tube effects loop on board

[2deaf]

I know this is an old thread but I'd like to use the tubenit's illustration in post 27 (that appears to be Merlin Blencoe's) The 1 meg grid resistor and the 15k  grid leak seem to be odd values. Almost seem to be in reverse. I looked at the real world photo example he posted and it appears it's the same as the layout. Why a 1m resistor at the input? Nice compact and handy layout though!

The 1M/15K voltage divider is there to attenuate the "in" signal down to line level.  I don't know what the "in" signal is, but it's going to take a divider like that if it's real hot and you don't want to overdrive the Send half of the ECC83.

[swingarm]

Thanks 2deaf, that makes sense! In fact now that you mention it, Merlin points this out in his book under sec 12.9 when he says " we might want to add a potential divider immediately before the driver triode to ensure the signal is not clipped, thereby preserving the amp's existing tone."

[tubenit]

here is an FX loop for Marshall type amps.   From this thread :  JCM800 Build (el34world.com)


With respect, Tubenit

[catalin gramada]

Be aware. Any cf. buffer develop dangerous voltage at startup whilst cap start charging. It needs to limit this voltage to a value not more than you ss device conected is supplied to protect the effect device or whatever ss line out device is used.Two zenners at 15v back to back will do the job. Merlin pointed that in his book as well. Otherwise design you buffer as a regular gain stage and get the signal from cathode but get a higher output impedance still. It doesn't matter if zenners are fitted in amp or in jack cable but should be done to suppress dangerous voltage startup transients.Small phisical size will be preferred as usually have the least residual capacitance, which may be important as time is in parallel with next stage input.
Some pedals have allready its inputs protected, but who wants to take the risk...?

[catalin gramada]

Now, is right as should be