Hi
I have sourced parts for a trainwreck express build. At first i will use cheap marshall transformers from a jcm 900 ( i have these lyng in a box) if i get the amp up and running, i will upgrade the trafos. Or at least the OT.
The OT has a primary impedance of 1.7k, used for 4 el34s
i guess i could use this for an express amp, using just a pair of el34s, if i hook up an 8 ohm speaker to the 4 ohm tap.
But the express design actually calls for an OT with a 6.6 primary. Will i be ok, trying it out with the 1.7 jcm OT anyway?
Best regards
Kasper
I have sourced parts for a trainwreck express build. At first i will use cheap marshall transformers from a jcm 900 ( i have these lyng in a box) if i get the amp up and running, i will upgrade the trafos. Or at least the OT.
The OT has a primary impedance of 1.7k, used for 4 el34s
i guess i could use this for an express amp, using just a pair of el34s, if i hook up an 8 ohm speaker to the 4 ohm tap.
But the express design actually calls for an OT with a 6.6 primary. Will i be ok, trying it out with the 1.7 jcm OT anyway?
Best regards
Kasper
JCM900 OPTs are rumored to be fragile. But using the 8ohm speaker on the 4ohm tap with only 2 EL34s would make sense if is a 100wt tranny. The 6.6primary on the express may have been since it was "marketed" as being for either el34 or 6v6.
Kasper,
I did a "Junkbox Trainwreck" for local guitar god.
I used a spare JTM45 Output Tranny I had on the shelf (Raa = 3K4) with a pair of Philips 6CA7 which are the same as EL34.
Your Raa= 1K7, 100W tranny will be fine with a pair of EL34 if you do as you suggest, connect an 8 Ohm speaker to the 4 Ohm tap (or 16 Ohm Speaket to 8 Ohms tap etc.). Putting a double load on a secondary tap like this can loose a little bass, for a Trainwreck that is a good thing, too much bass makes them very muddy, hence the 2n2 coupling cap into the final preamp stage (Express preamp).
JCM900 Output Trannies were typically Raa= 2K
JCM800 Output Trannies were tpicallly Raa 1K7
I've ONLY ever had to replace one JCM900 Output Tranny. It was intermittent. When unsoldering the old one, one wiring terminal came out in 2 pieces, It was that terminal which was intermittent, the tranny was fine.
Cheers,
Ian
I did a "Junkbox Trainwreck" for local guitar god.
I used a spare JTM45 Output Tranny I had on the shelf (Raa = 3K4) with a pair of Philips 6CA7 which are the same as EL34.
Your Raa= 1K7, 100W tranny will be fine with a pair of EL34 if you do as you suggest, connect an 8 Ohm speaker to the 4 Ohm tap (or 16 Ohm Speaket to 8 Ohms tap etc.). Putting a double load on a secondary tap like this can loose a little bass, for a Trainwreck that is a good thing, too much bass makes them very muddy, hence the 2n2 coupling cap into the final preamp stage (Express preamp).
JCM900 Output Trannies were typically Raa= 2K
JCM800 Output Trannies were tpicallly Raa 1K7
I've ONLY ever had to replace one JCM900 Output Tranny. It was intermittent. When unsoldering the old one, one wiring terminal came out in 2 pieces, It was that terminal which was intermittent, the tranny was fine.
Cheers,
Ian
Part of the TW Express mojo comes from the 6.6K primary. At clipping this causes high screen current and that 25W screen dropping resistor gets real hot. The big sag in preamp B+ adds compression and touch sensitivity.
Real Marshall JCM900 transformers have poor insulation which tends to breakdown. Don't forget to install the diodes from EL34 plates to ground. They prevent the voltage spikes that will damage the OT.
Real Marshall JCM900 transformers have poor insulation which tends to breakdown. Don't forget to install the diodes from EL34 plates to ground. They prevent the voltage spikes that will damage the OT.
If the amp works and i Can manage to run it at full tilt without squealing or oscillation, then i will upgrade the OT to 6k6 primary. Any suggestions for a suitable OT available in the EU? I Can see you guys are in the States. It is soooo expensive to order parts from your part of the World, when you live here i do 😉
Best regards
Kasper
Best regards
Kasper
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