I've been tweaking my 50W non-HRM dropping string, and I have my voltages where I'd like them to be with the exception of the output tube plate voltage: 495V (6L6's) Here's what I have with 121V AC line voltage, under load:
Dropping string is 390R choke resistor, 8.2K, 22K, 2.2K, 150K FET
I've been through a bunch of threads that helped get the downstream voltages where I want them, I just need some ideas to lower the output tube plates. My PT is from an early '70's Bassman 10 that was the donor amp.
I thought I read somewhere about using a zener diode somehow to achieve this.
Thanks!
Mark
"- Yeah, can we have everything louder than everything else? Right!"- Ian Gillan
Yes the best way is to use a zener diode on the CTof the PT.. Weber sells them I have used a couple for some EL-84 amps...They will drop about 30v on Plates..
Tony
" The psychics on my bench is the same as Dumble'"
In my limited understanding, I would think that the higher PA plate voltage would provide a cleaner power section with more headroom, witch seems to be desireable in a D'style build.
With the Weber zener installed your voltages will still be a llittle high like 470v (which is fine) so your first dropping string will need to be more like a 5K or 6.8k in that area to get you down to about 290-300v for the PI for max headroom.... From there you can continue on down with the 22K 2.2k 150K.. Good Luck!!
Tony
" The psychics on my bench is the same as Dumble'"
talbany wrote:Yes the best way is to use a zener diode on the CTof the PT.. Weber sells them I have used a couple for some EL-84 amps...They will drop about 30v on Plates..
Tony
Cool, thanks Tony. I knew I saw it somewhere!
If you do that it will drop everything else too, so plan to rework the dropping string a bit after you install that.
OR you could just tweak it to make it sound good with the voltages as they are now...
I've read where some can hear a difference in amps with the higher than the average plate voltage. Guess i want to bump it down and hear for myself I'll play the amp for a while with these new tweaks before I try the zener trick.
Mark
"- Yeah, can we have everything louder than everything else? Right!"- Ian Gillan
You can use a string of zener diodes to increase your wattage handling capacity. It can also be placed in line after the standby switch. Make sure the cathode end is facing the towards the switch.
Another way is the R.G. Keen method using a mosfet and zener diode. See attachment below.
Ken
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Blindog wrote:I've been tweaking my 50W non-HRM dropping string, and I have my voltages where I'd like them to be with the exception of the output tube plate voltage: 495V (6L6's)
Man, I'd like to have your problem. If I were in your shoes, I'd tweak the amp to run KT88s, leave that PA voltage where it is, and enjoy. I'm running my amp with 88s at 432V, and wish they were a bit closer to 500.
Seriously, I think the power amp is happy with the additional headroom. The plate voltages for tone that you need to concern yourself with are mainly in the preamp and PI, IMHO.
You can obtain similar zener diodes in any flavor you like from Mouser. I've used them to drop the B+ on a Rose Morris AC30 from 395 to 320 with no trouble at all. It's best to find a nice quiet place with some steel around it to soak up the heat because it will throw the excess voltage off as heat. That and using a good heat sink compound and you're good to go. It's like dial a voltage.
I did obtain a Weber V-dump and I didn't much care for dumping all that excess heat into the power transformer.
If you believe in coincidence you're not looking close enough-Joe leaphorn
Blindog wrote:I've been tweaking my 50W non-HRM dropping string, and I have my voltages where I'd like them to be with the exception of the output tube plate voltage: 495V (6L6's)
Man, I'd like to have your problem. If I were in your shoes, I'd tweak the amp to run KT88s, leave that PA voltage where it is, and enjoy. I'm running my amp with 88s at 432V, and wish they were a bit closer to 500.
Seriously, I think the power amp is happy with the additional headroom. The plate voltages for tone that you need to concern yourself with are mainly in the preamp and PI, IMHO.
Interesting, I never even thought about that possibility.
You just reminded me that I have a pair of GE 6550's in my tube box that I should try out. What kind of tweaks did you do to your amp? Where do you like your bias set?
Thanks!
MArk
"- Yeah, can we have everything louder than everything else? Right!"- Ian Gillan
I used a 25v 25w Zener on a 6V6 amp I built.
I used an old Hammond organ PT which put out over 480v.
I used the chassis to mount the stud type Zener and it worked great on the CT.
It does get pretty hot.
If you use a stud mount be sure it is the negative ground type (cathode to case) and use heat sink compound on it when mounting it.
Blindog wrote:
Interesting, I never even thought about that possibility.
You just reminded me that I have a pair of GE 6550's in my tube box that I should try out. What kind of tweaks did you do to your amp? Where do you like your bias set?
Thanks!
MArk
Mark,
I have a pair of Tung Sol 6550s, and I very much prefer KT88s in this amp...either the Winged C or Pentas. The 6550s seem to have a quality that I just don't care for; they kind of smack you in the face, and not in a good way. But that might just be this particular brand.
Blindog wrote:
Interesting, I never even thought about that possibility.
You just reminded me that I have a pair of GE 6550's in my tube box that I should try out. What kind of tweaks did you do to your amp? Where do you like your bias set?
Thanks!
MArk
Mark,
I have a pair of Tung Sol 6550s, and I very much prefer KT88s in this amp...either the Winged C or Pentas. The 6550s seem to have a quality that I just don't care for; they kind of smack you in the face, and not in a good way. But that might just be this particular brand.
I'll play the amp for a while with these new tweaks before I try the zener trick.
