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Post subject: Excited about new patient
Posted: Tue May 20, 2014 7:56 pm
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Aspiring Musician
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Although this isn't mine, I still get wound up to be able to do some maintenance. Besides the obvious, a new 3 pronger is in order also.

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Post subject: Re: Excited about new patient
Posted: Tue May 20, 2014 9:07 pm
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A real piece of history, upnorth.

If that amp could only talk......

8)

Arjay

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Post subject: Re: Excited about new patient
Posted: Wed May 21, 2014 12:18 am
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That circuit is in pristine condition. I wonder if you could disassemble those old Astron Minimites and insert a new filter cap into the cylinders? Would be interesting. Prolly need to use modern 16mfd/475VDC caps. As the 20 or 22mfd/500VDC maybe too large.

The Sprague 25mfd/50VDC ATOMS will easily fit into the 25/25 Astrons.

http://www.tubesandmore.com/products/C-SA


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Post subject: Re: Excited about new patient
Posted: Wed May 21, 2014 1:06 am
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+1

The F&T 25/50's are also pretty small.

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Post subject: Re: Excited about new patient
Posted: Tue Jun 03, 2014 1:09 pm
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Aspiring Musician
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Just a question about bias/plate voltage on this Tweed Deluxe. The plate voltage is 315v's and the bias is 39mA's. I know it's cathode biased but that seems low/high. I replaced all the bias and filter caps, (22uf instead of 16), the 250 ohm ohmite bias resistor, the two 1.5K grid stoppers, the 22K and 4.7K in the PS and the two 220K resistors. Everything else was in spec. New 3 prong cord also.

It's really quiet and that's good. Nothing on the schematic for voltages.

B+ is 347v's

Thanks for any input.

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Post subject: Re: Excited about new patient
Posted: Tue Jun 03, 2014 4:18 pm
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Aspiring Musician
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Okay! Everyone can laugh now. :lol: :lol: :lol: :lol: :lol: :lol: :lol: :lol:

I just built one of those homemade current limiters with a 300 watt bulb and was trying it out. It works great except for the fact that it doesn't allow an amp to use the full household current. :oops:

With it in line, 315 plate volts, 347 B+

Without it, 334 plate volts, 366 B+

I still have a high bias reading but from what I gathered, that's somewhat normal.


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Post subject: Re: Excited about new patient
Posted: Tue Jun 03, 2014 5:07 pm
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upnorth wrote:
Okay! Everyone can laugh now. :lol: :lol: :lol: :lol: :lol: :lol: :lol: :lol:

I just built one of those homemade current limiters with a 300 watt bulb and was trying it out. It works great except for the fact that it doesn't allow an amp to use the full household current. :oops:

With it in line, 315 plate volts, 347 B+

Without it, 334 plate volts, 366 B+

I still have a high bias reading but from what I gathered, that's somewhat normal.


Very nice job on the placement and soldering of the new components.

Current limiter is only for ramping up the amp and checking for shorts. Need to pull it to run amp at usual operating conditions. But, you knew that. It is a limiter, after all. :D

Now, the idle current should be less at 334VDC, than 315VDC. Kinda auto adjust. 39mA with 315VDC = 12 watts per tube. Which is about 85% of max rating of 14 watts. This is pretty "warm" and typical Class "A" idle settings. You need good 6V6GT's. Strong NOS mil spec GTA or GTY would be nice. Or JJ 6V6S. Watch for red plating. At 334VDC, the tubes should be passing about 30-32mA, for similar idle dissipation.

How's the tone?


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Post subject: Re: Excited about new patient
Posted: Tue Jun 03, 2014 6:35 pm
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Upnorth...nice work! And thanks for the post. I certainly get a little misty eyed when I think back on my old 59 Harvard...tweeds have a mystique and tone all there own.

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Post subject: Re: Excited about new patient
Posted: Tue Jun 03, 2014 7:56 pm
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BMW2002Ti wrote:
At 334VDC, the tubes should be passing about 30-32mA, for similar idle dissipation.

How's the tone?


At 334VDC I still have 41mA's bias. When the PV was 315VDC it was at 39mA's.

21.9v's across the cathode resistor

There's no red plating with strong Sylvania 6V6GT's but I know that seems high, 13.7 watts. I could try experimenting with the cathode resistor.


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Post subject: Re: Excited about new patient
Posted: Wed Jun 04, 2014 10:08 am
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upnorth,

With Class "A" biased output tubes, you are going to find that changing the cathode-tied resistor may not make that big a difference in current passed. The strength of the 6V6GT's determine much of the specific final idle current passed. If you put in weaker 6V6GT's, you prolly will get lower idle wattage, without changing any resistor or plate voltage.

One other issue. You really need to subtract the ground-to-grid dissipation from the ground-to-anode dissipation to find the actual idle output of the tube. And because the cathode resistor is 250-ohms, you will get more wasted power --- than if the cathode was tied directly to ground -- as in fixed biased amps. A measure of the voltage drop across that 250-ohm resistor will give you an idea of the wattage lost. One of the reasons why cathode biased amps are less efficient than fixed bias amps.

What also maybe an issue is the signal-to-grid resistors loading the grids. These are those 220k-ohm pairs. By lowering the resistance of these pairs to, say 180k-ohms, you drop the idle bias. Tungsol originally recommended 100-500k-ohm resistance, depending on application or Class of output stage.

This will affect the amp's tone. So you may need to experiment a bit. But, really, if the amp sounds fine --- no early OD or muddy, ill-defined bass --- AND there is no red-plating, you may want to let sleeping dogs lie. All this means is the max output of your amp is close to 30 watts.


Good luck, there! Great amp. :D


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Post subject: Re: Excited about new patient
Posted: Wed Jun 04, 2014 1:46 pm
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Aspiring Musician
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BMW2002Ti wrote:
upnorth,



--- AND there is no red-plating, you may want to let sleeping dogs lie. :D



Thanks, I agree and that's exactly what I'm going to do. I was posting before I read this.


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Post subject: Re: Excited about new patient
Posted: Fri Jun 06, 2014 12:40 am
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BMW2002Ti wrote:
But, really, if the amp sounds fine --- no early OD or muddy, ill-defined bass --- AND there is no red-plating, you may want to let sleeping dogs lie. All this means is the max output of your amp is close to 30 watts.


+1

And a few extra watts is not a bad thing -- better to drive that vintage Jensen P12N that you'll eventually install into sonic nirvana.

:wink:

Arjay

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Post subject: Re: Excited about new patient
Posted: Fri Jun 06, 2014 9:00 am
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I'd guesstimate about a 1-2 watt loss of potential output power across that 250-ohm resistor.


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Post subject: Re: Excited about new patient
Posted: Fri Jun 06, 2014 6:11 pm
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Thanks for the much needed input. The amp sounds pretty good. Starts to break up at about 3 with humbuckers. The power tranny stays really cool on this amp which seems strange for a 5E3 doesn't it? 22uf/500v caps worked out well. One thing, it's micro-phonic on a few single notes only. I've changed all the tubes tubes, did the tap test throughout, tried a different speaker cab, even changed the rubber grommets on all the tube sockets and it's still there. You have to work for it a little, but it's there. Tried different guitars too. I do get a considerable amount of static from pin 1, V2 to the .02uf/600v & 100k resistor when following the wire with a chopstick.


Last edited by upnorth on Fri Jun 06, 2014 6:57 pm, edited 1 time in total.

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Post subject: Re: Excited about new patient
Posted: Fri Jun 06, 2014 6:50 pm
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Sounds like it maybe a soldering issue. Which may cause a microphonic effect. Perhaps, re-flowing that line off V2 would help? Including the pins of V2.


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