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Old 4th October 2007, 02:16 AM   #1
m6tt is offline m6tt  United States
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Question adding reverb to a PA head

I am still retrofitting an old McGohan PA head. Having more than a few Accutronics, Gibbs and OC reverb tanks lying around, I am trying to add reverb to the beast. The original circuit is fairly simple, I'll try to scan the schematic which Jim McGohan was kind enough to mail me. It uses 1/2 12ax7s as the mic/guitar pre-amps, followed by volume controls. These combine using resistors and drive both halves of a 12ax7 with some kind of local feedback connecting the output of the last half to the cathode of the first. Very sketchy caps (.005/330k high pass) are used to connect the first half to the second, seems like a Q-cut, and maybe impedance lowering. This is followed by the treble and bass controls which drive of all things a 12av6 (the 7-pin AA5 tube, diodes unused). This tube drives the 12at7 and on to the 6ca7/el34s.

There is a well placed octal socket available which was used once for a relay of some kind (although nothing is soldered to the socket, it says priority relay on the chassis). I have a 6v6, and I was hoping to drive the reverb with that.

My proposed circuit uses one half ax7 with cap coupled output into another 1/2 ax7 for the reverb, as well as 1/2ax7 general pre-amp for the clean signal. On the reverb signal path, the ax7 is followed by a 6v6, which is transformer coupled into the reverb tank. At this point the signals would be out of phase, but after recovery with the other 1/2 ax7 available (out of two total tubes), I believe it would be in phase. The output of the clean channel and the reverb circuit are combined through 330k resistors followed by the tone control. This drives the 12av6 and subsequently the inverter and power stage.

Is a 6v6 a reasonable tube to drive a spring reverb? or should I steal a precious 6sn7 from my in progress hammond m2 repair? Will removing the Q-cut/Z-lowering stage take away to much gain or is such a thing expected to be around unity? Finally, why would the designers have used a 12av6 at all in this design, especially right before the PI and power stage? I will try to get access to a scanner and see if I can post some images.

Thanks for helping a newbie, you all are the best!
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Old 4th October 2007, 04:23 AM   #2
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you could always use a 12at7 or 12bh7 as a driver instead of a power tube. A fender reverb transformer is around $14 from almost any place online. I believe it is 25k:8 ohms, the high impedance is needed because of the 12AT7 being two paralelled triodes. Try googling the reverb circuit of a fender twin reverb or something similar ( minus the tremelo). It only uses two tubes, one 12AT7 for driver and one 12AX7 for the recieveing end.
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Old 4th October 2007, 04:30 AM   #3
m6tt is offline m6tt  United States
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I would have done that, I have an amp with a 12au7 side doing SE through a transformer into a 4-ohm Gibbs tank. Unfortunately, all I have is an octal socket. The original Fender reverb apparently used a 6k6 through a transformer. Apparently the 6k6 is a low dissipation 6v6 in a few aspects, enough so that apparently the 6v6 can be subbed for a 6k6. Do you know of any usable octal dual triodes that can do this that aren't hifi, but good enough for a reverb? Alternately, I was thinking about using a different pentode, but I can only really get good tubes if they aren't used in very much.
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Old 4th October 2007, 06:11 AM   #4
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Hi,

Got an ECL86 laying around? Use an old SE transformer from a 50C5 line-opped radio. Perfect load Z for the pentode section (even trioded) and the triode is like a 12AX7 and makes a great recovery tube... all in one bottle

Cheers!
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Old 4th October 2007, 06:29 AM   #5
m6tt is offline m6tt  United States
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no ecl86s around, just a mullard ecc86/6gm8 ...Essentially this amp has 3 9-pin sockets, one of which is phase inverter (12at7). There is one 7-pin socket (12av6), used as post-tone stack amp(?). There are 4 available octal sockets, 2 are output tubes, 2 are sockets on the top of the chassis for non-tube octal socketed items. One octal socket is empty and not connected to anything. One octal socket contains an extra microphone transformer, of which this amp does not need two for the way I plan on it being used. Long story short, probably an AX7 in the first position and second as well, or a DW7 driving the reverb tube but then I am short a recovery section.
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Old 28th December 2007, 10:59 PM   #6
m6tt is offline m6tt  United States
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well, it's been a while, here's my progress so far--

I axed the second input channel, and I intend to use that 1/2 ax7 for reverb preamp. This will lead into 1/2 6SL7 (ideally, have been using sn7) and out through a fender reverb tranny. I have completed the reverb return, and it works really well (I found a 1954 RCA receiving tube manual at the dump, used cap values etc. from there). The only things that might be an issue are audio phase and time constants (I don't really understand time constants yet, but I believe they are related to phase). I have no problem trying to determine relatively whether the phase is inverted or not at a given point, but I am worried about capacitor count shifting the phase by less than 180. Is that bad in a reverb channel?

The tone controls on the head so far seem alright, in that the guitarist it's being built for seems to like them, and they provide a wide range of control (I think they're a baxandall variant being driven by a cathode follower...might be fender style, but no mids...haven't dug into that part of the amp too much yet).
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Old 29th December 2007, 02:29 AM   #7
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hey Geek I like that idea! Typically those old radios came with 4 ohm speakers, so putting an 8 ohm tank on one of those OPTs would be even better on a triode. I just finished my own build of a deluxe reverb section, and have it spliced in with my own amp. I don't like it much- WAY too much 'verb on tap. Maybe I don't need as much drive, this would mean that I could use 1/2 of a 12at7 ( or 12DW7) for the driver and the other half for the recovery. This would cut costs down... and I really don't like having that extra gain stage between my tonestack and the amp just to compensate for tonestack insertion losses.
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Old 29th December 2007, 02:45 AM   #8
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no such thing as too much reverb on tap...I guess it depends on style I suppose. My amp, which used to have a 12dw7 side driving a transformer into the reverb blew the transformer (disconnecting the reverb tank is just like leaving the speaker unattached...mine had a bad RCA cable). I dropped in a fat 50c5 transformer from a store PA (somehow driven by a 50c5)...never sounded better, although I like using a 12ax7 (GE fatty plates) in that spot for the super-saturated surf sound.
Unfortunately in this amp, I only have 3 noval sockets, but I have another 3 octals available not including the power tubes (it has a solid state recto). I want to be able to dial in everything from the slightest taste of reverb to dick dale.
Another note, I just finished another amp with a diode fuzz on the main channel after the reverb is split off...The notes come in fuzzed but the reverb is clean...it sounds pretty nice for retro psychedelia.
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Old 29th December 2007, 03:52 AM   #9
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isnt using a 12ax7 as a driver a little hard on it since it cant dissipate as much? I guess your right about taste, I just know a lot of people that will never use all the reverb that a fender can provde, most people I know never take it above 4 or 5.

Hey is it better to you to have just a 'verb control, or both reverb level ( or "mix") and also depth?
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Old 29th December 2007, 04:09 AM   #10
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I just have a reverb control...it's weird though, not having dug into the amplifier's internal circuitry too much (it's fairly complex), it seems like it must adjust grid resistance or something...it's a 1 meg pot, and with 1 meg resistance between the red and white wire, I have dick dale style reverb. 500 k gives me a reasonable resonance, and anything less is barely perceptible.
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