# Dual amplifier guitar amp - tube & transistor - possible?

#### RogueRose

Joined Oct 10, 2014
375
I want to build a guitar amplifier and would really like to build it with both tube and transistor amplifiers. There is no problem with fitting the hardware into the box so that won't be a problem.

I think I would make a selector switch which would restrict power to one amp when the other amp is selected so that both amps can't run at the same time.

Is there any problem with the line in going to two different amplifiers - will it reduce the signal strength at all if this is done?

Obviously the speaker will need to be connected to both amps so I have concern about any feedback going into the amplifier which is not powered at the time. Is this s legitimate concern and if so what would be needed, a diode(s) or what?

I think it would be nice if the tone controll could be consolidated into one set so that each amp uses the same tone controls, IDK if that is senseable or not as the controls may need changed when switching between tube to transisotr. Either way, would it be possible for the build to use the same tone controls for both amps (I'm thinking bass, mid and treble).

Before I get into more detail here about size of speaker and wattage, I'd like to know if the above are possible as this may determine the size. I was originally thinking about using a small 8" driver for a practice amp but kind of have the urge to build something a little larger.

#### blocco a spirale

Joined Jun 18, 2008
1,546
This is possible, you can have one pre-amp/ tone control circuit feeding both power amplifiers simultaneously but you must have some means of switching the loudspeaker between the outputs of the two power amplifiers (they can't both be connected a the same time) and the tube amp should have a dummy load connected across its outputs when the loudspeaker is disconnected. Whether or not you decide to switch the power to the tube amp off when not in use is determined to some extent by its power output/ consumption, but it's easy enough just to add a separate power switch for it. The solid state amp will probably use very little power when not in use so could remain powered.

#### crutschow

Joined Mar 14, 2008
26,032
Note that the two amps require very different voltages, so two separate power supplies will likely also be needed.

#### crutschow

Joined Mar 14, 2008
26,032
This is possible, you can have one pre-amp/ tone control circuit feeding both power amplifiers simultaneously .............
Not if he wants one amp to be completely tube, and the other completely solid-state.

#### blocco a spirale

Joined Jun 18, 2008
1,546
Not if he wants one amp to be completely tube, and the other completely solid-state.
True, or perhaps he wants to build a hybrid front-end e.g. Solid-state eq and reverb with tube preamp/ over-drive stages.

#### #12

Joined Nov 30, 2010
18,216
Either way, I would start with a transistor impedance lowering stage. Hi-z input as a pre-amp, then tone controls, then drive the 2 power amplifiers. I have sneaked in op-amps for an audiophool that never figured out there were transistors in the amplifier. It can be done.

#### blocco a spirale

Joined Jun 18, 2008
1,546
Lots of guitar amplifiers that many believe to be all tube have solid-state front-ends but the marketing departments either keep quiet or invent a new name for it that implies tubes are somehow involved throughout. In fact, there are many fully solid-state guitar amplifiers that have names that would normally be associated with tube amps; Marshall Valvestate, for example. Most people who buy them know there are no valves in there (ok, some have a 12AX7 in the signal path) but would probably buy something else if they were called Marshall Transistorstate.

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#### RogueRose

Joined Oct 10, 2014
375
I have been looking at some tube amps and am amazed to see a 5w 10" tube amp for $500! that seems pretty excessive from what I have seen as other amps rated at 50/100 watts 12" are close to the same price. Can anyone help shed some light as to why such a power rating difference and how it correlates to heard sound/volume? I know I will need some pots for adjustment of the following. Gain Volume Master Vol treble middle Bass reverb I have access to some used Spectrol and/or Bourns precision pots for what I think is a fair price (about 2-3x price of standard generic chinese pots). IDK if it is worth the extra$ but the are supposed to be super smooth and solid. Does anyone know anything about these? These are 10K, 2K and 50K. Would these be useful in this project at all?

#### #12

Joined Nov 30, 2010
18,216
Can anyone help shed some light as to why such a power rating difference and how it correlates to heard sound/volume?
Study decibels and how humans hear. Check out speaker efficiency, too.
These are 10K, 2K and 50K. Would these be useful in this project at all?
You can design circuits to use what's available, but I think designing for the right sound and buying the right parts is the only way to go.
I have some 1K wirewound pots from 1973 and they are still flawless, but almost nothing I design needs them. The resistance is too low for most jobs and the high resistance pots are not made in wirewound.

#### Lestraveled

Joined May 19, 2014
1,946
I have been looking at some tube amps and am amazed to see a 5w 10" tube amp for $500! that seems pretty excessive from what I have seen as other amps rated at 50/100 watts 12" are close to the same price............. I agree, those kits are ridiculously priced. Buy a Fender Super Twin and be happy. #### #12 Joined Nov 30, 2010 18,216 those kits are ridiculously priced. Buy a Fender Super Twin and be happy. Or make your parts list, buy the parts, and pretend it's a kit. I made a version of a Fender Princeton by starting with a 1 by 10 plank. #### Attachments • 162.7 KB Views: 5 #### blocco a spirale Joined Jun 18, 2008 1,546 I have access to some used Spectrol and/or Bourns precision pots for what I think is a fair price (about 2-3x price of standard generic chinese pots). IDK if it is worth the extra$ but the are supposed to be super smooth and solid. Does anyone know anything about these? These are 10K, 2K and 50K. Would these be useful in this project at all?
Design it first and then think about what type of pots to use.

The power output depends on what you intend to use it for, if it's just for home practice then 5W will be more than sufficient but for live performance you will need at least 50W to be heard over everyone else.

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