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In Reply to: RE: What to do with 412 volts posted by BofService on March 08, 2017 at 17:28:26
There might be some derating built into them. You could build a SE el34/kt66 amp like Eli suggested. With biasing at 60ma. Then see how warm they get with running 6550 tubes with the bias set at 70ma. If they get pretty hot after an hour you probably need to back down. But, el34 or el37 and kt66 sound really nice at 60 to 65ma.
The PT is a big potted thing, surplus, but I don't know if it is military or not. It weighs 14 lbs. Regulation seems quite good. The B+ is rated 500-0-500. Actual (at the rated 115v) is 515-0-515, 535-0-535 at 120vac.
I really like those big potted trannys, moving them around is how i get my exercise. If it were from the Navy it would have an "anchor" logo stenciled on it. I would think it must be derated some, maybe 5% maybe more? Have you decided on what kind of amp you want to build? I am on SEP kick now so i would vote for 6L6 type amp with fixed bias. That way you could experiment with higher power tubes like 6550 as you can try different tubes in the family with just adjusting the bias. If you use the "RH" type circuit it only requires a single 12at7 driver tube but you can also use the 6sl7 or 6sn7 with the rh universal circuit.
Good thinking DAK. Last year I built an RH "Universal" that allows me to experiment with the 6L6 family, including the 807 and 1625 (of which I have several). It turned out good, and I appreciate Alec Kitic's design and documentation efforts on that amplifier.
On this amp I'd like to build with 300Bs and CCS 5842 drivers. Since the B+ from my transformer seems well regulated, that encourages me to think it will work with any tubes that run well at 60ma (or less), 300B included.
Last night I used PSUD to design a separate power supply using the PT's 235-0-235 winding and a couple chokes I have on hand. The supply gives me about 256vdc with .005 v of ripple at 28ma. This should work nicely for a 10M45 CCS and a 5842 per channel. My thoughts are to use the TubeLab "Simple 45" schematic for the driver stage. There are all manner of examples out there for how to do the OP stage. I'm looking at the "sjselectronics SE104it" schematic as a model for the OP right now. I have a pair of Hammond 125ESE OPTs I can use for breadboarding, and invest in some better iron later. I think I have everything else on hand except the OP tubes.
I'd consider trying the TubeLab PowerDrive topology, but I don't quite understand how it works, nor how I would implement it with 2 separate power supplies, nor whether it could work without a negative supply. Also there is the issue that the Toshiba 2SK2700 MOSFET has been deprecated. Which of the analogs are people using to replace it?
Awesome, it always helps to have a plan in place to help guide your project along. But, regarding, SS stuff i am not a convert, yet. I still find that even using an LM317 device to be a hassle since it is much less reliable than a resistor.
Thanks DAK. I thought of that as well, and if I am using the lower voltage winding at well under its rated current that might give me some breathing room on the higher voltage section's current. The hot fingers test is probably appropriate here.
I was overly optimistic in thinking KT88s. The 6L6GC/KT66 suggestion is excellent, given the near perfect fit of Edcor's CXSE25-3.75K O/P "iron". ;> )
Add Tubelab's PowerDrive, for a positive g1 regime (Class "A2"), and the capability of the suggested O/P trafo will be utilized. Some magnetic headroom has to be reserved for GNFB, as unassisted UL mode is unlikely to exhibit an adequate damping factor.
I double checked the GZ34/5AR4 data sheet. The type is OK fed by 500-0-500 working into a choke I/P filter. :> )
So, avoid the large 5R4 forward drop and use a 5AR4 to rectify the "tall" B+ rail.
A 5V4, 6106, or 6087/5Y3WGTB is quite adequate to rectify the "short" B+ rail.
Thanks for the great suggestions Eli. I had been assuming SE, as PP requires 2x the tubes and around 2x the current. I really appreciate you researching the OP tubes and rectifiers. I used the Edcor CSXE series on another amp, but I believe a lower impedance.
The transformer is indeed potted, chokes as well. It has a "Spec" number (14513) but I don't know if they are military or not. They are black, and the PT weighs 14 lbs, 6 oz. I didn't mention before that the 5v filament windings are each rated for 3A. One of the 6.3v windings is rated at 2A and the other 2.5A.
Still quite a few options out there, but hard to anticipate what the PS would do near its limits.
The KT66 draws 1.3 A. of heater current, while the 6L6GC draws 900 mA. of heater current. Also, the 6L6GC can safely dissipate a bit more power.
Seems like the 6L6GC is the O/P tube to use. Jim McShane shows a "reissue" black plate TungSol 6L6GC-STR for $21.50/tube. Yum!
With some 'A2 action, 12 WPC is not an unreasonable target.
"But, el34 or el37 and kt66 sound really nice at 60 to 65ma."
so do 300Bs.
Have Fun and Enjoy the Music
"Still Working the Problem"
when i started to collect tubes about 25 years ago. Compared to tungsol 6550 and such, the 300b was still an expensive tube. i could have blown my whole tube budget on just a couple of 300s. there would have been little fun in that.
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