Friday, April 22, 2011

Mystery Amp: Modded Bassman RI in Sonny Jr Cab


A buddy of mine bought this amp on eBay: It is a modified Fender Bassman chassis circa 2003 in a Sonny Jr 410 cabinet. The eBay seller did not misrepresent the amp at all, and my friend got the amp for only $750, which is a smokin’ good deal.

The seller said the chassis had been modified by Gary “Sonny Jr” Onofrio himself. I contacted Onofrio and sent along photos for his comment. He said he had no specific memory of working on this amp, and had no idea how it ended up in an SJ 410 cab. He said he may have modded the amp back in the timeframe when he was transitioning from the 410 model to the Super Sonny amp. He took on custom work during that time of low amp sales.

When examining the Bassman chassis it is easy to see the mods. They are the standard tweaks that are typically used whenever adapting a guitar amp for use with blues harp: Convert to tube rectifier, larger than normal coupling caps, and lower idle plate current in the power tubes. There is nothing secret or magical in the tweaks here. Pretty standard stuff.

When bench tested the amp produced 32 watts just as it began to distort on the scope, and 39 watts at peak output.


One thing did stand out. Bassman RI owners often use low-gain 12AU7 tubes to calm the amp down and reduce feedback, and this amp was no exception. They get bad advice from yokels on the Internet who claim the AU7 tube is the magic bullet for the amp. In fact, the 12AU7 tube is incompatible in circuits designed for the 12AX7, such as the Bassman RI. The 12AU7 tube draws more current than the 12AX7, so the cheapo ¼- watt carbon film resistors found in many PCB amps get stressed, sometimes burning open and causing a nasty hum and damaging downstream components. In this case the tech changed the plate load resistors to a higher value. Good call.

This amp had a nasty sizzling noise that usually indicates a bad preamp tube or a bad solder joint. After resoldering several connections the noise went away.

The amp is loaded with four Alnico Blue speakers, which I believe are rebranded Eminence 1028s. We tried several different preamp tubes and finally settled on this combination: 5751 in the preamp socket, 12AX7 in the secondary gain stage, and 5751 in the phase inverter.

How did the amp sound? Good! Very good, in fact. It has less of that reedy edginess typical of the stock Bassman RI. It has a bit more wamth, with good note definition but not much crunch, which is normal with 4x10 harp amps. Nice big sound. I need to borrow the amp and play it in a performance setting to wring it out more, but I am initially impressed. (I'll be using this amp at the Sunday Blues Jam my band hosts at Ziggies Saloon in Denver this Sunday night.)

I am so impressed I plan to buy a used stock Bassman RI for a project amp. I’d like to see just how much tone can be wrung out of this amp. I suspect the amp can rival the expensive custom 410 amps for a lot less money.

NOTE: Many thanks to Bruce Collins at Mission Amps in Denver for helping with this article.

10 comments:

Peter said...

750$ is a smoking good deal for any Bassman RI.
Good luck finding another one, this must have been a real distress sale.
I have seen a number of these modified RI Bassman over the years. I have heard one by Jonh Kinder and others. They always sounded fine to me, certainly better than a standard RI Bassman.
The choice of speakers is interesting as they are all the same, something that Sonny Jr amps have moved away from over the last few years as the more current crop of amps have a blend of speakers.
I am glad to see you exploring a 4x10 amps since your more recent searches for the holy grail of tone have been with smaller amps. At the end of the day, 4x10 push the air unlike a small amp and they are great for getting that classic chicago blues tone.

Rick Davis said...

Peter, I still prefer the tone of my Mission Chicago 32-20 amp. On Sunday I played this Bassman amp on a stage I am very familiar with, playing with my band. I also listened to other players playing this amp. The Bassman has a harsher tone and about the same volume before feedback.

tmfharpking said...

What makes a RI Bassman a worhty harp amp is to rid it of the PC board and circuit.
Install a Hoffman kit http://hoffmanamps.com/
The blue Eminence AlNiCo speakers can stay,but the rest has to go.
The Hoffman kit includes all turretts,lugs,resistors and even a bias pot. My tech installed it in one day-it makes all the difference,as it becomes a true point-to-point circuit.

Rick Davis said...

Hi Tom-

Opinions about the Bassman RI are all over the map. I've heard from lots of players about this article. One well-known pro player tells me all that is needed to make a Bassman into a killer harp amp is to install 12AU7 tubes in the input section. On the other hand, a maker of major custom harp amps tells me there is NOTHING you can do to a Bassman RI to make it sound good. He said he "detests" these amps.

Your solution sounds like a good one, but it may not make economic sense. The kit from Hoffman amps is about $200, and it would be a rather complex job for a tech to install. It might make sense for a hobbyist who could do it himself, but to have a skilled amp tech who really knows harp tone (i.e. Skip Simmons, Bruce Collins, et. al.) would cost a considerable amount of money. The conversion could easily make the project more expensive than a new custom harp amp of similar configuration.

tmfharpking said...

My amp tech,Chris Davis of Hall and Oates fame,charged me 1.75 hrs to install the Hoffman kit,set bias @40ma,add filter caps for preamp stages and flyback spike eliminator circuit-grand total$319.33-you can easily obtain a RI Bassman amp used for between $750.00-$900.00 . Still less than any custo made 4x10 amp.

tmfharpking said...

This is exactly what I am speaking of-

http://cgi.ebay.com/Hand-wired-Tweed-Fender-Bassman-Reissue-/330557577496?pt=LH_DefaultDomain_0&hash=item4cf6c45918

Ray said...

Heck, just get a Sligo: http://sligoamps.com/
I bought a standard spec Bassman clone from him, paid $1000. Sounded great to me. My buddy just bought an upgraded version, Fat Stack tranny, better speakers, etc, for $1500. Thing is TOUGH! Sounds great. Brand new. Last fo 'eva.

ijones7 said...

Rick, I would like to swap some 12au7s in my harp players Bassman RI, but would also like to replace the 1/4 watt plate load resistors to a higher value. Which components are these on the schematic if I plan on replacing them for V1, V2, and V3? R8, R9? Will 1/2 watt resistors be adequate?

Rick Davis said...

ijones- I asked Bruce Collins; here is his reply:

"I just replaced four burned 1/4 watt plate load resistors in one of those re-issue 5F6 amps!!!
Betcha he was trying to use 12AU7s too.

The 12AU7s draw quite a bit more current then a 12AX7 or 12AY7 and 1/4 watt is not very safe.

I'd replace R8, R9, R14, R16, R28 and R29... they should all be 1/2 watt resistors to be safer.

Tell him to snip only the body of the old resistor off, leaving the actual wire leads of the old resistors in the printed circuit board. Then bend them up straight.

Make tiny loops in the new resistor leads so he can drop those over the little nibs on the PCB. Solder the new resistor carefully to them.... not too much heat or those left over lead nibs in the PCB will melt the solder under the PCB and fall out of board!!"

ijones7 said...

Rick, Thank you and Bruce! That was exactly what I was needing to know! -Ian