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Post subject: Re: Trying to buy my 1st Vintage Amp
Posted: Wed Dec 07, 2016 10:36 pm
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Yes I agree its odd.
I really don't know but it makes sense to me interpreting the schematic that the grids P3/P4 should be at a negative voltage due to the path resistance to ground when compared to the cathode(maybe just to suppress noise w/o any input signal). While the cathode P6 should be at zero because it is direct to ground w/o a resistor in the path. The plates P2/P5 are getting between 90-100vdc.

The input caps are at zero on both sides.

All the caps are good= <1vdc to ground, the 1st 5 caps are .000 and the sixth one(nearest PT) is .051

Each V1 triode is about the same, Cathode= 0vdc Grid= -.7 and -.69 Plate= 94vdc and 96vdc and the other side of the caps are .000
W/O any input signal, volume@ zero, nothing plugged into any of the inputs
This is w/ a tube I just received today, better than what I had but not as good electrically as the micro-phonic tubes were. Unfortunately one of the pair of tubes I received today was micro-phonic.


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Post subject: Re: Trying to buy my 1st Vintage Amp
Posted: Thu Dec 08, 2016 2:06 am
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OK. Guess those coupling caps are ok. The new 6SC7 should work fine.


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Post subject: Re: Trying to buy my 1st Vintage Amp
Posted: Thu Dec 08, 2016 10:18 am
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Yes, the new 6sc7 is working out ok for now.
Played for about an hour total through the amp, with each of my guitars. Its a darker sound than I'm used to and starts to breakup early on the volume knob 6/7. With amp volume on 12, the clean tone on my Strat ends at about 4 depending on pick attack. The mic input is more to my taste, with a slightly higher frequency and more breakup. No red-plating or over-heating issues, no smoke or open flames. There is something that's a little loose somewhere that resonates mainly on an A note and less on an F#; I maybe just need to use tube dampers on all the tubes. I had the upper panel off, so replacing that may help fix the issue.

I still need to tweak some things w/ the tone stack and the overall loudness is really to high even at low volume.

I put a cheap .01 no-name cap on the tone pot to ground, how does this value impact the tone?
May play w/ the value of the bright cap, its 500pf and will likely try around 100-300pf or no cap.
I may also remove the grid stopper resistors on the 6v6's. They were an add-on to the circuit anyhow.
May replace the on/off tone pot with a new one.

I really don't know how I might address the overall loudness.


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Post subject: Re: Trying to buy my 1st Vintage Amp
Posted: Thu Dec 08, 2016 1:42 pm
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sfceric64 wrote:
There is something that's a little loose somewhere that resonates mainly on an A note and less on an F#; I maybe just need to use tube dampers on all the tubes.


Make sure your speaker is snug to the baffle board.

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: Trying to buy my 1st Vintage Amp
Posted: Thu Dec 08, 2016 3:51 pm
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Quote:
Make sure your speaker is snug to the baffle board.
roger that, I will go through every bolt/screw to make sure they are at least hand + tight.


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Post subject: Re: Trying to buy my 1st Vintage Amp
Posted: Thu Dec 08, 2016 7:20 pm
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6SC7 amps are noted for darker tone than 12AX7 ones. This is why Orange Drop and silver mica caps seem to work nicely in those older amps.

For tone mods, I find it best to download Duncan's Tone Stack Calculator. Saves a lot of experimenting time and effort.

http://www.duncanamps.com/tsc/

Good luck! :D


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Post subject: Re: Trying to buy my 1st Vintage Amp
Posted: Fri Dec 09, 2016 9:47 am
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I've never figured that Duncan amps tone circuit curve model out. It may seem more complicated than it is to me. Input frequency and how to remove extra inputs either R or C have always prevented me from using it, but it looks very easy if you know what value to put where.
It would be really cool if it produced a sound and not just the freq. curve/center bias.

I have always just thought it was as simple as:
increase R or C to decrease frequency
decrease R or C to increase frequency
As heard when comparing the Mic input to the Instrument input

Without a more precise way of measuring (just using a DMM) I think I need to keep it as simple as possible.


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Post subject: Re: Trying to buy my 1st Vintage Amp
Posted: Wed Dec 14, 2016 9:16 am
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New question about PSU caps; ok so now that I decided the amp is stable and I wanna be proactive.

If I change the 2nd & 3rd caps to 16uf instead of the 20uf currently installed, what should I expect from the swap as far as noise/hum, voltages & frequencies?

I am considering this change as opposed to altering tone stack values to start with because I tagged the last cap w/ the iron earlier in the rebuild and covered w/ epoxy(don't want to run the chance of a leak) and it seems like a good place to slightly alter the tone.


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Post subject: Re: Trying to buy my 1st Vintage Amp
Posted: Wed Dec 14, 2016 9:26 am
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I've changed amps from stock 16 mfd caps to 20 mfd caps. Not too much difference. Maybe, a tad better bass with the 20mfd caps.

As for tone stack, changing one component will effect the others. So, overall shape of the frequency curve will change. Note, you need to click onto each component of the stack (C1, R1, etc) to change the values.


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Post subject: Re: Trying to buy my 1st Vintage Amp
Posted: Wed Dec 14, 2016 12:00 pm
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Thanks that's what I thought, slight gain in highs going from 20uf to 16uf.
Ordered some of those orange drops you suggested as well to try out.

Quote:
As for tone stack, changing one component will effect the others. So, overall shape of the frequency curve will change. Note, you need to click onto each component of the stack (C1, R1, etc) to change the values.
So, if I make a value zero, it is actually removing it from the circuit completely. Trying to get it down to a single tone pot and volume pot. I get errors and the program closes. I haven't figured out how to use it yet.
I'm assuming the following using the Fender circuit:
Zsrc is the guitar signal freq
R5 load is the Volume pot


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Post subject: Re: Trying to buy my 1st Vintage Amp
Posted: Wed Dec 14, 2016 3:04 pm
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For Tone Stack Calculator:

If you are using Windows 7, 8, or 10:

Run the Program Compatibility Troubleshooter.

1.In the search box, type run programs.

2.Select Run programs made for previous versions of Windows, and then select Next.

3.Choose the program you want to fix. If you don't see the program in the list, select Not Listed.

4.Select Next, and then follow the instructions in the troubleshooter.



If the troubleshooter can't fix the problem, try changing compatibility settings manually.


Change compatibility settings manually.


These steps will not work with apps that came built-in with Windows, or with apps you get from the Windows Store.

1.In the search box, type Tone Stack Calculator, right-click the search result, and select Open file location.

2.Right-click the program icon, select Properties, and then select the Compatibility tab.

3.Select one or more of the settings (see the table below for more info).

4.If you want to apply the settings for that program to all accounts on the PC, select Change settings for all users.

5. Administrator permission required. You might be asked for an admin password or to confirm your choice.

6.When you're done, select OK.

-----


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Post subject: Re: Trying to buy my 1st Vintage Amp
Posted: Wed Dec 14, 2016 3:53 pm
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I'm not having a windows compatibility issue. I just don't understand how to apply my #'s to the program's inputs. Trial and error usually ends in the program becoming unresponsive, flat lining the curve.
Its not that important to me at this point, I do appreciate the attempt though.


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Post subject: Re: Trying to buy my 1st Vintage Amp
Posted: Wed Dec 14, 2016 5:38 pm
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BTW... I did find two NOS Tungsol mil spec 6SC7GTY, in my stash. Both are round black plates. In case you are interested.

1957, 44th week. And a 1951, 36th week.

Image

Image


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Post subject: Re: Trying to buy my 1st Vintage Amp
Posted: Wed Dec 14, 2016 5:44 pm
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Are you doing this? Double-click on the component to get window to insert new values.

I double-clicked onto C1 to get this option. All of the labeled C and R's can be edited this way.

Image


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Post subject: Re: Trying to buy my 1st Vintage Amp
Posted: Wed Dec 14, 2016 10:55 pm
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Yes I have changed the different values. The main problem I think is that the circuits available, don't fit the single tone pot, R/C circuit. For instance the Fender curve is based on the TMB tone circuit and if you try to zero out to many of the components it doesn't work out so well. I think its a great tool, just don't think it fits my amp.

I have calculated the "low pass cutoff frequency" of my amps components, to be 454.959Hz.
Freq. =1/ (2Pi)*(R)*(C)
Changing either the resistor or cap value will increase or decrease the above cutoff.
If I sub a .033 cap for the .05 cap the new freq would be 689.331Hz, .022 --> 1033.997Hz.
Cap changes make more sense than resistors, for instance if I wanted to get the same freq w/ the .05 cap I would have to drop the 7k resistor down to the low double digits. I didn't make all the calculations for the resistors and they are harder to find.

The bright cap 500pf, is about the right value, maybe could be 100-200 less and be ok throughout the sweep of the volume pot.

I have ordered 2 new PSU 16uf Caps to start the tone quest portion of the amp works. As long as that gets me a little higher w/o picking up any hum its a good start.

I ordered the orange drop caps to sub in/out to see what flavor I like the best.

I am also gonna try a lower 1st bypass cap w/ an 18uF 25vdc.

I'm still waiting for a few more cathode bias values as well.

I really appreciate you looking through your tube stash, you didn't have to.
I have a couple ready to go and a new pair of vt-107's for Christmas, but I'll shoot ya a message later.


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