quality) :


PR1 : bias, PR2 : balance


NewHT5-2To check balance, measure voltage between blue wire and ground, and compare to orange wire and ground (blue and orange wires go to 12BH7 grids 1, triode #1 and triode #2).

To bias the amp, measure between R33 (red arrow) and ground (or voltage drop across the resistor R33), you should read 46mV (470V at the anode) : bias PR1 pot !





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