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Old 27th December 2016, 03:35 AM
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Greg. Greg. is offline
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Join Date: Dec 2005
Location: Bristol, UK
Posts: 3,378
Default Re: Upgrade ideas for 300B PP

Hi Lyndon,

Well, it looks like you have bought Gerry's amp so congratulations. Before you start any upgrade process, you need to know what Gerry or any previous owner has done. I'm not sure of your amps heritage but certainly Richard has previously been involved, possibly even the previous owner. If that is the case, undoubtedly I would expect some tweaks, or upgrades to have already been carried out. Furthermore, if it was previously Richards, it is also likely the previous owner to him was Neal Gibbons, a member here although ages since he contributed. He is also a tweaker so may have carried out some of his own modifications. I'm being highly speculative here based on my own little knowledge and I could be completely wrong. Maybe Gerry or Richard can confirm. My main message is, using the original schematic and build spec, check to see what has previously been done and exactly what it is you have. Is it altered or is it original?

I am happy to share with you my own 300B PP spec but please bear in mind that this is based on my own personal take on what can get the best out of this amp. Your preferences may be different so you may not be inclined towards all that I have done. The secret is to make one change at a time so you can assess the corresponding differences to sound you hear. Also be aware that I have never owned an original amp like you now have. I just loved it so much, at a much later date I built a clone albeit I used a very similar PCB and the chassis is of the original dimensions.

You will need to refer to the original schematic and parts list. Here we go........

C1,R3,R4 not fitted. Converted to battery bias using one per channel AAA NiCad battery (NiCad now hard to find. NiMH or alternative easier but they can bring a brittle edge to the sound whereas NiCad sound cool).

C2/5 to lowest esr Evox Rifa.
C3 (V1/V2 coupling) to VH Audio teflon V-Cap. When I did it it was pretty affordable. Will cost you an arm and a leg now.
C4 to Black Gate FK. Probably no longer available but gives you an idea what to go for.
C6/7 to Black Gate standard. Same availability situation applies.
C8/R10 and feed back loop omitted as of no value to me with the speakers I use. Likely to be the same for you.
C9/10 to low esr Evox Rifa.
C14-17 omitted owing to diode upgrade.
R1/5/6/8/15/16/17/18 to equivalent Kiwame.
R2 Vishay S102.
R9 to 12w non-inductive Mills.
R12/13 to 1K5 25W aluminium clad Please note that the value of this resistor depends on your intention on 300B plate dissipation. I have deliberately increased value to reduce dissipation to give my 300B's (plate life) an easier time (see more on this below).
D1-4 to 8A 1200v Fairchild FRED's. The reduced voltage drop and fast soft recovery these produce is the reason for omission of C14-17 snubber. However to further assist this voltage reduction and kindness to the 300B plates, I used Welwyn W22 120R voltage drop resistors immediately in line between L1/2 and C11/12.
Rectifier is Mullard GZ37.
I also introduced a DC heater circuit to feed V1/2. I am having some difficulty uploading this schematic to the thread so if you PM me your email address, I'll pass onto you the schematic and detailed instructions on how to implimemnt this additional circuit.

Valves have been the great and affordable Electro-Harmonix 300B gold grids coupled with some NOS options for V1/2. Your choice. I majored on Tung Sol and Jan Phillips.

Thereafter you can mess to your hearts content on the quality of input sockets, speaker output terminals etc. In terms of wire I used IPL Acoustic HU1 for hook up and their IC2 for signal wires.

Finally, I bypassed C2/5/9/10/11/12 appropriately. Again, if you need the minutiae, let me know and it can be detailed.

Hope that helps. It all depends on the type of sound you desire. I appreciate this is a complicated issue for discussion. If you would like to talk it over on the phone, just PM me your phone no. I'll respond.

For me, this worked very seriously. It took the original -300B to another level. In a sad kind of way, I have moved on from this amp and now use a commercial Devialet so my WAD 300B PP will be up for sale. Obviously, anyone interested, please give me a shout.


Last edited by Greg.; 28th December 2016 at 01:42 AM.
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