What are the pre-amp limitations for a EL84 or 6V6 PP power section?

[drfunkphd]

Hello guitar amp gurus,

I am trying to design an amp that has a push-pull power section and wanted to know what limitations do I have using EL84's or 6V6's?

I want a pre-amp section that I can drive a KT66/6L6 or EL34/6550 power section with, too, but am looking to build a solid and simple front end that I can translate to lower and higher wattage applications.

Is it really more or less a matter of adjusting/using different PT's for output voltages appropriate to the back end or do I need to adjust the amount of signal coming from the front end or is it a combination of both?

I know that EL84's and 6V6's want to see less voltage (in the 350-375V range) and I realize that they require power trannies with less voltage, but if I were to put the same pre-amp circuit in that amp as one with a PT designed for 6L6's or EL34's, would I need to limit the pre-amp signal going to the power section of would the decreased voltage from the PT compensate for this?

I am thinking I will have to change cathode resistors to keep the gain the same, am I right? Or am I looking at changing a lot of values to make the preamp sound similar with increased or decreased B+?

Thanks for any and all input.

[drfunkphd]

Some further reading has led me believe that the secret may be in the phase inverter. I should still need to compensate for any changes in B+, too, right?

[tmod]

Someone else will probably chime in with spesifics, but generally you have three things to consider: bias, anode voltage, and signal swing. All three can pretty easily be put on switches. Generally, smaller tubes want smaller signal swing, less anode voltage and need less bias. The easiest way of dealing with the last is to use cathode bias, and switch between different values of cathode resistors. Anode voltage can be dealt with by either using a PT with dual primaries, or with voltage dropper circuit. Of the top of my head I would use two parallel cap multipliers set up for different B+, but others might have better ideas. Adjusting signal swing isn't hard, but may take a bit more thinking to make adjustable. When I use smaller bottles for PA with LTPPIs I use tapped anode resistors (ie 2x47k instead of 100k, take signal from the junction for half signal voltage). Easy to use a DPDT to choose between two different signal levels (remember to put the coupling cap before the switch to avoid swithching dc if you plan on ever throwing the switch while on), a bit more involved if you want many.

[GregS]

You probably will have to adjust the amount of signal coming from the front end as the larger power tubes do require larger signals to drive them. For El84's or 6V6's you will probably have to attenuate the output of your pre-amp somewhat. Otherwise you can be at full volume and overdriving the power amp with your volume control only on 1 or 2.

Greg

[drfunkphd]

I guess if I placed a master volume either pre-PI or post-PI, I could probably achieve that to some extent.

[GregS]

That's what I have done with my 6V6 build - put in a master volume pre-PI. Originally I had the main volume control before my final pre-amp stage and would be at full volume and power amp distortion with the volume at about 1. It was hard to back it off to get a clean sound before it just cut out. The master volume really improves the situation.

Greg

[tmod]

I guess if I placed a master volume either pre-PI or post-PI, I could probably achieve that to some extent.

You could do that. A master volume simply reduces the volume to the PI or output tubes. However, if you want the amp to sound more or less the same with either types of tube I would use switched presets, or possibly both switch and master volume. The reason is that a master volume is a pretty imprecise intrument. You might have trouble finding the same range of tones with small bottles as with larger bottles.

If you design the amp around, say EL34s, decide what percentage of the signal swing you would need to drive, say, 6V6s equally hard. Split the plate resistors in the PI to this ratio, and then use a master volume for fine tuning. By using the hi/lo switch you'd get approximately the same level of PA overdrive with the master in the same position regardless of which of the two output tubes you decided to use on that occasion. To me that's much more intuitive. Also, remember that you'd need some switching anyway for bias, etc. You could easily get a rotary switch to move all the presets with just one know change.

You should also be aware of the different drive requirements for different tube types of similar "size" (ie 6L6 vs EL34, 6V6 vs EL84). Particularly EL84s need much less signal than the other common types...

[drfunkphd]

Well, through experimentation and education, I got it figured out. I decided that it's better to design the PI around the output tubes, not to mention the power supply and pre-amp, than try and make an omnipotent "takes all tube types" kind of amp, which may mean building another amp...not really that upsetting.

Throw another design on the "to do" pile.