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In Reply to: Pat Morford's Ultimate Preamp posted by MichaelL on April 03, 2001 at 17:48:50:
I did. It's a fairly standard SRPP circuit, with perhaps the only unusual feature being the very large output cap.I preferred the sound taking the output from the lower section anode, and losing the cathode bypass.
My simple 12b4 RC-coupled circuit sounds much better in my system, however. YMMV, of course.
Regards,
Jim
Follow Ups:
I guess I have to admit the huge output cap
and bypasses intrigue me. What caps did you
use?I am also intrigued by your design. Do you
have a link?Thanks,
Michael
Hi Michael,
Try this one too: http://www.the-planet.org/linestage.htm
I built it with a regulated power supply and it sounds really good. Not bloated or loose at all. Maybe a little on the dry side. I may try it without the regulation and see if I like that any better.Jim,
I'd love to get a schematic of the 12B4 linestage you are running. I have a boxful of them and would like to give them a whirl.Thanks,
Tom
Hi Tom,I agree with your comments about the 5687 sounding a little dry. I've tried these and their sisters (7044, 7119) at just about every imaginable operating point and circuit configuation. Very detailed and dynamic in most instances, but in all, just a little, well ... dry. I got the "juiciest" sound running them really hot and high in a SRPP, 400v B+, 22ma.
Still like the 12b4 better in my system.
Give me a day or so to scribble someting up and scan it, then check your email.
Regards,
Jim
Look bij 6080 amp - powersupply regulated.Thanks.
Hi , Maybe a stupid question but I like to know if the powersupply
what give out 250 V - 150 ma in the same configuration can be use for a amp that works on 250 V - 85 ma.Thanks for advice;
Best regards;
Peter.
Yes
I've been looking at Gary Kaufman;s 5687
design as well. I'm wondering about the
very different plate voltage differences
in these designs. Morford's is at 330
and Kaufman's is at 85vdc. Am I getting
this right?Michael
Yep, 85v is right. I attended a Bottlehead meeting in St. Louis a few weekends ago and had Carter Hendricks, the resident guru, look over the schematic to see if there was anything I should change. He told me that most people tend to run the 5687 plate voltage a little too high. He thought that 85v was a good plate voltage for this simple 1 stage design. He did suggest adding some cathode bypass caps to warm things up a bit. I'll let you know if I draw any conclusions about the sonics after they break in a little. I need to buckle down and do the math to see if I have the right values.If I remember, Pat's is an SRPP circuit(?) That explains the big difference in plate voltage. The top section only sees a portion of the 300v from plate to cathode. I had been pondering building that, but the 50uF caps made me shy away from it. I suppose if I lashed together all my old oil caps, I might be close enough to give it a try.
Tom
FWIW:I was just toying with a single 6DJ8 in a test jig I threw together. With a pair of 70v speaker matching transformers, the thing works as a headphone amp running a plate voltage as low as 10 volts! Sounds better at 80 volts though. Might even be worthy of some budget SE OPTs. Hammond 125_se?
Posted by hifitom (i) on April 04, 2001 at 03:00:05
> He did suggest adding some cathode bypass caps to warm things up a bit. I'll let you know if I draw any conclusions about the sonics after they break in a little.
What value did he suggest or did you end-up using? Also,
did you leave the 360 cathode resistor at that value?By the way, the design doesn't have a grid leak resistor?
I'd think it might be hum prone on account of that.Michael
I have 120uF in right now and used 270s as cathodes resistors. Looking at the tube data, it's hard to tell where the 100v line really is on the chart, but I think I'm in the ballpark. I used 18K plate resistors and am pulling about 9ma/side with the 300 volt B+. I added some 430s as gridstoppers and they were just a guess on my part. No traces of hum at the outputs and I'm running with AC heaters. I really didn't play much with the circuit after it was completed...too busy listening to the results. I would like to roll some different caps (Musicaps in there right now) to hear how the overall tone would change.The one thing I don't understand, when I was breadboarding this one the HV bounced back and forth at the top of the regulator tubes. I've never used regulators before, so could this be a normal part of their function? You could see it with an ammeter bouncing at about 10Hz or so. Voltage is now rock solid at the plates. Maybe someone can tell me if I had/have some oscillation problems with this supply.
Tom
If you have two regulator tubes in series and they are firing back and forth visually you may need to increase their current a bit.Also they tend to oscillate if you place any caps across them.
The RCA RC-30 or similar tube guides have some good info.
As a cute side note, most of the regulator tubes contained a bit of radioactive gas to help trigger them. Some of the gases I bet have long since lost their radioactivity. I wonder if there is any degradation in performance from this?
Thanks Gary. I'll do some poking around and see if the problem still exists and if I can fix it.Tom
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