• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Some doubt about my first Tube amp (Heater).

tnt76

Member
2005-05-25 12:33 pm
Italy
Hello!
This is my first post :cool:

I'm writing from Italy and please, pardon for my bad english...

I've made my first amp and this forum is the more useful i can found on the web.

Well, my amp is based on the Ciuffoli's Power follower outline and it's working huge now:D
At the moment i'm driving the follower with a tubes preamp recycled from a cheap kit based on the 12AU7 tube.
In the meantime i'm recovering all the material for building a line stage as the input stage of the famous Triodino (almost in Italy :D ). I found a matched pair NOS Sylvania 6SN7GTB... they should sound good, isn't it?
You can found here: schematic

I'll drive it only with my tweaked CD player, because all my software is on the CD (i'm a young boy;) )
Wath do you think about this schema?

But, my main doubt is about the heater supply. From the kit i recycle the transformer (my budget is low,unfortunately:( ) , but the secondary for the heater is @12.6V, so i'm going to make a DC supply with a regulator.

Wath about this?
Another doubt is about heater to cathode voltage. I have to raise the reference for the heater, this is clear, and if i leave the heater PSU floating? Is still necessary?

Please don't damn me for all this questions....:smash: :smash:
 
tnt76 said:


But, my main doubt is about the heater supply. From the kit i recycle the transformer (my budget is low,unfortunately:( ) , but the secondary for the heater is @12.6V, so i'm going to make a DC supply with a regulator.

If you're concerned about having only 12.6V available to drive the 6.3V 6SN7, you could consider connecting the heaters in SERIES (6.3V + 6.3V = 12.6V)
 

EC8010

Ex-Moderator
2003-01-18 7:57 am
Near London. UK
Welcome to the forum. Don't worry about your English - it's better than my Italian.

I've looked at the circuit. That switch before the choke isn't going to last long. Each time you switch the amplifier off, the choke will generate an arc across the switch and burn it out in very short order. I can see why it's there, but solving the problem properly is a bit more expensive. As a temporary measure, I suggest that you only switch the amplifier off using the mains switch. Then, after a few seconds, switch off the HT switch.

I doubt if your heater transformer can supply the current needed to feed 6SN7 (600mA as opposed to 150mA). Never mind, it shouldn't be too hard to find a cheap 6V or 6.3V transformer. I wouldn't worry about DC heaters on a power amplifier. The audio signal is big enough that heater hum won't be an issue. Besides, the 2A3 unavoidably generates heater hum. Yes, you really ought to connect the centre-tap of your 6.3V heater supply to about 90V.
 

tnt76

Member
2005-05-25 12:33 pm
Italy
zobsky, you give me an excellent tip...

but... EC8010 is right
:bawling:

my transformer is only 10VA... i checked now :( so it can give only 0,8 A, damn!
I have to find another...
but i prefer DC heaters, from the schematic that i linked, i'll take only the first stage.

In the circuit i'm usign now, with DC heater i reduced a lot of hum....
 

PRR

Member
Paid Member
2003-06-12 7:04 pm
Maine USA
www.diyaudio.com
> i prefer DC heaters, ..., with DC heater i reduced a lot of hum....

Correctly wired AC heaters don't hum.

Heater leads tightly twisted, routed away from grid-circuits, tight to a steel chassis.

Back in the Golden Years, everything was AC heat and we got mighty-fine recordings. It seems to be a forgotten art.

I consider DC-biasing an option, or a problem-solver. On well-insulated heaters, it should not be needed. Heater insulation is never perfect, and if you mass-produce amplifiers you are sure to get a few leaky heaters. It is cheaper to find a positive DC bias than to weed-out leaky heaters.

Note that poorly-filtered "DC" makes much worse buzz than AC. I ran into that on a recent project for friends: knowing that they would NOT twist-and-route the heater wires properly (one dude even made a PCB: you can't run AC heat through a PCB), I showed a DC heater supply but without much filtering. Instead of 6V 60Hz, it showered the circuit with 0.4V 120Hz, 0.2V 240Hz, 0.1V 480Hz, etc. Small amounts of midrange buzz couple into circuits better, and annoy the ear more, than large amounts of pure 60Hz. I had to add some more filtering to get the ripple harmonics down.
 

tnt76

Member
2005-05-25 12:33 pm
Italy
works in progress

well, thanks for all this cool suggestions :)

Actually my Hybrid /Integrated class A amplifier introduces itself in this manner:D :D

I'm waiting for a new 300VA toroidal transformer for the mosfet section and a new 20VA (0-6,3 X2) for the heaters supply.
At this time i'm testing with a regulated DC supply taken on loan from another devices for the high 6SN7 and another small self made for the lower ( 7905 fixed regulator with two diodes in series on the ground).

The high voltage section is mosfet regulated with the virtual battery operation mode (it takes more than 7/8 minutes to catch up about 332 V)

I've tested it with some of my preferred audio CD:eek: :eek: :eek:
The 6SN7 vs 12AU7(visible at the top of the image) sounds so wonderful that i can't believe all of that difference!!!
I've dicovered a lot of particular i never listed before...

I'm very happy
:D :D

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