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Post subject: Re: Trying to buy my 1st Vintage Amp
Posted: Tue Nov 01, 2016 10:00 am
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Good choice for a DRRI. Give the speaker 30 to 40 hours of vigorous play and it'll really begin to sing.

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: Tue Nov 01, 2016 11:17 am
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Two very nice speakers. That 12F150 is one of the most efficient I've used. Bass and high notes. The 12A150 is my favorite. I like the tone of the alnico magnet speakers in vintage Fender amps.

Good luck with both! :D


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Post subject: Re: Trying to buy my 1st Vintage Amp
Posted: Tue Nov 01, 2016 9:24 pm
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I was a little hesitant using the 12A150 in the DRRI, never really pushed the amp past 6/7. With the 50watt F150 I will test my neighbors patience after a due warm up period.

As a side note got a voice mail from Patrick at Mercury this evening; he said the PT definitely had a short. I will call him back in the morning to confirm the re-build and any options I may have with it. So maybe a little longer wait, I imagine they are super busy having just moved to a bigger building and w/ the 57' re-issue builds.


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Post subject: Re: Trying to buy my 1st Vintage Amp
Posted: Thu Nov 10, 2016 5:08 pm
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Well the new Weber will be here this weekend and the PT will be in the mail headed my way soon.
I am ready to get this project finished.


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Post subject: Re: Trying to buy my 1st Vintage Amp
Posted: Fri Nov 11, 2016 11:35 am
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Very cool I do hope you the best
mud


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Post subject: Re: Trying to buy my 1st Vintage Amp
Posted: Fri Nov 18, 2016 6:09 pm
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Well the new speaker has been getting a work out in the DRRI.
Got the PT back from Mercury late Wed. and addressed a few issues w/ it regarding the leads. I should have asked if they installed new leads w/ the re-build, they do not. So after a couple pieces of shrink tubing and one extension I re-installed it in the chassis in between passing storms.
Ran an unloaded test to check the basics, checked out fine 5.6 Vac to the heater filament of the 5y3, 6.6 Vac to the lamp and other tube heater filaments and was pushing 392 Vac to the secondary. About 30 Vac hotter then when it went out for rebuild.
Loaded up the old 5y3 and the rest of the tubes to let it run a while, ran fine for 30 minutes and I had to shut down do to storms in the area. Better safe than sorry, the B+ was 375 Vdc and the 6v6's were seeing 373 on the plates(about the same numbers as before the re-build).
Saturday plan to start the day w/ a PSU resistor swap to 15k from the 10k. That should drop the plate voltage of the 6v6's down to about 248, if my math is any good at all.
I'm hoping to keep the Cathode resistor at 250 Ohm and adjust the other dropping resistor if need be to get the front side tuned in.


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Post subject: Re: Trying to buy my 1st Vintage Amp
Posted: Fri Nov 18, 2016 6:43 pm
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Nice! I'm sure that MM rewound tranny is going to last, well into the 21st Century.
:mrgreen:


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Post subject: Re: Trying to buy my 1st Vintage Amp
Posted: Sat Nov 19, 2016 4:49 pm
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BMW2002Ti wrote:
Nice! I'm sure that MM rewound tranny is going to last, well into the 21st Century.
I certainly hope so; it wasn't a cheap endeavor.

Well the plate voltages dropped slightly but only for V1(-4v) & V2(-12v). The screen grid dropped for V3 & V4 (-20v) which should reduce current and increase sag from what I understand. I still have to get a better handle on the idle bias of the output tubes(currently running at 37/39ma). I will probably try a few different values from 290-330 ohms to get the bias around 26-29ma. This should be the end of my issues and I can move on to playing.


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Post subject: Re: Trying to buy my 1st Vintage Amp
Posted: Sun Nov 20, 2016 2:56 pm
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sfceric64 wrote:
BMW2002Ti wrote:
Nice! I'm sure that MM rewound tranny is going to last, well into the 21st Century.
I certainly hope so; it wasn't a cheap endeavor.

Well the plate voltages dropped slightly but only for V1(-4v) & V2(-12v). The screen grid dropped for V3 & V4 (-20v) which should reduce current and increase sag from what I understand. I still have to get a better handle on the idle bias of the output tubes(currently running at 37/39ma). I will probably try a few different values from 290-330 ohms to get the bias around 26-29ma. This should be the end of my issues and I can move on to playing.


Glad to hear it working out and than as you say "can move on to playing" and that what it is all about
mud


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Post subject: Re: Trying to buy my 1st Vintage Amp
Posted: Wed Nov 30, 2016 11:27 am
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Well maybe by Christmas I'll get this thing as close as I can w/ what I have for components.
I've changed the 2-10k dropping resistors to 12k & 15k, the latter helped drop enough voltage to V1/V2 to 94/170 vdc. I changed the plate resistors for V2 from 220k to 300k/285k dropping the voltage down to 155vdc.
I also increased the cathode resistor to 330 ohms, this helped drop the bias to .31ma, almost but not quite where it needs to be. I will order a few more resistors up to about 500 ohms to get the bias down to my target.
In an attempt to lower some noise in the upper levels of the volume controls, I replaced the original 500pf domino cap, .01uf cap to ground and the 100pf cap across the plates on V2. I am considering re-wiring the heater lines to all tubes w/ an insulated modern version of green cloth covered wire in an attempt to lower any ac voltage that may be adding to the noise floor. The amp is really quiet up until about 7-8 on the volume knob.

The amp plays and sounds pretty good, I don't think it will sound as it did originally. The extra plate voltage just doesn't allow the same tube operation. The PA/PI is providing more distortion than the output tubes, however from 9-12 w/ an aggressive pick attack the amp has plenty of punch/growl. The pots on the amp & guitar are critical to achieving a good balance tonally. As a novice guitar player, it definitely reminds me of how poorly I actually play.I find myself w/ the strat volume around 3-5 to be clean/crisp. The lows, mids and highs are pretty tight throughout the volume range. The tele doesn't have all the twang most associate w/ it. A bucker guitar sounds pretty nice to me, but revealed some issue w/ its pots. I will clean them before next use. I also will add a little Fender drive up front after I get the bias dialed in better.


I have a few more 6sc7's on the way to try in V1/V2. I have been using modern EH6v6gt's throughout the recent testing and the set of nos 6v6's I have are closely matched bias wise. The nos 5y3wgta is to efficient in the amp, providing about 390 B+. So I have been using the NU5y3gt that came w/ the amp, it is less efficient and provides 380 B+.

Aside from adding a NFL, installing a choke or replacing the PT w/ a lower output one any ideas on making it perform any better?


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Post subject: Re: Trying to buy my 1st Vintage Amp
Posted: Wed Nov 30, 2016 3:19 pm
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I can't remember if you check the input stage coupling caps for leaking DC voltage? Those early Sangamo caps are notorious for passing DC. This will completely change the OD profile of the amp.

Anyhow, if you need to change caps, the Jupiter Red series or Russian K40Y-9 PIO caps are close to the original tone.

http://www.partsconnexion.com/capacitor ... stron.html

http://tubes-store.com/index.php?cPath=35_41_82&page=3


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Post subject: Re: Trying to buy my 1st Vintage Amp
Posted: Wed Nov 30, 2016 4:59 pm
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BMW2002Ti wrote:
I can't remember if you check the input stage coupling caps for leaking DC voltage? Those early Sangamo caps are notorious for passing DC. This will completely change the OD profile of the amp.

Anyhow, if you need to change caps, the Jupiter Red series or Russian K40Y-9 PIO caps are close to the original tone.

http://www.partsconnexion.com/capacitor ... stron.html

http://tubes-store.com/index.php?cPath=35_41_82&page=3


Yup I didn't have a way to test them, so they were all replaced.
Just trying to dial in the best sound I can hope for now with as much stability as possible @ as close as I can figure out how to get to original specs.
I really don't wanna change to a lower output PT or rectifier, install a choke, nfl, different PI type, or run off a variac, etc...

The B+ is 395
The plates of the output are about 370 and its running @ about 90% dissipation at idle or 30-31ma.
The PI & PA are running a little above spec at about 158vdc and V1 is right at 95vdc

The more I think about it, I think the noise in the upper range of the volume pots are probably just the reaction of the OT to higher voltages and not the AC heater lines. Dunno for sure or how to test it.


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Post subject: Re: Trying to buy my 1st Vintage Amp
Posted: Wed Nov 30, 2016 5:53 pm
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Prolly need a scope to really seeing if the pots are generating noise or if you are getting oddball distortion, at high volume. Scope with signal generator would be nice, to check how symmetric your output sine wave is.

There is one thing one may want to try. Replace one of the input coupling caps with an 0.02mfd/600VDC one. Then, you can go between INPUT 1 and 3, for instance, and compare high volume distortion with 0.05mfd and 0.02mfd input caps.

The only other thing, I can think of... is putting a global negative feedback resistor into the circuit. From secondary of the OPT to the PI stage. The narrow panel Pro put 100k-ohm from secondary to input grid one side of the 12AX7 phase inverter. I'd prolly choose a 500k-ohm pot, so you can dial in the GNFB that sounds the best. Kinda like the PRESENCE pot used in some transition era (blonde and brown era) amps.

You'd need to play with the polarity (which grid of the PI) of that feedback to place the pot. But, you'll know for sure. The wrong side (to equal positive feedback) will give you a howl from the speakers you'll never forget. :)

Your amp:

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


Brownface Pro:
http://ampwares.com/schematics/pro_5e5a.pdf


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Post subject: Re: Trying to buy my 1st Vintage Amp
Posted: Wed Nov 30, 2016 9:08 pm
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Not really sure I understand the effect(increase of voltage) of a GNFL to the PI cathode, as in the 5e5a Pro.
Does it increase/decrease PI distortion or increase/decrease the output signal to the output tubes or does it alter the PI driver gain?
I looked at the 5c3's GNFL for reference earlier and it is directed to the output tubes grid vs the Pro's to PI cathode.

I would be interested in something that increases output tube distortion at less than near max volume.
The main distortion I'm getting is via the PI currently, the paraphrase PI seems to do a pretty good job of that.
I get good bark from heavy pick attack and decent sustain, although its not the warm creamy output tube distortion...maybe I just need to let the tubes burn in some?

I was also trying to better understand the effect of screen grid voltage vs plate voltage/bias to drive the output tubes into a better clipping condition. Currently running the screens grid 77vdc lower than the plates would narrowing the gap via the 1st dropping resistor increase clipping at lower volumes? While most of the vintage specs I've seen are about a 50vdc difference and more modern amps seem to be even less of difference. But I guess that's apples and oranges trying to compare non cathode biased amps.


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Post subject: Re: Trying to buy my 1st Vintage Amp
Posted: Thu Dec 01, 2016 12:23 am
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The idea of global negative feedback is to reduce output distortion. Does this by feeding a tiny portion of output signal (off secondary tap of OPT) back to an earlier part of the amplifier's circuit. In most amps, this GNFB is applied to one-half of the phase inverter duo-triode.

Yes, I believe some early tweed and woody Fenders fed GNFB back to one or the two output tubes. This proved not as effective as applied to PI.

Aiken has a very good tech article on GNFB:

http://www.aikenamps.com/index.php/desi ... e-feedback

In general... the greater the voltage difference between the screen grid and the anode plate = greater the compression and less the headroom effect of the output tube. The screen grid helps to maintain steady electron flow from the cathode to the anode. Help prevent internal instability within the tube, that can lead to destructive parasitic oscillations.

A resistor or choke is added to insure that the screen voltage is always below the anode plate voltage. That the majority of electrons flow to the anode and not temporarily through the screen grid (when the screen voltage > anode plate).

A 5-10 VDC difference ---- which is around 500-1k-ohm screen resistor, for 6L6GC and EL34 with 450-475VDC on the plates ---- has been found to be effective in maintaining this voltage difference. And not adversely changing the output tube's tone.

Short screen resistor lead lengths and using a fire-proof resistor will help maintain stability, even in the most strenuous situations.

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.


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