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lth1 28th September 2010 12:05 AM

Need Help with Tubelab SE
I am having trouble with a new build. I get no voltage across the 10 ohm resistors R18 and R29. Have changed out the Fred DSE112-12A diodes D2 & D3, Q1 and Q2 the Toshiba 25K3563, and U2 and U3 the CCS ic's. Pots R12 and R23 have no effect on the output at R18 and R29. Have no idea how to proceed.

Lee Hankins

Ty_Bower 29th September 2010 12:36 PM


Originally Posted by lth1 (
I get no voltage across the 10 ohm resistors R18 and R29.

First I'll just mention that I haven't built the Tubelab SE, so my usefulness is somewhat limited. I see R18 and R29 are intended to be used as current sense resistors. When the power tube is drawing current, you should be able to calculate that current draw based on the voltage drop across R18 and R29. This voltage will be on the order of millivolts, so be sure your meter is set to the millivolt scale.

What kind of tubes are you using? Are they new? Are they known to work in a different amp? What kind of power and output transformers do you have? Have you verified B+, B- and heater voltages?

Have you gone through George's checkout procedure?

If you truly have no voltage across R18/R29, then you must have one of the following problems:

* No high voltage at B+
* Open circuit somewhere at R18/R29, T2/T3, or the tube socket V4/V5
* Bad power tube
* Negative bias supply is driving output tube into cutoff

Keep a printout of the schematic handy. It is invaluable for troubleshooting.

Can you post any photos of the wiring in your assembled amplifier?

lth1 30th September 2010 08:41 PM

Hi Ty, thank you for coming to my assistance. I worked most of the afternoon on the amp and was able to get everything working, reduced the B-from 197v to 148-50v by increasing R5 from 270 to 3K ohms. Started the amp this mourning and nothing was right, B+ 230V instead of the normal 277V, B- was 148V, but the left channel again could not read a value across R29. Waited a few hours and tried again, B+ was up to 277V everything else was the same. Came back after lunch and tried again, B+ now down to 230V again. It is time to trash this build, do not have the time nor the knowledge to progress further.

I do have another board that has all components except the tube sockets on the back side of the board, works perfectly, will preceed with this board.

Because I am using the Hammond 270FX with this board also, the B- is 197V. Is increasing R5 to 3K ohms the correct was to reduce the B-, or is increasing R6 or the Caps a better solution.

Ty, again thank you for your assistance.

Lee Hankins

Ty_Bower 1st October 2010 12:58 PM


Originally Posted by lth1 (
Because I am using the Hammond 270FX with this board also, the B- is 197V. Is increasing R5 to 3K ohms the correct was to reduce the B-, or is increasing R6 or the Caps a better solution.

I'm not sure. I don't fully appreciate how the bias scheme on this amp works. I can tell you that increasing R5 or R6 will make your negative bias supply take longer to stabilize after power on. Generally, that's not a good thing. You want your bias to come up right away, or at least long before the tubes start to conduct.

Ordinarily, I'd say that R11 and R13 set the "window" in which you can adjust your bias, and the trimpot R12 sits in the middle. Using the trimpot you can chose any bias you want, from one end of the window to the other. It should just be a simple voltage divider. If the B- is supposed to be -150, then using the values shown on the schematic the window ranges from -72 to -13. That should give you enough leeway to bias a typical 45 (-50V), 2A3 (-45V) or 300B (-61). If you B- is running around -197, the window runs from -95 to -17. While this isn't exactly the same as the amp where the B- is at the "correct" value, I'd think the divider still gives a useful range and the trimpot should allow you to dial in the correct bias point. From this perspective, I'd say your bias supply is fine, and you shouldn't need to do anything to "fix" it.

Now, I did start out by saying that I don't fully appreciate everything that is going on with this design. R14 and Q1 are in there doing something. At a quick glance it looks to me like Q1 is really just another amplification stage, and what you are doing with the bias supply is adjusting the bias on Q1. The signal to the power tube is taken off the "cathode" of Q1, and R14 is the cathode resistance for Q1. If that's the case, you'd want to adjust the bias on Q1 so that it's cathode is sitting exactly at the right voltage to properly bias the power tube V4. I dunno. I'm sure I've got something wrong in there. Go read George's technical write-up on this amp's theory of operation. I know he has it somewhere on his web site.

Theory of Operation - Tubelab SE
Theory of Operation - Powerdrive
Powerdrive Cookbook

Evenharmonics 1st October 2010 03:42 PM

lth1, those varying voltages, are they measured with all the tubes in place?

lth1 1st October 2010 06:27 PM

Yes, all tubes are in place, and all tubes are good and strong.

Lee Hankins

Ty_Bower 1st October 2010 06:54 PM


Originally Posted by lth1 (
B+ 230V instead of the normal 277V... I am using the Hammond 270FX

What values did you use for C4 and C5 (the two main caps in the power supply)? Did you use the 150 ohm resistor for R4, or do you have a choke instead? If you used a choke, what are its specs?

lth1 1st October 2010 07:48 PM

Ty, C4 = Obbligato Film Oil 2.0uF 630V; C5 = Panasonic CE 180 uf (measured 171) 450V plus ASC 30uf film in oil (measured 33) for 204uf total.

R4 is the Triad C-14X 6H 151.8 ohm choke.

Ty_Bower 1st October 2010 08:07 PM


Originally Posted by lth1 (
I do have another board that has all components except the tube sockets on the back side of the board, works perfectly...

So you're saying one board has all the parts mounted topside, and the other has them mounted bottomside? I don't suppose there is any possibility you accidentally installed the mosfets or CCS ICs the wrong way around? I believe if you mount the components topside, the mosfets and CCS parts have their tabs facing each other, as showing in George's assembly photos (last photo on the page):

If you mount the components bottomside, they need to have their tabs facing away from each other. Russ' amp is the only photo I've found:

Your B+ voltage readings are curious. Considering the parts in your power supply, I'd imagine your 270FX power transformer should make about 400VDC with no load. If you're saying it should be running around 277VDC, there must be some kind of load on it, probably about 110mA is my guess. If you saw it pulled down all the way to 230VDC, there must be quite a heavy load on it indeed - maybe as much as 200mA. I can't fathom why there is no voltage drop across R18/R29. The current has to be going somewhere.

lth1 1st October 2010 09:06 PM

The board that is having the problems is the normal top side component one, all stock built except for R4, C4 & C5. The bottom side component board is just fine, because of the different orientation I had to custom build heat sinks for the SS components and since it is using the same PT the B- is also 197V, did not try to drop the B- on this board.

Did not have any problems figuring the proper orientation of the tabs on the bottom side component board.

The B+ voltage is measured across R30, which is 277V on both boards. The 270FX no load voltage at T1-4 and T1-4 is 557VCT or 278-0-278.

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