music, guitars & other obsessions

TOS Redux

An update to the Tweed Overdrive Special build.

I never completed the amp, even after getting everything wired, the front panel laid out, chassis made, etc. It just sat on the shop floor months after months as I went through various other projects, but about a week ago, I dusted off the chassis and decided to start from scratch, as I was never entirely satisfied with how the amp was housed inside the existing Houston Mini chassis with P2P wiring. In addition, it was going to be a dual channel (stereo) amp, don’t think I need that anymore…

The following pictures show the new build process:

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The schematic and layout were based Tubenits design, I made several modifications:

1) substituted 5965 for 12AY7 for V1;
2) substituted 12AX7 for 5751 for V2;
3) substituted 12AT7 for 12AX7 for V4;
4) installed 3PDT switch to select tonestack position – original position after V1a or after IRF820 source follower.

In the next few days, I will going through the bench tests and measurements.


As usual, there were a bunch of mistakes in the wiring that needed to be fixed, it is always the simple stuff – the tone and volume controls for the OD channel were shunting the output to ground! Also had to adjust some of the wires to fit better with the wire flow, even found a few turret joints un-soldered! Oh well, at least it is working now.

I had to lift the filament with DC to reduce the ac hum, otherwise, it was too noisy, which surprised me as PP amps tend to be less noisy than the SE amps. The B+ voltages were quite a bit higher than the ones shown in the original schematic, and I think may have something to do with the sound which it is a bit on the clean/aggressive side, I was expecting a round, vowel-y OD sound, will have to investigate further. On the other hand, the PPIMV works like a charm even down to low settings. Not sure if I need the VVR in this amp.

Also moved some of the tubes around, I found the 5965 was very microphonic in the V1 position, putting the 12AX7 there gave too much gain and made the clean channel dirty, so at the end, the best compromise was simply swap the 5965 with the 12AV7, but I will try more tubes in the V1 position, as it sets the tone for the rest of the amp, and I am not sure 12AV7 is best in that position as you seldom see design with it there.

When I completed the TOS, I was so disappointed with the sound – in fact, I thought it was by far the worst sounding amp that I ever built or auditioned for that matter. With all the raving at Hoffman, I just could not understand how it could possibly sound so bad. Months passed, I did nothing while I got busy with other projects.

To get such bad sound, I naturally suspect that I got some wiring wrong or perhaps a bad tube somewhere – but all the voltages checked out ok, and no obvious mis-wiring was found, so could it be poor design? Hard to believe given the experienced builders that hang out at Hoffman. Then one day, a light-bulb went on in my head – could it be the output transformer? We know that it has a major influence on the sound of the amp, and here I am using a “hi-fi” transformer salvaged from the Houston Mini 2, perhaps I should try another OPT and see… Well the date of reckoning arrived this morning, when I received the cheapo Chinese OPT that I ordered off Taobao…

Swapping out the transformer was a breeze, probably the easiest “mod” that can be done to an amp, just pop in the new OPT and solder a few wires – nothing to it. Well, the result was simply spectacular! Gone were the nasal, thin sounding BAD over-driven sound that I remembered. With the new OPT, I can easily dial in a number of great sounding “patches” – from clean, cleans to growling Marshall-y overdrive. Immediately, the amp moved from the bottom of the heap to the very top, just like that. No modification to any other part, no tube changes, wiring changes, nada, zilch!

So the conclusion here is simple – never use a “hi-fi” transformer in a guitar amp – it will sound like crap. And the flip-side, never use typical Chinese “hi-fi” transformers on your serious hi-fi gear – they will sound muffled and crappy. But for musical instrument, the off-the-shelf Chinese OPTs are hard to beat, they got just the right amount of LF/HF roll-off and distortion characteristics for such application. So it is no old-wives tale, there is something to this “under-power or bad” OPT “theory” floating among the various guitar amp building sites after all. Very glad that I tried it out!


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