300B AC heating?

Ray P · Post by **Ray P** » Mon Sep 16, 2013 8:30 am

pre65 wrote:I'd be frightened to cost 8 quality 300b valves.

It helps with my aim to not have any money left by the time I need to be looked after

Ray

Paul Barker · Post by **Paul Barker** » Mon Sep 16, 2013 5:57 pm

Ray P wrote: Paul, apparently there are no transformers in Bruce Rozenblit's 25Hz supply.
Ray

Makes sense. Only need 5v.

Ray P · Post by **Ray P** » Mon Sep 16, 2013 6:41 pm

Paul Barker wrote:
Ray P wrote: Paul, apparently there are no transformers in Bruce Rozenblit's 25Hz supply.
Ray
Makes sense. Only need 5v.

Hi Paul,BR has also released an update to the original circuit to address the possibility of 'bass pumping' occurring.

http://tubehifi.websitetoolbox.com/post ... nt-6383815

Incidentally, we're fast approaching the tenth anniversary of our trip to Seattle.

Ray

Dave the bass · Post by **Dave the bass** » Mon Sep 16, 2013 9:46 pm

Ray P wrote:.... to address the possibility of 'bass pumping' occurring.

I hope it doesn't happen anymore too, I'm already pumped up quite a bit thank you all the same!

DTB

Andrew · Post by **Andrew** » Mon Sep 16, 2013 9:50 pm

Ray P wrote:
Andrew, if you're developing a 25Hz supply please remember my requirement, sufficient capacity to run four 300Bs!

Ray

Sorry Ray mine is a DC supply.

I never did find out why BR's has to be AC, but he seems to know what he's doing so I guess he must have a good reason.

Andrew

Ray P · Post by **Ray P** » Mon Sep 16, 2013 11:06 pm

Dave the bass wrote:
Ray P wrote:.... to address the possibility of 'bass pumping' occurring.
I hope it doesn't happen anymore too, I'm already pumped up quite a bit thank you all the same!

DTB

I'm sure Mrs Bass will be relieved.

Ray

Ray P · Post by **Ray P** » Mon Sep 16, 2013 11:24 pm

Andrew wrote:Sorry Ray mine is a DC supply.

I know Andrew, just a hopeful jest

Regarding AC heating being necessary: In BR's OTL amps the output valve cathodes are connected to the output and, of course, in a 300B the cathode is also the filament. To give cathode followers a high resistance to operate into BR uses a current sink arrangement; the current sink has a high AC impedance but a low DC impedance and it is this that prevents the use of DC heater supplies. I'm paraphrasing the relevant chapter of BR's book and I don't fully understand the technicalities but hope that helps.

Ray

Paul Barker · Post by **Paul Barker** » Tue Sep 17, 2013 7:37 am

Yes it looks Andrew like a specific situation.

Once I realised it had to be solid state I was turned off to the idea of using it in one of our more standard applications.

for me the benefit of AC heating is the least interference path. As soon as we introduce a path which is as complexed (and contains as much silicon) as a DC heater supply it is game over for me.

someone may say that to generate your own clean supply at 20hz or so prior to your transformer you too have a shed load of solid state Paul.

And I would have no retort.

Off to a slight tangent, the most likely direction my efforts to generate an independent clean supply would take should I ever get it off the ground, may well be a DC motor speed controlled (via stabilised DC power supply absolutely loaded up with solid state) driving a 3 phase generator, (full rotary balanced 3 phase generation, not just pulling a third phase with a capacitor and then passing the output through a motor to try and equalise it, which the usual rotary converters do) and make my B+'s 3 phase choke input. Heater supplies would have to be from another supply as I wouldn't want to pull an imbalance from the phases. 3 phase choke input B+'s a Go Go.

Seems a lot easier to clean the mains we have. but if anyone has a spare 3 phase motor in the shed I can experiment with?

pre65 · Post by **pre65** » Tue Sep 17, 2013 11:32 am

Dave, will you be using separate 5V transformers, or a 5V secondary on a HT transformer ?

Dave the bass · Post by **Dave the bass** » Tue Sep 17, 2013 11:45 am

pre65 wrote:Dave, will you be using separate 5V transformers, or a 5V secondary on a HT transformer ?

Separate 5V filament TX's probably Popz.

DTB

pre65 · Post by **pre65** » Tue Sep 17, 2013 11:47 am

Dave the bass wrote:
pre65 wrote:Dave, will you be using separate 5V transformers, or a 5V secondary on a HT transformer ?
Separate 5V filament TX's probably Popz.

DTB

Good lad.

pre65 · Post by **pre65** » Tue Sep 17, 2013 11:55 am

My original 300b (built by some "bod" in Germany) used small 6V AC transformers with dropping resistors.

Alex Kitic · Post by **Alex Kitic** » Fri Oct 18, 2013 7:06 pm

There are several arguments why AC on the heaters leads to better sound. If adequate circuitry is applied, hum should be reduced to a minimum, and the minimum will depend primarily on voltage. Lower voltage means lower audible hum, thus 2A3 leads the way. With 10V or 20V direct heated tubes, hum is too high and DC on the heaters is absolutely necessary. The gain of the tube is another important factor.

But the hum issue is actually most closely related with the efficiency of the speakers! If your speakers are highly efficient, like 98dB, you will probably need DC on the heaters of direct heated tubes, regardless of voltage or gain. On the other hand, with normal speakers, like 88dB/W/m, you should be ableto enjoy even 6B4G at 6.3V AC heating. Of course, maximum SPL levels will suffer... so choose your poison wisely

pre65 · Post by **pre65** » Fri Oct 18, 2013 7:11 pm

Alex Kitic wrote:On the other hand, with normal speakers, like 88dB/W/m, you should be ableto enjoy even 6B4G at 6.3V AC heating. Of course, maximum SPL levels will suffer... so choose your poison wisely

88db is not "normal" for us on here.

Alex Kitic · Post by **Alex Kitic** » Fri Oct 18, 2013 8:29 pm

pre65 wrote: 88db is not "normal" for us on here.

I can only envy you: freedom of choice is not the strongest asset of life in my country.

My only consolation is that AC sounds better than most DC solutions.

Has anyone tried heating tubes with "electronic transformers", AC high frequency transformers for use with halogen lamps? High frequency might be less audible or annoying?

audio-talk

300B AC heating?

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