Cornford Hurricane cathode resistor

[retgaz]

Hi
I've got a Cornford Hurricane in for repair with burnt out cathode resistor and power transformer. I know that Cornford schematics are rarer than hens teeth but I was wondering if anybody could tell me what value the resistor used to be. I suspect it may have been 220ohm from whats left of the colour code (now different shades of brown) but can't be sure.
I'd be gratefull if anybody has any other info on this amp (a voltage chart would be ideal) that I can use to check things as I put the amp back together. Cornford have supplied me with a replacement transformer but can't tell me the value of the resistor, supply me with a schematic or any other info.
Thanks

[Enzo]

Hey, 220 ohms is a place to start at least. What power tubes are used? Look for similar amplifiers and see what they used.

[retgaz]

Hi Enzo
Thanks for the reply. The power tubes are a pair of EL84s. The resistor is in parrallel with a 100uF capacitor. I'll try your idea of checking other amps. Thanks

[Enzo]

220 sounds quite reasonable then.

[Chuck H]

Vp on that amp is supposed to be about 380V to 390V. 220r would seem reasonable, but... All the reports I've read indicate that the value is lower. As in NOT reasonable. I've read 100r (tube eater) and 120r to 150r (slightly better). IMHE with most modern EL84's in "my" design (355Vp) 130r for a hot bias = very warm, soft attack, short tube life (about 150hrs). 180r = longer tube life and poor overdrive (excess crossover distortion). I use 130r and am glad the tubes aren't too expensive. At 390Vp I probably wouldn't go under 160r unless the tone suffered.

HTH

[pdf64]

I think 220 ohms is at the upper end of normal for a pair of el84s, 100 ohms being the minimum value; all depending on the plate voltage of course.
However, rather than a shared resistor, I think that a cathode resistor for each tube can be very beneficial, in that if one tube fails to conduct normally (eg screen grid resistor blows or the tube just wears out) the other tube can still work normally.
With a shared resistor, the remaining operational tube would run super hot, with the potential to damage the output and power transformers.
With independant cathode resistors, each would require bypassing, with a reasonable rule of thumb being to double the normal shared resistor / cap values (or quadruple the value in the case of 4 x el84).
Pete

[Zouto]

Hi, I found some useful (spanish) information & gut shots of the hurricane
Ver Tema - Tutorial: Reducción de potencia para amplis cathode biased | Guitarrista
so 220 Ohms seem to be Cornford's standard value.
Zouto

[retgaz]

Thanks for all the help. There's a lot of good info there.
I've been gathering info as I've been working on this.
The early Hurricane had a TECM 323C power transformer that gave 30v more on the secondaries than the later TECM 323E. The later models had a 150R cathode resistor, so 220R seems reasonable for the early model.
Thought this info may be usefull to somebody in the future.
Cheers

[Demonoid]

Hi
I've got a Cornford Hurricane in for repair with burnt out cathode resistor and power transformer. I know that Cornford schematics are rarer than hens teeth but I was wondering if anybody could tell me what value the resistor used to be. I suspect it may have been 220ohm from whats left of the colour code (now different shades of brown) but can't be sure.
I'd be gratefull if anybody has any other info on this amp (a voltage chart would be ideal) that I can use to check things as I put the amp back together. Cornford have supplied me with a replacement transformer but can't tell me the value of the resistor, supply me with a schematic or any other info.
Thanks

I know that Cornford schematics are rarer than hens teeth