Tube Amps / Music Electronics
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|2/5/2005 11:25 AM|
I powered up a Mission Tweedy Deluxe yesterday and have a few questions that some of you may be able to help with.
1. On startup all voltages were within tolerance (B+ 362 volts) on the build schematic but after playing for 12+ hours B+ has now fallen to 350 volts. I am using the NOS 5Y3GT that was supplied with the kit. Is it common to see a change like this? Do I have a weak rectifier and should I consider a replacement? Can anything be damaged by low B+ voltage?
2. My cathode resistor checks out at 276 ohms and I am calculating bias current as follows:
Voltage drop across cathode resistor: 20.3 volts
Plate voltage: 344 volts
Cathode voltage: 20.3 volts
I = 20.3 / (2 * (344 - 20.3)) = 31 mA
P = (344 - 20.3) * 0.031 = 10 w
Any comments on these calcs and acceptable ranges of idle power dissipation or bias current? I currently have EH 6V6GTs in the amp.
3. I am currently driving a Celestion Blue and using a Weber MiniMass to attenuate volume. Clean tone is gorgeous but a big chunk of the mid-volume range on both bright and normal channels yields somewhat ratty distortion (to my ears). Cranked all the way up, both channels provide over the top raging distortion and sound good. The amp overdriven sounds better on my Strat than on my humbucker equipped guitar. Has anyone experienced similar distortion issues and come up with tube changes or other solutions that smoothed this out? I don't have experience with any vintage Deluxes so I am not sure if this is an inherent feature of this amp or something that can be tweaked.
The build went well, parts are top quality and the documentation is very good. Bruce puts together very nice kits.
Thanks in advance for your help.
|2/5/2005 12:38 PM|
i built mine about a month and a half ago and have been experiecing the same thing. i too find my strat to sound best with this amp (fender amp, fender guitar, go figure). i am usually a HB or P90 player so i am trying to get it the way i like for those guitars.
your voltages and idle current (= watts) look good. i think you have a little room left. you may want to try a 250 cathode bias R. this will up your current a bit. from all the tweaking i have done, i found the a low voltage, high current power section sounds best to me (smoothest). keep in mind not to surpass about 12 watts per tube at idle. i am a newby too so take all this with a grain of salt.
alot of people suggest tweaking the coupling and cathode caps for some tone tweaking which i have done. i found stock (lots of bass), it sounds best clean. but it sounds a little tighter when overdriving with some bass reduction.
i am still totally searching and have not given up. part of me thinks the circuit just sounds this way, the other part says somethings not right.
i could share some mp3s if interested, maybe we could share findings.
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The Ultimate Tone, Volume III by Kevin O'Connor
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|2/5/2005 12:59 PM|
I built a slightly modded design based on suggestions from Bruce to tighten up bass response so that it would offer a comprimise between HB and single coil PUPs. I dropped capacitance on both 12AY and 12AX cathode bypass caps to 4.7 uF from the orignal 22 uF and also installed an RC network (220 k resistor and 0.0022mF cap in parallel) connecting the common lug on the normal volume pot to the shielded cable to pin 2 on the 12AX7. I powered up with this change in place so don't know what the original sounded like but clean tone on both HB and Strat are excellent. Dynamics are simply kick ass with a huge range of tone control and character. Very nice sound.
My only issue is the transition from clean to moderate distortion. Distortion seems ratty and it dies away abruptly. What tubes are you using.
If you have the time please send me some MP3s. I live in the boonies so have old copper dialup but reasonable file sizes I could handle.
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