Blues Jr - Mains wiring affects internal voltages and bias

[haknuts]

In Norway the AC mains is rated at 230VAC. However this voltage varies depending on your supplier, distance to distr.transformer and net load (+/- 5%) . At home my mains is at the higher end of that scale. B+ then was 340VDC, whilst the schematics expects 329VDC. Obviously this affects the amps performance. Rewiring to 240VAC lowers B+ to 324VDC. Check your mains and B+. Rewire if needed. Keep safe.

[Stevem]

Don't be too surprised if you play the amp normally at half way up or more and the PT burns up in short order!
That 230 and 240 volt wiring on the primary couples to very little of the secondary and as such you are powering the amp through far lass secondary wire mass!

[haknuts]

Interesting theory Stevem! If I understand you correctly, Your approach is that the primary windings are those between white/black and black.

It is my understanding of both map and the terrain that the primary windings are between violet and black. S6 is N/C, S2B is the switch and P4 goes via the fuse to the other pole S2A of the switch. P9 & 10 are latched at the PCB.

Now what confuses me is that if you look at how the PT is visualized here, it may seem like the 240VAC config involves more windings than at 230VAC. This does not make sense to me.


Link to the Jr. (II) schematic: http://support.fender.com/schematics/gu ... ematic.pdf

[ToneMerc]

Don't be too surprised if you play the amp normally at half way up or more and the PT burns up in short order!
That 230 and 240 volt wiring on the primary couples to very little of the secondary and as such you are powering the amp through far lass secondary wire mass!

Not exactly, the primary is wired in series across the entire core using both windings starting at the thermal fusible link. The second winding determines the discrete primary voltage because it multi-tapped.

Thus, no,worries.


TM

[haknuts]

ToneMerc:
Would you agree with me here "...what confuses me is that if you look at how the PT is visualized here, it may seem like the 240VAC config involves more windings than at 230VAC "

[haknuts]

ToneMerc:
Would you agree with me here "...what confuses me is that if you look at how the PT is visualized here, it may seem like the 240VAC config involves more windings than at 230VAC "

I would expect that in order to keep the secondary at the rated level, a higher input voltage would require less primary windings

[ToneMerc]

ToneMerc:
Would you agree with me here "...what confuses me is that if you look at how the PT is visualized here, it may seem like the 240VAC config involves more windings than at 230VAC "

I think what's confusing you is that for 230V or 240V operation one only needs to use either of the bottom two taps on the second winding, which leaves the top(white) winding unused.

However if I'm correct, you can wire the leading tap of both windings in parallel and grab the white lead, which will configure the primary for 100V(Japan) and would make this a true export PT.

TM

[martin manning]

I would expect that in order to keep the secondary at the rated level, a higher input voltage would require less primary windings

No, it's the other way around. The turns ratio pri/sec needs to go up to reduce the secondary voltage, so you need more primary turns for a higher input voltage.

That 230 and 240 volt wiring on the primary couples to very little of the secondary and as such you are powering the amp through far lass secondary wire mass!

This makes no sense to me. The entire secondary is wound around the same core as the primary, so all of it is in use regardless of how much of the primary is actively carrying current.

[haknuts]

Great discussion, and I think we have estabelished the facts regarding the PT The biasing of the output tubes of the Jr have been widely discussed, but I have not seen any discussions reg biasing the preamp tubes? My Jr is modded (27K R52) for 70% bias at normal voltages. At 340VDC B+ i still had 85%. So, an unmodified Jr that runs at 100% bias at normal (329VDC) B+ should be expected to be overbiased at 340VDC B+. How will this situation affect the preamp tubes, as they also will experience higher vpltages from Y and X?

[martin manning]

Preamp tubes are cathode biased, therefore "self-compensating" to some degree, and not in any danger of exceeding dissipation limits. Their tone will be affected by changes in supply voltage, however, and it will take more signal to over-drive them when the supply voltage is increased.