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Old 4th June 2018, 12:59 AM   #241
colnago55 is offline colnago55  United States
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Below is my attempt at an output wiring diagram for KT88ís and a Hammond 1650N output transformer.

Iím assuming that the screen inputs on the output transformer will not be used in triode mode.

Does this look right?

Thanks, Jacques
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Old 4th June 2018, 02:37 AM   #242
TheGimp is offline TheGimp  United States
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Yes, that looks good.

I use a 10 R resistor in the cathode to give easier reading of the cathode current as most of my meters are not real accurate in the low mv range. The 10R resistor does not give much feedback, but gives 10X the voltage for setting the cathode current.

Apples and oranges.
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Old 4th June 2018, 02:42 AM   #243
TheGimp is offline TheGimp  United States
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This is the basic bias supply I'm looking at.
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Old 19th June 2018, 11:37 AM   #244
Sprags is offline Sprags  United States
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Quote:
Originally Posted by Tubelab_com View Post
My boards can be ordered by sending payment via Paypal to the Tubelab email address (see below). Make sure that Paypal has your correct shipping address on file, or provide it in the comments box. State which board or boards you want, and how many in the comments box, or send a separate email with this information within a few minutes of sending payment, so that I can match it with the order.

The SSE, SPP, and TSE boards are $35 USD each. Each board makes a stereo amp with power supply.

The board described in this thread is called the UD or Universal Driver board. It is $25 due to its smaller size. Each board makes a single channel driver. Two boards are required for stereo, and a power supply for the amp is needed.

A suitable power supply schematic will be posted soon, and a PC board for it should be available within a month or two depending on PCB delivery times. I buy my current boards in batches of 100 with a 4 week delivery time to get an acceptable price. That may not be an option this time.

Shipping for 1 to 4 boards in the USA is $8. International shipment is $15.

I do not post my complete email address in plain text format in an attempt to control spam. Even still I get 20 to 30 useless emails a day. My personal email address that I have had for over 25 years gets over 100 spams a day, many loaded with evil payloads.

The address is "tech AT tubelab.com" remove the AT and replace it with the appropriate symbol without the spaces or quotes.
Will the Universal Power Supply board that is available at the DIYAudio Store work as the power supply for this amp? If I am way off base please excuse my ignorance. I'm a complete idiot when it comes to electronics.
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Old 19th June 2018, 08:35 PM   #245
colnago55 is offline colnago55  United States
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See post 202 in this thread for a schematic that George posted for a mono-block power supply for this amp.

I assume that the power supply you are referring to is not for a tube amp.
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Old 21st June 2018, 04:43 PM   #246
TheGimp is offline TheGimp  United States
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George, you may have gotten the notice but in case you didn't:

"Mouser Electronics received a lifecycle change notification from the manufacturer regarding a product you had interest in or purchased within the last two years. Please click on the part number to view product details and similar products for replacements.
Manufacturer Part # Mouser Part # PO# Part Status
Bourns 3309P-1-102 652-3309P-1-102 13901034 End of Life
Bourns 3309P-1-104 652-3309P-1-104 13901034 End of Life
"
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Old 24th June 2018, 03:41 AM   #247
TheGimp is offline TheGimp  United States
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I'm being stymied by having to build a bias supply. I can't find my perfboard that is in a box that I hid from myself.

I ordered some from Amazon today figuring I'd find the stashed supply immediately after placing the order, but that hasn't worked.
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Old 25th June 2018, 06:44 AM   #248
dch53 is offline dch53  Australia
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Default Simulation of Feedback options

Iíve been simulating various feedback options for the UDB with 6550 outputs.

The open loop schematic is shown below.

Open Loop schematic.png

Iíve run simulations of the following feedback options:
470K output anode to driver anode (1dB),
output anode to driver anode plus second LTP anode to first LTP anode 540K (lowest recommended), additional 1.5dB,
output anode to driver anode plus second LTP anode to first LTP anode plus global (100R/2K7, additional 9.3dB)
cathode ( 0.5H, 4dB) and
cathode plus global (100R/2K7, additional 10dB).

The Universal Driver Board is easily capable of swinging the voltage required for cathode feedback.

Output power for these options into 8Ω is shown below:

THD Comparison.png

and for less than 10W

THD Comparison less than 10W.png

Observations:

The lowest recommended resistor (470K) from the output anode to driver anode only provides 1dB of feedback. This provides some improvement in THD up to about 45W but provides no improvement over open loop above that.

540K between the LTP anodes only adds a further 1.5dB but no improvement in THD.

The addition of 9dB global negative feedback is worse than open loop below 6W but provides significant improvement above that.

Just adding 4dB of cathode feedback provides comparable performance to the output anode to driver anode plus LTP anode and global feedback below 40W and significantly better performance above that.

Adding 10dB of global feedback to the cathode feedback reduces THD to less than 0.1% below 10W and below 0.3% all the way to 60W.

Conclusion:

I'll try, measure and listen to all the options, but cathode feedback looks to be the way to go.

There are at least 2 vendors of the specialist toroidal transformers required. Iím planning on adding bias servos to ensure the output tube quiescent currents are as close as possible.
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Old 6th July 2018, 11:31 PM   #249
colnago55 is offline colnago55  United States
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Default Bench Test

Quote:
Originally Posted by Tubelab_com View Post

I start with a board containing no tubes, but wired up, no output tubes in place either. Power everything up and check that you have your screen voltage at the output tube sockets. Measure the voltage at each output tube grid and turn R37 and R43 to verify the the voltage can be adjusted over a range of negative voltages with the most negative voltage being enough to cut off your chosen output tube.

Pot R10 is an offset adjustment. it compensates for imbalance between the sections in the tubes. Probe pin 7 of the empty V1 socket. R10 should cause pin 7 to swing from about 1 volt negative to 1 volt positive depending on your meter's internal resistance. The exact voltage is not important as long as it swings on either side of zero. Set it somewhere close to zero volts.

R4 and R13 do adjust the current through the tubes, however it's easiest to set them by adjusting for plate voltage.

Power down, install V1 only, then power back up. Allow for some warm up time and probe the plate voltages. These are easiest to probe on the right hand ends (viewed from the top with silkscreen writing correctly oriented) of R6 and R7. I set these with R4 for about 100 - 110 volts with a 450 volt supply, around 120 volts for 500 volts, so I'm guessing you are looking at 140 to 140 volts. Readjust R10 so that both voltages are equal.

Power down, install V2, power up, wait for warm up, then adjust R13 for about 450 volts on the plates of V2. (ends of coupling caps closest to V2). Readjust R10 for equal voltages on V2's plates.

Install the output tubes, set the bias by measuring the voltage across the 1 ohm cathode resistors.

This should get you operational. These pots can be tweaked for minimum distortion if measurement equipment is available. All adjustments interact somewhat, and the distortion should be optimized at where your average listening level is, not max power since you rarely hit max power. I aim for about 5 watts with 87 dB speakers.
Hi George,

Above is the start up and adjustment procedure that you documented in post 178 for EvanC:

Itís been very helpful to have this guide to get my board up and running on the bench.

Do any of the values need to change when using EL34ís for output? Iím using 6CG7ís for the driver board. My output transformer is a Hammond 1650N Ė 4300 primary.

Iím at the point where I need to make the adjustments. Iím getting sound, but if I set R13 to 450 volts the sound cuts out as I raise the bias much above 15ma. I donít have an oscilloscope and am using cheap multi-meters. Should R13 be set lower? Are my meters too cheap to get a good reading? Can I set up this amp for optimum sound with just multi-meters or do I need a scope?

Thanks, Jacques
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Old 11th July 2018, 07:34 PM   #250
wanders is offline wanders  United States
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I finished two KT-88/Hammond 1650-RA mono-blocks. Theyíve been up and running, generally without incident, for several weeks. However, I did lose one 5AR4 and had a close call with a second. In reading about possible causes for rectifier tube failure, it was mentioned that too much high voltage filter capacitance could be a cause.

The power supply, high voltage circuit finishes with a ď100uf Temco, Aux cap.Ē I looked around for this component and the closest I could find was a very large automotive capacitor, too big to be useful in a finished amp; so, instead, I installed a Vishay-BC 100uf 500v electrolytic cap. Wonder if that was mistake.
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