JTM45 Build

[martin manning]

You might need to experiment a bit, but half of the amp rating of the CT fuse would be a good place to start. Each half of the secondary carries half of the current coming in at the CT.

[Littlewyan]

The other thing people suggest is using silcon diodes as protection. The issue I have with this is the diodes will keep the amp going if the valve rectifier shorts, so unless you notice the sudden tone change you wont know. Plus all the voltages in the amp will suddenly rise.

[Reeltarded]

I have news.

You would not notice the tone change.

[Littlewyan]

In that case I'll just stick with the fuses. They should protect the PT enough if the Valve Rectifier short circuits. I may end up using the standard Centre Tap fuse just to keep it original, not sure yet.

I was thinking earlier, Ken F never used B+ fuses in his Trainwreck Amps. I thought the reason may have been that his Power Transformers were always way overrated so he just thought they could easily handle fault current long enough for the mains fuse to blow.

Getting the faceplates sorted out with Michael at Modulus Amps at the moment.

[Roe]

the HT fuse is normally 0.5A slo-blo on these

[pdf64]

Get rid of that awful standby arrangement and quick blow B+ fusing can be used; plus the likelihood of the rectifier blowing is probably greatly reduced.

[Littlewyan]

I want to keep the amp looking like the originals, which I've never done before in an amp build! Hence why I may keep the centre tap fuse.

I believe two common issues with using a standby switch in an amp with a valve rectifier are:

1. Have the reservoir capacitor on the cold side of the standby switch (think thats right) so the capacitor doesn't get charged until the standby switch is on. So always have the reservoir capacitor on the hot side.

2. Turning the standby switch off with the mains power still on. This can cause a nasty flyback voltage from the mains transformer winding which can fry your rectifier valve. To try and prevent this you can either A) always turn the amp off using the mains switch or B) Install silicon diodes in series with the rectifier anodes to reduce the reverse voltage. When I'm not using my amps I just turn off the mains switch, even during set breaks as I'm pretty confident my amps will turn back on again ok . TOUCHES WOOD TOUCHES WOOD!

One good thing about this switch in Marshall amps is it can stop arcing from occurring in the Cathode Follower.

Just read the Marshall Handbook and they recommend that you always switch off the standby switch first when powering down an amp. Why on earth would you do that :S. I cannot think of a good reason for doing this. Turning off the Mains Switch first means the capacitors throughout the amp are drained and you don't risk getting a flyback voltage off the mains transformer. They also tell you to use the standby switch when taking a break from playing the amp. Again, bad news. Does make you wonder when Amp manufacturers give out bad instructions like this.

[Reeltarded]

I leave all mine in but leave them on all the time..

Except for when I need a switch to defeat NFB..

[billyoung]

When a tube rectifier "starts up" the filament takes a little time to heat so if it is connected to cap (stand by on ) would be a bit gentler on the tube. Makes you wonder about the stand by switch ?

[lovetone]

My take on this is build it as original why try to change whats worked for more than 40 years.

My 1966 amp has only blown 1 rectifier since 1973 Also fuse tech has moved on and the fuse's respond much quicker. The 2A mains fuse used to be anti surge with a spring like coil. Use a T2AL you should have no problems.

Also i've been repairing these amplifiers since the late 70's and never replaced a mains transformer.

Geoff

www.voltamps.co.uk

[Littlewyan]

Geoff you're right. I'm going to build it as an original.

One thing I found when looking through Marshall schematics, it seems they only fuse the secondary windings on their top of the range JVM amps. Interesting they don't do it on the TSL or DSl series! As I'm building an old amp I want to keep it looking original, but if I was building a new amp design then I would definitely add in a lot more protection.

[lovetone]

On the JTM45 The fuse on the earth to the centre tap is effectively the same, I use this method if i'm using a centre tap supply otherwise the HT from the rectifier.

I never fuse the heaters thats just looking for trouble, the fuse holders nearly always overheat. The first JVM 100 watt amps had them soldered to the PCB I had one in for repair 3 months old and the heater fuse's had got that hot they where unsoldered from the board.

Build the JTM45 as original I don't think you would have any problems.

[Littlewyan]

They had heater fuses soldered to a PCB and didn't think that would be an issue? Makes you wonder! Its also difficult to fuse heaters effectively as when the valve heater first turns on its effectively a short circuit so will drag a lot of current through the fuse, so you have to use a big slo blo, but then by the time you've made the fuse big enough to cope it ends up being pointless as it wont blow in time to save your PT anyway.

[pdf64]

I think RG Keen advises automotive type blade fuses for the heaters.

[Roe]

some people add a 100+k power resistor across the standby switch and install diodes on the rectifier tube socet in order to help with gz34 tube life. New gz34s don't last as long as the old mullards did, esp. if you increase the mains filtering from 32uf to 47uf or 64uf