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Old 28th Mar 2015, 5:08 pm   #61
ColinTheAmpMan1
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Default Re: Valve tremolo circuit

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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..
You're not kidding. It took me a while to get around to buying it, but my curiosity got the better of me and I parted with about £147 including P+P to get the one with the schematics, as I felt the need for them.

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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.
If you mean the "Vox AC30 circa 1959" on P12, I agree with you on the misnaming of the "Blue No.1" connector between the 2k2, 22k and 1M resistors on the cathode circuit of the second half of the ECC83 pre-amp; it is incorrectly labelled "Blk No.1". An easy mistake to make, I feel. I haven't checked this diagram with the schematic in Jim Elyea's portfolio, but I will do so asap.

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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.
I wasn't suggesting that the cans didn't have a "Black No.1" connector, just that the circuit diagram that we are referring to in Stephen's book shouldn't have one. There is an earlier circuit diagram that does have a "Blk No.1" point.

This whole mystery is fascinating. It seems that Dick Denney used the "Wurlitzer"-style tremolo circuit for the earlier AC15s and all of the AC30s. Later AC15s had a simpler tremolo using just one ECF82 and no potting, as the circuit was not so unusual. It seems that there was even a third circuit using an EF86, too. I'll have to see what I can dig out on that one. Furthermore, after Dick Denney had jigged around with the tremolo so that it would provide some degree of vibrato too, Vox didn't bother to hide the components. A bit of guesswork on my part, but I may be close to reality.

I am still interested to see what your reverse-engineered circuit looks like, if you can scan it. I wouldn't mind betting a pint of best that it won't be too far away from the circuit in the later Vox AC30s.
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Old 28th Mar 2015, 8:45 pm   #62
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Default Re: Valve tremolo circuit

Dick and his Research and Development boss, Derick Underdown were known for 'borrowing' circuits from various sources and modifying them to give the sound they wanted. I was in the works one day about 1965, when a sample amplifier was delivered from America built by the Thomas organ Co., with whom they had a manufacturing agreement. Dick plugged in his guitar, played about 6 chords and yelled out "what the *$*# have they done to it". Vox had spent many hours engineering distortion into the amp and Thomas, thinking it was faulty, took the distortion out and gave it the cleaner American sound.
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Old 28th Mar 2015, 9:36 pm   #63
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Default Re: Valve tremolo circuit

I have compared the Circuit diagram in Stephen Grosvenor's book on P.12 (Vox AC30 circa 1959) with a couple of similar ones in Jim Elyea's portfolio of schematics. These are schematic #15 (AC.30 Amplifier No.2; JMI drawing OS/007 dated 1-1-60) and schematic #16 (AC 30 Amplifier, JMI drawing OA/030 dated 1.3.60). These two schematics only differ slightly from one another, as I'll detail later, but the circuit diagram shown by Stephen is pretty much the same as OA/030 but with a few errors/differences. These are:

Stephen shows four ECC83 and one ECC82 whereas OA/030 shows four 12AX7 and one 12AU7.
The heater wiring for the GZ32 rectifier is incomplete.
The electrolytic capacitor connected to "Red No.2" is unmarked and should be 32uF.
The cathode bypass capacitor on the first ECC83 triode in the "normal" channel is shown as 25uF, 25V whereas OA/030 has 25uF, 50V.

The difference between Stephen's diagram (aka OA/030) and OS/007 are:

The choke is 10H, 250mA for OS/007 (10H, 150mA for OA/030).
The ECC83 triode which has connections "Red No.1" on its anode and "Blue No.1" on the junction of two cathode resistors has a 2k2 connected to the cathode and 22k connected to chassis; OS/007 has both of these as 2k2.

My next task is to compare the known tremolo circuit shown in later AC30 variants with this incomplete circuit, if I can retain my sanity!
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Old 28th Mar 2015, 11:09 pm   #64
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Default Re: Valve tremolo circuit

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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.
That may have been a one-off, if you remember seeing more than three valves . The "Vibravox" circuit that I mentioned, when fitted to AC1 or AC2 amps took its HT from the amp HT via a 33k, 3W resistor and (presumably) its heater power from the amp's heater chain. When fitted in other amps, an additional power pack was used with a combined HT/LT mains transformer, reservoir capacitor, smoothing choke and what seem to be two metal rectifiers in series. No valve rectifier on the circuit diagram I have, I'm afraid!
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Old 29th Mar 2015, 2:53 am   #65
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Stephen shows four ECC83 and one ECC82 whereas OA/030 shows four 12AX7 and one 12AU7.
Aren't they the same valves, though? I.E. isn't an ECC83 just the Mullard numbering for a 12AX7, and ECC82 a 12AU7?
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Old 29th Mar 2015, 3:20 am   #66
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Yes, I remember them by alphabetical order T,U and X for 1,2 and 3.
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Old 29th Mar 2015, 12:26 pm   #67
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Aren't they the same valves, though? I.E. isn't an ECC83 just the Mullard numbering for a 12AX7, and ECC82 a 12AU7?
They are indeed, but ECCxx tends to imply European origin and 12Axx US origin. Since the schematics in Jim Elyea's portfolio are pretty much copies of the original Vox circuit diagrams, it seems to me that he probably changed the original ECCxx designations to the US 12Axx ones. I was simply giving as much info as I could, without copyright problems.
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Old 29th Mar 2015, 2:22 pm   #68
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Default Re: Valve tremolo circuit

Hi Colin,
I do really appreciate your help. First thing first. I don't have the original hardware so there is nothing to actually look at. I've rebuilt it very close to the original, apart from the tremolo section as I mentioned earlier, the Vox Ac10's circuit came closest and sold it to a family member. However I did map out the original schematic from memory and with the help of some inside pictures. I made a drawing of the tremolo section as that is the part I need mostly. I've include a scan of it and also the Ac 10's circuit for comparison. Another section is the tone stack that I would need to grapple, but that is another story and probably not pertinent to this forum, or this topic anyway. So with that in mind, would you consider to look at the drawing and let me know if it would work this way. As far as mapping the circuit when looking at the available pictures, this is as accurate as possible. Certainly the values of components are correct.
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Old 29th Mar 2015, 10:00 pm   #69
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Default Re: Valve tremolo circuit

Hello,

The use of 2 EF86's and modulating the screen grid is very interesting, I think this circuit would work. I suspect the two 330K resistors between the two anodes and the decoupling capacitor would provide some damping, thus avoiding the thump you can get with tremolo devices.

I would like to try building this circuit myself as an experiment when I have some free time
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Old 30th Mar 2015, 9:15 am   #70
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I don't know what make of amp it was but I repaired a valve amp in the mid 80's (USA origin) and the tremolo speed control was a double gang pot with one gang to set the speed and the other appeared to be in a low frequency filter which tracked with the speed of oscillation and removed the thump. It had a very wide depth to the modulation without and noticeable thump. Only in my possession for a couple of hours so I didn't look into the detail.
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Old 31st Mar 2015, 3:18 pm   #71
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Default Re: Valve tremolo circuit

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I would like to try building this circuit myself as an experiment when I have some free time
I think that making this circuit would certainly prove or disprove whether it works or not. I have no formal training in electronics, so I tend to "borrow" ideas from other circuitry rather than designing my own. That means, of course, that I couldn't tell you with any certainty that this circuit will or won't work. I do have a couple of observations, however, of (to me) unusual portions, or things where I can't see a purpose in them being there.

The "TREM ON/OFF" switch is doing the normal thing (stopping the phase-shift oscillator from oscillating) and should work, if everything else does.

Why the lower end of the 2M "SPEED" pot goes (via a 100nF cap) to the phase-shift oscillator screen grid puzzles me.

There appears to be no "DEPTH" control. I would suggest that this might be necessary, even if only as a pre-set such as on the Vox AC30 circuit. You will note that there is one on the AC10 circuit, although it seems to be adjusting the DC voltage on the screen-grid of the EF86 pre-amp/modulator valve in this case (this puzzles me as to how it works, too).

The direct connection of the phase-shift oscillator to the screen grid of the modulator valve seems a bit odd. If you look at post #39 from me in this thread, you'll see the circuit of the Linear TP1 stand-alone tremolo. This couples the oscillator to the screen-grid of the modulator valve too, but via a capacitor. Perhaps the voltages on the anode and screen grid of the modulator are compatible with this arrangement.

I can't see the point of the 1M resistor which appears, together with an unspecified cap (which is surely the output coupling cap), to be connecting the modulator EF86 anode to it's own control grid.

I would suggest that the two 330k resistors, with the associated 50nF cap might be connected as a filter to decouple the phase-shift oscillator from the HT rail, rather than as shown. The Linear TP1 has such a filter, though more extensive.

I think it is good to remember that the waveform of these phase-shift oscillators is not a pure sine-wave, or even remotely so. The method of adjusting the frequency of the oscillator will unbalance it, but a phase-shift oscillator isn't intrinsically the best producer of sine-waves. Also, a sine-wave isn't necessarily the best for producing the tremolo effect, anyway. This implies harmonics which can cause unwanted effects such as "ticking" in some Fender tremolo circuits.

It is a great pity that it seems impossible to find a schematic for this amplifier; it would most certainly solve the whole problem!
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Old 31st Mar 2015, 3:22 pm   #72
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Default Re: Valve tremolo circuit

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Originally Posted by vidjoman View Post
I don't know what make of amp it was but I repaired a valve amp in the mid 80's (USA origin) and the tremolo speed control was a double gang pot with one gang to set the speed and the other appeared to be in a low frequency filter which tracked with the speed of oscillation and removed the thump. It had a very wide depth to the modulation without and noticeable thump.
That seems unusual indeed! It is much easier to put a high-pass filter in the circuit to just attenuate anything below about 5Hz or so. The circuits I have seen which have a twin-gang pot for the speed control simply vary two of the resistors in the oscillator phase-shift network to lessen the unbalancing of the circuit when only one is varied.

It is a pity you didn't get to examine this amp more extensively.
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Old 31st Mar 2015, 5:06 pm   #73
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Default Re: Valve tremolo circuit

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Originally Posted by ColinTheAmpMan1 View Post
I can't see the point of the 1M resistor which appears, together with an unspecified cap (which is surely the output coupling cap), to be connecting the modulator EF86 anode to it's own control grid.
Hi Colin

The 1M resistor along with the 1M grid to ground resistor is to define the voltage gain of the EF86 amplifier to 2. Just think of the EF86 as an inverting op-amp if it helps.

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Old 31st Mar 2015, 9:58 pm   #74
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Default Re: Valve tremolo circuit

Colin, this amp didn't have a tremolo depth adjustment, so this must have been preset by the values. The two 330k and the 50n cap are connected exactly as the AC10 if you care to take a look. The 1m connecting from the 10nF decoupling cap to the next stage back to the input grid of the preamp EF86 is to provide a negative feedback to reduce gain and provide some compression. Same solution was implemented on the other channel that didn't have the tremolo circuit connected. It had again 91k anode resistor, 390k screen grid resistor that in turn connected through a 50nF cap to the ground. The cathode resistor and cap was again the same.
The '100nF' cap that connects to the lower leg of the 2M speed pot might have a higher value as it was huge, maybe 2mF? (Not an electrolytic cap though)
The next stage was provided by one half of an ECC83, followed by a tone stack between the power stage. The tone stack had a massive gain loss, so the amp was relatively tame, but had a wonderful tone even at lower volumes.
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Old 2nd Apr 2015, 4:16 pm   #75
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Colin, this amp didn't have a tremolo depth adjustment, so this must have been preset by the values.
Yes, I see from some photos that I found that there is only a speed control. A pity, but maybe impossible to arrange a depth control with this circuit. Depth on the complex Vox vib/trem circuit was a pre-set, so there are parallels.
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The two 330k and the 50n cap are connected exactly as the AC10 if you care to take a look.
Yes, I can see that is so, but it still puzzles me. Maybe the fact that this group of components in the AC10 circuit connects to the modulator anode and that the cathodes are coupled directly together is suggesting some kind of parallel modulation idea. Not something I recall seeing before, but perhaps not a million miles away from the complex vib/trem circuit in the Vox AC15 and AC30 which employs a push-pull system on the double-triode modulator? You will notice that the network of C7,8,9 & 11 with R16,18,19 & 20 constitutes an "anti-thump" filter, which your hand-drawn circuit lacks.
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The 1m connecting from the 10nF decoupling cap to the next stage back to the input grid of the preamp EF86 is to provide a negative feedback to reduce gain and provide some compression. Same solution was implemented on the other channel that didn't have the tremolo circuit connected. It had again 91k anode resistor, 390k screen grid resistor that in turn connected through a 50nF cap to the ground. The cathode resistor and cap was again the same.
Yes, I see that, too, now. Although the idea of throwing away the gain of both the EF86 pre-amp valves seems a little crazy, especially if, as you have said, the tone-stack is horribly lossy! That DC connection between the oscillator anode and the pre-amp/modulator screen-grid is still odd to me, but might be perfectly ok. As you have said, the other channel has a 390k screen-grid resistor decoupled with a cap. That would be similar to the circuit used in early Vox amps which used an EF86 as the first pre-amp valve in the "normal" channel. Incidentally, this EF86 wasn't "tamed" by negative feedback.
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The '100nF' cap that connects to the lower leg of the 2M speed pot might have a higher value as it was huge, maybe 2mF? (Not an electrolytic cap though).
It still puzzles me as to what function it has. If, as you say, it was huge but not electrolytic, just how big physically was it? I know that high-voltage paper-in-oil capacitors can be physically large (95 x 45 x 90 cm) but they are 3kV DC operating and only 1 MFD and would be wasted here. Perhaps, given the lack of the "anti-thump" filter, this is doing something similar, as it is associated with the speed control?
Quote:
The next stage was provided by one half of an ECC83, followed by a tone stack between the power stage. The tone stack had a massive gain loss, so the amp was relatively tame, but had a wonderful tone even at lower volumes.
Most tone-stacks are pretty lossy. It is not unknown to get a significant gain-boost function by simply switching the bottom end of a conventional guitar-amp tone-stack away from ground. This is effectively changing the value of the middle control (or fixed resistor) to infinity.

I don't know if you've see the tone stack calculator on "Duncan's Amp Pages" (http://www.duncanamps.com/tsc/) but it is quite fun to play with.

I'm not giving up on finding the circuit diagram of the Dallas amp yet!
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Old 14th Apr 2015, 1:50 pm   #76
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Default Re: Valve tremolo circuit

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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.
Surprise, surprise! I went browsing following another thread and found the abomination that I spoke about. Here is the link:

http://www.vintageradio.me.uk/electronics/germ_trem.htm

It was actually "Practical Wireless" in March 1963 and designed by P.L.Taylor. The surprising thing is that the writer of this web-page describes it as working with "only a whisper of noise" and doesn't seem to have included the optional high-pass filter by the appearance of his "breadboard". Perhaps the schoolboy that I used to be was a pretty dire electronics constructor? I'd like to think that I have improved in that respect over the years.
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