Fast Acting or Slow Blow on B+ in Crate GT500H/VC50?

[stoneattic]

So while you guys have been helping me with a Peavey CS800S in the "Other Amps" forum I picked up another project, a Crate GT500H. The GT500H is supposed to be the same as the VC50 (Vintage Club), which seems to have a pretty big following. The inside of mine appears to match the pics I've seen posted of the VC50 so I believe that's the case.

The on/off switch, standby switches and tube heater light, but no sound. So I did the pre-amp out to another amp's power amp and vice versa. Nothing. So I pulled the chassis and found that F2 fuse is open. I pulled the tubes and was setting up my current limiting bulb and found that I didn't have any 1/2A fuses, which was the fuse that was in there. Here's my quandary:

The PCB is silk screened "F1/2A, 250V"
The original (blown) fuse is a 1/2, 250V 312 and appears to be a fast acting
The VC50 schematic shows 1/2A SLO-BLO

A few other bits of info I notices:

The PCB is silk screened "2.5A, 250V" and has a SLOW BLOW paper sticker on the PCB
The VC50 schematic shows 2A SLO-BLO

Rev 7 on VC50 schematic states "CHG FUSES PER E960366"

The GT500H designation was before the aesthetic change to the VC series, so is older.

So I'm thinking that they changed the fuse call outs over time either because there were issues with the original fuse ratings or due to circuit changes as listed in the schematic revisions. I assume the correct thing to do would be to put a 1/2A, 250V fast acting in there, but was hoping to get some feedback from those who know WAY more than me.

As always, any thoughts, suggestion, etc., are appreciated.

Thanks!

[g1]

You didn't mention if the blow fuse was fast or slow?
Being that F2 is the HT fuse and I've never seen a fast blow used for HT, I'd put a slow-blow in there. Others may think differently.

[stoneattic]

The blown fuse appears to be fast acting. I only see a little bit of thin wire in there, not the coil I'm used to seeing in a slo-blo.
In my limited experience, slo-blo seems more common as well, which is why I'm asking those with much more.

[g1]

Are there any numbers/letters on the old fuse to designate type?
Some slow-blows have a thin wire with a 'blob' in the middle.

[stoneattic]

The only things I can see are:

1/2A 250V 312 around the metal on one end
nothing wrapped around the metal on the other end
on one end what looks like an LF as kind of a logo, I'm guessing LittelFuse, and Mexico
on the other end the same logo and the UL and CE symbols

I believe the 312 is a Littelfuse designation for a fast acting:
http://www.littelfuse.com/products/f...fuses/312.aspx

[nevetslab]

I agree with g1....use a 1/2 Amp Slo Blo Littelfuse 313 type or equiv. Slo Blo / Time Delay fuses are typical in HT fuse circuits.

[stoneattic]

So of course I didn't have amp 1/2A fuses laying around and now that Radio Shack is no longer around in my area I need to order some. But I do have some Buss MDL 3/8A, 250V (slow-blow). So thinking that a 3/8 slow-blow is (kind of) half way between a 1/2A fast acting and a 1/2A slow-blow, I used my current limiting light and gave is a go.

The light acted normally, so I plugged my favorite guitar in and it sounded glorious! I was thinking this is my new main amp and all of my others were now dead to me! After about 15 minutes of playing I was thinking that the volume was not as loud as I expected and realized that I was still plugged in through the light. So I plugged directly in and played some more. All is fine, but it sounded better with the light, though there was now plenty of volume. Variac or power scaling time?

I ordered some 1/2A fast acting and 1/2A slow-blows. I know the fuse should not have gone for no reason, unless maybe it was the original fuse and Crate realized on the later revisions, as shown on the schematic, that a slow-blow is more appropriate. My plan now is to re-flow the solder joints on the PCB, especially the PCB mount tube sockets, then check the bias on the EL84s.

Thanks!

[nevetslab]

I've had a couple amps come thru blowing the 'standard' 1A TD HT fuses, with no faults found in the output stage. Changed power tubes, re-biased, still had the same issues. I hadn't checked the power tube grids coming out of Stand-by but after recently looking into the turn-on characteristics of some Fender Twin Reverbs, which always have a noticeable thump of pop switching out of Standby, I've found the negative driver tube's plate output produces a large transient, and it drives the neg side of the output tubes into full conduction briefly. That sort of condition may well be the cause of HT fuses blowing. On these amps (not Fenders), the HT fuse would blow switching out of S/B. For the time being, on those amps, I had switched to T2A (time delay 2A 5 x 20mm GMC size fuses).

[g1]

A big sag resistor in the B+ line might get you some of that 'bulb limiter' compression effect.