Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: theundeadelvis on October 24, 2011, 01:50:10 pm

Title: What power transformer would work?
Post by: theundeadelvis on October 24, 2011, 01:50:10 pm: Hello folks! I'm getting ready to embark upon building a new amp. A simple little 26 watt KT66 amp. I'm trying to find a suitable power transformer. My preference is a standup type that does not require the chassis cutout. Using the following schematic, can anyone point me in the right direction? Thanks in advance!

(http://i90.photobucket.com/albums/k267/theundeadelvis/PhatAssschem.jpg)
Title: Re: What power transformer would work?
Post by: stingray_65 on October 24, 2011, 02:35:30 pm: http://www.edcorusa.com/products/595-xpwr007_120.aspx (http://www.edcorusa.com/products/595-xpwr007_120.aspx)

Edcor xpwr007-120 is what is called out on your scematic.

It's a stand up like you want, reasonable price too (my opinion)

The only issue I have with Edcor is slow processing time, WEEKS!

I'm all about that instant gratification stuff :icon_biggrin:

I like the looks of that schemo, parallelled input, tweed tone stack and a JTM45 power section.

a simple switch and relay you could go from a parallel to hi gain, LOTS of cool mods to be made on a simple cicuit like that!

Add a 9 pin socket and Mod later, add a gain stage, or trem,1 tube reverb... endless!
Title: Re: What power transformer would work?
Post by: theundeadelvis on October 24, 2011, 04:41:01 pm: Thank you stingray! I Googled that part number and didn't find anything, so thank you for that link. I like the looks of that Edcor, bummer about the slowness though.

Quick question, the schematic calls for 300 volts on the secondaries, but that Edcor says it's 275V. Is that still workable? It is a cool little simple amp. I looking forward to getting it built. Can't decide on whether to use a master of VVR on it though. I post a report once it's done! Thanks again!
Title: Re: What power transformer would work?
Post by: DummyLoad on October 24, 2011, 10:00:16 pm: question, the schematic calls for 300 volts on the secondaries, but that Edcor says it's 275V. Is

Edcore overbuilds. Off the cuff, hammond 300V around 200mA should do.

-DL
Title: Re: What power transformer would work?
Post by: theundeadelvis on October 25, 2011, 01:39:22 pm: Thanks DL!

Stingray, how would you get more gain with the switch you mention? Also, I may add an extra tube, just not sure if for a one tube verb or maybe a tube effects loop. Thanks again!
Title: Re: What power transformer would work?
Post by: HotBluePlates on October 25, 2011, 11:25:25 pm: Quote from: theundeadelvis on October 24, 2011, 04:41:01 pm
Quick question, the schematic calls for 300 volts on the secondaries, but that Edcor says it's 275V.

And 399v / 1.414 = 282v

Which means 300-0-300v would give you too much voltage, unless the PT sags quite a bit. So 275-0-275v works, especially if the circuit doesn't draw the full current rating of the PT, or if your line voltage is a little higher than 120vac.
Title: Re: What power transformer would work?
Post by: theundeadelvis on October 26, 2011, 01:40:18 pm: Thank you HotBluePlates! I ordered the Edco last night. Hopefully it won't take weeks, but if it does, at least I was warned.
Title: Re: What power transformer would work?
Post by: Boots Deville on October 26, 2011, 02:04:52 pm: Quote from: theundeadelvis on October 26, 2011, 01:40:18 pm
Thank you HotBluePlates! I ordered the Edco last night. Hopefully it won't take weeks, but if it does, at least I was warned.

It most likely will. I've ordered from them twice in the past five months and both times the order shipped exactly a month and a day after I placed the order.
Title: Re: What power transformer would work?
Post by: DummyLoad on October 27, 2011, 03:02:57 am: Hopefully it won't take weeks, but if it does, at least I was warned.

the last two times i ordered from them, they took 6 weeks and 8+ weeks respectively. i quit buying from them - after two months i lost interest
and the amp i bought the iron for never did get built.

this plan idles at ~94mA and that load with a 600VCT @ 170mA rating secondary will give close to what is specified in the schematic. see attached PSU designer simulations. one at no-load (1mA is very close to no load - need some value for sim to work) and another at 94mA load not including preamp load, which isn't significant. notice the no load sim is very close to 1.414 ROT calculation - 300V x 1.414 = 440V - no brainer. now have a look at the loaded sim.

the transformer is a hammond 272FX. you get similar results with the 272BX - it may sag some.

peace.

--DL
Title: Re: What power transformer would work?
Post by: theundeadelvis on October 27, 2011, 07:00:19 pm: Thanks again DL! Maybe I'll go ahead and order a Hammond, and use the Edco to build a second amp a couple months from now when it gets here!
Title: Re: 1 watt resistor enough
Post by: theundeadelvis on October 31, 2011, 04:18:24 pm: Quick question. The layout I'm using has a 220K resistor parallel to the first filter cap (C4 in the schematic). I'm assuming this is just a bleeder resistor for the filter caps? It does not specify wattage. Would 1 watt suffice? Thanks!
Title: Re: What power transformer would work?
Post by: DummyLoad on October 31, 2011, 05:35:07 pm: ohms law... P=V^2/R = 400V*400V/220K = 728milliWatts. use a 2 to 3 watt resistor.

peace.

--DL
Title: Re: What power transformer would work?
Post by: theundeadelvis on October 31, 2011, 06:34:35 pm: Thanks again DL! That's what I was afraid of. I have one 220K metal film, but not sure if it's 1 or 2 watts. May have to head to the local electronics store and pick one up tomorrow.