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Post subject: 1965 Deluxe Reverb
Posted: Wed Sep 19, 2018 9:27 pm
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Received this thing in the mail yesterday. All original except for the power transformer.

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Long overdue for a cap job. Can see some bulges forming.

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before and after
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50 MFD bias filter cap. The AB763 schematic has a 25 MFD cap, I guess there were variations.
Also, note those two resistors attached to the light, you'll see them later.
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Really cool thing to see a relatively clean 53 year old amp.
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The renowned blue molded coupling caps. These ones didn't go anywhere.
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So after the cap job I fire this thing up. Sounds incredibly. Makes my 65 DRRI sound like a line 6 with a pillow for a grille cloth.

Then it starts to buzz and smoke comes out of every hole in the chassis.

Open it back up.

Remember those resistors from earlier?

Completely burned up.
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Replace them with the same resistance. Doubled up to raise wattage.
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Figured the older resistors that were there had seen better days and just died from age. Then the replacement ones burned up as well. Uh oh.

Replace them again, using more resistors to raise wattage well above what it should be getting from the ~7 V heater coil circuit. I leave the chassis out while I turn it on so I can see what's going on. It keeps crackling just like before, and I notice that the light in the tremolo bug flashes with each pop. Weird.

Then I notice one of the power tubes glowing blue. Replace the tube. No more smoke and crackling.
My best guess is that there was a short between the plate and a heater pin of the tube. This was putting the 400+ Volts from the main power loop across those resistors, which ground the heater circuit to the chassis. 400 Volts across 200 Ohms makes for a lot of smoke.

Just got back from playing my first gig with the thing. Not sure how I'll ever use another amp.

Next step, clean and tighten the tube sockets. After that, it'll hopefully be ready for another 53 years of playing.

Thanks to Bob Pavelka, wherever you are, for keeping this beauty in such good shape.
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Post subject: Re: 1965 Deluxe Reverb
Posted: Tue Sep 25, 2018 4:12 am
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Location: Coastal Bend, Tx.
Congrats on owning one of the finest amplifiers ever created.
You didn't say but I'm assuming you replaced that -Bias capacitor?
Nothing like a bad bias circuit to fry output tubes and hopefully you reverted to the spec resistors in the lamp/heater circuit.
Also, you didn't mention what the idle bias was adjusted to or what the -bias voltage was. I can only assume it was w/in spec.
With proper care and maintenance, that amp will last FOREVER.


Post suffered from new member six day lag syndrome.


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Post subject: Re: 1965 Deluxe Reverb
Posted: Tue Sep 25, 2018 6:19 am
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You should really take a few minutes to de-solder that entire artificial center tap and start from scratch. Not to be critical but, to nice of an amp to have that under the hood. Twist one end of the resistors and use a ground tab to the PT screw.

UN


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Post subject: Re: 1965 Deluxe Reverb
Posted: Tue Sep 25, 2018 2:09 pm
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The idle bias current was set at 4 mA. This seems incredibly low to me. I measured the plate voltage and it was pushing 480 V, which is high even for a DR.

By my calculations thats 16% maximum plate dissipation for a 6V6GT. I didn't notice any crossover distortion though, the amp sounded sweet.

I put in a set of RCA blackplate 6V6s and set the idle current at 17 mA, which is close to the 70% dissipation limit. Sounds even better.

What's the lowest bias current people have heard of sounding good?


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Post subject: Re: 1965 Deluxe Reverb
Posted: Tue Sep 25, 2018 2:48 pm
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Part of your issue *may* be today's wall-outlet voltage. Your amp -- and all other Fenders built in the sixties -- was designed to operate on 117 VAC. Contemporary commercial AC power measures significantly higher than that, often close to or exceeding 125 volts. Your 480 VDC plate-voltage measurement is a good, though not necessarily conclusive, indicator that there's a pretty hot primary voltage coming out of your wall socket. Further investigation is warranted.

As for the low bias-supply current, the reason for that is anybody's guess. But you're now getting close to normal operating parameters with the BP RCA bottles so I don't think there's any reason for concern there. If anything, the idle bias may be a skosh low -- I normally run my 6V6GTA's at around 21.5 mA.

Congrats on a great find......I think the amp is in very capable hands now.

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: 1965 Deluxe Reverb
Posted: Tue Sep 25, 2018 2:55 pm
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:mrgreen:
Pants187 wrote:
The idle bias current was set at 4 mA. This seems incredibly low to me. I measured the plate voltage and it was pushing 480 V, which is high even for a DR.

I put in a set of RCA blackplate 6V6s and set the idle current at 17 mA, which is close to the 70% dissipation limit. Sounds even better.

What's the lowest bias current people have heard of sounding good?


60-70% of max dissipation (8-9.5mA per tube) is a good ballpark, for this amp. The original 4mA would most likely indicate a very worn set of 6V6GT tubes.

Yes, to changing out that bias cap. Sprague Atom 100mfd/100VDC would be fine.

Your amp came with a nice speaker. My 1968 drip edge came with a so-so Oxford 12K5. I replaced it with a Weber alnico (12A125). So much better vibes!

:mrgreen:

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Post subject: Re: 1965 Deluxe Reverb
Posted: Tue Sep 25, 2018 4:29 pm
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BMW2002Ti wrote:
Your amp came with a nice speaker.


Not original to the amp. While a Jensen *might* be possible, it certainly wouldn't be a C12N which is what it appears to be. More likely would be a C12Q or perhaps a C12R. In any case though, the speaker would be identified as a Jensen specifically designed for Fender with a gold-and-brown label.

Down to cases though, the C12N is a truly great speaker for a Deluxe Reverb -- superior tone and more-than-ample power-handling capability.

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: 1965 Deluxe Reverb
Posted: Wed Sep 26, 2018 7:41 am
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That is a super nice blackface DR. With the C12N, must be a tone king. Guess I'll take the old DR out and put it through its paces.

One of the best amp types, ever created.

:D


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Post subject: Re: 1965 Deluxe Reverb
Posted: Wed Sep 26, 2018 9:27 am
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BMW2002Ti wrote:
One of the best amp types, ever created.


+1!

Never bettered, never equaled.

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: 1965 Deluxe Reverb
Posted: Wed Sep 26, 2018 5:40 pm
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Pants187 wrote:
Figured the older resistors that were there had seen better days and just died from age. Then the replacement ones burned up as well. Uh oh.

???

There is nothing in your previous posts or pictures that suggest you have replaced that bias supply cap. Replace it, clean up the artificial center tap resistors like UN suggests. A blue glow in a power tube is normally not indicative of a problem.


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Post subject: Re: 1965 Deluxe Reverb
Posted: Wed Sep 26, 2018 8:05 pm
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vinyl wrote:
???

There is nothing in your previous posts or pictures that suggest you have replaced that bias supply cap. Replace it, clean up the artificial center tap resistors like UN suggests. A blue glow in a power tube is normally not indicative of a problem.


Bias supply cap had been replaced by this point. There's also no way that cap failing would cause a high voltage across the resistors in question.

Blue glow may be normal in tubes, but intermittent arcing accompanied by loud pops and burning resistors is not. Replacing the tube in question completely solved the issue.

The two 100 ohm resistors were replaced with equivalent resistances, and more resistors means more wattage can be dissipated in case another tube shorts out.


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Post subject: Re: 1965 Deluxe Reverb
Posted: Thu Sep 27, 2018 4:26 pm
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Your pic of after doubling up the resistors.
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That sure looks like the original bias supply cap.

I'm merely questioning why you would power up a valuable amp like this before replacing the bias supply cap, and as you state, burning up 2 sets of resistors, which means besides the resistors, all of the tube heaters have been subjected to over 400 volts of voltage.

If that cap failed in a way to drive the power tubes into cut-off, then the plate voltage might have risen to the point that one of the tubes might have arced over in the manner you suggest.

Enjoy your amp. It is a fine example of a blackface deluxe reverb.


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Post subject: Re: 1965 Deluxe Reverb
Posted: Thu Sep 27, 2018 7:26 pm
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Unless there was (is) some abnormally-audible noise generated by the PT's heater winding, I have no clue as to why someone added that artificial center tap. Blackface amps are traditionally pretty quiet.

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: 1965 Deluxe Reverb
Posted: Thu Sep 27, 2018 9:36 pm
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We all learn from what gets posted here. Including me.


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Post subject: Re: 1965 Deluxe Reverb
Posted: Thu Sep 27, 2018 11:02 pm
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After the first set of resistors burned, replaced the bias cap with a sprague 50 uF cap. Resistors still burned.

My understanding of the bias circuit is that the diode acts as a halfwave rectifier, filtered by the cap. (Yeah technically there is a solid state rectifier present in a 65 DR).

A short in the cap would result in the bias being grounded, which would saturate the tubes.

If the cap failed into an open circuit, the negative part of the sine wave would be present, which would alternate the bias between whatever the pot is set to and 0, saturating the tubes

Just got back from a gig, the amp sounds a lot cleaner with the new tubes and bias. With those center tap resistors burned up, there was an incredible hum. Are you saying they aren’t there in the original design?


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