Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: newguitarsmell on April 26, 2020, 11:57:33 pm
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Hey all you smart guys - I am putting this amp in a cab with a 15" speaker . I will be using this amp for both keyboards and guitar.
I want to
1. tighten the bass response -
2. increase the headroom for more clean tones at higher volumes
any thoughts? How could I modify this to give me more bottom to top balance?
Thanks
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one thing i would suggest is a fender type negative feedback, make it adjustable instead of a fixed resistor that way if you want to use it for keys you crank the nfb and turn it down for guitar. perhaps a zobel network across the ot primary. i have never used these things because in my guitar amps i want grit but both of these things from what i have read will tame distortion.
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Higher volume equals higher output power. No way around that.
This requires bigger power and output transformers. Essentially, it means building a different amp.
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Use the higher-voltage tap(s) on the HT winding, with SS diode rectification in a 2-phase FW rectifier. (Ditch the Weber copper cap)
Make sure you're using higher voltage filter caps to cope with the increased power rail voltage (Weber sells up to a 600V rating)
Change the Output Transformer to a bigger tweed bassman type (Weber sells a WO18343 with 2, 4, 8 R secondaries: https://www.tedweber.com/w018343. There is a Heyboer equivalent upgrade on their site https://www.tedweber.com/hy018343)
Build a one-channel AB165 circuit (12AT7 LTP inverter, choke filter) You can then use the spare 12AX7 for a 2nd input channel, or as an extra stage in a one-channel amp, for a range of purposes (including incrementally bumping up the clean gain)
YMMV
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After you address the power to output conundrum...this is a separate issue:
How could I modify this to give me more bottom to top balance?
I would try an EQ between the keys and the input of the amp.
Also, since you have 2 separate input paths, I would think about dedicating one for guitar and the other for keys and modify each of those input channels to get you closer to what you want.
-As it is now, is the keyboard inherently louder than the guitar? ...with all else being equal?....(i.e. with both instruments volume controls set to 10)
- Is the bass response ok for guitar but muddies up with the keyboard?...or the other way around?
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When separating the channels like SG suggested, separate the the cathodes on first pre-amp tube (maybe use a 5751) , lower keys input's side to 1K5, 4.7uf, also lower coupling caps to .047uf, think about using 6L6's, change rail r's to lower values, 1K and 4.7K, change the 5watt cathode bias R to 10 watts. Try lowering the cap on C5 to 4.7uf. Try one at a time, hear how you like it...
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I already have a 40w OT in there so that is done. I think separating the inputs is an excellent idea.
Amp isn’t quite done so I will keep working on it and do some further voicing when I get the speaker..as I think that might also effect the overal output.
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I want to
1. tighten the bass response -
Thanks
You can reduce the values of C2+C3 and C7+C8 to 0.047 (slight reduction) or 0.022 (preferred) and this is a very common modification to tighten up the bass response.
If you want more headroom Id suggest adding a switchable NFB or considering a different amp. The tweed deluxe is a breakup machine and its hard to change its overall nature vs working off a better starting platform.
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Thanks.