Vox Berkeley II

[lowell]

http://music-electronics-forum.com/a...y-ii-v1081.pdf

This amp has some minor issues that I'll be repairing. I wanted to start a thread to discuss why this amp sounds the way it does - specifically how much the transformer coupling plays a part in the sound. I read RGs article here Thomas Vox Solid State Amps. So the driver transformer was used as a PI basically. Being that the Germanium transistors may not have had an NPN counterpart, the transformer could drive each one out of phase in push-pull.

Do we think there is any transformer saturation in this circuit? Is there a way to test this? Is transformer saturation when the field collapses due to too much voltage or current?

I am going to scope through the amp and see where the overdrive is produced, but I'm mainly checking to see if the Germanium output transistors are a big part of the sound.

[R.G.]

The transformer does a few things:
1. It forces low feedback conditions. Transformers are at least a second-order LC filter, and the economics force you to not design the transformer to put the LC time constants out of the way for feedback issues. The only good way to deal with this is to use low feedback. This prevents the feedback from hiding the eccentricities of the amplifier.
2. It is a single ended transformer stage as used in these amps. One polarity output transistor is driven by positive transformer action when the driver transistor pulls down on the primary, but the other polarity can only be driven by the stored volt-seconds in the transformer core. These work in different parts of the core's b-h curve, impressing some asymmetrical distortion on the drive to the output transistors.
3. Clipping is asymmetrical when you hit it, as getting the idle current on the driver transistor to clip at the same relative peak current as the driver transistor saturates on the opposite polarity is essentially impossible. I think this is one reason the limiter circuit was introduced in the bigger brother amplifiers; Buckingham, Guardsman and Beatle.
4. The output stage is, as a UK writer put it "limply driven". There's not a lot of current drive for the outputs, and the output transistors have high beta falloff at high currents, which makes for a form of soft clipping as the effective beta droops at higher currents. This has the potential to sound GREAT!! for musical amps.

There are others but those are the big ones.

By the way Lowell, I can't reply to your last pms, as the box is full. you can email me.

Use the id keen, and send it to the domain I use for my web page, geofex.com.

[lowell]

Thank you! Thats awesome info...the amp certainly exhibits the sound of asymmetric distortion! And yes thanks for the email. I pm'd you for your email but guess that didnt go out. Yah my box is full, and I dont want to delete anything.

Is there a way to fix this without deleting messages?

I'll send an email.

[loudthud]

This is just a hunch I have about what makes a good sounding solid state power amp. This applys to simple bang the rails designs, not chipamps with mixed mode feedback that never actually slam the rails.

The open loop output impedance is on the high side. Because the drive to the output transistors is floating, you don't get the lower impedance of the emitter follower. Combine this with low global feedback, and it is much the same as a black face (tube) Fender. Not much damping factor and a higher impedance when clipping occurs.

The single ended drive of Q7 does not clip symmetrically. This produces a non-symmetrical square wave at high clipping. Symmetrically clipped square waves sound hollow. The capacitor coupled output (C22) removes the DC component that otherwise would be produced. This, and a poorly regulated power supply, adds a certain low frequency component that gives the amp more of the feel of a tube amp. The low feedback also allow the gain to compress slightly as the power supply sags.

IMHO, the driver transformer saturation (no proof of this) occurs on low frequency transients like pick attacks and this affects the tone of signals that otherwise would be well above the low frequency saturation of the transformer.

[lowell]

Good stuff.... Pretty wild that solid state is so demonized, when its really the CIRCUIT, not using ss devices, that dictates the sound. Inspires me to wanna build a ss amp that could pass a blind fold test. This amp sure as hell does!!

Is that what Quilter is doing? I have a few musician friends in the midwest that are liking those alot.

[J M Fahey]

Not this way, definitely.
Circuits are not available, but being a Class D design, I guess that they precede it by a front end which clips gracefully and does all the nice things, and then the Class D design cleanly amplifies that.

As of the transformer driven design, here in Argentina a very popular power amplifier design, which was being used when I started, 40 years ago, and is still used, is this, maybe you can guess why, it sounds quite good when overdriven, if driving a large 4x12":


one added bonus is that FAPESA (our local branch of Philips Holland, same as VALVO/Mullard/Miniwatt/etc.) liberally published lots of data to help construction,including hard to find specs of driver transformer:


another added bonus is that amps are easy to repair: besides being very simple, the driver transformer separates power transistors, which are the most stressed and the first to die, from the rest, so absolute worst case, a semi experienced Tech just replaces both transistors plus a handful of resistors, not much troubleshooting needed, and the amp comes to life.

EDIT2: another bonus is that power transistors are barely given enough drive current, which works as a de facto short circuit protection, not bad.
As of symmetrical clipping, the FAPESA circuit allows for driver transistor idle current setting with enough range to achieve that, if scoping full power into a load.
In general, a circuit which works much better than what it looks on paper.

[lowell]

Love simplicity!!! Thanks for sharing. Seems like a class A transformer driver is the key component to asymmetric clipping.

[R.G.]

As of the transformer driven design, here in Argentina a very popular power amplifier design, which was being used when I started, 40 years ago, and is still used, is this, maybe you can guess why, it sounds quite good when overdriven, if driving a large 4x12":
...
one added bonus is that FAPESA (our local branch of Philips Holland, same as VALVO/Mullard/Miniwatt/etc.) liberally published lots of data to help construction,including hard to find specs of driver transformer:

Juan, thanks for posting that. It's very similar to the Thomas Organ Vox driver circuits. I've been trying to reverse engineer the driver transformers for the Vox amps for some time, and I think I have most of it down.

My technical Spanish is far worse than your technical English. I can order in restaurants and talk about the weather, but "winding" and "turns" and "area" don't turn up much in those conversations. Could you translate the winding data? Pretty please?

I do know that the Thomas units have more primary turns, and do a different layering/filar mechanism. The primary turns are about twice that, and the secondaries less (which indicates a higher current step up ratio, more current for the outputs), and quite possibly a different stack. I speculate that the core is butt-stacked, no interleaving, with a spacer.

If I were winding that one, I'd probably go trifilar, winding primary and both secondaries at the same time.

[J M Fahey]

Ok, here it goes:

"Exciter" (Driver) Transformer
Build data:
Primary turns:275 t
Secondaries turns : 275 t each
Trifilar winding (you guessed right )
Primary enamelled copper wire diameter: 0.36mm (~ 27 AWG)
Secondary enamelled copper wire diameter: 0.29mm (~ 29 AWG)
I guess the difference comes from primary carrying DC full time.
EI lamination #62 , 16mm center leg, stacked 21mm
It's an old US core size convention, center core 0.62" = 5/8 inch ; stack height would then be ~13/16 inch
Oriented grain silicon steel.
Block mounting (all E on one side, all I stacked on the other) "no need to add an extra gap" (because surface irregularity and punching errors are supposed to be enough) ... personally adding a single printing paper thickness to the gap makes me feel safer and is definitely more consistent than trusting lamination punching dies sharpness or dullness.
Primario: primary
Secundario: secondary
Image EI dimensions in millimeters.

I suggest you wind one (or a couple) and build one amp , using MJ150xx of course, forget about any modern 2N3055 , the Philips suggested BD183 is a very strong "2N3055" and we used original RCA 2N3055H .

These amps turn up often at my bench, most desperate (even sent by Techs) , because "they repaired them 10 times and the d*mn beasts keep blowing" ... of course they use modern 2N3055 .

If I get one such amp as a trade in I plain junk the board and substitute one of my own, based on IRFP240/250 or 4 x IRF640, to profit from the relatively high and stiff +/- 45V rails ; FAPESA/Philips also offered a big beefy power transformer design , a core size I use today for my 300W amps, go figure.

These amps were originally designed to drive a step up transformer and power railway and bus station or school/church/supermarket PA systems and such.

The only problem is (for us) to get 50 ohms thermistors, the standard solution is to use fixed 22 ohms resistors instead but amps then show some crossover distortion ... not a problem in a distributed PA system.

[olddawg]

I guess it's entirely subjective, but I wish someone could tell me what is magical about these amps that wasn't magical in the 70s and 80s. I remember them being door stops that we bought to take the speakers out of. I remember buying Super Beetles for $80 with the head, cabinet, and frame. Now the tube models have been desirable throughout all of these decades. But these old germanium Voxs? Really?

[lowell]

As you said its subjective. I love compressed grainy frothy warm overdrive. This has it! On the other hand I HATE twin reverbs. Tone ...to each their own! And need I mention you wont find a solid state amp in a store made from 1980-recent that has this. (im sure there are some exceptions) because manufacturers were trying to eliminate this sound in search of clean headroom. All kinds of negative feedback and diode clipping etc... Causing cold stale harshness.

[loudthud]

I have a Cambridge Reverb somewhere. I think it's a half power, single 12 (or 10?) version of the same amp. I'll dust it off and give it a try.

[J M Fahey]

I guess it's entirely subjective, but I wish someone could tell me what is magical about these amps that wasn't magical in the 70s and 80s. I remember them being door stops that we bought to take the speakers out of. I remember buying Super Beetles for $80 with the head, cabinet, and frame. Now the tube models have been desirable throughout all of these decades. But these old germanium Voxs? Really?

Magical?
I don't believe in magic so any mention of it in a Musical/Tech context will bring the same respect as homeopathy/astrology/gemology or Reiki.

Now as of the sound, it was the Beatles' sound,period.
You want to sound like the Beatles? ... you get a Vox ; you don't, go get something else.

SS? Be my guest, they played clean basically.

No, they were not used by Jimi Hendrix, Jimi Page or Ritchie Blackmore nor any of the other Eric Clapton wannabes so, in that sense, no "magic" attached to them, ... but that's something else.

And of course, to Americans it was just a foreign, "Limey" amp, why would they respect it?

Plus they were *expensive* .

The today so revered Tweeds were in every pawnshop, sold also for 80 bucks ... or less, nobody wanted them.

Some had the tweed spray can painted black to look like the new shiny black Tolex which became all the rage.

As of SS in general, Americans were quite happy buying and using *tons* of Acoustic/Kustom/Peavey in the 70s, until the second British invasion swept them with their Marshalls.

Santana himself played Woodstock with a Gallien Krueger, I saw him in a World Tour using early Boogies but the full backline was Peavey, Queen used a couple Tube AC30 and completed the wall with slaved AC30SS , go figure.

Now for those who want "magic" , fine, go watch a couple Videos and get what the Guitar Gods use.
Maybe some of that godlyness is sticky and gets attached to the new player

[olddawg]

In truth. I believe the Beatles used a lot of different stuff, especially later on. And... I think Page even used a big SS Vox from time to time. Hendrix played through Dual Showman at the "Rainbow Bridge thing." (Still sounded like Hendrix as does Santana through anything. Tone is in the hands) I don't remember tweeds being door stoppers. There certaintly wasn't such a reverence for old used gear back in the day but there has always been a demand for tweeds, Magnatones, Valvcos, and such. But SS Vox and Kustom amps.... ? Yeah different beast altogether. I was just curious as to what the deal (magic) was. Working ones are scarce now (which usually makes things desirable) and I remember not caring for them as a kid. We also threw out Silvertone 1484s as well in the 80s. Nobody thought they were worth new tubes and went in the dumpster.

[lowell]

For me every amp can have its place. If I was gonna dedicate time to be a funk guitarist I'd use a solid state amp. Probably something cheap like a Fender blah blah 65 or whatever. I wouldn't even go the tube route at all. For RnB, Blues, rock etc gotta have that warm compressed OD. Same with bass amps. I dig that Chicago gospel scooped mids slap sound for certain things...but it really doesn't work well when playing Motown. Amps, guitars, pickups - all tools to achieve a specific sound. My fitty cent.