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PostPosted: Wed Dec 19, 2012 9:53 pm 
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OK I'll admit it, I'm in over my head. I've been asked to measure B+, and I don't even know where to stick my meter (guessing - pin 3 of output tubes?). High voltage on the heater wires....how would that happen...seriously, I haven't a clue what it even means let alone what I might have done to cause it.

I need a little handholding here. I've been staring at it for 3 nights, since I blew the bulb, and done nothing.

Near as I can tell, and I've looked everything over, this should be right as per the layout.

Here are some pics I just took.
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PostPosted: Wed Dec 19, 2012 11:11 pm 
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I'd thought the 6V6/KT66 bias toggle looked odd in the pictures but it's looks like it's just oriented differently than in the layout diagram.


Last edited by frethead on Wed Dec 19, 2012 11:18 pm, edited 1 time in total.

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PostPosted: Wed Dec 19, 2012 11:17 pm 
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OK the heater wires are the two green ones that attach to the light bulb socket from the power transformer. And then the red and black twisted wires that connect to there as well and continue down with connection to each tube socket. Its looking like there's too much voltage there and its blowing up the light bulb.

In a previous picture it looks to me like outside green wire might be shorting to the inside one need a better picture of this. Is the outside wire touching the piece of metal between the two red coloured washers

If you measure ac voltage across those two wires it should be in the 7vac range with no tubes plugged in. (look at coco's power up instructions in the manual) Be sure those aren't shorted together


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PostPosted: Thu Dec 20, 2012 12:03 am 
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Good call. See the pic below, the main wire wasn't but there was a single loose strand in behind that I hadn't seen, you can maybe barely see it in the pic.
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I'll suck up my courage, stick another bulb in it, and go give it another try.


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PostPosted: Thu Dec 20, 2012 12:14 am 
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OK that was it. Each leg should measure half the AC? ie., each side measures about 3.4vAC x2 = almost 7, just as per the manual - right?

I'm still confused about which are primaries and secondaries. I know I read it as I went through, scrolling back 50 pages looking for it gets to be a drag. Recommendation: for newbs like me, give the instructions in colour of wire coming off the transformers.


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PostPosted: Thu Dec 20, 2012 12:30 am 
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Aaaand, I still don't know where B+ voltage points are to check.


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PostPosted: Thu Dec 20, 2012 1:05 am 
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B+ is taken off the standby switch. It feeds the centre tap of the OT. It is connected to one end of t he 2K5 5W resistor on the board. You can see it in the schematic.

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PostPosted: Thu Dec 20, 2012 1:41 am 
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OK I have sound output, sounds really good but very low headroom.

The overdrive switch works when pulled. When pushed back in, sometimes it cuts the sound. Pulling it back out re-engages the sound, and if I keep playing when I push it back in, it doesn't seem to cut sound.

The bias points won't give my meter any reading at all..mA setting, right? I can move the decimal on my meter, but no readings of any sort spit out. For the readings below, I have the bias pot spun all the way down/left.

Some tube pin readings. 6V6's are a touch high. V4 is giving me very wacky readings, and on pin 7 my meter won't slow down from jumping around to give me a solid reading at all.

V1 - Pin1= 152, Pin3 = 1.29, Pin6 = 129.3

V2 - Pin1 = 183, Pin3 = 1.33, Pin6 = 107.3

V3 - Pin 1 = 281, Pin2 = 67, Pin3 = 70.6, Pin6 = 191

V4 is an AT7. Pin1 = 130 (??), Pin2 = 70.5, Pin3 = 71.0, Pin6 = 423 (!!), Pin7 = no reading, Pin8 = 28.7

V5 - Pin3 = 463, Pin4 = 436, Pin5 = -43.3, Pin6 = -43.3

V6 - same readings as V5.

Also, I screwed up the inputs (??) The bright vol works when I plug into the normal input, and vice versa. Link works fine.


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PostPosted: Thu Dec 20, 2012 10:54 pm 
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The 82k and 91k resistors connect to pin 6 and 1 of v4 (are you sure that these are the right values) and what is the voltage measurement on the wire side or those resistors. (the blue one that goes to the cap can)

As for your bias measurement you need to use a volt meter as your measuring across a 1 ohm precision resistor

The input side of the transformer is where you input the ~120 vac from the wall (north America) the primary and secondary's are the output side 6.3 ac (secondary voltage) for the heaters. Primary 300 vac or higher used to be rectified via diodes or tubes and filter caps to a dc voltage B+ that the amp uses to operate.

Edited cause I screwed up my explanation of primary and secondary's :bugeye:


Last edited by sazafraz on Fri Dec 21, 2012 8:23 am, edited 1 time in total.

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PostPosted: Fri Dec 21, 2012 1:44 am 
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OK those 82K and 91K, here's what happened: I was using my meter to measure them, NOT reading bands.

The 82K read 90.9K, so I put it where the 91K goes. Then, I thought I was short an 82K, as the value I have left is 93K. We figured out that I had them in the wrong spots. Via email I was told to just sub in the 93K where 82K belongs, and another one would be shipped to me. I looked for one around town but couldn't come up with one.

I just remeasured them, for about the 9th time, and get the same readings.

I missed the point about using mV on my meter for biasing, just found that in the instructions. V5 is 16.5 and V6 is 16.8. I'll fiddle with that later.

B+ is 455 at the junction of 2K5 resistor from power switch.

Pin 1 as instructed by sazafraz, 135.3. Pin6 is 424. Pin 4 I can't get a reading, even moving to the junction with the cap instead of right off the pin. Meter seems to climb and climb then go haywire just before it hits 400.

I am seriously chomping at the bit to get this thing tuned up and running right :) Appreciate the help.


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PostPosted: Fri Dec 21, 2012 10:21 pm 
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What I wanted was a measurement on the other side of the two resistors, Where the ht3 wire connects to them (from the capacitor can)

Is the 1meg resistor and the jumper wire on the socket of v4 (can't see it well from the pictures)

Try putting a different tube in the and retaking the measurements a 12ax7 will do your just taking voltage measurements

And pin 4 is a ac heater connection


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PostPosted: Fri Dec 21, 2012 11:02 pm 
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yeah I just remeasured, after putting in a different V4 (12AX7A)

Pin 1, on the cap can side 417
Pin6, on the cap can side 421

OMG is that a jumper from pin 3 to pin 8? oops. Firing up the iron.


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PostPosted: Sat Dec 22, 2012 12:17 am 
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OK fixed...and readings are closer but not quite right on. I didn't re-measure everything, just V4, which is an AX7A. Here's where we're at:

Pin 1=280.2 Pin2=70.5 Pin3=71.8 Pin6=284.7 Pin7=63.0 Pin8=71.9

So, just pin6 looks high compared to the build guide chart.

It now has the headroom and volume I expected.

Remaining Issues:

Bright and Normal inputs are reversted. *suggested* put a word about that in the manual and even on the layout, where they are all side-by-side. I had a moment of dyslexia obviously and looked at the front panel wrong way around.

Bass pot appears to be wired backwards. Treble and Mid pots are either EXTREMELY subtle, or not working.
**EDIT** played with them some more, and I *think* the treb/mid are just really really subtle. Bass is wired correctly, it just has a different response than what I am used to. Really affects the whole spectrum, not just the bass end.

Still have an issue with the overdrive pot cutting in and out, whether engaged or not. There appear to be spots on the rotation where it's OK, and others where it's not....this is admitted based on fairly minimal time of observation.


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PostPosted: Sat Dec 22, 2012 10:26 am 
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Have you put the 12at7 back in and redo the measurements ? (more likely to be due to the similar plate resistor values 93k an 90.9 k)

The overdrive pot sounds like it got over heated and has dead spots (my best guess)

As for the rest of your issues I've never played one of these amps so I don't know what there supposed to do or sound like. There are others on this forum that have experience with the subjective side of this amp then I do .

Glad you headed in the right direction now. :D


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PostPosted: Sat Dec 22, 2012 12:00 pm 
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I haven't registered a lot of time with my Triwatt yet but noticed similar behaviour with the treble and presence controls - pretty subtle.

I haven't experienced the amp cutting out when engaging or disengaging the overdrive, but there is a significant volume jump when the overdrive is engaged. I can see why you'd want the overdrive to be controllable via a footswitch for solo boost in this case - mine isn't wired for footswitch so it's a matter of trying to dial the amp in again when you engage the overdrive which is a pain if you're trying to use the amp for both clean and dirty tones.

As mentioned in another thread by another member, I also noted that my amp cut out when I had it cranked full volume and played aggressively on the lower notes - if I was heavy with pick attack, the amp would cut out for half a second and then come back to life (didn't happen with a lighter touch). I re-flowed some of the connections and tried the full volume test again and didn't experience it cutting out again so perhaps it was a cold solder joint but I was also playing a guitar with lower output pickups on the 2nd test. I don't think I'll ever be cranking this amp anyway as it's simply too loud already for my needs (already been asked a few times to turn it down), but in my opinion this amp sounds best when the power tubes are providing some crunch so I'm half tempted to check out how to build an attenuator - not sure why some vendor's attenuators are worth $500+ but it looks like some of the designs incorporate re-amping to ensure retention of higher frequencies. In any case, it would be great to be able to get the 6V6's working hard but at "somewhat-ear-friendly, jamming in the basement with drums" type levels.

At this point, I may just use the Triwatt as a pedal platform although I've never been big on the whole pedal thing (I've only ever really plugged straight into the amp). Following the lead of one of the other members in an old post, I set all of the controls to 12 noon with overdrive disengaged and started tweaking with minor adjustments from there and you can dial in some cool clean tones pretty quickly.

I may begin a search for a 5watt-10watt kit capable of SLO type gain and pushing lower end.


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PostPosted: Wed Dec 26, 2012 10:51 am 
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Before you try an attenuator or build one, try a PPIMV first. A friend of mine had a hotplate, and I have a BBQ Mass, and we both agree that the PPI master volume mods we did to some traynors sound better than using our attenuators with them. I was really blown away, and the mod is NOT hard. you can use the footswitch hole for now if you want, to see if it works for you and then drill later


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PostPosted: Wed Dec 26, 2012 12:02 pm 
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Why the talk to PPIMV and attenuators? The MV on the Triwatt is as affective as any. When it is dimed it is totally out of the circuit, it's not adding anything tone wise to the amp.

As Zaphod has eluded too on the forum before the Triwatt actually has more gain on tap then a Soldano SLO. You got to learn how to use the amp, it's not as simple as a Fender or Marshall.

The treble and presence controls are subtle, that's the Hiwatt design. If your having troubles getting tones or dialing things in read up on the DR103 because that is what the Triwatt is essentially.

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PostPosted: Wed Dec 26, 2012 10:11 pm 
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The trouble is that, as far as dirty sounds go, the Triwatt sounds best to me with the power tubes being pushed and adding a really nice warm crunch which I can then tailor with the overdrive. Without the 6V6 adding some crunch, the overdrive doesn't have as much girth or grunt to my ears but in order to get to the point where the power tubes are being pushed, the amp has to be cranked pretty loud. I own a Jet City JCA20H and had thought the Triwatt would be similar in overall volume but it seems to be not the case - Triwatt is (or seems to be) much louder than the JCA20H. It also appears to be easier to push the EL84's (JCA20H) into saturation than the 6V6's perhaps.

In any case, I haven't yet tried changing the value of the coupling caps for the overdrive so perhaps that may address some of my concerns.

To be clear, the Triwatt is a great amp and I'll be using it for clean tones and for applications where I'm able to crank it into power tube distortion, but I'd like to be able to get the same sort thick crunch going with the Triwatt at lower volumes or I'll need to look at augmenting it with a 10w tube head that I can push hard without the volume concerns.

Apologies to keto - didn't mean to hijack this thread.


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PostPosted: Wed Dec 26, 2012 11:06 pm 
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Try to add a little OD just to get that crunch. The MV can control the loudness at a reasonable level. Make sure the B+ is correct to manual and bias as well. To high a voltage will not help your cause!

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