Valve tremolo circuit

[MrElectronicman]

Colin, some nice circuits you have there. I must try a couple.
Also intrigued more and more with the circuit that Tommy posted, looks like I will be trying that one as well.

[ColinTheAmpMan1]

Unfortunately, Richard Dorf's article is a 2,686 KB pdf file and I can neither compress it enough to post, nor can I post rar volumes. If anyone is interested, please PM me and I should be able to attach it to an email.
Colin.

[ColinTheAmpMan1]

Originally Posted by Dr Wobble

The Wem dominator had a tremolo effect on it using an ECC83 I think.Quite a simple circuits as I recall,Andy

None of the Watkins Dominator schematics that I have show a tremolo circuit, but the W/14/T and W/20/T certainly did. The single-triode phase-shift oscillator (half ECC83 - what a surprise) had both speed and depth controls and seems to have modulated the audio by the simple expedient of direct coupling the cathodes of the oscillator and the first amplifier stage (the other half of the ECC83). There was a high-pass filter included, which I suppose was pretty much essential to suppress "thump" from the oscillator.

In either "Practical Wireless" or "Practical Electronics" when I was in my teens, there was a circuit for a transistor tremolo unit using red-spot and/or OC71 devices, IIRC. This was a horrible thing which used a multivibrator as the oscillator and attempted to convert the square-wave to an approximation of a sine-wave with a low-pass filter network. This was doomed to failure and the "thumping" was enough to convert any amplifier that it was attached to into an early drum-machine - but only kick-drum. With a bit of luck that one has been lost forever.

Regards, Colin.

[MrElectronicman]

Originally Posted by ColinTheAmpMan1

If anyone is interested, please PM me and I should be able to attach it to an email.

PM sent

[tommytoneism]

Hey guys,

So I'm building the PW trem circuit and I have a new ef80 on it's way.

I did have it working very briefly (but not very well) at the weekend but since nothing, before the new valve turns up I was wondering if maybe I've missed something else?

I'm not convinced of some of the voltages in a couple of places: 79v to the anode of the EF80 and 50v the grid2..?

Like I said in previous post the pre amp is working great ..just no trem?

when it was working I was getting some swing in voltage on the cathode which I gather is the sinewave? but now it's static??

Anyway clucthing at straws.

more here on Merlin's vibrotron for you electronicman:

http://ax84.com/drums/bbs/dm.php?thread=292675

thanks

Tommy

[MrElectronicman]

Originally Posted by tommytoneism

I'm not convinced of some of the voltages in a couple of places:

Looks like there is a problem somewhere, you should have much more voltage on both the anode and grid2. Did you check grid-1? you should have a small negative voltage there, if there is a positive voltage, you may have a leaking capacitor or a wiring error.

[MrElectronicman]

Hi Tommy,

I just connected up my circuit, and mine does not work either now, must be something in the air.

The voltages I have are:
HT supply 250 VDC
EF80 Anode 108 VDC
Grid 2 80 VDC
Cathode 1-3 VDC depending on the rotation of VR2 (2K)

I will try to find out what the problem is.

[tommytoneism]

Ah!

OK well that gives me hope, I'll tear into it and check some of the components.

good luck with yours

I'll report back with my findings..

Thanks again

Tommy

[MrElectronicman]

Hi again Tommy,

Got my circuit working now, I had purloined the screen bypass capacitor for something else.

New set of voltages
HT 250 VDC
Anode varying between 140 and 170 volts. At least using my DMM.
Grid-2 77 volts
Cathode 1-3 volts in sympathy with the frequency (2-7 Hz)

Based my observations.

Check the screen bypass capacitor c10 16uF, I used a 22uF

Ensure there is a decoupling capacitor between the HT power supply and the EF80 per the diagram C11 32uF, I used a 50uF.

Some of the resistors I had used originally were quite old and, as they were carbon comp types, they had drifted up in value by quite a bit. The circuit is quite critical on the values of both the capacitors and the resistors being the same. Thus the 0.1uF should all be the same value and the 500K resistors the same. Does does not matter so much if they are 470K as long as they are the same.

[ColinTheAmpMan1]

Hi,
I have found the document with the Spice analysis of the Vox vibrato/tremolo circuit. It is attached.
Regards, Colin.

[cservonyec]

Hi Colin, it seems you know your stuff.

I had an amp made by Dallas, a Dallas Rangemaster Model 5412 Combo Amplifier. It had a tremolo very similar that you've described, as it was a type where the LF was done with an EF86 valve and it was modulating the screen grid of the preamp that was also an EF86. Unfortunatelly on this amp the tremolo didn't work for some reason, when turned on, you could hear the thumping but no input signal.

Since I've stripped the amp and used it's parts to build another one, but I did not map out the schematic. Big mistake!!! Later I've rebuilt it as close to the original as possible, using pictures from the amp's inside. The tremolo is almost identical to the Vox AC10 with an EF80 instead of EF86, but that is cathode coupled, whereas the dallas was defenitely screen grid coupled. I couldn't work out the circuit for that, and built it with the AC10's circuit. I would love to build another replica of this amp with the original tremolo design. I have a schematic that I sort of mapped out, and lots of pictures.

Would you consider to take a look at these and perhaps together we could figure out the original schematic?

[ColinTheAmpMan1]

Hi, I could possibly help, but what exactly do you want of me? I don't live anywhere near Northumberland - I'm in London and so can't actually look at the hardware.

Have you had a look at this site?:

http://www.vintagehofner.co.uk/britamps/restof.html

Regrettably it doesn't seem to have any schematics.

My copy of "The Tube Amp Book" by Aspen Pittman also doesn't list any Dallas amps, so that's no help. The circuit in that book also shows the Vox AC10 as having tremolo achieved by using the pentode part of an ECF82 as the oscillator, its cathode commoned with the cathode of the input EF86 channel 2 preamp valve. The triode section of the ECF82 is used as the channel 1 input preamp. Dick Denney also put a triple C/R filter in the output from the EF86 to try to tame "thumping" from the tremolo oscillator. Not exactly the best way to get the job done in my opinion, but who am I to criticise the revered Dick Denney?

The difficulty is that if you have completely re-constructed the circuit, there is nothing that will tell us what the original components were. Educated guesses could be made and the result, when (and if) built might well work, but there is no guarantee of that.

I don't wish to appear negative, anything but. It looks as if what you want is a schematic of the original Dallas amp, is that correct?

Colin.

Nosing around more, I see you have been to another forum (http://music-electronics-forum.com/t4768/) on this topic. Someone actually posted a schematic for a Marshall 10-watt amplifier, which seems to be very much like your Dallas beastie. Is the attached circuit of use?

[vidjoman]

I remember getting a Vox standalone tremolo unit to service about 1960 and although I lived about 300 yards away from their works, and Tom Jennings brother lived in my road, they would not help with a circuit diagram. All the parts had been covered in paint making it impossible to diagnose the fault. I recall that there were about 5 valves and mostly ECCxx types. The fault was cured with new valves but I never did work out any sort of circuit.

[ColinTheAmpMan1]

Yes, while there were fairly simple tremolo circuits in many early Vox amplifiers, the original tremolo circuit used in the early (1959) AC30 was definitely "hush-hush". The amplifier's circuit diagram was bristling with odd connections marked as "Yel No 2" and suchlike and a whole chunk was missing. I have heard that they even potted some components in cans like electrolytic capacitor cans to preserve secrecy. I think this was because Dick Denney actually purloined the tremolo from a Wurlitzer organ circuit and didn't want to be caught out. He did eventually show the circuit in later AC30 models, but by then he had jigged it around a little to achieve vibrato as well as tremolo and so felt he had got around Wurlitzer's patents. This may be all hearsay (says Colin to avoid any litigation from anyone).

[Omegaman]

Colin,
I have in the workshop an original 1958 Vox AC1/15 which I have just repaired. Large chunks of the tremolo circuit are indeed potted in capacitor cans with coloured wires emerging. I reverse engineered the circuit and rebuilt one of the 'cans' after removing the contents. Dick Denney didn't reckon on the man with the drill and hammer investigating...!
The circuit has nothing special going on as I found when I drew out the diagram from the amp.

Regards

[ColinTheAmpMan1]

Howard, have you seen the books "A Service Engineer's Guide to the Vox AC30 Valve Amplifier" by Stephen Grosvenor and "Vox Amplifiers: The JMI Years" by Jim Elyea? Stephen has redrawn a lot of circuits for the AC30 and its variants, as well as other Vox amps and add-ons. He has the partial circuit for the 1959 AC30 with the "secret" tremolo (although I have just noticed that he mis-identifies "Blue No. 1" as "Blk No.1" - there is no Black No.1).

Jim Elyea's book has an option to include a portfolio of schematics, some of which are rare birds indeed. Schematic #56, dated "3-3-58", is the original JMI "Vibravox" and has notes to the effect that certain components are hidden in cans. A part of the wording reads: "the hermetically sealed cans No. 1,2&3 must not be opened to the air". The circuit diagram doesn't identify the components in the cans, it has the connections very similar, but not identical to those on the 1959 Vox AC30 circuit.

Schematic #11 is the "AC/15 Amplifier No. 2", dated "4-12-59" and includes the "Vibravox" with all components. This simply notes that the "Vibravox" components are shown within a dotted area and are connected to the rest of the amplifier with a B9G plug-and-socket.

These two tremolo circuits are not the same! That in schematic #11, the "AC/15 No. 2", uses one ECF82 valve, while that in schematic #56, the "Vibravox", uses two 12AX7 and one 12AU7. Further to this, Jim's schematic #16 (drawing OA/030) is of the 1960 AC30 and has the tremolo circuit without showing the "mystery" components, only the connection points. On this circuit, "Blk No.1" is another chassis earth.

The main text of Jim's book states that the potted components were originally in three cans; this changed in early 1960 to two cans. By summer of that year, a tag-board was potted into a single aluminium box filled with Araldite. By the end of the year, potting ceased on the AC15, but not on the AC30.

I would be interested to know if your reverse-engineered circuit is the same as any of Jim's schematics or is something different. I am disinclined to scan my copy and post it here for copyright reasons, but if you can send me a scan of yours by PM, I''ll do a comparison.

As I said, Jim's portfolio of schematics has some rare birds and includes drawing A/026, which shows the perplexing R74 referred to on drawing OS/010 which is "removed when fitting unit". This resistor doesn't appear on any other AC30 circuits. For reference, drawing OS/010 is the "optional brilliance unit", otherwise known as the Top-Boost circuit.

[vidjoman]

So the one I saw must have been "schematic #56, the "Vibravox", uses two 12AX7 and one 12AU7" as I recall it had several valves as well as it's own power supply, so the rectifier was probably one of the valves.

[kalee20]

Originally Posted by Mike Phelan

Note that tremolo is not the same thing as vibrato. Tremolo is an amplitude variation, vibrato is a frequency variation.

That's true, but they can sound very similar.

Narrow-band FM, with a small modulation index, has sidebands of very low amplitude except the first (carrier frequency plus modulation frequency, and carrier minus modulation). And that is the same as amplitude modulation. The only difference is the phase of the sidebands relative to the carrier, which is quite different. But the ear is insensitive to phase! So it sounds the same.

If the vibrato swings the frequency a lot, and the rate of swing of frequency is low, then the sounds are recognisably different. But if the rate is quick, and the change in frequency is only a few percent, you can get away with a tremolo circuit for both effects.

[Omegaman]

Colin.
I would like to get a copy of the Elyea book, but the one with the schematics is a bit difficult to get hold of at sensible money.
The circuit of the AC30 in Grosvenor's book is wrong. Nothing like the one I did a little while ago. I reverse engineered the circuit from a completely untouched 1959 model. All 3 of the modules do in fact have a red, green and a black wire. It is the purple, yellow and orange wires that distinguish which module is which. The modules are standard capacitor cans with a small pcb potted in some of the hardest potting compounds I've ever come across!
All of the later AC15 circuits using an EF86 are different in the tremolo dept. from the original AC1/15.

[Ed_Dinning]

Hi if you are still stuck send me a PM and I'll see what can be done. I live just to the south of Newcastle.

Ed