Keep Blowing HT Fuse

[Jack M]

Hey Guys,

Any idea's why the HT fuse keeps popping?

The amp seems fine except for some faint ghosting around the 12th fret but.. once you get the volume
above 7 the HT fuse blows! Do these amps suffer from slight ghosting at volumes say around 5 or 6?

The amp has BC 32uF's on the cap board and a BC 32uF in the preamp, it also has Valve Art KT66's
in it and it is biased at 34ma 497V if I remember correctly.

I've swapped out the KT66's with another brand new quad and I have set the bias to 30ma and gone back
up as high as 38ma with the same results!

Any one experience this before with the 45/100?

Best - Jack

[Flames1950]

It's pretty common for the amps to ghost -- it is a 100-watt amp with only 32uF of filtering on the mains and only 16uF on the screens, after all. After you get used to it it works pretty well with the overall sound even.

Is your HT fuse a fast or slow blow? With fast blows I used to pop a fuse every couple of days, but no problems with slow blows. (The fast blows are nice for initial testing though, while you make sure your tubes aren't going to smoke anything.)

[Jack M]

Hey Flames,

Thanks for the quick reply!

I have a 1 amp fast blow fuse in the HT holder I didnt think it would be safe to use a slow blow for the HT!

The ghosting it isnt that bad really, I was just curious to see if this was a related to the HT problem.

I guess I could go with a mix of 32Uf's and 47Uf's on the cap board to clear up some of the ghosting?

What are your thoughts?

Best - Jack

[Flames1950]

You can sub the 47uF's for the 32uF's. I have never done it personally -- I hate to mess with the soft feel of the stock values.

The startup current of the amp can wear those fast blow fuses out pretty quick. Maybe not a big deal on an amp with a fuseholder on the back for the HT, but when I gotta yank the chassis everytime, it's a pain in the tush. Almost every vintage amp out there specifies slow-blows anyway, but George likes the faster protection of the fast blows. And again, the fast blows are nice at first to be sure those tubes you just got shipped to your door aren't shorted out!

[Jack M]

Hey Flames,

Right on!

I have to get some 1 amp SB fuses and give it a try, I will let you know how it turns out!

Thanks again for the heads up and your time!

Best - Jack