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Post subject: Re: 5E3 Circuit
Posted: Sun Feb 26, 2012 1:26 pm
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BMW2002Ti wrote:
As far as I know, the AA (AB)763 SR have interactive stages. To test the theory, just pull the input 12AX7 (7025) on the usused channel and see if can hear a difference.

With many of this era Fender amps, you can pull the tremolo/vibrato 12AX7 and hear a difference in the NORMAL channel's response.


http://www.webphix.com/schematic%20heav ... _schem.pdf


Seems to me that the reason you hear a difference when you pull the vibrato input tube is because the second stage of the vibrato input and the second stage of the normal input share a cathode resistor between the two halves. Pulling one tube will leave the other tube with an 820 ohm cathode resistor all by itself instead of a 1500 ohm resistor a single tube should have. The bias of the second tube half will be altered which will affect the sound. I don't really consider that "interactive" the way the Tweed design was.

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Post subject: Re: 5E3 Circuit
Posted: Sun Feb 26, 2012 2:34 pm
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I agree with Bill, about the bias and voltage drop across the cathode resistor. But IMO what we are hearing, mostly, is lack of bleed-off of the preamp signal. The two channels are summed just ahead of the phase inverter circuit. At this point, part of the signal is allowed to divert back into the unused channels circuit. With the tube removed , there is an open circuit. Thus, nowhere for that part of the signal to go and all of the signal gets to the power amp. Kind of the same reason jumpering channels works, but that's another subject. Beemer, that looks like good stuff ! I used to use Pro-Gold, but when that was discontinued I went with the D5 and S5. Haven't been chem shopping for awhile. Thanks, Art

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Post subject: Re: 5E3 Circuit
Posted: Sun Feb 26, 2012 10:20 pm
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On the repro tube chart in the amp it says you can use a 5y3gt or 5v4gt rectifier tube and a 12ay7 or 5751 in V1. I understand that you can swap these 2 preamp tubes with no problems, but can I just switch out the 5y3 for the 5v4 I ordered from KCA NOS or is there an adjustment I need my tech to make before I do? Thanks guys.


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Post subject: Re: 5E3 Circuit
Posted: Sun Feb 26, 2012 10:46 pm
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A 5V4 puts outs more current 5Y3. It'll be a much more stouter tone. You won't need to adjust the idle bias on the output tubes, because the output tubes are cathode-biased. They are somewhat self-leveling.

http://www.webphix.com/schematic%20heav ... _schem.pdf


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Post subject: Re: 5E3 Circuit
Posted: Sun Feb 26, 2012 11:28 pm
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Yeah, it's a rectifier with less sag and will give a little tighter sound. I just want to try it and see how it sounds. BMW, it sounds like your saying I can just drop this thing in and it will be okay?


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Post subject: Re: 5E3 Circuit
Posted: Sun Feb 26, 2012 11:33 pm
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Okay, thanks. I'll give it a shot and see what happens. Thank you very much and I'll post back about what I figure out, if anything.


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Post subject: Re: 5E3 Circuit
Posted: Mon Feb 27, 2012 12:13 am
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Yeah, 5V4 is a drop-in for most 5Y3 driven circuits. You may want to check the output tubes for overheating (esp "red-plating"). Some 6V6GT are more sensitive to voltage & current change in cathode-bias setups, than others.


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Post subject: Re: 5E3 Circuit
Posted: Mon Feb 27, 2012 7:55 am
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BMW2002Ti wrote:
Yeah, 5V4 is a drop-in for most 5Y3 driven circuits. You may want to check the output tubes for overheating (esp "red-plating"). Some 6V6GT are more sensitive to voltage & current change in cathode-bias setups, than others.


If this is a problem and the OP is not hung up on the NOS tube thing, the JJ 6V6S would be a good choice and are good sounding tubes. They are spec'd to handle up to 450 VDC plate voltage and 14 watt max plate dissipation vs 12 watts. Those 2 extra watts can come in handy when dealing with a cathode biased amp, even more so if the amp is running in class A. :D

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Post subject: Re: 5E3 Circuit
Posted: Wed Feb 29, 2012 8:55 am
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+1, The JJ 6v6S are fine sounding, pretty hardy tubes. One of the better new production tubes.

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Post subject: Re: 5E3 Circuit
Posted: Wed Feb 29, 2012 9:47 am
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What I have in there now are GE 6V6GT's. My amp tech said RCA's are a little more hardy and to watch for red plating in the GE's, but that they should be alright with the 5V4 rectifier. Maybe I should drop some JJ's in before I make the swap just to make sure I don't ruin some NOS tubes. I haven't tried the rectifier swap yet but I threw a 5751 into V1 and it sounds good. Seems a little brighter than the 12AY7.

If the 6V6's don't like the 5V4 rectifier, would it be apparent right away?


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Post subject: Re: 5E3 Circuit
Posted: Wed Feb 29, 2012 10:39 am
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The GE 6V6GT should be fine. Best way to be sure is measuring the idle bias current. The 5E3 circuit is set in Class "A." push-pull topology. Prolly best not to run the tubes past 75%-80% maximum rated dissipation for Class "A" push-pull.

If your amp is putting around 350 VDC onto the anode plates of the 6V6GT, then you should be running about 18-20mA per each 6V6GT. This would be about 6.5-7 watts of idle dissipation per tube (13-14 watts for both). As long as the tubes are healthy, this should be ok. And you may not need to change the cathode-tied resistance.

One other trick is to go to lower resistance on the grid-to-ground resistors (220K-ohm ones tied to the 6V6GT grids). Like a pair of good tolerance (5% or less) 180k-ohm ones. This will lower the idle settings of the 6V6GT, also. Tone is the ultimate criteria, once the tubes are stable. So jockeying the grid-to-ground and the cathode-tied resistance maybe neccesary. You cannot drop or raise the grid-to-ground value too much, without changing the coupling cap value from the phase inverter to the 6V6GT's. As the tone will suffer greatly (time-constant issue).

http://www.webphix.com/schematic%20heav ... _schem.pdf


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Post subject: Re: 5E3 Circuit
Posted: Wed Feb 29, 2012 3:54 pm
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s3mamill wrote:
I threw a 5751 into V1 and it sounds good. Seems a little brighter than the 12AY7.


That is my experience also. :D

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Post subject: Re: 5E3 Circuit
Posted: Wed Feb 29, 2012 6:19 pm
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Beemer, It's been my impression, that the 5E3 Deluxe is a cathode biased-non negative feedback ClassAB amp. Do you know where I can find the load line data for these tubes ? Art

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Post subject: Re: 5E3 Circuit
Posted: Wed Feb 29, 2012 6:33 pm
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aclempoppi wrote:
Beemer, It's been my impression, that the 5E3 Deluxe is a cathode biased-non negative feedback ClassAB amp. Do you know where I can find the load line data for these tubes ? Art


http://www.hifitubes.nl/weblog/wp-conte ... -6v6gt.pdf

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Post subject: Re: 5E3 Circuit
Posted: Wed Feb 29, 2012 6:54 pm
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Thanks Bill ! You're a wealth of information ! Art

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