I wasn't sure what I wanted this to be, so I built a mongrel using what I have and now I'd like to change it to suit me better. Here is the basic design with a few changes annotated in red.

- I eliminated the single input. I didn't reference both sides of the preamp tube to ground. I only used a single 1M resistor. I'm pretty sure I don't need it, but I'm not entirely sure. I know that most schematics have that resistor but I'm not sure what it does, or what happens if you don't include it.

- I had an output transformer that is about 4.5k Zout, so I used that

- I had some worn-out 6L6 tubes that I had leftover from the previous build. I also used a higher value cathode resistor. 240 rather than 150.

- I had a bridge rectifier and a power transformer that I knew would get me up to about 305 VDC so I used that.

- Everything else is per the schematic.

- The pre-amp tube is a brand new Groove Tube Gold (JJ) ECC83. The PI tube is a NOS Raytheon that I know is working reasonably well.

- The worn 6L6 tubes are 6l6GC STR430 from a Mesa amp of some sort.

- I haven't biased the amp properly yet. The reason for that - I'm going to order a Weber Copper Cap and have switchable rectifiers. Sag and No Sag. I just thought I should point that out because I'm not sure how poorly it might be biased.

- Finally - I don't have a proper cab for it yet. I've been hooking it up through an 8" Celestion speaker in the box of a cheap solid state practice amp. Specifically it's Celestion Super 8 Red label.

Now here is the main problem so far. The amp sounds darker than I'd like. I want to have a little bit of sweetness and some more high-end. It's also boomy.
I went to the store and listened to the Marshall DSL40C and I think that's more along the lines of what I was hoping for. I'd like it to sound better than that to be honest, but I don't know what I'm capable of doing on my own.
I'm looking for a plan of attack. I would be happy to replace the output tubes with EL34s (the PTs can definitely handle it)
I don't know what to do about speaker. I've never had a cab before. A 4x12 is not a possibility. A 1x12 is probably the best I can manage, but not right now. Maybe as a Christmas present to myself.

Any ideas or advice would be appreciated. Thanks for reading.
you could try reducing the size of the bypass capacitor at the input stage.

the input resistor serves to filter out radio frequencies and parasitic oscillation. You need one, but not that big. from the looks of it, you have the input stage in parallel, meaning you should have halved that resistor to around 470k or used two 1M resistors in parallel. The grid resistor also serves as a low pass filter of sorts because the 12AX7 has a very small miller capacitance. The higher the the resistor used, the lower the frequency response.
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Thanks bustapr. I've got a lot of progress to report. I found a good matched pair of 6l6gc tubes. Amp is fairly loud and not nearly as dark.
I think I want more gain, so I'll probably be cascading the first two stages together and I guess I'll need to add a gain control.
Some ideas for you to get more gain and some less low end.

You mention cascading the two tubes rather than using them in parallel, good for more gain.

To keep the bass from becoming overwhelming I would look at using a smaller, or no, bypass capacitor on the first stage.

I usually like center biasing the first stage, the typical 100k plate 1.5k cathode or something with that ratio would be about center biased. The higher your plate resistor the higher your gain, to a limit.

I'd add a 68k resistor to the input tube just to cut out and radio frequency noise

Gain control is simple, basically a potentiometer after the coupling cap off of the first tube stage.

If you want the gain control to not turn the volume off completely just put a resistor in series with the lower half of the potentiometer to set a minimum gain.

If you want more high end you could bypass the plate resistor of the second stage with a small value capacitor. I wouldn't do it at the first because it might make the amp too bright.

You can adjust the 0.01uF coupling caps to the power tubes to adjust the bass response as well

I would suggest ordering a few different capacitors and resistors so that you can tweak the circuit to your taste, play it for awhile, and then tweak it again.
I think I might just take all of your ideas to start with, Xgamer.

I'm learning a lot. The tubes that I found that worked are NOS GE 6L6GC. I didn't know much about them and I thought they were all worn out. I was wrong. I did a search on those tubes to read what people thought about them and the words that people were using are very similar to what I'm hearing.
Oh I messed up. I forgot to mention that what I'm hearing since I put the GE tubes in is very different from what I was hearing when I had the other tubes installed. Those Mesa tubes were reading 12mA and 3mA when I did the shunt current check. The GE tubes are 42mA and 42mA.
The Mesa tubes, at least one of them but probably both, are bad. If your going to tweak the amp for a bit I would suggest getting a set of JJ or other tubes while you work and make sure the amp is okay. Once you get to a point where it works with the JJ tubes you could throw the NOS tubes in there. I've burned up too many expensive items.

Do you have a light bulb limiter? If not you may want to build one to help save transformers and other expensive items.
I don't think I've ever heard off a light bulb limiter.

I showed the amp to my wife and she was pissed about the volume. I need more gain so that I can get to it at lower volumes.
I think I'd like to start by buying a pair of 6v6 tubes, and yeah those should be used while I'm working on the circuit.
I'm definitely going to cascade the first two stages. I think I'll also have a lower B+ once I get my new rectifier installed.

I have about 4500 ohms of Zout. Should I take any special precautions with 6v6 tubes, since they seem to like a slightly higher Zout?
A light bulb limiter allows you to power on an amp and protect it in the event that you have a short somewhere. You wire up something so that you have a light bulb in series with input power to the amp. When you first turn the amp on the light bulb will be very bright and then dim. If it remains brightly lit then you have a short in the amp somewhere. You want something like a 40W bulb

Something like this: http://www4.picturepush.com/photo/a/10715222/1024/Anonymous/AmpCurrentLimiter1.jpg

Do you have the part number of the transformer so that I can take a look at it? I can see how you should adjust your circuit to work with 6v6s if it is necessary at all.
The OT came from a Bogen CHA33 that I stripped down, but I don't think it was the original xfmr even so. On the primary it has blue, red, brown. On the secondary it has two different taps that are each a different shade of green, a black ground wire, and I think the others were wh/black, yellow/red and maybe yellow? (Trying to remember) It had 6l6 tubes installed in it but I ran out of leads in my search for info on that OT, so on advice from a friend I applied 120 across the entire primary, measured secondary voltages and used ratios and speaker assumptions to figure out which windings would give me an impedance suitable for a 6l6 power amp.

Near as I can tell, I've got 4, 8, 16 ohms, 170v and maybe a 2 ohm winding? It's impossible for me to say.
Alright, I have an idea of what that OT might be. I'm not going to be near a computer long enough to run the numbers tomorrow, but I'll try to get back to you sometime Tuesday.

I vaguely remember working on one of these amps before and the OT being 5200 ohms rather than 4500, but that is easy enough with the schematic of the amp.
Alright, the laptop I normally use to run my simulations of an output stage isn't working for some reason. I've got the software downloading to a different computer so I'll work on those numbers today and tomorrow.

Should have something after work tomorrow with graphs and numbers.
Quote by XgamerGt04
Alright, the laptop I normally use to run my simulations of an output stage isn't working for some reason. I've got the software downloading to a different computer so I'll work on those numbers today and tomorrow.

Should have something after work tomorrow with graphs and numbers.

That's cool.

I implemented many of the changes that you suggested.
Then my filament transformer blew and I learned something new about the way transformers are rated. I screwed up.
I patched in an external 6.3V filament voltage and tried to play through the amp, but there was almost no volume.

At first I thought that maybe I needed to "re-simplify", so I got rid of a few of the changes I had made (such as the input resistor). That didn't work.
Eventually I realized that I'd made a mistake. I had installed two 100k resistors in parallel in the place where I used to have a single 120k resistor. I guess I must have screwed up all of my voltages. I put 70k of resistance in the circuit (ahead of the plate resistors of course) and tried again. Success.

Now I'm waiting for my new filament transformer to be delivered, but I have one question for you in the meantime.

What size of resistor should I use for my "minimum gain" resistor? The one that's in series with the bottom leg of the gain pot?
I picked a 100 ohm resistor for no particular reason. It almost worked. At full CCW I get some relatively clean sound. As I turn the gain up, at about 8 am on the gain pot the sound DECREASES down to an almost silent, staticky mess. After that point it starts to get louder and more distorted. I don't know what I screwed up.
Looking at the numbers so far your going to be limited to how many different output impedances you can have if we use 6V6 tubes. The load lines for the transformer you have are far too steep to get any real power out of a 6V6. That isn't a problem though, because we can adjust the transformer impedance somewhat. If we hook up an 8 ohm speaker to the 4 ohm tap then the transformer impedance now becomes ~9k which we can work with.

If you wanted to use either 6L6 or 6V6 then we'll have to do a bit more to make sure that the biasing works for both of them easily, but it is something that can be done.

I can draw up a schematic that shows how I would hook the stages up and use that to illustrate how to add the minimum resistance for the gain stage. It likely won't have all of the values populated and be more to see how it could all be connected.
I still don't know how to read the charts in the tube data sheets. Or did you say "for that transformer"?

I am planning this build to be capable of as many different set-ups as I can, within reason. I'd like to be able to run EL34s as well.

I think I had a misconception about 6V6 impedances. I thought they were about 5k plate to plate. Then I thought 6L6 was about 4k and EL34s were 3.5k. You said 9k and I was thinking "that can't be right" but then I looked at the tube data sheet and it specifies 8k.

But you're talking as if the load lines tell a more complete story. I'm interested in that.