Marshall JCM600 heater fuse blowing

[g1]

V103b is the cathode follower. It's cathode voltage is fairly high with respect to the heaters. Some 12AX7's are more prone to failure in that spot. What kind do you have in that position?
Doing away with the fuse may just allow some more expensive component to burn/blow, so it is better to find the cause and correct it.
The use of DC heaters for those tubes is to keep the noise/hum down. Using AC there may be noticeable in terms of hum.
Also, it is possible this is unrelated to the previous problem. The power tube AC heaters are running off that fuse too.
And if you still have 7.7VAC on the heaters, it may be that the fuse rating is borderline.

[pontiacpete]

They are all JJ ax7s.
Power tubes are JJ el34s.
Are you suggesting that the fuse could be bumped up a bit? 7A maybe?
Or should I be thinking about lowering both heater voltages?

[g1]

Well, I don't think bumping it up to 7A slow blow from is going to allow some catastrophic failure that wouldn't happen with the 6.3A . It wouldn't be the first time I've heard of nuisance blowing in Marshall heater circuits, some of them are fuse right on the borderline with nominal line voltage.
I came across this on another forum: "I've noticed that the heater fuse on the JCM600 models has a tendency to
surge-blow at cold turn-on. I believe that these suffer from an under-value fuse. You can watch the element groan and arch it's back till it almost touches the glass and comes probably within a few milliamps of popping, and then recovers. Don't recall off hand what the stock value is (6.3A Slo-Blo w/ (2) 5881 I believe), but it should
really be 1 amp larger than it comes supplied."
With all that being said, I sure don't like the idea of 7.7V heaters. I think it will reduce tube life. A couple power resistors could drop the voltage to an acceptable level and limit the turn on surge current. But before doing it, verify that is really the heater voltage when ALL tubes are installed.

[Mick Bailey]

Fuse quality and construction for a given rating can be an issue where equipment is borderline on blowing the fuse at startup due to very close ratings. I posted elsewhere some simple trials I did with fuses - all fuses don't behave the same way and exhibit different failure characteristics. Usually this doesn't matter, but where equipment is fused so closely that it's right on the point of blowing from a cold start, those differences can mean that the fuse lasts or doesn't.

[pontiacpete]

Thanks for the help guys.
I have seen the wire in the fuse move when turned on, thought I was imagining that.
With all the tubes installed, measuring from the PT there is 7vac for the heaters. The DC voltage is running the same @ 7v.
The 7.7 was taken with one tube pulled, at that time I didn't think to just measure from the PT.
If I want to drop the voltage would I need some low ohm 10watt resistors?

[Mick Bailey]

There are a few different ways - anti-parallel diodes, series capacitor in the AC line, or a resistor. 3W is usually OK for a resistor as the voltage drop is so low. You can calculate the value, or just remove the fuse and use a pair of clip leads to fine-tune the resistance. You'll probably only need about 0.25 ohms.

Most amps I see are running around 7v on heaters - Fenders especially. They seem to run for many years like this and I rarely drop the voltage, otherwise I'd be doing this on every amp that comes through the door.

[g1]

Yes, agree with Mick, 7V is a lot better than the 7.7V mentioned earlier. You are about 10% high which is within reason.

[pontiacpete]

Much thanks again, I will put a 7A fuse in and send it on it's way.