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Post subject: Re: Some TLC for my 1965 6G15 Reverb Unit
Posted: Sun Mar 05, 2017 8:22 am
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The vintage Gibbs/Hammond types as used in older Fender amps typically measured much lower than the modern equivalents built by Accutronics and Belton. As an example, the two tanks I had rebuilt (for my '64 Vibroverb and '68 Deluxe Reverb) showed a DCR at the input side as 0.81Ω and around 80Ω at the output.

Arjay

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Post subject: Re: Some TLC for my 1965 6G15 Reverb Unit
Posted: Sat Mar 11, 2017 4:48 am
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Kinda curious how your Unit comes out sounding. I broke out my old unit and plugged it into my brownface Pro. The combination of that amp's three tube tremolo/vibrato section and the stand alone reverb unit is something else. Very deep, wet tone.

:)


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Post subject: Re: Some TLC for my 1965 6G15 Reverb Unit
Posted: Sat Mar 11, 2017 8:39 am
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BMW2002Ti wrote:
The combination of that amp's three tube tremolo/vibrato section and the stand alone reverb unit is something else.


+1

Hammond reverb and harmonic tremolo......a match made in heaven!

8)

Arjay

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Post subject: Re: Some TLC for my 1965 6G15 Reverb Unit
Posted: Sun Mar 12, 2017 8:56 am
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BMW2002Ti wrote:
Kinda curious how your Unit comes out sounding. I broke out my old unit and plugged it into my brownface Pro. The combination of that amp's three tube tremolo/vibrato section and the stand alone reverb unit is something else. Very deep, wet tone.

:)


(If that was directed my way)...I am totally curious too - but it might be another week. All the components have arrived (including a new Fender badge for the grill when this is all done) but I spent yesterday soldering together the Surfy Bear FET-based reverb unit and getting it operational - than having some fun playing with that heavy wet drip. That way I can take my time with the real thing here and not be rushed to get the job done plus I now have a halfway decent reference of what I'm shooting for tone-wise. And now today is Mother-In-Law's B-day as luck would have it so chances of sequestering myself in the garage without interruption are pretty slim. If I'm not back in 7 days would someone please call in the air strike... :lol:

Again thanks for all of your help so far, I have way more confidence (and info!) going into this now than I did before.


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Post subject: Re: Some TLC for my 1965 6G15 Reverb Unit
Posted: Sun Mar 12, 2017 9:42 am
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Stoopy wrote:
If I'm not back in 7 days would someone please call in the air strike...


Copy that, snake and nape will be inbound to your pos. Keep the head down 'cause it'll be danger-close.

:mrgreen:

Arjay (certified USAF weapons controller)

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Post subject: Re: Some TLC for my 1965 6G15 Reverb Unit
Posted: Sat Mar 18, 2017 1:38 pm
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Hold your fire Arjay! (but thanks for the offer it was almost needed!)

Dealing with the 6K6 cathode circuit first as planned....yanked the strange parallel resistors and little cap and put in a 1K 5W wire-wound Resistor and 25uF/50V Spraque axial cap so it's back to spec.

Even tho they were the wrong spec I put the 6V6 and 12AX7's temporarily back on place before firing it up (and built a current limiter) just in case anything blew up. Sounded terrible - very rough, grainy and overdriven.

Also the 6V6 got very very warm - wanna say hot. I saw the cathode heater glowing orange but nothing else, should it get uncomfortably warm to the touch?

I then swapped back in the 6K6 and 12AT7, sounded better but the tone control would yield grainy overdriven distortion when above 3

Lastly swapped in the suggested 5751 tube for the 7025 (which had one of those EH 12AX7's in it) and MOST DEFINITELY that improved things, sounds very clean, but now the reverb is much less pronounced and doesn't get "drippy".

So... progress, but not out of the woods yet. Will keep at it.


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Post subject: Re: Some TLC for my 1965 6G15 Reverb Unit
Posted: Sat Mar 18, 2017 1:39 pm
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Oh Arjay - I forgot to say, thanks for your service.... and for playin' along 8)


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Post subject: Re: Some TLC for my 1965 6G15 Reverb Unit
Posted: Sat Mar 18, 2017 2:28 pm
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No problem, Stoopy.

:wink:

Did you try the 6K6/5751 combination by any chance?

Arjay

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Post subject: Re: Some TLC for my 1965 6G15 Reverb Unit
Posted: Sat Mar 18, 2017 4:46 pm
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Retroverbial wrote:
No problem, Stoopy.

:wink:

Did you try the 6K6/5751 combination by any chance?

Arjay


Yes - at the moment I have, from left to right (or V1 - V3):
1 - RCA 6K6GT where it should be,
2 - next to that I have an RCA 6679/12AT7 in the 12AT7 socket,
3 - and now a JJ 5571 in the 7025 socket (where there was a EH 12ATX7). This made things much more civil but there is still some dirt in the tone, less so than before, but also less reverby at the same time.

Only thing I don't have still is a real 7025.

Tomorrow I will swap reverb pans just for fun, then also take care of those weird parallel caps on the 7025, and swap out that green capacitor between the mix and tone pots.

Progress shot... at this point I am glad that I haven't broken anything or zapped myself. :)
Image


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Post subject: Re: Some TLC for my 1965 6G15 Reverb Unit
Posted: Sat Mar 18, 2017 4:55 pm
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Stoopy wrote:
Tomorrow I will swap reverb pans just for fun, then also take care of those weird parallel caps on the 7025, and swap out that green capacitor between the mix and tone pots.


Post a sitrep once you give the other reverb pan a listen.

Arjay

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Post subject: Re: Some TLC for my 1965 6G15 Reverb Unit
Posted: Sun Mar 19, 2017 9:40 am
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Swapped in the Accutronics reverb pan and Accutronics is brighter and cleaner, the original pan is darker and swampier with just a skosh more grunge - but overall there is still lots of dirt that shouldn't be there. Turning the Dwell and Mix anything above 4 or 5 is grungy. Tone is around 3. The Mix control seems to contribute to most of the grit in the sound. Also have a little bit of hum that wasn't there prior to putting in the 5751.

For consistency'as sake I'll stick with the Accutronics from this point out until things are sorted out.

Swapping the reverb pans did provide an opportunity for me to verify I didn't have the input/output leads backward and actually I believe I did. It does sound more reverby now with either pan but still nowhere near what it should be because of the dirt/distortion that's still there.

Couple of dumb questions if you don't mind:

1: For the 250 pico-farad cap between the TONE and MIXER controls, does polarity matter and if so which way is which? Being a newb I'm having trouble telling from the schematic.

2: Also, spotted this orange guy in the picture below and going by the other pictures I see, plus schematic and the Mojotone diagram, it looks like this orange capacitor in parallel with the 100K resistor from the 7025 might be another "added feature" that normally isn't there. Opinions on any effect this would have? I am tempted to yank it unless there is any beneficial purpose it may provide.

Image

Much thanks - Erich


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Post subject: Re: Some TLC for my 1965 6G15 Reverb Unit
Posted: Sun Mar 19, 2017 9:56 am
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Polarity will not matter for that green chicklet cap between the tone and mixer controls.

As for that orange thingie, I have no idea WTF it's in there for. Perhaps additional isolation for that tube's cathode follower......?

Pull one lead off and see what happens.

Arjay

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Post subject: Re: Some TLC for my 1965 6G15 Reverb Unit
Posted: Sun Mar 19, 2017 1:18 pm
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Thanks Arjay.

Disconnecting one end didn't seem to make much difference so I put it back and re-tested, no effect really. Go figure.

I replaced the funky parallel 2x100uFd caps, the single 25uFd cap, and the 250pFd cap bridging the tone and mix controls, and that definitely helped bring things further into line. Can't say what really did the trick since I did all 3 at once but the issue with the Tone control introducing mega-loads of dirt is gone.

Although as much as I hate to say it, it still is not right, although it sounds better, there is still a grainy overdriven sound that becomes more obvious with more dwell and mix (up to 6, still within reasonable levels). Single notes sound a bit reverby but little to no real "drip" at 6 like my 6G15 clone does with the same pan, and chords just sound grainy. Turning the volume on the guitar down helps but then there's no punch. Muted notes with that much dwell and mix should definitely drip but they do not, nowhere close. I am testing using a recent model MIM Strat with stock pickups. Nothing really hot.

Not sure what else to do at this point and the 6K6 tube getting that hot is a concern - unless that's normal but I'd be surprised. Uncomfortable to the touch and can't keep a finger on it for more than 3-4 seconds.

Haven't touched the filter caps yet although I have 3 x 40uF/500V Sprague Atoms and a 10K/5W wire-wound resistor ready to go in.

Here's a progress shot for now, at least it looks a little cleaner and more consistent.

Image

In the meantime I will give all the pots and connectors a good going-over with some DeoxIT GN5, heck some of the pots are scratchy so who knows...

And thanks for all of your help so far, Gents!


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Post subject: Re: Some TLC for my 1965 6G15 Reverb Unit
Posted: Sun Mar 19, 2017 2:46 pm
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It's a stumper, that's for sure.

How about this......try subbing a 12AY7 for the 5751 and see if that doesn't tame that graininess.

Arjay

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Post subject: Re: Some TLC for my 1965 6G15 Reverb Unit
Posted: Sun Mar 19, 2017 5:07 pm
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My WAG on that ceramic disc paralleling the 100k-ohm resistor --- this resistor ties the two halves of the 7025 duo-triode together. The "top-half" of the 7025 (on the 6G15 schematic) --- is the reverb section. The "bottom-half" of the 7025 is the non-reverb signal, which is routed around the reverb tank.

http://ampwares.com/wp-content/uploads/ ... ematic.pdf

The 100k-ohm resistor mixes the two halves. That ceramic disc is acting like a high frequency bypass --- feeding more high frequency non-verb signal into the verb'ed signal. If your signal is weak or grungy --- try removing this cap. What is the pico-farad of that cap? It SHOULD be a silver mica, if you want to keep it in the circuit. It is important to final tone.

As for other grungy issues --- my bet is one or more of those coupling caps is leaking DC. Sorry, about this as MOLDED are nice caps. But, if they leak DC --- they are no good in the circuit. And can cause pre-mature tube failure.

You can test this by measuring the DC voltage on the GRID SIDE of each coupling cap. There should be less than 1-2 VDC on this side. Hundreds of DC voltage on the anode plate side. If the cap is leaky --- this is what choices you have:

1.) For the best DC blocking over the long haul --- Sprague Orange Drop SBP-716P 600VDC rated mylar (polypropylene).

http://www.angela.com/sbe716porangedrop ... citor.aspx


2.) For best tone --- PIO mil spec Russian metal clad 600-1000VDC rated K40Y-9.

http://www.ebay.com/itm/022uf-0-022-630 ... SwL7VWnpbC


Measure VDC onto each anode plate, to be sure that are within +/- 20% tolerance of schemo specs. Measure the VDC on the cathode of each tube for same tolerance specs.

I'd use a strong, quiet 12AX7A or 7025 in the output section. The best IMHO? Raytheon black plate, mil spec 12AX7A or RCA 7025 (black plate is the best).

Raytheon of the 1960s were either orange-red label (US made) or yellow (Japan made). Both are excellent tubes. MUST be strong and QUIET testing, for this application. You NEED ALL the quiet, strong gain of a good 12AX7 here to get the cleanest verb tone.

The 12AX7A and 7025 have spiral wound heater filaments to reduce 60Hz heater hum noise. Only the 12AX7A of the 1950 - early 80s are truly spiral wound tubes. Any Chinese or current Russian made tube with this nomenclature means nothing.

http://www.tubemongerlib.com/gallery2/d ... +-+USA.jpg

The 12AT7 is not as critical in the stand alone Reverb Unit as the reverb driver in most Fender amps. Does not need to be mil spec. Quiet is important.

My favorite 6K6GT is the black plate, long glass envelope Sylvania 6K6GT. Best clean, deep shimmering reverb tone. IMHO. The 6K6GT is critical. This is the tube that drives the reverb tank. It must be strong testing. Some ppl have subbed in a 6V6GT for the 6K6GT, to get a little more gain. You can try this --- as both tubes use approximately the same voltage setups.

The 6K6GT is a single-ended, cathode-biased, Class "A" amp. So, you do not need to re-bias to use the 6V6GT.

http://i.ebayimg.com/images/g/frIAAOSw~ ... -l1600.jpg

Good luck! Keep us posted on those DC readings! :)


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