The new dumble modded bassman build video series... begins!

[sluckey]

I noticed this transformer (290YX) has 186v secondary. Will you be using a Bridge rectifier to increase the B+ .

Good point. He must use a full wave bridge with that PT. If he tries to do a full wave conventional rectifier his unloaded B+ will only be 263V.

[pompeiisneaks]

I'm following the typical fender bassman schematic, for PSU excepting the caps which are more 'marshall' topology. The transformer states 186VAC but that rating is between center taps, so it should be about 372VAC unloaded out the transformer. After rectification through that at 0.9 it should be about 334VDC. Oddly the bassman schematic I've seen shows 330VAC into a full wave rectifier with 445 at the output so I'm confused there.

BUT I'll know more for sure when I do first power up, if I'm super low voltage then I'll have to see.

If someone smarter than me can help that make sense, I'd love it. The transformer I bought was specced as a Bassman Classic 70 (I didn't see one called silverface bassman or the like) the only other that seemed in the ballpark was the T290EX which does just say "Bassman" in the description, but it's at 640VAC which would be WAY higher than the expected. Unless I'm confusing this badly somehow?

If I need another transformer, I think that one would work for the other dumble I have up next to build, going to do a 102, since I got donated a chassis, pcb and some NOS components by forum members (THANKS!) and I had suggested I call that one 'Stone soup'

Any ideas on if the transformer is wrong for this build?

~Phil

[sluckey]

Yes, it's wrong for your schematic. It will only put out 263VDC unloaded if you use your schematic.

However, if you use a FWB you can get 526VDC unloaded. It's a very simple mod to your layout. Just requires a couple jumpers, and feeding the HT AC differently. See attached pic.

I seem to remember you having a similar problem with this when you were building your Vox AC-30. I'm also attaching a rectifier.pdf that you should take a look at. Maybe commit to memory or nail it on the wall above your bench...

[pompeiisneaks]

Yes, it's wrong for your schematic. It will only put out 263VDC unloaded if you use your schematic.

However, if you use a FWB you can get 526VDC unloaded. It's a very simple mod to your layout. Just requires a couple jumpers, and feeding the HT AC differently. See attached pic.

I seem to remember you having a similar problem with this when you were building your Vox AC-30. I'm also attaching a rectifier.pdf that you should take a look at. Maybe commit to memory or nail it on the wall above your bench...

I fully understand how the rectification works. I also fully understand how to wire both of them up.

What I don't understand is how to determine the right transformer.

On the bassman schematic I was looking at it showed 330VAC on the windings, but 445VDC out of a bridge rectifier, not a fwb.

I don't understand how the transformer specs are to be translated into the output voltages. I see some transformers that just list the VAC output, but others list it as 186 - 0 - 186. I thought this one, that seems to list on the datasheet the 186-0-186 with 372VCT meant it was 372VAC output, but now I think I might actually be getting the difference. The Fender ones show a 330 - 0 - 330 right? it's just showing 330 on each leg but I misunderstood that to be 330 output.

The problem for me is that the math STILL doesn't make sense.

My understanding was a bridge rectifier is .9x input VAC
FWB is 1.4 times

so if you're inputting 330VAC X 2 = 660 then .9 times that is NOT the 445VAC output the fender shows....

Or I'm still just misunderstanding how the transformer ratings work.

The input VAC for 445 should be about 495VAC no? but instead I see either 330 or 660, either one doesn't make sense.

that's what I'd love to understand.

~Phil

[pompeiisneaks]

Also I think your picture is wrong sluckey. You show HT on the second AC connection. In either rectifier, the output is still at hte left of where the AC input is, but the difference is that in a FWB you input the AC on the middle of there like that picture.

On Bridged, you input AC at the right and don't ground the right... correct?

Also for FWB I'm pretty sure I then do NOT ground the Center tap.

So what do I expect for my output voltages on this amp?

330 x 1.4 ? = 462?

this is the part that seems to bite me every time... and every time I try to google it, nobody has a good 'howto' on translating transformer stats into real world output from differing rectifiers, either that or I've had it explained but I keep forgetting it or getting confused.

~Phil

[sluckey]

I don't think you do fully understand how the rectification works. For example, you say "My understanding was a bridge rectifier is .9x input VAC... FWB is 1.4 times". Guess what. bridge rectifier ***IS*** FWB. One and the same. One is spelled out, the other is abbreviated.

There are different multiplication factors for voltage output for any type rectifier. That factor will be different depending on the type filter/load you apply. That could be a totally resistive load, or choke input load, or cap input load. We mostly see cap input load in guitar amps.

That Hammond pdf does a good job explaining the different configurations an how to calculate the voltage out. Your 290YX transformer is listed as 372VCT. That's the same as 186-0-186. IOW, there is 372V all the way across the HT winding, but only 186V from each winding to the center tap. Here's the datasheet...

https://www.hammfg.com/files/parts/pdf/290YX.pdf

Now look at the rectifier.pdf. The last diagram on the left is for a FULL WAVE Capacitor Input Load. This is what your Bassman schematic shows. Notice that V (Peak) D.C. = 0.71 X Sec. V A.C. V(Peak) is what your caps will charge up to id no load is connected. Doing the math for your 291YX PT shows .71 x 372 = 264VDC (unloaded). Not nearly enough for your amp!

Now look at the third diagram in the right column. This is for a FULL WAVE BRIDGE Capacitor Input Load. Notice that V (Peak) D.C. = 1.41 X Sec. V A.C. Doing the math for your 291YX PT shows 1.414 x 372 = 525VDC (unloaded). This is what you need for your amp.

The pic I posted above shows the simple mod you must do to your PS board if you want to use this 290YX PT.

[Guy77]

Thanks Sluckey.
Phil the good news is that the approx 500V B+ you will get from this amp should make for some very cool overdriven tones !
I would make sure your caps can handle the high voltage B+ you will be good to go!


Cheers
Guy

[pompeiisneaks]

I don't think you do fully understand how the rectification works. For example, you say "My understanding was a bridge rectifier is .9x input VAC... FWB is 1.4 times". Guess what. bridge rectifier ***IS*** FWB. One and the same. One is spelled out, the other is abbreviated.

There are different multiplication factors for voltage output for any type rectifier. That factor will be different depending on the type filter/load you apply. That could be a totally resistive load, or choke input load, or cap input load. We mostly see cap input load in guitar amps.

That Hammond pdf does a good job explaining the different configurations an how to calculate the voltage out. Your 290YX transformer is listed as 372VCT. That's the same as 186-0-186. IOW, there is 372V all the way across the HT winding, but only 186V from each winding to the center tap. Here's the datasheet...

https://www.hammfg.com/files/parts/pdf/290YX.pdf

Now look at the rectifier.pdf. The last diagram on the left is for a FULL WAVE Capacitor Input Load. This is what your Bassman schematic shows. Notice that V (Peak) D.C. = 0.71 X Sec. V A.C. V(Peak) is what your caps will charge up to id no load is connected. Doing the math for your 291YX PT shows .71 x 372 = 264VDC (unloaded). Not nearly enough for your amp!

Now look at the third diagram in the right column. This is for a FULL WAVE BRIDGE Capacitor Input Load. Notice that V (Peak) D.C. = 1.41 X Sec. V A.C. Doing the math for your 291YX PT shows 1.414 x 372 = 525VDC (unloaded). This is what you need for your amp.

The pic I posted above shows the simple mod you must do to your PS board if you want to use this 290YX PT.

First off sorry that was a typo, I meant Full Wave rectifier vs FWB rectifier. I.e. the way mine is now is a full wave, and you're recommending I change it to bridged. (which I will 100% do, but was just confused as to 'why' and how my math was broken. )

https://www.electronics-tutorials.ws/diode/diode_6.html

Sorry, I do understand that getting the terms right is very important.

On the other hand, I have read that pdf many times and completely forgot the difference between unloaded, resistive load, capacitive load, and choke fed variations there. That does make total sense. Sometimes I feel like I forget more than I learn.

what I don't understand, is that you've got one AC connection in that picture, but there are two ac leads. Also my understanding was that HT/B+ comes from the left. I.e. attached picture of what I 'thought' would be the right way:

Thanks for the explanation, that makes perfect sense. Please do confirm I'm right about now NOT connecting the center tap, I'm like 95% sure that's right but don't want to blow up something.

ps1_LI.jpg

~Phil

[sluckey]

Haha, I think I may have contributed to the confusion. In my pic you think the red wire on the left is labeled "HT" and the red wire on the right is labeled "AC". I can see how that would happen. But in fact, I used a single label "HT AC" to represent ***BOTH*** of the two red wires from the PT. Those wires are often referred to as HT (High Tension) and they are always AC (Alternating Current). That pic shows you exactly how to connect your PT wires to those four diodes to make a FWB.

It seems that you equate HT to be the same as B+. I don't. Never have seen anyone else do that until I started hanging out in guitar forums. Can be a confusion factor.

Oh, and you won't need the center tap.

[pompeiisneaks]

Excellent, thanks as usual for clearing up my brain fog.

my understanding of HT is that it comes from the term "High Tension - Haute Tension" which just means high voltage, but I never knew if it meant AC or DC. I thought it was the same as B+, and wikipedia seems to link the two, but wikipedia is also not always right heh.

https://en.wikipedia.org/wiki/HT_(vacuum_tube)

Either way, I understand for sure now.

Thanks again!

~Phil

[martin manning]

Oh, and you won't need the center tap.

But you can run the CT to the center of a stacked reservoir, like the Fender 75 amp that the 290YX transformer is intended for.

I thunk HT is British. Merlin Blencowe uses HT for the DC supply all the time.

[sluckey]

Oh, and you won't need the center tap.

But you can run the CT to the center of a stacked reservoir, like the Fender 75 amp that the 290YX transformer is intended for.

That's true. But with 600V filter caps I doubt he'll be stacking 'em.

[pompeiisneaks]

Oh, and you won't need the center tap.

But you can run the CT to the center of a stacked reservoir, like the Fender 75 amp that the 290YX transformer is intended for.

That's true. But with 600V filter caps I doubt he'll be stacking 'em.

Oh yeah someone asked that earlier and I forgot to reply, but sluckey's right, all of my filter caps are 50uF 600V. They're those huge film caps that should outlast everything else in the amp. I've got small jumpers between each side of the halves of the cap so it distributes the current into both pins (there are 4 pins, two per side) Cost a bit more than electrolytics but not tons more, and should be way longer life. I could have gone for lower voltage ones in the later stages, but just figured it was faster to choose all 5 as the same instead of hunting through mouser.com for different ones.


~Phil

[Guy77]

But you can run the CT to the center of a stacked reservoir, like the Fender 75 amp that the 290YX transformer is intended for.

That's true. But with 600V filter caps I doubt he'll be stacking 'em.

Oh yeah someone asked that earlier and I forgot to reply, but sluckey's right, all of my filter caps are 50uF 600V. They're those huge film caps that should outlast everything else in the amp. I've got small jumpers between each side of the halves of the cap so it distributes the current into both pins (there are 4 pins, two per side) Cost a bit more than electrolytics but not tons more, and should be way longer life. I could have gone for lower voltage ones in the later stages, but just figured it was faster to choose all 5 as the same instead of hunting through mouser.com for different ones.


~Phil

Awesome Phil! Nice that you got the 600v caps. Getting those caps was a great move. Most people are running 500v caps for B+ that feeds into that cap from the choke but you got the big 600v one and it will go to good use.

[pompeiisneaks]

Here's Video 4...

After many hours of hard work, I made a major boo boo... drilled and set the holes for the sockets on the WRONG SIDE of the amp... I'll fix it soon



~Phil