Developing the DIY "Champ" for first time builders

[rp]

Yes, switchable tube vs. diode is what I'll do.

Get yourself one of these. Comes in handy, you can check your voltages and tone before making extra work for yourself modding. IME you're going to prefer one over the other most of the time and find you won't need a switch and all that.

http://www.mojotone.com/amp-parts/vacuu ... -Rectifier

[gktamps]

RP - the amp is already wired with an ss rectifier, so I'll have to add a tube socket either way. Would be great if I decide the tube isn't my preference, though - just pop this in the socket and go - no switch. I've got bare tube bases from other projects, so I'll fab one of these up. Thank you!
Greg

[rp]

RP - the amp is already wired with an ss rectifier, so I'll have to add a tube socket either way. Would be great if I decide the tube isn't my preference, though - just pop this in the socket and go - no switch. I've got bare tube bases from other projects, so I'll fab one of these up. Thank you!
Greg

It's a class A SE so you do know tube or ss won't make much difference but you can see ~40V more on the B+ and that will change the tone. Watch your plate current and IME if you stray too far from 470R you should skip the ss rec idea.

edit - oops just realized you are going the other way. So depending on your volatge now it might sound really good going a bit lower. So try a tube. A NOS 5y3 will drop a lot more than a modern one.

[Zippy]

Zippy - Yes, switchable tube vs. diode is what I'll do.
It's too loud for my small music/work room (and house) when turned up past 1 o'clock, so I've been playing it at around 11-12. My wife thinks it needs a much "bigger" knob than it has (= more effective).

As I mentioned, this is my test/learning tool, not my holy grail amp. With such a simple circuit, small changes seem to make a big difference.

I'll try different values with my capacitor substitution box to get some clues, then put in two caps - one on either side of an on-off-on switch.
Also, is there any advantage to altering the negative feedback resistor for this amp, or any cautions about doing so?

Hey Greg,

You're right about small changes having a big difference - at least an audible one. Similarly for the negative feedback resistor - over the course of history, the feedback resistor ranges from 22k to 33k to 100k and beyond (no feedback resistor). Let's say it's an "opportunity". That's yet another place one might use a switch, trimmer, or pull-pot to dial in, or remove entirely, the feedback resistor.

More resistance = less feedback = less "tame".

And while we're at it, why could this not become your "holy grail" amp?

[gktamps]

RP - thanks for the cautions. RJ spec'd the transformers for use with a solid state recto, and I measure 314 v rectified, so with a tube, B+ would be around 265 v, eh? I think I've got some NOS 5Y3s from my father.

Zippy - hmm. Push-pull pot is a brilliant idea. Thanks!
OK, since you asked about my "holy grail" amp... I look at guitars and amps as tools, and I like to have a lot of tools. I used to think one amp could do almost everything I wanted (a la Mesa), but I've been finding that some amps just do their thing very, very well, and have so much character they must be part of the toolbox. So, I suppose, this could become one of those really great tools, but my holy grail amp is an ever-changing target.
To that end, the sound I'm really digging is the Retro-king 18 watt amp, with PPIMV (despite the varied opinions on that), and switchable rectifier. I have been thinking of building something along those lines as a matter of fact... Then there's the Rocket...

Hmm. This could spell trouble.

Cheers,
Greg

[rp]

I really wouldn't bother with the tube rec given the already low B+, unless you just feel like messing around. Sounds done to me, time to enjoy the new amp.

I never a-b'd ss and tube in a SE amp but since you don't get sag all you are really playing with here is a ~265 and ~314 B+ and you'll also have a lower plate V on the preamp tube. You could just do this too with a reverse zener on the pt ct and avoid extra holes. There are plenty of posts here about it. Again I wouldn't bother, 40V less will likely make it browner and some less gain. At 300V Champs are brown enough and I doubt you want less gain. i initially thought you'd be around 390V like most 5F1s, and dropping to 340-360V with a 470R @ ~ 40mA sounds much better to me.

Try some NOS tubes - or ANOS from ebay. Get yourself a coke bottle 6V6G like I put in my 5C1 - sounds great, looks good.

[Zippy]

RP - thanks for the cautions. RJ spec'd the transformers for use with a solid state recto, and I measure 314 v rectified, so with a tube, B+ would be around 265 v, eh? I think I've got some NOS 5Y3s from my father.

Recall that the spec'd voltge drop is based on the max current-carrying capacity. An amp running fairly low current draw won't drop max volts. I'll be interested to learn your observed voltages.

This whole amp was designed in a modern day take of "What would Leo do?" regarding parts count, etc. You can read the early part of this thread regarding rectifier, voltage, and so on.

Zippy - hmm. Push-pull pot is a brilliant idea. Thanks!
OK, since you asked about my "holy grail" amp... I look at guitars and amps as tools, and I like to have a lot of tools. I used to think one amp could do almost everything I wanted (a la Mesa), but I've been finding that some amps just do their thing very, very well, and have so much character they must be part of the toolbox. So, I suppose, this could become one of those really great tools, but my holy grail amp is an ever-changing target.
To that end, the sound I'm really digging is the Retro-king 18 watt amp, with PPIMV (despite the varied opinions on that), and switchable rectifier. I have been thinking of building something along those lines as a matter of fact... Then there's the Rocket...

Hmm. This could spell trouble.

That's the spirit! Welcome to The Amp Garage!

[gktamps]

OK, then. Here's what I've been up to with this amp.
First was to bypass the 22k NFB resistor and temporarily replace it with a resistor comparison box. I ran through lots of different values, from 47 ohms to 1 megohom, amp about halfway up in volume. Then I tried a 100k pot. I was expecting this to act like a presence control, but that wasn't the effect. Compared to no negative feedback, volume increased with most levels of negative feedback, and sounded good with resistance values anywhere between 10k and 100k, but I couldn't discern much difference in frequency response so typical of presence controls. I see JR's design for the Supre version of this amp has a 10k NFB resistor, so I am simply going to add a 10k pot in series with a 10k resistor on the board and not bother with the option of removing NFB.
Then, I temporarily rigged a 5Y3 into the circuit with a switch, and AB'd the difference between the solid state and tube rectifiers. As predicted, volume with the tube was significantly reduced, as was overall attack and bass response. To me, it sounded a little bit like a tired old amp in need of filter caps - not really the tone I was looking for.
By the way, this little amp has plenty of bass response. Turn it down to about 1/3 and it's quite a clean little amp with nice tones. Bring it up and it gets a bit tubby sounding, but that seems to be what these little amps end up doing.
After doing these experiments I gathered the nerve to try to figure out why my 18 watt Marshall 1974X clone (professionally built and sold by the builder) doesn't give me the tone that seems to be typical of the amp, according to many sound clips, and experienced users on the 18watt forum. I had previously done some wiring cleanup and repairs to sort out some noise, and to tidy up the poor solder joints and change a couple component values, and re-tubed with NOS, and though that really improved the amp, I think it still lacks the magic it should have. So, I'm buttoning up the Eagle Premium, will make a cabinet for it, and will leave any future mods for a later date.
After I get the 18watt sorted out, I would like to build another amp - any suggestions?
Thanks for all of your help!
Greg

[RJ Guitars]

Hey TAG builders. It's been a while since I posted anything on this thread but I did build another simple Eagle that I wanted to share with you.

In this case I went with all Octal tubes and I actually changed the power transformer to reduce the voltage... there is more to that story as well.

It's very cool sounding especially clean. It's a little too buzzy if you dime it out and hit it hard. Not surprising for the voltages involved.

I am curious what every ones thoughts are about the 6SL7 as a preamp tube.

rj

[M Fowler]

rj,

Looks good and I like the 6SL7 tube.

Just need a good non-microphonic one though.

[overtone]

I am curious what every ones thoughts are about the 6SL7 as a preamp tube.
rj

To my mind the 6SL7 in V1 is a very juicy tube, but yes, I too can hear a background buzz in the quiet spaces when dimed.
Buzz is the right word, not hum.
Not sure how to tackle that. I have tried over thirty different tubes and so far it seems to be inherent so I just live with it.
I tried a couple of those red based RCAs which were a tad quieter, but then one was microphonic anyway...
Mistakenly I had some ECC33s in the 6SL7 bin which did not buzz so much, but they are said to be closer to the 6SN7 anyway, so less gain. Maybe I should give those a whirl again.

[RJ Guitars]

...
Mistakenly I had some ECC33s in the 6SL7 bin which did not buzz so much, but they are said to be closer to the 6SN7 anyway, so less gain. Maybe I should give those a whirl again.

I was getting ready to post an update when I reread your last post. I had already put a 6SN7 in it and liked the amp a lot better. It still gets just a little bit ratty when dimed using a set of hot humbuckers, so I guess even the 6SN7 is over-driving the output tube. (q1) Can we assume that when an output tube is operating at a lower voltage it can be over-driven with substantially less signal... (did I just discover the origin of the "Brown sound" fifty plus years after Fender did it b4 me)? I think most of us believe this to be true but it wouldn't hurt for me to quantify that idea with some technical info.

I measured voltages and to my surprise the 6SN7 really knocked the plate voltages way down there.

168V - Out of the rectifier
165V - Into OPT
145V - On the Output tube screen
101 - v1b (6SL7)
95 - V1a (6SL7)

76 - v1b (6SN7)
72 - v1a (6SN7)

I have to get better at understanding load lines to help myself go further with this. I want to understand the relationship between the plate resistors and the cathode resistors. My intuition tells me that when I am operating at these lower voltages I can't use the generic 1.5K cathode resistor? Anybody able to tutor me a long and shed some light on these issues?

thanks,

rj

[gktamps]

RJ,
After fiddling with mine and playing it awhile, I have found that with a 12ax7 preamp tube and a 6v6 power tube, the sound is nice and clear turned down low, but gets honky and a bit muddy turned up, and the overdriven sound is unpleasant to me. As a result, I haven't bee playing it much at all, since clean tone playing at low levels isn't my bag at the moment. I might try a 5751 input tube, or a 12at7 and see how that sounds, then play around a bit with putting in a simple tone circuit. It may be, as you seem to be suggesting, that voltages are the main contributor to the tone of this amp.
Cheers,
Greg

[overtone]

...
I have to get better at understanding load lines to help myself go further with this. I want to understand the relationship between the plate resistors and the cathode resistors. My intuition tells me that when I am operating at these lower voltages I can't use the generic 1.5K cathode resistor? Anybody able to tutor me a long and shed some light on these issues?
...

RJ
I am not qualified to help you on load lines, but by coincidence I am packing books and pdfs on the subject right now to study again over the next three weeks of evenings while working away in London. Enclosed are two of these pdf documents for you to peruse at your leisure.
With regard to the relationship between the plate and cathode resistors I think page 6 (Table 7) of the enclosed RCA Resistance Coupled Amplifier Charts is the kind of thing that you are looking for. It is the kind of thing you can keep in your back pocket and impress the girls at the pub with. The first pages explain the terminology.
Best, tony

[RJ Guitars]

thank you Tony,

I am not sure which kind of girls you are hangin out with at those pubs but if technical documentation lights up their eyes... go with it. I am slightly envious of your London assignment and the pub offerings they have there in food and drink that we don't. Specifically the Bulmers or Magners cider... or maybe the Swedish brewed Kopparberg pear cider, I could get fat on that stuff for sure! Did I mention the ginger beer?

Oh yeah amps - When I built the first of these simple rascals I set out to make it a learning tool for first time builders. I also knew that old hack builders like myself would also reap great benefit from learning what each simple little piece of this amp was doing. I have a tendency to just start swapping tubes, resistors, and capacitors to hear the sonic differences then try to technically understand the results. That works to some degree but I also think it has to be good to understand what each component is doing and then intentionally make a change and see if the result goes as planned. Worst case is I learn something no matter the result.

Starting with that first gain stage I want to know what I can do with the anode and cathode resistors to change the sonic characteristics of this amp. I have been reading up in my Merlin book again and he must have spent far too much time studying and not enough playing guitar and impressing girls... unless he also found the kind that love the technical details of analog electronics??

Thanks again for the documents. I'll see how these jive with the teachings of Merlin then form some experiments and try them. Stay tuned.

rj