Super Twin Reverb blowing fuse

[nickb]

Second: why they bothered to design in this huge cap is beyond understanding. Any "extra bass" is incremental, iow not such a difference that anyone could hear it. The old standard 25 (or 22) uF cap is plenty. If you opt for overkill cap value here, 100 uF is way more than enough.

A big cap on the first stage reduces hum coupled from the heaters to the cathode. That could be what they had in mind when they selected that value.

[nickb]

With the limiter removed, it will start ok a few times, then the fuse will blow. On the limiter I cannot see weird behavior of the bulb even after 20 starts. Except some times when it takes a bit more longer to dim (less than 3sec.).

When exactly does the fuse blow? Is it in operating the power switch ( amp in standby). When coming out of standby? Does the time between switch closures have any impact on the problem?

[Leo_Gnardo]

A big cap on the first stage reduces hum coupled from the heaters to the cathode. That could be what they had in mind when they selected that value.

I've experimented with various bypass caps, with hum reduction part of the reason. In a typical Fender preamp (1k5 cathode R, 100K plate) I found that values over 5 uF added nothing to hum rejection. However hum could be reduced by the usual methods: 1) Dress grid wires away from filament wires. B) Select pre tube that doesn't internally pick up hum from its filament. III) Humdinger pot. D) float filaments on DC, or just an uncharged cap like some 60's Ampegs. 5) Modify amp for DC filaments in sensitive preamp stages.

Of course YMMV, so do whatever works for you and your amp.

Way big cathode bypasses = the big hammer approach. Hey if it works for you, no complaint from me.

[yann]

When exactly does the fuse blow? Is it in operating the power switch ( amp in standby). When coming out of standby? Does the time between switch closures have any impact on the problem?

The fuse blows right at operating the power switch, amp in standby.
Whatever how long I wait between tests, after some tries (less than 5) the fuse blows. It's an instant blow, I mean there is no sound, not even the power bulb indicator coming to life.

[Tom Phillips]

What is the timing of your On - OFF - ON - etc. switching?

[yann]

Power on. Let the amp warm for 3 minutes. Power down.
I wait one hour.
Then same operation.

[nickb]

The fuse blows right at operating the power switch, amp in standby.
Whatever how long I wait between tests, after some tries (less than 5) the fuse blows. It's an instant blow, I mean there is no sound, not even the power bulb indicator coming to life.

OK, disconnect both of the red transformer secondary wires and try again.

[Tom Phillips]

Power on. Let the amp warm for 3 minutes. Power down.
I wait one hour.
Then same operation.

OK. That should be no problem for a properly working amp.
And just to be clear, since I don't see it mentioned in this thread, you have an export model Fender Super Twin and are powering it with 230VAC directly from the wall plug. Is that correct?

[yann]

OK. That should be no problem for a properly working amp.
And just to be clear, since I don't see it mentioned in this thread, you have an export model Fender Super Twin and are powering it with 230VAC directly from the wall plug. Is that correct?

Yes, correct.

[yann]

With the new fuse it seems to survive to over 10 power cycles.
I think I am done with this amp now

[Pedro Vecino]

I did not doubt that the type of fuse was related but under these circumstances it might be advisable to install a CL-60 thermistor in series with the current input.

[yann]

I did not doubt that the type of fuse was related but under these circumstances it might be advisable to install a CL-60 thermistor in series with the current input.

Yes it would probably be worth to do it but the fact is that I don't have one in stock and the next order is not planned for now. Also, the stock fuse never blown in the last 40 years until the guy put this EL34 instead of 6L6GC by mistake.
So I hope the right fuse will do its job as expected