Solid State Guitar Amp Forum | DIY Guitar Amplifiers

Solid State Amplifiers => Amplifier Discussion => Topic started by: syndromet on December 04, 2012, 02:32:21 PM

Title: I think I want a Pignose.
Post by: syndromet on December 04, 2012, 02:32:21 PM
Hi, guys! It's been a while since last I contributed with anything here, as I've been busy getting kids, building a house, building a career and playing with tubes. I finally have some spare time again, so my interest in building guitar related gadgets have come back. I'm currently building a Hiwatt DR504, fixing an Ampeg SVT 6 pro and producing a small run of guitar pedals for sale.  :dbtu:

After seeing this video, I think I'm falling in love with the pignose 7-100, and I'd love to build a high quality version of this wonderful amp. I see the amp already have been discussed on this forum a few times before, but I can't find any info on the transformers used in it. I was wondering if any of you guys who actually understand this kind of stuff knew what impedances the transformers are? If not, I'm hoping some of the experts here can make an educated guess.

I'm guessing the transformer between the preamp and the power section is has a 1:1 ratio, and that it's function is purely to isolate the two stages from each other. When it comes to the transformer on the output section, I  guess it's there to match the impedance to the speaker load. What would be the primary impedance on something like this? Is there anyway to calculate this stuff?

Pignose video:
http://www.youtube.com/watch?v=_q0nImsfMvE

Schematic:
http://music-electronics-forum.com/attachments/2913d1218923726-pignose-model-7-100.gif

It's good to be back.
Thanks in advance!
Title: Re: I think I want a Pignose.
Post by: J M Fahey on December 05, 2012, 05:31:09 AM
Hi syndromet, glad to have you back :)
That kind of power amp is *real* dated, 60's technology, so not much info remains.
That's a typical "transistor radio" power stage, only it's somewhat beefed up to around a couple watts, what a car or table top radio would have; "pocket" "Spica" ones were only about 250 mW or so, but the basic circuit was the same.
From what misty memories I have, the driver transformer was 6:1+1  , and the output one was "48 ohms:8(or 3.2) ohms"
They were available *everywhere* as surplus, for peanuts ... in the early 70's :( .
Here is the schematic of an Argentine record player, *very* popular in my youth (don't ask).
(http://electronica.yoreparo.com/foros/files/pag_272__a_.jpg)
As you see, the transformers were specified only as driver and output.
It was supposed the supplier knew what you were talking about.
Unfortunately, I think the only option is to salvage them from some *very* old radio or record player found in the attic (not even pawnshops or Salvation Army will have them) *or* bite the bullet, get one Pignose and copy it.
Sorry, "reverse engineer" ;)
Title: Re: I think I want a Pignose.
Post by: tonyharker on December 05, 2012, 08:40:26 AM
Very often these http://www.ebay.co.uk/itm/Eagle-LT700-LT44-Radio-Output-Inter-stage-Tapped-Audio-Matching-Transformer-/120788551407 are used as replacements but not actually having tried them I cannot comment.  It maybe worth a try :)
Title: Re: I think I want a Pignose.
Post by: J M Fahey on December 05, 2012, 01:29:35 PM
Yes, these will work, although probably supply around 100 to 200mW, which is fine for a transistor radio, not enough for a Pignose.
Buy buy a pair and experiment anyway.
The "48 ohm primary" I mentioned earlier would provide around 2.5W RMS , reasonably close to what a Pignose offers.
Title: Re: I think I want a Pignose.
Post by: Enzo on December 05, 2012, 07:08:41 PM
The transformer after the preamp is not there just to isolate the stages.  Look again.  The output stage is push pull, the format very similar to a tube power stage.  The transformer acts as a phase inverter or splitter.
Title: Re: I think I want a Pignose.
Post by: syndromet on December 07, 2012, 08:03:16 AM
Lots of great info here, guys!  :dbtu:
I noticed that the amp is actually very similar to Brian Mays Deacy-amp, which in my opinion is one of the coolest sounding rock amps ever. I think I'm shifting my focus from the Pignose to a Deacy, for this build, and then maybe build a Pignose later. I also have a speaker somewhere that looks a lot like the original Deacy, and if memory serves me right, it has a 6,5" dual core speaker like the original.

Thanks for the link to those transformers. Looks like the right stuff for a deacy-build. I don't mind if I reduce the wattage, as I'm looking for an amp that will record great at low volumes.

I see that the Deacy use one AC125, one AC126 and two AC128 for transistors. I have a bunch og russian GT308's. Would it be any problem replacing the 125 and the 128s with these? I don't know the spec of the AC126, but from the schematic I'm thinking this transistor have a lot more gain than AC125 and AC128, so I'll probably just buy an AC128 for this position.
Title: Re: I think I want a Pignose.
Post by: syndromet on December 07, 2012, 08:04:38 AM
The transformer after the preamp is not there just to isolate the stages.  Look again.  The output stage is push pull, the format very similar to a tube power stage.  The transformer acts as a phase inverter or splitter.

Could this be the reason why these amps sound somewhat tube-like? I don't think I've seen this configuration with transformers in bigger SS amps? Why not?
Title: Re: I think I want a Pignose.
Post by: J M Fahey on December 07, 2012, 12:26:13 PM
1) you'll need the AC128 or similar at the output, because they are *Power* transistors, handle more power and current; the others are "small signal" (preamp) ones.
I have made tons of "Mini Fahey", my version of Pignose with AC127/AC128 .
They come embedded in a special rectangular aluminum block to handle heat.
2) yes, the output tramsformer and low feedback contribute to a more tubey sound.
But this configuration is not practical in larger SS amps.
First thet got rid of the OT and kept the driver one, as in Standel, Acoustic, VOX, and a couple Gibson, Sunn and Silvertone but biasing becomes very difficult, so they usually show horrible crossover distortion (which helped to criticize "horrible SS sound") but as soon as PNP drivers were developed, transformers dissappeared.
I guess a driver transformer must have cost between U$10 and 20, while a PNP driver transistor cost less than a Dollar and was technically 1000 times better, so ....
Title: Re: I think I want a Pignose.
Post by: syndromet on January 29, 2013, 07:40:06 AM
Well, here she is!

http://www.youtube.com/watch?v=KF25MUgBOo8
Title: Re: I think I want a Pignose.
Post by: joecool85 on January 29, 2013, 12:46:18 PM
Well, here she is!

http://www.youtube.com/watch?v=KF25MUgBOo8

No sound here at work, but it looks good!
Title: Re: I think I want a Pignose.
Post by: J M Fahey on January 29, 2013, 01:02:35 PM
VERY good, congratulations ;)
 :dbtu: :dbtu: :dbtu:
Title: Re: I think I want a Pignose.
Post by: gbono on January 29, 2013, 06:32:23 PM
The transformer after the preamp is not there just to isolate the stages.  Look again.  The output stage is push pull, the format very similar to a tube power stage.  The transformer acts as a phase inverter or splitter.

Could this be the reason why these amps sound somewhat tube-like? I don't think I've seen this configuration with transformers in bigger SS amps? Why not?

Mmmm Acoustic 150, Sunn Concert series, etc all had transformers driving the output sections. Why not? Cost
Title: Re: I think I want a Pignose.
Post by: Roly on January 29, 2013, 06:38:21 PM
Everything JM said about transformers.  Apart from scrounge/recovery, I can't imagine where you would get such transformers at these power levels today; a couple of hundred milliwatts maybe, but even a couple of watts ...

That's a lot of shred for a tiny box.  :lmao:   Very well done.   :dbtu:
Title: Re: I think I want a Pignose.
Post by: syndromet on May 13, 2013, 05:01:32 PM
The honeymoon is over, and I still love this little amp. It compliments my Hiwatt clone wonderfully, and records really, really well. She's definitively a keeper! Thanks for all the help along the way, guys!

http://www.youtube.com/watch?v=GsOEeTl-WSc