audio-talk

26s back to individual DC boards, no virtual cathodes, bias resistor connected to the +ve filament pins.

Valves are in balance to within 0.25mA of each other.

Warning up with TV then a quick listen. Unfortunately I'm at work early...

... But hey it all works...

Scary. You'd have to be mad, mad I tell you mad, mad. Nobody around here would... Nurse, nurse, time for my meds...

I'm looking for suggestions for plugs and wire to connect an off-board power supply...500v ish and 300ma ish. I've been using GX16 avaiation plugs for low voltage stuff but they're not up to this kind of voltage. I've never done a remote ps for valves before so I'm looking for some pointers. I've u...

So, back to the original topic for a second. Once the virtual cathodes are out I'll implement the led warning and report back.

Watch out for imbalanced anode currents, If the two valves are not an exactly matched pair. DC offset will reduce transformer inductance. Unless the transformers have gapped cores. Ideally needs presets to indiviually adjust grid bias and equalise the anode currents. I had this aggro with a GEC Wil...

Well I'm convinced something has changed for the better. Unless I like the sound of one valve at a higher bias than the other (consequence of series heating) those virtual cathode resistors were holding the amp back. Hard to believe quite honestly, given where it was, but that was an unexpected resu...

In my most humble opinion, getting the filament heating "right" is the key to directly heated valve performance. To that end, each directly heated valve should have it's own separate filament supply. Makes sense. Nick tried to tell me that several times. I was listening honest, but nice to be able ...

Hmmm as always there is a little more to it. The outer valves are at the -ve end of the heater voltage. Each valve has a voltage gradient on the filament, but the midpoints of the filaments are different. The outer valves have less bias and unsurprisingly are running slightly higher DC current (6mA ...

Switched over to my favourite valves (old Cunningham globes) and the variation is a little greater. One channel they are identical, the other one is 5.5% high the other 3.5% low. The sound is every bit as good, first impressions are the bass is tighter, more tuneful, follows the micro sounds better....

Left channel now done. Both valves within 0.02v (one high, one low), that's 1.3%.

Ok I've converted the right channel 26 pair to series, removed the virtual cathodes, moved the 10R measure resistors to the anodes, disconnected the unused board from the transformer. I'll trim the board for 3v. Will report back after listening probably tomorrow, wife not well poor love so off to s...

Ok I've converted the right channel 26 pair to series, removed the virtual cathodes, moved the 10R measure resistors to the anodes, disconnected the unused board from the transformer. I'll trim the board for 3v. Will report back after listening probably tomorrow, wife not well poor love so off to so...

Andrew you asked for a refresh of the current circuit, see below. The same pattern is used for all valve pairs, with the same Virtual Cathode resistors (200R) and the same measurement resistors (10R).

1) with a DC heated DHT using Andrews DC board, is there a benefit to using virtual cathode resistors When this question was asked, my answer was "as there is no disadvantage to doing so, and some theoretical justification, then may as well" Now there is a downside, that justification is removed. 2...

Can we please revisit the virtual cathodes? I see the sense and purpose of these in a AC heated circuit, turns single ended noise into common mode noise etc, but I'm not seeing the value here. The DHT heaters contribute very liitle to the noise in an amp, so there's no noise rejection benefit; ther...