Marshall DSL 100 - Mod to Switchable (50/100 watts)

[Jecarroll7]

Have a DSL 100 coming in for transformer/choke upgrades and they're asking for a switch to flip to 50 watts (without removing tubes).

I don't want to reinvest the wheel, was wondering if anyone has seen a good way to approach this?

[Chuck H]

After you finish putting in the Mercury Magnetics transformers...

Two tubes can be removed from the circuit by lifting the cathodes from ground. Along with the removal of two tubes from the circuit you should, ideally, alter the secondary impedance to reflect a more accurate primary impedance on the remaining tubes. This could be done by switching the actual impedance selector (if the desired impedance is available) or altering actual speaker load. Neither of these is easily achieved, along with the circuit removal of two tubes, with a single switch.

Another option might be to simply ground the grids of two tubes (in the your case the grids should be taken to the head of the bias supply). Effectively leaving them in the circuit but giving them nothing to amplify. There was a discussion here a while back about whether or not this alters also requires an impedance adjustment. I think it wouldn't. Unless someone else has better info I think this would be the simplest thing to do.

I'll note that a half power switch isn't going to do much to make the amp less loud. The audible difference between 50 and 100 watts is not half as loud/twice as loud. It's 3dB, or consider the difference between 120dB and 117dB. That would be the actual difference. To reduce volume in an effective way a more aggressive power reduction method is needed.

[Jecarroll7]

I like your 2nd option Chuck, although on your first option has me curious, hope you don't mind a could dumb questions...

- Any idea how much the impedance would be impacted to the OT's primary and secondary?

- How is this different from physically removing the two outside tubes from the amp as many of us do, (impedance wise?) I did note you said ideally.

[olddawg]

I would say think long and hard before embarking on this adventure. (Replacing stock transformers with boutique equivalents? Disconnecting tubes?) You will probably will not turn a profit and the customer will not be happy in the end. I would educate the customer and would suggest that he simply buy a smaller amp or a power soak with the smaller amp the preferable option. There are no magic bullets. He's gotten some bad advices from someone.

[Chuck H]

It's just the same as removing two tubes. So the mismatch is 100%. In the case of removing two tubes the remaining two running into half the primary impedance that they should be. Since transformers work on ratios you can double your speaker load to present double the primary impedance to the tubes. Thereby correcting the mismatch. Alternately you can change the impedance switch on the amp to half of the speaker load. As in a 4 ohm impedance setting into an 8 ohm load. Now you don't need to mess with the speakers. The only rub being if your using a 4 ohm cabinet, because the Marshall impedance switch doesn't have a 2 ohm option.

Since extra money is being spent on special transformers it's a good idea to observe correct impedances. Just pulling two tubes or disconnecting two tubes doesn't do that. Even if it is common practice.

[jamesmafyew]

The 100W JCM900 series amps had a "high/low" power switch on the back, which was actually a pentode/triode mode selector. So power would be cut down to about 25W, without needing to flip the impedance switch or remove tubes. Most people hate how it sounds though.

The JCM2000 TSL model (as opposed to DSL) has a "Virtual Power Reduction" switch on the front panel. It inserts a 680K resistance in series between each of the .022 phase inverter coupling caps and their respective power tube grids. What this does is form a voltage divider with the 220K grid leaks (or 'bias feed' resistors if you prefer), and prevent the power tubes from ever breaking up -- the phase inverter runs out of headroom at about 25W total system output. The large series resistance also interacts with the power tubes' input capacitance to knock off some extra high end, which incidentally sounds to many people more like "cranked" tone. Some people hate the VPR switch, some people like it.

Point is, this 'feature' has been offered on other modern Marshalls and in no case does it actually remove tubes or switch impedance. You might ask the customer for some clarification about where his inspiration for the half-power thing came from.