Hi/Lo Gain Switch on DC30 Clone?

[nee]

Greetings all! I'm new to the board but have learned much through reading the information here.

I am considering adding a "Hi/Lo Gain" switch onto my Matchless DC30 clone. The plate resistor on channel 1 (12AX7) is 220k, and on channel 2 (EF86) it's 330k.

What I thought I'd do is fit a switch which would throw 2 x 220k resistors, one for each channel, in parallel with the plate resistors, bringing the resistance down to 110K for channel 1 and 132K for channel 2.

Is there anything I need to be aware of that might potentially cause damage by doing this? Would there be switching noise and how could I alleviate it?

I've also thought about making the power tubes (EL84) switchable between pentode and triode. Is this a matter of simply inserting a switch in the correct position, or is there other circuitry involved?

[Andy Le Blanc]

anything involving B+ volts will create noise...... pop.... scratch
its the different potentials on switch contacts....... a gain boost or cut can also
be accomplished by switching diffent values or types of bypass caps in the
chathode circuit.... the voltages are much lower and you can use diffent cap
values to voice the gain stage responce..... on the trainwreck side of the
forum theres is a neat way to use the plate circuit..... a fast \gradual mode
that switches between coupling caps at differnt taps on the plate resistor....
make sure you dress your amp with safety in mind .....
if your voltage shy, mod the chathode side..... increasing the bias resistor
reduces gain in general terms .... switching between a couple three
arrangments of resistors with or without a bypass cap or between different
caps is pretty easy without any complicated parts of wireing.....

I like the detail that a triode or triode wired tube can give..... for HIFI....
but that detail never seemed to suit my expectations for guitar...
I'd wire it in and play it for while before makeing it permanent....

[dartanion]

Remember that the DC-30 has parallel gain stages using the 220K shared between plates. It is already a fairly low gain stage which is plexi like in nature. One easy way to add a high/low gain switch would be to play with the voltage divider directly after treble pot and before the FX loop jack/PI. This voltage divider is dumping gain before the PI. Vary the amount of gain being sent to ground with a switch or pot for high/low gain.

On the EF86 side, you can mess with values there too, but pentodes are a bit different beast and the different variations that I have seem on this channel are all pretty similar. Again, in this channel the volume pot and the 470K resistor are acting as a voltage divider too, so you can play with that.

Too often there are posts about increasing or decreasing gain and making everything switchable or adjustable, it's a tweaker's nature Some other simple solutions, use lower gain tubes in the plexi like channel. 5751 would be a good alternative or 12AY7 or 12AU7 for even lower gain. As well, signal attenuation is often over looked when voicing an amp. Cascaded 12AX7 channels most often dump gain between stages using voltage dividers, and tonestacks suck gain too. So these are good places to look for possible changes.

[nee]

Thanks for these replies - they make much sense.

Rather than just reduce the overall gain of the front end, my idea was to reduce gain right at the very front of the amp so the PI and power tubes are hit with as much of an undistorted signal as possible. I figured that reducing the plate resistors on the input tubes would be the ideal place to do it: I want to make it difficult to drive the preamp tubes into distortion.

I should say that I love the way this amp sounds straight out of the box, hence the thought of making the option switchable.

[dartanion]

I would suggest trying a 12AY7 in place of the 12AX7 for the paralleled triode stage. That might just give you the right balance of clean to dirty on that channel. If too clean, try a 5751.

[Wayne]

I haven't actually seen the schematic (maybe later when I've got time to look for it), but just at a glance - if the amp has parallelled triodes at the input, why not just make the drive signal to one of them switchable? Grounding one grid should provide half the gain.

W

[Wayne]

Dbl

[dartanion]

You can certainly do that Wayne. I did a parallel/single triode switch on some amps, and these really isn't that much of an effective gain boost with parallel triodes. With that said, I hardly ever switch it to single triode mode. The biggest gain jump is cascading the paralleled triodes into the next triode.

[nee]

So I take it that making the plate resistors switchable isn't a common method of modding an amp! I found this thread which deals with a similar scenario:

https://tubeamparchive.com/viewtopic.php?t=3851

Thanks for all your help everybody!

[nee]

Double post.

[dartanion]

So I take it that making the plate resistors switchable isn't a common method of modding an amp! I found this thread which deals with a similar scenario:

https://tubeamparchive.com/viewtopic.php?t=3851

Thanks for all your help everybody!

Yes, you can change plate resistors, and this a common modification. In the linked thread, they're talking about Dumbles which are a whole different beast than a DC30. You can go ahead and do what you plan on doing. You'll have your work cutout for you keeping it from making a loud "POP" when switching.

If you are an advanced builder, it would help to state so much when you only have 3 posts. I think we all tend to approach new folks with kid gloves. No offense intended if you took it that way. Although, the advice given so far is useful.

[nee]

No offence taken whatsoever. I'm pretty new to amp-building, so any advice anyone is prepared to give is most gratefully received. Considering the high voltages inside an amp chassis it's wise, I think, to assume that a new member has limited knowledge.

Like most (all?) the people on here I'm just intrigued by what can be achieved by merely swapping a couple of leads around, or changing this component for that one. We of little experience have nothing but admiration for you guys - you've probably forgotten more about tube amps than we'll ever know!