[S Clark]

After blowing one of my 5AR4 on one of my old Knight KB-85 amps, a friend and amp designer gave me a set of 5U4's to run in place of them.  What's the difference?   

[zacster]

A 5U4 will provide a lower voltage out, plus it does not slow start like a 5AR4.  You should be OK with it, but I'd have your amp designer friend check out the bias/operating point you get on your power tubes.

[S Clark]

Can you say what the sonic result of the lower output voltage might be?

[JohnR]

One thing to be aware of is that the filament current is 3A, vs 2A for the 5AR4

JohnR

[S Clark]

One thing to be aware of is that the filament current is 3A, vs 2A for the 5AR4

JohnR

So what is the effect of the higher amperage?  Does it stress other bits of electronics? Although I can follow a schematic, I have absolutely no clue what each part of a tube amp circuit does.

[aerius]

It's not the bias I'm worried about, it's the power supply.  According to the 5U4 spec sheet, the maximum size of the first capacitor, if the first element of the power supply filter is a capacitor, is 4uF.  For a 5AR4 I believe it's around 40uF, so there's a good chance the 5U4 isn't going to work if the amp was designed for a 5AR4.  Or if it does work, the 5U4 may arc over or have its life drastically shortened.

[S Clark]

Ok all you E.E. types, here is the link to the schematic.  So with this bit of info, now can anyone tell me if running these 5U4's is a bad thing?

http://web.mit.edu/glettler/www/projects/antiques/a_kb85_schema.gif

[ghpicard]

IMO, replacing an indirect heated cathode rectifier with a direct filament type is not a good move.
The slow turn-on of the 5AR4 would give all of the tubes in the amp enough time to heat up before it starts conducting. This, besides preventing cathode stripping, helps your filter capacitors too.
If you look at the schematics, they state 435 VAC on the rectifiers anodes. With the 5U4s you will have around 1.4 times that (about 630VDC) in the positive rail a while after start and until the rest of the tubes (all indirect heated types) catch up. Not nice when you have 500VDC (and less) rated capacitors in the filters.

I wouldn't run the risk.

[ltr317]

It's not the bias I'm worried about, it's the power supply.  According to the 5U4 spec sheet, the maximum size of the first capacitor, if the first element of the power supply filter is a capacitor, is 4uF.  For a 5AR4 I believe it's around 40uF, so there's a good chance the 5U4 isn't going to work if the amp was designed for a 5AR4.  Or if it does work, the 5U4 may arc over or have its life drastically shortened.

Are you sure that's right?  The first electrolytic cap on my Magnavox console amp after the 5U4 rectifier is 40uF.   

[doorman]

FWIW, I have  Decware Zen that's happy with either the 5U4, or 5AR4.
Several years, no problems.
Don

[DerrO]

Note that the 5U4 first cap max value is indeed 4uf ....... BUT  a 5U4G is 40uf  as  is a 5U4GB.

Beware as other have mentioned  about the current draw of the 5u4 series over the GZ34/5ar4

Cheers.

Deeee

[ltr317]

Note that the 5U4 first cap max value is indeed 4uf ....... BUT  a 5U4G is 40uf  as  is a 5U4GB.

Beware as other have mentioned  about the current draw of the 5u4 series over the GZ34/5ar4

Cheers.

Deeee

Thanks for the clarification.  The amp does use a 5U4 either GA or GB.   

[JohnR]

One thing to be aware of is that the filament current is 3A, vs 2A for the 5AR4

JohnR

So what is the effect of the higher amperage?  Does it stress other bits of electronics? Although I can follow a schematic, I have absolutely no clue what each part of a tube amp circuit does.

No, it just draws more current from the transformer. It's not like it's a huuuge amount (5 watts) but personally I think I would just get a gz34, I thought they were available pretty cheap (haven't bought tubes in ages)