Bass State B65 transformer specs

[tristanc]

Hi, I'm trying to modify a Japanese B65 for use in the UK by replacing the power transformer with something suitable - either an approved part or something that comes close 3rd party. Marshall won't deal with me directly nor will they give out the specs. This amp was destined for the skip before I saved it thinking a quick PT swap would be a piece of cake. Alas...

Does anyone know the raw secondary voltages or the target +/- DC rail voltages? Or indeed know of a drop-in replacement or suitable PT?

I've been advised the size is likely 100VA. And that the PT would have a similar winding to that of the ValveState 8080's transistor winding.

Thanks,

Tristan

 

[SkyMonkey]

There have been similar threads for Valvestate guitar amp transformers in the past.
Not many successes.
Dagnall transformers seem to have been the norm in those amps.
Hard to find now though.
The schematic for a B65 is elusive, but the 8080/8100 is reportedly the closest you'll get.
Have a look at this short MF thread on the subject.
I have downloaded the Zip and scanned it with Bitdefender. No problems found.

Has anyone got a Schematic for a Bass State B65

Hi I would be most grateful if anyone has a schematic for a Bass State B65 Amplifier. I have a very lively output stage but no amplification from the inputs. It seems that an voltage rail may be missing. So if you can supply a manual both myself and my students would be most grateful. Cheers...

www.marshallforum.com

 

[SkyMonkey]

There is one on Gumtree right now, for £50.

Marshall Bass State B65 - 65W Combo Amp | in Clapham, London | Gumtree

B65 Combo Amp Pre-loved and underused, great for home recording usage and small venue gigging Collection from around SW8

www.gumtree.com

 

[tristanc]

£50 gives me the upper limit on what I should pay to get a replacement PT.

I see I have a few options:

- use a step-down TX to go UK mains to Japanese 100V - about £60 or so
- find a 230Vac : ~[24 ct 24] or ~[35 ct 35] at ~100VA TX (something that gives <50Vdc after rectification, is cheap and fits the space in the chassis)
- I see in the other post linked to here the pointer to TAD for a potential replacement. (~£90 or so)
- find a variac I can borrow to get the actual output voltages to help with the 2nd option above…

 

[South Park]

you have to match up the transformer voltage to the transisters that are being used . you can take a reading off the transformer output

 

[tristanc]

Yeah, I suppose I could do something without a variac:

- disconnect the secondary connections from the PCB
- connect UK mains to the primaries
- measure the unloaded VAC across the secondaries
- measure the primary VAC
- use the ratio VACpri:VACsec to calculate what VACsec would be with a nominal 100VAC supply, as in Japan.
- then assume loaded is a bit lower (divide by 1.3? I think Merlin has an approach to estimate unloaded to loaded voltages)

 

[Pete Farrington]

connect UK mains to the primaries

Even with secondary unloaded, I strongly advise against this.
Why not test ratios with 100V AC from the variac to the primary?

 

[tristanc]

Even with secondary unloaded, I strongly advise against this.
Why not test ratios with 100V AC from the variac to the primary?

If only I had one... I'll ask around.

I agree, using mains is always risky - but in this case I only have to disconnect the 3 secondary wires, isolate them and attached a DMM.

I may have a signal generator capable of outputting 2Vrms - that might allow me to get the turns ratio from 2ndary -> primary.

 

[Pete Farrington]

If only I had one

Oops, sorry I misread your post.

I may have a signal generator capable of outputting 2Vrms - that might allow me to get the turns ratio from 2ndary -> primary.

Yes, a decent sig gen should be able to drive a PT primary, and allow you to get a turns ratio.

 

[tristanc]

A variac has been sourced! I'll then power up to 100Vac, take readings from where I can (no schematic or layout) and report them back here in case anyone else wants to save a 20 year old transistor amp....

 

[tristanc]

OK, for the benefit of anyone else looking in the future:

Japan PT spec: 100Vac Primary -> 33-0-33 Secondary

I then measured ~+/-43Vdc at a couple of places close by the rectifier. But without removing the board and tracing the circuit I have no idea where those readings where.

I'll now be looking for a cheap replacement (100VA or so) that physically fits.

 

[tristanc]

Had a quick look, it seems there are a couple of options for PT:

- a 115/230VAC to 36V CT - just run it a bit over voltage
- use a 115VAC to 16VAC CT and connect 230VAC instead to get 32VAC CT (likely a bit over).

Both are ~100VA and roughly the right size. Tempted for the latter option as it will be closer to 33VAC.

 

[Pete Farrington]

use a 115VAC to 16VAC CT and connect 230VAC instead to get 32VAC CT (likely a bit over).

I suggest not to attempt that. The transformer’s magnetic core seems likely to saturate, causing it to lose inductance and draw heavy primary current.

 

[tristanc]

I suggest not to attempt that. The transformer’s magnetic core seems likely to saturate, causing it to lose inductance and draw heavy primary current.

Good point. I’ll look for the 230:36 option.

 

[tristanc]

For those in the future looking to replace the PT in a Bass State B65, the Hammond 186F36 seems to work well. And is cheap.

Low Voltage Solder or Quick Connect Terminals - 2.4 VA to 102 VA (186-187 Series)

Hammond's 186-187 series low voltage power transformers include combination solder and quick connect terminals with choice of single or dual primary. Their split bobbin construction eliminates the need for an electrostatic shield.

www.hammfg.com

You need to drill out the rivets of the old PT, drill two new mounting holes for the new PT, then snip off the existing spade connectors and solder the wires appropriately. The change is invisible from the outside.