Adding a preamp?

[Tekmek]

I am looking at using this schematics:
DIY Push-Pull (PP) 6V6 / 6V6GT Tube Amplifier Schematic

but I would like to add a preamp, maybe input selection, volume and tone control.

Can I add a tube preamp to this amp without altering the output quality?

How does one go about planning a preamp?

thanks for any advices!

[indianajo]

This place in Canada has PCB's for a clone of pcb's for the dynakit pas2 or 3. 4 ea 12AX7's. The switched tone controls are not available, I suppose you might be happy with a PAS2. (I am) Be aware the actual amp has a steel box enclosing the high gain wires & parts from the 60 hz power supply, then it all is enclosed in another ungrounded steel box. Ground comes from the RCA jack rings to the power amp. Schematic is at various places on the net as the PAS3, do a search.

[Tekmek]

I was thinking of adding something very simple in the same chassis as the amp itself, ain't that a good idea?

[ticknpop]

The power supply will likey not be large enough to power a tube preamp - especially the heater current needed and do you have space? solid state fet or opamp would be smaller , but the voltages are probably wrong in your amp. A passsive volume control might fit and needs no voltage or current IF you have Enough GAIN in the power amp - try a cheap 100 or 250 k volume control with short cables near the power amp to see if you have enough volume.

[Tekmek]

Well...

as the project isn't started yet, I surely can manage to have ennough space...

how do we know (before implementation) if we'll have ennough gain?
I'd be happy if I can ommit preamp and still have volume & tone control, but somehow I doubt it.

Maybe I'll be looking at having 2 power transformers housed in the same chassis?

[indianajo]

Transistors are the wrong voltage for a tube amp transformer. You can buy as much tube transformer as you can afford. If hammond manuf doesn't make enough now, you can buy a used Hammond organ company H-AO-71 chassis with a big enough power transformer to run two 5AR rectifiers, 17 signal tubes and 6 power tubes (4 6bq5, 2 7591). People scrap H100 organs all the time. It has a 25 V winding if you are **** bent on transistors. Nothing wrong with transistor inputs, it is all in the details of layout, circuit, etc. My love of tubes has more with the way they take lightning strikes (so what?) and don't blow up speakers after a shorted wire the way split supply transistor amps can without $500 in protection circuits. I've had lightning hit the PAS2 and ST70, shorted the switch cap and the switch, nothing else. One H182 tube organ lived on top of a hill in Cincinnatti 42 years (a definite lightning hot spot), worked when they were tearing down the building around it.

[ChrisA]

Originally Posted by Tekmek

I was thinking of adding something very simple in the same chassis as the amp itself, ain't that a good idea?

Something simple should be very easy. But you said something about tone controls. That gets slightly into not-simple.

The simplest preamp is just one tube. A 12AX7 has two triode sections and is small and only uses 100ma of heater current. That will be the limit, the power supply needs to power the extra tubes. It likely can handle one more small tube, maybe two.

The way to plan this is FIRST figure out you power budget. How many small tubes can the current power supply handle.

[chrish]

Do you really need a 'preamp'? If you simply want to select various input sources and change the volume, you don't need a preamp. As ChrisA mentioned, it is the tone controls that become the problem. For what it is worth, tone controls may not be required. If your source and speakers are good, you don't really need it.

[Tekmek]

How about tone control the old-radio way, with only one pot/cap right after the volume?
Then I can do without preamp altogether?
Selection and volume shouldn't be very much more complex than that...

ticknpop mentionned something about having ennough gain.?

[indianajo]

You need about 50x gain for a magnetic phono cartridge or a little less for a dynamic mike. If you are not playing LP's, 78's, or recording live music, then 1.6v PP input sensitivity for full volume is fine. CD players, Radios, computers all put out that much voltage. Then an extra gain stage on your power amp could give you sharper slopes on the filters than passive filters, but not be extremely necessary.
When I'm using the tube ST70 amp, I mostly use a disco mixer (upgraded op amps and power supply) to drive it. That way I can leave volume up on all three, LP turntable (magnetic phono cartridge), FM radio, and CD player, and when changing from one to the other, not walk around the piano to switch the inputs. The tube PAS2 preamp lives in the attic, banished for its 100 times greater power use than the disco mixer. The tube ST70 is still the most trouble free amp I own. Both transistor amps have issues when the room temperature drops below 62 deg F- Both are waiting for me to construct a working sine wave generator and finish repairing a third transistor amp all over the coffee table.