output transformer questions

[jbrew73]

i'm thinking about building an st70 or maybe eico amp. i have a power tranny but i need output trannies. i have 1 hammond 1645 and 1 handwound brand 60 watt(similar to hammond 1650n). i don't think finding a match for the handwound will be possible so what are your opinions on the 1645? should i sell them both and get trannies from ned?

[mesherm]

I used a pair of Hammond 1650G OPTs in a triode connected ST70 clone and it sounded great. However, since I don't believe in chassis hacking, I did have to make a set of aluminum adapter plates to mount the trannies in a stock ST70 chassis. If money is an issue I would buy another 1645 and spend a little of the extra money in beefing up the power supply capacitance.
If cost is not an issue and you want to build an true ST70 clone then use Ned's trannies. I have a set in one of my amps and they look and sound great.
FWIW, I built an ST70 for a friend and used a power supply from Cyrus Chong, cyruschong@yahoo.com. It is much improved from the original ST70 power supply and mounts under one of the OPTs where the bias circuit usually goes. It supplies a lot more filtering and has a zener diode regulated bias supply. I was much impressed. The only problem was what to do with the hole for the cap can...lol

[jbrew73]

any more opinions on using hammond iron verses neds?

[Shannon Parks]

any more opinions on using hammond iron verses neds?

Ned's Dynaclones will perform better, particularly in the sub 50Hz region due to them having more primary inductance. Since these amps you mention use up to and around 20dB negative feedback, I would recommend DIYers to stick ones that won't cause feedback issues without a lot of testing.

That said, you can get another 1645, and 'slug the pole' on the Hammonds with a larger NFB bypass capacitor. With a little effort and testing, it can make a fine amp. It just won't have the specs.

Shannon

[EWBrown]

Those Cyrus Chong cap boards are also very good, I've used both the ST-70 and MKIII versions. On my first ST70 renovation, I just left the original can cap in just for looks, it's not connected to anything.

I believe he is working on some ST35 boards, but they use a pentode/triode TV "plinker" tube rather than a 7247 or 12DY7. More similar to SCA-35 boards, but without the expensive 7199.

/ed B in NH

[Shannon Parks]

Hi Guys,

Does Mr Chong have a website?

Shannon

[EWBrown]

He's on E-bay,

Check here: http://search.ebay.com/_W0QQsassZtubes4pro

He currently has an ST70 and an ST35 cap board listed. As well as some GP jacks and rubber feet. My last purchase was three "blank" ST35 boards. (I have three ST35 carcii also "in process" of a full restoration - next winter...)

Like the butcher who accidently sat down on his meat slicer machine, I am getting a "little behind" in my work... Yellow_Light_Colorz_PDT_04


CC also has a real nice cap board for the original ST35. THat's where is saw the (still in development) ST35 driver boards in the photo. I think they use a 6GH8 "plinker" for the VA/PI duties.

BTW, did those "tiny-trans" ever show up?

I figure they might be great for triode PP or 6EM7/6DN7 service.

/ed

[Shannon Parks]

BTW, did those "tiny-trans" ever show up?

I figure they might be great for triode PP or 6EM7/6DN7 service.

/ed

They sure did - thanks Iron Man! Yellow_Light_Colorz_PDT_18

I keep waffling on this design. I scrapped the one I found that I had almost finished (a monoblock) and worked on a stereo design. Now I think I'll scrap that one and go back to the original monoblock one. It is small enough to put them side by side and share a PT. All those trannies you sent will be perfect. One change I did to the schematic was to add a lithium 3V coin cell to the LM334Z base (flipped). We'll see how this works.

Shannon

[EWBrown]

I was wondering about having the conjoined cathodes of the phase splitter at a somewhat higher DC potential than the LM334Z currently (pun intended) allows for, either by using an LED or resistor to increase it above 2.6 (or so) VDC. Actually the existing circuit sounds great, definitely far better than the original "VM" amp did. And I didn't bother attempting to match up the 6EM7s, I just grabbed four out of teh dozen I have and plopped them in and gave it a whirl. It's never going to be a "threat" to a well built ST35 or 70, but for a design using cheap parts it perfornms amazingly well. WIth a "Foreplay 2" preamp up front, it really delivers the goods.

I've built up two "turret board" 6FD7 versions of the same circuit, but haven't yet added the rest of the necessary parts to complete it.

I also have about 8 of the "cheapie OPTs" that were originally supplied with various 11BM8 / 11MS8 kits, about 9600 ohms P-P to 8 ohms. I'll have to figure out a good use for them... Yellow_Light_Colorz_PDT_09 Probably something using 6AQ5s...

/ed B in NH