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Nikon1975 23rd January 2010 08:25 AM

Power Transformer for Tubelab SE
For testing, can I use a 360-0-360 (on the high voltage side, as it is rated 280 mA) to drive the 300B ?

From my experience with the KT88, with 50 ohm resistors on the secondaries, 22uF input cap, using the 5R4GB I should get around 400 V on the B+. But I am not sure of what will happen with the B-.

I can use 5000 ohm load.

Please advise,

Best Regards,


w5jag 23rd January 2010 04:37 PM

You could omit C4 and configure the power supply as a choke input.

My Tubelab SE is configured for choke input to drop the voltage down to where I wanted it.

Rectifier tubes with larger voltage drops can also be very useful to lower voltage.


Nikon1975 24th January 2010 07:08 AM

I thought about it, but it looks like chokes for L-input are different from chokes for C-input, althought I cannot understand what the difference is.



rknize 24th January 2010 06:59 PM

You need to use a very large choke on a choke-input filter. It also needs to be a good one or it will probably buzz annoyingly.

SGregory 25th January 2010 01:57 AM

Buzzing of the choke is the problem I had. I was using a very nice 10H electra_print. I am not sure what the difference is between a choke for a CLC or LC is, but there has to be some guidelines.
You could also use a couple of resistors just prior to the rectifier. If you look up your rectifier sheet it will give a list of recomended values for either CLC or LC configuration.

Nikon1975 10th February 2010 03:09 PM

So, I still did not buy a power transformer for my tubelabSE. I am using the 360-0-360 with no problems now. I was ordering a 320-0-320, but I saw the specs of the 300B, and it looks like this tube likes to be driven at 400 V plate and 80 mA.
From the graph the distorsion is lower at this working point.
I understand that the power drive comes to work in the transients, but if I put a test tone, he does not add anything to the distortion of the tube.
I have the Golden Dragon 4-300BC (C stands for Carbon, I guess) and they should also have a higher voltage rate of the standard 300B.
This I cannot get with a 320-0-320.

What do you think ? Should I go for the 320 or keep it as it is ?

Please advice.

Best Regards,


rknize 10th February 2010 03:23 PM

My simulations show more like 363V of B+ when using 360-0-360, a 5R4GYB, and a CLC filter of 22uF, 10H, 120uF. It should work fine, but you will need to adjust R14 and R25 accordingly (36k).

However when using the specified 5AR4, you will get more like 435V of B+. Be careful.

Nikon1975 10th February 2010 03:44 PM

I am running the SimpleSE with an external psu, as I have one with very good components. If I decide to run the 300B around 400V, maybe I could just add the bias part at the PSU and use the same for both amplifiers.
I have all the parts for this.

I cannot decide, and I really would like to have a finished product, now it's Frankenstein style.


jrenkin 10th February 2010 03:57 PM

Am I misunderstanding something about power supply design?

If you have a slightly over voltage transformer, couldn't you just adjust R5 and R6 to get the desired bias voltage and add an RC filter after C5 to get the desired plate voltage?
Or add resistors before the rectifier.

With simple modifications and PSD II it seems like anything in the range of 320-0-320 to 380-0-380 or even more would be manageable, although not necessary. So why not work with what you have?

Nikon1975 12th February 2010 02:03 AM

My point is what is the working point you recommend with the 300B ? What load ?
Because at the beginning I went straight for 3500K 350 V 70mA, but when I saw the data-sheet of the 300B it looks like 5000K 400V 80mA, will reduce distortion significantly.



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