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Homebrew Equipment A place to show, design and discuss the weird and wonderful electronic creations from the hands of individual members. |
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28th Mar 2015, 5:08 pm | #61 | |||
Octode
Join Date: Jan 2012
Location: Wimbledon, London, UK.
Posts: 1,465
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Re: Valve tremolo circuit
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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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28th Mar 2015, 8:45 pm | #62 |
Dekatron
Join Date: Jun 2013
Location: East Sussex, UK.
Posts: 3,327
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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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28th Mar 2015, 9:36 pm | #63 |
Octode
Join Date: Jan 2012
Location: Wimbledon, London, UK.
Posts: 1,465
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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! |
28th Mar 2015, 11:09 pm | #64 |
Octode
Join Date: Jan 2012
Location: Wimbledon, London, UK.
Posts: 1,465
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Re: Valve tremolo circuit
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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29th Mar 2015, 2:53 am | #65 |
Dekatron
Join Date: May 2008
Location: Derby, UK.
Posts: 7,735
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Re: Valve tremolo circuit
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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If I have seen further than others, it is because I was standing on a pile of failed experiments. |
29th Mar 2015, 3:20 am | #66 |
Dekatron
Join Date: Nov 2003
Location: Heckmondwike, West Yorkshire, UK.
Posts: 9,643
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Re: Valve tremolo circuit
Yes, I remember them by alphabetical order T,U and X for 1,2 and 3.
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29th Mar 2015, 12:26 pm | #67 |
Octode
Join Date: Jan 2012
Location: Wimbledon, London, UK.
Posts: 1,465
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Re: Valve tremolo circuit
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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29th Mar 2015, 2:22 pm | #68 |
Retired Dormant Member
Join Date: Mar 2015
Location: Widdrington, Morpeth, Northumberland, UK.
Posts: 3
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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. |
29th Mar 2015, 10:00 pm | #69 |
Heptode
Join Date: Feb 2011
Location: Washington DC, USA
Posts: 619
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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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David |
30th Mar 2015, 9:15 am | #70 |
Dekatron
Join Date: Jun 2013
Location: East Sussex, UK.
Posts: 3,327
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Re: Valve tremolo circuit
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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31st Mar 2015, 3:18 pm | #71 | |
Octode
Join Date: Jan 2012
Location: Wimbledon, London, UK.
Posts: 1,465
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Re: Valve tremolo circuit
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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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31st Mar 2015, 3:22 pm | #72 | |
Octode
Join Date: Jan 2012
Location: Wimbledon, London, UK.
Posts: 1,465
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Re: Valve tremolo circuit
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It is a pity you didn't get to examine this amp more extensively. |
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31st Mar 2015, 5:06 pm | #73 | |
Octode
Join Date: Jan 2003
Location: Guildford, Surrey, UK.
Posts: 1,960
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Re: Valve tremolo circuit
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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. Ron |
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31st Mar 2015, 9:58 pm | #74 |
Retired Dormant Member
Join Date: Mar 2015
Location: Widdrington, Morpeth, Northumberland, UK.
Posts: 3
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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. |
2nd Apr 2015, 4:16 pm | #75 | |||||
Octode
Join Date: Jan 2012
Location: Wimbledon, London, UK.
Posts: 1,465
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Re: Valve tremolo circuit
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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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14th Apr 2015, 1:50 pm | #76 | |
Octode
Join Date: Jan 2012
Location: Wimbledon, London, UK.
Posts: 1,465
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Re: Valve tremolo circuit
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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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