I Need "How to" help- HT Fuse on '65 Bassman

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I Need "How to" help- HT Fuse on '65 Bassman

Postby jerryjg on Fri Feb 05, 2016 6:45 pm

First of all, I am not a tech, but I have a friend who is ann E.E., and has experience with amps.

Is an HT fuse something that is straight forward, or should I be seeking a wiring diagram for such. I have searched the web to no avail.

Can anybody direct me to a diagram for installing an HT fuse on my 1965 Fender Blackface Bassman head?

Any advice or ideas besides don't need one or don't do it. I've done my homework, and I am going to install one if at all possible.?
Marshall MKII 100 Lead, 80's Peavey Classic VTX, 1966 Ampeg Mercury, 1973 Marshall Superlead 100, 1974 Hiwatt DR504,1965 Fender Bassman, 1971 Traynor Bassmaster, 1969 Sunn 100S
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Re: I Need "How to" help- HT Fuse on '65 Bassman

Postby pdf64 on Fri Feb 05, 2016 8:00 pm

Great idea, give that old beauty better protection from tube shorts etc.
Identify the HT centre tap wire from the PT to the chassis 0V, eg red/yellow in this layout http://bmamps.com/Schematics/fender/bassman_ab165_schem.pdf

Unsolder it from the chassis (a big bertha iron will be needed); check that it has continuity to the HT winding end legs, and nothing else that comes off the PT.
Move it to one end of the fuseholder (the most recessed contact if it's mounted on the back panel).
May need to extend the wire, if so ensure that the joint is suitably insulated, eg heatshrink.
Connect the other fuseholder terminal to the previously used chassis 0V solder point.
In the 1st instance, fit a T0.5A anti surge / slo-blo / slow acting / time delay fuse.
May be able to get away with a quick blow / fast type and / or a lower value, eg F0.5A; it's a balance between spurious fuse blows and good protection.

I edited a Super Reverb schematic to show such a fuse, amongst other mods, see http://s963.photobucket.com/user/pdf64/media/PDFBFCombinedMods_zpsca05d439.png.html?sort=2&o=11
My band:-http://www.youtube.com/user/RedwingBand
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Re: I Need "How to" help- HT Fuse on '65 Bassman

Postby jerryjg on Fri Feb 05, 2016 11:37 pm

pdf64 wrote:Great idea, give that old beauty better protection from tube shorts etc.
Identify the HT centre tap wire from the PT to the chassis 0V, eg red/yellow in this layout http://bmamps.com/Schematics/fender/bassman_ab165_schem.pdf

Unsolder it from the chassis (a big bertha iron will be needed); check that it has continuity to the HT winding end legs, and nothing else that comes off the PT.
Move it to one end of the fuseholder (the most recessed contact if it's mounted on the back panel).
May need to extend the wire, if so ensure that the joint is suitably insulated, eg heatshrink.
Connect the other fuseholder terminal to the previously used chassis 0V solder point.
In the 1st instance, fit a T0.5A anti surge / slo-blo / slow acting / time delay fuse.
May be able to get away with a quick blow / fast type and / or a lower value, eg F0.5A; it's a balance between spurious fuse blows and good protection.

I edited a Super Reverb schematic to show such a fuse, amongst other mods, see http://s963.photobucket.com/user/pdf64/media/PDFBFCombinedMods_zpsca05d439.png.html?sort=2&o=11




You are awesomely awesome. Thank you so much!
Marshall MKII 100 Lead, 80's Peavey Classic VTX, 1966 Ampeg Mercury, 1973 Marshall Superlead 100, 1974 Hiwatt DR504,1965 Fender Bassman, 1971 Traynor Bassmaster, 1969 Sunn 100S
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Re: I Need "How to" help- HT Fuse on '65 Bassman

Postby stratele52 on Mon Feb 08, 2016 10:35 am

You don't need a HT fuse. Be sure you have the right fuse in transformer's primary and you are in bussiness.

So many old amps use too powerfull fuse because user did not find the right one.
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Re: I Need "How to" help- HT Fuse on '65 Bassman

Postby jerryjg on Sat Feb 13, 2016 7:23 pm

stratele52 wrote:You don't need a HT fuse. Be sure you have the right fuse in transformer's primary and you are in bussiness.

So many old amps use too powerfull fuse because user did not find the right one.


well, I am using higher rating tube screen resistors - going to be at least 1k 2w, and probably 1k 5w - there are definitely recommendations for this as to control oscillation, althouhg thats a whole nuther can of worms...so there will not be any "fuse" of the resistor if the tube fails.
Marshall MKII 100 Lead, 80's Peavey Classic VTX, 1966 Ampeg Mercury, 1973 Marshall Superlead 100, 1974 Hiwatt DR504,1965 Fender Bassman, 1971 Traynor Bassmaster, 1969 Sunn 100S
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Re: I Need "How to" help- HT Fuse on '65 Bassman

Postby pdf64 on Sun Feb 14, 2016 5:34 pm

Just to note that whilst the screen grid resistors may act in some degree to suppress oscillation, the control grid stoppers (eg 1k5 in series with socket terminal #5) are the main players in that role.
I think that all Bassman models after the 6G6b (eg aa864 onwards) had power tube control grid stoppers fitted.

However, the screen grids take a beating when heavily overdriven, and higher value resistors can help stop them failing.
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