Vacuum Tube Power Amp Input

[MrCayman]

I would greatly appreciate help from the brilliant minds on this forum to educate me on this topic. I've successfully built a couple of guitar amplifiers over the past year and for my next project, I'd like to undertake construction of a push-pull vacuum tube power amp for use with guitar preamps (Marshall JMP-1, Friedman IR-X and future preamp projects). I'm thinking the Minimalist 18watt drawn by Mark Huss would be a ideal starting point (see image below). My questions include the following:

1) If I build the power amp portion of the design, starting with the highlighted volume control, how likely am I to experience issues with input voltage or impedance mismatching?

2) Would it be advisable to add a low gain tube stage -- perhaps a 12AU7 or cathode follower -- at the beginning of the signal path to ensure the power amp can handle a wider variety of input signals? I ask because I've noticed the Marshall EL84 20/20 and similar power amps include this additional stage.

3) I would think it's advisable to keep the 22nF coupling cap after the volume control to block any incoming DC voltage, but I don't always see that included on power amp schematics (if I recall correctly). Any downside to including it as drawn, even though I'm adopting the circuit for another use?

4) Any other recommendations for making this project a success?

Thank you all very much for your help!

18minwatt.jpg

[pdf64]

It all depends on the designer's goals.

Usually, these kind of power amps are expected to be operated clean and be tonally fairly neutral, with the vast majority of tone shaping and overdrive coming from the preamp unit.

Is that what you want?

Or are you wanting the power amp to shape the tone significantly, and perhaps overdrive it heavily?

DC blocking caps are always needed to couple together stages that are at different DC levels. Could you provide links to any schematics you've found that don't seem to do that?

FWIW my experience with preamps and power amps back in the 90s is 'everything sounds better through a vintage Marshall'.
I ended up building a pair of JTM50 based Marsall circuits in a 19" rack chassis.

[LOUDthud]

The 22nF cap connected to pin 2 of the phase inverter is needed because there is DC on that node. The gain of that power amp is pretty high because there is no global negative feedback so the extra stage is not needed. You will probably find that the Volume control is useful

[MrCayman]

Thank you pdf64 for your insightful questions. I do love the growl and sustain of classic Marshall output sections. So, a more transparent power amp isn't what I'm going for in this design.

I see the Marshall EL84 20/20 power amp circuit begins with an ECC82/12AU7 input stage (see attachment). Similarly, the Mesa Boogie and Peavey 50/50 both begin with ECC83/12AX7 input stages (see attachments below).

Am I correct in assuming the purpose of this triode stage is to improve impedance matching and ensure the power amp can handle a wider variety of signal strengths from various preamps and effects units it will encounter? If so, I'll include it in my build.

I think what's confusing me is that some people reportedly plug external preamp outputs directly into the effects returns of amps. Furthermore, some amps have buffered effects loops while other designs simply add input and output jacks between the preamp and PI sections (see Steve Luckey's AX84 November Project, schematic also attached below). I'm not knowledgeable enough yet to understand the advantages or disadvantages of these different approaches.

Finally, as I review these schematics, I clearly see the coupling capacitors before each PI stage. I must have been recalling the triode input stage of these power amps which don't begin with a coupling cap. Sorry for the confusion.

Thanks again!

[MrCayman]

Thank you LOUDthud for your response. Good points! The addition of a global negative feedback or presence control would be something I would want to add to this design. You're all giving me lots to consider.

[mhuss]

Haven't seen that schematic in a while, lol.

Best approach is to breadboard your proposed circuit and tweak it until it sounds good to you.

[TUBEDUDE]

You could compicate things with transformer coupling, if you just have a spare one.