Any discrete transistor based amps that are Ruby like?

[Joe Kramer]

Legendary Pignose uses output transformer. There is rather clean schematic on a Russian guitar forum:

There ya go.  The Pignose and the "Brian May" amp made from tape-deck guts was sort of what I had in mind.  Besides sounding "legendary," I think that would definitely "look cool" done up on turret board.  

Thanks for the schemo!  Anyone have a general idea as to the transformer ratios used for that?

Joe

[PRR]

> rather clean schematic on a Russian guitar forum



Very standard pocket radio plan, plus preamp, and presumably scaled to lower impedance to suck more current and make more power.

Four transistors, two transformers. Transfomer specs not given..... AH, on page 2, measurements:

Transformer 1:
primary cell: R = 41 ohms; L = 0,51 Tt; Q = 1,36
secondary housing: R = 49 ohms; L = 0,53 Tt; Q = 1,39
halves of the same secondary housing
Core 7mm * 5,5 mm = 38,5 mm ^ 2
 
Transformers 2:
primary cell: R = 7,5 ohm; L = 54,5 mH; Q = 2,1
identical halves of the primary
halves of the secondary housing is not the same
secondary housing 1 (lower scheme): R = 1,1 ohm; L = 8,5 mH; Q = 2,45
secondary housing 2 (upper scheme): R = 0,4 ohm; L = 0,47 mH; Q = 2,6
core 11mm * 7mm = 77 mm ^ 2
 
The conversion coefficient is not measured excuse. At work once, at home no oscilloscope / generators.

DC voltages:

Modes for DC (B), relative to the total wire:
voltage 8,8
voltage preamplifier 8,6
base of first transistor 0,7
emitter of first transistor 0,16
Resistor 47 Ohm 0,018
collector of the first transistor 7,2
base of second transistor 2,4
emiter second transistor 1,8
collector of the second transistor 8,5
voltage on the diode 0,67
the resistor 2,2 Ohm 0,067
collector output transistors 8,7

Mechanical translation; clarification welcome.

[Brymus]

Hey Paul,
Any educated guesses as to what the values of those trannys are ? ( i would like to build that and have a ton of old warts and pulled trannies)
And why the transformer driving the PP stage ?
Was that to save a couple of transistors ? Or is there some other benifet to that ?

[Joe Kramer]

This thread has info on the similar Deacy (Brian May) amp:
http://www.diystompboxes.com/smfforum/index.php?topic=38129.0

Here's an actual radio/tape deck circuit:
http://www.pisotones.com/Deacy/Deacy.pdf

Regarding those transformers, Mike Irwin (Vsat) says:

That schematic is  the same one shown on Fig.4, page 171 of the 1961 Mullard manual.
The transformers are identified in the text as Colne 06005 (for T1) and Colne 06006 for T2.
The equivalent Hammond transformers are listed in the back of the manual as 57318 and 57319.
Maybe someone has an old Hammond catalog that lists the specs?

Text also states that this amp is the same as the one shown in Fig.13, page 160, (but with the preamp added).
Fig.13 has some further transformer info:
T1 is 3.5:1+1, Rp < 130 ohms, each half of the tapped secondary has Rs < 40 ohms.

T2 is 3.1+3.1:1, each half of the split primary has Rp < 1 ohm, Rs < 0.2 ohm.
Speaker is 3 ohms.
The OC81D driver and OC81 matched pair were supplied as a set by Mullard.

Somewhere in my garage, I have the PCB from an old tape deck with four 2SB172s and two transformers.  With any luck, it's exactly this same circuit.  

[PRR]

> an actual radio/tape deck circuit:

Actually a crystal phonograph. But overlooking the gigundo input resistor (crystals don't like loading), different NFB scheme, being PNP etc, and omitting the bias diode, it IS the same circuit.

> With any luck, it's exactly this same circuit.

Aside from source adaptations, and voltage and power output, they ARE all the same. Driver transistor, driver transformer, 2 transistors, output transformer.

> the PCB from an old tape deck

Tape head needs LOTS of bass boost and has known output. This was usually done as a 2-transistor EQ-ed preamp before the volume control. Then you might get away with just 3 transistors after the volume control. The tape preamp will need radical mods, so radical that you might best do a low-gain low-Z JFET preamp instead.

Rather than try to duplicate specific originals, and assuming we are unlucky at the garbage dump, use in-stock parts.

Hammond 146B  48 Ohms CT  3.2 Ohms    
Dimensions:  0.88 in L x 1.63 in W x 0.81 in H    
Power Rating:  700 mWatts  
$17.29  
Low stock, expensive

Xicon  42TU048-RC
48 Ohms  8 Ohms  
460mW
$2.57

Xicon 42TU120-RC
120:8
460mW
$2.57

The 120 part is probably a safe bet. At 9V supply you could get a whole Watt of output. This is twice the rating, and the rating is for 300Hz, two octaves up from a guitar's bottom note. Loud bass will be "colored"; guitarists know how to handle that.

BTW: the OP mentioned "simplicity". There's over 60 legs in this plan, a lot more than a '386 amp. More involved build.

Driver:
Xicon 42TM016-RC
600CT:600CT
200mW
$1.82


Output devices: the Pignose's TIP41C was a shock, you could build a 50W amp with those! However the mass-production cost is just 23 cents, and they are nearly bullet-proof in this plan. The other devices may be '5088 etc.

[Brymus]

Has anyone breadboarded any of these yet ?
Thanks for all the tips Paul,I definetly want to try messing with this circuit for awhile at least.

[Joe Kramer]

I just finished tracing out the PCB I scavenged from an old tape deck.  It uses four 2SB175s and two transformers.  The output section is indeed very similar to the old Mullard schemo, without some of the compensation and feedback elements.  But the funny thing is, the two-xstr preamp is essentially a Fuzz Face circuit with a few different resistor values.     Looking forward to hooking it up for a test today.  If it works, maybe I can post a rough schemo. . . .

[PRR]

> two-xstr preamp is essentially a Fuzz Face circuit

There are not that many basic ways to wire two transistors. Maybe a few hundred ways, but a dozen or so plans stand out. This plan with Q1 Base biased from Q2 Emitter is neat and stable and tends to work with huge part tolerances. And as cheap as it gets.

It would be good to re-read R.G.'s "Technology of the Fuzz Face".

As a tape or phono preamp it might be tarted-up with an R-C network from Q2 Collector to Q1 Emitter to set the correct EQ curve and define the gain. This can get fancy: the input stage of the NeoVibe is the same framework but with a Q3 as Cathodyne to deliver complementary outputs and more internal gain.

The difference between a Clean amp and a Fuzz amp isn't really the amp, but the signal.

The tape head puts out about 5mV. It can be amplified over 100 times, to 500mV, and still be clean (assuming 6V-12V power).

The guitar puts out 20mV-500mV. If the amp has a gain of 4 or more, the loud plucks come out bent. If it has a gain of 100, even the soft decays are bent.

Same basic amp, bigger signal.

The Fuzz Face also taps-down the output because 2V-3V out would slam some guitar-amplifier inputs too hard. The 8K2+470 divider drops the several-Volt output to like a tenth-Volt, which amps accept gracefully. That would be counter-productive for a tape-player.

Fuzz boxes often have a "Gain" trim so you can set your input clipping point from 500mV down to 20mV, to get mostly-clean or mostly-dirty range for your particular pickup, arm, and style. A tape player will obviously pre-set any gains for correct (and clean) output from the specific tape head used.

[Joe Kramer]

Hi PRR,

Thanks for your observations and info, very helpful.

My PCB-tracing adventure was a success in terms of deducing the circuit and identifying the relevant input/output/power connections.  After hooking it up, it produces sound, although nothing inspirational.  The preamp circuit does in fact behave like a Fuzz Face, loading the input, distorting heavily, and cleaning up with the guitar volume backed down.  The circuit runs off of -4.5 volts and I nearly burned my finger on one of the output xstrs when I tried increasing voltage to about -6.  Some internal oscillation occurs when the guitar volume is up all the way, stops when it's backed down.  As you mention PRR, there is a compensation network, but it's connected between the two collectors of the input preamp. 

I'm guessing now that the only parts of this PCB of value are the xfrmrs, and it will probably be necessary to rebuild it more-or-less from scratch with higher powered xstrs similar to the Pignose.  Meanwhile, I have this rather nice wood intercom cabinet with an old GE 8" 15 watt speaker waiting for the right amp to power it.

Joe

[PRR]

> The circuit runs off of -4.5 volts and I nearly burned my finger on one of the output xstrs when I tried increasing voltage to about -6.

I bet the bias for the output transistors' bases is a voltage divider? Two resistors, no diode?

The bias should set the bases "barely ON". For Ge this may be 0.15V, or 1/30th of 4.5V. Go to 6V and you get 0.2V. I ferget the constants of Ge but I'll guess that 10mV 0.01V causes current to double. Therefore the 0.05V change of bias makes 2*2*2*2*2 or 32 times the idle current! With the 1.5 times supply voltage, this is nearly 50 times the idle dissipation!

What you want is a bias diode. Then idle current will be more nearly constant as supply voltage changes.

Or just leave the supply voltage alone. While you get "more power" at 6V than at 4.5V, if you really want more power you don't bulk-up a flea, you get an ox. You can make a lot of bedroom noise with the stock amp at the stock voltage. Hot-rodding these parts won't make is a large-room amp.

The PCB will be about the same for any reasonable hot-mod. The bias-diode can go in the lower bias resistor holes. You may have to twist-up transistor legs to get in the holes. At an extreme you extend wires to heatsink-mount devices.

If you must beat this iron, considering the small supply of small-Power Ge today, I'd be thinking a couple TO220 Si PNP, just because they are easy to get, and any Si diode will do for bias (Ge diodes vary a lot). If you "can't" go Si, then I would leave the voltages and devices alone. Love it for what it is.

[Joe Kramer]

Thanks again for your help and knowledge.

Yes, the bias for the base of the output xstrs is a voltage divider, 2.2K hot and 100ohm cold.  You are saying replace the 100ohm with a diode?  It must be a GE diode, correct?  And this will allow me to up the voltage to, say, -9 volts without frying the output xstrs?  BTW, I say 9 volts just because it's convenient voltage.  If it would buy me any more clean headroom, I'd be interested in going even a bit higher.

I realize no amount of tweaking will turn this into a all-purpose amp.  I'm thinking only of something with a unique character for recording and so forth.  As for changing out other parts, I'm not averse to going Si if it means a bit more stable behavior.  I can always plug through some of my outboard effects for the Ge sound.  Then again, once I start changing out parts, it will mean lots of tweaking and so might as well breadboard from scratch at that point.  Or just go buy a Pignose.   

[PRR]

Once you start modifying, it is hard to know when to stop.

I'm in favor of using it AS-IS, doing the minimal mods to make it guitar-friendly. Straighten out the tape EQ (if any), fiddle the gain. 5V wall-warts are around (look behind the PC). You might not be able to use your standard 9V warts because the PNPs are wired positive-ground.

You can still get the Xicon transformers and some fat Si parts, build a modern re-interpretation on vero or perf.

[brett]

Hi
A few comments. 
A couple of years ago I worked  a fair bit on push-pull amps including Deacy amps.
OTs were always an issue.  That 1/2 W OT from Mouser looks excellent and would remove a lot of the hassle of the output stage.
The Pigface was indeed a great little amp.  Low input impedance, a simple preamp, simple biasing of the driver transformer and a couple of robust TIPs driving the OT.  You could probably put 12 or more volts into it for more power.

However, I've never seen a correct schematic of it.  Even the one that was attached inside some Pigface amps was incorrect (deliberately??).  Some errors were simply scaling factors (e.g. 56 ohm instead of 560 ohms).  After much SPICEing I worked out all of the major errors.  If I can find my close-to-correct schematic, I'll post it.  Or maybe I already did.  Search?

Here's a link to a circuit I posted a while back: http://www.diystompboxes.com/smfforum/index.php?topic=53860.0
Note that the old schematic is a Mullard design and resembles a Deacy in broad terms, not in detail.
cheers

[brett]

Hi

However, I've never seen a correct schematic of it.

The Pignose schematic linked above at the Russian site DOES looks correct to me.
There are only a couple of minor differences to old circuit boards and schematics that I've seen, and these are probably due to changes over time.
From memory, the TIP41s used to be TIP31s (the older transistors are less robust - 3A vs 6A - but either would do), and the 680 ohm resistor parallel with the driver transformer used to be 560 ohms, which might shift the DC bias point just a bit.

thanks for the link.

[PRR]

Thanks for the experimental reports. Hand-winding DT and OT may be the best option above a part-Watt.

The Pignose plan in this thread (from the Russian site) looks likely.

It's been in production forever, so there are likely several revisions.

> the 680 ohm resistor parallel with the driver transformer used to be 560 ohms, which might shift the DC bias point just a bit.

It is across the winding which is probably 60 ohm DC resistance. No significant change in DC bias point.

That resistor is not "essential". Cheap radios omitted it. But then the driver transformer is working between the near-infinite impedance of the driver collector, and the radically varying impedance of two class-B bases. Transformers are affected by source and load impedance. So response varies with frequency and level. In non-obvious ways. Throwing the "nominal" resistance across a winding reduces output but makes it more consistent with frequency and level. This has to be judged "by ear". Start with a resistor somewhat higher than the winding nominal impedance, then try 1/2 that and 5 times that, see where it goes.

There is an additional factor, deep in class-B transistor amp action. If you idle the output devices cold, and drive with constant voltage, small signals don't get through: "crossover distortion". A roughness on faint signals. If the driver impedance is constant-current, crossover distortion is much less. So for this factor alone you want a large damping resistance.

(I would not expect 680 versus 560 to be any real difference; maybe they got over-stocked with 680s and were trying to use them up.)

There isn't any elegant answer to all the tradeoffs. That's a reason amp design shifted to transformerless drivers (and outputs). A different set of problems to solve, and some early transformerless amps were awful. But designs improved, and transistor prices fell, and transformers mostly vanished.

So this is technical archeology, re-discovering the past.

[Joe Kramer]

I guess we've totally hijacked this thread by now.    Great info, but all the same, apologies to the OP.

My thanks as well Brett for posting the link to your previous thread on the Deacy/Pignose.  Your descriptions of the sound and behavior of the amp pretty much clinch the deal for me.  I'll probably attempt a Pignose based on the Russian schemo.

That 1/2 W OT from Mouser looks excellent and would remove a lot of the hassle of the output stage.

Do you mean this one: Xicon 42TU048-RC/48:8/460mW?

Or this one: Xicon 42TU120-RC/120:8/460mW?

BTW, Mouser carries the TIP41C for 43 cents.  

[brett]

Hi
oh yeah, apologies for shifting away from simpler Ruby-like amps 

Joe: in general, you'll want a low impedance ratio (or you'll lose too uch power), so the 42TU048-RC/48:8/460mW is the one I'd go for.

PRR: thanks for the information about the 560/820 resistor.  I didn't know any of that.  As for cross-over distortion, I haven't seen any on my scope or heard any.  The bias diode keeps the TIP41s turned on a smidge, removing crossover distortion (and making it a Class AB amp ?).   
cheers

[Joe Kramer]

Joe: in general, you'll want a low impedance ratio (or you'll lose too much power), so the 42TU048-RC/48:8/460mW is the one I'd go for.

Thanks Brett.  I'll be using this amp with the aforementioned intercom/extension cabinet that looks from the 60s, and which contains a "full-range" 8-inch speaker with an alnico magnet, rated for 15 watts.  I want enough power to push it a little, but I'm afraid of frying it with too much.  I tried it with my Valve Jr. head and it sounded very good, but after about a minute I got spooked and disconnected it.   

Another question about the Pignose schemo, if I may?  The speaker is hooked to the CT of the output xfrmr.  Does this mean it's running at 4 ohms then?  My speaker is 8 ohms, so I'm assuming I would use the full secondary winding of the Xicon xfrmr instead?  As for the preamp out, I have no need for it. . . .

Thanks for your help!

So this is technical archeology, re-discovering the past.

Well said, and so true!  Going backwards technically isn't always justified, mainly when it comes to practical matters, I think.  But when it comes to aesthetic/artistic matters, the technology of the past seems to retain something human and organic that is missing today.  Few people want a refrigerator from the 1960's, but a guitar, amp, or fuzz pedal is a different story.