Changing 100K to 50K pot?

[buddam]

My KEL84 runs as a power amp, with a 50K pot in the Cathode follower pre II. Aim to make the same change on new Chinese KT88 integrated amp (from Andrew I). Wondered what the issues are before I make the replacement.

I could by-pass the KT88's volume pot entirely and run it as a power amp, but wondered whether I'd have to make the preII passive. Does the fact of a powered preII interfere with the pre-amp setion of an integrated?

Cheers

Adrian

[andrew ivimey]

Not in the least Adrian.

1. Pre2 is a lovely preamp with 600 ohm output through the transformer. It could drive almost anything.

2. So, you could take the pot off the front end of the KT88 and just put a 100k resistor across the input. (I wonder .... is that a pentode or triode input? It is a pentode valve so prob. pentode. So, I'd be more inclined to keep the input impedance high. Idle speculation .... 1meg resistor input grid to earth?)

But if you keep a pot, and I don't see why 50k or 100k could harm things, (50k will prob. mean you will get to LOUD quicker!) you will find that by using both the pot on the pre2 and the pot of the KT88 you may actually find a 'sweet spot' as you juggle the impedance of the input.

Keeping a pot in circuit will not do any harm at all.

good luck!

[buddam]

Is that 100K across the input necessary? Why not just treat it as moving the pot out of the integrated chassis and connecting it via longer cables straight to the signal input tags on the pre-amp board. There's certainly no additional resistors on my KEL84 as power amp.

Whatever, I have a thick head which gives me the opportunity to try it tomorrow

Adrian

[Richard]

Originally Posted by buddam

Is that 100K across the input necessary? Why not just treat it as moving the pot out of the integrated chassis and connecting it via longer cables straight to the signal input tags on the pre-amp board. There's certainly no additional resistors on my KEL84 as power amp.

Whatever, I have a thick head which gives me the opportunity to try it tomorrow

Adrian

Hi Adrian,

In theory you're right but in practice it may be safer to fit them. Looking at the example of Kel84, we are talking R2/3 which are 1M grid leaks. They hold the grid to ground under DC conditions and prevent rising bias which would cause runaway and destruction.

Whilst there is another DC route connected, such as a pot or another source across the input, the gridleaks are somewhat superfluous. If the pot wiper failed its DC connection though there would be no grid leak. So, fitting the highest value allowed is a sensible precaution.

With the pot removed from the amp there is a greater risk that the amp may be run without a source/pot/pre connected (say when checking voltages on the bench or a poor DC connection through an interconnect). Much better to fit the gridleaks. The max value for these is dependant upon the valve and given in the data. For ecf80 it is 500k fixed or 1M cathode biased for the pentode section. When used in a power amp it is common to use lower, such as 100k, as some preamps may prefer it.

Rich

[buddam]

Thanks Andrew and Richard. It's now working a treat with a 50K pot in the pre and one in the Sound Luster (a Panasonic replacing an industrial carbon pot). Very happy, even before any coupling cap and other mods (currently they are WIMA polyester).

Adrian