805 Power stage.

[pre65]

Hi-after giving most of my amps a bash yesterday (except 45 cos that's for Mo ) i came to the conclusion that i prefer ECL82 Doddington with the VoFos.

300b came close but the accentuated mid/upper mid vocals were a tad unreal,but with some material quite interesting.

Now as to my "transmitter" valve aspirations,still thinking about that !

On the question of raising the heater potential for the 6D22S diodes,is it just a case of running the HT to one side of the 6.3V AC connection,or is there merit in dropping it a tad (say from 900v to 500v) via 2 resistors.

The EM516 diodes came in the post from Germany this morning.Not bad service me thinks.

[Nick]

On the question of raising the heater potential for the 6D22S diodes,is it just a case of running the HT to one side of the 6.3V AC connection,or is there merit in dropping it a tad (say from 900v to 500v) via 2 resistors.

YEs, that will do it, or just don;t reference it anywhere, the secondary will float up to the B+ through the capacitance between the heater and the cathode.

The idea of creating a center point via a couple of resistors highlights one of the things that need to be remembered with higher B+, check the max voltage ratings of resistors, you might find its less than you expect.

Then work out the dissapation of resistors at those sorts of voltages. 100k across 900v will disapate 9w.

You have to think about aspects that can be safely ignored at lower voltages.

[pre65]

Hi-i attatch a power supply drawing i did earlier.

Does it have any obvious faults,or perhaps anything of merit ?

Would one 6D22S do instead of 2 ?

[Lee S]

Why are you feeding unsmoothed DC from your BR into a pair of paralleled valve diodes? Not sure what that is going to do Philip, apart from drop more voltage across the diodes. The smoothing circuit looks OK to me though. What are the 6D22S meant to be doing?

Cheers

[pre65]

Hi-the main reasons are to have a slow warm up ( the 6D22S is quite slow-about 30 secs) ) and a way of reducing the voltage peaks from the bridge.

Several Chinese transmitter amps and 211 amp circuits i have found use a FW valve rectifier after a bridge.

[Lee S]

Interesting... Sounds like a good idea then. If that's the case, then it all looks OK to me.

[Nick]

Looks ok, I would use 220k on the caps instead of 100k.

As to how many rectifiers to use depends on the current you intend to supply.

Personally I would use a hybrid bridge instead of a SS bridge followed by the valve rectifiers, unless the current is such that one valve will do.

Are those diodes fast or soft switching? Maybe some snubber caps across them would be worth doing.

[James]

Maybe think about a smallish value resistor in each rectifier valve leg to help balance the currents. This will help prevent one valve hogging all the current and flashing over.

J.

[pre65]

Hi Nick-a hybrid bridge ?

What a brilliant idea ! So neat and solves the problem James predicted.

See Version 2 diagram.

[pre65]

Hi-so with the hybrid bridge,could you also use a FW valve rectifier in place of two diodes ?

In a FW valve rectifier (ie GZ34) does the max voltage rating apply to each anode or both anodes combined ?

For example,if rated at 1,000V is that 500V + 500V or a max of 1,000V on each side ? (don't know if i am explaining myself properly here !)

[Nick]

Per plate, but you need to check the current ratings as well, but than depends on what you intend this supply to power.

[pre65]

Hi-thinking again about 805 amps is there any reason why 805 in A2 (as per the schematic shown) should be problematic ?

I had envisaged replacing the 6SJ7 (in triode mode) with 1/2 6SN7 (thanks Andrew I) or its single version 6J5 and using a trioded KT66 as the driver.

The "magic eye" part of the circuit will not be used and i think this will enable me to use the output transformers,the KT66 and most of the power supply i already have .

[Nick]

Hi-thinking again about 805 amps is there any reason why 805 in A2 (as per the schematic shown) should be problematic ?

No, other than you are starting with a circuit that is unproved, you don't know some of the important features, and you are suggesting changing it.

The driver (kt66 should be ok here) cathode follower will have its idle current set (just as Paul suggested) by the DC resistance of the choke, so you will need a choke with whatever resistance gives you whatever current you want to run that stage at.

What current is your output transforers gapped for? The DC across the choke and the DC generated by the 805 cathode circuit will govern the current in the output stage.

I don't know what swing the 805 will need on its grid, but you might not have enough gain with a single 6sn7 stage.

I would suggest building this on a breadboard first, this is not going to be a build plug in and listen amp.

In effect there is no circuit there that you would directly copy, you would be starting from scratch (well I would anyway).