stereo SE kt88 build ... abdellah diyaudioprojects design

[jmillerdoc]

I have a 30H 40mA Hammond choke I found in a box of stuff I forgot about today. I was wondering what disadvantage there would be in trying to use this insted of the 5H described earlier on the power supply to the preamp stage of the amp. Any thoughts of concerns with trying this here. I have used this filter in many of my preamp designs in a CLC cofiguration for a dead silent supply. I have used it on several SRPP designs with the 5670 and 5687 tubes as well as a cathode follower preamp with a 5687. I adapted it from the simple 5687 preamp at diyparadise.com to all of my other preamps I have done lately. What is the difference in trying to use it here? Seems to me it would work just fine but I have been grossly incorrect in the past.....
Thoughts, comments, rebukes welcomed.
Jeff

[scitizen17]

There is additional discussion concerning this topic in this thread:

KT88 SE Abdellah (modified by Alex Gendrano) - AudioKarma.org Home Audio Stereo Discussion Forums

Originally Posted by jmillerdoc

I have a 30H 40mA Hammond choke I found in a box of stuff I forgot about today. I was wondering what disadvantage there would be in trying to use this insted of the 5H described earlier on the power supply to the preamp stage of the amp. Any thoughts of concerns with trying this here. I have used this filter in many of my preamp designs in a CLC cofiguration for a dead silent supply. I have used it on several SRPP designs with the 5670 and 5687 tubes as well as a cathode follower preamp with a 5687. I adapted it from the simple 5687 preamp at diyparadise.com to all of my other preamps I have done lately. What is the difference in trying to use it here? Seems to me it would work just fine but I have been grossly incorrect in the past.....
Thoughts, comments, rebukes welcomed.
Jeff

[jmillerdoc]

Hello,
I just realized that my Edcor power transformer 6.3v secondaries are not center tapped. The schematic at the beginning of this thread uses a center tapped 6.3v secondary with the CT connected to float the voltage closer to the HT. The circuit at the DIYaudioprojects does not show a CT 6.3v secondary and is just connected to the filaments without attention to this. Is there any issues I might have without a CT 6.3v as above? I built x2 rectifier/filter modules with a bridge rectifier and x2 4700uF caps on either side of a 1.2R resistor (what I calculated to get the voltage back down to 6.3v after rectification assuming about a 2.1a draw on 1 KT88 and 1 6N1P). I have x2 6.3v secondaries rated at 3 amps each so I was going to use one for each side (L/R) having each run a KT88 and a 6N1P. Also, if I rectify and filter the filament supply, how would it be possible to float the DC higher to the HT?
Thanks for your help with this.
Jeff

[jmillerdoc]

Can somebody tell me if a ripple of 0.020v Peak to trough is very much or likely to be audible? This is what I get with my PSUD2 simulation with a CLC of 47u-10h-102u. If I add about 100uF more I can drop it to about 1/3 of this value. Problem is it pulls the MAX on the current on the 5u4gb and about 0.2a over max on the 5u4g.

Also, how huch would it hurt my 5u4g to run it over that much? Will it immediately explode on me or will it just wear out alot faster?

I like the looks of the 5u4g better than the gb!
Jeff

[Scott594]

You caught me while doing the final design on own version of this amplifier in monoblock with DC filaments and a fully regulated supply. I havn't chimed in here yet, but have thought this through a bit.

The drawing at the beginning of this post shows the filament center tap grounded, thus keeping the filament voltages close to ground. This helps keep the internal filament (heater technically) in the proximity of it's cathode voltage which shouldn't exceed 50V or so above ground. I'm not sure if this has any real advantages, as in the event of a filament-cathode short, unpleasant things will happen either way. It's only really 3.15V different than grounding one side of the 6.3V winding. Either may slightly reduce any switching noise from the rectifer tube which could be coupled to the 6.3V secondary winding from the 5V one.

What I'm doing on the filament supply is using a $12 computer Switch Mode Power Supply - 12V 5A, then regulating to 6.3V using a LM7805 3 terminal regulator with a couple of diodes in the reference circuit (~.7v per) which gives 6.3 - 6.4 VDC regardless of load. I'm using a separate regulator for each tube. I know some hate using any silicon devices, but in power supplies they shine.

Which brings us to ripple. Shunt supplies, as you are finding are power hungry, and rapidly reach the point of diminishing returns. Fortunately, that is usually sufficent, especially for output stages (I doubt you'll hear 20mv unless you terminate the inputs and put your ear to the speaker). What I have done with my setup is add a separate RC from the main B+ to supply the driver stage. This could work for you, dropping the ripple level to the driver plate where it really counts, and adding virtually no additional load to the main supply.

[scitizen17]

In case anyone is interested, I did some research on the best type of switch to use for the standby switch. The folks at NKK Switch recommended this switch for breaking the B+ line. They indicated that "the biggest issue is that when the switch is actuated there is a significant change in voltage causing a big arc even with the small current." This switch has internal magnets that extinguish the arc. It may be overkill, but I bet it lasts a long, long time.

S822D-RO NKK Switches Toggle Switches

[scitizen17]

When I run PSUD2 with:
Hammond 374BX, 5U4GB, 10uF-10H-200uF, 180mA load, I am showing 217mA draw on the rectifier. Changing C1 to 47uF, I show 221mA.
The 5U4GB datasheet shows the typical DC output current as 275-300mA.

Originally Posted by jmillerdoc

Can somebody tell me if a ripple of 0.020v Peak to trough is very much or likely to be audible? This is what I get with my PSUD2 simulation with a CLC of 47u-10h-102u. If I add about 100uF more I can drop it to about 1/3 of this value. Problem is it pulls the MAX on the current on the 5u4gb and about 0.2a over max on the 5u4g.

Also, how huch would it hurt my 5u4g to run it over that much? Will it immediately explode on me or will it just wear out alot faster?

I like the looks of the 5u4g better than the gb!
Jeff

[alexg]

Originally Posted by scitizen17

In case anyone is interested, I did some research on the best type of switch to use for the standby switch. The folks at NKK Switch recommended this switch for breaking the B+ line. They indicated that "the biggest issue is that when the switch is actuated there is a significant change in voltage causing a big arc even with the small current." This switch has internal magnets that extinguish the arc. It may be overkill, but I bet it lasts a long, long time.

S822D-RO NKK Switches Toggle Switches

Actually, if you use 5ar4 for the rectifier, you don't need a standby switch, The B+ will ramp up slowly and will give your tubes to warm up before the full B+ is applied.

You can also make some soft start devices for slow ramp up of B+.

HTH.

[alexg]

Originally Posted by jmillerdoc

Hello,
I just realized that my Edcor power transformer 6.3v secondaries are not center tapped.

Just put one 100R resistor on each leg of the 6.3V secondaries, and ground the other end of the resistor. This will reference the 6.3V secondaries to ground, much like having a center tap. I usually match my resistors if I need to do this, less hum.

I this SE KT88 circuit, referencing the 6.3V secondaries a few volts above ground is not necessary.

[88man]

Alexg, have you tried the amp without the LC circuit to the driver plate? Can I use a choke with 26R?

Thanks!