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Tubelab Discussion and support of Tubelab products, prototypes and experiments

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Old 5th September 2018, 05:18 PM   #291
MellowTone is offline MellowTone  Australia
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Tubelab Universal Driver Board, 2015 version
Whoop whoop, definitely throwing my hat in the ring for this one. Super interested in building an amp with a pair of the UDB boards. I think I might finish rebuilding my "Industrial SSE" then skip the SE board and go straight to the UDB. Already impulse purchased some 6CG7's...

Either way, restarting from first post in the thread and shall ask questions when I make it back around to the end. Just deciding whether or not to let my itchy trigger finger buy the UDB boards prior to the PSU information and PSU boards themselves becoming a little more clear...
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Old 10th September 2018, 02:06 AM   #292
TheGimp is offline TheGimp  United States
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I had to pull the board off the workbench for other projects for now. Spent the day pressure washing the driveway cracks and filling them with crack filler.

It will probably be a week or two before I get back to playing. Summer is winding down and the garden needs work along with trying to get some work done on the house before winter.

Two years till I sell and move on.
If there are no dogs in Heaven, I don't want to go there.
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Old 21st September 2018, 02:23 PM   #293
colnago55 is offline colnago55  United States
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I decided to go ahead and invest in a Rigol DS1045Z. My plan is to use it to the scope to maximize the settings on the UDB board.

I am using 6CG7ís and KT88ís. Per Georgeís startup instructions I have set the plates in V1 at 110 volts and the plates for V2 at 450 volts before installing the power tubes. I am used a signal generator ap on my iPhone set at 1000Hz with a dummy load.

I am assuming the goal is to adjust the bias settings and the V2 plates to keep the sine waves from distorting.


1. Do I need to purchase a real signal generator or can I get by with my iphone ap?
2. What is the best method for making adjustments using a scope?

Although Iíve looked at a couple of articles and YouTube videos about how to use the scope Iím learning by doing. Any suggestions would be most appreciated.

On the bench with these settings the amp sounds great

Thanks, Jacques
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Old 3rd October 2018, 09:42 PM   #294
wanders is offline wanders  United States
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Default Stress to R6 & R7

I've had two udb monoblocks (Hammond 1650R, KT-88) up and running satisfactorily since mid July. During that time the driver boards have been mounted on a piece of plywood, socket side up. In disassembling the first amp to transfer it to the final chassis, I removed the udb and inspected the under side. Clearly the R6 & R7 (2W, 43K) resistors are damaged. (A new resistor is shown alongside.) Also, the solder joints seem iffy as well. Of course I have used these two resistors as test points for setting voltages throughout building and testing; so perhaps that testing has been stressful.

Anyway, I need to do something about this - at minimum replace them with heavier resistors (3W, etc.) and refresh the solder joints.

Comments appreciated.
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Old 5th October 2018, 11:07 PM   #295
Tubelab_com is online now Tubelab_com  United States
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at minimum replace them with heavier resistors (3W, etc.)
The resistors in my board don't look toasty at all, but there are lots of variables at play here and yours are obviously not happy. If you have about 450 volts of B+ and maybe 110 volts on the plates of V1, then there are 340 volts or so across the resistors. The usual bias current for V1 is about 10 ma split between the two halves, or 5 mA per resistor. This makes each resistor dissipate 1.7 watts, which is a bit close for a 2 watt resistor, but it works in my board. Not all boards run on 450 volts, or 10 mA through V1.

My boards have the resistors on the top side, and they are spaced about 3/8 inch above the board, so the heat can get out. Your resistors are mounted pretty close to the board and have limited airflow. Heat rises, but it's blocked by the board, so it has nowhere to go but build up in the resistor, the solder joint, and the PCB.

I would got to at least a 3 watt resistor, and space it away from the board. Mouser has some 5 watt metal oxide film resistors, but 43K is not stocked. They have 39K and 47 K in stock.

The current trend is toward resistors that are much smaller than an resistor of the same wattage rating that was made just a few years ago. The laws of physics haven't been repealed, stuff 2 watts through a resistor that's half the size of an older 2 watt resistor, and it will run hotter.

Modern high temperature parts can take the heat, and modern no - lead solder takes more heat to melt, so all looks good on paper. Trouble is that none of the resistor makers even dream of building tube amps where several resistors and some hot tubes are all stuffed into a box. They stick one or two into a power supply......

As you have seen the hotter resistors will transfer heat up their leads to the solder joint......it's even part of the new resistor's temp spec. The solder joint and PCB is part of the resistor's heat sink.

For this reason I allow as much lead length as possible to transfer less heat to the solder joint. Heat rises, so if you are putting the resistor on the bottom, space it a half inch or so below the board. If given the choice of two different 3 watt resistors, go for the larger of the two. The extra surface area will help shed some of the heat.
Tubelab, it's 5 year mission. To explore strange new tubes, to seek out new circuits and topologies, to boldly go where no tube has gone before......
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Old 5th October 2018, 11:56 PM   #296
wanders is offline wanders  United States
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Thanks for the explanation. FWIW, the resistors are mounted higher than they appear in the photo, but being on the underside with a solid surface an inch or so away they're getting a lot of trapped heat. I ordered some 3W, 43K replacements from Digikey; and on the final chassis the resistors will be open to the air. Hopefully those two adjustments will do the trick.

For info, along with a couple of friends, we displayed the amps with a diy pre and speakers at Burning Amp in SF. The system was well reveived (although the Nelson Pass exhibit managed to get most of the attention.).
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