Full B+ voltage @ 5842 plates / coupling caps

[jamicomon]

Hey everyone,

It's been a few months since my last post.

I recently re-did the checkout for my tubelab SE as I am now using a step-up isolation transformer to increase my sagging B+ (around 315v).

I increased my input voltage from 115v to 130v.

I used so .1 ohm resistors on 6.3 and 5v windings to drop the voltage.

I initially had 300v plate voltage on my left 5842, but now I have full b+ on both 5842 plates. (Even after going back to 115v input and even with the 5842 tubes removed)

I replaced both CCS ICs.

Anything else I can check?

 

[JonSnell Electronic]

Sorry, no idea what circuit you mean. Maybe someone can recall your older post?

 

[kevinkr]

Pushing the power transformer pretty hard, not a good idea for the long term as the transformer probably running pretty close to saturation.

As I recall the TSE uses CCS loads on the 5842 and running with the tubes removed will result in the full supply voltage present on the plate pin..

What power transformer are you using and what are its rated secondary voltages?

Are you using a tube rectifier?

Did you search for your old thread?

http://www.diyaudio.com/forums/tubelab/266349-tubelab-se-bias-current-jumping.html

http://www.diyaudio.com/forums/tubelab/265284-started-tubelab-se-checkout-r5-went-into-fusion.html

Note: You will need to ask George to merge these threads if it makes sense.

 

[jamicomon]

Thanks you JonSnell Electronic and kevinkr for the quick replies.

Since this is a new problem, I started a new thread.


This is for my tubelab SE.

The power transformer is a hammond 376x:

640V C.T.
@ 173ma.

5V C.T.
@ 3A

6.3V C.T.
@ 5A


Also, by increasing the primary voltage of the 376x, I am using less current overall.

The load on it stays the same (higher voltage/less current).

The temperature of the 376x seems to be lower that way.


I am using a sovtek 5ar4 rectifier.

I checked all nearby resistors and everything checks out.



Does that mean my 5842s are dead?

 

[kevinkr]

Something seems to be very wrong here..

Increasing the primary voltage should result in higher current, not less, unless you are deliberately reducing the plate current. Internal losses in power transformers increase substantially when operated at currents and voltages outside of their intended range of operation.

The secondary current rating seems a bit low for this application..

You might try replacing the tube rectifier with diodes or at least something other than that Sovtek 5AR4. (I found both JJ and Shuguang generally performed better.)

I doubt there is anything wrong with your 5842..

I wonder if there is anyone in your neck of the woods who could help you debug your amp..

 

[jamicomon]

So I woke up this morning and decided to tinker with my amp.

I took out the PCB and retouched my soldering for Q1, Q2, U1, U2 on both sides of the board.

Now I can adjust to 175v plate voltage on both channels.

I don't know if i will still mess with the step-up isolation transformer.


Thank you all for your time.

 

[jamicomon]

I put the output tubes back in and started to adjust their current.

On the input tubes, the right side plate voltage adjusts fine.

The left side input tube can't go below 188v plate voltage.

And now the mosfets (U1 and U2) get insanely hot and start smoking even though they have a massive heatsink with heatpipes.

I might try replacing the mosfets next...

 

[jamicomon]

I replaced my 2sk3566 mosfets by 2sk3565 which has higher input capacitance.

I can now adjust both input tube plate voltages to 175v.

They still get quite hot but at least they don't smoke.

Everything is now working as it should.

 

[jamicomon]

BTW I tested all of these with a parts tester and they all test good.

All their values are as they should be.

 

[Evenharmonics]

Good to hear success stories. Let us know what you think of the sound.