Help with tube project

Started by blackcorvo, October 24, 2009, 12:37:01 AM

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you really did a nice job with this amp. keep tweaking it until you are completely happy with the sound.
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Quote from: Cliff Schecht on December 05, 2009, 01:11:09 PM
First off, I've used 2 6AU6's in the preamp of the tube amp I posted in the November Effects Submission thread and I must completely disagree that they sound bad in a preamp. Gibson and a few other manufacturers also used these tubes in a few of their weirder small amplifiers to much success. Good sounding little tube IMO and gives a very pleasing distortion when pushed.

Well, i did a try with it as a pre-amp but i didn't like the sound too much... i dunno if its my tube or what it was but i could never get a clean tone in a nice volume... and since my idea was to try the 6AU6 as power, and my 12AU7 got broken, my only option was to use the 6AU6 in the power and the 12AX7 in the pre-amp. but thanks for your comment, i didn't know Gibson used these tubes :o

Quote from: birt on December 05, 2009, 04:59:50 PM
you really did a nice job with this amp. keep tweaking it until you are completely happy with the sound.

I will, once i have it built inside an appropriate box.
She/They as of August 2021

Cliff Schecht

You can't really get a lot of clean drive out of a single pentode. Distributing the gains correctly across a few stages is really the trick and is what most everybody does in practice (there are exceptions). A single low power triode or pentode like any 12A*7 or 6AU6 just doesn't give you enough gain in a single stage, especially with any kind of headroom. My old Silvertone 1331 (schematic can be found online) uses a single 6SJ7 in the preamp and a 6V6 in the power amp and while it gives out a nice sounding 5 Watts, it definitely ain't a clean sounding amp. My recent 6AU6/807 amplifier (posted in Nov submission thread) used to only have one 6AU6 in the preamp but I quickly found that this configuration just couldn't drive the 807 with any headroom. Using two 6AU6's brought this amp alive though (along with a fixed bias on the 807) and it sounds fantastic IMO. I'll post some songs recorded with these two amps (807 amp and Silvertone) in a thread soon.


sorry for the long time without checking stuff here, i was kinda distracted with other stuff going on here, specially the christmas day.

about the amp, i gotta wait a bit. i'm thinking of buying a new pre-amp tube to this, but 12A*7 and others are kinda pricey around here so... anyone knows of any other model of COMMON, twin triode or (non-twin) double triode that i could use, that haves 12v or 6v heaters?

thanks for any info.
She/They as of August 2021


Hey guys, whassup?

i've been playing a bit with the amp, and decided to make thigs a lil different. I built a JFET based pre-amp, since that's what i'll have here for now, and got some nice results in sound (i think they are better than my other tries with the 12AX7).

also, i found out that the 6AU6 alone can be used as an amp for pc speakers and other stuff. maybe you should build a small 1 transistor pre-amp to drive the tube better, but i liked my results here without any pre-amps.

here are the schematics:

Power Section


I'm using a 9v supply for it... one that makes around 15v o__o

and i'm thinking of maybe, when building it for real in a nice box with the speaker and stuff, i should make a voltage doubler recticifier from the 12vAC to get around 24v for the pre-amp, for better headroom. For now, the tests made me happy enough to make some samples (don't mind my stupid playing...) :

-Before Boost control was added (10k resistor instead)-

On Youtube:

On 4shared:

-After Boost control was added-

All controls at max:

Made after using the correct lightbulb in series with filament (yup, i have 2 here that have the exact same appearence, but one is only 1w, while the another is 2w. the filaments needs 1.8w to work properly... ) After the correction, i got a bit more power, less noise and better quality in sound :
She/They as of August 2021


> 6AU6 ...  I must completely disagree that they sound bad in a preamp

Getting a triode to a "happy" state is tricky enough for many folks. And triodes tend to self-adjust to the resistors you use.

The added (G2) variable on a pentode makes hasty design/fiddle of a pentode stage really tough. The plate impedance is so high it won't self-adjust. Manual adjustment is non-intuitive. It is surely a lot easier to find a "bad" pentode plan than a "good" one.

> Gibson and a few other manufacturers also used these

Yes, plagiarism is always a good starting point. The amp mentioned above was "designed" with PaintShop and the schematic of the early Champ. I erased the 6V6 pin-numbers, pasted "6AU6" over "6V6", fudged the "power stage" load and cathode values. The preamp is 100% stolen.


hey guys,
sorry for "disappearing", i got busy with some stuff here, then i was on a little family trip, so i only was able to get back here now.

Quote from: PRR on February 04, 2010, 10:26:02 PM
The preamp is 100% stolen.

you mean my JFET design or your 6Au6 design? i got a bit confused...


about this project, i'm still playing with it, and i decided to get the V+ for the pre-amp from the B+ of the power section, through a voltage divider using resistors. i'm using right now a 100k resistor, connected from the B+ to a 150K resistor to ground, that haves the pre-amp voltage divider (using 11k instead of 4k7 resistors) connected in parallel to it through a 1N4148 diode (did that because it seemed to add a bit more of stability to the V+ and Vref voltages).

i used a 1000uf in parallel with the 150k, and i'm getting around 35v for the V+.

it gave me a really nice clean tone, but since its too late now (2AM) i cant make any tests. i'll do them later and then let you know what i got.
She/They as of August 2021


Hello again! I'm back with nice updates to this amp :3

I gave up of the stupid idea of using Jfets in the preamp, and i went back to the 12AX7. The amp has been put apart for a looog time, and only last week i started playing with it again.
The preamp is kind of based on the lo-gain side of the JCM800, just without a tone control - still thinking about what to use -,  and the 6AU6's G2 is polarised via a LED (I remembered seeing something about using them on gain stages with triodes, in place of the cathode resisto, and the idea popped in my mind: why not try one on G2?).
It's cool because the LED gets a bit brighter when you hit the strings. Higher volume = more LED fun. :)

Here are some voltages measured just a sec ago:
B+                    = 256v
B+1                  = 254v
B+2                  = 254v
6AU6's pin 5      = 250v
6AU6's pin 6      = 251v
6AU6's pins 2+7 = 4,13v
12AX7's pin 1    = 196v
12AX7's pin 3    = 1,84v
12AX7's pin 6    = 152v
12AX7's pin 8    = 0,9v

And here is the schematic:

As you can see, I have also come up with a better name for it.
I chose The Emerald because I'm thinking of putting the LED on 6AU6's socket, so it'll make the tube light up as it gets warmer, in a green color, like an emerald :D aaaand it sounds cooler too XD

So, that is it for now. I'll try to make some sound samples tomorrow!!!
She/They as of August 2021


hello, very good project.
I would like to build that amp since the 6AU6 are cheap and easy to get.

what would be the final version without the LED?

Thank you.