Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Misc Schematics and Layouts => Topic started by: darkbluemurder on March 03, 2010, 02:32:18 am
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Hi,
This was my first amp project. I wanted to build an amp with a Vox top boost preamp but with octal power tubes. The basis for this build was the Top Hat Club Deluxe schematic on Schematic Heaven. I used a JTM45 power transformer, a 50W Hammond OT with a primary impedance of 7.3k and a Hammond choke 194C. I quickly replaced the 6v6 tubes with EL34.
Later tweaks evolved into the circuit attached. The most important changes were:
1. Changed the plate resistor for V1 to 100k. This lowers the gain slightly (there is still enough) but drastically improves in retaining high end when turning down the guitar volume.
2. I added a 250kB "load" pot to increase the 150k load resistor before the PI input. It can be adjusted for the guitar used to find the sweet spot where the amp really begins to sing but does not get muddy.
3. For more overdrive capabilities I added a treble shift switch to the tone stack. I used a 1MA push-pull DPDT pot for the treble control to add a 500pf cap in parallel to the 50pf treble cap.
4. I changed the 1.2k bias resistor in the phase inverter cathode to 820R. To my ears it adds a lot more cream to the tone.
5. I used a cut control with a 0.0047uf cap (actually it is now a 0.0033uf).
6. This may be one of the most important changes: I ditched the cross line master volume and put a LarMar PPIMV in instead. This has three distinct advantages: a) when dimed it's out of the circuit, b) it sounds better at low settings and c) the cut control remains effective at lower settings.
After three more builds this is still my go to amp for the country rock band I play with.
Cheers Stephan
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Hi Stephen,
I was wondering if you could repost a larger version of your schematic? I can't read the detail in the one you posted.
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Hi conger,
here it is. Unfortunately I cannot post the original file because it is a hair too big (142 KB). I resized it by a lower amount than the first one. Hope this works better.
Cheers Stephan
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Couple of questions:
Why the 0.1uF bypass cap on V1? You already have 25uF bypassing all frequencies.
Do you find it noisy/hummy? Taking all the B+ from the B2 node means that you dont have the benefit of successive stages of filtering for the preamp.
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I heard that a 0.1uf poly cap can make the sound more transparent when parallelled with an electrolytic cap. So I tried it and left it in even though I did not notice much of a difference.
The amp is virtually hum free - better than all of my Fender amps. I paid careful attention to grounding and layout and it worked. I have not even elevated the heaters yet.
Cheers Stephan