Metro - 50w - First Power Up - Using Dim Bulb Tester

[Janglin_Jack]

I have progressed with my testing - (I actually do some testing now) from the first amps I built, where I just flipped the switch with my fingers crossed. I do test voltages now. With my latest build, I completed it and then it went to storage before I tested it and fired it up. I have it back in my possession and decided it was time to fire it up. I have measured 120v and the power indicator lamp lights when I turn it on. I am getting 3.15v on the heaters....in another post I was missing a ground wire for my bias network. I fixed that and I am getting -50vDC on Pin 5 on the power tubes and the bias pot adjust that value. I am getting ~ 350v AC at the rectifier.

At this stage I have only had preamp tubes in. I remember I used to use my variac to bring the voltage up slowly to form the caps. Do I do that with the power tubes installed and a load box? I have a dim bulb tester now, but I haven't used one before. I understand the concept and the bulb will light if there is a short. Am I missing any steps before I put in the power tubes....I am not sure why I am so scared to turn it on...it never used to bother me before.

Am I missing some test steps before I can form the caps and test the rest of the voltages and finally bias properly? Help me out as I am worried I am going to fry something.

Mike

[danman]

Normally you would plug into the light bulb limiter for the very first power up without tubes. If the bulb only flashes for a brief moment and then goes dim, you are safe to remove the limiter, install the tubes and power up. You don't want to be plugged into the limiter once the tubes are in because they draw enough current to light the bulb brightly depending on which wattage bulb you use. Since you have already powered it up once, the caps have seen current and should now be formed. No point in trying to bring them up slowly on a variac at this point. I would throw the tubes in, bias it up and give it a try. Also remember that the limiter affects your voltages in the amp so don't make the mistake of trying to take voltage readings while plugged into the limiter. They will always read quite a bit low while plugged into it.

If you ever need to form old caps in the future, using a variac is helpful but it only controls the voltage. What you really want to control is the current flowing through the caps. Installing a temporary 100k resistor in the B+ line will limit the current to a very low level and allow the caps to form slowly and safely. Here is a great thread on forming the caps using only a 100k resistor.
http://forum.metropoulos.net/viewtopic. ... ilter+caps

[Janglin_Jack]

Normally you would plug into the light bulb limiter for the very first power up without tubes. If the bulb only flashes for a brief moment and then goes dim, you are safe to remove the limiter, install the tubes and power up. You don't want to be plugged into the limiter once the tubes are in because they draw enough current to light the bulb brightly depending on which wattage bulb you use. Since you have already powered it up once, the caps have seen current and should now be formed.

If I recall my steps correctly, I did some measurements with no tube and a few with preamp tubes. I left the preamp tubes in and started to try to "form" the caps. At that stage is when my 100w bulb was lit and didn't seem to shut off. So after a few seconds I panicked and shut it down. I wasn't sure if I had an issue or not. Based on the voltages I was getting, do you think it is OK to put in all the tubes and try to get it biased up? Like I said, in all my other builds I never had any concerns before and just went for it.

I remember one time a different amp I built, it was like 3am and my wife and kids were sleeping. I attached a speaker and took it off standby and the squeal of positive feedback scared the living shit outta me and woke everyone up. Reversed the OT leads and was in business.

Mike

[danman]

The light may have been lit because of the current draw from the preamp tubes. Try this...plug into the limiter, remove all of the tubes and then power up and put the standby switch in "play" mode. The bulb may light briefly but if everything is wired correctly, the bulb should go dim pretty quickly with no tubes installed. If the bulb stays dim, go ahead and remove the limiter, install of the tubes, set the bias voltage for the highest negative voltage and power up. If you don't get the positive feedback squeal, go ahead and check your B+ voltages, set the bias and then give the amp a test play.

If you used new caps in the build, they will form instantly once power is applied the first time. If your amp ever sits for several years unpowered or if you decide to use older NOS capacitors in a build, the 100k trick in the link that I posted is very easy to do on the 50 watt models. You must use this process the very first time you apply electricity or you will miss the opportunity to form them.

[Janglin_Jack]

Hi Dan,

I plugged it in - no tubes with the bulb limiter. Power on - the 100w bulb doesn't light. Take it off standby, light goes bright. I waited maybe a count of two and then shut down. Should I wait longer than two seconds? You noted it will probably light breifly...how long would you say is briefly?

Since my heater wiring is right and I measure 325v off the transformer - (pre rectifier). The first thing after the standby is switched is the rectifier. I my other builds I have used a tube rectifier GZ-34. I have used a SS plug in the rectifier slot.

In this build I made my own with diodes. Can someone point me to a picture or diagram of how a 50w 1987/1986 rectifier should look. Mine is wired in a diamond shape, the PT wires attached on in the middle of each set of diodes...a ground wire on one end and output on the other end.

The Metro instructions I have show using a bridge rectifier so I can't see what is going one with the individual diodes. The Unicord schematic looks like it uses two diodes one each connecting to the PT and then joining together and wired to the main filter cap. (I am not a schematic expert - but to me that looks like they use only two diodes.

I saw a Ceriatone diagram that puts two diodes in a row and then joining with a wire output to the filter cap.

Since I am not 100% on this one section, I suspect this could be the issue. I have mine grounded and none of these other seem to ground. I suppose I have it double grounded as the transformer is 350 - 0 - 350. So maybe my rectifier design is for an amp without a center tap for ground.

I think I was getting ~450v so there is some rectification happening, but maybe I am losing some current with the way I have it wired, (probably should be closer to 490v (350v x 1.4 = 490v). Hence my bulb tester is "seeing" a short?

A detailed picture would be great if someone has one.

FYI - on EL34 world site...mine is wired like what they describe as a "full wave bridge rectifier". EDIT - I found this conversation about exactly what I have done. The transformers are different, but this is exactly what I have done. https://el34world.com/Forum/index.php?topic=22725.0

Do I need to use new diodes? EDIT: I have 1000V 1A 1N4007 diodes. Are those OK? Now I just need to know exactly how it SHOULD be wired and I think I am in business.

Mike

[Janglin_Jack]

I used four 1N4007 diodes. I couldn't quite make a diagram the way I wanted to but the two leads from the PT goes through two diodes in series and then joining up to connect a lead to the filter cap.

PT---1N4007-->---1N4007--->\ B+ to filter cap
PT---1N4007-->---1N4007--->/

Now, without tubes I can turn the power on and take off standby and the dim bulb tester doesn't light at all. I am going to start over and test voltages and hopefully I won't have anymore issues to report.

I would still like to see how a SS rectifier is wired. Do I need two pairs of diodes the way I have them? Hoping to get this confirmed before I more forward.

Mike

[Janglin_Jack]

Well, I moved forward. I went over all the voltages and was getting 490v on the plates. I couldnt find my bias probe, so I added some 1ohm bias resistors at the power tubes. At first I thought I wasn't getting a mV reading, but it turned out it was 1mV. I spun the bias pot and dialed it into 33mV/33uA. According to my calc that should be 490v*.00331=16.2w or right at 65% of EL34 25w. I used my load box, so I was still worried when I attached it to my speaker cabinet. No feedback squeal when I turned it on!!!! Whisper quiet. I played it at very modest volume so I couldn't crank it up and hear the roar. Maybe tomorrow for that test.

So in all, that bulb limiter may have saved my build. Not sure if that double grounding issue could ruin anything, but it was great to see that it works. I learned a bit on how to use it. I found a few resources that spell out some tips on trouble shooting using the dim bulb tester. Thanks again for the help Dan.

Mike

[danman]

Your welcome. Happy to hear that everything turned out good!