First attempt at design, suggestions please.

[distorted_frequency]

i want to design a power amp with a presence and volume knob. i plan on using it with a ADA mp1 pre. i hope it will have a 12ax7 input/preamp stage, 12ax7 phase inverter and 2x6V6s. tonight I started reading into all the tube theory and i hope i correctly designed the input/pre stage. i attached the drawing. hints, tips, suggestions very welcomed!

Rob

[drlowlow]

The 33K resistor should be between the signal contact of the jack and the 1M resistor. I think 68K is a more appropriate value instead of 33K.

You might want to check the values of your cathode and plate resistors as they are very uncommon for guitar amps. I would recommond 100K for the plate resistors and between 820 to 1.5K for the cathode resistors. These are common values that worked well in many different amp schematics.

You will need to add a bypass capacitor between the two amplifier stages.

I recomend that you move the volume control to after the bypass capacitor from stage one to stage two. I think 1M would be more appropriate and you will need to wire one side of the resistor to the output of the bypass cap and the other side to ground (the way you have it drawn up now it is just a resistor). The wiper will then be conected to the grid of the second triode. If you leave the way it is set up you will need to wire it posterier to the bypass cap but I think you end up with alot of distortion as you are really pushing a big signal into the second triode.

Don't be afraid to look at schematics http://www.schematicheaven.com/index_HTML.htm
from amps that are proven to work. the truth is that there are truly few topographys that work. There are a few ideas that are genuinely original but you need to master the understandings of how the circuit works before you are going to get to those. the best starting point is to take a proven design and alter it to meet your goals. There are plenty of amps out there that are based on your tube complement or similar, the most obvious one is a Fender Deluxe. Good luck.

[tubeswell]

33k is alright at the start (33k is basically what you get when the 2 x 68k in a typical fender input are in parallel). It is also okay to have the 33k and the 1M wired the way around shown in your schematic. That way the 33k acts as a grid stopper instead of being part of a voltage divider. A single 33k this way will probably work best if its connected right on the grid pin (pin #2) of V1A. You will need a coupling cap between the plate of the first gain stage and the grid of the next gain stage to stop DC from enterig the following grid. 1nF - 4n7F is a typical range for this cap - 2nF is commonly found in fenders. It will need a 400V minimum rating. You don't want DC on the grid, you only want AC. You will also need a coupling cap after the plate of the next gain stage (and indeed after the plate of every gain stage -unless you're doing a direct-coupled pair CF stage) for the same reason (I think this is what the previous poster was referring to as a 'bypass' cap).

560R is probably too low for a cathode resistor, you might find the signal cutting out when the amp is driven hard. 820R is a better minimum value. (Ideally you need the grid slightly negatively biased with respect to the cathode. If the cathode is too close to ground potential, you'll get the signal cutting out when a bigger voltage swing is applied to the grid). 1.5k in combination with a 25uF bypass capacitor is a typical fender value that gives a nice full sound with single coil p'ups. a 1k/.68UF combination is what you typically find in Marshal amps for a more midrangey sound.

Like the previous poster says, 100k is the ideal plate resistor value for a 12AX7. If you want more gain, you need to increase the plate resistor value and/or decrease the cathode resistor value, and or, reduce the buffering/attenuation between gain stages.

In addition, unless you have some attenuation/buffering between the gain stages, you might find the signal onto the grid of he next stage contains a little too much unwanted extraneous noise (like hiss and other weird noises). You can make a voltage divider here to achieve this attenuation by putting a 390k - 470k resistor in series in the signal path before the grid of the second gain stage (but after the coupling cap I said to put in earlier), followed by something like a 470k - 1m resistor to ground. You don't need to worry about losing signal strength - with two or three gain stages before the PI or output stage, you should have plenty of signal to drive the output stage nicely.

[d95err]

Since you'll be feeding it from a preamp and not from a guitar, the 1M input impedance is not necessary. Lower input impedance will result in less noise. Something like 33k-100k would probably work fine.

EDIT: Wait a minute - two cascaded gain stages before the phase inverter? This will generate lots of distortion, especially if it's fed a line level signal (I'm not sure that even one gain stage is necessary if you're using a typical long-tail PI). I think you're having way too much gain here.

[distorted_frequency]

I appreciate the information, especially the "Why" things are done. i tried to avoid looking at schemmys because i wanted to think and not just copy.

keep in mind this is only for a Power Amp, i will be running a ADA MP1 Pre Amp into it. i figured i would still need one tube to buffer the signal between the pre and the Power section. i looked at some schematics on schematic heaven and every power amp have at least 1 or 2 pre tubes at the input. So i do i keep this or discard it and start my circuit straight from the PI. what do you guys recommend for the PI if thats the route i go? also if i get rid of the buffer how do i get a presence knob in the circuit?

also i was wondering about using a Single KT88 for my Power section. will it sound as nice as a push/pull pair, but less wattage?

thanks for the help!

[d95err]

Typical standalone poweramps start with a volume control, then a tube gain stage, then phase inverter and power tubes. If you have the tube gain stage before the volume control, you will distort the signal hard if it's a line-level signal. (I don't know what the ADA MP1 is - is it a rack preamp or a 9V stompbox type of device?).

If you want to be able to accept both line- and guitar level signals, I suggest having two separate inputs. One that starts with a cathode follower (line level), and one that starts with a gain stage (guitar level).

A single ended (SE) poweramp will sound different from push-pull. If it's "nice" or not is a matter of taste. SE will have less power and more hum. Since an SE poweramp does not have a phase inverter, you may have to add an extra gain stage to drive the powertube properly. In this case, your dual gain stage design might work (if you put a volume control before them), with some changes. E.g. you may have to put an attenuator stage between them to avoid excessive gain.

Another question: What is the purpose of the variable resistor in series with the signal on your schematic?

[distorted_frequency]

Typical standalone poweramps start with a volume control, then a tube gain stage, then phase inverter and power tubes. If you have the tube gain stage before the volume control, you will distort the signal hard if it's a line-level signal. (I don't know what the ADA MP1 is - is it a rack preamp or a 9V stompbox type of device?).

If you want to be able to accept both line- and guitar level signals, I suggest having two separate inputs. One that starts with a cathode follower (line level), and one that starts with a gain stage (guitar level).

A single ended (SE) poweramp will sound different from push-pull. If it's "nice" or not is a matter of taste. SE will have less power and more hum. Since an SE poweramp does not have a phase inverter, you may have to add an extra gain stage to drive the powertube properly. In this case, your dual gain stage design might work (if you put a volume control before them), with some changes. E.g. you may have to put an attenuator stage between them to avoid excessive gain.

Another question: What is the purpose of the variable resistor in series with the signal on your schematic?

MP1 is a rack pre. i wanted to try to fit this power amp in a 1 space and make a really small semi-quiet practice rig.

im not sure about the VR in the signal i figured VOL. my lack of experience and misunderstanding of knowledge is making this quite the task.

[drlowlow]

it is possible to fit pre-amp tubes in a single rack space but I think it is strongly not recomended to do it with power tubes. I would error on excess room for the tubes to breathe, don't want thermal cookdown.

[distorted_frequency]

As long as i design the enclosure around that i don't think i will have a problem. there are a couple company's making single space power amps(marshall, mesa) out there and i have never heard of thermal cookdown problems.

[Don Moose]

Another (odd) idea depends on the front-to-back depth of the MP1. If it's relatively short, you could either stand the big bottles up, or have the power amp chassis get a bit taller (a whole rack space even, to make some breathing room.

The Mesa 20-20 and Marshall EL-84-20/20 both use EL-84 output tubes, which aren't much bigger around than a 12AX7 - it takes a 9-pin socket. The octals are rather bigger and the KTxx tubes are extra wide.

[distorted_frequency]

Another (odd) idea depends on the front-to-back depth of the MP1. If it's relatively short, you could either stand the big bottles up, or have the power amp chassis get a bit taller (a whole rack space even, to make some breathing room.

The Mesa 20-20 and Marshall EL-84-20/20 both use EL-84 output tubes, which aren't much bigger around than a 12AX7 - it takes a 9-pin socket. The octals are rather bigger and the KTxx tubes are extra wide.

i thought of this, it may work because my idea was not to house the MP1 and power amp in a conventional rack case. i was going to build a head shell(23"x11"x8") and permanently attach the power amp in it, thus leaving the possibility to change out pre amps. most 1 space preamps are under 10.5"(ada mp1, engl e530, marshall JMP1; i checked). so i could swap for almost any 1 space pre. similar to that tech 21 or atomic power amp thing for digital modelers but; tube and in a head shell!

how does a SE power amp vs a PP change the tone? im not sure which i would like more as i have only ever owned PP. ill probably just do el84s so i can get it in a 1 space. all pre outs are line level i believe.

so as a recap before i go to the drawring board: VOL->Gain Stages(1 or 2)->PI->Powertubes

[distorted_frequency]

Ok, so I spent more time studying schematics and heres what i came up with! I still need to do a power supply and the OT wiring but i hope to get this part correct before moving on. I decided on 6v6 because they're small enough to fit in a 1 space. once again criticism wanted and appreciated.

thanks,

Rob

[Bruce / Mission Amps]

Ok, so I spent more time studying schematics and heres what i came up with! I still need to do a power supply and the OT wiring but i hope to get this part correct before moving on. I decided on 6v6 because they're small enough to fit in a 1 space. once again criticism wanted and appreciated.

thanks,

Rob

What is that 56K resistor supposed to do... being grounded?
I see some issues.... check my attached file

[distorted_frequency]

its supposed to go to the tip of 4ohm output on the OT. its on the schematic for an amp kit i built, and references for the PI and power section (i also referenced a Mesa 2:90, Marshall 9100, and the power section of a JCM800) i haven't seen it on any other amps so i will probably omit it as I see no reason for it.

[Bruce / Mission Amps]

its supposed to go to the tip of 4ohm output on the OT. its on the schematic for an amp kit i built, and references for the PI and power section (i also referenced a Mesa 2:90, Marshall 9100, and the power section of a JCM800) i haven't seen it on any other amps so i will probably omit it as I see no reason for it.

OK... I was being rhetorical... there is no reason to not use some negative feedback. It is used in most every guitar amp made these days. Yes, you'll find amps with no NFB.
The 56K is your "NFB resistor", and with the NFB resistor grounded, as drawn in your original schematic, it will do nothing because the NFB voltage can't go anywhere... it's grounded out.
That's why I crossed off your ground lead and drew in a little 2K7 resistor under your phase inverter circuit.
This is a very elementary description so bear with me... the two resistors, the 56K and the 2k7, now form a voltage divider.
The result is that the 56K resistor is not grounded and it feeds a little, sampled AC signal from the OT, back into the phase inverter, ... hopefully at most frequencies, 180 degrees out of phase, ....shutting the gain down to eliminate some distortion products.
I think if you recheck some of the other amps you mentioned you'll see that the 56K resistor is not grounded at the phase inverter or anywhere else.