Buenas noches amigos del porno valvular,
Y despues de este homenaje a Luis Buñuel ................ no se si os suena el nombre de Santiago Alvarez, es el ingeniero que trabajando en Marshall fue pieza clave en los diseños de entre otros los modelos JVM410H, AFD100, YJM100. 2203KK; pues resulta que buscando informacion acerca del SV20H me he encontrado un post en el Marshallforum donde el mismo Santiaglo Alvarez asegura que estuvo involucrado en el diseño la serie Studio, que los tres modelos comparten el diseño de la etapa de potencia y que dado que operan con una tension de placas cercana a los 300vdc (280vdc como he descubierto luego para ser mas exacto) es seguro poder intercambiar las valvulas de potencia por otras valvulas como 6L6/5881 en concreto el comentario es:
I did design the 20W amp that was used in the mini-Jubilee, the original Origins and some other amps that did not see the light. That design DOES work with any octal tube, even 6V6. I don't know what circuit is in the Studio series but I'd be very surprised if it isn't just the same as the mini-Jubilee with the preamp modified accordingly. In fact those amps were already planned when I was still in Marshall and that's what was being done, just a preamp change (which is actually the same if you think about the 100W amps, same power amp, different pre).
Regardless of circuit changes, the amps will work with the other tubes. Actually the EL34 is harder on the transformers than tetrodes like the 6L6, 5881 and so on. The KT88 and 6550 draw a bit more of heater current but that was taken into account when specifying the power transformer.
In Marshall's defense it is normal that if the amps come with EL34 they told you to only use those but that doesn't mean it won't work or will be dangerous to use other tubes. I'm sure if you ask if you can use a 12AT7 in the preamp they'll tell you it is designed for 12AX7 too.
Y mas tarde:
Hope it is clearer, also, with "any tube" I mean any octal tube with pinout similar to the EL34 and a minimum anode voltage of at least 280V also know as 6L6, 6V6, 5881, KT66, KT77, KT88, KT90, EL34, 6CA7, etc. etc. just in case someone plugs in there an octal preamp pentode or something and it doesn't work but I guess this was pretty much clear...
El enlace al tema del Marshalforum es este, alli Santiago Alvarez es el usuario Santiall, el comentario que cito esta en la pagina 2:
https://www.marshallforum.com/threads/sc20-sv20-power-tubes-what-is-the-deal-exactly.107083/
De otro hilo en otro foro consegui verificar que la tension de placas es de 280vdc y Rk (bias resistor) de 150 Ohm compartida.
http://marshallroadhouse.com/viewtopic.php?t=21968&start=30
Teniendo en cuenta que es un diseño de etapa de potencia con dos EL34s en push pull de clase AB1 a 280vdc y Rk de 150 Ohm, estáis de acuerdo en que se podría utilizar cualquier valvula de base "octal" de las que comenta Santiago Alvarez?
Os animo a que expongáis vuestras impresiones.
Y despues de este homenaje a Luis Buñuel ................ no se si os suena el nombre de Santiago Alvarez, es el ingeniero que trabajando en Marshall fue pieza clave en los diseños de entre otros los modelos JVM410H, AFD100, YJM100. 2203KK; pues resulta que buscando informacion acerca del SV20H me he encontrado un post en el Marshallforum donde el mismo Santiaglo Alvarez asegura que estuvo involucrado en el diseño la serie Studio, que los tres modelos comparten el diseño de la etapa de potencia y que dado que operan con una tension de placas cercana a los 300vdc (280vdc como he descubierto luego para ser mas exacto) es seguro poder intercambiar las valvulas de potencia por otras valvulas como 6L6/5881 en concreto el comentario es:
I did design the 20W amp that was used in the mini-Jubilee, the original Origins and some other amps that did not see the light. That design DOES work with any octal tube, even 6V6. I don't know what circuit is in the Studio series but I'd be very surprised if it isn't just the same as the mini-Jubilee with the preamp modified accordingly. In fact those amps were already planned when I was still in Marshall and that's what was being done, just a preamp change (which is actually the same if you think about the 100W amps, same power amp, different pre).
Regardless of circuit changes, the amps will work with the other tubes. Actually the EL34 is harder on the transformers than tetrodes like the 6L6, 5881 and so on. The KT88 and 6550 draw a bit more of heater current but that was taken into account when specifying the power transformer.
In Marshall's defense it is normal that if the amps come with EL34 they told you to only use those but that doesn't mean it won't work or will be dangerous to use other tubes. I'm sure if you ask if you can use a 12AT7 in the preamp they'll tell you it is designed for 12AX7 too.
Y mas tarde:
Hope it is clearer, also, with "any tube" I mean any octal tube with pinout similar to the EL34 and a minimum anode voltage of at least 280V also know as 6L6, 6V6, 5881, KT66, KT77, KT88, KT90, EL34, 6CA7, etc. etc. just in case someone plugs in there an octal preamp pentode or something and it doesn't work but I guess this was pretty much clear...
El enlace al tema del Marshalforum es este, alli Santiago Alvarez es el usuario Santiall, el comentario que cito esta en la pagina 2:
https://www.marshallforum.com/threads/sc20-sv20-power-tubes-what-is-the-deal-exactly.107083/
De otro hilo en otro foro consegui verificar que la tension de placas es de 280vdc y Rk (bias resistor) de 150 Ohm compartida.
http://marshallroadhouse.com/viewtopic.php?t=21968&start=30
Teniendo en cuenta que es un diseño de etapa de potencia con dos EL34s en push pull de clase AB1 a 280vdc y Rk de 150 Ohm, estáis de acuerdo en que se podría utilizar cualquier valvula de base "octal" de las que comenta Santiago Alvarez?
Os animo a que expongáis vuestras impresiones.