Toroidal transformer - why?

telehack

I have an early ‘64 AA864 bassman that started out life with a 110v power transformer. It now has a UK voltage toroidal transformer which looks to be of Chinese manufacture. I’ve never seen one of these in a guitar amp. Are they cheaper than traditional ones? Lighter? I know it won’t affect tone, but is there anything to be gained by putting say a Hammond 290ex in there? Thanks in advance. 

ICBM

Lighter and more efficient. I wouldn't be so quick to think it won't affect the sound - it will, since it will have different regulation from the stock type, which will affect the dynamics when driven hard.

I assume it's been fitted by bolting it through a couple of round plates on opposite sides of the original rectangular cut-out - in which case no permanent damage to the chassis should have occurred. If so I would definitely put it back to stock using a correct modern replica transformer, both from a performance and value point of view.

That's a really great and becoming quite valuable amp, and as long as it hasn't been hacked in other ways it is very much worth it.

Kittyfrisk

Pretty sure Laney used toroidal transformers in their TT50 & 100 amps, GH's & I believe LC30's too. Noratel  transformers I think.

telehack

Thanks, @ICBM. The transformer is mounted on to the bottom of the head cabinet. I believe the circuit to now be original but in the past someone installed a send/return on the back panel. Also there’s a filled in hole on the front panel, so not collector grade. Love the amp though.

randella

Kittyfrisk said:

Pretty sure Laney used toroidal transformers in their TT50 & 100 amps, GH's & I believe LC30's too. Noratel  transformers I think.

There's certainly one in the VH100R I'm working on. It's pretty substantial.

ecc83

The most usual advantage of the toroidal transformer is their lower profile for a given power (VA)

They also have a lower radiated hum field. They are often touted as being "more efficient" than E/I traffs but in practice this only obtains when they are run close to their VA limit, in a 'class AB valve amp that will not happen (might tho' for a pure class A hi fi amp)

Toroids do however tend to have better 'regulation' so there will be less PSU sag when you really give it the beans.

I see no benefit in swapping it for the Hammond 'square' jobby?

Dave.

HAL9000

I’ve certainly heard, from someone who should know, that toroidal transformers produce less hum.

ecc83

I do of course bow to the great knowledge of friend ICBM. I did not know the amplifier was a valuable rarity and so yes! If IC says put it back to stock? Do it!

Dave.

ICBM

telehack said:

I believe the circuit to now be original but in the past someone installed a send/return on the back panel.

That's a bit stupid - there's no point on an amp which doesn't develop significant preamp distortion.

ecc83 said:

Toroids do however tend to have better 'regulation' so there will be less PSU sag when you really give it the beans.

I see no benefit in swapping it for the Hammond 'square' jobby?

That's why I would swap it - you don't really want an overly-well-regulated PSU in an amp like this, a lot of the 'magic' in them is the sound when the power stage is overdriven. (Even if you're using it for bass, as I've discovered with my Bassman 50.)

HAL9000 said:

I’ve certainly heard, from someone who should know, that toroidal transformers produce less hum.

Yes, they do - the radiated electromagnetic field is much smaller - but it's only really an issue if you get a single-coil guitar very close to them. The circuit in a Bassman is not inherently hum-prone.

ecc83 said:

I do of course bow to the great knowledge of friend ICBM. I did not know the amplifier was a valuable rarity and so yes! If IC says put it back to stock? Do it! 

Not so important if it's had the front and back panels drilled - all collector value is gone really, since they're not *that* rare, although certainly less common than the later AB165 version. It was one of Fender's biggest-selling amps at the time, since it was one of the few proper bass amps made. (Although many pro bass players used the more powerful Dual Showman.)

It would actually be interesting to do it though - just to see how much difference it really makes.

TheMarlin

If you wanted to revert, Mercury Magnetics do great Bassman replica power supplies, and 240v versions too. They get a huge thumbs up from
the Gretsch fanboys. 

telehack

Thanks for all the input. To make things stranger, the original power transformer is still in there, making the amp heavier still. I don't know why I didn't notice that. I've uploaded some pictures:

View post on imgur.com

The last repair person left his phone number in marker on the chassis! Still in business in Nottingham. I'm not sure what's going on with the two extra jacks on the back that I assume are preamp out, amp in. There's a closeup in the pictures I uploaded.

ICBM

Ah - it’s not a replacement transformer… it’s being used as an autotransformer to step the mains voltage down to 115/120V - the secondary taps are the wrong voltages for a valve amp. It looks like the transformer from a big solid-state bass or PA amp.

The jacks on the back do appear to be a preamp/power amp insert loop I think - but badly done, the floating caps with wires attached are bad practice and a broken connection waiting to happen.

Almost all the internal caps and resistors have been changed. Also, the ‘original’ transformer is itself a replacement since it has a 1966 1976 date code. (619, 19th week of 1976 - the other code corresponds to a 1970s one not 1960s.)


Edit - reading that back it sounds a bit critical... it's not meant to be, other than of the two unsupported caps - the changed resistors and caps have very little, if any, bearing on the performance, and none on the value since it's already been drilled. The slight surprise is that there are still a few original ones, you would think that whoever did it would just have changed the lot if they thought it was necessary to change most of the ones they did.

It's perfectly OK to use a big mains transformer as a stepdown autotransformer, if it has a dual-voltage primary as this does, and if it's adequately rated - it looks like it should be more than enough. The non-original transformer actually looks like it's from 1976 not 1966 - my mistake. Either way it looks like the correct type.

hywelg

telehack said;

The last repair person left his phone number in marker on the chassis! Still in business in Nottingham. I'm not sure what's going on with the two extra jacks on the back that I assume are preamp out, amp in. There's a closeup in the pictures I uploaded.

Who is it? Not Dave Hunter by any chance? He's @DJH83004 on here

telehack

@hywelg - sorry no. I was misinterpreting the phone number. It doesn't seem to belong to anyone current.