Marshall Amp Forum - View Single Post

stargazer76 · 06-19-2009, 02:25 PM

Very interesting thread here! I also have seen Liam's post outlining all that must be done to convert a 900 from 5881/6L6's to EL-34,s. Most say that both tubes are interchangable, as long as you can bias them properly. This may require the replacement of one or three resistors. I had to change R28 to 33k to achieve this.
Now, Bob at EuroTubes told me that all that extra stuff is not necessary to conversion. Also, where is the schematic evidence for replacing all of these components? R26-R27 from 150k to 220k? Perhaps. R28? Probably to a lower value. You want the bias dc voltage at pin 5 of your output tubes to be about -30vdc to -45vdc, tubes NOT installed. My 900-4100 is set at -34.8vdc, and the bias current is 36.6mA with a plate voltage (EL-34,s) of 488vdc. I only replaced the three resistors.
But, is there something to Liam's call for the replacement of these many components? Something we may be missing?

06-19-2009, 02:25 PM	#10 (permalink)
stargazer76 Member Join Date: Mar 2008 Location: Dallas, Texas Posts: 42	Re: JCM 900 6l6 to EL34 Conversion problems Very interesting thread here! I also have seen Liam's post outlining all that must be done to convert a 900 from 5881/6L6's to EL-34,s. Most say that both tubes are interchangable, as long as you can bias them properly. This may require the replacement of one or three resistors. I had to change R28 to 33k to achieve this. Now, Bob at EuroTubes told me that all that extra stuff is not necessary to conversion. Also, where is the schematic evidence for replacing all of these components? R26-R27 from 150k to 220k? Perhaps. R28? Probably to a lower value. You want the bias dc voltage at pin 5 of your output tubes to be about -30vdc to -45vdc, tubes NOT installed. My 900-4100 is set at -34.8vdc, and the bias current is 36.6mA with a plate voltage (EL-34,s) of 488vdc. I only replaced the three resistors. But, is there something to Liam's call for the replacement of these many components? Something we may be missing? __________________ JCM 900-4100 JCM 800-2204 1960A/G12T75 1962RI Stratocaster