An amp question

[Jeffj85]

Hello again. I'm working on the 12k5 amp from sophtamps and decided to throw together a little gem to boost the volume. My little gem build works great nice and clean but when I feed it the 12k5 amp i get a buzz. When I run the lg I'm feeding it it's own filtered power, but when I run it with the 12k5 I share the filtered power source of 12vdc. And ideas? 

[GibsonGM]

I would try to get the Gem on the same power supply with the 12K5.  The 2 separate PS's are probably causing some ground looping and other noise/buzz issues....can you set the 12K5 up to give you the 12V you need?

[Jeffj85]

When I hook the little gem into the 12k5 amp I am using the same supply, I'm starting to think i don't need the transformer of the 12k5 amp when i'm boosting it through the gem.

[petemoore]

  Separate filtering for the supply of the amp and preamp might help.
  Note that the Little Gem is a full amplifier by itself, so there's a lot of gain, the noise could be inherent to ''the amp drives the amp'' chain.
  A well filtered 'preamp' [lil' Gem], or lower gain preamp might be worth a try.

[Jeffj85]

I know the little gem is a full amp, I'm trying to use it as a power amp and the sopht 12k5 amp as a pre amp due to ts low volume output. Could it be, because the 12k5 amp uses a transformer to put out 8 ohms that I'm getting the hum. I know that's why we filter dc from a wallwort. 

[markeebee]

I'd try eliminating the output section (V3 and transformer) of the 12k5, and using just the preamp section running into the LG.  Similar to the Sopht Ruby Tubey. 

For a start, disconnect C2 of the 12k5 and connect the LG there at the junction of R4 and V2.  See what that does.  You might need to monkey with the value of the LG input cap to reduce 'fartiness', and you might want to leave out the LG gain control and leave pins 1 and 8 open.

I'm not sure that the LM386 will really be 'louder' than the 12k5 output valve.  You might want to try something with a higher output like a TDA2003?

[Jeffj85]

Hmm sounds like a good idea. I'm a total noob what do you mean by v3?

[Ripthorn]

When you are running the 12k5 amp into the LG, the gain levels of the LG might be causing you signal to start to hit the rails, so that you actually get a little bit of clipping on the signal from the LM386 being pushed so hard.  You might see if you can increase the supply voltage or something.  If you want more volume from the 12k5 amp, see if you can feed it a higher plate voltage via charge pump or something.  In tubes, more volts=more volume.

[markeebee]

Ok, sorry if I'm teaching you to suck a bunch of eggs.....

The 12k5 amp, like pretty much every guitar amp, has a preamp section and a poweramp section.  The preamp shapes the sound by introducing some distortion and equalisation, but doesn't increase the 'loudness' of the signal a huge amount.  The power amp takes the shaped sound and makes it bigger so that it can drive a speaker (but also shapes the sound a little as well).

It's not really practical to increase the 'loudness' of a poweramp by running into another amplifier in the way that you've been doing.  There will be all kinds of mismatches between the impedances and gain levels (you can find out more about that at a whole bunch of sites on the interweb - Geofex is a really good place to start looking).  If you want it to be louder, really, you have to tweak the power amp as Ripthorn suggests or just use a different power amp with more output.

So, if you look at the schematic at the Sopht site, everything to the left of C2 is the preamp, and everything from C2 to the speaker is the power amp.

And, if you take a look at the Ruby Tuby schematic at the Sopht site, everything to the left of C3 is preamp, and everything from C3 to the speaker is power amp.

So, to retain some of the nice valvesound of the 12k5 amp, but make it as loud as the LG, I would add the Ruby Tuby poweramp (which is really similar to the LG) to the 12k5 preamp.  It'll sound different to the 12k5, but plenty of people like it just as much.  You can try it and, if you don't like it, return it back to a standard 12k5 and tell me I'm a twat.

[Jeffj85]

Nice terminology. I understand what your saying about the preamp section I didn't set out to build the ruby tuby I wanted to build a full tube amp. Is it possible that I'm getting really low volumes from the output due to my speaker size? It's about 10" and 8 ohm. I really don't want to use a solid state. 

[markeebee]

That speaker should be fine, no problem there. What transformer are you using?

[Jeffj85]

I'm using a junker transformer I bought from a local electronics shop it's a 1200 ohm to 8 ohm with a center tap to bring it down to 600 ohm to 8 ohm. I'll report more on the transformer when I get home from work. 

[Jeffj85]

I've done some reading and have found builds that used a 70v OT (I'm guessing that the OT stands for Output Transformer). The transformer I'm using is an audio transformer manufactured by Mode Electronics I've taken a photo of it,http://i769.photobucket.com/albums/xx339/jeffjs/c0aafb1b.jpg and it is 1.2k primary and 8 secondary and the  primary is center tapped. I'm not entierly sure if this is a good transformer for this application. Maybe thats why I'm not getting much volume at all, I can hear my strumming over the amp.

[frequencycentral]

That transformer looks tiny! Also, having the centre tap it would appear more suited to push/pull designs than single ended designs like the 12K5 amp. I believe that centre tapped transformer have an air gap (visible in your pic), whereas transformers for single ended amps lack an air gap. You might have bette luck with a Hammond 125ASE.

I've never used space charged tubes, or built a Sophtamp. I've built quite a few tube amps running off 12 volts using 'kinky' power supplies though, such as charge pumps and SMPS's to get higher volatges. I'd agree with Ripthorns comment wholeheartedly: more volts = more volume. Though as you're using a 12K5 you are obviously limited to 30 volts maximum.

Have a look at these anyway:

http://www.diystompboxes.com/smfforum/index.php?topic=73222.0

http://www.diystompboxes.com/smfforum/index.php?topic=78302.0

[Ripthorn]

I would wager that going from 12V plate to ~27V on plates would make a fairly large difference, since you are then able to more than double your voltage swings (in theory).  That means you get an extra 6 dB (I think, or is it 3? I can never remember which is which in circuits, only in air), which is really noticeable.

[markeebee]

I think that transformer has a part number 60-282? 

If so, it's really not the right type of tranny.  It's meant for matching small signals at line level, and is not suited to use on the output of an amp.  Even a low output amp like the 12k5 can develop, oh, fifty times the nominal rating of the transformer.

I don't know enough about how a transformer behaves to say for sure that this is the cause of your low output.  However, the transformer will almost certainly fail prematurely, if it hasn't already 

Definitely you should use something like the Hammond that Rick mentioned.

[bancika]

IIRC you can use a regular 120:12V transformer for 12k5 output.
It has 10x stepping ration which means that for output purposes it will essentially work as 800:8ohm transformer which is pretty close to 600:8 data sheet calls for.

[Jeffj85]

Would any old 120 to 12 ac tranny work for me? And as for running the plates at ~27 would the heaters work @ 27 also?

[Ripthorn]

Would any old 120 to 12 ac tranny work for me? And as for running the plates at ~27 would the heaters work @ 27 also?

Heaters always, and I mean ALWAYS, have to stay at their stated voltage (typically 6 or 12 V depending on how the heaters are wired).  Failure to do so can ruin the tube quicker than about anything except for a hammer. The plate voltage is all that should change.  The amount of volume increase depends on the the plate voltage because that governs how wide of a voltage swing it can accurately produce.  Think of it as being similar to an opamps supply voltage.  The higher the voltage, the greater the output signal can be, meaning a louder output.

[bancika]

Would any old 120 to 12 ac tranny work for me? And as for running the plates at ~27 would the heaters work @ 27 also?

yes, any 120:12 transformer will give you impedance step down ratio of 100. As far as power rating is concerned, 12k5 outputs 50mW or something, so pretty much any power transformer will handle that...