Black face Dual Showman ??s

[THX1136]

I'm working on my Dual Showman as it had "quit" on my son who has been using it. He said he was adjusting a pedal and the amp just went dead - no noise, just stopped working. I figured the problem was in the voltage rectifying portion of the circuit. I had the same thing happen to me back in '76 or so.

Went in and replaced all the diodes since they are inexpensive even though the ones that were in checked okay. Turned the amp on briefly and got a loud buzzy, spitty sound mostly 60 Hz and associated harmonics. Quickly switched off. Repeated this a couple of times and then just let the fuse go to observe whatever might happen. A bit of smoke from the fuse holder and that's it with the "noise" dying away somewhat slowly. (I assume this is an indication of the filter caps discharging?)

Tubes are known good - power and pre. No evidence of leakage from the big electrolytics in the power section (the 3 20s and 2 70s). Went over the whole circuit with a magnifying glass and could find no cold solder joints. Checked all components in circuit referenced to ground and across each component - no obvious open circuit. Checked across all pins of all the 6L6s referenced to ground and got close readings on all pins for each tube compared to the others.

It sounds like a problem going to ground to my uneducated mind. Can't find any obvious signs of failure. I only have a digital VOM to diagnose with. Short of desoldering a leg on each component leading to ground and checking each what are logical places to look into? I'd rather fix it myself if possible since money is somewhat tight for me right now. I realize I may have to just take it into a tech somewhere, but I'm not quite ready to give up yet.

Thanks in advance for any insight anyone can offer.

[Tom Phillips]

Well it sounds to me like you are just trying things without following an organized troubleshooting procedure. If you are intent on working on the amp yourself check out the Tube Amp Debugging page at http://www.geofex.com/ampdbug/ampdebug.htm

If you are having fun, that's fine. Otherwise you may be better off getting together with someone who has more experience or dropping off the amp at a known good tech.

One thing to keep in mind is that newly or recently replaced parts do no mean that they are known good parts so don't make any assumptions like that when troubleshooting. Go by the symptoms and the facts supported by your measurements.

Regards,
Tom

[THX1136]

Thanks for the link Tom. I am definitely a novice at all of this. I know some on how to evaluate components as to whether they are good or not, but using just a VOM is limited I'm sure. In the past I've recapped my Pro, Beatle and Royal Guardsman, but that's easy stuff compared to troubleshooting what I thought was an obvious thing that isn't that obvious. I appreciate your input. Thanks.

[6267]

Make sure that there are no accidental bridges between tube socket pins. Is the hum affected by the volume control? Is it still present when you pull the input tube? Generally, I try to isolate components by swapping tubes around/replacing/removing them, messing with the knobs, trying different cables, guitars, etc. If you can get the problem to change, you are on the right path. I thought my amp had a grounding problem for months because I only have one guitar...turns out my cheapo guitar had the grounding problem, and the amp is dead quiet!

If the hum is 'before' the volume control, you can *generally* assume it's in the preamp/input section. If you can't change the volume of it, you can start to assume power supply or power amp problems. If you had one dead or otherwise not working power tube, the amp will definitely hum a lot. Check filter caps too if the hum is not affected by the volume control...the fuse suggests power tubes or filter caps to me, but most likely there is some kind of short to be causing that.

[THX1136]

6267, the amp only stays on for a very short while before the fuse blows. The sound is loud although the channel volumes are at a minimum setting. The power tubes that were in after the replacement of the diodes in the rectifying circuit were taken from a working Mesa Boogie.

I was thinking the same thing you suggested - the problem being in the "power" end of things. After gathering info from the link Tom posted I'm going to approach the whole thing systematically. The "noise" it made after the diode replacement just seems to indicate a short somewhere. Back to square one and starting logically to go through things.

Thanks for your reply!

[acorkos]

"known" good tubes? tubes are always suspect. known good tubes can go bad. first thing i would try is to remove all the tubes and see if the fuse blows. if not, add them back one at a time until you install the one that causes the fuse to blow. this is something you can do without even busting out the soldering iron, and if the fuse is still blowing, you've at least probably ruled out the tubes.

[loudthud]

Check that there is about minus 50V on pin 5 of each 6L6. It should be there before the tubes warm up while the amp is in standby mode.

[6267]

It does seem like a good test might be pulling all the tubes and checking voltages at the socket. They will all be a bit high, as much as 20% or so because the transformer will be unloaded. You'll want to make sure the B+ isn't somehow there but partially shorted and thus low, and that the bias voltage is present. Heaters should be 6.7-7.5 or so unloaded, and the bias voltage should be present. Make sure there are no accidental solder bridges under those diodes or anywhere you recently did work...it could be the fix caused a new problem (this is so common it's one of my first checks when there's a problem). The "sputtery" nature of the sound strongly suggests a bad solder connection or a over-solder short...the diodes are all good for sure, right? Even though they're new, make sure none of them went short. Good luck!

[THX1136]

The diodes checked good before I put them in. On close scrutiny with a magnifying glass the solder joints look good on the diodes and the rest of the "board" they are on. I can obviously pull them all again and check as that isn't that big of a deal. I did check the underside and all is as it should be - no solder bridges. That would seemingly take a ton o' solder to do that as the solder points are quite far apart. The spittiness in the sound was on top of the loud buzzy hum.

Like I mentioned I'm planning on going through a orderly check process soon. Going to pull the tubes and see what happens and proceed from there. Thanks for the info everyone - it will all be helpful as I proceed with the work.