audio-talk

I think the idea was not negative bias as such, but just a way of making it less positive at times as a means of control.

I believe Hammond do a 10v @ 10A (CCS) centre tapped filament transformer for 833. I suppose they were really for transmitter use, but what's the thoughts on using them for hi-fi use ?

I've got the U19 rectifiers and the 833c carbon anode valves now, and the transformers / chokes from Canada should be next week sometime.

You can make a negative bias supply which keeps you in class A1 the whole time for low power and use driver valves suitable. But which are not suitable for class A2.

OR you can use driver capable of driving grid current which you have done up to now. You can bias this negatively but allow it to go into A2. BUT in that case your negative bias supply has to be strong enough to hold steady when the grid current comes into play.

The two exclusive paths are realively easy for you, the middle ground allowing for both is difficult.

My view is bias at 0v except for the 5v which occurs at the filament if you ground one side, so your bias can be -5v or +5v or 0v depending where you connect the filament transformer (which would be fine) to ground.

A mechanism to reduce power output would be to lower B+ voltage a means to raise it would be to increase same.

In this instance make a powerful driver of low impedance and be sure that the transformer, if IT coupled as your friend, has low dc resistance.

As always, my preference for this would be direct coupled CF. If you use a low dc resistance inductive load on the CF grounded, the bias will be positive (no bad thing for the 833 (all suffixes) ) you can mitigate the positiveness by the filament connection to 5v less. You end up at a good operating point controlled by B+ keep an eye on cathode current and stop where you feel comfortable.

If you want less simple you make a +/- driver supply and level change your CF at will to any bias you desire.

I've unpacked the carbon anode versions (833c) and giving them 30 minutes on filament only.

I think they are of Chinese origin, but not 100% sure.

After 10:30 I'll see how they sound with music.

This sounds like the right way to spend an evening . Pics of these anodes would be very cool too, when you get chance Phil .

The only 833Cs around these days are from China, unless you can find hen's teeth.

The 833C looks like the ideal candidate for HF AC heating. At 10V 10A, it is perfectly feasible with a 150W unit, and modifications are easy (removing a couple of turns from the secondary.

At high current draw these electronic transformer units really come into play: cost, size, heating... and noiselessness in HQ DC class.

I guess the 833A is not that picky when it comes to heaters voltage as the 813? 5% would be OK?

Phil is not confident in the voltage assumptions. The way to do it would be for me to make one and check voltage on scope. But all 833 equipment is at my lockup and I have zero mobility. Laters.

Paul Barker wrote: Phil is not confident in the voltage assumptions.

You are so right about that - it almost hurts. It's assumptions - but for the time being they are rather consistent, at least when it gets to calculating the voltage per secondary turn, and subsequent result.

I think Philip should first revamp his GK71 amp, eventually modifying it to match the RH813 due to be posted probably this week.

Looks like the Hammond "iron" I ordered is in the UK, but as I'm away in West Yorkshire for a few days (watch out you locals ) I've asked for delivery next Monday.

Seems Hammond consider this a LARGE order.

I could be trying two 6D22s damper diodes as half wave rectifiers with my Hammond 1500-0-1500 transformers.

I will need a dedicated 6.3v transformer for the heaters.

Any potential problems with this arrangement ?

Just make sure the heater transformer is wound to handle 2kV between its primary and secondary.

The Hammond 735a 3,000v ct mains transformers have been delivered.

Big beasts, the other items will follow shortly.

Nick wrote:Just make sure the heater transformer is wound to handle 2kV between its primary and secondary.

Yep, I'm going for a couple of Hammond 266N12, 6.3v @ 8A, a bit over the top current wise but only a few pounds dearer than the 5A version so worthwhile.

Today could be the last day of 833a in it's current form, so I'm giving it a play today.

The HT PSU is being "robbed" to make a new dual mono PSU for the GK-71 amps, similar to what was used on the old GM70 amps a few years ago.

So, 833a will be dormant for quite a while till I get the urge to use all the big and heavy Hammond iron that's waiting for the new PSU.

Interesting ... sounds like a worthwhile exercise though . What voltage are you aiming for with the GK-71's ?
It would be good to hear what they can do at 700 or 800V .