When did the PI voltage supply change in 1969?

[shakti]

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.

[mhuss]

I don't know when it changed. There are not a lot of documented amps from that time to draw from.

[shakti]

Well, there's this one which is serial # 727, and has the later B+ line:
http://www.amparchives.com/folder/124/

Date codes on pots and caps put it at no earlier than second half of 69, but that would be consistent with the rest of the looks. Skinny font Traffolyte Hiwatt badge, so probably among the earliest with a Traffolyte badge rather than metal badge.

The first "The Who" amps that are reasonably well documented are #852 and #853, then 892 and 902. All of those look like they could be real judging only by the exterior and the serial number plate. Serial #742 is obviously not correct - could be a genuine Who amp, but definitely not serial #742 (Harry Joyce wired and 1971 date codes on the pots).
Serial #1095 is an obvious fake.

http://www.thewho.net/whotabs/gear/guitar/cp103.html
Perhaps 852/853 and 892/902 are the first batches delivered late 1969 and early 1970? If so, I think it's fairly safe to assume they have the later B+ line.

[OldSchoolDave]

Not sure if this helps or hurts, but this is SN 520:

20180106_2nd_try_800px.jpg (504.01 KiB) Viewed 4341 times

Dave

EDIT: Would help to show the correct tube !

[shakti]

Actually it does help - but this one is clearly before the change. That flying resistor is also something you only really see on early (usually pre-69) amps with a shorter turretboard.

[gtomax]

Hey shakti! Meet SN 538, a DR201 (4xKT88) owned by Tich from Dave Dee Dozy Beaky Mick & Tich.

Components are around 30th week '68 throughout and unusual preamp - 8+8uf filter, shared cathodes on v1 + v2, 22k presence pot...sounds killer

Some other fun things too - factory DI out and factory half power switch to name some.

538 preamp

538.jpg (101.99 KiB) Viewed 4311 times

[Alnico]

Well, there's this one which is serial # 727, and has the later B+ line:
http://www.amparchives.com/folder/124/

Date codes on pots and caps put it at no earlier than second half of 69, but that would be consistent with the rest of the looks. Skinny font Traffolyte Hiwatt badge, so probably among the earliest with a Traffolyte badge rather than metal badge.

That one's mine, I think it's probably early 1970. I also have serial nos. 424 and 675 which both have (different) Traffolyte badges and they seem to be original. No extra screw holes as I recall. Both of those are 50s, so probably not relevant to your power supply question.

[shakti]

Interesting, Traffolyte badge on a 4xx serial number, that's early for a Traffolyte badge, isn't it?

[shakti]

Hey shakti! Meet SN 538, a DR201 (4xKT88) owned by Tich from Dave Dee Dozy Beaky Mick & Tich.

Components are around 30th week '68 throughout and unusual preamp - 8+8uf filter, shared cathodes on v1 + v2, 22k presence pot...sounds killer

Some other fun things too - factory DI out and factory half power switch to name some.

538.jpg

Nice! Is that the one from The Music Locker?

[shakti]

Hey shakti! Meet SN 538, a DR201 (4xKT88) owned by Tich from Dave Dee Dozy Beaky Mick & Tich.

Components are around 30th week '68 throughout and unusual preamp - 8+8uf filter, shared cathodes on v1 + v2, 22k presence pot...sounds killer

Some other fun things too - factory DI out and factory half power switch to name some.

538.jpg

Nice! Is that the one from The Music Locker? I think I need to try all the different ones eventually. I have a DR103, will be finalizing a "vintage" CP103 and a "vintage" Jimmy Page model sometime this year, but a DR201 is one I've never tried.

[Alnico]

Interesting, Traffolyte badge on a 4xx serial number, that's early for a Traffolyte badge, isn't it?

It is, but appears to be original. The head box still has the original Rexine and there are only two screw holes where the badge goes. I have a repro metal badge and that doesn't fit the holes.

Here is it with s/n 675, which has the thinner badge.

[gtomax]

Hey shakti! Meet SN 538, a DR201 (4xKT88) owned by Tich from Dave Dee Dozy Beaky Mick & Tich.

Components are around 30th week '68 throughout and unusual preamp - 8+8uf filter, shared cathodes on v1 + v2, 22k presence pot...sounds killer

Some other fun things too - factory DI out and factory half power switch to name some.

538.jpg

Nice! Is that the one from The Music Locker? I think I need to try all the different ones eventually. I have a DR103, will be finalizing a "vintage" CP103 and a "vintage" Jimmy Page model sometime this year, but a DR201 is one I've never tried.

Yep, came over from TML. The 201 is great! I echo the "really above a certain level, loud is loud" the 201 isn't THAT much louder perceivably than a 103. The main difference is the 201 will make its cab walk across the room like a busted washing machine 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

I have 3 Hiwatts -> This 69 metal badge DR201, a 50w PA from 72 in mint condition with original cover, tubes, the works, and a DR103S I built a long time ago (pre Royal amp clones becoming available) that kills. They're all wonderful.