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Old 5th December 2010, 09:18 PM   #1
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Default Hammond 1650n and Dynaco mark III circuit.

Hey All,

I was looking for a circuit to build a pair of mono blocks around the Hammond 1650n OPT. I was thinking of using the Dynaco Mark III. Hammond says they are capable of 60 watts and can be paired with KT 88's. But I have read in some threads that they may be to lightweight for the MARK III circuit. Triode's replacement for the Mark III weighs 12 pounds while the Hammond weighs 8. This would lead me to believe what I have read may be correct. So could the circuit be used with more conservatively run-and therefore long lasting-KT 88's. or a different topology entirely. And to use KT 88's would you just give them less bias voltage to decrease their output?

Kevin
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Old 5th December 2010, 10:19 PM   #2
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well, the power ability varies with bandwidth . the below sample is 60watts from 20 to 20k, and 120watts from 30hz to 15khz. so you can limit power by limiting bandwidth (input coupling capacitor)

mark III sample
Transformer specs are: 4300 ohms CT primary with screen taps, 4,8 and 16 ohm secondary. 20 Hz to 20 KHz response at 60W, within 1 db, 30 Hz to 15 KHz at 120 watts. Max DC per side (suggested) 120 ma. Exact same dimensions as the original A431 Dynaco transformer: Height 4 3/4 inches (120 mm) Width 4 inches (101 mm), Depth over covers 4 1/2 inches (114 mm), mounting centers are 3 inches (75 mm) by 3 1/2 inches (88 mm).
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Old 6th December 2010, 12:05 AM   #3
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The MK3 uses global NFB. The O/P trafo has to have magnetic headroom. Otherwise, the LF error correction signal saturates the core.

If you don't want to pay $126.95 for "DynaClone" trafos, Edcor's $92.87 model CXPP100-MS-3.3K will do very nicely.

The Hammond 1650N would be a good fit for a pair of the EL34/6CA7/KT77 group. In things Dyna, that means a ST70 stereoblock or MK4 monoblocks.
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Old 6th December 2010, 12:33 AM   #4
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Dynakitparts sells the A-431 OT clones for $120 ea
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Old 6th December 2010, 02:14 AM   #5
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Thanks All,

The only reason I'm using the 1650n is that I found one used on eBay that I got for $38 and another new that I found on the same day for $69. Just like I was given a 101B amp out of an old Magnavox console. And then I found a 101A (which is nearly identical) for $22.50 on eBay. I'm cheap. I am aware of the Triode OPT but I don't want to invest that much money in these amps. I just like building them for fun.

Hammond specs the 1650n at 60 watts with a frequency response of 30 to 30k. So If I understand the saturation problem correctly, at lower power output the frequency response would be better than that.

As always Eli you explain the problem succinctly and with true clarity.

The Mark IV is rated at 40 watts from 10 to 20k. If I have this right if I build a Mark III as it approaches full output it will lose bandwidth. If I build a Mark IV it will keep the wider frequency response but at a lower output. So from a non-purist perspective it isn't a huge difference because there isn't much material below 30 cycles anyway.

The real question is this. Would the KT88 Mark III produce significantly more distortion above 40 watts or would it just lose bass performance?

Kevin
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Old 6th December 2010, 02:27 AM   #6
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Or put another way. Is it worth compromising the frequency response for another 2db of output?
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Old 6th December 2010, 02:42 AM   #7
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Kevin,

It's not mere distortion. It's a sonic catastrophe, when the O/P trafo core saturates. Build with EH 6CA7s, along MK4 lines. Work backwards from the speaker, in terms of voltage swing. To get near 60 W., employing 1650Ns and KT88s, you will need a B+ rail well over 500 V. It was not an accident that I pointed you at 3.3 KOhm primary Edcor "iron", for use in a KT88 amp.
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Old 6th December 2010, 04:18 PM   #8
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Thanks Eli. I didn't mean to try your patience. Mark IV's it is!

And once again, thanks for all your help. Kevin
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Old 11th December 2010, 10:09 PM   #9
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Eli,
So, in your opinion you can't build an audiophile quality amp with the 1650n. Or any Hammond transformer for that matter? Would it make more sense to sell them and buy something better? Like Triode's Dynaclones?

Kevin
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Old 11th December 2010, 10:43 PM   #10
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You can build a highly satisfactory amp with the Hammond "iron", as long as you do your homework. Many nice speakers will work well with the 35 or so WPC you get from the EL34/6CA7/KT77 group.
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