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Post subject: 5E3 Deluxe Build Questions
Posted: Fri Jun 21, 2013 9:47 am
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Hi, I'm a new amp builder and needed a few questions answered on a 5E3 deluxe build I completed recently. First off, the amp works great in all aspects except one. With 6V6s tubes, the amp will start oscillating (sounds kind of like tremolo) when turned up above 8 which keeps going until I turn the amp down. With 6L6s the same thing happens, but much more subtly and only at full volume. So I talked to my amp repair guy and he thought the power tubes might not be getting sufficient power and suggested increasing the value of the cap after the rectifier and checking the cathode resistor and the bypass cap for the power tubes. So here's my questions. First, I don't know all my terms and can't properly follow a schematic so I don't know which caps and resistor he is talking about. Also, do you have any idea what could be wrong? Thanks a lot!


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Post subject: Re: 5E3 Deluxe Build Questions
Posted: Fri Jun 21, 2013 1:43 pm
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Do you have a link to the schematic? For example, did you use resistors on the output tube control grids, like 1.5k ohm?

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Post subject: Re: 5E3 Deluxe Build Questions
Posted: Fri Jun 21, 2013 5:19 pm
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shimmilou wrote:
Do you have a link to the schematic? For example, did you use resistors on the output tube control grids, like 1.5k ohm?

The problem is I don't know what the output tube control grid is. I'm not learned in my terms yet.
EDIT: Nevermind, I did put 1.5k resistors on the output tube control grids.


Last edited by DrewMeyer on Sun Jun 23, 2013 2:56 pm, edited 1 time in total.

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Post subject: Re: 5E3 Deluxe Build Questions
Posted: Fri Jun 21, 2013 5:24 pm
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DrewMeyer, IMHO, it's sounds like you have some coupling going on between sections of your power supply. This is the part of the amp that powers the output tubes, the gain stage, and phase inverter. What it sounds like is possibly "motorboating" from positive feedback in these circuits.

You prolly need to go over those electrolytic caps in the power supply. I have seen many "new" or not-too-old ones go prematurely bad. If this doesn't solve the issue... there are other ways to isolate sections from each other.

Good luck!


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Post subject: Re: 5E3 Deluxe Build Questions
Posted: Sat Jun 22, 2013 1:29 pm
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BMW2002Ti wrote:
DrewMeyer, IMHO, it's sounds like you have some coupling going on between sections of your power supply. This is the part of the amp that powers the output tubes, the gain stage, and phase inverter. What it sounds like is possibly "motorboating" from positive feedback in these circuits.

You prolly need to go over those electrolytic caps in the power supply. I have seen many "new" or not-too-old ones go prematurely bad. If this doesn't solve the issue... there are other ways to isolate sections from each other.

Good luck!

Oh that would be an easy fix. Is there any way to check if they are bad without replacing them?


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Post subject: Re: 5E3 Deluxe Build Questions
Posted: Sat Jun 22, 2013 3:01 pm
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You might start by checking those filter caps to ascertain that they're correctly polarized within the circuit -- that is critical to optimum performance of the filter stage (dittos with the idle-bias cap, which should be reversed, ie: "+" tied to ground).

Arjay

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Post subject: Re: 5E3 Deluxe Build Questions
Posted: Sun Jun 23, 2013 3:00 pm
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Retroverbial wrote:
You might start by checking those filter caps to ascertain that they're correctly polarized within the circuit -- that is critical to optimum performance of the filter stage (dittos with the idle-bias cap, which should be reversed, ie: "+" tied to ground).

Arjay

All my bias caps have their negative ends tied to ground. I put them facing the same way as my electrolytics as per this diagram: http://2.bp.blogspot.com/-m1XpM-ob8Y0/T ... 080508.jpg Is that a problem?


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Post subject: Re: 5E3 Deluxe Build Questions
Posted: Sun Jun 23, 2013 3:08 pm
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There's only one bias cap. And it appears to be oriented correctly.

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Post subject: Re: 5E3 Deluxe Build Questions
Posted: Sun Jun 23, 2013 6:52 pm
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Drew, your build is a Ceritone? If so, I'd consider upgrading the filter caps to F&T or at least Sprague Atoms. Same for the power tube cathode bias cap. JMHO, I'm not a fan of star grounding. I would definitely want to see the power tube cathode resistor, bypass cap and the first filter cap grounded at the power transformer lug. Again JMO!!! Art

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Post subject: Re: 5E3 Deluxe Build Questions
Posted: Sun Jun 23, 2013 10:12 pm
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aclempoppi wrote:
Drew, your build is a Ceritone? If so, I'd consider upgrading the filter caps to F&T or at least Sprague Atoms. Same for the power tube cathode bias cap. JMHO, I'm not a fan of star grounding. I would definitely want to see the power tube cathode resistor, bypass cap and the first filter cap grounded at the power transformer lug. Again JMO!!! Art

I based the build off that layout, but didn't use a Ceritone kit. But I could probably use an upgrade to F&Ts. What exactly is star grounding?


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Post subject: Re: 5E3 Deluxe Build Questions
Posted: Mon Jun 24, 2013 6:48 pm
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Drew,

Star-grounding is tying ground wiring to as few chassis points as possible. To reduce the chances of ground loop induced hum -- due to slight potential differences in each ground wire.

http://www.aikenamps.com/StarGround.html


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Post subject: Re: 5E3 Deluxe Build Questions
Posted: Tue Jun 25, 2013 10:21 am
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I'd check the wire routing too. Sometimes the way the wiring is run can cause parasitic oscillation.

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Post subject: Re: 5E3 Deluxe Build Questions
Posted: Tue Jun 25, 2013 12:54 pm
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63supro wrote:
I'd check the wire routing too. Sometimes the way the wiring is run can cause parasitic oscillation.


+1

Try to find some detailed photographs of a modern clone 5E3 chassis such as a Victoria or an Allen and replicate that lead-dress configuration.

Arjay

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Post subject: Re: 5E3 Deluxe Build Questions
Posted: Tue Jun 25, 2013 1:19 pm
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I always look for the simplest thing first. :wink:

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Post subject: Re: 5E3 Deluxe Build Questions
Posted: Tue Jun 25, 2013 1:33 pm
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That often pays the biggest dividends.

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