Hopefully Paying It Forward

[HomeBuild]

I'm relatively new here and I've been learning a lot about building my own amps, so it's time to give back.

I've been building #102 for quite some time now. My obsession with replicating details tends to slow me down some. I'd like to thank the member who created the pictorial layout of 102 and accompanying schematic. Even though I'm basically a "newb" at this, both visual aids have been very helpful to me and I'm sure to others as well. This is great forum, thank you all.

What I found interesting was the power transformer in both the schematic and layout. The layout labels the PT as Dual Showman technically by number and yet the schematic labels the PT as "unknown". In my build I'm using replica Twin iron with a center tap on the secondary power legs. I decided to change to the half-wave style rectifier in my quest for 102 accuracy and still ground the center tap. Turning the amp on for the first time gave me a dose of humility. Normally I use a dim bulb tester for first time power ups, but I was feeling confident I did a good job. Well, the amp came to life and died as fast as a Slow-Blo fuse. After consulting with a learned friend, I started to realize maybe 102 had a transformer without a center tap because it had lower voltages than usual for a 100W PT and a half-wave rectifier was used to boost the voltage. After blowing a few more fuses, I decided to take some voltage readings. I powered up again leaving the standby engaged (not operational). Voltage at the rectifier board before the diodes was 742 VAC for my PT. Staying to the "left" of the standby switch, voltage at the second 220 MF Filter cap was a little over 1000 VDC!! Yikes! I powered down in a hurry and set up the dim bulb tester.

My very good friend, who was an authorized Fender amp tech in his day and currently a vintage radio restorer, later informed me half-wave rectifiers multiply secondary voltage by 1.414. There was a reason a half-wave rectifier was used and I had no idea. I always told myself "I know enough to be dangerous" and that axiom came true.... But I love to build amps!

There's more... I noticed my guitar made no music through the amp during the dim bulb test. So my mentor by phone had me start reading DC voltages on the B+ chain... Nothing past B+2. I pulled up the filter board after discharging the filter caps and I forgot to add a jumper between B+3 and the 22K. After making that correction, my eye for detail caught a little change in color in the first 20 MF where the + lead enters the body of the capacitor. A quick check with a VOM read like the cap was dead, fortunately I had a replacement. So I unsoldered my CT and tried again, this was at the point where I still didn't understand about half-wave rectifiers and I used up some more fuses when the standby was off (operational).

I started to learn that the creator of the amp used a half-wave rectifier because 102's PT was not the usual Twin iron like us emulators use. I went back to the 124 schematic I've seen and noticed the rectifier was 6 diodes and full-wave configuration. Not wanting to spend the rest of my life finding an "unknown" transformer, I resigned myself to use the correct rectifier system for my available PT. My 742VAC now became a 480-ish B+ and lower down the line. Dim bulb off, switches up and the amp came to life finally. I was very fortunate I made a jumper error during the build with the correct rated fuse, think about 1000+ VDC traveling down the chain and the possible damage it could have done. Fortunately the only casualty was my pride and a 20 MF electrolytic.

So I've learned to not build everything like I see it because there are sometimes reasons for what engineers do that are not terribly evident to the novice. My advice is don't blindly follow a layout unless you have more knowledge. If this saves anyone from harm to person and possession, then hopefully I have paid it forward!

[Structo]

I know there was a schematic floating around that had, I believe a bridge rectifier.
But I don't think anybody builds them like that.

Just go by what the #124 layout and schematic show as those are proven to be correct.

Regardless of what type is used, you should always use a current limiter when first energizing a new circuit.

What cost a couple bucks to build could could potentially save hundreds of dollars of components.

[MKB]

IMHO the power transformer is possibly the least important major amp component as far as tone goes. If a PT can provide the proper voltages and currents, and does not have too much or too little sag during transients, it should work OK in most any particular amp.

I'm used to thinking of two different HT rectifiers in tube amps, either a bridge rectifier or a full wave (center tap) rectifier. Not really sure why these two were used interchangeability other than the full wave rectifier is easier to implement with a tube rectifier. You can make a tube bridge rectifier, but it isn't easy and takes a few additional transformer windings. But I digress.

In general, if a transformer B+ winding has no center tap, it was designed for a bridge rectifier. This can be confirmed by reading the full secondary voltage and multiplying it by 1.414; if this puts you in the ballpark for your desired B+, use a bridge rectifier with this transformer.

If the transformer has a B+ with a tapped secondary, it could use either rectifier type (Some Marshall 100W amps and the Fender red knob Twin use a center tap secondary with a bridge rectifier). To find out for sure, measure the full secondary voltage and multiply it by 0.707; if this is in the ballpark for B+, use a full wave rectifier. If it is too low by say 40-50%, use a bridge rectifier.

These rules are useful as you can really use either rectifier type with a center tapped secondary, and either roughly halve or double the secondary voltage to get certain results. I built a pair of hi-fi amps using 211A transmitter tubes, and used a standard 100W (4XEL34) tube amp transformer designed for a full wave rectifier. I used a tube bridge rectifier on the secondary and got over 1kV for the tube B+, which was perfect.

The above are generalizations, there is much more to power supply design than this. But these suggestions can help prevent bad results when using a surplus power transformer in an amp build. In my 102 build, I had a nice Triad power transformer I wanted to use, but the B+ calculation showed that a full wave rectifier was required to get the proper voltages. I rewired the bridge on the supply board for a full wave rectifier, and everything worked great.

Also IMHO, anyone that is going to build an amp should budget in a variac to power up the amps for the first time. It can save a ton of money and trouble if the amp is first powered up with slowly increasing voltages from the variac, while monitoring the critical voltages in the amp. You can often find and fix a problem before anything is harmed or explodes.

[vibratoking]

With all due respect, your comments and conclusions lead me to believe that you are still confused. There are quite a few rect possibilities. You may want to take a deeper look.