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Old 23rd January 2016, 08:12 PM   #1
hewo is offline hewo  United States
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Default Increased hum from changing 1st condenser vc508 schematic

Installed replacement 1st condenser 100uf appliance run capacitor (polypropylene, non-electrolytic)

But hum comes bigger, why?

Run capacitor has sealed pigtail leads eight inches long untwisted, soldered into original capacitor's pcb landings.

How can high tension dc fed by bridge rectifier introduce bigger hum?

Can proximity wiring affect 8" untwisted run capacitor, either influx into ht or efflux out ht into susceptible wiring?

Or could it be the swapout to one meg audio taper of gain and level pots?
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Old 23rd January 2016, 08:32 PM   #2
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the bigger the capacitor, the bigger the currents through the mains transformer. This indicates me that a magnetic coupling between the mains transformer and (likely) the output transformers is inducing hum. It is also possible that the wring between rectifier and 1st capacitor is part f the ground plane of your circuit. A photo or exact layout would be helpful
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Old 24th January 2016, 04:35 PM   #3
hewo is offline hewo  United States
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Default rescind appliance run capacitor install

Rescinded appliance run capacitor install.
It is not the problem of hum invite.
It was 96.4 but factory 88.2 is accurate enough per schematic call.
The meg pots are amplifying way too much.
So I Rescinded both, instead using twenty kilo linear for level.
That reduced hum, as the preamp tube does not trump dc heat.
I'm wondering how much gain resulted by those meg pots? Did not get chance to plug in guitar, amp runaway in looped feedback without input, meaning gain is out of control, likely in the burrbrown2134.
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Old 24th January 2016, 04:52 PM   #4
kevinkr is offline kevinkr  United States
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So this is a guitar amp, looks like I am moving it again. Guitar amp threads belong in Instruments & Amplifiers.

You might get a little help over there, but changing the gain control pots is probably not the right thing to do.
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Old 24th January 2016, 05:20 PM   #5
hewo is offline hewo  United States
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Default pot does manipulate performance

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Originally Posted by kevinkr View Post
So this is a guitar amp, looks like I am moving it again. Guitar amp threads belong in Instruments & Amplifiers.

You might get a little help over there, but changing the gain control pots is probably not the right thing to do.
By changing pot temperament, the upstream signal can be throttled more pronounce'fully.

It's no different than changing guitar pot values on a particular signal generator (pup).

Realize pot can be viewed as an obstruction to signal growth. If we constipate the signal chain with a pot meg ohm, upstream signal will "bloat", meaning amplitude grows. The reciprocal would be to "unbloat" the upstream signal, say by pot tenth meg ohm, whereby signal can thrive into earth fullspeed, of which particular frequencies get trashed more readily and with purpose too for signature appeal remainder signal unearthed.
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Old 24th January 2016, 06:14 PM   #6
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Have a look here;
http://www.guitarnuts.com/amps/vc508..._schematic.pdf
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Old 24th January 2016, 06:31 PM   #7
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About changing the cap in the PSU : the legs of the cap should be virtually non-existent, so eight inches is an astronomic lenght
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Old 24th January 2016, 06:38 PM   #8
Enzo is offline Enzo  United States
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Come on, hewo, you've been around long enough, tell everyone it is a CRATE VC508, unless it is something else, and put up a schematic so everyone can see what you are talking about:
http://bmamps.com/Schematics/crate/C..._Schematic.pdf

You surmise changing the cap increased the hum, then only as a parting thought you mention changing pot values. What else did you change? Not sure what the new cap was going to do, but as DC it won't radiate much from its wires, or pick up anything. In post #3 you add you apparently changed the op amps. Anything else?

Quote:
It was 96.4 but factory 88.2 is accurate enough per schematic call.
What does this refer to?

Pot values changed? Oh dear... Look at the first stage, gain pot P3 is 250k, and combines with 2.2k R5, that ration sets the gain of the op amp. By quadrupling that value you upset the designed gain of the stage. And later, the volume control P1 is part of a voltage divider comprising R6, R11, and the tone circuits, plus P1. Looking just at R6 and P1, they form a 10/1 voltage reduction in signal level leaving the tone controls. If you made p1 a 1 meg then you have reversed it to a 1/10 voltage reduction of signal.

So looking at the circuit, the output from P1 is meant to be a line level signal, about 1 volt. The signal is shown as 30v p-p under test conditions at the preceding plate, so with a 1 meg pot, instead of about a volt all the way up, your signal level would be a lot closer to that 30v level. That would seriously overdrive anything in the line out jack, as well as hugely overdrive the following grid. Note the plate of that following triode only expects about 10v p-p, but if you start with more than that at the grid, it won't be happy.

So it is vastly different from changing the pots on a guitar. They affect impedance, but not overall signal levels or gain.

So what you did was increase gain in the first stage, but also increased signal level at the master volume. I don't doubt the amp is not stable.
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Old 24th January 2016, 06:39 PM   #9
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Quote:
Originally Posted by hewo View Post
By changing pot temperament, the upstream signal can be throttled more pronounce'fully.

It's no different than changing guitar pot values on a particular signal generator (pup).

Realize pot can be viewed as an obstruction to signal growth. If we constipate the signal chain with a pot meg ohm, upstream signal will "bloat", meaning amplitude grows. The reciprocal would be to "unbloat" the upstream signal, say by pot tenth meg ohm, whereby signal can thrive into earth fullspeed, of which particular frequencies get trashed more readily and with purpose too for signature appeal remainder signal unearthed.
Even with Google Translate, I can not understand what you are saying...
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Old 29th January 2016, 07:47 PM   #10
hewo is offline hewo  United States
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Default vc508 absurd gain actually okay

While the vc508 had those incredibly high signal, the speaker voiced incredibly maybe because the ot is an over rated edcor at 18va?

Anyhow dials had to be babied to stay away from overheating el84.

I restored the factory gain pot but settled on volume pot of twenty instead of ten kilo ohms.

The appliance running cap 96.4uf is well worth it! The eight inch leads are not deleterious to creation hum, how do I know, scope. So I will recap all powering electrolytics on tubes with such appliance running caps, they're significant breathability, airy.

Also experimenting with inductor on tone wiper earth dump, about 1k, 42gauge windings over iron core slug. Looks like highs get reflected back upstream instead of annihilated. I also will experiment tone pot change, go bigger, like meg linear. Trick is to tailor ranging of it for useful signatures (not dark!)
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