On going saga with Tubelab SE

[lth1]

I really need some help trying to figure out what is going on with my two Tubelab SE boards. The following is an email I sent to George concerning the first board that was operational until installing the Hammond 270HX PT.

I also have two additional questions. First does anyone know of any better quality higher turn count pots for the Tubelab boards, and second does the B- need to be lowered to the -150V as indicated on the schematic.

Hi George;

Hello from Homer, Alaska. Let me start out with stating right out front that I do not understand electrically everything that is going on with your design, do not have enough knowledge. I have been building speakers, amps, preamps, and a lot of kits or circuit boards projects for over 20 years now. Have only had one problem project (a 832A amp build from schematic) until now. I just do now understand what is going on here.

I purchased two Tubelab SE boards, after completion only one was operational, used the forum but did not get enough information to correct the problem with the second board, will deal with the second at a later date. The first board (components on the reversed side) has not had a problem until yesterday.

The hardware used:

Hammond 270FX: 550VCT @ 173mA; 5V @ 3A; 6.3VCT @ 5A no load voltage = 303VAC, voltage with load = 176VAC

Hammond 270HX: 550VCT @ 200mA; 5VCT @ 3A; 6.3VCT @ 6A no load voltage = 301VAC

OPT: James JS 6113 HS, Electra Print BE5KB, Edcor GXSE10-8-5K

C4 = Obbligate 2 uf

R4 = Triad C-14X

C5 = 2 X ASC 50 mfd 370VAC X386S, for a total of 100uf

Specs with 270FX: B+ = 277VDC @ 28-30mA; B- = -199VDC with 270 ohm R5, with 820 ohm 12W at R5 B- = -180VDC; 175VDC at 417A WE

There was not a problem until I replaced the 270FX with the 270HX PT. I did this in order to use both the 45 and 2A3 on this board (higher 6A 6.3 voltage for the 2A3). Right at startup this first board started acting like the second board, no voltage reading across R29, B+ @ 330V and reading at R18 and R29 not adjustable via trim pots. Removed the 270HX and reinstalled the 270FX.

With the 270FX installed the B+ was higher by 10V to 287VDC, R18 and R29 now became "interconnected", they each effected the others channels voltage, change one side up the other side goes down and .217 became the highest voltage possible, but the amp did play without any apparent problems. After several minutes everything turned to normal and was able to adjust both side to 287VDC @ 29mA. Left the SE running for break in, checking approximately every 30 minutes. At the 2.5 hour check the readings at R18/R29 were back around 21mA and "interconnected" again.

It has been 5 hours since shutdown, restarted the system, specs. are: B+ 283VDC @ 21.7mA (both channels), B- = -184VDC, R18 and R29 both .217vdc, and both 417A's near 175 volts.
System sounded normal.

George as you can see I need help. This is almost the exact same problem that I was having with the second board; on the second board I changed out D2/D3, Q1/Q2, and U2/U3 with no change, absolute no reading across R18 and R29. But, this first board does have reading of .217, will not go any higher, and the pots R12/R23 are "interconnected".

Cordially,
Lee Hankins
lth1@ptialaska.net

[rknize]

Some "interconnection" is expected when making bias adjustments. The is due to the B+ sagging under the load of the tube you are adjusting up. It sounds as if your B+ is not staying stable. When you say you measured "287VDC @ 29mA" on both sides, where are you measuring B+?

The best way to help is for you to take voltage measurements when the problem is occurring at the plate, grid and cathode of all tubes and at the source, gate, and drain of both FETs. Also report B+, B-, and the filament supply voltages at the time. From that we might be able to help you further.

[lth1]

Hi Russ thank you very much for the response.

The 287VDC @ 29mA is the B+ measurement, B- is -199VDC with stock 270 ohm resistor R5.

Before I installed the 270HX Xfmr, the normal specs with the orginal 270FX xfmr were B+ = 277VDC @ 28-30mA, B- = -199VDC, and 175VDC +- 1V on the 417A's. The amp was broken in for about 10 hours at these specs.

Will get the values you requested shortly.

You are correct that there is always some interconnection between R12 and R23, but now .217 V is the maximum for either side.

The B- at -199V bothers me. In the instructions and schematic it is -150V, and it has to be transformer dependent, design based on 500VCT xfmr. Should I adjust the resistors R5 and or R6 to drop the B- closer to the designed -150 value.

Lee Hankins

[Ty_Bower]

I'd take a good look at the bias trimpots, R12 and R23. If your B- is -184VDC, you should have -90VDC on one side and -16VDC on the other. With this much range of adjustment, I have a difficult time figuring why you can't dial in the right point to get your tubes biased where you want them. If I hear what you are saying, you've got the trimpot turned all the way over to the -16VDC side. Under those conditions you should be getting much more than 21.7mA through the 45 tube.

Maybe you could check the voltage across R14 and R25, or we can figure it out if you've measured the voltage at the grid of the output tube. I'm guessing you should have somewhere from 90 to 165 volts across it (4.5mA to 8.25mA) depending on where the bias pot is set.

[rknize]

Originally Posted by lth1

The 287VDC @ 29mA is the B+ measurement, B- is -199VDC with stock 270 ohm resistor R5.

But where are you taking your B+ measurement? There are components between the power supply and the plates of the output tubes.

Will get the values you requested shortly.

You are correct that there is always some interconnection between R12 and R23, but now .217 V is the maximum for either side.

Agreed. That's why it is important to see the actual plate voltage (at T2-2/T3-2) and the actual grid voltage (at R33/R34) on the tube. Then we can see why it is not drawing much current through R18/R29.

The B- at -199V bothers me. In the instructions and schematic it is -150V, and it has to be transformer dependent, design based on 500VCT xfmr. Should I adjust the resistors R5 and or R6 to drop the B- closer to the designed -150 value.

It's an indication that not much current is being drawn from the bias supply. It's a "long" RCRC filter that will sag towards 0V as it comes under load. This tells me that the FETs are not flowing as much current as they should. What is the voltage spec on your coupling cap?

[Ty_Bower]

Originally Posted by lth1

The B- at -199V bothers me.

I'm really not convinced that is the problem. The trimpots offer such a broad range of adjustment, you should be able to compensate. I don't think the 2SK2700 mosfets care - if I'm reading the datasheet correctly, they can handle up to 900 volts from drain to source.

Can you pull the power tubes (but leave the rectifier tube in) and stick your meter probe in the grid pin of the 45 socket? What voltages do you measure (referenced to ground) at the grid pin as you turn the trimpot from one extreme to another? I'm expecting you should see about -22V to -99V. George actually has you go through this exercise in his checkout procedure:

Next, put in a rectifier tube. Power up the board. After the rectifier tube warms up you should have B+...

Put a meter from the grid of the output tubes to ground. One meter for each tube. Power up the board. Each meter should read a negative voltage. With ONE HAND BEHIND YOUR BACK carefully adjust one of the bias pots (R12 or R23). The negative voltage for the corresponding output tube should change.

[Ty_Bower]

Originally Posted by rknize

It's an indication that not much current is being drawn from the bias supply. It's a "long" RCRC filter that will sag towards 0V as it comes under load. This tells me that the FETs are not flowing as much current as they should.

Lee said his power transformer made 303 volts with no load. Multiply by sqrt(2) to get peak rectified DC voltage = 428 volts. The power supply filter has R5 (270 ohm) and R6 (10K ohm). If you pull 22mA across those resistors, you'll drop 229 volts. That brings B- right to 199VDC, just as Lee noted.

I think 22mA is probably about right. You'll draw 0.8mA through each voltage divider (R11/12/13 or R22/23/24). That leaves 10.2mA for each mosfet, which seems about right. I can't find a 2SK2700 for LTspice modeling, but someone suggested the SPA11N60C3 was pretty close. It draws anywhere from 5mA to 9mA, depending on where you set the trimpot.

[lth1]

Off to see a few Doctors, be back with requested information in a few hours.

Lee

[lth1]

Tubes In: Left Right
278.3 277.4
-60.0 -55.8

Tubes Out:
321 322
-19.6 to -20.25 to
-101.6 -109.3

C9 Cap is .22uf @630VDC

At this point we entered the Twilight Zone. Replaced the 45 tubes and all readings became normal; B+ = 278VDC @28-29mA, then all of a sudden the right channel went to zero, left up to .39V. Then both channels were adjustable to a max of .217V, but the pots became reversed, i.e., turning right increased voltage, left decreased voltage. A few minutes later back to normal with correct rotation of the pots, then right channel went to zero, left up to .39V.

Right pot: left = -16.37 to -88.9
right = -18.49 to -100.1

Left pot: left = -16.5 to -88.4
right = -18.61 to -100.0

[lth1]

Russ, here are answers to your questions, I hope.

Cap = .22uf 630v

B+ is being taken across R30
B- across R7
T2-2 = 268V
T3-2 = 269V

R33 = -52V
R34 = -50V

Q1(Mosfet) = 180(left) 278 (center) 183 (right)
Q2 = 182 278 186