[Harp-L] Bassman (And Other Amps) Mods by Ron Holmes

[Ray Beltran <raybeltran@xxxxxxxxxxx>]

On a recent out-of-town trip, I had an opportunity to play through a  
friends Bassman LTD that had been modded by Ron Holmes. After using  
the amp for the gig, out of curiosity I asked Ron what he had done to  
the amp. He was quite detailed with his explanation, as he can be.  
Ron has given me permission to post these mods, which I'm sure you'll  
agree is quite generous of him. He's cut down on working on amps,  
since he is now heavily involved in manufacturing. So he says it's  
cool to post the details, and let you all have at it. I liked the  
amp, but it wasn't my cup of tea. Nothing wrong with it, but for my  
tastes, I like an amp that is "stiff", and doesn't give it up too  
easily. However, I appreciate that there's "different strokes for  
different folks", and if anyone can benefit from this, then it's all  
good.

As far as tube compliment goes, the amp was set up with 12AY7 in V1,  
and a GZ34 tube rec. All other tubes were normal, although Ron says  
that he spends a good deal of time listening to the amp with  
different tubes of the same type to see if a particular tube sounds  
best in the amp.

In these quoted e-mails, Alan is the owner of the amp, and Sonny Jr.  
is a reference to Gary Onofrio.

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Hi Ray,
You are welcome to edit and post my comments. The stuff is tried and  
true. I either double or quadruple the coupling cap values (say from  
0.022 uF to 0.1 uF, 400 VDC). I like the Vishay/Sprague 225P film- 
foil polyester cap series which are chubby but not as long as the too- 
bright film-foil polypropylene caps. The polyesters (Mylar, etc.)  
sound closer to the old foil and paper coupling caps than the ultra  
clean and bright polypropylenes. At least for harp use. Always use  
clear RTV or similar adhesive to support the bodies of the somewhat  
larger caps. The sealant and board should support the new caps and  
not just the poor too-small copper foil traces.

For a long time Sonny Jr. has been encouraging players to try  
different tubes in their amps. This sequence I use after tweaking the  
amp’s electronics. Alan’s amp was stock factory. I lowered the B+ to  
preamp stages by increasing the Metal Oxide Film 1 watt dropping  
resistor from 10K ohms to 39K ohms, 2 watt. I change first two stage  
plate resistors to 220Kohms, ½ watt carbon film. Cathode bias  
resistors to 2.7Kohms or 3.3Kohms. Technically this increases the  
gain but actually makes first preamp more compressible and mellow.

The first plate voltage ends up being around 85-90 volts DC. All main  
plate and cathode bias resistors should be changed to ½ watt carbon  
composition resistors. They sound different, faster, and fuller than  
the hideous carbon film resistors now covering the planet. Usually  
right before the phase splitter circuit I increase a coupling  
resistor to 1 megohm or better. Usually it replaces a 470Kohm, ½ watt.

Goal is to drop circuit gain output from preamps alone by ½ or 2/3.  
The different tubes also lower gain depending on which tube. All  
signal coupling caps need to be 0.1 uF. Including phase driver to  
power tubes. Don’t mess with 6L6 biasing to try to lower anything.  
Those you want running normally. Just progressively lower each  
stage’s gain so the accumulative effect is lower overall gain, a huge  
round sound and right vibe.

You should end up with the volume or gain control ½ to 2/3 all the  
way up and not at 2. The factory’s choices were based on economics  
and not premium sound. The PC work has to be done gently and  
methodically so as not to tear up thin foil traces. With care and  
good solder braid it can be done.

As I am focusing solely on manufacturing I am not doing any more  
service work or mods at this time. But, there are skilled folks out  
there that can follow this plan (with schematic in hand) and do these  
modifications. Carefully!

Best Regards,
Ron Holmes

In a follow-up e-mail, he wrote:

Hi Ray,
Yup-no mods or repair for the time being. Advice is as far as I go  
for now. Alan’s amp was tube based, just wrong stuff for harp. The  
list I rattled off applies to all Fender reissues as well as Carvins  
and others. They all do the same stuff with same Chinese parts.  
Probably all are made in same Chinese factory.

I think Alan’s was an eyeleted board that tried to copy the  
originals. Have done so many Fenders that they all sort of merge.  
Rules are the same. Replace all coupling caps with double or triple  
the values using film/foils,,,,,etc. The so called “real reissue”  
Fender Bassmans and others  that use the black board still use some  
bad (sonically) parts. They copy the original cosmetics but not the  
parts used inside. They have been built by the accountants and not  
the engineers.

The reissues can actually sound good. Even the trashy “Hot Rod  
Deluxes”, Blues juniors, and so on can sound good following the basic  
outline I give. I loved doing the mod work and really enjoyed making  
the amps come alive-but couldn’t reach critical mass to earn a living  
at it. So, the new stuff will have to pay the bills.

Best-
Ron

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If you have a chance, drop Ron a line to thank him for supporting the  
harmonica community by making his mods available.

Ray.

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My Music – www.resgraphics.com/music