interesting observation re:filtering in some old amps

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interesting observation re:filtering in some old amps

Postby dai h. » Tue Apr 14, 2015 11:05 am

from "Tazin" (probably among the more (most?) knowledgable guys on old Marshall details) over on metroamp bbs:

http://forum.metroamp.com/viewtopic.php ... 1&start=15

kind of funny that I had a 50W set up like that (if I understand correctly) with split cathode setup but four filtering stages instead of the usual five (I had from rectifier out to preamp--40-30-20-16uF, 0.47uF instead of 0.68uF, 6.2k instead of the usual 10k-20k for preamp B+ of 385V which was the same as a 5F6-A IIRC) when I was chasing the..erm...Hedwig von Brownstein tone some years ago and experimenting. *I think*(could be false memory or something) I noticed the seemingly odd B+ wire positioning before on some tag board pics but never really made the mental connection that it was mis-wired. I have noticed other boards with mixed feature oddities which I suppose could easily happen from confusing or perhaps interruptions (different people or perhaps a weekend) during wiring.
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Re: interesting observation re:filtering in some old amps

Postby David B » Wed Apr 15, 2015 12:29 pm

How about that, QC department let a few out there :lol:
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Re: interesting observation re:filtering in some old amps

Postby dai h. » Fri Apr 17, 2015 10:22 am

heyho!

well, I tried looking for some of these apparently mis-wired amps in my chassis pics collection, and so far I've found two. Both have the dark perf board (with the holes), lots of Dubilier blue/white-ish caps, and the B+ wire piece in question has red (cloth-looking) insulation. Can't be absolutely certain, but I'm guessing in both cases, no one noticed and never bothered to correct the wiring?

One is a lead with a number of obvious alterations (red marks not redone for the re-worked joints, different parts values). The other is a Bass which I think belongs to (or did belong to) Glen K.(the Trainwreck playing virtuoso sort of player guy). The Lead has two RS branded 33uF450 axials on the board, which I'm guessing were a replacement for the dual 32uF w/ground lead out the other end seen in Marshalls. The Bass amp has a can on top of the chassis (don't have a picture with the value marking showing though, unfortunately).

So I gather the filter decoupling the stages connected consists of the filter plus a parallel network of 10k + the other filter cap. Have no idea if the parallel network would do anything (seems shorted out by the "normal" filter).

(not directly related, but something that I've wondered about that looks a bit weird from a conventional point of view.) Sometimes a preamp filter is lying directly on top of a B+ dropping R and really close to one of the tone stack 22nFs. So wondered if the heat would start affecting those (more leakage for the alu electro and change in value for the 22nF film?).
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Re: interesting observation re:filtering in some old amps

Postby dai h. » Thu Apr 23, 2015 8:59 am

another thing (besides reducing a filter stage) to experiment with (looking for that vintage something) is to try lowering the value maybe 50% or more (over time, the liquid inside aluminum electrolytic capacitors can dry up, reducing value). The "50%" is just a rough guess from reading things like figures measured from things like console restoration. The lowest preamp filter value I've tried (FWIW) is 3uF (two 1.5uF films). No difference in hum as far as I could tell, and the films seemed to smooth out the tone slightly (subjective I guess). Also think about the 5F6-A (which is meant to be a bass amp). That has 8uF stock for the preamp filter if memory serves.
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Re: interesting observation re:filtering in some old amps

Postby neikeel » Sun Apr 26, 2015 4:36 pm

I am sure that is one of the features of a vintage amp - less efficient filter caps that are smooth (er tired).

Most amps I end up recapping are louder, brighter and tighter - not sure I always like the results, so going lower might be the answer, wonder how accurate the 50% guesstimate is?
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Re: interesting observation re:filtering in some old amps

Postby dai h. » Sun Apr 26, 2015 6:44 pm

perhaps someone could share if they've taken measurements?

for a triode set up like that in 5F6-A, Marshalls, there are four (circuit loops).

a. input loop from guitar pickup through "hot wire" of the shielded cable, then through the amp's 1Meg, then back to PU by way of the shield (which is a shield/return path)

b. from B+ source (towards power transformer) through plate side to cathode, then back towards power transformer (through ground) (I think of this like a resistor across the power supply with bias affecting the R value)

c. output loop from plate through local filter cap, then to cathode by way of ground

d. output loop from plate through output coupling cap(the coupling cap is tapping into loop c.), then 1Meg pot then back to cathode by way of ground

since the output signal travels through c. it seems like the cap should do something. I think it can affect the amount of low frequency extension (bigger = more extension, not a "boost" in resonance) and a bigger one would seem like less distortion (less voltage drop through the cap. If smaller, then I think that means more reactance(?) towards lower freqs, so more voltage drop = more distortion). I doubt there is any absolute right or wrong but seems in general, smaller = more coloration, bigger = less.
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