Cabability

themanthatwalks · 2009-01-25T10:09:20+00:00

Hey guys. I have a 4 ohm cab that I use with my Fender bassman. the cab has 4 16 ohm speakers in parallel. Now I was wondering if I could mod the cab to swi...

Cabability

AlanRatcliffe

Neither did I! That's why I used a 4 ohm example ?

themanthatwalks

thanx guys! by the way its a 4X10 cab.

The reason is that relatively few amps are 4 ohm compatible. So if I want to connect them I would have to do something to change the impedance.

Gearhead wrote:
BTW I can see the point of going to an 8 ohm 2x12 cab to reduce volume, but I don't think you'll get proper tone out of a 4x12 running on 16 ohm from a Bassman. The output tubes now see a much higher inductance, which gives a lot less distortion than you're used to. Plus, the volume will be similar to the 8 Ohms, 2 speakers.

If I go higher (like 8 ohm 2X10) for the Bassman. What diff will this do in tonality and volume?

Thanx again for the helpies ?

Gearhead

I'm no expert on tubes or speakers but there's a few things I can tell you:
1. Changing from 1 of your speakers at 16 ohms to your two parallel speakers at 8 ohms running from the same amp and output transformer terminals, expect five differences:

the volume will increase by 3dB because of the higher speaker efficiency
the volume will increase another 3dB because the amp sees a lower impedance and produces more power
the distortion will increase because the tubes will see a lower output impedance
the colouration of the sound by the speakers will decrease because of the lower swing
the mids will get a small (1dB) boost if the cab is closed back (which it presumably is not) and the speakers are the right distance apart (which most of the time is also not the case)

2. These effects you will notice as well when changing from 2x10@8ohms to 4x10@4ohms. When changing to 4x10@16 ohms, you're going to not get the +3dB for lower impedance but -3dB for a higher one. Same for the distortion: identical to the 1x10. The increased efficiency effect and the decreased coloration will be more pronounced when compared to the 1x10.
3. Tube amps tend to have multiple outputs (or, more correctly, the output transformers have multiple taps on their secundary). You normally get 4 ohms and 8 ohms but 16 ohms may also be there. This is to get the designed load on the tubes with different cabinets. Expect the following effects when changing from 4x10 @4ohms on 4ohms taps to 2x10 @8ohm on 8ohms taps:

both the efficiency effect and the reduced colouration through output swing will be there
the tubes see the same load so there is no change in distortion or output power

4. If you deliberately make a mismatch, you can play with the effects if you understand the above.

AlanRatcliffe

Hold on, Hold on! DO NOT run a valve amp at anything other than the rated impedance. They are a completely different animal than a solid state amp because of the transformer coupling between valves and speakers. The transformer is designed so that each of these two sees the correct impedance (4 ohms only with a '66 Bassman). Changing speaker impedance does not make the valves see a different impedance, because the valves only see the impedance of the transformer.

I'm sure other more edjumicated electronical types like Karel can explain it better, but if you try and run a 4 ohm amp into an 8 ohm load, the speakers will only draw half of the power available from the transformer. The transformer always puts out the same power regardless of load, and if it's not used by the speaker it "feeds back" into the transformer and burns it out (usually taking the power tubes and sometimes the rectifier with it). This is why the quickest way to blow a tube amp is by not connecting a speaker (infinite impedance)...

A transistor amp is the other way round - if you use a lower impedance speaker load, the amp tries to put out more power than it is rated for - often causing overheating to the point of thermal cutout (or damage to the power trannies if the amp has no thermal protection).

MoJo-JoJoe

themanthatwalks

MoJo JoJoe wrote: Now may we ask why you'd want to do this? If the tranny has taps for 4 and 16, then running at 8 makes non sense .... unless of course you want to try to take some sort of sonic advantage of mis-matching loads on the output tranny. I wouldn't, but then it's not my amp.

I am looking at getting a amp with lower wattage, and for instance If i get a vox pathfinder with 8 ohm then i would like to have the option to use the cab if id want ? I love the bassman but man to loud.

Anyway. I considered just putting in a switch that takes 2 speakers out and runs the other 2 parallel thus 8 ohm
(R1R2)/(R1+R2)

and if possible another setting that can do the series and parallel mixup for 16 ohm..

Thanx for the life saving alan...amp saving ? otherwise i would have had a BBQ. Nice when you have meat.. not nice when you have notmeat ?

Any suggestions in turning down the volume of the bassman without modding the amp itself. I considered doing something with the bass channel but then there goes 1966 authenticity.

AlanRatcliffe

A single 4 ohm speaker would bring the volume down a bit, but your best bet to tame an amp is to invest in a speaker attenuator like a THD Hotplate, Marshall Powersoak, Ultimate Attenuator or Weber Mass. These lets you run the amp hot for the tone of a wound up power section, keeps the amp happy but lets you turn the volume down.

themanthatwalks

Alan Ratcliffe wrote: A single 4 ohm speaker would bring the volume down a bit, but your best bet to tame an amp is to invest in a speaker attenuator like a THD Hotplate, Marshall Powersoak, Ultimate Attenuator or Weber Mass. These lets you run the amp hot for the tone of a wound up power section, keeps the amp happy but lets you turn the volume down.

Thanx Ill have a look

5 min later

I had a look ?
looks cool - sounds nice - very interested- wow expensive - rather buy smaller amp - :'(

MoJo-JoJoe

themanthatwalks

MoJo JoJoe wrote: - THD Yellow Jackets or similar (there are a couple)

Do I understand correctly. The yellow jackets Is like a conversion that lets your amp "see" the 6L6 tubes when you are in fact running smaller tubes. these smaller tubes still work at optimum level but your output is reduced ?

MoJo-JoJoe

AlanRatcliffe

MoJo JoJoe wrote: I don't think you're going to be doing your cab any favours if you run a tiny output like the Pathfinder into it. The voice coils of speakers need to move to keep them cool.

The general consensus is you are fine down to about 1/4 watt. Below that and most speakers' tone changes radically. Speaker sensitivity tests are usually done at a mere 1W. Overheating isn't a problem at low power as there is less power being turned into heat energy.

You can blow a speaker with too little power just like you can with too much power.

The problem with underpowered amps blowing speakers is more a PA one - because of the way PA amps distort, which actually stop the voice coil from moving fully, causing it to overheat. Guitar speakers are also made with a wider gap between the voice coil and the magnet, which lets them cope with distortion better.

I think your idea of taming the Bassman a bit will be your best route, but you won't get it down to a whisper without affecting its tone.

Very true. So the first choice is always to use an amp that produces the power output you need for the room you are playing.

Personally I like Ted Weber's design the most

Me too. I must get an attenuator for the Vox soon and have decided on a Weber.

http://www.tubejuiceattenuators.com/

Haven't seen those before! Any good?

Gearhead

MoJo JoJoe wrote: Yes, they are adapters which allow you to fit EL84 tubes into the 6L6 sockets in place of your 6L6 tubes.

Followed the links and read around, if I understand correctly there's nothing to move from 6L6 to 6V6? That would not really change the sonic character. Looking at the tube data, I guess it would also not be so hard for the THD guys to make one...