Clones vs originals

[Scumback Speakers]

I think you're preaching to the choir here, Jim. Almost all the guys on this board (and certainly the ones on this thread) use Metroamp quality parts throughout. And a lot of people, the participants here included, try to take it even one step further by using as many NOS parts as possible.

I wish I could agree with that statement. I've bought 8 used Metro clones, only two came to me with all the parts George recommends, or supplies. It seems once the stock values are changed even slightly or if the circuit is modded, the other six clones found some of the absolute cheapest parts they could to mod them.

Of course, those cheap parts came at the price of decent tone, and that's why I bought them for less than what kits sold for without being assembled.

So while I would love to think that everyone on the Metro board has your level of parts installed, it's not reality.

So let's leave it at "You're preaching to 1/2 the choir, Jim."...or maybe just 1/4th of it based on my Metro clone purchases.

[shakti]

Fair enough!

[Roe]

Interestingly, builders of vox and fender and dumble clones experience much the same problem with newer OTs: they simply have to much (low) bass

[shakti]

Roe,

thats is indeed interesting. It would be easy to A/B output transformers and see if that's the whole story, but I don't think I am going to mess around with that on this particular amp (the 70 Super PA). What I willt ry to do if I find the time before it's going back to its owner, is to transplant the pots into my own 69 build and see what difference that makes. That, and try snubber caps on the rectifier or a different rectifier altogether.

I forgot to mention that the older amps also seem to work better with fuzz, probably because there's less deep bass and they seem to focus better in the mids. Again, it's subtle, but the newer amps with more (deep) bass seem to get slightly more splattery when hit with a fuzz. When I hit the Super PA/Lead conversion with a fuzzface, it sounds eerily close to Hendrix' sound on the BBC Lulu TV show from January 69 (playing Voodoo Child SR, Hey Joe and Sunshine of Your Love). My clone gets just that tiny bit more blurry. Not by much, but I can hear it.
Interestingly, once I add a non true bypass wah in front, and run it through a Uni-Vibe preamp, the differences are almost unnoticeable. Presumably those two factors shave off some of that deep bass from the front end, so it's not there for the amp to work with. Makes me wonder what a (much) smaller bypass cap on V1 cathode can do...

[Roe]

Shakti, some people who have swapped OTs report that clones tend to sound cleaner with a flatter response. I've personally tried this with an ac50 (which had a lofi OT)

I've also tried running an original plexi preamp into the power amp of a plexi clone and vice versa. They sounded similar but the power amps differed more than the preamps.

A quick and dirty solution for getting almost corrects pots: I get alpha 50k and 500k pots which tend to read around 45k and 450k. I then add 1/4w CC resistors (usually 120k and 1m2) in parallell to the pots, ending up with values around 33k and 330k. Still, the taper is not correct

[shakti]

Quick update; haven't had a time to mess with the rectifier or swap pots yet, but came to think of two things;

- forgot that this SL Circuit has just a 0.68uF bypass cap on V1 cathode, so that would probably rule out the possibility to use that cap as a way to remove some of the excessive low bass on the clone(s).

- the original Super PA/Lead conversion is slightly cleaner sounding and appears ever so slightly lower output. Perhaps the output transformer has a higher primary impedance? I suppose there is some variability to them, and even though it is suppose to be nominally 1.7k and the M e r r e n clone is supposed to be 1.7k, there is the possibility that the original is higher. That would translate to less distortion, less output, and less bandwidth...at least as far as I understand it? I will try to measure primary impedance if I get a chance. Can I do that with the transformer in situ?

[Roe]

yes, but remove the output tubes when measuring. apply a low voltage ac signal, eg. 6.3vac, at the secondary and measure the ac voltage at the primary

[shakti]

Yep, will do later.

I did just try to put the pots from the 70/69 conversion into my clone. First of all, the pots in the vintage amp measured way off. The two volume pots were above 1M5 and 1M3 respectively, likewise with the bass pot. The treble pot was almost 330k and the mid pot not that way off at 28-29k.

More interesting was that with the vintage pots installed in my clone, it had an EQ balance much closer to the vintage amp. It was very obvious on the bass pot, which could now be turned up quite a lot, and it still didn't feel like it was cluttering up as much and some of the deepest bass seemed to be gone almost entirely, or at least it appeared that way. It had an almost "drier" and more mid focused sound like the vintage amp. Of course, this wasn't true A/B comparison, and I didn't put the new pots in the old amp to see how that sounded, but it was most interesting. Since it was more mid focused it seemed to sustain more at lower/cleaner settings. I liked it a lot.

I seem to recall someone modifying their new pots to achieved higher values, and possibly modifying the taper as well. Has anyone tried that?

[Tazin]

Like George mentioned, the wire type, transformers, and potentiometers are big players. I know the tapers in the old pots are slightly different than most modern units. This is one reason why anything from 0 thru 5 on the dial has a different response when comparing the originals to the clones. You also have to remember that Marshall sometimes used LOG pots where LIN pots were normally used. The difference in low end is mainly due to the transformer(s) construction and materials as far as I can tell. Wire strand size/gauge and number of strands per bundle mainly effect it's current carrying capabilities....But when you couple in all the other factors like; pre-tinned, not pre-tinned, copper impurities, capacitance per/gauge, inductance, stretch factor due to spool winding, etc., it allows for a lot more variation between wire types.

[shakti]

Bump for info on how to modify the pot values and tapers. I seem to recall there being someone on Ebay offering the service? I really think it would be worthwhile to try modified pots to shave off some deep bass and have it more focused in the mids. I suspect the treble pot is the biggest player in this, but I would want higher reading 1M pots as well.

Tazin, the info about wire is also interesting. For all the pink wires to the pots I have stranded wire that is not top coated, and it appears similar to the wire in the old amp. I got it from M e r r e n. But all the other wire is newer top coated wire.

[shakti]

Found this in an older post:

Potentiometers are often overlooked as a source of variability because they're regarded as variable carbon comp resistors and that's where the discussion ends. The truth is different brands of audio taper pots from different eras vary considerably in rates of taper and resistance, both of which affect the overall picture.

One thing I've learned is when dealing with guitar volume and tone pots, I want to be assured that the pot delivers at least the rated resistance. I've changed plenty of guitar volume and tone pots for 'reworked' pots that deliver the full rated resistance (or more), and the differences are plainly audible. For example, many '500k' volume pots give maybe 440-450k. These are often installed unchecked. When the pot is reworked to deliver 500k+, suddenly the pickup gives greater output and clarity.

There is no reason why this observation doesn't apply to every pot within an amplifier, which is a worthwhile note to DIY amp builders.

When dealing with a pot that gives a substandard reading, one can modify it to increase the total resistance. Here's a video that illustrates how to disassemble Bourns and CTS pots, and the physical difference between log and linear taper:

http://www.youtube.com/watch?v=rUkrpqEmXb8

As for modifying the pot to increase the resistance, I believe it's a matter of scraping the carbon conductor with an Xacto knife. I've purchased reworked CTS guitar pots inexpensively from one Ebay seller (strat100), who I imagine would be willing to modify one's amplifier pots for a nominal charge.

Thinking about this now; construction, inherent capacitance, overall resistance and taper of the old pots I suspect are a big player in the difference between old and new amps. A new market for someone with the know-how and dedication to have vintage spec "Plexi Pots" made, perhaps? It appears Alpha pots may not be so easy to modify, while CTS pots should be possible to modify.