More Watts or Less?

[BluesDude]

This is related to my other recent post. I am considering a Marshall 18 watt amp.

I have been looking at the Weber 6M18 kit. I noticed that the 6M18 and the 6M36 circuit and parts list are almost identical. Looks like the 6m36 has a second set of Power tubes (along with some minor circuit mods) and a larger OT.

https://taweber.powweb.com/store/kits_60b.htm#6M18
https://taweber.powweb.com/store/kits_60b.htm#6M36

It looks like it would be fairly easy to mod the amp to a half power/full power (18/36W) version by switching off one set of Power tubes and some minor circuit mods. It seems to me that this would let the amp be more versatile for both home/practice and gigging (more power if needed).

What is your opinion (or experience). Does the idea look sound? Have others done it?

Thanks.

[d95err]

First, consider the weight. The 36 watter will weigh a lot more because of heavier transformers. So, if you're going to use the 18W mode most of the time - do you really want to drag around that extra weight just for another 3dB?

One practical option if you build the amp as a head is to use difference speaker cabinets for different occasions. E.g. a single 12" cab for rehershal and small gigs and a 4x12" for the really big gigs. That will give you a bigger volume span than the 18/36W option.

[BluesDude]

First, consider the weight. The 36 watter will weigh a lot more because of heavier transformers.

Your point is well taken... Extra weight is not a plus.

It looks to me like both the 36w and 18w use the same PT. However, the OTs are different. I suppose that must mean the PT is over speced for the 18 watter.

[d95err]

Your point is well taken... Extra weight is not a plus.

It looks to me like both the 36w and 18w use the same PT. However, the OTs are different. I suppose that must mean the PT is over speced for the 18 watter.

If it's the same PT, then the weight difference will be negligible, as the difference in OT will not be that significant. I may have exaggerated the weight thing a bit. The difference is a lot bigger between say a 50W and a 100W amp.

To switch out two power tubes you need to lift their cathodes and also switch to another cathode resistor (twice the resistance). Also, you should make sure you get twice the output tranformer impedance ratio. If you have an output impedance switch you can just set it to one step lower than the speaker load (e.g. 4 ohms for an 8 ohm speaker cab). If you're output transformer has multiple tap you could switch on the primary side.

(I hope I got this right; I can never remember if it's twice or half the impedance, but this seems logical).

I still think working with different speakers is an easier and more flexible option though. For instance, if you have a 2x12" cab, switching out one of the speakers has the same effect as a half power switch on the amp.