Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Electricmonkey on November 03, 2019, 09:44:15 am
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I have a 275V@160mA ; 6,3V@3A power transformer and I want to build a single ended amp with EL34 as power tube.
Inspirations are Allen Chihuahua/Carr Mercury/Champ Reverb.
Pretty sure about the topology but some points are still not clear for me.
A little bit lost in filtering B+, should I connect the OT on the first cap, thought it was better to benefit from the choke.
Not sure about V2 cathode resistor, a little bit lost in math concerning // 12AT7. Do I have enough B+ for this type of reverb since I'm not looking for the surf type you have with a stand alone reverb.
The 7k OT is here to keep the amp under 10W (8.something if my calculation is good), with a more suitable 5K max power would be 12.something W.
Possible tweaks/add on : tone stack defeat switch, unbypassed cathode resistor on V1a, negative feedback.
What are your thoughts?
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A little bit lost in filtering B+, should I connect the OT on the first cap, thought it was better to benefit from the choke.
Connect as shown in your schematic. The choke must be rated to carry the entire amp current draw.
I think your B+ will work with that reverb circuit, but don't expect Dick Dale performance.
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EL34 at 380V can pull a lot lower load than 7k. And you have way more current than is needed. 5k and 76mA gets significantly more power out.
This part is drawn wrong:
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I have a 275V@160mA ; 6,3V@3A power transformer and I want to build a single ended amp with EL34 as power tube.
Inspirations are Allen Chihuahua/Carr Mercury/Champ Reverb.
Pretty sure about the topology but some points are still not clear for me.
A little bit lost in filtering B+, should I connect the OT on the first cap, thought it was better to benefit from the choke.
Not sure about V2 cathode resistor, a little bit lost in math concerning // 12AT7. Do I have enough B+ for this type of reverb since I'm not looking for the surf type you have with a stand alone reverb.
The 7k OT is here to keep the amp under 10W (8.something if my calculation is good), with a more suitable 5K max power would be 12.something W.
Possible tweaks/add on : tone stack defeat switch, unbypassed cathode resistor on V1a, negative feedback.
What are your thoughts?
Probably no big deal but EL34 datasheets say 700K max for g1 and you have 1M. I don't like the way VR5 loads down V3B more and more as you turn the volume down. If you want to load down V3B like a Twin Reverb, use a 50K pot wired up normal. If you don't want to load down V3B, use a 500K pot wired up normal.
I wouldn't remove the cathode bypass cap on V1A (C2) because it shunts 60Hz noise to ground. You can turn down VR3 if you want less gain.
C1 (250pF) is going to make the amp very trebly. Move it to between R4 and VR1.
You can apply NFB to V3B if you separate the cathodes of V3A and V3B. That's what I would do.
I would use an un-bypassed 470R resistor for the cathode of V2. It will chew up a lot of current at idle, but it will really drive that tank at your lower B+ voltage.
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see attached - i'd also add a third RC network for V1a-b (call it A') split R21 to 8.2K for A and A'.
--Pete