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bereanbill 8th April 2009 12:09 AM

Noisey gain boost
Hi again ! Thanks to everyone who has responded to my previous threads...I have gained some good insights.
Anyway...I have a VOM amp from an old reel to reel. Single ended with a 6V6 output, 12AX7 preamp, and 6X4 rectifier. Very common and basic...both power and preamp are cathode biased, but the 12AX7 was not bypassed, so I took this as an opportunity to add a cap with a switch for a gain boost. The boost is reaaly cool, but there is a lot of noise. The first half of the 12AX7 has a 1K cathode resistor, the second half a 1.5K. I only bypassed the first half with a 22@25 'lytic. I've tried moving the switch and the ground as well as trying a different cap...thinking I had a bad one. No difference. Would it help to replace the switch with a jack and control the boost remotely ? Should I switch the bypass cap to the second half of the 12AX7? Any suggestions would be appreciated...thanks !:bawling::bigeyes: :(

pointy 8th April 2009 02:49 PM

have you tried the set up without the switch (ie. just the cap in bypass)

pointy 8th April 2009 03:19 PM

what I mean is if it works fine with just the cap you can then control the boost down before the amp.

in the old days they would not use this type of boost as you have found as so as you put a switch in,........ it all goes tits up.

so they would go for anode boost effect etc.

but now with the better graphic equalizers in most homes.........its easy

bereanbill 8th April 2009 09:36 PM

More gain...good, noise...bad
Hi Pointy...thanks for the reply. No I did not try it without the switch...I need to clarify the application here. I am mainly a guitar amp junkie who really dislikes modern tube amps. I will try removing the switch, but it defeats the purpose of the boost. That's why I thought using a remote switch might help....:idea:

mashaffer 8th April 2009 10:51 PM

It might help to know how the unit is wired at the input and the grounding scheme used. For example is shielded cable used from input jack to grid of first tube? Is star ground, buss ground or "wherever it fits" ground used? Is the noise high freq., broadband white noise, or hum?

What I am thinking is that the amp was designed with a certain level of gain in mind. With higher gain more extensive measures may be required to keep the noise in check.

pointy 8th April 2009 11:21 PM

it may be worth your while waiting for someone else to post on this subject.

but as far as i recall the way to use the cathode bias bypass as a bass boost would be to fit the cap in place and then use pre-amp controls to tame it down.

it was thought that it messed up the valve capacitance but it was found to be something more like the switch started acting in part like a capacitor.

up to the 1970s designers would not use it and as far as i know no real
way has been found around it.

i would have normally thought if it was that the switch acting as a capacitor that you could just put a small resistor in between but from what i have heard it does not work out like that.

so i think its best to wait for someone with more current information.

bereanbill 9th April 2009 12:51 AM

Ok...I took out the switch and the hum dropped considerably. But this is not a good thing as I wanted to have a switchable gain boost. The input to the grid is shielded 2 conductor wire, the ground is at the same spot as the cathode resistor...a terminal lug, and the noise is hum that increases with volume. :scratch: :goodbad:

mashaffer 9th April 2009 01:03 AM

Can you give us pics of the underside? I would start looking at the heater wiring (is it tightly twisted, routed around the edge of the chassis, kept away from the inputs etc.). Are the PS filtering caps grounded at the same point as the signals or a separate location close to the power transformer? Also look at the power supply filtering. Your circuit may need better filtering. It is also possible that the routing of the wiring to your switch is less than optimum. Keep the wires short and away from heater and PS wiring. If you have to cross other wires do so at a 90 degree angle.

Another thing to look at is the heater winding reference. If the heater winding is grounded the hum might be improved by elevating the heater somewhat. Easiest way is to connect the heater winding center tap to the cathode of the output tube instead of the chassis ground.

pointy 9th April 2009 05:58 PM

you can do the same thing so you have the boost but you would need to do it at the anode.

what happens is the contacts of the switch gets a small cover of dirt grease or dust and then acts as a intermittent capacitor.

if you change to the anode the increase of volts will over come this effect.

now days you may be able to use a mosfet or transistor as a switch
but i have no knowledge of anything working at Kr bypass.

bereanbill 9th April 2009 09:30 PM

So Pointy, are you saying bypass the plate load resistor and put a switch on it?

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