When did the PI voltage supply change in 1969?
Posted: Tue Jan 29, 2019 5:48 pm
Greetings,
after a long absence from the forum I'm back. I've still been playing Hiwatts, but am gearing up for another round of building/modding. I have a 1972 DR103 with matching SE4123 cab, as well as CP103 that I built back in 2011 with a 69(?) Sound City cab with the 17000 Gauss Fanes (12187). I also had one of the very first Royal amps Jimmy Page clones, but sold that a few years ago.
Cool as the clones are, nothing beats a vintage amp. So I just bought not one, but two vintage "donor" Hiwatt amps! One of them is a slave amp from 73. Everything is intact except the headbox is in tatters and the front panel is cracked. The other is 1972 PA amp in excellent and near untouched condition. I feel semi-guilty about modding that one, but not many people use the PA amps anyway. The slave amp really does deserve to be built into something more "proper".
I am planning to do one as a "The Who" style CP103 and the other will be a "Jimmy Page" model. These are unobtanium as original models, so a vintage based clone is as close as anyone is going to get. So stay tuned for more close scrutiny as to the "actual" circuits of these amps! The Page amp is a black hole unto itself - what is the original accurate circuit, and how much is after the fact mods and when did those appear? But that will get a topic of its own come the time.
What I am trying to pin down right now is this; at some point in the second half of 69 (as close as I have been able to pinpoint it), the layout of the B+ line in Hiwatt amps changed. Prior to this, the pre-phase inverter "Master volume" tube received its supply from the same point as the PI, but with its own small filter cap (typically 3-8 uF) and 100k resistor. All the Reeves-built Sound City amps had this AFAIK, and many early DR103s did. Clayton's '69 non-Who CP103 did as well, and the Page amp (debut in July '69) had this layout. But some time presumably towards the end of the year the layout changed, and the voltage supply for the pre-PI tube now got its supply from further downstream, and was filtered by the 16+32 uF cap that served V1 and V2 as well.
Does anyone know at around which serial number did this change? Or was it a transition/overlap thing? The reason why I am asking is that I want to build a replica as close as possible to what the earliest "The Who" labeled CP103s had. The first amp with a "the Who" faceplate appeared in December 1969 (can be seen along with non-The Who faceplates at the London Coliseum Dec 14, 1969 Blu-Ray). By Leeds in February 1970 all three amps on Townshend's side have "The Who" faceplates. So presumably these first amps are all late 69 or very early 70.
I know this is probably splitting very fine hairs, sound-wise, but I am trying my best to be as accurate as I can. When I built my CP103 I based it on Clayton's amp and used the earlier layout. It is a very brilliant sounding amp and I don't suspect one with a different voltage supply to sound very different (if at all), but that's part of the experiment.
There's not much to glean from studying photos of alleged Who amps. Many of those are obvious fakes, presumably built on existing PA or slave chassis like I am going to do.
after a long absence from the forum I'm back. I've still been playing Hiwatts, but am gearing up for another round of building/modding. I have a 1972 DR103 with matching SE4123 cab, as well as CP103 that I built back in 2011 with a 69(?) Sound City cab with the 17000 Gauss Fanes (12187). I also had one of the very first Royal amps Jimmy Page clones, but sold that a few years ago.
Cool as the clones are, nothing beats a vintage amp. So I just bought not one, but two vintage "donor" Hiwatt amps! One of them is a slave amp from 73. Everything is intact except the headbox is in tatters and the front panel is cracked. The other is 1972 PA amp in excellent and near untouched condition. I feel semi-guilty about modding that one, but not many people use the PA amps anyway. The slave amp really does deserve to be built into something more "proper".
I am planning to do one as a "The Who" style CP103 and the other will be a "Jimmy Page" model. These are unobtanium as original models, so a vintage based clone is as close as anyone is going to get. So stay tuned for more close scrutiny as to the "actual" circuits of these amps! The Page amp is a black hole unto itself - what is the original accurate circuit, and how much is after the fact mods and when did those appear? But that will get a topic of its own come the time.
What I am trying to pin down right now is this; at some point in the second half of 69 (as close as I have been able to pinpoint it), the layout of the B+ line in Hiwatt amps changed. Prior to this, the pre-phase inverter "Master volume" tube received its supply from the same point as the PI, but with its own small filter cap (typically 3-8 uF) and 100k resistor. All the Reeves-built Sound City amps had this AFAIK, and many early DR103s did. Clayton's '69 non-Who CP103 did as well, and the Page amp (debut in July '69) had this layout. But some time presumably towards the end of the year the layout changed, and the voltage supply for the pre-PI tube now got its supply from further downstream, and was filtered by the 16+32 uF cap that served V1 and V2 as well.
Does anyone know at around which serial number did this change? Or was it a transition/overlap thing? The reason why I am asking is that I want to build a replica as close as possible to what the earliest "The Who" labeled CP103s had. The first amp with a "the Who" faceplate appeared in December 1969 (can be seen along with non-The Who faceplates at the London Coliseum Dec 14, 1969 Blu-Ray). By Leeds in February 1970 all three amps on Townshend's side have "The Who" faceplates. So presumably these first amps are all late 69 or very early 70.
I know this is probably splitting very fine hairs, sound-wise, but I am trying my best to be as accurate as I can. When I built my CP103 I based it on Clayton's amp and used the earlier layout. It is a very brilliant sounding amp and I don't suspect one with a different voltage supply to sound very different (if at all), but that's part of the experiment.
There's not much to glean from studying photos of alleged Who amps. Many of those are obvious fakes, presumably built on existing PA or slave chassis like I am going to do.
!
That's more my fault than its, I'm running through a 4x12 unported Marshall straight cab (with four of the new 100w 231's that are SO GOOD). I've got to port the cab or find another vintage Hiwatt cab for it, the 13-ply birch really helps keep it stationary 