Fender late 1960s Bandmaster Recap
Edit: He says that it is a 1968 silver face but early enough that it is built like
a black face. He thinks that it is an AB763 drip edge.
A new friend tried recapping his 1968 Fender Bandmaster and it sounds to
me, from his description as if it is oscillating. Are these sensitive to main psu cap
ESR/ESL? Would adding an ohm or two in series with the new caps be the likely solution?
I'm looking at it tonight, just wanted to get suggestions in advance.
Thanks!
Edit: He says that it is a 1968 silver face but early enough that it is built like
a black face. He thinks that it is an AB763 drip edge.
A new friend tried recapping his 1968 Fender Bandmaster and it sounds to
me, from his description as if it is oscillating. Are these sensitive to main psu cap
ESR/ESL? Would adding an ohm or two in series with the new caps be the likely solution?
I'm looking at it tonight, just wanted to get suggestions in advance.
Thanks!
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I'd believe he missed a ground on one of the decouplers (filter caps on the preamp nodes). There is nothing esoteric in these amps, no super sensitive circuitry. In that era they were made with 20% tolerance resistors for Pete's sake. Or perhaps moved a ground.
Check the polarity of the negative bias supply cap. Its a common mistake that noobs don't put the positive to ground.
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Grounds are good, caps are in correctly. Thanks for the suggestions.
It would not show the problem on my test bench. He played it with his guitar and speaker that caused the problem at his home. I put a scope on the speaker and it clipped at 20 Vpeak that works out to about 49W into 4 ohms - seems fine.
It would not show the problem on my test bench. He played it with his guitar and speaker that caused the problem at his home. I put a scope on the speaker and it clipped at 20 Vpeak that works out to about 49W into 4 ohms - seems fine.
A curious point. This looks like it is the 763AB design and as show here
there is a 500pF cap, into a 1M ohm load. Plugging those values into
this high pass calculator yields an F3 of 318.5 Hz :
High Pass Filter Calculator
318.5 Hz is darn high.
Comparing to the Hot Rod Deville here we can see that a .022 uF cap into
the same 1M ohm is used with the calculator yielding an F3 of 7.24 Hz which
is rather low.
The Twin Reverb as another comp uses a .001 uF cap for an F3 of 159.2 Hz, also
rather high but much better than 318 Hz.
.01 uF would provide an F3 of 31.8 Hz which is a good fit for even a bass guitar.
there is a 500pF cap, into a 1M ohm load. Plugging those values into
this high pass calculator yields an F3 of 318.5 Hz :
High Pass Filter Calculator
318.5 Hz is darn high.
Comparing to the Hot Rod Deville here we can see that a .022 uF cap into
the same 1M ohm is used with the calculator yielding an F3 of 7.24 Hz which
is rather low.
The Twin Reverb as another comp uses a .001 uF cap for an F3 of 159.2 Hz, also
rather high but much better than 318 Hz.
.01 uF would provide an F3 of 31.8 Hz which is a good fit for even a bass guitar.
300Hz? What is the slope of that rolloff? How many db down at 80Hz? (two octaves down) What sort of pre-emphasis in the bottom end might there be before that cap?
It would be a first order 6 dB/oct slope until the other RC networks come into play but those are probably much lower. So, we calculate the -3dB point for the 300 Hz example, then at 150 it would be 9dB down, and at 75 Hz 15 dB down. Having thought about it more, it was probably done to limit speaker excursion given that these were usually open back cabinets, and to allow them to play louder before OT saturation comes into play. I think that they would sound better with a lower cutoff.
I'm thinking now that .005 uF is best for lead with a 64 Hz -3dB point.
It is interesting that several Orange amps have a selector switch for that cap probably to match the amp to the speaker cabinet and what the player wants - deeper response vs. max SPL. See the F.A.C. selector here:
http://www.orangefieldguide.com/OFG_SCHEM/orangeotrschem.gif
I commented about it in this thread:
http://www.diyaudio.com/forums/instruments-amps/273590-orange-other-amp-design-features.html
Forgot to provide links in post #5, Twin Reverb schematic:
http://www.thevintagesound.com/ffg/schem/twin_reverb_ab763_schem.gif
Hot Rod Deville:
http://support.fender.com/schematics/guitar_amplifiers/Hot_Rod_DeVille_schematic.pdf
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That's the "bright" cap. On other fender amps its switchable. I had a Deluxe Reverb with it and I removed it.
No, it is not a bright cap. It is an interstage coupling cap.
If it were removed there would be no signal going to the power amp.
I roughly measured the bass response and it does not roll off until about
40 Hz even with the 500 pF cap. The feedback design looked odd from
the start, and looking again it is boot-strapped which raises the input
impedance and lowers the cutoff even with the small cap.
The 820 ohm feedback going to the 100 ohm resistor at the bottom of
the cathode path actually feeds the grid resistors on both sides, it would
be a common mode signal if not for the .1 uF in the path to the negative
input. Seems that makes it primarily negative feedback but also provides
some bootstrapping - this is my theory for now, have not done the full
analysis or simulated it.
Not going to worry about that cap.
40 Hz even with the 500 pF cap. The feedback design looked odd from
the start, and looking again it is boot-strapped which raises the input
impedance and lowers the cutoff even with the small cap.
The 820 ohm feedback going to the 100 ohm resistor at the bottom of
the cathode path actually feeds the grid resistors on both sides, it would
be a common mode signal if not for the .1 uF in the path to the negative
input. Seems that makes it primarily negative feedback but also provides
some bootstrapping - this is my theory for now, have not done the full
analysis or simulated it.
Not going to worry about that cap.
Put a dummy 4 ohm load on it and drove the input with a function generator.
He said that the Normal channel was not right and I found that just tapping
the first stage grid of that channel caused bursts of oscillation. Checked the
wiring, carbon comp resistor values, voltages around the pins and everything
looked okay. Noticed that the 1 M grid leak resistor was grounded to the
input jack and they looked very dirty. Removed the jack nuts, cleaned things
up and it is fine now, no oscillation.
Tone controls also did not work in the Normal channel and I noticed that the
6800 ohm at the bottom of the tone stack was grounded through the case of
the pot, soldered right to it. Backed off the tone nuts and reseated them, now
the tone controls work!
He got this amp fairly cheap and it has been worked on a lot. He recapped all
the electrolytics with a kit and those look fine now. Many of the resistors have
been replaced with carbon comps from Radio Shack that just do not look right.
Is there a modern recommended 5% carbon comp resistor for these old amps?
The soldering job from the factory does not look very good with too much
solder blobed onto the eyelet board - the eyelet board is warped.
Any suggestions for replacements for the paper coupling caps, they are blue
with white lettering and they look original?
He wants to learn and I think a full rebuild/cleanup makes the most sense.
He said that the Normal channel was not right and I found that just tapping
the first stage grid of that channel caused bursts of oscillation. Checked the
wiring, carbon comp resistor values, voltages around the pins and everything
looked okay. Noticed that the 1 M grid leak resistor was grounded to the
input jack and they looked very dirty. Removed the jack nuts, cleaned things
up and it is fine now, no oscillation.
Tone controls also did not work in the Normal channel and I noticed that the
6800 ohm at the bottom of the tone stack was grounded through the case of
the pot, soldered right to it. Backed off the tone nuts and reseated them, now
the tone controls work!
He got this amp fairly cheap and it has been worked on a lot. He recapped all
the electrolytics with a kit and those look fine now. Many of the resistors have
been replaced with carbon comps from Radio Shack that just do not look right.
Is there a modern recommended 5% carbon comp resistor for these old amps?
The soldering job from the factory does not look very good with too much
solder blobed onto the eyelet board - the eyelet board is warped.
Any suggestions for replacements for the paper coupling caps, they are blue
with white lettering and they look original?
He wants to learn and I think a full rebuild/cleanup makes the most sense.
Sorry for the bad information it was late when I posted. I pulled out the schematic and you are correct, the "bright cap" is much smaller at 120pF. The 500pF is a coupling cap and because the input impedance of the LTP phase inverter is high due to the bootstrapped grid leak resistors, I believe doubling the grid leak resistor value is a good approximation. So in this case a 2Meg input impedance. This sets the -3db break point to around 159Hz. Maybe that's correct or my analysis of the circuit is wrong, someone will correct me.
How are you measuring the frequency response? What settings do you have the tone controls at while measuring? I only ask because when comparing to a 1kHz input sine wave the bass frequencies will be higher because of the mid scoop in those circuits. There is free tone stack software you can download that will help show where you should be at.
Nice! It seems like you are getting your buddies amp all hooked up
RadioShack doesn't sell carbon comp resistors that I know of, do you mean carbon film? I usually use carbon film when doing repairs as plate loads and in the grid circuits, 1 watters. The only time carbon comp is good is right on the pin of tube socket for a grid stopper. In that case I use these: http://www.mouser.com/ds/2/303/res_od_of_oa-180619.pdf
As for the coupling caps, those molded blue ones sound great to me. If they don't leak leave them in there. If they are leaky I usually replace with Orange drops, I find they have the best clean fender tone.
Problem with the Normal Channel tone controls is back, odd thing is that
they both stop working together. Obviously might be a bad pot but both
at the same time? I'll hit them with some deoxit and maybe wire the 6.8K
resistor directly to the ground plane.
they both stop working together. Obviously might be a bad pot but both
at the same time? I'll hit them with some deoxit and maybe wire the 6.8K
resistor directly to the ground plane.
Does that amp have a brass ground buss that all the controls go through? Remove all the pots and jacks, clean everything well and check all grounds.
Should be a nice sounding amp when working 🙂
Should be a nice sounding amp when working 🙂
DeOxit fixed the treble control, 250K ohm Bass control measured ~100 ohms at one end of rotation and ~10 ohms the other way. Opened it, could not find the short but it was gone after I removed the center contact, cleaned it and reinstalled it. Works fine now.
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