Think I can get away with the 15W Edcor OPTs?

for the DIY ST35, the Dynakit and every other PP EL84

Postby whitelabrat » Thu Jul 17, 2008 10:27 am

Sorry I didn't mean to scare anyone. Folks... don't try this at home.
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Postby whitelabrat » Thu Jul 17, 2008 6:30 pm

Success! Yeeha!

Ya'll were correct. I swapped the primaries and ultralinear taps on the left channel. I also found a loose spade on the right channel OPT primary. Music!

First impressions. I'm a bit shocked at how well the little Edcor's sound. I was expecting the low end to be very flat, but quite the opposite. Thick bass. A bit flabby, but that may be caused by other infuences. Otherwise it made my cheap bose test speakers sound better than they should.

Didn't someone do some voltage readings around the board. I may need to do some tweaking. The B+ off the rectifier was 375v and after the choke it dropped to 350v. What do most folks see after the choke?

Now I need to finish the chassis. It's scrap that I salvaged from an old Sony all-in-one stereo. It was free so I used it. I intend to support it with some decent quality pine (the 2x4 on the back is temporary). I also need to enlarge one of the holes for the front left EL84. Very sloppy I know, but it will do for now. Did I mention free?

Image
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Postby whitelabrat » Thu Jul 17, 2008 6:35 pm

This is the picture I took before I swapped the left channel primaries and ultralinear taps. I should organize the wiring a bit better. That's a Hammond 373BX power transformer and a Triad C-14X choke by the way.

Image
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Postby EWBrown » Sat Jul 19, 2008 6:44 am

Dynaco's 17.5 WPC rating was a bit optimistic, the various manufacturers and designers had various ways of playing the "ratings game".

The cathode current on the EL84s is 35 mA (this combines the total the plate and G2 current consumption) so the 75 mA primary current rating is fine, even for loud-signal current peaks.

Heathkit rated their AA-151 EL84 PP / UL integrated amp at 14WPC which is more honest, and under pentode mode operation, EL84s, 6BQ5s, and similar tubes can go up to 25 WPC, at the upper limit.

After seeing your under-the chassis photos, I can be just about 99.9% certain that DEcware employs Edcor to make their "custom" SE OPTs for their smaller SV83 and EL84 based SE amps. The blue plastic bobbins, and the two labels are identical... Yellow_Light_Colorz_PDT_11

/ed B
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Postby DerekVa » Sat Jul 19, 2008 3:23 pm

whitelabrat wrote:Success! Yeeha!

Ya'll were correct. I swapped the primaries and ultralinear taps on the left channel. I also found a loose spade on the right channel OPT primary. Music!

First impressions. I'm a bit shocked at how well the little Edcor's sound. I was expecting the low end to be very flat, but quite the opposite. Thick bass. A bit flabby, but that may be caused by other infuences. Otherwise it made my cheap bose test speakers sound better than they should.

Didn't someone do some voltage readings around the board. I may need to do some tweaking. The B+ off the rectifier was 375v and after the choke it dropped to 350v. What do most folks see after the choke?

Now I need to finish the chassis. It's scrap that I salvaged from an old Sony all-in-one stereo. It was free so I used it. I intend to support it with some decent quality pine (the 2x4 on the back is temporary). I also need to enlarge one of the holes for the front left EL84. Very sloppy I know, but it will do for now. Did I mention free?

Image


For the benefit of those of us who are also planning on using Edcor transformers, can you post the correct connections?

Also, you might want to try and find some furniture-grade plywood scraps for the sides - less chance of warping.

-D
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Postby whitelabrat » Sat Jul 19, 2008 8:23 pm

OK. Since the tabs on the bobbins are numbered, I'll save myself the trouble of drawing out a diagram. Here the wiring:

Left OPT:
J3 tab 1 <-- OPT tab 1
J3 tab 2 <-- OPT tab 2
J3 tab 3 <-- OPT tab 7

J2 tab 1 <-- OPT tab 5
J2 tab 2 <-- OPT tab 4
J2 tab 3 <-- OPT tab 9

J1 tab 3 <-- OPT tab 3

Right OPT:
J4 tab 1 <-- OPT tab 1
J4 tab 2 <-- OPT tab 2
J4 tab 3 <-- OPT tab 7

J5 tab 1 <-- OPT tab 5
J5 tab 2 <-- OPT tab 4
J5 tab 3 <-- OPT tab 9

J1 tab 4 <-- OPT tab 3
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Postby whitelabrat » Sat Jul 19, 2008 8:26 pm

I should also mention:

OPT tab 7 = 8 ohm to speaker (Red/Hot)
OPT tab 9 = COM to speaker (Black/Ground)
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Postby whitelabrat » Sat Jul 19, 2008 8:44 pm

That's an interesting association of the Edcor transformers and Decware. It would prove these little OPT's to be quite a bargain if it were proven to be true.

I swapped out the Motorola 5U4GB for an RCA that looked to be in better condition. The B+ went up to 380v, but at C10 it remained at 350v. At C12 I was reading 295v so I jumpered R33 which brought it up to 321v. I'm guessing I'll have to adjust the bias on the EL84's to get the best out of them. Sounds fine as is though.

I swapped out the Raytheon 12AU7 for a Dumont 12BH7 and the LF got tighter. Nice. That's a keeper.
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Postby Ty_Bower » Sat Jul 19, 2008 9:04 pm

I wonder if Phyllis could tell us for certain?
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Postby Ty_Bower » Sat Jul 19, 2008 9:10 pm

whitelabrat wrote:I swapped out the Raytheon 12AU7 for a Dumont 12BH7 and the LF got tighter. Nice. That's a keeper.


Funny you should mention that. About a year ago, I think I felt similar things, but I didn't manage to express it quite as succinctly. My preference, whether 'U7 or 'BH7 wasn't as clear to me either. I guess it boils down to what you're playing tonight, and how you want it to sound.

I think the 'AU7 gives the appearance of having more bottom end. It sounds fuller, or deeper and richer. I don't think it actually renders the lower octaves "better" - in fact, I fear it may be the opposite. It could be that it is sloppier on the bottom end, but the slop gives the bass more body in a way that sounds full and appealing. It may be doing this at the expense of the mid and upper ranges. The effect is particularly noticeable on piano, acoustic guitar, strings, and vocals. I've been listening to a lot of Natalie Merchant, Indigo Girls, Sarah McLachlan, and Lorena McKennitt recently.

I think the 'BH7 really brings out the detail in the mids and highs, but the bottom nearly suffers. It loses some of the richness that it had with the 'AU7. I'm guessing the 'BH7 is more "accurate", and overall I prefer to have the detail. But, I can't help but miss what I'll call the warmth of the 'AU7.
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