Cornford Roadhouse 30 resistor mod help EL34 to 6L6

Fortheloveofguitar

Hey all 

Does anyone know the basic resistor mod so you can use 6L6 valves in an original EL34 model

There were issues with earlier EL34 models with the valves running too hot and causing issues. Mine is an original and I prefer 6L6 anyway 

i have the schematics and I think it’s resistor R7 that needs changing but there’s no rating on the diagram and obviously need to know what I need to change it to

All of this info was on the Cornford forum but as it’s now shut its becoming extremely difficult to obtain tech advice. The Cornford users FB page members aren’t very technical minded either 

Many thanks 

ecc83

I don't know the amplifier but it should be fairly easy to look up the 6L6 data and find the cathode resistor (I assume it is not FB?) and, to a lesser extent, the G2 resistors needed?

You will need to know the HT voltage. Note, data sheets usually give a pretty "hot" bias for lowest distortion so err on the high side.

And don't be mean! Fit a 10 watt R so it never burns out if a valve borks.

Dave.

Collings

There are two versions of the roadhouse. The 30 is cathode biased and the 50 fixed bias so I would assume this makes a difference to what is required regards circuit/component change.

Fortheloveofguitar

Collings said:

There are two versions of the roadhouse. The 30 is cathode biased and the 50 fixed bias so I would assume this makes a difference to what is required regards circuit/component change.

Mine is the 30 cathode bias 

ecc83

Fortheloveofguitar said:

Collings said:

There are two versions of the roadhouse. The 30 is cathode biased and the 50 fixed bias so I would assume this makes a difference to what is required regards circuit/component change.

Mine is the 30 cathode bias 

,Ok, for  Va= 360V you need about a 250 Ohm cathode resistor. A 5W component would do but loke I say, go nuts.

If they are not already I would fit 1k 1W metal film , one per valve G2.  If you cannot find 250R easily, 270 is a common value and you can allways shunt it if you think the amp sounds a bit "cold" (it won't)

For 360Va the book says about 24W but that at 4%THD (IIRC)  but 4% is bloody HI FI in the gitamp world! Still be pretty clean at 30W betcha betha.

Dave.

John_A

I really wouldn’t bother, it will sound very little different with 6l6s. 

Fortheloveofguitar

John_A said:

I really wouldn’t bother, it will sound very little different with 6l6s. 

Not sure about that. My old MK50ii was much better with 6L6 and my Carvin legacy is much better with 6L6 than stock EL34

ICBM

John_A said:

I really wouldn’t bother, it will sound very little different with 6l6s. 

Yes, but you can kill two birds with one stone because 6L6s are rated for a higher dissipation than EL34s (30W vs 25W) so you can run them proportionately cooler.

The schematic shows 180R 7W for the 30W cathode-bias model with EL34s. On that basis, assuming they were running the 34s pretty hot at 25W at idle, with a B+ of around 420V (the PT secondary is listed as 300VAC), that gives roughly 60mA per valve, 120mA total, so a cathode voltage of around 22V. I would be looking for around double that voltage with 6L6s to get the same sort of dissipation, so I would ideally go for 360R 10W, possibly as high as 390R, and probably not lower than 330R. You may have to experiment slightly though since if the current draw is different it may affect the B+ voltage.

If you can't get a 360 easily then you can just use an extra 180R in series with the existing one, if there's room to fit two.