Lil' Wreck -mini Watt amp - how lo can U go?

[El_Martin]

Hi there!

Finished a new built yesterday. My amps are getting smaller day by day

I decided to build a very small amp Trainwreck style w. only preamp tubes and very lo wattage. Wanted to go with a back to back solution for HV. Gathered some parts over time (12 AC wallwart, OT w. high primary )impedance)

Drew up a schemo based on TW preamp and some kind of reverb driver as SE power (!) stage. Trimmer (25k) for Rk of third stage.
Mounted parts point-to-point from socket to eyeletstrip. ratsnest!
Firing it up w. crappy old tubes for testing. Siemens

BiG SURPRISE!
It worked! off the bat, kind of a home run. Yeehaw!

Voltages are too low. B+ is around 170 V. Little 12 V transformer does not take the load too gracefully ...

As regards sound: loud enough for me, kind of a desktop or office amp
creamy sustain, more Dumble than Trainwreck.
If you dig into the strings, lil thingy sounds like a vomiting cow. Sound goes off for part of second and sets in again. Doesn't clean up well. Not the Twin clean. (did someone expect it, really?)

Could be blocking distortion or total breakdown of B+ due to hi impedance source.

Have to decide which way to go now:
1. adjust Ra/Rk pairs to lower voltages
2. increase voltages by using 220V->10 V as back to back tranformer
3. insert Your favoured idea here and answer me

some pics?
There goes!

Ciao
Martin

[El_Martin]

Got some guts, man?

sure!

Oh, what a mess!

Ciao
Martin

[El_Martin]

update:

Fixed a loose grounding of the preamp-> Problem of intermittent sound solved. Screams! Still too loud for bedroom levels...
Installed bright cap over volume pot.

Voltage are a bit low on the plates. 100 to 170 Volts.
Bias points have to be adjusted...

it's fun...puts a smile on your face, w/o sacrificing sanctity of the home

Ciao
Martin

[roberto]

Ciao Martin,
I've built something similar for a friend of mine. That "pony" screams like a damned! The first stage was a plexi like 100k 820R//680n, then like a trainwreck.
Great job!

[Andy Le Blanc]

awesome......

its when you take a leap on faith.....

love to see the look on the face of the sound guy

[Doug H]

Looks great! Very cute!

I was going to build a 10W Liverpool with 6bm8's but built a Vox instead. 10W is still a little too loud for me with a Wreck. I'm going to revisit the idea with a pair of EL91's for 5W instead.

Have fun! Little amps are a blast.

[El_Martin]

Thanks for the input!
I appreciate it.

Doug, I run a couple of these little amps
SE: 0,2W + 3 W (EL84) +10W (any octal)
PP: 10 W (ECL82) + 15 W (TW Liverpool El84)
Currently I prefer my GA-5 variant w/o tonestack (Hoffman) but Lil' Wreck is right on track

Playing in a living room doesn't need that much power. You knew that, I'm sure

Have to fiddle a bit with voltages/bias points etc. Then some kind of feedback will be inserted. Might be positive to compensate for different damping of triode amps compared to pentodes.
If this will not be successful, small pentodes w. low voltages shall be my best bet.

Ciao
Martin

[flood]

hey there! it looks great, but i do have one question (a very noobish question) - when using a wall wart in back-to-back configuration, how do you ensure that no HT is conducted by the chassis? i see you are using a metal chassis, and i haven't got this one figured out myself yet. thanks for your input!

[El_Martin]

Well, flood, that's no noob question. It's serious.
Currently I connect the personal earth via two (!) flying leads to the chassis.

Generally I use ground fault circuit interrupter switches with this/my builts.

Once implanted in a wooden (insulated) housing, the connector can/iwll be changed to insulated types. Plexi plates for front and back will be applied.

But one should take care...HT is not meant to be messed around with

This one is more of an experiment. If I will do it again, I shall use a little toroidal transformer for heaitng and HT/HV...

It may look like a toy, but it is not a beginner's project. Champs or the like will be better for this purpose.
It is a highgain amp and the crammed parts will be a major pain (you know where
You'll have to improvise a lot...
But I just had to try it...something forced me

Ciao
Martin
edit: now I can write "two" correctly

[flood]

Well, flood, that's no noob question. It's serious.
Currently I connect the personal earth via twho (!) flying leads to the chassis.

Generally I use ground fault circuit interrupter switches with this/my builts.

Once implanted in a wooden (insulated) housing, the connector can/iwll be changed to insulated types. Plexi plates for front and back will be applied.

But one should take care...HT is not meant to be messed around with

This one is more of an experiment. If I will do it again, I shall use a little toroidal transformer for heaitng and HT/HV...

It may look like a toy, but it is not a beginner's project. Champs or the like will be better for this purpose.
It is a highgain amp and the crammed parts will be a major pain (you know where
You'll have to improvise a lot...
But I just had to try it...something forced me

Ciao
Martin

hi there martin,

thanks for the informative reply. i'm currently looking at a whole bunch of low-wattage amps as projects. i'm very inclined towards thee doug hammond firefly, which sounds fantastic - i've had transformers wound for this amp, but even using two trannies back to back would work well.

another amp that interests me is the niteglow, which uses an ecl82 in SE configuration. i salvaged the ECL82 and an OT for it from an old telefunken radio that i picked up at a frankfurt flea market - it worked perfectly, no idea why i tore it up in the first place!

ideally, i'd like to wreck that circuit, adding a 12ax7 to increase the overall gain. i've always wanted to attempt my first build using a 12V AC adapter, but prudence advised me otherwise. i'm just waiting to hand in my thesis so that i can get down to some serious building.

i'd be greatful if you could post a schematic to this amp, and maybe a soundclip; i'd love to hear what it sounds like! regards, adi
your wreck looks very interesting though - are you using

[El_Martin]

Hi!

...working on the schematic...

maybe sound clips later, have to prepare a few things and learn to play that six string thingy

Ciao
Martin

[flood]

learn to play that six string thingy

Ciao
Martin

hey, i have one of those too!

...now if i could only remember what they're called

[El_Martin]

Hola hombres, que tal?

Been working on the little one. Thanks to Josh/Krash (Revelationsamps), I've totally changed the biasing of the 12 AU7 in the SE output stage.

Now Ra is about 8k Ohm and Rk is 820 Ohm. Ua is 160 Volts. 4,4 V over Rk=820 Ohm equals less than one Watt static. I didn't want to go to higher voltages, since I'm using a 100 V OT...

Here's my schematic.
Please note: This is NOT a fully documented project. Since the pairing of back to back transformers vary a lot, one will have to change/check bias points of the SE output carefully. If I would build it again, I'd go for one of Hammonds universal PP OTs. They should handle a wattage this low even in SE and one can experiment with different plate loads.

Ciao
Martin
EDIT: latest front pic added