845 Project

[Ali Tait]

Your floor is starting to look like Nicks!

[pre65]

The heatsink i made for the cathode resistors was getting really hot,but i had not noticed for fear of getting my finger too near 950 volts !!

So using some alloy channel i made up a new heatsink and mounted it on the output transformer with standoffs.

The heatsink is now mildly warm rather than bloody hot.

http://i111.photobucket.com/albums/n...45heatsink.jpg

The original heatsink is on the right of the new one.


Philip

[pre65]

Hi-i have also made some changes to the 300b amp that drives the 845.

I have now removed the output transformers (they had been bypassed !) and installed a DC heater supply for the 300b,to try and improve the hum.

Phil Ramsey had advised me to aim for 4.8V instead of 5V and using a 5v Hammond transformer,Shottky diodes and a cap thats what i got.

So the 300b is much quieter now but it has shown up a bit of hum on the 845.

Now looks like i will have to try a current regulated supply for the 845 heater or bite the bullet and go for a Tentlabs module,which is current and voltage regulated.

http://i111.photobucket.com/albums/n...0bDCsupply.jpg




Philip

[pre65]

Update time.

The 845 heater hum needed to be reduced so i decided to experiment with AC.

Andrew I supplied me with a large 10V CT transformer,twin secondaries and made for RCA Victor !

Tried it with the centre tap first and hum was worse ! Then tried it with the two 50R resistors i had used for the DC as hum buckers and the hum was reduced,but still slightly more than my basic DC supply.

However,with the AC heating the sound seemed a bit more "open" ,so this morning i had a chat with Nick Lucas and as a result now have a couple of 5 Amp Tentlab heater modules winging their way to me.

As my amps are monoblocks Andrew I has let me have another of the large transformers and they should run the Tent modules perfectly,giving 10.2 V under load on the 230v tap.

[steve]

Hi Philip,

I am interested in your results. I have found AC heating more open with past amps. In my home design & build 845 I got the hum down to 9mV p-p using DC heaters throughout. (Measured hum at speaker output with a 47K resisistor between IP & GND) but I think the sound would benefit from AC heating.

I thought the tent labs were DC modules also? but based on current sources. Still let me know if they make an improvment and measure your hum level with a scope and let me know.

[pre65]

Hi Steve-i'm in the process of moving things around on the breadboard to make room for the BIG transformers and Tentlab modules.

From what i have read the Tentlab modules are voltage AND current regulated.

My simple DC heaters for the 845 were used with two 50R resistors instead of a hum pot,and were slightly better than using a conventional hum pot.

Hopefully pictures and a report will follow later !

[pre65]

Well it's all back together and working !

Have played a few CDs that i am familiar with and the difference is quite noticable. That old cliché about a veil being lifted is my description ! Normally i am not able to describe differences ( if i can hear them ) but this is so much better.

I can only imagine what it would sound like if i could afford another Tentlab module for the 300b driver !!??

http://i111.photobucket.com/albums/n...Tentmodule.jpg

[pre65]

Today is an 845 day,and now the 300b amp has had a "makeover" with the fixed bias ripped out and converted to cathode bias.

So, we have 6CG7 driving 300b, then connected to the 845 modules (via 2-1 step up transformer) and driving my KLS3 speakers.

http://i111.photobucket.com/albums/n...5/845setup.jpg

Future plans are to try the 845 with cap coupling (no transformer) and then try GM70 in place of 845.

The future really is for the VoFos and amps like my 300b, KT66 SET and 832a P-P but what a big and powerful sound the above system gives, and subtlety as well ! Pity it takes up so much room.

[andrew ivimey]

My feeling about GM70s is that they are easier to drive than 845s (and of course they have plenty of grunt). So, sort the heaters and you also have bedside reading lights.

[Ali Tait]

Almost anything is easier to drive than 845! (and 300b)