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Hey guys,

I'm a newbie here, so bear with me if I'm asking some stupid questions. I fired up my Triwatt tonight for the startup tests and I've run into a couple issues. All preliminary tests checked out fine until installing the tubes.

First, when trying to set the bias it appears that the with the bias pot at full counter clockwise the reading is about 12 mV. When turning it all the way clockwise the value actually DECREASES to around 6 mV. I went ahead and set it at 12 and checked all the other voltages at the preamp and power tube pins. They are all within 10 to 20% of what the manual says. I looked at the bias pot and it looks like it is wired like the large picture of Zaphod's amp. I know that the power tubes are good because I pulled them out of a working amp. Any ideas?

Next, after turning the volume up a bit there is a loud hum. I am wondering if this is related to the bias thing, but I don't know. I swapped out another set of preamp and power tubes and I get the same hum. Also, I am having the squealing issue with the OD up past 3 o'clock like some other builders have mentioned. I have attached a pic of my build and any help would be greatly appreciated. Thanks!

I should also mention that when the pic was taken the high voltage PT secondaries and the red wire to the left side of the zener diode were not wired in yet. They are now... obviously.

Can you not bias it higher by turning the pot the other way, or was that as much as possible?

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Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

No, those seem to be the two extremes. Any ideas?

Recheck all the bias connections with the Zener, bias swiTch and bias resIstor. Maybe the bias resistor Is wired reverse to what is required?

_________________
Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

I just rechecked everything and it looks correct, unless you see anything in the photo that looks amiss. I also bought a new set of 6V6s over the weekend and it is the same issue with those as before. Is there any chance that I could have overheated something?

What's the voltage at the junction of D6, C22 and R45? The solder joint at this turret looks a bit suspect in your photo.

In the photo, there seem to be turrets that weren't soldered, I'm assuming you have soldered everything.

_________________
Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

Coco: yes, everything is soldered in now. I took that picture before I soldered the high voltage secondaries and the red wire to the zener diode.

Mitch M: Thanks for taking a look. I'll check that when I get home tonight.

I checked the voltage at the junction of D6, C22, and R45, and it was -56.3 VDC. I checked it at all three leads coming into the joint and they were all the same, so I assume the solder joint is ok.

I checked voltages at all socket pins and the only one that was more than 15% different from the values in the manual was V4 Pin 7. Mine read at 37.7 while the manual says 64.4.

All this was done with the amp biased at 17.0 mV, which is the highest I could get it. Also, the bias pot was still set completely counter-clockwise. Turning the pot clockwise continues to decrease the voltage reading at the test points. I have checked and rechecked everything I can think of. Could there be something wrong with the pot?

What are the voltages at each end of R46 as you rotate the bias pot from one end to the other?

Also, is the bias reading pretty much the same for both V5 and V6? I mean with the bias maxed, are you reading 17mV between TP1 and TP2 as well as between TP3 and TP2?

I rechecked the schematic and it looks like I may have had the bias switch on the wrong setting. Oops! I switched it to what I thought was the KT66 side and started getting readings that looked correct: about 14 to 31 mV from one extreme to the other (although the pot is still operating "backwards" like I have previously stated). Just to make sure, the 6V6 setting should mean the switch is pushed toward the power transformer side of the chassis (assuming the wiring is done per the layout drawing) and the middle terminals of the switch should connect to the terminals of the switch that are not connected to anything, right?

This still doesn't explain the bias pot working "backward". My amp is wired so that terminal 1 of the bias pot is connected to R46 and jumpered to terminal 2. Terminal 3 is connected to the middle bias test point and to ground (which is per the layout drawing). I was looking at this thread (viewtopic.php?f=14&t=3135) and it looks like Zaphod has wired it differently, where terminal 2 is connected to ground, jumpered to 3, and connected to the middle bias test point and his terminal 1 is connected to R46 only. Which wiring scheme is correct, and could this account for my pot working backwards?

I didn't think to question the bias switch, either. Too obvious!

Yes, double throw toggle switches usually work "backwards" to how you might think. The terminals that are connected are opposite to the setting of the paddle.

So in the 6V6 setting, the center terminals should be connected to the side of the switch that has no wiring and the paddle should be pushed towards towards the power transformer. I'm reading this from the layout drawing. I haven't actually built a Triwatt, but I've built a couple of amps that are Hiwatt inspired.

There isn't a right or wrong way to wire the bias pot. It's personal preference. If you think it works backwards, just change the jumper so it's between terminals 2 and 3 instead of 1 and 2. The more bias voltage you have, the less cathode current you'll get. Some people prefer to increase the bias as you rotate the pot clockwise, others prefer to increase the cathode current.

I think I was headed in this direction when I suggested to Recheck all the bias connections with the Zener, bias switch and bias resIstor. Maybe the bias resistor Is wired reverse to what is required. Well the switch can be confusing too!
We've. Had a few people comment on the trim pot being "reversed". I think it's a preference as Mitch suspected. change it if you prefer.

So did this solve your issues, or most of them?

_________________
Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

Thanks for all you help guys. All the other amps I have biased before increased as the pot was turned clockwise and I thought I had something wrong. If it is just a matter of preference I will keep it like it is as long as I know it is working properly.

I got a little more time to play the amp and troubleshoot the other issues I originally mentioned (hum and squealing) and I have a little more info on that. I figured out that the hum goes away when I plug the OD footswitch in and use it to turn the OD on and off. I have attached a couple pictures of the the wiring as it stands now. Did I wire something wrong with the switch possibly? Grounding issue maybe?

After spending some time playing with the OD footswitch connected, I can say I am really loving the amp. For me, it seems to really excel at the cleanish/edge-of-breakup type tones. But it really does have a lot of gain on tap if you want to take it there too. I'm also really pleased with how well the master volume works. I think there are a lot of great tones to be had in this amp. Thanks Stephen for such a great design!

Anyone?...

Try moving around the yellow,orange and purple wires on the OD switch. All three of those wires are very sensitive, especially the orange one I find.

Try moving the wires around with a non-conductive device while the amp is running. This way you can hear the effects while the amp is on.

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I have experimented with that a little bit. It did affect the squealing issue with the OD pot past 3 o'clock, but I could never get the squeal to completely stop. What I am more concerned with is the loud hum issue even when the volume pots are low. Moving these wires did not affect the hum issue.

I don't have the OD switch in my amp, but if the hum goes away when the footswitch is plugged in, then you could try grounding the jack. chances are you might have a ground loop happening and the jack is eliminating it. If you can get a grounding jack that would also do the trick (grounds out when nothing is plugged in it) usually more common on older amp, but that probably won't fit nice into the pcb..

For the squeal, I had it pretty bad and slowly corrected things until it went away almost completely unless the OD, the preamps, and the MV are all pretty cranked to top.

I shortened all leads coming or going in the OD circuit - moved leads around with a chopstick and any signal leads that affected the sound of the squeal I replaced with shielded wire. I then ran everything under the board. Each change gave incremental improvements.
Don't give up!