AMPEG B15 '64 ish mains transformer question .

[Chimike]

Hoping I can get some info on my B15 mains transformer. Connections all seem correct, and agree with circuit diag under amp.  Tube rectified, non-pcb version. Amp is run from a step down transformer, and  is working fine,( just need to lower the hum level a tad )

However, there appears to be two additional leads attached to the transformer , blue and blue/white , taped off, and tucked away. Can anyone throw light on why they are there ?

I don't want to guess !

Thanks,

Mike.

[Aidan63]

possibly taps for different impedance speaker connections, not required in this specific application, so not connected - the transformers maybe used in different applications not all of which need all the wires to be connected

I watch a lot of Psionic Audio (US amp tech) on YT and he'd definitely be able to answer the Q; I don't know if any of the knowledgeable old school valve amp technicians in the UK are on this forum and would interact, check the recommendations in repairs and maybe contact someone who works on these

[Chimike]

Thanks, but just to confirm, it's the power transformer, not the output transformer I was referring to.

, although I do realise  it may be a replacement, but def a proper, potted, B15 replacement, if so.

I'll check out Psionic Audio JIC.

Edited November 9, 2024 by Chimike
spelling.

[BassmanPaul]

I'm a little confused. Both transformers in my B15NF are potted units.

 

As the leads in question are coming off the power transformer it should be easy to identify what they do.  Simply untape them and and do resistance checks to the other primary leads to see if they are a primary tap or are off the secondary. Obviously do these checks with the amp unplugged.

[Chimike]

 Thanks,

I'll do that, and report back.

[Chimike]

Hi,

So, taped up leads, blue and blue/white show no connections to other leads from Transformer. Measure 4 ohms across leads. This is similar to the

Primary leads , black/white and black  , which measure 3.8 ohms . I'd love to think I have a dual primary transformer, but I'm not willing to risk my

old amp on a guess.

BTW. Have done a cap job, and also fitted the protection diodes to the rectifier ( as you have suggested in the past ). Did the same on my SB12, but lost the bias 

'till I realised I had to move that connection as well !

[Chimike]

So,

Continued searching, and found a mention of a similar transformer on a '63 B15 on music electronics forum .

Guy named Boroman , in Germany, was recently asking the same question as I have re the Blue and blue white taped off wires.

Conclusion  was that it was indeed a dual voltage transformer. He was advised to remove the death cap ( done that ),

And add a resister into the neon pilot light circuit before making connections for 240v.

Only problem I have now, before I try it ( on a variac ) is that there is a " thing " that I don't recognise in line with one side of the mains,

unsure what it does, or what it's voltage rating is.

Will take photo, and show it to my tech friend , when I next see him

[BassmanPaul]

Upload the photo here so we can identify what it is. (Hopefully :D)

[Chimike]

 

Not the best photo in the world. It's the silver thing between the pink condenser and the white resistor. It's shown on the circuit diag under the amp,

( B12N AND B15N ) between the on / off switch and the mains fuse. Any help greatly appreciated .

 

 

 

 

 

[BassmanPaul]

I'm sorry but all I got was a blur. Maybe take another photo and post it.

 

[nekomatic]

Do you have a link to the circuit diagram? I searched for one and found this which shows what looks to be a thermistor as it’s labelled with ‘cold’ and ‘hot’ resistance values. Is that performing some sort of soft start function perhaps?

[Chimike]

That circuit diagram is similar to mine, at least, on the mains input and primary side. Seems that the  symbol is an old way of showing Resistors, so you're  probably right as to it's " soft start " function .

I'll measure it !

I'll also get a better picture ,will help to confirm.

Thank you Both.

 

[Chimike]

Better pic !

Seems these were used on the '61 B15 ,the again on the '64 version, that used Silicon rectification .

Maybe my late '64 valve rectifier amp was built using the early '64 boards that had the "thermister type thing ".

Later B15's did away with that component, so I think I'll do the same. Also, there were two versions of the PT108 mains transformer,

part no 300...- 1 for 117v operation, and part no 300...- 3 for 230v operation. Unfortunately, the circuit diagram

I found on the Ampeg website with that info is a bit smudged, but I think I'm sorted now.

Will try the 230v connection on the transformer, through my Variac, and keep a good eye on the output voltages, JIC.

Many Thanks for the help.

Mike