power supply tranny

[memphissound]

I’m in the process of spec’ing parts for a tube guitar amp. Around 40 watts, drive-boost, and reverb. I’m not breaking new ground here, it’s mainly just clone stuff from my TAB by Aspen Pittman, but I want to understand what’s and why’s of things. So I’m sizing my power transformer now and I’d like some reassurance that I’m doing this right. Here are the specs so far…

a) power-amp: 2x 6l6’s or EL34’s in AB/push-pull (400-425V B+) ~ 200-250mA+25mA for screen2
b) pre-amp: 4x 12AX7’s (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. It’s 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 don’t see a viable alternative for the tranny here.

But it could also be that I’m confused.

Can someone help a brother out?

peace,
memphissound <><

[tubelab.com]

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

[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.

[Trout]

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

[amperex]

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.

[Trout]

Id Love to see the schematic and underside photos, That amp sounds very interesting

[memphissound]

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 <><

[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.

[memphissound]

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 <><

[amperex]

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.