Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Tone Junkie on August 14, 2013, 04:59:23 am
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Anyone done any measurements on carbon comp. resisters in old amps that there willing to share the results. Im looking to build a 5d6-A bassman and I understand the old ones sound wonderful and I have seen that people have copied the values from the schematic and were not as happy. Im thinking resister drift has a lot to do with this Im just looking for some averages.
If you have what you think is the holy grail and don't want to share. I understand just give us a few fender values to work with. I have a few but not enough to go all the way yet. Marshall values are good to.
Thanks Bill
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Some of them old resistors drift with the temperature. So just putting in different values won't give you the mojo.
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TJ, the only ones I ever measured were in the old Harmony. Some were still very close, I replaced the ones that were way out of whack (drifted upwards). What I replaced went back to schematic values, the amp sounds much much better than it did before.
I think that the reason it's so difficult to build 'em "just like they used to" is that the transformers are beefier, the caps and resistors are better quality, we're more careful of layout and all that. I really don't know if CC resistors "add that special something" or if I'm just hearing the resistor noise mixed in with the audio signal. (more likely) And the caps in the old ones are probably way out of spec too. Maybe the clones today actually sound just like the originals did in '65!
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Bill, I am not convinced resistors are the key element. I have a 62 Bassman with that mojo. Strange how some amps just have that little extra. When going through it I found all the the coupling caps and bypass caps had drifted to lower values. Much lower values in a couple of places. The amp always has a little hair on it and it is very nice. I did change a few resistors with DALE RN65, 1 watt where you are supposed to keep the CC. Like the plate resistors. The amp got much quieter, but the tone stayed the same. The resistors had drifted up, but not out of tolerance.
Considering what John mentioned, I believe the transformers and ripple current have something to do with it. For what it is worth, I built a 5F6a and was not very happy, but kept on and when I put an old Supro OT in it there it was. For what it is worth.
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Hey Bill, I was thinking of that very thing, just to have a record of the effects of time on the old components and as ED said, if they haven't drifted too much is it really going to make much difference and when there isn't many amps around how can you compare.THanks
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Two or three key things give old amps that 'mojo'.
1)transformers
2)tubes
3)speakers
The resistor drift is a waste of time IMHO.
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My experience is the same as everyone else here, they drift upward, are noisy and even new ones are on the high side.
My first build was from David Allen, an Old Flame actually his second head he sold (super Reverb topology). He send me all CC, the vintage wire and all vintage specs. The amp sounds fantastic even better than when new in the mid nineties. All CC, not much noise either but that's rare in a vintage CC amp. I think the drifting can be for the better or worst, that's why some of those BF amps are killer with just new filters, IMO.
I wonder if one could just measure each old CC and go off that value trying get the closest to the schematic...? OT's probably have a lot to do with the tone too.
I don't use CC much anymore except sometimes on the plates as they say that's the best place for them.
I had an old V4 Ampeg that had 2 100K's in the power rail that measured like 5K, and 22K so there ya go for drifting upward.
al
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Yea I wasn't going to use CC resisters I like carbon film. I was just going to try equaling things out like some of the old ones, just to try.
Use good quality iron and I guess in some places low quality Iron has the sound (LOL). I have enough cathode cap, and coupling cap boxes now I can just dial it in . I guess I have been doing to much reading and had gotten to some cork sniffer sites were they really hadn't built much and were just discussing there idea's. Ever notice cork sniffers will give you this is the holy grail speech when they probably havn't built an amp in there life.
I probably better finish the 2 I have in my amp chassis now to get to that bassman. I havn't built any of the tweed variety yet and realized my collection is lacking, So many amps so little time.
Thanks Bill
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I cannot find the video, maybe someone else can point us to it. The new reissue fender Bandmaster 57 video shows how they created the amp. They used 3 originals, which all had their own character. They analyzed all 3 to come up with the new bandmaster. Very cool video as they play all of them, 3 originals and the new one. Fender really did a good job with this amp and the assumption from the video is they did exactly what you are referring to. While all 4 are slightly different, the new one stands on common ground. Not sure what they did as I have not compared the original schematic to the new one, but it may be interesting to see what they changed for the new one.
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Good call Ed might be interesting.
Thanks Bill
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Good call Ed might be interesting.
Thanks Bill
I saw your same question on amp garage. Put all your resistors in the oven for 8 hours and dip them in shellac and you will have mojo to spare. :l2:
There is some validity in wall current differences but a variac can get you there. I use one often while tweaking only because it is fun.
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Hey Bill, I took some readings of the FI SONIC to see how the resistors/ caps faired after 48yrs and the usual suspects were dead.
All electrolytes were dead
The coupling caps faired well.All the mustard and polyester caps were spot on.
The Wire wound resistors were good.
The CC were all over the place.
Attached is schem with the measured values, I will be leaving as is and decide to change once fired up. Thanks
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They really are all over the place aren't they Timbo mostly high but a few low. I think I will just stick with kiwames or dale resisters I have a bunch
Carbon films are good and seem to stay were you want them.
Thanks Bill
I think I will go with the Valco PT. and a under rated OT and see what happens.
Thanks Bill
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ROCK ON :headbang:
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Anyone done any measurements on carbon comp. resisters in old amps that there willing to share the results.
I measured every cap and resistor in a '67 Princeton Reverb that sounded especially good to me.
Most coupling caps measured dead-on (or very, very close). Most cathode bypass caps dried out and dropped to 1-5uF.
Resistors were all over the place. Some close to spec, some a little high, some a little low. Almost all were still within the marked 10% tolerance.
I can't confirm this story, but I think I might be able to find one or two authoritative sources to back it up...
When manufacturers produce carbon comp resistors, they mix a carbon material and binder into a paste, with a target resistance in mind. The paste is formed into the shape of the body, leads are attached, and an outer coating is applied. Next, the resistors are measured to determine the value to mark on the body. The process is in-exact enough that the resistor value has to be marked on the body only after verifying where the particular resistor measures.
For all resistors that get the "100k" color bands, a tolerance has to be determined and this happens after-the-fact when sorting the resistors for marking. Basically, all resistors measuring 95k-105k get marked as "100k, 5%". Those measuring 90k-95k and 105k-110k get marked as "100k, 10%". Resistors measuring 80k-90k and 110k-120k will likely be marked as "100k, 20%" ... except that 108k-114k could be marked as the low end of "120k, 10%".
Since in the old days manufacturers charged more for 10% or 5% tolerance resistors as compared to 20% tolerance, there is no economic or manufacturing-process incentive to have 10% or 20% resistors that measure exactly 100k. Therefore, it is highly likely the resistor values are "hollowed out" and the 100k 10% resistors in Fender amps never measured exactly 100k but were either high or low.
While carbon comps can and do drift with age, and do change value some with temperature rise, from what I've seen it's most common to have resistors that don't exactly measure on their marked values but are reasonably close to the marked tolerance when you consider the above.
Things are different these days, as cutting a spiral in carbon or metal film deposited on a rod is a more exact and repeatable process, and 5% resistors sell in such huge quantities for transistor gear that 5% tolerance is generally cheapest in carbon film resistors (and largely same-as other tolerances in the metal films I've been using lately).
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> cutting a spiral in carbon or metal film deposited on a rod
That's not essential. You put a known number of rods and a measured puff of butane in a drum, heat it, spin it, you have a known quantity of tightly bonded carbon on each rod. Gas-meter accuracies beyond 1% are routine in chemical and pharmaceutical factories. There's probably a cheaper brand of gas-meter with 2% slop. After a million operations it wears to 5% slop and can still be used for 5% resistors. If the slop is consistent they just re-mark the dial to get the right value.
Spiral may be useful for very high values so they don't have to meter nano-scopic quantities of gas. Though I'd suspect they'd instead dilute the straight gas with exhaust gas down to 1% concentration to get two decades more-higher values with cheap meters.
The exotic use of spiral-cut is to measure each part and auto-cut the spiral to the right value. That's practical but hardly cheap. I doubt they do it for the penny-resistors we buy for a dime.