4 Tube Amp From Scratch

[TubeNoob]

Hey MooreAmps - thanks a lot for pointing this out - anything that 1) improves control of tone 2) makes it sound less like a Fender, is super cool for me - already have a 'fender' sound. Looking for something different. So what you're suggesting - would that be pot from the grid input of the pentode to ground? (that's how i did the master vol on my princeton as i recall).

Cheers

[mooreamps]

Hey MooreAmps - thanks a lot for pointing this out - anything that 1) improves control of tone 2) makes it sound less like a Fender, is super cool for me - already have a 'fender' sound. Looking for something different. So what you're suggesting - would that be pot from the grid input of the pentode to ground? (that's how i did the master vol on my princeton as i recall).

Cheers

How abouts this.... Put a 100K or 200K on top of the volume control.. It's not perfect, but a workable compromise..

-g

[TubeNoob]

hey MooreAmps - thanks for the reply on that! Can u tell me whats happening by doing that? is it forcing more of the signal thru the tone stack instead of the vol control? ----just trying to understand!

for what it's worth, my princeton build does allow for a nice variety having both a preamp-2nd-stage gain and (the mod of a) 6v6-input 'master vol' to ground - nice to balance the gain/master vol combo instead of just havin one or the other, way more variety.

[nashvillebill]

(Disclaimer: I am not an expert; I'm just a hack. RG, JMF, and the others are the real experts. But perhaps my perspective may help)

I have an old Sound Projects (Lectrolab) R200B guitar amp, made in the very early 60's. (I may have posted a schematic on this forum if you search) It's got a 50L6, 35Z5, and two 12BF6's with their heaters all wired in series, direct to the 110 volt AC. At least though there is a power transformer so it's not a "true" hot chassis widowmaker... 155VAC secondary yielding a B+ of around 160. Again, this is hardly an ideal setup, it works but that's perhaps the best I could say about it.

Anyhow, I bring this up not because this is a sterling example of "how to wire heaters" but rather to relate my story with respect to this amp. It sounds...like a 2 watt practice amp with an 8 inch speaker. So I think to myself...gee, let's put in a better 10" speaker? Well, can't easily do that, the enclosure is just half an inch too small. Okay, well...I could make a bigger enclosure, no problem, I've got a woodworking shop downstairs. But then I continue thinking...okay, I'm going to build a new cabinet, but you know, if I'm gonna do that, I should get a proper power transformer, and do away with the 110 volt series filaments. Okay, that means new tubes, well you know if I'm gonna get new tubes I might as well get new sockets. And while I'm there, maybe redo the circuit a little, perhaps that would get rid of some of the blocking distortion I think I'm hearing. Might add a better tone control circuit, of course that means I should get a new chassis. Might have to get a new OT with the new tubes too, and....WHOA. Reality check!!

In reality, I would've just "converted" this to a new amp--and I would've destroyed a semi-collectible amp in the process. It would be just as easy (perhaps easier) and not much more expensive (if at all) to just build a new amp from scratch, with a proper design and proper components from the start. A new PT, with the right specs, can be had for 20 to 30 bucks, for example.

Periodically, I'll see things on CL like an old Webcor tube tape recorder, and I'll think to myself--hey I could use the PT and maybe the OT...then I go through the whole thought process all over again, and come to my senses, and pass on buying a conversion project.

So the little practice amp sits in the closet unmolested. Maybe I'll put it up for sale on CL and build a new amp with the proceeds.

[mooreamps]

hey MooreAmps - thanks for the reply on that! Can u tell me whats happening by doing that? is it forcing more of the signal thru the tone stack instead of the vol control? ----just trying to understand!

OK. Re-draw the schematic in the way as described, and see what happens when you turn R5 up to max...

-g

[TubeNoob]

@MooreAmps - updated schematic - i can't run a simulation, but it seems like it will force more of the signal thru the tone control since the signal will see more resistance going into the vol control. is that even close?!

@NashvilleBill - thanks a lot for your post. i guess i should clarify a couple things, i'm not so much converting the radio as using what parts i can - i do have a nice separate isolation PT and am just trying to use what i have on hand because, unfortunately, i don't have bread to spend on stuff at the moment and here in Italy it can take a month and a half to get anything from outside the country. So this is allowing me to work on something instead of nothing. Also, this radio was just a chassis and some tubes, no casing; not like its a semi-collectible amp like yours, and it will be just the head so i can route it into different cabs. Above all, it's one helluva learning experience!

I'm going to be testing that autoformer soon per RG, and if that thing won't let me use these tubes, I'll be swapping out the 50B5 and I guess the 35w4 for tubes i can wire in parallel per JMFahey's suggestion.

[J M Fahey]

Now that I think about it (slow brain), ¿why don´t you buy (or get for free) an old Geloso amplifier?
Built better than tanks and killer sounding ones.
Great Iron and chassis.
I´m sure you can get one for free, there are thousands of them collecting dust in cellars and places like that.
Or you might ask at any Church nearby, they were the standard Rome-issued amplifier.
I repaired and modded tons of those.
Their universal multi tapped output transformers are gorgeous.
Made to be easily wired from 1.5 to 500 ohms, and all intermediate impedances.
I still remember the labels "per "X" ohms, unire tra loro le uscite YYYY"
"Noi gli Argentini, quase tutti parliamo algo de Italiano"

[mooreamps]

@MooreAmps - updated schematic - i can't run a simulation, but it seems like it will force more of the signal thru the tone control since the signal will see more resistance going into the vol control. is that even close?!

Yes, close enough... I am dissapointed some of the others are not helping you with this.... They seem to be off into their "own little worlds"...

On the schematic, you will see a grid leak resistor, R11 V3. Insert this same resistor on the same location for V1. Also, in front of R1, insert a 4.7 uF cap in series with R1.

-g

[J M Fahey]

I am dissapointed some of the others are not helping you with this.... They seem to be off into their "own little worlds"...

Yeah, sure.
That´s why this topic has 69 posts, of which 9 are yours.
I wonder where all the others might have come from ["scratching my head" smilie icon]

[Chuck H]

On the schematic, you will see a grid leak resistor, R11 V3. Insert this same resistor on the same location for V1. Also, in front of R1, insert a 4.7 uF cap in series with R1.

TubeNoob, don't do this as described. It may actually damage your amplifier.

Either forget about the 4.7uf cap or put it first in line ahead of any resistors. Then put a 1M resistor to ground behind R1. So it's:

cap (or no cap)- 1M resistor to ground- 68k resistor (R1)- tube grid.

[J M Fahey]

And .... why 4.7uF in series with a 1 Megohm resistor?
Cutoff frequency around 0.032 Hz !!!
Ridiculous !!
Not even whales hear sound that low in Frequency !!
Nor Sismographs !!!!
And if per chance that grid conducts (on an over 2 V peak signal), the blocking distortion will turn your amp off for a couple days.
You might as well come back next week to finish your solo.

[TubeNoob]

Not even whales hear sound that low in Frequency !!
Nor Sismographs !!!!

- oh man, JMFahey, that was some funny stuff, thank you.

I am more than blown-away at the support i've gotten here so thank every last one of you.

Great idea on the Geloso's, I have a friend with one that is missing the front dial glass (wonder how possible it is to find that?) and is missing the speaker (really nice cab tho!). If its not restorable, I might just see if there's a way to get it. And i will certainly see about finding other Geloso's. I researched him and his company a few months back when i saw my friend's radio. Quite a history.

E si, io so che gli Argentini parlanno un poh d'Italiano! e anche, luro parlanno Spagnolo con un accento piu o meno Italiano, no?! :-) Saluti!

But hey, i'm an American trying to learn not just Italian but Electronics - my brain is imploding!!!

Happy new years everyone!!!

[R.G.]

Yes, close enough... I am dissapointed some of the others are not helping you with this.... They seem to be off into their "own little worlds"...

Nope, I've been watching.

Historically, you don't like it when I point out technical issues with what you say, Gary. I started writing a post this morning to try to sift a little wheat out of the pile of fly specs and chaff, but then I decided, shoot, it's New Year's Eve - why rain on Gary's parade? Go ahead, friendly neighborhood rocket scientist - you got the ball, run with it.

If you get into deep water, just remember that you're buying a car in January, so that will make your technical understanding and ability to communicate whatever you do know much better, right?

ROFLMAO.

[mooreamps]

if you have nothing to contribute to the project, then why are you here ?

[Chuck H]

He's our guardian. Sort of making sure no one puts an eye out. I never asked my parents "why are you here?" They were just an omnipresent entity that kept any situation from completely $h!tt!ng the bed.