pentode to triode

[space hopper]

Hi all

I am thinking of modifying the input of my KAT6550 from pentode to triode. The pre I am using has to much gain. It worked pretty well with the quad 2’s I used with it before but now with my 6550 the pre’s volume control is severely restricted. I have tried putting a potential divider on the input of the 6550 but it sounds a little ‘’muffled’’ to my ears. I have found a section in the Morgan Jones Valve Amplifier book which recommends the above mod.

Based on the steps outlined by Morgan I intend to do the following:

1) disconnect C4/5 and R6/7

2) connect grid 2 to the anode

3) change the feedback components (divide the resistance and multiply the capacitance by 4)

Morgan states that the cathode bias resistor may need to be recalculated?

Any thoughts on this would be much appreciated

[Richard]

Hi Spacehopper,

Not sure my circuit uses the same part numbers you refer to.

This looks fairly simple but is really quite a redesign. I don't know if the pentode section will make a very good triode. See if you can find the curves and go from there.

You could triode strap it but you'd need to set it up and check it on a scope for stability and bandwidth and set feedback accordingly.

The cathode resistor is the feedback resistor in this amp which may work out for you or not when it's triode strapped.

I'd look at using an ecc double triode instead if you really need go this route.

It may be worth having a look at your Pre output spec and the divider you were using. It shouldn't be muffled if you get the right values in the divider. Which Pre is it?

Rich

[space hopper]

thanks richard

it's a trilogy 901

not sure of the specs but it looks like a an ecc88 cascode arrangement, it drove the quads no problem. maybe fixing the output of the pre may be the way to go.

[Richard]

I don't know that pre but rather than put a divider between or change the output of the pre you can easily change the input of your Kat to be similar to the Quads (1.5M and 1.2v sens);



'Scuse the drawing , it's of a Kit input so the component numbers may be different, but all you need do is change the R7/8? gridleak from what will be 100k on yours to 330k.

Then add a 680k series R between the input cable and junction of the gridleak and gridstopper as shown. Put it after the input cable not on the phono socket end to stop any HF losses due to cable cap. (I'd also suggest those small metal film Rs from Maplin as being cleaner sounding in the signal path than carbon.)

This will give a higher 1M input impedance and lower the sensitivity to 1.5v.

Rich

[space hopper]

thanks for taking the time richard it's appreciated

i'll give that a go

[Baggy Trousers]

This was just what I was looking for when I was inside the box trioding the 6550s (and then untrioded them as I preferred the original sound ). A simple alteration to the input and 1,5V should be just about right. Good stuff and what makes this BB indispensable to duffers like me.

Thanks from me too.

Richard.

[Richard]

You're welcome, let us know how it goes .

If it's simply a case of lowering the sensitivity to 1.5v and you wish to keep 100k input impedance use 68k instead of the 680k and 33k instead of the 330k.

(I'd keep to 100k input impedance for use with an active preamp with low output impedance such as PreII or a solid state pre. For a valve pre or a passive go to 1M as per the drawing.)

Rich

[space hopper]

hi richard

i ended up with the 33k 68k option and now have much more volume control

thanks once again