KiT34 + GZ34 + KT77

[Markm]

I'm looking into using this combination. This tubes would be JJ current production. The issues I forsee are:

1. Lower voltage drop across the rectifier tubes, leading to to need to recalculate the potential divider to maintain the 375V to the ECF80.

2. Change to cathode bias resistor value for the KT77.

Is it OK to feed the KT77 with the extra voltage provided by the GZ34?

Is there something else I'm missing or things I should be changing at the same time?


Mark.

[Richard]

Hi Mark,

Looks fine to me. I personally wouldn't worry about the extra few volts to the ECF80s. The KT77s will cope easily but you will probably need to change the output valve cathode resistors to 470R (if they are the circuit 390R at present) and check plate dissipation.

Whether it will be an improvement though may be another matter. I much prefer old style winged C Svet 5U4s in my 5-20 amps compared with genuine Mullard GZ34s which just sit here in the spares box

Rich

[Markm]

Looking at my notes I think that the cathode resistor is currently 470R.

At the moment the the 5U4s are Edicrons, I don't know how they rate, but I thought that the KT77s might appreciate a stiffer supply. I'll put the Svets on my list for next time.

If the ECF80 can take the extra voltage then this is going to be much less complicated than I feared.

Thanks for your input.

[Richard]

Both sections of ECF80 have max plate voltages of 250V and are run at nominally 125V in this amp so a rise of perhaps 10V won't cause trouble.

The circuit I have says 390R but I seem to remember the output cathode resistors being changed. KTs (66, 77 and 88) draw more current than EL34s though so you may need a higher value; check plate dis and use 470R or 560R.

The voltages and TX load look good for KT77 and similar to the typical UL spec on the data sheet,

http://www.mif.pg.gda.pl/homepages/f...086/k/KT77.pdf

Rich

[Markm]

I notice for the KiT34 there are no resistors between the transformer and the rectifier for plate loading. I think that the GZ34 requires around 125 ohms but I don't know the how much of this is provided by the transformer secondary. When calcuting Total input resistance per plate given by

Rs = Rsec+N(squared)Rpri+Ra,

where:

Rs = plate supply resistance per plate,
Rsec = transformer secondary resistance,
Rpri = primary resistance,
Ra = added series resistance per plate, and
N = transformer step up ratio per section.

Is the transformer information published or better measured when disconnected from the circuit?

http://www.jj-electronic.sk/pdf/GZ%2034%20S.pdf
http://www.jj-electronic.sk/pdf/KT77.pdf

[Richard]

Several instances of rectifiers flashing over at switch on have been reported. The rects are shared on these amps.

The mains TX will not be very low impedance but the value isn't known. What may be more important in this configuration is to balance the work the rect sections do.

The reading's in Radio Designers Handbook but 47R or 100R would seem sensible. The value can be varied to adjust voltage but will run hot so they need to be 3W+ wirewounds.



Rich

[Markm]

Well I should receive these tubes this week. In the spirit of balancing the work of the regulators and increase the rating I came across this. My only concern being that the current rating would be above that of the transformer. Do you think this would be useful, or safe for the transformer.

[Richard]

Hi Mark,

That would work OK and would share the work between the 2 valves. There would be no extra loading on the TX and voltage out of the valves would be the same.

I think you could even get away with just one valve (2 thermionic diodes) when fed in this way. However,

Plus points

Sharing the load between the 2 valve rects
Possibly only needing one valve rect for this load
Single 1200V ss diodes could be used instead of the seried ones shown

Neg points

There isn't a problem between valves, it is between sections within a valve
No longer purely a valve supply and switching effects may be introduced, if not forwards then maybe backwards into the TX or radiated
Complexity and cost for extra parts if you wish to minimise noise

Overall, could be a good idea from the outset to give valve smoothing without needing 2 valves but not needed and possibly a worse result implemented now.

Rich

[Markm]

Thanks for that Richard, it's tied a few loose ends together for me.

Mark.

[Markm]

I've been living with the KT77s for a while (not the GZ34s yet). They are very good but the top end is less delicate that the EL34s I had before. I would like to be able to swap between valves and easily adjust the cathode bias resistor value.

I was thinking of using a higher value resistor in parallel with a variable resistor or potentiometer of some kind.

Any suggestions of the type of variable resistor or resistor value to run in parallel to provide a useful range for el34s kt77s and 6ca7 and similar tubes?

Many thanks,

M