Metropoulos Forum

Hey guys, I've got my Bluesbreaker board installed (and a Heyboer JTM45 output transformer) and am now checking voltages without the tubes in.

I appear to have good heater voltage (about 3.4 VAC is ok right?) and am getting approx. -44 VDC on pin 5 of the output sockets, which looks right according to my chart. If I'm understanding correctly this means that the voltage that the power transformer is supplying to pin 6 of the rectifier socket is making through the borad and to pin 5 of the output sockets.

I then flipped the standby and tried to measure the VDC on pins 3 and 4 of the output sockets. I'm getting nothing (well actually about .09 VDC), and I ought to get something in the vicinity of 390 VDC right?

Where should I start in troubleshooting this... any ideas?

thanks!
Eamon

Actually, -44VDC is produced by the bias supply so this has nothing to do with the main rectifier. This means that the bias supply is working OK.

Measure the voltage on pin 6 of the rectifier. If nothing you might have a bad rectifier tube. If you have good voltage there, check the wiring from there. Also check your HT fuse. If this is blown you won't have voltage on any of the tube plates.

Hope this helps.

Jon

Doh! Yeah, I guess I should start with the simplest and most obvious thing the HT fuse.

I'll double check for sure when I get home but I'm pretty sure I was getting voltage on pin 6 of the rectifier. If so, I'll double check how many volts.

thanks!
Eamon

Ok, so I've made some progress... I spent some time with my meter and the schematic that George sent me and figured out I had wired the filter cap and choke wrong. I got that figured out and now with no tubes in (except rectifier of course) I am getting voltage on both pins 3 and 4 of the output sockets.

Here's my question: I'm getting about 450 volts... is this too high? I was expecting more like 390 based on the chart I have, but it's for 50 watters, not JTM45's. Is this OK? I plan on running the set of KT66's I got from George. If this is too high, do I need a different rectifier (I have a GT5U4)?

thanks everyone, I'm learning something every time I work on this.

Eamon

450 with no tubes in the amp is about right with a tube rectifier. This voltage will drop considerably once the tubes are in. With a solid state rectifier the voltage would be anywhere from 500-510. My Super Lead clones with a brand new set of JJ E34Ls set at proper bias has around 485 volts on the plates. So it looks like now you're sitting pretty.

Before you put the tubes in the amp, make sure you set the bias control all the way to the point where it gives you the highest available negative voltage on pin 5 of the power tube sockets. In other words, rotate it to where you have the highest achievable number on the meter. This will put your power tubes at full cold bias. Then bias the amp and you should be done.

Jon

Ok so I installed the rest of my tubes, connected the speaker cable from the combo, and flipped the power on - all knobs turned to zero. I then flipped the standby switch and there was fairly loud kind of crackling, popping sound. I touched the speaker cable and a very LOUD hum came from the speakers. I immediately turned the amp off.

Could this have been caused by a bad speaker cable? The cable does appear (according to my meter) bad - I tested it and it seem to cut out intermittently. It's a fairly cheapo cable with the molded plastic cover so I can't open it up to inspect it. I'm going to wire up a fresh one cable and plug, but was afraid of turning the amp on again if the crackling and LOUD hum might have been caused by something else...

Eamon

Did you remember to run a wire from the common speaker terminals to ground?

That could cause what you're describing.

George

I'm not at the amp right now, but that's probably the issue. I don't remember running that wire. I think I ought to replace the speaker cable/plug too though - it's probably not good for there to be an intermittent connection to load.

thanks!
Eamon

Well, I guess I've still got a problem...

I checked and I do in fact have a wire running from the common speaker terminals (the one closest to the chassis right?) to a ground lug next to one of the output sockets. I verified it for continuity too...

I also wired up a new speaker cable and plug - the old one was definitely dodgy.

Unfortunately, once I flipped the standby I immediately got the same low frequency LOUD hum. I think all my voltages are ok on all the tube sockets, so where to next? Have I maybe wired the taps from the output transformer wrong?

I can take pictures of anything in the chassis, if necessary...

thanks
Eamon

Take pics of the chassis and email them to me.

flemingmras1@comcast.net

Jon

Do you maybe have the output tranny leads switched going to the speaker jacks?
If this is the case, it would cause the amp to go into a positive feedback runaway condition, where your feedback circuit is now emphasizing everything that it is supposed to be canceling out..
When I modded my AC568 Bassman to earlier Bassman spec I didn't realize Fender had switched the OT leads at the jacks, this gives you the sound you're describing. Makes you think all your filter caps took a crap, but replacing them ain't gonna help.

OK, here's how I've wired the output transformer...

The color codes for the Heyboer JTM 45 OT are as follows:
primary- blue/ red CT/ brown
secondary- black/ yellow 4/ green 8/ orange 16

So for the primary leads, I have the blue and the brown leads going one each to pin 3 of the output sockets. The red lead goes to the 32uf filter cap.

For the secondary leads I have the yellow going to the 4 ohm terminal of the impedance selector, green to the 8 ohm terminal, and orange to the 16 ohm terminal (along with a wire going to the 27k resisitor on the board). The black lead goes straight to the common terminal of the speaker jacks (ones closest to the chassis) along with another wire going to ground on the chassis. There is also a wire from the selector to the speaker jacks.

I thought I had verified with my meter which pin was which on the selector, but maybe I got it wrong. If that part is correct, does the rest sound right?

thanks guys!
Eamon

Is there any chance that something to do with the tremolo circuit is the problem? Without the switch turned off it's really "on" all the time right?

I was thinking that if I put in a dead plug (no wires attached) to the tremolo on/off jack it ought to break the circuit. I could then try firing it up again. Anyone think this could be related?

It really does sound like a low frequency feedback...

thanks guys, I know I'll get it figured out eventually and it will be worth it!
Eamon

At first thought it just sounds like too many black wires going to those speaker jacks, but I haven't had time to really suss it out in my head.