Old Fend. Transf. Primary Colour Codes

[oc disorder]

Hi
I have an imported from Europe Fender Studio Bass amp with the power transformer wired to 220 volts.
The transformer number is 014679 and the EIA code is 606-102.
I wish to change it to the 240 or similar tapping as the heater voltage is
a little high.
I thought this would be straightforward but am slightly confused
It has 6 wires on the primary side
Red/Black
Black
Yellow
White with Black stripe
Green/Black
Black/Yellow

The Green Black and Black Yellow (y stripe) are joined as they should be
for 200v+ the 220 connection was to Yellow and White/Black.

I assumed the 240 or 260 tap was the unterminated Red/Black and Black.

When I hooked that up the heater voltage was higher..???

I haven't been able to find any schematics with that transformer (014679)
and I'm hoping someone may point me in the right direction.

At the worst I can use wire wound resistors or a couple of diodes to drop the heater voltage but would be neater to get the primary wired right!

Thanks

[Alex R]

If you disconnect the primaries and check across them with a good ohmmeter, the 240v pair will be the ones with the biggest resistance between them (these will be quite small resistances, in the one ohm area). Or disconnect the secondaries too and just experiment with primary connections till you see what you want to see on the secondaries.

[Enzo]

The only prints I find don;t show that transformer, they use 012405. That is a split primary, and the schematic note shows the alternative wiring for "210/250v 50/60Hz"

This one does not show 220 and 240v arrangements as the two primaies are simple windings.

In this case grn/bk and y/bk are wired together to series them, and 220 would be across red/bk and bk.

The drawing shows an internal thermal fuse in the transformer, with yellow wires bringing it out. (As opposed to just wiring the fuse in series with a winding.) The thermal fuse leads are yellow.

You list six wires, but are all of them windings? I am not assuming that the six wires are two windings with a tap each.

Can you apply 120v to a single primary? If you have 120v, wire it across red/bk to grn/bk, of coursze disconnected from all others. Now mete4r the remaining wires to see how many of them are actually windings. For example if the yellow wire is a thermal fuse it will not show a voltage difference from the end of whatever winding it is bonded to.


And after all that, the question remains: just how high are these heater voltages? 10% is a reasonable margin. And of course only readings taken with tubes count.

[oc disorder]

Thanks Alex R and Enzo for the replies.
I must confess I was hoping for a short cut reply.
I separated the windings and measured the resistance.
It seems with this transformer 014679 that it it already set to its maximum
input voltage so like the alternate primary connections in the 012405 transformer it must be the setting for 210 to 250 volts.
It has 220 volts printed on the back panel.
Measuring the preamp heater voltage with the o/p valves out I get 6.7 volts.
This doesn't change upon fitting the 6 o/p tubes but the heater voltage drops from 7.3 volts to 6.98 volts on the output tube heater supply.
I guess I should fit 2 diodes to get a .5 drop to 6.38 .
Interesting to note the previous Danish repairer has fitted 2x8 amp fuses
in the heater supply and disconnected the mains fuse (doesn't have one)
and has two HT fuses instead.
It has an ultralinear o/p transformer and a metal plate with a standard 9 pin tube socket where I presume the 6CX8 used to reside in a former life.
It must have a thermal fuse as the Yellow and Black wires have negligable resistance and I now presume the extra tapping is for the USA 110 117 120
selection.
I was a bit concerned as the previous work suggested there may have been a problem in that area and seeing the owner has just bought $300 worth of tubes I thought it worth getting the heaters as close to 6.3v as reasonably as
possible.
Thought I would post my diagram of my findings in case its of any use.

Thanks for reading.

[Alex R]

Those are not catastrophic heater voltages, I'm always seeing 6.8vAC and stuff like that. Is there a problem with the amp? If not I'd leave it and see. Before putting diodes in there remember the current draw you're dealing with.

[Sredna]

Old thread, new problem!

Im sitting here with a Fender Studio Bass amp with the 014679 power transformer in 230V land, the heaters have about 6.7 but that seems ok.
Now I get ~530V plate voltage (should be 500V) and my cheap chinese 6L6GCs have great problems with that.

It seams that this transformer is already set to its highest input voltage (230V -> Yel & Black/Yel, Bl/Yel + Bl/Green joined).

Any ideas out there how to get down the plate voltage?

Thanks in advance!

[g1]

The idle current of the power tubes will affect the plate voltage.
What do you have for idle current of the power tubes?
What exact problems are you having with the tubes running at 530V ?

[Sredna]

Thanks for fast repy!

In this amp the bias is fixed with a balancer between the pairs, bias voltage is about what the schematic says (-61V).
The idle current varies between the power tubes but lies around 20-27 mA, that should be ok.

The problem is that when I insert new chinese 6L6GCs (a pair) they start to redplate and idle at 35-39 mA,
the other (old) tubes sinks down to about 16-20 mA. I also get a strange feedback when I turn up the master volume, kind of motorboating.

It just occurs with these new chinese tubes and not with the old ones (mixed sorts, Fender original, old Sovtec etc.).
I know that lots of new 6L6GCs are relabeld tubes that dont really meet the specs of real 6L6GCs.

I have been poking around with zeners on the negative side of the rectifier bridge and got the the plate voltage down to about 500V,
the new tubes behave well with idle currents round about 20mA.