From: balon@cup.hp.com (Tom Balon)
Subject: Re: Q:Bassman tube amp mods?
Date: Wed, 12 Aug 1992 22:01:56 GMT
Organization: Hewlett-Packard, Cupertino, CA
Keywords: bassman tubes amp mods


	Hi,

	I've done many of these type mods. It isn't as simple as just
	replacing the output tubes. There's much more to it. I run a
	small shop in Santa Clara that does amp/preamp rebuilds and
	repairs on all types of tube amps, I do custom mods as well. The
	bassman is probably the *most* modified amp of all time. Given
	it's cheap used market price ( about $80.00 used )

	Any bassman can be made into a fine guitar amp. Since your starting
	with an amp that's 29 years old, you'll need to do a lot of
	work just to get the amp back to it's original factory fresh
	performance. Any amp 15 years or older should have it's power
	supply caps checked and replaced if necessary. At 29 years, I'd 
	skip checking them, I'd replace them ASAP since a failure in
	your power supply can cause *serious* damage to your amp. 
	
	What you want is a great sounding amp thats going to be *reliable*.
	Just hot rodding an old amp is fine, but remember, you wouldn't 
	put a hemi in a 30 year old vette and go screamin 90 mph on twisty
	roads without checking the suspension and brakes. You may end up 
	with a heap of burning wreckage. 

	Describing a complete mod here would take too long. So I'll just
	give the basics.

	Replacing the 6L6's with EL34's will not give any more 
	overdrive or change your sound much *unless* the amp already has 
	plenty of gain to get the distorted sound in the first place.

	You'll probably want to add another gain stage to the amp. Adding
	a 12AX7, using the first triode as a voltage amp and the second
	triode as a cathode follower works well. The low impedance output
	of the follower makes a good spot to put an effects loop. I usually
	place a master volume between this last gain stage and phase splitter
	This works better than the bias circuit method described in the 
	Tube Amp Book.
	
	Replace all the electrolytics in the cathode bias and HV and bias
	supplies they'll be shot for sure if it's a '63. This will clean
	up the sound a *lot* and improve gain and freq. response. 

	Boogie generates their distortion sound by doing fancy channel
	routing using LDR's (light dependent resistors) in the preamp stage.
	They run one half a 12ax7 with it's operation point set to be
	on the non-linear portion of the characteristic curve, this generates
	the distortion, they feed this into the other 1/2 of the 12ax7 
	set for high gain, they end up cutting a lot of the high's to
	reduce edge and harshness.  
	
	BASIC CIRCUIT: 1/2 12AX7 ( 82K plate, 1500 cathode bypassed by 15uf ) 
	other 1/2 12AX7 ( 275K plate with 3.3K cathode bypassed with 0.22 uf) 
	B+ is around 397. 
	The two stages are coupled using a 0.02uf cap and a voltage divider 
	using 275K and 68K to gnd. The grid of the second triode is connected 
	between these two. There's a 500pf cap across the output of this
	stage to gnd. They use a standard 0.047 cap for coupling to the
	next stage. 

	Hope this helps. I can send you a schematic of the boogie gain stage
	if you like.

						 Tom
						 Tube Amp Service
						 Santa Clara, Ca.
						 (408) 243-7745


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