Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: dankinzelman on January 06, 2021, 06:13:26 pm
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Hi everyone,
I'm taking on my first ever major restoration project on a 1970's era Italian tube amp, the FBT 500R. Unfortunately, it's based on unobtainium EL503 tubes, but the amp came with a strong pair and sounds really nice!
I will be mostly using this amp for a Fender Rhodes Mark I.
It mostly seems to be working really well but there are a few problems and some improvements I'd like to make.
1. Engaging and disengaging tremolo causes a nasty thump in the speaker. The thump volume is dependent on the channel 2 control settings (ie if I crank up the bass and/or volume, the thump is much louder), and occurs regardless of the tremolo intensity pot position.
Static cathode voltage on the first cathode with tremolo DISENGAGED is about 1.46v. Engaging tremolo with intensity at minimum brings cathode voltage to ~2.6v and the thump, and there is a slight volume drop and increased distortion with tremolo engaged (even at minimum intensity).
Increasing intensity causes cathode voltage to swing between ~2 and 3.2V and results in major subsonic cone oscillations, leading to a whooshing sound and nasty distortion which corresponds to tremolo oscillations (and the beginnings of red-plating on the output tubes which synchronizes to cone movement). Tremolo effect becomes audible only in the highest ~20% of the intensity pot's travel.
Tremolo speed is slightly higher than I'd like, and usable control range for both speed and intensity is bunched into the last ~15% of pot travel.
I'd like to slow the tremolo, improve usable control range, eliminate the subsonic cone oscillations and associated distortion and get greater tremolo intensity at maximum setting.
2. Everything fed from the 'D' node of the power supply seems significantly below reference voltage (see schematic - pencil notes show my voltages). Is this something to be concerned about? Can I change the 10k node resistor to a lower value to bring the voltages up? Would this improve headroom/noise and/or gain?
3. I'd like stronger reverb. I temporarily clipped in a 25uf bypass cap on the cathode of the second half of the ecl82 and it made it a lot more powerful sounding, but also a bit distorted.
4. I don't know how to choose a target plate dissipation based on the datasheet (attached). All articles and videos I've seen simply give a recommendation for more common tubes, but I haven't found any clear explanations of how to work out target dissipation from the sheets, and the datasheet doesn't seem to cover voltages any where near where this amp operates.
5. I'd like to install a master volume pot. Are there any disadvantages to using a variable resistor between the downstream side of the coupling caps following the phase inverter?
6. For my own curiosity, why are target plate voltages so different on the first ecc83 on each channel? I would assume that, since the cathode and plate resistors are equal on each channel, we would expect the same voltage on each plate. Am I missing something? I'm totally new to circuit analysis...
Thanks in advance for any help.