Changing output transformer ?

[dubtrub]

I have a Bassman 100 (supposedly 100 watts) with a replaced OT of unknown make model or impedance with too little headroom when using a bass guitar. Useable maximum volume is around four on the knob before it breaks up too much. That would be great to have that early breakup for a standard six string guitar but that is not what this amp is used for. The amp schematic shows it to have 4 ohm output. So based on that, when I installed speakers in the cabinet I bought a pair of Peavey Black Widow 8 ohm extra heavy duty bass speakers and wired them to 4 ohms. Those Peavey's weigh 20 lbs each and lifting the amp in and out of the back of a Jeep creates a hernia, so my thoughts are, change the OT to a brand new 8 Ohm and build a new baffle for the speaker cabinet and run one 8 ohm speaker. This would make transporting much more user friendly and hopefully create more useable headroom for a bass guitar.

The Weber site does not have a replacement output transformer for the Bassman 100 with an 8 ohm tap but they do have a Marshall 100 watt OT with multiple taps There is a difference on the input impedance of 1900 ohm on the Marshall OT vs. 2000 ohm on the Fender replacement. Question is, would this be a compatible replacement choice?

Here's the Fender Bassman 100 replacement OT rated at 80 watts. https://taweber.powweb.com/store/022889sch.jpg

This is the Marshall 100 watt OT. https://taweber.powweb.com/store/ot100sch.jpg

[MWaldorf]

Weber has a generic OT for 4x6L6 Fenders in the 100W range: W022756 125P34A. I'm not sure what the ohm reading is, but they list it for amps ranging from the Showman to the Super Six Reverb. Isn't the Bassman speaker cabinet 4ohms?

[dubtrub]

Isn't the Bassman speaker cabinet 4ohms?

Yes, that's where the second problem comes in. I want to run just one of the Peavey's and it is 8 ohm. I know it is said that it can safely be done (run 8 ohm speaker with 4 ohm output) but if I'm going to buy a replacement OT, I'd prefer to have a multi tap so I can get the correct impedance match. I've got too much money invested to buy an additional $150 four ohm speaker. The transformer is much cheaper and either way, the existing OT needs to be replaced. I cannot find a direct replacement 8 ohm OT for the Bassman 100, they are all 4 ohms.

I'm only seeing a 4 ohm 80 watt OT on the Weber site. Also, the part numbers you listed are for the power transformer not output transformer but it is on the same page.

[MWaldorf]

Isn't the Bassman speaker cabinet 4ohms?

Yes, that's where the second problem comes in. I want to run just one of the Peavey's and it is 8 ohm. I know it is said that it can safely be done (run 8 ohm speaker with 4 ohm output) but if I'm going to buy a replacement OT, I'd prefer to have a multi tap so I can get the correct impedance match. I've got too much money invested to buy an additional $150 four ohm speaker. The transformer is much cheaper and either way, the existing OT needs to be replaced. I cannot find a direct replacement 8 ohm OT for the Bassman 100, they are all 4 ohms.

I'm only seeing a 4 ohm 80 watt OT on the Weber site. Also, the part numbers you listed are for the power transformer not output transformer but it is on the same page.

Oh power transformer, output transformer, what's the big deal?

FWIW, I ran my '67 Twin Reverb into a single 8ohm K-130 for years with no ill results to the speaker or the amp.

[Mr. Bill]

Look at the Magnetic Components site, they have a 100 watt Fender output with 4-8-16 ohm taps. The part number is 40-18013. I have used their output transformers before and have been impressed with the cost/value and they are made right here in Chicago.

As for the clean headroom question, there are other factors involved that will effect this. Power supply voltage, output bias and tube gain to name a few.

[dubtrub]

Look at the Magnetic Components site, they have a 100 watt Fender output with 4-8-16 ohm taps. The part number is 40-18013.

That looks like what I need. Thanks!

[vibramutant1965]

i like mercury magnetics , they are a little more money, but they give good free advice and the producfs are simply better.

[zarfnober]

Magnetic Components PT's and OT's are made the same way they were back in the day, and are hand wound. Mag Comp has been making transformers for over 50 years. They used to make, and have reissued, the OT for the Supro Thunderbolt(and all Valco) and many other popular amps. Nothing against Mercury, but they are made differently; different wire, plastic bobbins and they use mylar instead of paper wrapping. Perfectly fine for new designs, but did the original Fenders sound bad? You may prefer that, maybe not. But the MagComp OT's are made the same way Fender had them made way back when. And Mercury's are pretty darn expensive. MagComps are about as close to original as you can get. FWIW I built an 18 watt kits with their PT and OTand let people that owned reissue Marhsall 18 watt(hand wired models too) play thru it and they liked the tone better.

Great thing about Mercury is they are making OT's for a lot of amps that haven't been available for years. Good for them, used a set in a Silvertone Twin 12, sounded fine. But slightly different than original.

For MagComps, got to www.triodestore.com nice people and good service.

Give them a try, you'll be happy,

Rocco

[dubtrub]

As for the clean headroom question, there are other factors involved that will effect this. Power supply voltage, output bias and tube gain to name a few.

When I had the amp opened up a few months ago to change the electrolytics and new tubes, during the biasing, all the voltages were checked against the schematic. Are there some tweaks outside the schematics that I could do to gain more volume and cleaner headroom?

Here's how the A/B comparison between my 40 watt Bandmaster with two 12s went in comparison to the 100 watt Bassman with two 15s.

a. I played a standard six string guitar through my Bandmaster with volume maxed, then plugged same guitar into the Bassman with volume maxed. The Bandmaster was twice as loud and had a nice musical breakup. The Bassman would go all the way to max but the volume was anemic. I did the same test with both amps volume control on four and the Bandmaster again was dominant. The Bassman seems like it's only generating about 25 watts of power.

b. Next I did the same test above but this time I used only the Bandmaster 2 x 12 cabinet for both amp heads and again the Bandmaster smoked the Bassman. I couldn't run the same test with a bass guitar for fear of blowing the speakers in my all original 65 Bandmaster.

I've never played or even heard a Bassman 100 before I bought this one so I have no idea what power they should generate but I have played through an old 50 watt Bassman and it will eat my Bandmaster for lunch. Shouldn't a 100 watt Bassman have even more power, like compared to an 80-100 watt Fender Twin or Showman?

I know the output transformer currently in the Bassman 100 is a replacement as it has vinyl covered wires spliced into cloth covered wires and the solder joint wrapped with electrical tape. It is a huge transformer and from photos I've seen on the internet, it looks to be correct. Plus there aren't any additional holes drilled in the chassis indicating a different size transformer being replaced. Without the necessary tools and knowledge to check the ohm rating or measure it's output, I can only assume that it isn't producing efficiently.

Just for the record, the amp didn't have any more power before the tube and electrolyics change. Actually that is why everything was changed. At low volume it sounds fantastic. Since it takes a forklift to move the Bassman 100, it doesn't seem worth while for such low volume. Other than the obviously changed OT, everything else looks original and untouched. Your thoughts?

[Mr. Bill]

The Bassman 100 with 4 output tubes should be louder than the Bandmaster. It should be as loud as a Dual Showman or a Twin Reverb.

The reasons for a loss of power could be related to the output transformer or not. A Fender transformer will have a part number stamped into the metal cover, maybe something like 022XXX. Does yours have a Fender part number and does it match what is listed on the schematic?

I will look up the schematic for your amp and see what I can find out, but until then, check the part numbers of the output transformer.