trinityamps.com

I finished assembling my TMB. I begin the "Final checkout" (page 70) and per the instructions (bottom page 72) I repeated the power supply voltage checks on page 48 ( my manual/revision is 2.0) as suggested aka all of the filament voltages. I then installed the rectifier (tube) and checked the B+ voltage and it seemed reasonable (I didn't write it down). I then checked the plate voltages, again I recall them being reasonable and I didn't write that down (I'm going to start doing that). Everything seemed ok so per the manual (page 73) I attached a 4 ohm speaker (only thing I have for testing) set the transformer switch to 4 ohms, installed all the of the tubes, all pots all the way down and powered up. No smoke, heaters slight orange glow, all seemed good. At this point I connect a guitar cable and ran my thumb across it... got a small zap. Stop the bus! I rechecked the circuit and found that I missed a ground, corrected that (powering off, checking for residual voltages from the filter cap and all that). I started over with the guitar cable check. I turn the volumes up slightly and could hear some sound from each channel. I proceeded to plugging in a guitar. I got sound, both channels, but very faint. That was enough for that day. I felt I had some progress, some success.

The next day I read the troubleshooting section and decided that I would go back and recheck some voltages. Unfortunately somewhere along the way I disconnected my speaker load and had the amp on and the standby switch on doing voltage checks, which were high... not good :/ I powered down immediately. During that time I had no guitar connected, no signal applied. I reconnected my speaker load and powered up again to check voltages but with I discovered I had blown a fuse. I did a visual inspect for shorts, replaced the fuse and powered up again. When I turned on the stand by I could hear some sound out of the speaker for maybe 10 seconds - again no signal applied, I hope this suggested my power transformer is still ok, but I blew a fuse again.

At this pointed I've pulled all the tubes and am taking a break to decide what to do next. I'm thinking rechecking the power supply voltages and work my way forward again. I'm open to suggestions. Is there any way to validate that I've not blown my output transformer ? Any suggestions?

You can test the OT as follows:

If you suspect your OT for some reason, you can do a resistance check to get an idea of it's health.

Disconnect all the wires from the circuit, then use a digital ohm meter to measure the following resistances. Clip the probes to the leads, do not hold them with your fingers.

Primary
Brown - Red: 275 ohms
Brown/White - Red: 160 ohms
Blue - Red: 331 ohms
Blue/White - Red: 166 ohms
Brown - Blue: 606 ohms
Brown/White - Blue/White: 326 ohms

Secondary
Black - Orange (16): 1.0 ohms
Black - Green (8): 0.7 ohms
Black - Yellow (4): 0.6 ohms
Note, it is very hard to measure low resistance.

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

Thank you Stephen,

I haven't tested the output transformer yet, but I probably will eventually. However I did make some progress today. As I said previously I pulled all of the tubes. I started to go through the power supply check on page 48. The AC voltages with all of the tubes out checked out just fine. E.g. Plate voltage on the rectifier socket, the AC voltages for the heaters on all tube sockets. I was more systematic and wrote down all the voltages as I went this time. By the way, the manual is just a bit inconsistent/confusing between page 48 and 72. One discusses checking the plate voltage for 290, the other 580 between the pins. It kind makes sense if you know that the 290 transformer taps are out of phase with either other and would sum to 580, but for someone without some background in electronics it might be confusing. Just a minor suggestion for doc improvements. I then moved onto the DC voltage check of the rectifier. I plugged in the EZ81 and powered on. I happened to be looking up the tube from the underside as I was going to make the DC measurement off the power supply filter cap. At first I saw the glow of the filament, I was looking for this. Then I started to see some sparks/arching inside the tube, then the indicator light when out. Sure enough, the fuse blew again. So my conclusion is that my rectifier tube has some sort of issue/failure and I'm planning on ordering a new one. It's quite possible that for that short moment when I had no load during my previous testing, I drew too much current and damaged the rectifier tube.

Once I get the new tube, I think I'll do the DC check, but with the rest of the circuit disconnected to ensure some short or failure downstream from the DC supply doesn't blow the fuse and/or rectifier tube again. I'm wondering if there is some way to know how much of current draw there would be from the DC supply and temporarily add a fuse inline there as well. At $12 to $16 a pop, I don't want to go through many more rectifiers.

Open to suggestions.

You can do the OT checks Stephen mentioned quite easily if all the tubes are out and nothing is connected to the speaker jacks. There is nothing connected to the OT in this configuration. The primary coil's center tap is available at the standby switch and the other two ends are on pin 7 at V4 and pin 7 at V5. The secondary coils are available between ground and the impedance switch.

With no tubes in the amp except the rectifier, there is no load on the power supply, so if the rectifier is arcing it looks like it has developed a fault.

You can do a general check for shorts in the wiring fairly easily. With the power off, no tubes in the amp, even the rectifier, and the standby switch closed, measure the resistance between the standby switch and ground. You should see a fairly high reading, several hundred K ohms. The only things connected to the power supply in this configuration are the filter capacitors and you should see the resistance reading getting higher as your meter charges the capacitors.

Oh thank you, this is also very helpful. These are things I can check while I await my tube order and with the minimal of additional solder/re-solder.

Finally getting back to this. I was checking things over and noticed I have missed a section of the ground buss between the two 56K ohm resisters. Even when I connect these (temporarily with a jumper) I'm finding that I'm getting several K ohms then slowing rising (due to the caps in the circuit) NOT "several hundred k ohms" as described.

Perhaps the missing ground buss is the problem and caused the circuit to draw too much current causing the fault in the rectifier and blowing the fuses? Maybe several K ohm (not several hundred k) and rising is ok/normal?

In fact, thinking about this a little more.

350 volts / 3K ohm is about 117 mA draw by the circuit. Googling around the EZ81 can source about 100mA so maybe this is high? I'm not sure if I saw 3 or 4K. 4K would make for 88 mA, more in the ball park at least from a max perspective.

350 volts / several hundred Kohm = is about 1 mA....a very tiny amount of dc current to draw from a supply? Doesn't seem enough to drive the rest of the amp?

The several hundred kohms I mentioned is with no tubes in the circuit so no load. Your actual resistance could vary. The key thing is you saw the reading rising as the filter capacitors charged and you didn't read a dead short.

Did you do the output transformer checks?

If everything reads good, and all the wiring is right, you should be able to go through the startup procedures again.

Great thanks so I did just that. I installed my replacement rectifier (tube) and fresh 2 amp slow blow fuse. Powered up. No smell, no smoke, no sparks in the rectifier tube and no blown fuse, even after taking it out of standby. I proceeded to check the DC voltage at the fiter cap and the plate voltage on the tube sockets (none installed except the rectifier). All read about 420. Now the manual says that these volages will be higher than what is documented on the schematic (or in the manual) with no tubes/no load, but that seems very high compared to the documented voltages; some of the voltages are much lower; as low as 155 on pin 1 of V1 for example.

I don't know if this is an indicator that something is still wrong or if this is normal and I can proceed by installing tubes and re-checking the measurements. Advice?

If none of the tubes are in (except the rectifier) nothing will be drawing current, therefore there will be little or no voltage drop across any of the resistors. Even the ones where it's supposed to be 155 with tubes in place.

420 volts sounds reasonable in this state. You should be able to continue with the startup procedures.

Well it's been along time coming, mostly because of my limited time, but my 18 watt TMB came to life a couple of nights ago. Last night I also got a proper 2x12 cabinet. Thanks for all the help guys. A few observations:

1) TBM channel is working, does get a little squeally with the volume/gain all of the way up. There seems to be a jump on the gain knob from ok to squeal, nothing in betweeen. You know that sweet spot where you can hold a note forever. Maybe I just need to work the tone controls a bit more to find it.

2) The TMB channel does not have the amount of gain that I expected. From the kit description "This amp does classic rock crunch to metal". Out of the gate this amp will not do rock (as in Aerosmith or AC/DC), it more clean with very light clipping and some compression - and doesn't come close to metal. Not without a pedal in front of it. I recall there's some notes somewhere - I think the schematic - to get more gain/grind/compression. I'm going for a JCM800 or even JVM tone range. I like gain/compression, lots of it.

3) It's not as loud as i thought it would be. My gigging amp is a PRS custom 2 channel 20 watt. To keep up with my band members I play with my 20 watt on about 12:00. (exact same speaker cab), but all the way up the TMB is not nearly as loud.

4) My normal channel just squeals. I'll work my way through the troubleshooting section, but again open to any advice.

All in all building this amp has been a great experience. I made this purchase to learn more about tube amps. I've learned a few things but I've got a feeling that now that it's built and somewhat working, I'm going to learn even more now getting it tweaked out to more of what I'm looking for.

Ok, I need to take some words back. I initally tested with a PRS EG2 (older strat style) with Fender Noiseless single coils. I re-tested with a guitar with some higher output pick ups. Seymour Duncan Pearly Gates, and yes with gain all the way up I can get into a 'classic rock' range of grit. Still, not hard rock, not metal. Maybe early ZZ sort of sound, but not 80's ZZ with a ton of grind. Anyway.. it's all subjective. For me, more is better, and it's a kit so I would be able to tweak to get it it there right... and without a distortion pedal ideally.

Onto my VRM module, maybe adding an effects loop, mayby tweaking the tone stack... and if I can figure out how, more gain/compression.

Again super thanks to all who chimed in on my thread and helped me get to this point. If you're ever in Troy, MI I'll buy you a beer.

If the TMB is not enough gain, you may want to look into cascading the two channels. TMB into the Normal channel. We can help.
Also, at least jump the two channels.

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