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Tubelab Discussion and support of Tubelab products, prototypes and experiments

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Old 6th January 2019, 10:37 PM   #351
boywonder is offline boywonder  United States
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So I've got the various boards built and tested....

For the B+ and MOSFET +/- supplies I used some Peter Daniels (audiosector) power supply PCBs I had stashed. The boards and mirror imaged and set up for bridge rectification for SS amp supplies. These works great for the MOSFET supplies.

For the B+ supplies they are both wired for B+ and full wave rectification...and the creepage clearances look reasonable. The present plan is to make mono blocks and snap these apart..one in each amp.

I have a pair of the power transformers shown below and a pair of Triode PA060 transformers. I also have a pair of 4A Hammond 6.3V CT filament transformers. I would prefer to use these power transformers but unfortunately these power transformers only have a 12VCT filament supply....can I successfully use this to power the 6CG7 filaments? ......say by grounding (or biasing up to 75 volts or so with a B+ voltage divider) the CT and connecting the ends of the winding to H1 and H2 and not connecting anything to H3?..not quite clear on the filament windings.....


I'm also assuming that I need to ground MOS-2, DRV1-2, DRV2-1 and B++-1 to a signal ground along with the ground side of the inputs.
Attached Images
File Type: jpg KT88 UDB.jpg (713.8 KB, 156 views)
File Type: jpg KT88 Power Supply boards (2).jpg (685.3 KB, 153 views)
File Type: jpg KT88 power transformer.jpg (567.3 KB, 150 views)
File Type: jpg KT88 mosfet supply boards.jpg (655.7 KB, 151 views)

Last edited by boywonder; 6th January 2019 at 10:54 PM.
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Old 9th January 2019, 01:46 AM   #352
Tubelab_com is offline Tubelab_com  United States
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Quote:
connecting the ends of the winding to H1 and H2
If you mean pins 12 and 14 when you say "ends." NO, don't do that you will feed 12.8 volts to the 6CG7's making them very unhappy.

The transformers you have are fine for powering up one mono block each. I can think of a few ways to do this, but let's go for the easiest. We are going to power up driver heaters with one half of the winding, and the output tube's heaters with the other half.

Each half supplies 6.4 volts at 4 amps. The driver board needs 1.2 amps (2 X 600 mA) and the output tubes need 3.2 amps (2 X 1.6 A), so there is enough power for all the heaters. Look at this as two separate 6.4 volt 4 amp windings. They just happen to be connected together which is not a problem here.

Connect one end of the winding, pin 12 to H1.

Then connect the CT, pin 13 to H2. That will supply 6.4 volts to the board to light the 6CG7's. Now connect the CT (pin 13) also to either pin 2 or 7 on BOTH output tubes. Also connect the CT to the resistive divider that elevates the heaters to about 70 volts. It is usually good to put a capacitor across the resistor in the divider that goes to ground. It helps cut hum. I use .33 uF 250 volts because I have a big bag full of them but anything over .1 uF and 100 volts is good.

Connect the other end, pin 14 to the other heater pin (2 or 7) on BOTH output tubes.

It is usually best to run a separate pair of lightly twisted wires from the transformer to the driver board, and to each output tube. You can chain the output tubes if your wire is good for 4 amps.

Quote:
I need to ground MOS-2, DRV1-2, DRV2-1 and B++-1 to a signal ground along with the ground side of the inputs.
All of them are connected together on the PC board. Electrically only one of them needs to be connected, but I find that this method eliminated ground loops and hum:

Run a piece of shielded cable, or a twisted pair of wires from the input connector on the board to the input connector of the amp. Make sure that the ground side of the input connector (IN 2) goes to the ground side of the input connector.

Run a wire from MOS 2 to your +/- 160 volt board's ground connection. This

Run a wire from B++ 1 to your B+ board's ground (negative) connection.

You will need a method to measure the current in each output tube for setting the bias. I put a 1 ohm 2 or 3 watt 1% resostor in series with each cathode. Measure the DC VOLTAGE across this resistor, them 1 mV is equal to 1mA. Set the tubes to the desired idle current by adjusting the corresponding bias pot on the board. Somewhere between 40 and 80 mA (mV).

Connect one end of the 1 ohm resistor to pin 8 of the KT88. Run a wire from DRV1-2 to the other end of this 1 ohm resistor.

Repeat with DRV 2-1 and the other KT88.
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Old 9th January 2019, 04:16 AM   #353
boywonder is offline boywonder  United States
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Thanks George! Completely clear on the heaters...technically I'm slightly shy on heater current with those transformers but I'll see how it goes.

I randomly pulled out a few 6CG7's and measured the resistance from pin 4 to pin 5 and then either of those pins to pin 9....it looks like pin 4 to pin 5 is around 2 ohms and every one of the 6CG7's had no connection from those pins to pin 9. I also discovered that a couple of the leftover tubes from the CJ premier 11a are 5751s, and these are around 14 ohms pin 4 to pin 5 and around 7 ohms from either of those pins to pin 9....which makes sense since they are 12V filaments.

I'm also completely clear on the 1 ohm cathode R for the output tubes..or thereabouts...

Next question......I'm assuming that I'm tying the screen to the plate with a 2W 100R resistor for KT88 triode connection? Or tie both the screen and suppressor grids together then 100R to plate? I've seen it done a few different ways for triode wiring...

Last edited by boywonder; 9th January 2019 at 04:23 AM.
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Old 9th January 2019, 02:14 PM   #354
boywonder is offline boywonder  United States
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After re-reading George's post above, I have plenty of heater power from the transformer.....since I need 6.3V everywhere, I get 8A out of the two windings at 6.3V....nice!
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Old 9th January 2019, 05:13 PM   #355
Tubelab_com is offline Tubelab_com  United States
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Quote:
6CG7's had no connection from those pins to pin 9
The 6CG7 should have no connection at all to pin 9 and the 6FQ7 should have an internal shield between the two sections connected to pin 9. I have however found several exceptions to this rule, and plenty of tubes wearing both numbers with or without the shield.

Pin 9 is the heater center tap on 12 volt tubes like the 12AX7, 12AU7, and as you found, the 5751.

The 5751 does work well for the input tube in this board, but it must be paired with another 12 volt tube like the 12AU7, 12BH7 or 5963, and a different heater wiring scheme would be required.

Quote:
I'm tying the screen to the plate with a 2W 100R resistor for KT88 triode connection?
Yes.

Quote:
Or tie both the screen and suppressor grids together then 100R to plate?
The suppressor (beam plates) is tied to the cathode internally in the KT88, 6550, and 6L6GC.

You can connect pin 1 directly to pin 8 if you ever want to run EL34's in this amp. The metal ring around the base in some KT88's and 6550's is connected to pin 1, so this ring should not be externally grounded (say with a base tube clamp) if you want to do this.
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Old 10th January 2019, 03:08 AM   #356
Pooch is offline Pooch  United States
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I for one sure don't see a problem raising the price of boards. When you look at what they get for tubes and boutique caps these days, the boards are a walk in the park. I've built all 3 of your boards and love them, but if I didn't have a few 45s in the first place, I would have skipped that build. I'd like to see something that sounds good and uses cheap, unpopular tubes. What that might be, I have no idea.
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Old 16th January 2019, 11:15 AM   #357
emi23 is offline emi23
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Can the LTP/driver boards be used without the source followers and without a separate +/- 150v power supply?
I ask because I am designing around a 400v B+ and 7k6 output transformers (from an EL84 amp) so a pair of KT88 UL will be biased heavily into class A and far from grid conduction, so the source followers won’t be required for their purpose. It would be handy to leave it out since I’m building monoblocks. My Tubelab SPP has been working lovely since I built it back in 2014, but it is time for an upgrade, and I’d like to reuse the transformers.

Also, is it possible to use a CCS in the tail of V2 instead of at the plates, to ensure perfect balance? Or have I misunderstood how it works.

Great to see you working on new designs and reading about the development here George.
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Old 16th January 2019, 05:03 PM   #358
Tubelab_com is offline Tubelab_com  United States
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Quote:
Can the LTP/driver boards be used without the source follower
Yes, simply omit the mosfet, it's source resistors, and place a jumper in the PC board between the gate and source pins where the mosfet should go.

I have done this when one driver board is used to drive multiple pairs of output tubes, and a separate bias adjustment is desired for each tube. In this case I also bypass the coupling cap, and omit all of the bias parts. There are mosfets, coupling caps and bias parts on each individual output board.

Even if the output tubes are in deep class A it may be possible to drive them toward or into the positive grid area. This will alter the charge on the coupling cap causing a bias shift and an associated recovery time. This can cause "blocking distortion" and is why I created the PowerDrive (mosfet) concept in the first place. If the amp has sufficient power reserves such that it will rarely if ever see clipping, you could leave the mosfets out, if not consider keeping them. Distortion caused by clipping or blocking is far worse than whatever real or perceived distortion is created by entering A2.

Quote:
and without a separate +/- 150v power supply?
The positive supply is not needed if the mosfets are removed. A negative supply is still needed for the CCS in the tail of the first pair (V1). It needs to supply the full tail current which is from 2 to 15 mA depending on the tube being used. It should be -20 volts or more. It can be derived from the heater supply, or from the output tube bias supply if it is capable of the current needs of V1.

Quote:
Also, is it possible to use a CCS in the tail of V2 instead of at the plates
It IS in the tail of V2. See the schematic in post #2 of this thread. Early versions of the driver board (2005 - 2007) had a CCS in each plate of V2 for gain / drive voltage reasons. It was unstable and was eliminated. Since the cathodes of V2 operate in the 90 to 125 volt range, no negative supply is needed for this tail, it's tied to ground.
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Old 16th January 2019, 10:35 PM   #359
emi23 is offline emi23
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Many thanks George, and apologies, I was working from a different schematic you posted.

Since you haven't suggested it, I don't suppose it's possible to derive the +150v from the B+, and the -150v from a negative full-wave rectifier from the main power transformer, a bit like a bias supply when you don't have a bias tap? The mosfets require 10-20mA and the CCS 10mA so that would be 25mA for each rail? If it's not possible to do this I will use the 50V bias tap for the bias supply and CCS.

How does my load line look? I've got two different sets of KT88 ultralinear curves so I've plotted both. I'm working with Hammond 370HX 550V@ 230mA and 1650FA 25W 7,600k
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File Type: gif kt88ultralinear load lines Va 375v.gif (19.2 KB, 36 views)
File Type: png KT88 ultralinear load lines GEC Va 375v.png (771.9 KB, 12 views)
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