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Custom V4B build from scratch...

K

kiljoy

Jun 28, 2008
45
0
Auckland, New Zealand
#1
Hi guys, I'm about to embark on something of a mammoth task which is to build an early 70's style V4B from scratch with some power section modifications. I wanna keep the preamp as is but switch out the 7027s for KT88s and feed them the right power via a bigger power tranny to get the output to 200w. I have a good tech who will be doing the actual build as I'm fairly ******** with a soldering iron.

Are there any hints, tips or advice anyone can share?
 
#5
I agree its much better to get an old blown up one and mod that. I do that all the time you get a decent pair of transformers and a good choke
some better faster diodes a few muticaps and replace all the lytics
and you are there. I always copy the Hiwatt Dr 201 output Schematic
and I always use matched Gec KT88 tubes in fact the tubes always come before I buy a suitable amp. There is no point in going off half cocked.
:ninja:
 
#6
1) Thats a great idea! A really great amp that could use a hundred more watts!

2) Why not just get an old SVT? They will weigh about the same and the preamp sections are nearly identical. It would probably be cheaper too.
 
#7
Thanks for the replies fellas. I'm in New Zealand which happens to be the arse end of the world so even if I did find a V4B on ebay or the like it'd cost about twice as much in shipping to get here and that's if the vendor could even be bothered to ship here because most don't.

I prefer the sound of the V4B's preamp to that of most SVTs, It's a bit dirtier and less subtle. I just need extra volume, headroom and bandwidth which is what I'm hoping to get out of the KT88s.

Is there a transformer upgrade anyone can recommend? My tech has an old guy that custom winds them but I think he may be quite pricey. Also recommendations on chokes, caps and any other related stuff would really help me out.

Cheers!
 
#9
Not to be discouraging but that is going to cost a small fortune in labor and parts. Probably more than the price of an SVT shipped all the way around the world? One way or the other, good luck!
 
#11
You will need a power transformer capable of delivering 600V for the plates and 400V or so for the screens. Negative 60V bias and 6.3V AC at about 10A for heaters. Output transformer rated at 200W with a primary impedance suitable for paralleled KT88. IIRC about 18/1900Ω plate to plate. These will be big, heavy and expensive. Be prepared that you will end up paying more for this beast that you would pay for a commercial offering. The other side of the coin is that you will know the quality of the components that you used.

Paul
 
#15
Ok, after much deliberation with my tech guy we've decided to can the build from scratch idea. It seemed to be a cost effective idea in the outset but ended up calculating far too high in price. So... I've just bought myself an Ampeg V4BH which I know is nothing like the original V4B but it was all I could find in this half of the world.

I'm still considering the upgrade to KT88s and potentially a transformer upgrade too. Does anyone know of any preexisting modification to do this?
 
#17
JimmyM said:
Why are you considering a tube change and a transformer change when you haven't even heard the amp yet? Who knows? It might be perfect as it is.
Click to expand...

I've played through a friend's V4BH once before and although I liked it I found it too 'guitar' sounding. I find 6L6's are very focused in the mids and don't sound good bs power tubes in bass amps (the Mesa 400+ is a perfect example). We've now decided on 6550 instead of KT88's as the tubes are mounted upside down in the chassis and will be more reliable.
 
#19
kiljoy said:
I've played through a friend's V4BH once before and although I liked it I found it too 'guitar' sounding. I find 6L6's are very focused in the mids and don't sound good bs power tubes in bass amps (the Mesa 400+ is a perfect example). We've now decided on 6550 instead of KT88's as the tubes are mounted upside down in the chassis and will be more reliable.
Click to expand...
I don't know if I buy that. My B-15's have 6L6's, and if you record a direct out from the OT with a Red Box, it sounds pretty flat IMHO. Plus if they were that heavy on the mids, guitarists wouldn't use them as prevalently as they do.
 
#20
My 70's V4 has been modded for 6550's. The tech that did it is a good friend and he said it's a fairly easy mod, one that he's done a lot over the years before 7027 became readily available again. I'm not sure how easy it would be for the V4BH, but that might be an issue you could PM someone like Jerrold Tiers on to get some better insight.

-> From what I understood when he explained what he was going to do to my head, in the 70s amps its mostly just an issue of changing out a few resistors and such. But there may have been a few details that slip my mind right now. And again, I dont know how that would pan out for a V4BH
 

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