[Harp-L] Re: typical bassman mods

["Steve Rowse" <steven@xxxxxxxxxxxxxxxxxxxxxx>]

>   Date: Thu, 5 Jan 2006 16:09:44 -0500
>  From: "Rufus Zee" <rufus.zee@xxxxxxxxx>
> Subject: [Harp-L] Typical Bassman mods?
>
> I'm not looking for anyone's hard-earned secrets -- just general things to
> consider to convert a bassman circuit to harp.
>
> What I'm looking to do is to build a Weber bassman-style kit, and mod it to
> be switchable from "harp-friendly" to "guitar-friendly". I know that often
> lower gain tubes are used in the front end for harp. I can likely get
> similar effect by limiting the front-end gain. But then again maybe I don't
> have a clue. Any hints, tips, schematics, pointers to great web pages etc
> would be much appreciated.>
>
> RZ

Hi Rufus. I'll try and give you an answer. Like Sonny Jr says, it's complex 
and confusing. But there you go,  you buy yourself a weber bassman kit quite 
cheap and you think 'I'll build it for harp, but I'd hate to miss the 
opportunity of playing through what they say is the best guitar amp ever 
built'. Here's my experience for what it's worth. I built myself quite a few 
amps over the years though mostly I'm a part-time repairman, then I took up 
the harp as an adjunct to singing and playing rhythm guitar and I thought 
I'd make myself a bassman-type amp for harp. I don't use kits because I'm 
cheap, also because I like to know I did as much of anything I do myself, as 
I could possibly have done. More about my personality later. This 
bassmannish-boy of an amp has a switch that parallels the voltage dropper 
resistor that feeds the preamp, with another power resistor. The two 
paralleled give the higher guitar voltages, whilst the single resistor drops 
the voltage by 60v to something more harp-friendly. This does bring down the 
gain a little and makes things a bit 'browner'. However it is not enough to 
stop feedback and enter the zone in which we have a true harp amp rather 
than a guitar amp. All it does is convert a good guitar amp into a boring 
one. For harp I have to swap in valves too. Which ones you use in which 
positions will depend entirely on the nature of your particular amp. There's 
no escape from getting hold of a few 12AX7, 12AU7 and 12AT7 types (for 
starters) and trying them around. It's worth it though. For me as a harp 
player used to guitar amps with all the knobs either on 1 or on 10, failing 
to cut through and howling with feedback anyway, the effect of valve swaps 
even on a low-preamp-volts bassman was stunning. Volume! No feedback! Big 
brown sound! But it's a journey. Before you lies a choice, is what Sonny Jr 
is saying to you. Years of solder burns, trashed chassis, failed 
experiments, expensive and abortive NOS purchase schemes etc etc? Or a 
couple of grand for a Sonny or a Meteor and no wait at all? You have to be a 
certain type of person to choose the former. You have to be a person like 
me. Five years of the solder burn route delivers the beginnings of an 
understanding of how far you have to go before you'll have got for yourself 
what the two grand or so would have bought in the first place. But I was 
already quite experienced in amp building and repairs before I started 
thinking about how to build an amp for the harp-playing I'd taken up.

Shall I answer your question now? I can't think of a simple switching method 
that would achieve everything you need to move from a guitar bassman to a 
harp bassman satisfactorily without using two completely different preamp 
channels. You need different valves. You can lower the gain of a 12AX7 by 
changing plate or cathode resistors or by introducing attenuation in the 
signal chain but none of that will make it the same as a 12AT7 as there are 
many other variables and also you would be taking it out of its optimum 
operating range before the gain dropped enough to be good for harp. You 
might need different coupling caps too. Mine aren't very guitar-friendly. Or 
maybe you need a 12AU7. Oh boy it's complicated. I endorse every word Sonny 
says.

Hey Weber sell kits for harp. They're probably better than a modified 
bassman kit. Why not build one of them, and a bassman too? Two heads, plug 
in the one you want. You'd still be saving time & money over the 
experimenter's route. Lots of time and money. Believe me, I'm an 
experimenter.

By the way everyone, I've got a funny little harp amp I modified on Ebay. Do 
a search for 'harp amp' in the uk if you're interested. Don't bid it up too 
far it's only a little one. These were some of the worst guitar valve amps 
ever made but they make a nice little harp practice amp, I've converted them 
before.

Happy soldering anyhow,
Steve

www.stevesamps.co.uk