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Old 17th January 2006, 06:35 PM   #1
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Default power supply tranny

Iím in the process of specíing parts for a tube guitar amp. Around 40 watts, drive-boost, and reverb. Iím not breaking new ground here, itís mainly just clone stuff from my TAB by Aspen Pittman, but I want to understand whatís and whyís of things. So Iím sizing my power transformer now and Iíd like some reassurance that Iím doing this right. Here are the specs so farÖ

a) power-amp: 2x 6l6ís or EL34ís in AB/push-pull (400-425V B+) ~ 200-250mA+25mA for screen2
b) pre-amp: 4x 12AX7ís (clean, drive, reverb recovery/mix, PI) @2mA each ~ 8mA
c) reverb driver: 1x 12AT7 in parallel ~ 20mA

Total current needs ~ 300mA

Did I do that right?

If so, it appears that Hammond only makes one power transformer that will handle that current rating. Itís the 278CX (shown here http://www.hammondmfg.com/263.htm ) with 454VA, 400-0-400 secondary, 465 DCmA (measured full wave 2 diode, and cap. input). The resulting avg. 360VDC, peak 568 VDC. I had originally planned to use a full-wave bridge and cap. input, but I donít see a viable alternative for the tranny here.

But it could also be that Iím confused.

Can someone help a brother out?

peace,
memphissound <><
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Old 17th January 2006, 09:05 PM   #2
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You only need 300 mA on musical peaks. Even if you abuse an amp like I do, you will likely be seeing this much current less than 50% of the time. You could look for a transformer that could deliver 300 mA continuous, but it would be overkill. For my guitar amps in the 40 to 50 watt range, I use the 273BX. 175 mA continuous duty. I have never seen one fail. For more voltage I use a 274BX. I use a full wave CT rectifier circuit. This allows the use of tube rectifiers (5U4 or 5AR4) or solid state, or a switch (like the Mesa dual rectifier amps). Transformers made for a full wave bridge are harder to find.

If you want a 300 mA transformer look at the pa060s from Triode Electronics. They also have guitar amp output transformers that work pretty good.

Another alternative is to look for a replacement transformer designed for a 50 watt Marshall or Fender. These are available from Antique Electronic Supply and others. www.tubesandmore.com
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Old 18th January 2006, 03:00 AM   #3
amperex is offline amperex  United States
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Default My 2 cents

I prefer a modest power transformer that has some voltage sag on peaks for a bit of compression. Thus, agree with reply above. The opposite is true when I design home entertainment amps.

My daughters project below should be finished tomorrow. It uses four EL84 & a modest Hammond 370FX for about 35-40 watts out.
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Old 18th January 2006, 03:53 AM   #4
Trout is offline Trout  United States
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Default Re: My 2 cents

Quote:
Originally posted by amperex
I prefer a modest power transformer that has some voltage sag on peaks for a bit of compression. Thus, agree with reply above. The opposite is true when I design home entertainment amps.

My daughters project below should be finished tomorrow. It uses four EL84 & a modest Hammond 370FX for about 35-40 watts out.


Where do you get the really nice front panel labels in your photo???
That one is nice!
Trout
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Old 18th January 2006, 04:05 AM   #5
amperex is offline amperex  United States
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Default Front panels.......

They are made on a laser engraving machine.

I should add, the small transformer feeds a small 10uF Solen input filter cap for lower peak to average current demands on the power transformer. Then, that feeds a LC to the EL84 plates thru the audio xfmr. Back to the 10uF input cap, that also feeds another LC to the EL84 screens & reverb driver, then to two RC sections to the other tubes. No electrolytics in the B+, all are Solen caps.

Top that off with NOS Raytheon 5751 windmill getter black plates, 5751 Sylvanias & an RCA 12AT7 reverb driver. The amps sounds excellent- musical. Cab is pine with a birch ply front speaker panel.
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Old 18th January 2006, 01:21 PM   #6
Trout is offline Trout  United States
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Id Love to see the schematic and underside photos, That amp sounds very interesting
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Old 18th January 2006, 01:59 PM   #7
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Default THANKS ALL

Thanks to one and all. Some of my thoughts were cleared up a bit after I started my drive home yesterday (clearer thinking in the truck I guess.)

Amperex - That is one SWEET looking front panel. What type of speaker arrangement are you going to use for that? And could I get a copy of the schem. just to see what someone else is doing?

peace,
memphissound <><
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Old 20th January 2006, 05:31 PM   #8
amperex is offline amperex  United States
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Default Schematic

The schematic is in my head & I should write it to paper. I built these before. It sounds like a Matchless DC-30 with slightly better defined clean & far better overdrive sonics per a few ears. Not a heavy metal amp though.
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Old 20th January 2006, 07:21 PM   #9
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Default Re: Schematic

Quote:
Originally posted by amperex
The schematic is in my head & I should write it to paper. I built these before. It sounds like a Matchless DC-30 with slightly better defined clean & far better overdrive sonics per a few ears. Not a heavy metal amp though.

What about the speaker compliment?


peace,
memphissound <><
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Old 20th January 2006, 11:54 PM   #10
amperex is offline amperex  United States
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Default Speakers

The amp uses a pair of new manufacture Jensen MOD 10" speakers. I have tested quite a few speakers. I find the Jensen MOD series to sound excellent. They are low cost speakers- go figure.

The MODs are better sounding than the reissue Jensen Alinco P10Q per our ears. However, those older vintage Chicago Jensen P12N & P15N are also great speakers. They way outperform the vintage Jensen 'C' series brown label.
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