Can you recap the exact changes you made so I can add them to the website?
Your scope trace is the most extreme example of double frequency distortion I've ever seen, but then again the phase inverter is driving a tiny 12AU7 power tube.
Your scope trace is the most extreme example of double frequency distortion I've ever seen, but then again the phase inverter is driving a tiny 12AU7 power tube.
Without the scope, I might have thought it was just 'overdrive distortion', not a problem! 🙂
To solve the buzz problem:
Added 100k grid stoppers right on the tube socket for the V3 tube.
Increased the grid leaks from 220k to 910k (I didn't have any small 1 meg resistors.)
I also added a switch (accessible from the back of the cab, on the chassis deck) to experiment with different 1st stage preamp cathode bypass cap values - 2.2,25,220uF (actually 2.2 in the OFF position, 27.2 and 222.2 uF in the two ON positions - caps are paralleled with the 2.2). I'm not sure I'd bother with that again, but I haven't played the amp much yet with different guitars.
No headphone out, but I added an external speaker jack which disconnects the speaker in the combo cabinet. Gotta keep that Frontman 15G vibe in there!😀
Doing it again, I wouldn't bother with the B+3 extra RC stage as the B+2 had hardly any ripple. It was a 'try this' sort of thing when troubleshooting and only took a couple of minutes as there was an empty lug on the terminal strip.
Otherwise, it's all as you designed it.
Thanks again for a fun project!
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Thanks for the details VictoriaGuy. One thing I would really appreciate is some sound or video clips. I'm not a good enough player to make my own so anything you could do would be greatly appreciated.
Also, can I use your pics on the Bassman Micro webpage with credit?
Rob
Also, can I use your pics on the Bassman Micro webpage with credit?
Rob
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I don't have either the gear and ability to make decent sound recordings, or the musical skill to make them enjoyable listening 😱, so can't help there!
Certainly, Rob - I have no problem with you using any pictures I've posted - it's a small repayment for the benefits I've gotten from your website.
The (Fender) case was a 'donor' from a Frontman 15G I picked up locally - the whole 'Micro' build was 'on impulse' after that bargain amp buy.
Bending aluminum to make a new sloped-front chassis was taxing without a proper metal brake. Next time, I'll try to find a sheet-metal guy to do that job.
I spent an hour or so learning to use Front Panel Designer and printed the faceplate on legal paper and had it laminated. That went behind the Plexi faceplate.
I used a RadioDaze RDC-100 8H choke (no longer available??) in the power supply.
I have a small WGS (Warehouse Guitar) speaker on the way to replace the Frontman speaker. The Bassman Micro sounds a lot better via the external speaker jack to my 15" Jensen!
BTW, the amp is pretty loud - plenty for 'living room' playing when other people are in the house.
Thanks again for the design and website, and to everybody here for the suggestions, which made it a better amp and improved my troubleshooting skills. (Also, a 'tip of the hat' to ValveWizard and his webpage on the cathodyne.)
Regards,
John
Nice work John 🙂
If you want to hear your V1 bypass switch make a difference you need to reduce your cap values considerably.
The valve wizard is a good read on this subject.
For starters your biggest cap need be no bigger than 10u, and the smallest between 0.33 and 0.68u.
If you want to hear your V1 bypass switch make a difference you need to reduce your cap values considerably.
The valve wizard is a good read on this subject.
For starters your biggest cap need be no bigger than 10u, and the smallest between 0.33 and 0.68u.
The Bassman Micro V1A bypass cap is extra large at 250uF because that is the size in the full size 5F6-A Bassman and Marshall JTM45. For switching purposes a .68uF bypass cap would de-emphasize the bass frequencies enough that it will be audible, especially when the amp is heavily overdriven.
So....
Using a cap bypass switch is a great idea. You can have a 250u for nostalgia and two more choices for tone 🙂
Using a cap bypass switch is a great idea. You can have a 250u for nostalgia and two more choices for tone 🙂
I'll take that advice about the bypass cap values, next time I have the chassis out of the case.
Thanks!
Thanks!
Just a quick follow-up:
I changed the switchable bypass caps on the 1st preamp stage to 10uF,0.47 uF and 0.1uF.
I also moved the NFB 'injection point' to move the tone controls and MV outside the NFB loop. NFB now inserts via 0.1cap and (larger) resistors after the coupling cap that follows the MV in the original schematic.
I changed the switchable bypass caps on the 1st preamp stage to 10uF,0.47 uF and 0.1uF.
I also moved the NFB 'injection point' to move the tone controls and MV outside the NFB loop. NFB now inserts via 0.1cap and (larger) resistors after the coupling cap that follows the MV in the original schematic.
A few days ago I swapped my parts box output transformer for an OT5PP transformer from Musical Power Supplies. It tightened up the sound, and is a lot better quality than the old OT.
A replacement for the stock Fender Frontman 8"speaker arrived today from Warehouse Guitar Speakers- the WGS-G8C.
The weight (1200g vs 480g for the Frontman speaker) and construction quality of the new speaker encouraged me when I unpacked it, and I wasn't disappointed when I fired up the amp.
The WGS speaker has made a huge difference for the better in the sound quality from the amp. I recommend Warehouse Guitar Speakers for your project, and not just for the sound quality. The price and shipping (to Canada) was excellent as well.
I already have a new 'victim' solid state practice amp (Traynor Guitar Mate 15) for my next project, and it will get a WGS speaker as well.
A replacement for the stock Fender Frontman 8"speaker arrived today from Warehouse Guitar Speakers- the WGS-G8C.
The weight (1200g vs 480g for the Frontman speaker) and construction quality of the new speaker encouraged me when I unpacked it, and I wasn't disappointed when I fired up the amp.
The WGS speaker has made a huge difference for the better in the sound quality from the amp. I recommend Warehouse Guitar Speakers for your project, and not just for the sound quality. The price and shipping (to Canada) was excellent as well.
I already have a new 'victim' solid state practice amp (Traynor Guitar Mate 15) for my next project, and it will get a WGS speaker as well.
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