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Old 20th December 2008, 11:27 PM   #1
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Default PP power supply design questions

I plan to build a 6l6 PP (el34, kt66, kt 88 etc.) amp and need some help in want to consider for cap and choke values, etc. My B+ to the plate will be about 360v in ultra liner or triode strapped modes using a ST-70 reissue power transformer.

I do have Duncan amp software but I am seeking help on what issues to avoid in power supply design.

Thanks.........
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Old 21st December 2008, 04:46 AM   #2
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Hi FD,

First things to decide, which will have a major bearing on the design and choice of components for the PS, are:

1. Will it be a guitar amp or a hi-fi amp?

2. If it's going to be a hi-fi stereo amp, do you intend to build a separate PS for each channel, or a shared PS that will feed both channels?

3. Will the rectification be done using a bridge arrangement (using an untapped power tranny secondary winding) or "full-wave" arrangement (using a center-tapped secondary)?

4. Is your preference for solid-state diodes or vacuum diodes?

5. Do you intend to use a choke-input filter or a cap-input filter?

6. What class of operation have you in mind - Class A or AB?
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Old 21st December 2008, 12:29 PM   #3
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And once you've decided what the topology will be, spend at least as much time thinking through layout and grounding- far too often, an excellent supply will be let down by second-rate grounding.
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Old 21st December 2008, 07:39 PM   #4
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Not to thread-jack, but when designing in PSUDII, is it desirable to get near critical damping or is overdamped voltage response OK? I understand that underdamped/ringing is undesireable, since the voltage gets a little squirrely, but is there a reason not to want slow response?

Is it desireable to have the supply voltage change quickly or slowly? Is there an easily indentifiable sonic difference between the two?

I am going thru the same exercise designing a PS for a PP EL34/KT88 amp.
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Old 21st December 2008, 07:54 PM   #5
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Ideally, the response time would be infinite, right? So, the longer, the better.
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Old 23rd December 2008, 11:31 AM   #6
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Hey Guys,

Let me add some detail to my request. I found a version of the “simple” HIFI stereo amp using KT77/6l6 tubes where the output section appears to act as a phase inverter. This has been discussed before using 6bq5 tubes. The KT77 article can be found here.


http://diyaudioprojects.com/Tubes/KT...Pull-Tube-Amp/


It operates in Class A with 350+/- on the output plates. The authors stereo version uses SS rectification. I plan to build a stereo version as well using tube rectification, a ST 70 power trans reissue rated at (360-0-360) 300 ma. My thinking is to use a cap input (to set the voltage)>5 H choke> 200 UF 2nd cap (supplying 350v) which connects to the PP OPTs. The balance of his power supply is one of my concerns. There is a big difference in the amount of UF in his design compared to historic schematics. Should I use more or less UF cap values in the R/C chain to the driver tubes?

My last issue is referencing the driver tube cathodes to the heater. I assume it is possible to use the center tap of the 6.3v heater or use two 100 ohm resistors across the heater wires as the author did. I would then tie this to a 250V reference if I am correct. The ST 70 reissue has two 6.3v heaters. Would I tie in both heaters?.

Thanks for your help.
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Old 23rd December 2008, 11:50 AM   #7
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There's so much wrong with that design, I don't know where to begin. First, the output stage as phase splitter throws away many of the advantages of push pull (hum rejection, distortion cancellation). The bible of tube electronics, RDH4, specifically slags this topology. Second, the 317 is a lousy CCS with limited bandwidth. Third, the overload recovery is dicey at best- 317s tend to oscillate under those conditions. Fourth the SRPP is fashionable but in this use has worse linearity than a simple grounded cathode voltage amp. Fifth, the lack of heater supply bypass to AC ground and the lack of HF differential bypass on the heater lines will negatively impact the noise performance.

You can do better.
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Old 23rd December 2008, 02:59 PM   #8
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What would be some alternatives?

I have a stash of 6sn7s, 6sl7s and 6L6 tubes. My OPTs are Peerless units (10K) from Heathkit W5 units. I want to keep it simple in order to build and test with simple tools and meters.

Thanks........
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Old 23rd December 2008, 05:49 PM   #9
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Well... if it were me, I'd design from scratch using a 6SL7 diff amp cascaded to a 6SN7 diff amp, and then a set of either 6SN7 or MOSFET followers to drive the output stage grid. But doing that design might be a bit much for someone with less gray hair than me. IIRC, Ray Moth had a design done right along those lines, and Ray knows what he's doing. You might want to ping him or do a forum search for his schematics.
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Old 23rd December 2008, 06:58 PM   #10
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The 317 may be a lousy CCS for high impedance loading, but
here we only tail low impedance cathodes... I'm sure here at
this point, lousy as it might be, a lot higher Z than a cathode.
I do not think this topology has any problem rejecting ripple,
as long as output tube GM's are reasonably matched.

-------------------------------------------------------------------

Secondly, the input totem is not exactly wired as an SRPP.
I'd have described it as a Mu-Follower. And as such, might
intentionally behave single ended, rather than balanced...

Tap bottom end of the upper cathode resistor: SRPP
Tap top end of the upper cathode resistor: Mu Follower
Tap middle of the upper cathode resistor: Anti-Triode

These latter two topologies are not balanced, and do not
require the top device to be precisely matched. And PSRR
improves if the top device is deliberately selected to have
a much higher GM. I'd have subbed a MOSFET or IGBT here
at this location for sure. But definitely no LM317 as a load!
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