THE SFA - 1W all tube guitar amplifier

[gtudoran]

Hello guys,

Long time no posts. Had a lot on my back and a lot of projects to be done, so after a while, i've managed to finish this project that was on my mind for some time now.
It's a small full tube practice amp that uses 6N16B double triode as an output tube (submini) and a 6N16B / 6N17B double triode as a preamp tube.
The schematic is based on Marshall 18W and the power stage is made in a self-split configuration.
Controls are: 1 volume and 1 tone knob and one switch that will enable a single or parallel preamp stage
For the HV it uses a SMPS module.

Project documentation is available here: https://dl.dropboxusercontent.com/u/15275178/OSEQ_UPLOADS/THE_SFA/THE%20SFA.pdf



Here you have some renders of the amp and also some views of the PCB







Regards,
Gabriel Tudoran aka. DeXTeR

[tonyharker]

Hi, I like your SFA amplifier.  I have a couple of questions - What voltage is the B+ and what advantage is it to use the 6N16B in parallel for the first stage?

Thanks & regards Tony.

[gtudoran]

Hello Tony,

- The B+ voltage can be dialed between 90 and 350V (a maximum of 200V is permited on the anode of 6n16b).
- Adding a parallel stage  it will add some distortion to the sound (the input stage is the same as the one from the 18w marshall)

If you have any other questions please do not hesitate to ask.

Regards,
Gabriel

[Luke51411]

I'm excited for this one. For setting the B+ do you just set it for where it sounds best?

[slacker775]

I'd be interested in one of these pcbs.   What's the cost on them?

[gtudoran]

@Luke - the B+ value will be set according to the primary load of the output transformer and secondary load of the transformer also the maximum voltage on the anodes of the power tube and not to exceed the maximum anode dissipation of the  tube (which is 1W per triode)

@Slacker - i will PM you with the details

Regards,
Gabriel Tudoran aka. DeXTeR

[duck_arse]

gtudoran - are you at all familiar with the English term "sweet fanny adams"? or the not so polite version? dextor- is there a connection?

[PRR]

> the B+ value ... primary load ...maximum voltage on the anodes ...maximum anode dissipation

For people who don't compute these every day:

The rated dissipation is 0.9W per triode or 1.8 Watts per bottle. In non-military applications you can round to 2.0 Watts.

The OUTput power is much less than the dissipation. For any self-bias amp, for any small triode amp, and especially for self-bias triode.

Suggested starting points for push-pull self-bias 6N16B power amp--

200V supply 22K load  9mA 750r bias (820r a good first-try)
170V supply 20K load 10mA 500r bias (about 0.25W output)
140V supply 20K load 12mA 270r bias
100V supply  7K load 13mA 150r bias (much lower output)

Supply voltage over 200V may be bad for the tube, and will require an impossibly high load impedance for good results.

0.25W may seem small compared to the monster Marshalls it is inspired by, but with a good speaker it can be really loud in a home situation. The 100V condition may be useful if there are other people in the house. You can leave the primary load near 20K, the bias resistor near 750r, and get down to flea-power at 100V supply. If the SMPS voltage knob is front-panel you have a wide range of maximum powers at your fingertip.

For Hammond 125A:
For 20K primary load:
4r load: 5&6
8r load: 2&4
For 7K primary load:
4r load: 4&6
8r load: 3&6

[gtudoran]

Sorry duck_arse, no i have to say i'm not (searched on urban dictionary but i do not see the connection).
I do not have any connection with "dextor" - Dexter was my nickname for a long period of time.
... I hope that i didn't bother you in some way.

gtudoran - are you at all familiar with the English term "sweet fanny adams"? or the not so polite version? dextor- is there a connection?

[gtudoran]

Indeed PRR and there is not much to say. The prototype was tested using a 12" / 100db SPL Celestion speaker and i can say that in an apartment there is now way of using the volume over 12 o'clock.
And after 10PM i can't use it over 9 o'clock.

> the B+ value ... primary load ...maximum voltage on the anodes ...maximum anode dissipation

For people who don't compute these every day:

The rated dissipation is 0.9W per triode or 1.8 Watts per bottle. In non-military applications you can round to 2.0 Watts.

The OUTput power is much less than the dissipation. For any self-bias amp, for any small triode amp, and especially for self-bias triode.

Suggested starting points for push-pull self-bias 6N16B power amp--

200V supply 22K load  9mA 750r bias (820r a good first-try)
170V supply 20K load 10mA 500r bias (about 0.25W output)
140V supply 20K load 12mA 270r bias
100V supply  7K load 13mA 150r bias (much lower output)

Supply voltage over 200V may be bad for the tube, and will require an impossibly high load impedance for good results.

0.25W may seem small compared to the monster Marshalls it is inspired by, but with a good speaker it can be really loud in a home situation. The 100V condition may be useful if there are other people in the house. You can leave the primary load near 20K, the bias resistor near 750r, and get down to flea-power at 100V supply. If the SMPS voltage knob is front-panel you have a wide range of maximum powers at your fingertip.

For Hammond 125A:
For 20K primary load:
4r load: 5&6
8r load: 2&4
For 7K primary load:
4r load: 4&6
8r load: 3&6

[J0K3RX]

Sorry duck_arse, no i have to say i'm not (searched on urban dictionary but i do not see the connection).
I do not have any connection with "dextor" - Dexter was my nickname for a long period of time.
... I hope that i didn't bother you in some way.

gtudoran - are you at all familiar with the English term "sweet fanny adams"? or the not so polite version? dextor- is there a connection?

Gabriel,

I think he was referring to "The SFA - 1W all tube guitar amplifier" more so the "SFA" part...  I don't see any connection either other than the initials being the same as "sweet fanny adams" which is not really that odd..  There was a hair band back in the late 80's early 90's called Sweet F.A. and perhaps there was some connection there?

[gtudoran]

Hahahahah J0K3RX - duck_arse explained it to me on PM ) funny as hell. As for the amp name ... the answer is even easier: is "The Small F6%$^ Amp"

And on another note, some preliminary tests that i've done today: SMPS oscillator OK / Filaments OK

[dan.schumaker]

Looks awesome, I'll be in on one too.

[gtudoran]

Well ... hes alive.
Yesterday i've finished the prototype of the mini-tube amp. Had some problems with a capacitor that was new (straight from the bag) - no signal passed that little bugger, but all is good now.
Here is a video with the amp (clean) and with various pedals in front(808 OD / Booster and Distortion).
Sound was recorded at the workshop using camera microphone (12" Celestion speaker in the cabinet)
So... this is the little giant, aprox. 100mW of pure tube sound (more then enough to annoy your neighbors)

[deadastronaut]

i thought it was sweet f... all too , (sweet fanny adams) bad minds think alike,



cool amp.

[Luke51411]

Sounds pretty awesome!!! 

[GibsonGM]

Nice job on the amp!!  It does sound good!

If you guys like "Sweet FA", check out the Sweet song "Sweet Fanny Adams"...my kind of classic guitar...

https://www.youtube.com/watch?v=9NKCR1jZbwk

[vigilante397]

This is ridiculously cool. This will definitely go on my things-to-hurry-and-build-before-school-starts-again list

[E-money]

I'd love one of those circuit boards (if you are planning on selling them).

Great work!

[J0K3RX]

DeXTeR ~

Of course, you know that "Small F#@king Amplifier" is a obvious name.... You are going to have to call it the "Sweet FA" now!? Besides it being a cool name it just kinda seems fitting now.. Call it Sweet FA and just let people wonder what it means... Sweet Fanny Adams or Sweet F#@king Amp or whatever.. Clever little marketing tactic