Need amp tech advice plz!


Senior member
Hi folks:
ok, so I bought a black heart little giant 5w tube head, an avatar speaker cab, and the 'bitmo' 'triple bypass' mod that's supposed to give it 3 different voicings, and some quality tubes (a sovtek 12ax7wb and a JJ el84).
Trouble is, the cabinet arrived a week after everything else, and since I had some time to kill with the head and the parts but no speaker to play them through, I decided like a total idiot to change out the tubes and install the mod before checking to see if the head actually worked.
Well, actually I did 'check' it, I also made the external attenuator from bitmo, and they say that if you hook it up and keep the volume low, you can run the head without a speaker and listen using headphones from the 'line out' of the attenuator. I did that, and it worked but sounded pretty weak overall through the headphones.
Well I got everything together, and it isn't good!  I KNOW this is my fault for not going one step at a time.....if anyone can offer suggestions as to what to check first, second, etc. that would be great.

1. switching from 5w to 3w there is a very loud 'pop', even with volume at zero. In either setting, volume is quite a bit less than I expected and less than that at the store. I was playing my LP with volume around 1 oclock and it was pretty clean and really pretty low volume. This may be due to the kit - one of the aspects of the mod is putting a 470k resistor between the lugs of the volume pot.
2. I get a lot of loud static with any touching of the input jack on the amp.
3. on 5w, I get persistent background static along with too much hum. On 3w, just too much hum, no static.
4. turning it off, after the loud 'pop' even at zero volume, the static continued by itself for a couple of seconds. I thought that was weird.
5. The 'mod' does work, changing the voicing of the amp, and the tone controls all work, too. So I know I probably haven't shorted anything in the mod itself.
6. The mod is mostly just a three way dpdt mini switch mounted next to the input jack, a mess of caps and resistors wired into it, then wired into the circuit board at a certain spot, plus replacing one capacitor for another and adding that resistor behind the volume control.
7. It really does have a sweet, awesome tone evn in its jacked up state. I was enjoying jamming on it even with that crazy background noise.

So, does this sound like an issue with the tubes, a short, or a fried component like a cap or a resistor(my soldering iron is kind of high powered for some of the fine soldering)? 

I have a multimeter so I can test individual components, I'm just kind of at the very edges of my electronics knowledge here and hesitant to muck around too much - if the problem is obvious to somebody here that would save me a lot of time and frustration. The picture shows the location of the mod switch, I haven't opened it back up yet. Any help would be really appreciated. Thanks guys.
Hi Tfarny,

Doesn't sound good - but don't worry.  I'm sure it can be fixed what ever the problem is.  We just have to go step by step to figure where the issue is.

regarding #1 - the pop might be normal, depending on what the power scaling circuit is doing.  the volume control is likely in the preamp section, so if the scaling is affecting the power amp, you'll get some kind of click/pop when you throw the switch even if the volume is off.  BTW - do you have a schematic of the head and the mod?  That would help diagnose things. It could be that you just need a .1uf ceramic cap between the lugs of the power switch to kill the pop - BUT that depends on what the switch is actually doing to the circuit.

regarding #2 - is the input jack grounded when there is no 1/4" guitar cord plugged in?  does it have a 100K resistor to ground?  does it have a 68K (or so) screen resistor before hitting the screen of the first tube stage?  If the answer is no to one of these, then that's the problem.  BTW, does the mod affect the first stage, or the second stage?  does the mod change the value of the cathode capacitor of the first stage, or does the mod change the coupling cap between the first and second stage?  Just trying to figure out if there is something that you could have done wrong when installing the mod.  Also, try pulling tubes, starting at the preamp, until the static goes away.  The tube that causes the problem to stop is where the problem starts.

3 - 60 Hz hum means that the power supply ripple is being inserted into the circuit somewhere along the line.  It could be grounding, it could be not enough decoupling capacitance, it could even mean that your heater wires are too close to a sensitive grid input somewhere.  It's tough to tell by just your description.  Again, try the pull the tubes one by one trick until the hum stops - that will identify where to look.  Here, a photo of the inside of the amp would help.

4 - the big pop tells me that you need a little capacitive bypass on the power switch - again 0.1 uf at 400v should do it.  The static persisting after the switch is off is normal - the circuit will still operate as the decoupling capacitors discharge.  however, I bet that the 60Hz hum stops right away when you flip that switch doesn't it?  :)

5 - that's good - hell that's half the battle.

6 - err - hmm - yea - do you have a schematic diagram for that?

7 - groovy.  I know we can fix the other problems, which will turn this amp into a usable tool for you.

To progress further, I really need to see that schematic - both of the stock amp and of the mod.  What do you have?

I don't know about the rest, but the "pop" when you change wattage happens on mine. There's no pop when I turn it off  - but it the sound does fade away after turning it off, but that's normal for a tube amp. I don't have a lot of hum or static though and my amp is louder than satan. I hope you get it worked out.
Well, first and foremost, I'd wait for CB to tell you how not to electrocute yourself.  If you modded the amp, then you are going to have some trouble finding the source of the hum (you don't know if it was there to begin with)  The hum sounds like something related to a grounding issue, like a shielded wire that the shield is not grounded on.  When building your own amps this happens and you do what is called chopsticking.  You move wires around with chopsticks.  It generally has more to do with the power wires being to close to the signal path and transformer squealing noises being picked up, but hum is also an issue.  The 1 Meg resistor to ground at the input is quite important for impedance to the guitar and if you messed around with stuff in that area, check the solder joints.  The 68k to tube 1's grid can go as low as 10k and that cuts down the "Mexican Radio" problems in amps.  That resistor is commonly responsible for a lot of noise, so you want to make sure the wire to the resistor and the resistor wire to the tube pin is minimized in length.  Actually, if you did add the mod, check the grounds and flow the solder joints well to be sure that you have a nice connection on all of the joints that were played with.  Also go over the wiring diagram and instructions again, I am sure you have done this already, and check that everything is where it is supposed to be.  I like to make a copy of the diagram and follow the actual circuit with a chopstick, while coloring on the diagram copy with a highlighter to make sure I didn't just pull a no brainer.  Last thing, if you have the original tubes, try them.  You might just have a bad tube, it does happen.  To me it sounds like a bad solder joint on a shielded wire.

Also, I know that they say you can turn on a tube amp without a speaker, but as rule, never do that.  And when you think, "OK, I have everything set up," that is when you should stop and check the cable to the speaker to make sure it is going to the correct impedance.  Good luck.

Thanks guys. Ok so:
step 1. don't kill self.
step 2. swap tubes (I have originals).
step 3. check grounding issues and solder joints.
step 4. check the solder and 1 meg resistor at the input (didn't mess with it but I'll check it).
step 5. learn to understand sentences like "The 68k to tube 1's grid can go as low as 10k and that cuts down the "Mexican Radio" problems in amps." - this might take a little time!  :tard:
Really appreciate the input, seriously.
No, actually... in this case... I'm wondering why someone would take a new amp, un-listened to, and modify it. 

So carry on... I'm gonna sit back and see where this all leads.  If the gene pool is drained a little... I can't see the harm done.
Is there a schematic for the mod? All I can find on the Bitmo site is claims of what it is supposed to do, but no schematic/details?
This morning I got nothing whatsoever - no sound except the pop when switching 5w/3w.
Ignored CB's comment. Confirmed that filter caps are self-draining on this amp. Went back to original tubes. Still nothing. Went to KISS principle - removed the mod from the circuit with the exception of the treble tone control cap - original was 470pf, was replaced with 300pf cap per mod instructions. My understanding, this will just slightly change the treble resonse as compared to the original. Anyhow I can't find the original cap so I just left it. carefully eyeballed the board for anything I may have screwed up.
Still nothing. Damn. Now taking it to an amp tech (Rudy's) - this is just a bit beyond me, I now realize.

Moral of the story: make sure your amp works before messing with it. I'm sure everybody already knew this, of course.  I suppose I already knew it too, but impatience got the better of me.
No elated really... since I have no idea what the mod is... or how its used in the circuit.

There was a bit of semi mis-information about a screen grid resistor up there someplace.  But do this -

On the preamp tubes, in almost every modern circuit, you need the following -

A grid load of some sort (a resistor to ground)
A cathode resistor, possibly with a cathode bypass capacitor in parallel with it
A plate load resistor, from which to develop voltage as the current flow in the tube increases

The hibjibberwash about 68k "screen" resistors can be ignored.  The input resistors are just a way of loading and isolating two inputs into one channel.  If you ran it "straight" into the tube's grid, it would work just fine (perhaps with a bit added treble response).

You also need some B+ voltage to make the tube work.

Checking on pins 1 and 6 of the preamp tubes for B+, should be anyplace from 100 to 250 volts DC or so, in reference to ground.
Check on pins 3 and 8 for a low resistance to ground - something like 3k ohms or less, down to about 680 ohms.  Under about 680 ohms on the cathode, a 12AX7 wont sustain bias, so.. thats about the lower limit, more or less, blah blah blah

Check to make sure B+ is NOT on pins 2 and 7, if so then you have coupling capacitor problems

Make sure you DO have 6.3vAC (or maybe DC in this amp, dunno) between pin 4 and 9, and between pin 5 and 9.  Thats the heater circuit.  It may have some fancy 12v heater - 12v between 4 and 5 with no connection to pin 9 at all... in which case it would be DC on those pins.


On the power amp tubes.  Make sure you have pin 8 grounded, or going thru a cathode resistor - low resistance, medium wattage, something like 270 to 680 ohms, in about 3 to 5 watt range.

B+ on pin three please

No DC on pin 5 - thats the grid (control grid)

Pin 4 should have B+ on it, thru a resistor of medium wattage, and low ohms... about 500 to 2k ohms, 1 to 5 watts

Pin 2 and 7 should have 6.3v DC or AC on it between them

If you got that, it'll work.... just figure out why you dont have that - like a blown fuse on the board in the amp....

And give the self draining caps a good 5 minutes or so to drain.  They do that (as I myself also do) by running a 1m resistor from B+ to ground.  It does take a bit of time to drain, its not instant.  Usually you're good in a minute or so... but give it time.

The 68k whosuhfudge that he was referring to is on this schematic here, R24.  They have since removed it, and made the shielded wire easier to understand in the diagram, but oh well.

Sorry, forgot to mention that the schematic I put up there has nothing to do with the amp that started the post.  Just an example of the 68k resistor in question...

Thanks guys.
Well, odd but good news - put it all back together, with orginal tubes, to take it down to the shop, tried it one more time, and voila, of course it works perfectly. Had to figure out why, so I went back and tested each tube more scientifically - turns out the sovtek 12ax7wb is totally dead, i.e. nothing works when it's in, AND also the jj el84 is the one causing the weird background noises. Maybe the 12ax7wb was working intermittently, that's all I could figure out. So, I'm back to (almost) stock state, need to  get new tubes but also just enjoy it for a while. The 'attenuator' works as advertised and damn do I need it, because this sucker is loud - wow. But I can get bedroom volumes with it cranked up using the attenuator. I guess they say these things 'suck tone' but it sure sounds good to me, even with the chinese tubes. LOVING the 5w at around 11 oclock, with the mids cranked up a bit, w/my new LP on the neck pup. Serious ear candy.
I'll keep the mod in a drawer for a while, maybe learn more about this stuff before I try to reinstall it.
That's cool - glad it's just the tubes that was mucking it up. Told ya the fudger is loud hahahah.  :rock-on:
the three watt setting, cranked up, is a lot, lot lot louder than 'bedroom' volume in a manhattan apartment bldg., unfortunately.
Patrick from Davis said:
Sorry, forgot to mention that the schematic I put up there has nothing to do with the amp that started the post.  Just an example of the 68k resistor in question...

Well considering how 12AX7's dont have screen grids... no wonder they took it out!

That 68k resistor is usually called a grid resistor or grid-input resistor or isolation resistor.  It serves two purposes really, or maybe three. What it does is provide a constant relationship between the grid input, pickup (or mic) and the grid load.  This is important in many designs in order to maintain bias on the input stage, depending on the pickup used.  In practice it isolates the grid itself from the pickup.  The second thing it does, is isolate one input from another, such that the instruments controls don't interact (too much).  You will still get some interaction if say... you have one guitar played with max treble cut and the other not.  In that regard, its a mixing resistor as well.  The third thing it does, maybe... is provide some protection in the rare case of grid shorting to plate.  This "has" been known to happen, and of course, when it does... you get B+ on the pickup for a brief moment right before it blows the windings..... a capacitor input would be great protection, but there are other problems with that... however the resistor input does offer some degree of imperfect protection under such conditions.

But its no screen grid resistor.