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Post subject: Re: Trying to buy my 1st Vintage Amp
Posted: Thu Dec 01, 2016 1:24 pm
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77VDC is too much difference between plate and anode. Prolly leading to too much compression of the tone. And lowered headroom. The dropping resistor in your amp splits the voltage supply in two --- before going to the screens. Not the best way to maintain constant voltage on those screens. That is why Fender went to screen resistors for EACH output tube on later Deluxe amps. And soldered them right on the sockets. Less chance of induced instability onto the screen grids.


I do get a lot of compression; I should have said in the earlier post that the 77vdc are the raw plate readings and don't account for the cathode's 21vdc. So relative to the actual plate voltage at idle the difference is 56vdc.
I will now probably look at variable 1st stage resistor values maybe down to 8k and increasing the voltage rating to 50 on the cathode cap. While the cathode bias helps prevent plate to screen imbalance, I don't wanna push my luck. The low resistance value screen resistor just seems like a second failsafe to prevent screen meltdown in the event the current swings are more than the screen can handle, maybe not necessary w/ cathode biasing.

Aiken's: What is Negative Feedback? http://www.aikenamps.com/index.php/what-is-negative-feedback
was very helpful; while it isn't directly related to a paraphase PI it helped me understand why Fender applied there GNFB to the output tubes in the 5c3 circuit. The 5e5a Pro was probably more successful linked to the PI tube because of the type of PI and each half inverting an equal portion of the signal.

I appreciate your inputs, they help me keep focus. Thanks


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Post subject: Re: Trying to buy my 1st Vintage Amp
Posted: Thu Dec 01, 2016 6:44 pm
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You are welcome! The only way to correct the compression would be to tap the voltage from before the 10k-ohm dropping resistor, in the PSU. Put 1K-ohm metal oxides on the sockets, between pins 4 and 6. Reconnect the voltage tap for the screens, to pin 6.

Too much work and this would ruin the original Deluxe circuit. You may want to keep an eye out for a nice Tungsol or Raytheon mil spec 6SC7GTY. Brown micanol base and smoked glass. These tubes have better tone throughout the volume range than commercial RCA 6SC7. Good ones will last 25+ years.


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Post subject: Re: Trying to buy my 1st Vintage Amp
Posted: Thu Dec 01, 2016 8:29 pm
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The only way to correct the compression would be to tap the voltage from before the 10k-ohm dropping resistor, in the PSU. Put 1K-ohm metal oxides on the sockets, between pins 4 and 6. Reconnect the voltage tap for the screens, to pin 6.

I will likely move back to a 10k or even an 8k(if necessary) as the 1st dropping resistor to increase the screen grid a little more. The current resistor is 11.7k actual. I will probably pick up a little voltage on V1/V2 by dropping back down while lowering slightly the cathode voltage.

After I get the resistor swapped and get some measurements and listen to it, I'll decide about the 1k plate to screen resistor. I actually appreciate some compression in my tone.

Unfortunately some minor tweaks are required to deal w/ the input voltage difference. I would have preferred it I'm sure if Leo & Co. had designed it based on 120vac.

I do currently have a 6sc7gty in the PI slot and will shop around for another, its not the best balanced but very sturdy so far. I just picked up a couple NOS Tung-Sol 6sc7 metal tubes to swap w/ the Philco (I think this is GE made, but not sure) I have in V1.

After I revert back to a lower dropping resistor, I think I just need to play it for 10-20 hrs before making any tone stack swaps. I will definitely swap the input cap back to the Sangamo .05 and try a couple Fender branded red/yellow .02 & .03 values at some point down the road. Of course the three guitars I own are all differently voiced as well, which leads me to currently liking the darkest toned/highest output to brightest toned through the amp at last listen.


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Post subject: Re: Trying to buy my 1st Vintage Amp
Posted: Thu Dec 01, 2016 11:29 pm
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Compressed tone is part of the character of a lot push-pull tweed Fender amps. Prolly one of the biggest changes done between this era and the blackface era. Most BF era push-pull amps have much taller headroom, less compressed tone.

Part of the charm of the tweed amp. Just can't run super hot pickups.


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Post subject: Re: Trying to buy my 1st Vintage Amp
Posted: Fri Dec 02, 2016 11:15 pm
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Well got the new 6sc7's in the mail today and I couldn't wait to listen to them. Plugged one of two into V1, powered up fetched the axe, connected cable and made way for the volume knob.
Of course this is where $@!& happens and all I get is a solid frequency feedback........try 2nd tube, ditto.........different frequency depending on which volume control. No input...

Return volume to zero, unplug axe/cable, remove back panel and take some measurements.
All the voltages look fine, even better than the old tube. I even get -.850 voltage close to what is expected from the grid to ground instead of the -.404 I had been getting.

Baffled and maybe Cursed can't decide which is worse at the moment; not in the mood to play is a bad day.


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Post subject: Re: Trying to buy my 1st Vintage Amp
Posted: Fri Dec 02, 2016 11:49 pm
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Well, both are microphonic. Are you able to put original 6SC7 tubes into the amp and all is okay?

Can you get $$$ back from seller?


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Post subject: Re: Trying to buy my 1st Vintage Amp
Posted: Sat Dec 03, 2016 1:03 am
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Thanks, that's the word I was looking for.
I sent a mail off to the seller, waiting on a response. Best I could do.(replied this morning w/ an apology and refund)
Sure the old one still works, just not well.(ordered a couple from the tube store, a little higher priced than ebay)

I asked for replacements in like kind tubes or money back, wait and see. (None available)

A pretty decent seller, reasonable prices I'll follow his listings.

I didn't realize how hard it is to find 6sc7's that aren't micro-phonic...now I see why the gty's are so expensive.

Did finally shake the baffle a bit in the wee hours.


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Post subject: Re: Trying to buy my 1st Vintage Amp
Posted: Tue Dec 06, 2016 7:36 pm
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Well after this eBay fail; I did realize just how bad the two 6sc7's that I have actually are. Both of the ones I have been using have grid issues. Vey apparent after putting the micro-phonic tubes in and getting the voltages off the pins. I have a new pair coming in on Wednesday, hope they sing strong. I'm really ready to play this amp. If these don't sing, I'm gonna try the new Tung-Sol re-issue type, anyone try them yet?


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Post subject: Re: Trying to buy my 1st Vintage Amp
Posted: Tue Dec 06, 2016 8:19 pm
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I have not tried the new Tunsgol 6SC7. Some hi-fi guys have and say they are ok. Tough tube. Tone is so-so.

I don't think I have any 6SC7GTY is my stock, I'll check.

BTW... what were the plate voltages on those 6SC7, again? I kinda remember them being quite sensitive to too much voltage. Also, any DC grid leak. I know you replaced the coupling caps. But, you may want to check for DC leakage onto the grids. It's been known to happen with brand new caps.


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Post subject: Re: Trying to buy my 1st Vintage Amp
Posted: Tue Dec 06, 2016 10:21 pm
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Quote:
BTW... what were the plate voltages on those 6SC7, again? I kinda remember them being quite sensitive to too much voltage. Also, any DC grid leak. I know you replaced the coupling caps. But, you may want to check for DC leakage onto the grids. It's been known to happen with brand new caps.


My two originals in V1 were:
1. plate 93.9 & 97 grid -.407 & -.41
2. plate 95.5 & 99 grid -.401 & -.414

The nos tungsol micro-phonic tubes in V1 were:
1. plate 93.1 & 92 grid -.85 & -.851
2. plate 93 & 92 grid -.85 & -.85

The Fender layout for V1 calls for 92 vdc & -.9vdc
As for the grid leakage test, wouldn't the different tube readings above indicate they were good to go?

Sourcing tubes is part of the fun aint it? I got one 6sc7gty and will pick up another sometime. I figure to try one of the Sovtek 6sc7gt (tung sol) re-issues as well. Tone wise, I don't have any comparisons to make, other than I like the tone produced by the metal 6sc7's of the F'd up tubes I have. Aint that a shame. I got a couple from the tube store on the way and hopefully they work out.


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Post subject: Re: Trying to buy my 1st Vintage Amp
Posted: Wed Dec 07, 2016 1:05 am
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0.85VDC is a little on the high side. I'm used to seeing 0.5VDC and lower. Usually around 0.1VDC for a good cap. Remember, this DC voltage is being placed onto a delicate control grid, which is designed to see mostly 10-100 mV of AC (signal).

You might consider another coupling cap. At least for the input stage 6SC7. IME, excessive grid DC voltage leads to noisy tubes. Hiss, white noise, & microphony.

You know what is a good coupling cap for some of these early Fender tweed amps? The lowly Orange Drop. The 716P. They are tough, cheap, and last forever. The tone is quite nice in these "brownish" sounding amps. Cleaner midrange.

http://www.angela.com/orangedropcapacitors.aspx

Mil spec tubes are sometime better because they are designed to withstand greater out-of-range voltages on their grid. And still perform well. Let me check my tube stash. Think that the 6SC7GTY are Tungsol mil spec black plates. Late 1940-early 1950s era. May even be the famous round, black plate specials. The best of their kind, IMHO.


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Post subject: Re: Trying to buy my 1st Vintage Amp
Posted: Wed Dec 07, 2016 8:16 am
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Quote:
0.85VDC is a little on the high side

It is a (minus .85 vdc) and the layout calls for minus .9
:wink:

So the original tubes were gaining .4 vdc vs the new ones. Doesn't seem like a lot unless it is looked at as 50%, then I think its a significant value shift.


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Post subject: Re: Trying to buy my 1st Vintage Amp
Posted: Wed Dec 07, 2016 8:30 am
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Oh, you are talking cathode voltage. I'm talking about the DC voltage on the signal grids. It must be close to zero DC. 1 VDC and higher means that the Astron coupling cap attached to that grid is leaking DC. And this will effect both tone and the life of that 6SC7. That coupling cap HAS to block DC voltage.

Prolly go down to storage and check to see if I have any GTY tubes, this weekend (Sunday night).

The OEM Astron caps in my 1955 Champ leaked 1.2 and 22VDC. I replaced them with some Jensen PIO I had in stock.

Photo is of new DC voltage on the grid.

Image


Photo of VDC on anode side of cap.

Image


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Post subject: Re: Trying to buy my 1st Vintage Amp
Posted: Wed Dec 07, 2016 9:49 am
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Oh, you are talking cathode voltage.

No, I was stating the grid to ground voltage(minus .9) referenced in the layout and the plate voltage(92) for V1.

But I appreciate the photos indicating how to check the caps leakage. caps measured .000vdc on the grid side on the first 5 of the 6 caps going from V1-V3, the last cap measured .051vdc

FedEx will be here some time today and I will open it up again and check as you have suggested.one is micro-phonic and one is good, both RCA's


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Post subject: Re: Trying to buy my 1st Vintage Amp
Posted: Wed Dec 07, 2016 6:41 pm
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Really odd numbers, on that layout. With no guitar or other devices like a signal generator plugged in, there should be no voltage (AC or DC) on the signal grid of the input tube. The very little stray voltage within the tube should not be at a 0.9VDC level. I wonder if they meant the cathode (pin 6) DC voltage.

To make it simple, just measure the VDC from one side of the associated coupling caps attached to the 6SC7 grids... and then the other. One side should read around 90-100VDC (like your anode voltages). The other should read very close to zero (under 0.5VDC). Power on. Volume at zero. NO guitar (or any other device) plugged into the amp.

The input caps (before the first 6SC7) should read zero VDC on both sides of the caps.

If the voltage on the low side is close to or greater than 1VDC. I'd change the coupling cap. :mrgreen:

http://ampwares.com/schematics/deluxe_5b3.pdf


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