You are not logged in.

  • Index
  •  » Ampeg Heads
  •  » Got myself a ´75 V4b with weird mods,anyone seen this?

#1 2014-01-26 12:30:26

New member
Registered: 2014-01-26
Posts: 1

Got myself a ´75 V4b with weird mods,anyone seen this?

Just got my first V4b sold as faulty.
The seller said it worked but had a loud hum
took a chance and bought.

Haven´t turned it on  but instead been going over all components
and changing the filter caps(originals in there)
and ruling out those things.

Some weird values on the resistors.

1.R41,42,47,48 has been lowered to 400Ω
all i read so far are if you changing them you rather go up in value(1kΩ)

2.R39,R40,R45,R46 are missing.

3.R37,R38,R43,R44 are 5.6kΩ instead of 47kΩ

4.D7-10 are missing

5.Has variable bias pot, seems ok.

All other components seems to be stock.

Anyone recognize this mod?
The power tubes are 6550 which what I have read doesn´t seems to have this kind of mod.

Also the 12DW7(V3) is changed to  12AX7
but all resistors around V3  are original.

I´m thinking of ditching all these mods
and make it stock and get some 7027 and a 12DW7.



#2 2014-01-30 23:45:29

From: Seattle Washington
Registered: 2006-09-03
Posts: 1848

Re: Got myself a ´75 V4b with weird mods,anyone seen this?

I have recently worked on a v4 with almost exactly the same mods.   strange.
Here are the ones to undo for sure.
R41,r42,r47,r48 should be definitely be 1k if you are using 6550s they can be 470 if you are using 7027 or 6l6,  but even then I recommend going to 1k

R37,R38,R43,R44. Should be returned to 47k.   5.6k is too low,  unless you don't mind an oscillating/red plating power section (you do mind)

Variable bias is ok... But bias the amp cold.  Do 60% max plate dissipation.  Even with the 6550s.   If you could fit kt88s in a v4 we could take the bias higher,  they take it quite nicely,  but the 6l6 and 6550s sound best at 60% in my opinion. 

The ones you can leave as is:

R30,R40,R45,R46.  Not really needed.   These guys can stay out or go back in,  doesn't make a difference.  They are such small values anyway.  You wouldn't notice a difference in sound,  and they just add heat to a part of the circuit board that already gets quite hot.

D7-d10.  No big deal,  these are the flyback diodes,   There is some discussion about their usefulness on this forum.   (Just search flyback diodes). I maintain, that I have seen amps with blown transformers that use them,  and that don't...but I see gobs of amps with blown flyback diodes.   
So,  to make a long story short...   They are there to protect the output transformer,  but blow up in the process,  (rendering your amp useless until serviced). I find little evidence to suggest that they are saving transformers,  and just cause amps to need service more often.   So,  you do the research and make up your own mind.

Change that 12ax7 back to a 12dw7.  The 12dw7 is in production again,  so no reason not to use it.


  • Index
  •  » Ampeg Heads
  •  » Got myself a ´75 V4b with weird mods,anyone seen this?

Board footer

Powered by PunBB
© Copyright 2002–2005 Rickard Andersson