Tubelab se with blown FREDs

[mulletronic]

Hello all,

I've been a happy user of tubelab se for a half a year now with a little or no problems. I have 300b tubes, a hammond 376x power transformer, transcendar opts, auricaps, the recommended choke and an extra cap on the psu. In addition to this I've added a heatsink with a small fan to the regulator due to a thermal shutdown I experienced when encasing the amplifier. After this I've had no problems with amplifier.

Last night the amplifier had probably it's longest run playing music all day and to the late of the night. Everything seemed ok and the amp was switched off. Next morning the amplifier did not start anymore. I checked and noticed that both fuses (on both lines) had blown. I changed the fuses, a brief buzz is heard from the power transformer and the fuses are blown again.

So I started digging. There were no visible burn marks or anything on the board. So I removed all tubes from the board, re-tried with no luck, fuses go again. Then I removed all power transformed secondaries from the board and metered the transformer output which seemed ok. After this I started looking for possible shorts on the board and finally noticed that the DSEI-12 FREDs were blown. The other passes everything though either way and the other almost the same.

I looked around the messaging board and found some discussions on people having problems with simple se blowing FREDs with certain power transformers (hammond was mentioned). It had something to do with some power spikes which fry the FREDs if you're unlucky.

I don't have any spares for these FREDs currently so a few questions about this problem

a) Is there anything to be done to avoid this issue in the future ? Is there something to be added to the circuit to prevent the power spikes frying the FREDs ?

b) Is there any alternative for these DSEI-12 FREDs which might cope better with the spikes ?

Getting these parts from Finland might be tricky so I'm ordering these probably from Mouser anyway. I'll probably pick up some spares for all the FETS and diodes on the board to be sure but I'd appreciate all feedback from you experts before I place my order.

Anyway I'm very happy with the amplifier and the sound and the amplifier seems more than enough for the fonken floorstanders it is driving. Big thanks to George by making this possible. Please see the attachment for the finalished (but no dead ) build.

Thanks in advance!
Marko

[SGregory]

It looks like you have a rectifier tube installed. If so, why don't you just use it and remove the FREDs.
There are some SiC rectifiers out there that would fit the bill nicely if you want SS rectification. A bit pricey, but do have better rating.

[mulletronic]

"The on board power supply uses FRED diodes for the bias (negative) voltages. This allows fast startup of the bias supply. The plate (positive) voltages are tube rectified. A 5AR4 is recommended because of it's slow warm up characteristics.".

The amplifier is "tubelab se" not "simple se" so what you are suggesting is not an option ?

[mulletronic]

Originally Posted by SGregory

It looks like you have a rectifier tube installed. If so, why don't you just use it and remove the FREDs.

I think the tube / SS is optional only in SimpleSE ? The amp is question is TubelabSE.

[tinitus]

Hey, maybe read about the hexfred issues in SimpleSE section
might be the same problem, I dont know, but worth a try

[rknize]

Use the Fairchild "Stealth" diodes instead...same as for the SSE. I have these in my TSE for the bias supply and they have given me no problems.

[SGregory]

Sorry saw the rectifier tube but didn't catch the two driver tubes. You should also check/replace the FETs. I blew a bias diode when I had a FET fail on my Tubelab SE. Failure mode was similar to yours. It worked fine for a long play period, came back and the fuses blew. I have never had a problem since.

IIRC the diodes used on the bias of the Tubelab SE are fine as speced. It wouldn't hurt to upgrade them as suggested by rknize. The B+ SS diodes on the SSE posed the problem due to a manufacturing change.

[mulletronic]

Thanks everyone for the input! I'll order some spares for the freds (the stealths you recommended) and fets and give it a try. I'll get the 2sk3563 FETs instead of the currently installed 2sk2700 ones because 2s2700 ones seem to be out of stock.

BR,
Marko

[SGregory]

Use a 2sk3564 instead as they are rated for 900V like the 2sk2700. the 2sk3563 are rated only for 500V. I think you can easily exceed 500V on your set-up.

[mulletronic]

Hi all,

To follow-up on this thread the amp works ok again after changing the freds. I did not change the mosfets so it seems only the freds were fried. Oh well.

br,
Marko