Minimum current needed for Express PT?

[lacrebob]

What are the minimum current requirements for an Express power transformer?


Assuming 300-0-300, will 200ma be enough? 225ma?, 250ma?

300ma seems like well more than enough.

Any difference in tone with all other factors equal?

[rooster]

180ma is enough.

[lacrebob]

that is what I was hoping to hear

How about tone though? Any change in tone if the power transformer was 250ma instead of 180ma?

[bigbeck]

www.antekinc.com/details.php?p=58

I used a few of these. They're cheap and they run cool. They're supposed to be better than conventional PT's. But I can't tell a difference. I used this one in my Express. It's 300ma.

[Cliff Schecht]

I'd like to see the guts of that amp. My first Express build used the Antek toroid and it sounds good and dandy but I built it in a 13" x 8" x 3" case which in retrospect was a huge mistake (not enough room!). I had to put a few band-aids on the thing to kill the oscillations so I'm hoping my current build is stable without using any circuit-fu.

[rooster]

As to 'tone' and the 180 ma PT vs. a 300ma PT? You are going to find that the 'tone' is more in the power supply caps and not in the PT that is supplying sufficient current for the circuit. Basically here the caps are like batteries holding a charge that the PT has delivered and the circuit needs. They all tend to store and discharge a little differently brand to brand, even given the identical values and materials.

The Solens cap, made of polester film, for example, is a far cry from electrolytic based Sprague 'blue' cap. It will pass current much quicker than the electrolytic type, making the amp typically 'brighter' sounding. Is this news to you? Typically, a good circuit designer like Ken Fischer has taken all this into account and built his power supply around what his circuit needs. And he made a deliberate decision to use the caps he did, is my point, if KF 'tone' is really your concern. You can use anything you like, its your amp, but just be aware that he did it the way he did for a reason.

Another case in point regarding power supply caps and circuits and how they might effect 'tone' is the '59 Fender Bassman vs. the Marshall JTM45, two pretty similar circuits. The preamp in this amp (Fender) used a 8uf cap. The smaller value stores less current than the 16uf cap found in the Marshall preamp. Another fact, the JTM45 is only seeing 22uf off the rectifier compared to the Bassman's 40uf. Sure, the two amps sound different, and the trannys are different, etc.. But, plugged into the same speaker box, have you ever wondered how much difference the power supply cap values alone might be creating? Eh, I think its worth thinking about and I think people like Ken thought about it and incorporated the results in their amps.

OK, long answer.

[rooster]

bigbeck - woah, that torroid is wicked looking sitting there! I see too that it is taking up a lot of real estate. But I also hear that they are quieter than the conventional 'E' core PT so more power to you. I have never built an amp with one of those but I have looked hard at them..

BTW, one odd thing that I ran into was a SLM Ampeg SVT transistor output/tube preamp bass amp using a torroidal that just hummed like a demon. This was when people first started usng them for guitar amps, '93. What struck me here was that I realized even the torroidal needs to be built well to be quiet - its not just the overall design that makes it so.

[Cliff Schecht]

Here's a pic of my first Express if you didn't see it in the original thread. Again, way too small of a chassis as the gutshots prove but this amp has been working like a champ since I built it. I can get away with sticking the V1 tube right next to the PT because the toroid really doesn't spit out any noise at all.

[img:600:450]http://imgur.com/0koSMl.jpg[/img][/img]

[bigbeck]

bigbeck - woah, that torroid is wicked looking sitting there! I see too that it is taking up a lot of real estate. But I also hear that they are quieter than the conventional 'E' core PT so more power to you. I have never built an amp with one of those but I have looked hard at them..

BTW, one odd thing that I ran into was a SLM Ampeg SVT transistor output/tube preamp bass amp using a torroidal that just hummed like a demon. This was when people first started usng them for guitar amps, '93. What struck me here was that I realized even the torroidal needs to be built well to be quiet - its not just the overall design that makes it so.

Just stand it on edge to take up less room. This amp is a push pull 6bm8. I built a ss gain stage on the input jack. It has the smaller 100VA (200V) toroid.
The other pic is a KT88 SE amp with an Atomic 16 front end and the good old 29.00 300V toroid. I think it's 150ma. This is a really nice sounding amp. The tone is more Marshall than Wreck. This is my favorite SE amp - next to the SE Wreck. Quite a bit louder than the SE Wreck too.

[mlp-mx6]

Cliff,

Looking at that picture the thing that makes me nervous is that it looks like you passed the power xfmr wires through holes in the metal without any rubber or plastic bushing. This could easily rub the wire raw and cause a short. YIKES!

Michael

[bigbeck]

I'd like to see the guts of that amp. My first Express build used the Antek toroid and it sounds good and dandy but I built it in a 13" x 8" x 3" case which in retrospect was a huge mistake (not enough room!). I had to put a few band-aids on the thing to kill the oscillations so I'm hoping my current build is stable without using any circuit-fu.

This chassis is 16" X 5 3/4" X 2". Lots of funny noises too. My main issues were the OT wires and finding the right tube for V1. I also used 500K pots and a 100K grid leak at R7. The shielded signal wires were also making noise. Everything is fixed now but the amp still makes noise with treble and volume turned up full. Like I would actually play it with treble all the way up. It would break my wife's fine china - and the neighbor's too.

[bigbeck]

Cliff,

Looking at that picture the thing that makes me nervous is that it looks like you passed the power xfmr wires through holes in the metal without any rubber or plastic bushing. This could easily rub the wire raw and cause a short. YIKES!

Michael

Those PT wires have a plastic coated woven jacket about 2" long where they exit the transformer. He'll be ok as long as he did a good job deburring the holes. It's still good practice to use rubber bushings.

Still,you could have a dead short across the primary or secondary and not damage anything as long as you're plugged into a GFI outlet.. I think it's cheaper and easier to buy a couple of grommets than carry a GFI around with you.

[Cliff Schecht]

I always chamfer holes after drilling them although in this case, I was using holes that were already in the chassis (it was a recycled chassis from somebody elses failed project). I still chamfered the holes to make sure they don't cut the wire and have now adjusted the transformer so that the fiberglass insulation protects each wire. It's still a safety hazard though so I'm really trying to retire this amp within the next few weeks after I finish my current Express build. Like I said, I'm surprised the amp has worked so well as long as it has .

[lacrebob]

Okay. Thanks for the info on electrolytic capacitors. I wonder how oil-filled capacitors would work as a replacement for electrolytic capacitors.

[Cliff Schecht]

Terribly, the ESR is much higher and they aren't meant to handle high current pulsating DC like electrolytics are. Conversely, electrolytics make terrible audio capacitors unless they are specially made for audio (Nichicon makes some fantastic audio electros) but these audio electros are terrible for power supplies because they aren't meant to handle a lot of current.