Stupid question about output jacks on BC30

[wiggy]

I did ask this in another thread but it was pretty much closed by the time I posted the question so I thought I'd ask it afresh.

 

I was wondering if someone could explain to me in simplish terms why, assuming that each of the outputs on the back of my BC30 (2 x 4Ohm, 2 x 8Ohm & 1 x 16Ohm) is wired to a separate tap on the OT, I couldn't plug an appropriate impedance speaker into each socket at the same time. The OT would be seeing the correct impedance at each tap so there would (on the face of it) be no impedance mismatch anywhere.

 

I'm not even vaguely thinking of trying it but, ignoring other issues such as limitations of power to drive that many speakers, would like to know why you couldn't theoretically do it.

[jefrs]

The 4ohm, 8ohm and 16ohm are indeed separate taps on the secondary. Loading more than one tap would throw an unknown reflected impedance back onto the primary. This load would be "complicated". I asked a chap that designs output transformers this question and got advised not to do it.

 

The total load for each tap is as stated on the label. The double socket taps are connected in parallel.

The 1x16ohm wants 16 ohm, this could be a pair of 8ohm in series.

The 2x8ohm wants 8ohm, this could be one 8ohm (which could be the internal pair of 16ohm in parallel) or two of 16ohm (in parallel)

The 2x4ohm wants 4ohm, this could be one 4ohm or two 8ohm (in parallel).

 

I have changed my BC30 speakers to a pair of 8ohm. I normally run them in series into the 16ohm socket, this is the most efficient tap. Or I can switched them to parallel and run them into one of the 4ohm sockets, this is less bright. Or I can switch them to mono and use either one into one of the 8ohm sockets (I suppose I could run them both individually into both 4ohm sockets). I use a switch like that found on the back of a Marshall cab.

[Andyroid]

Hi, no doubt someone has tried it and it probably didn't do the amp any harm (too low a load impedance is near enough ok for valve amps). Might not have sounded too good though. Here's my theory (bare in mind I'm a novice). A transformer has a ratio of input impedance to output impedance. The output taps are from the same coil and will interact with each other to create a total load impedance that when multiplied by the transformers ratio is NOT what the OP valves want to see. If you can calculate was the total impedance will be you may be able to choose a set of speaker cabs that WILL work when plugged into every socket.

 

A few weeks ago I emailed gibson about the BC30 transformers and am still waiting for a reply. My guess is about 5.6k primary, must be. Can't wait to find out though!

[jefrs]

The only flaw there is that too low an impedance will harm a valve amp: you can go up from the stated impedance, not down.

 

Going down in impedance worst scenario is a short circiuit, the amp finds this very easy to drive and passes a lot of current into it, bad news.

 

It is actually quite difficult to cook an output transformer, what normally happens is the valves red-plate when over-loaded, long before the transformer insulation melts and it shorts out. If a speaker blows they can short and set fire to an amp (I had one do that).

 

The exception is that some Fender 4ohm outlets are actually designed to 2ohm, and they will take handle a large mis-match i.e from 2ohm to 8ohm or thereabouts. I think the rule here is, if it ain't an old Fender, and it don't say so in the specifications, then don't under-load the outlets.

 

On an amp like the BC30/SoCal50 the transformer secondary expects no less than the tap value, but it is pretty safe to put an additional load on it - up to the value of the next tap, i.e the 4ohm socket might take 4ohm to 7ohm loads. Do bear in mind that the actual impedance of a speaker varies a lot with frequency, and that a typical 8ohm speaker has a DC resistance of about 6ohm.

 

Primary impedance can be misleading: the BC30 transformer is marked 22000ohm-0-22000ohm but it measures 48ohm DC resistance. The impedance varies a lot with frequency. The thing to look for with transformers is the intended valves, the intended load and the power expected. I expect the BC30 transformer to be similar specification to the Soldano60, a 5881/6L6 60 watt job with 4/8/16 taps and available from TAD as SL60A (they do not even state the primary impedance).

http://www.tubeampdoctor.com/advanced_search_result.php?search_in_description=1&keywords=transformer+soldano&x=4&y=7

[Andyroid]

Oh ok, it just shows how hard it is to learn by reading things on the internet. I think it was Jeremy Clarkson who said "All 72 billion facts on the internet are wrong!". Looking through my bookmarks list, I probably got my 'wrong' info from this site:

 

http://www.geofex.com/tubeampfaq/TUBEFAQ.htm#mismatch

 

This guy seems to know what he's talking about, but so does everyone with their own amp design website!

[jefrs]

Andyroid, it is true that there is a lot of rubbish on the internet, and what 'geofex' is saying you can do to a guitar amp is true, but it is not always best practice, nor is the reasoning behind some what he is saying entirely correct. Much of what experts post comes under the category of "lies for children", which is to say that the truth is more complicated. To understand what is really going on, you probably need a few degrees in electronic engineering, and 50 years experience in electronic design (in order to go far enough back in time to when valve amps were common). I am fortunate to know a few people that do have that knowledge, and whilst I am pretty well qualified myself, I do not always understand what they tell me.

 

What is true is that when you mis-match the load on a valve amp, strange things happen to the transformer and the output valves.

 

What is true is that you can mis-match the load on a guitar amp, but not by all that much, or you will damage it.

 

 

FYI Laney advised me in writing to connect a 5.6ohm load onto the 4ohm tap, not the 8ohm tap, as the correct way to connect their 2x12 cab (16ohm) onto their 1x12 (8ohm) combo, i.e. to go up on the load to the tap, not down. Both shorting and open-circuit will harm a valve amp.

 

 

BTW pulling a pair of valves from a quad is a bit like loosening the bolts on a wheel of your car because it will make a nice drumming sound as it goes around: it will work for a while.