10 V 3.25A heater supply

[pre65]

Hi-i have been thinking about building a current regulated filament supply for the 845.

Is this easy to do with the current involved ?

I see Tentlabs do a "module" to suit but it's quite expensive and still needs a transformer to feed it.

The "module" i made for current regulated filaments on PT15 was relatively cheap and easy to make,so i'm hoping the same will be true for 845.

I have heard that voltage regulated supplies "rob" the sound of life and that AC supplies can be more "powerful",but then the hum becomes a problem again ?

[nic]

If you're going to use a chipreg , use as large a heatsink as possible and go for a low-drop type such as the LT1086 . The main problem at higher currents is heat , you may find that your current sense resistor will need to be much larger in wattage than anticipated . I don't use chip regs for filaments anymore , preferring chokes in an LCL arrangement which I find makes a better sounding supply , it's more or less a passive version of the 'Ronanreg' . AC on filaments couples crud from the mains right into the signal path , there have been some interesting discussions regarding this on DIYAudio . I'd expect the 845 to be a hummer with it's 10V filament , DC will kill the hum stone dead and should sound better than AC if implemented correctly

ps Don't forget a NTC thermistor on the mains transformer primary . Thoriated valves such as the 845 appreciate a soft start on the filaments

cheers

Nic

[Nick]

Thoriated valves such as the 845 appreciate a soft start on the filaments

Which current reg will give them to some extent.

One thing I found trying to heat GM70 fills, its easyer to kill the LDO regs, the older 337 types will sulk and pass no current, and behave very strangely, but they keep going.

[nic]

Thoriated valves such as the 845 appreciate a soft start on the filaments

Which current reg will give them to some extent.

You're right , my brain is locked into choke mode

One thing I found trying to heat GM70 fills, its easyer to kill the LDO regs, the older 337 types will sulk and pass no current, and behave very strangely, but they keep going.

Sounds like a job for a nice , chunky , outboard LCL supply

cheers

Nic

[pre65]

Question time (Again !)

I have seen some Tentlab 845 heater modules on e-bay and was wondering how accurate the claims are,not only for the low hum aspect but the claimed improvements in sound quality ?

http://cgi.ebay.co.uk/ws/eBayISAPI.dll? ... :IT&ih=015

[pre65]

Hi-thanks to Andrew I i now have a 10v CT transformer to try out with AC heating on the 845.

Now in the process of wiring it up with the cathode resistors feed taken from the CT,but not sure of how to place the two 50R resistors that were used in place of a hum pot for the DC heating.

The diagram shows how it was,and how i think it should be,just need an "expert" to clarify my thoughts please.

[Nick]

If you are going from the center tap, just leave them out, bipassed cathode resisor to center tap.

[pre65]

Well AC heating is a bit noisier than the basic DC supply i was using.

Looks like i will have to start saving up for the Tent modules and transformers !!

So its back to DC and if i bring the Adires with the amp to Owston the hum wont be so noticeable.

[Nick]

Well AC heating is a bit noisier than the basic DC supply i was using.

Did you expect any different?

I would try using a hum pot instead of the center tap of the transformer.

One way of doing the current regulated supply is to use parallel current sources. Say, two set for 1A then the third adjustable to set the exact current to what you need.

I haven't tried that though, so there may be catches I haven't thought of.

[andrew Ivimey]

I wonder if the hum you are getting, Philip, is 100% from your heaters.

For hexample. I have always used AC with humbuckers on the 211s of my ongaku. No hum with 93dB speakers if 'tuned' in. So, you could use the centre tap for the cathode resistor and cap and the pot across the heater elements and the wiper of the pot to the centre tap, too. This might help.

With GM70s I had to use CD and indeed used CLC after a bridge rectifier. No hum and no tuing to do but I didn't like the sound of the amplifier; more variables, I admit.

[pre65]

When i had finished wiring up the new transformer i switched on the heater supply for the 845 on its own,nothing else,nor the 300b amp,and the hum was there !

Must try it with the two 50R resistors first as my hum pots are being used on the 300b DC supply at the moment.

Hum aside,it seems a tad more "open" with AC heating.

I might invest in a couple of Hammond filament transformers (one for each monoblock) as i will need a 10v transformer for the Tentlab modules.I seem to remember reading that running the filaments of two DHT from a single transformer was not good practice,although the new transformer has two 10v CT secondaries.

[Nick]

I seem to remember reading that running the filaments of two DHT from a single transformer was not good practice,although the new transformer has two 10v CT secondaries.

Should be ok with seperate secondaries.

[pre65]

Had the power off most of today while the leccy people cut branches off nearby trees so not had much time to play.

I've disconnected the centre tap and used the two 50R resistors again as the hum bucker.

Hum is a bit better now,only a smidge above what i had with DC.

Now to find a couple of Hammond 185D10 so i can make each monoblock self contained,and they should run the Tent modules when i can afford them.

[pre65]

Our man in Bedford (Andrew I) has offered me another of these massive 10v transformers,so i can use one on each breadboard.

They are made in Canada for RCA Victor,so would that have been for cinema amps ??

Using the 250v tap the heater voltage was down to 8.9V,but on the 230v tap i get 10.2v.

Just need a crane to lift them now.

[pre65]

I had a chat with Nick Lucas this morning and as a result have ordered a pair of the 5A Tentlab modules.

So,hopefully,in the next few days 845 hum will be a thing of the past.

Interestingly,Nick might pay a visit to Owston !