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In Reply to: RE: CARY 40M Special ... with Cap Input posted by drlowmu on January 25, 2015 at 17:01:11
I bought a used pair of Cary 40M SE (Signature Edition) 15 years ago, which gave me a lot of grief until a design flaw (see below) was discovered. Before learning the cause of my woes, I had many telephone conversations with Dennis Had and his chief tech Kirk Jenkins and over the course of these conversations, I learned a few things about the Cary operation. First of all, Dennis liked to change the circuits all the time and usually without changing the model numbers or updating the manual (if there even was a manual). For example, Kirk once said Dennis went back and forth between fixed bias and cathode-bias, sometimes preferring one, sometimes the other. So don't expect any consistency among different amps with the same model number. In fairness to Dennis, he really was trying to make his amps sound as good as he could make them. He had a passion about amplifiers that is typical with one-man shops but often lacking in bigger, more successful operations.The 40M amps used two types of input/driver circuits but both had a single 6SL7 and looked identical from the outside. Some used a split-load inverter that drove the output tubes directly. Others used a paraphase inverter. I haven't heard of any 40M's that were fixed bias, however. That would be a stretch for a 6SL7. With the later 50M and 50M Mk.II, Dennis used both fixed and cathode bias, but for the fixed-bias model he switched to a 6SN7 driver.
My 40M SE was an early split-load model with cathode-bias and a power supply very similar to what Jeff posted. I do question the 73ma load shown on the simulations. That sounds about right for a single output tube but not the pair. Dennis drove the EL34's hard! With the high B+ of 505 to 520 volts, depending on line voltage, my pair of outputs was usually in the range of 130 to 145ma. Yes, way over max ratings.
I tried several PS variations to lower the B+ but what ultimately worked best was putting a Mills 12W resistor between the rectifiers and the first cap. (Working from memory, it was either a 50R or 100R.) This dropped the B+ down to a more manageable 450-460. And there was minimal impact on the sound quality.
Now for the big design flaw. Output tubes in my 40Ms would only last for about 300 hours. It turned out this was not due to running the tubes so hard, but rather to a heater voltage of 7.3 volts. Adding some small value Mills 12w resistors to drop the heaters down to 6.15 or so solved the tube failure problem. Obviously I should have measured them as part of the initial troubleshooting, but I didn't. Live and learn.
So my advice to any 40M owner is check your heater voltage and make sure it is not over 6.3v!
Edits: 01/26/15Follow Ups:
Thanks,
I will forward this post of yours to the Gentlemen in Aussie Land.
What is bugging me is, on FOUR different 6SL7s he has plugged in so far last night, the self bias of the INPUT stage was at about 0.66 VDC across a 550 Ohm Rk. That means about 1 mA. of 6SL7 current and - the amp can only take 1.12 VAC of signal input before clipping the first stage.
Notice , by me eliminating the stock "C" input filter ( 600 uF ) and going to a 700 mHY / 10 Ohm "L" input filter, we advantageously reduce the B+ to the finals by about 60 VDC, making the finals' tube dissipation happier.
L2 is a lead-out modified EDCOR XC Series choke, 1.5 HY at 17 Ohms ... just about the right value and size. Both Ls will fit above-deck neatly where the too large 1,200 uF caps resided.
THANK you for your input Salectric!! Now to confirm self biasing of the input stage at 0.66 VDC with someone !! Ideas anyone ?? Que Pasa??
Jeff Medwin
You won't clip the input stage with that voltage unless you remove the feedback, assuming the most common implementation of GNFB applies.(With the feedback removed, some grid current will be drawn before you hit 0)
If you're seeing 7+V on the 6.3V winding, it may be a good idea to see if the primary is a 117V primary (Hammond). The whole amp may be happier with a 6V bucking winding added to the primary to bring 125V out of the average outlet down to 119V.
Edits: 01/26/15
CB,
Thanks !!! Basic stuff like that I didn't know, since I am an amateur and only do zero NFB builds. No need to learn it. Many Thanks.
Jeff Medwin
There's another potential problem with the 40M design regarding the 6SL7. At least in the version I had, the 6SL7 heaters were referenced to ground which put the heater-cathode voltage over the max rating due to the split load inverter. As I recall, the H-K was around 110 or 130v. I burned up a couple NOS 6SL7 tubes before figuring out that issue. Fortunately the tubes I liked best in that spot (Brimar CV1985) were not bothered by the high H-K voltage. Had I kept the amps, I would have added a divider network off the B+ with roughly a 65vdc tap and connected it to one leg of the heater supply, which would have allowed more flexibility with other 6SL7 types.
Yes, thank you. I read your post about this on another Forum, about midnight last night !! We will certainly LOOK into this !!
Turns out, the Aussie gentleman had new power transformers wound for this amp, some years ago, so ALL must be documented, before we re-configure the filter to the Finals !!! - or touch the amp in any way.
I wonder if there is an easy way to reach Dennis Had, and confirm from his notes what HE thinks should be the stock voltages on the 6SL7 of this 40M. ?
I just got spooked with 0.66 VDC self bias on the input stage. I wonder what voltages on the 6SL7 pins, Dennis Had wants it to be ??
Who can supply me with a contact email address ?? Thanks all !!
Jeff Medwin
you can search him on Ebay. he now made some small amp and sell them on ebay occasionally.
LT
Yes. Thanks. I was aware of that .
Please send me the URL for any one of his previous eBay amp auctions, so I can get his eBay Moniker, and easily contact him.
Gracias,
Jose Yeffie Medwin
sorry I don't have it. just do a search . I think he name some of his amp " inspiration or something's like that
,
Have Fun and Enjoy the Music
"Still Working the Problem"
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