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In Reply to: RE: Measure heater voltage---Important!! posted by Salectric on January 26, 2015 at 05:22:04
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
Follow Ups:
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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