Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: TIMBO on August 13, 2016, 05:46:25 am
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Hi guys, I have two amps of similar design, one takes pedals (boss blues driver and DD1) well and the other not so good.
This one
(http://i761.photobucket.com/albums/xx254/Timbo-08/001%204_7.jpg) (http://s761.photobucket.com/user/Timbo-08/media/001%204_7.jpg.html)
Plugged into the "guitar" input the sound is some what compressed and overdriven and the amp is not very loud, although I would assume around 10w.
This amp sounds great without pedals but still not as loud as the other.
This one
(http://i761.photobucket.com/albums/xx254/Timbo-08/001%206_4.jpg) (http://s761.photobucket.com/user/Timbo-08/media/001%206_4.jpg.html)
Takes the pedals well and is much louder, but I have to turn the treble way down and the bass up otherwise it sounds very thin and tinny.
I know the circuits aren't exactly the same but I not cluie enough to know why some amps take pedals well and some don't. :icon_biggrin:
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... I have to turn the treble way down and the bass up otherwise it sounds very thin and tinny. ...
Is the Bass control drawn in the correct place? A typical spot would be across that 0.01µF cap, with the Bass pot wired as a rheostat in series with the 330pF cap. The 0.01µF cap then sets the maximum bass available, and as the pot resistance is dialed down the 330pF cap is placed in parallel giving a total capacitance smaller than the 330pF cap. That means minimum bass is determined by the total value of the coupling cap in parallel with the Bass pot cap.
As drawn, it does something similar but also makes the apparent value of the Treble pot 270pF cap smaller, which will make turning up the treble very harsh.
Do you see the Bridged-T filter in this amp, between the Volume pot wiper and the grid of V1B? Try bypassing that (unhooking it and/or a wire straight from wiper to grid).
The T-filter has "mismatched" caps. The 470pF position is considered "0.5C" while the 3.3nF position is considered "2C" so the former is sized for C=1nF while the latter is sized for C=1.5nF. Overall, this probably just broadens the notch (which is perhaps not a bad thing). 1nF against 250kΩ implies a notch at ~636Hz, while 1.5nF against 250kΩ implies a notch at ~424Hz.
You could add R between the 3.3nF and ground to reduce the amount of midrange notch (strengthen mids a bit). Or you could keep the caps as they are and adjust the resistors to sweep the notch up & down (bigger R will move the notch frequency lower, and smaller R higher). I did both to the T-filter of my 25L15-type amp, and the moving notch frequency can change the apparent character of the amp a LOT.
The Treble and Bass controls are somewhat akin to the control in a Tweed Deluxe where midrange is left alone and simple high/low-pass filters control the response. The bridged-T is likely added to get a midrange notch would would have been typical for many 60's amps. You could remove the bridged-T for more of a 50's sound, or tinker the caps to tune the notch for a more 60's sound.
I've used 1nF in place of the 470pF, and 5nF in place of the 3.3nF with 250kΩ resistors to form a T-filter with good results. Again, varying the R's will sweep the midrange frequency higher or lower even though the caps are kept constant.
Hi guys, I have two amps of similar design, one takes pedals (boss blues driver and DD1) well and the other not so good.
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I know the circuits aren't exactly the same but I not cluie enough to know why some amps take pedals well and some don't. :icon_biggrin:
I don't know exactly why either. I do notice for your amp which doesn't play well, there is a lot of crap between the first grid and ground, no matter which input is used. For example, as drawn everything on the Bass input (except the 82kΩ) is in parallel with the 47kΩ resistor drawn at the Lead input. There's a lot of odd tone-shaping right at the input which may not be needed (classic amps which seem to "take pedals well" often have noting but a resistor or two here).
I was never much of a pedal user, though I'm starting to amass some now. I suspect this "takes pedals well" thing is overhyped or is too broadly applied. At a minimum, it tells you nothing about what's going on, and so also no way to fix it.
You may have seen "That Pedal Show" on YouTube (if not, stop NOW and go watch some!!). They did a series of 3 videos on "Ultimate Overdrives" which is fabulous, and also highlights that whether a given pedal gets a pleasing sound can be heavily dependent on the particular guitar (and pickup) and amps used. Similar results are demonstrated in a lot of their other videos, where a pedal sounds great or lousy depending on whether the amp receiving the pedal output is running very clean, already distorted, or then amp/speakers have a particular overall sound of their own.
It has also been instructive (and amusing!) to me that many pedals the show's hosts love get amazing distortion sounds, but often only when set in the relatively clean part of their range. Examples include the Klon (often set with the gain near off), King of Tone (they rarely mention it, but internal dip switches are set for Clean rather than OD or Distortion, so the pedal is used in the lower/lowest ranges of distortion available), any Klon-style pedals, and many of the other OD/distortion pedals.
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I want to see voltages. If a plate is jammed-close to rail or ground it will crap-out early and perhaps ugly.
However life may be too short to solve this question of "Why?"
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If a plate is jammed-close to rail or ground it will crap-out early and perhaps ugly
This is one of the things I look for, especially V1, I'm like you, I don't know whys or how's, but when I get my preamp cleanish, no hard clipping, just minor compression, the players that use pedals often comment, damn your amps can take pedals well. If I get *creative* and try to build distortion into the pre, I get ugly amp, ugly pedal.
any fwiw, keep learnin, it might rub off on me :icon_biggrin:
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Thanks guys, Some good reading HBP, I'm not sure if sometimes the guys that build these amp start to overthink them to get a edge on their competitors by adding unnecessary bits.
OK..........
The Diason is the one that is underperforming and voltages look to be in the ball park of the datasheet. http://www.nj7p.org/Tubes/PDFs/Frank/049-RCA_Cuningham/6GW8.pdf (http://www.nj7p.org/Tubes/PDFs/Frank/049-RCA_Cuningham/6GW8.pdf)
So I'm not sure what the problem is??
(http://i761.photobucket.com/albums/xx254/Timbo-08/013_4.jpg) (http://s761.photobucket.com/user/Timbo-08/media/013_4.jpg.html)
(http://i761.photobucket.com/albums/xx254/Timbo-08/014_5.jpg) (http://s761.photobucket.com/user/Timbo-08/media/014_5.jpg.html)
The bottom voltages are from a stock amp.
(http://i761.photobucket.com/albums/xx254/Timbo-08/001%206_7.jpg) (http://s761.photobucket.com/user/Timbo-08/media/001%206_7.jpg.html)
This is the circuit I'm talking about
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Hi guys, Something is not good with the power output of the Diason Tallboy.
The power output seems to be about half of that of an original amp that I used to clone this one from.
So I stuck them on the Oscope to try to paint a picture for you guys.
Both amp were set with volume @10 and bass/treb @5
wave gen @1k 1v and plugged into the "LEAD" socket
Smaller script my amp and large script the original
Bias setting are marked
Wave patterns were similar throughout both with some more clipping @ pin 8 on power tubes on the original at the points marked
(http://i761.photobucket.com/albums/xx254/Timbo-08/001%203_21.jpg) (http://s761.photobucket.com/user/Timbo-08/media/001%203_21.jpg.html)
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Both sets of signal voltages look fine to me. The slight differences could very well be due to different tubes. Would have been nice to see the signal voltages on the plates of the output tubes and also the signal voltages across the speaker.
Swap tubes between the two amps.
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Thanks sluckey, I'm unsure of what I can do with the Oscope as I have never had any real training on where I can do tests.
The OT is from a Hammond AO-63 similar to yours. I have not been able to track down a OT like the original that is marked 10k:15 and the only one that I have, came from the Hammond.
Do you know what the secondary impedance is for that OT
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(http://i761.photobucket.com/albums/xx254/Timbo-08/001%205_7.jpg) (http://s761.photobucket.com/user/Timbo-08/media/001%205_7.jpg.html)
(http://i761.photobucket.com/albums/xx254/Timbo-08/002%203_14.jpg) (http://s761.photobucket.com/user/Timbo-08/media/002%203_14.jpg.html)
Original amp
Measurement taken at speaker terminal with 15ohm dummylaod
Amp settings same as before
(http://i761.photobucket.com/albums/xx254/Timbo-08/003%204_10.jpg) (http://s761.photobucket.com/user/Timbo-08/media/003%204_10.jpg.html)
My amp.
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Maybe I'm seeing this wrong, but the a.c. voltage measurement at the output tube grids is 36-47v.
But the output tube bias is -8v. It's pretty normal to have crazy flat-topped waves with an input signal 4 times as big as the bias voltage...
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Did a bit more probing..........
I totally disconnected the reverb circuit and got new readings that were just a touch above the original, I also cleaned the input jack and that also brought the voltage up.
The output still did not increase. Also tried a number of speakers and no difference.
I am noticing that the volume is low up until about 5-6 on the Volume dial and increases from that point but still only half of the volume of the original, the original amp kicks in at about 3
Reconnecting the reverb circuit, this lowered the signal voltage back to my original measurements
Some of the resistor values at the lead input on the original amp were slightly higher and so was the signal voltage.
I notice that on most amps the grid resistor is 1M, so changing the grid resistor to 1M on pin2 v2a increased the signal voltage throughout the circuit, but had no effect on the overall output volume.