Fender Twin Reverb 65 RI keeps blowing 8 amp filament fuse

[LukeB]

Twin Amp

Hi Tom,
Here are some pics of the replacement 30A fuse holder, etc.
You can see the size difference with the original 5A holder. There are also corresponding size differences with the area of contact with the fuse within.

This may not make any difference to the operation of the fuse circuit, but I feel more comfortable having made the mod considering the issues being discussed.

Cheers, Luke

[olddawg]

I would crowbar it as has been mentioned before. If the transformer fails, it was bad.

[LukeB]

Hi All,
I've now had some time to evaluate the effectiveness of my filament coil soft start circuit and also heat sinking the power transformer.

The Soft Start circuit comprises two SL32 0R530 Thermistors by Ametherm (purchased through RS Components) wired in parallel and in series with the filament fuse.

These limit the inrush current for about 1.5 seconds from a cold turn-on protecting the fuse. The current flow through the thermisters heats them up and their forward resistance drops to about 0.1 ohms when the temperature stabilises after about 10 seconds.

The circuit works with a single thermister as well, but its is much hotter and although still within the components specification, by using two devices in parallel, they run much cooler. Cool enough to touch with out getting a burn.

I have operated the amp for many hours, through enough on off cycles to feel confident that I have solved the fuse blowing problem in my unit.
I have used the amp on three gigs without any issues.

A second addition I made to the amp was to attach two small heatsinks to help the power transformer run a bit cooler.
The transformer used to heat up enough after about two hours on standby, so that it was not comfortable to touch, but now with the addition of the heatsinks, it stays much cooler. After 4 hours of operation, it was still cool enough to hold comfortably.

I now feel confident that the amp is reliable. Something that I haven't been since the day I bought it.


I have attached two photos showing the mods for those that are interested.

Cheers,
Luke

[Pdavis68]

The transformer used to heat up enough after about two hours on standby...

My understanding is that leaving your amp on standby for extended periods of time can lead to cathode poisoning. Merlin discusses it in the "Standby Switches and Folklore" section here: The Valve Wizard

[LukeB]

My understanding is that leaving your amp on standby for extended periods of time can lead to cathode poisoning. Merlin discusses it in the "Standby Switches and Folklore" section here: The Valve Wizard

Really helpful and interesting article, thanks.
I must admit that although I had heard of cathode poisoning, I hadn't really given it much consideration because I hadn't heard it mentioned in relation to valve based guitar amp circuits.

Now that I'm no longer worried about blowing the filament fuse, I'll just turn the amp off during set breaks and avoid using the standby switch altogether.

Cheers, Luke

[diydidi]

LUKE B, do you have the RS part number please?

[LukeB]

Hi,
Sorry. I thought the parts came from RS but actually the RS parts were a different power handling and not as good.
The parts I used came from Farnell (Element 14), but DigiKey is also a distributor for Ametherm.

Cheers, Luke

[LukeB]

Fender Twin Amp Slow Start Circuit

Hi,
Sorry. I thought the parts came from RS but actually the RS parts were a different power handling and not as good.
The parts I used came from Farnell (Element 14), but DigiKey is also a distributor for Ametherm.

Cheers, Luke

Hi All those interested.
I have now had several months to evaluate the effectiveness of my modifications to my Twin Amp to reduce the filament inrush current on cold start.

I have now used the amp for many hours and done about 10, 3 hour gigs since I installed the mod and not a single blown fuse in all that time.

Also the power lamp always stays fully lit with none of the fading out that happened before.

Adding the heat sinks to the side of the power transformer allows it to run significantly cooler.

My conclusion is that the mod is a total success and Fender should add something like it to all their valve amplifiers.

It is a simple mod using two low cost components and it has saved me much concern.
Before, I could never be sure that I would be able to play a gig with blowing a filament fuse.

If anyone is having similar issues I would recommend you consider this mod.

WARNING. If you do not have any formal technical training, I would recommend you seek the help of a qualified technician.

Cheers,
Luke