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Post subject: Some TLC for my 1965 6G15 Reverb Unit
Posted: Sun Feb 26, 2017 9:19 am
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Greetings all, longtime lurker and first post, the name's Erich, nice to meet you, put 'er there... and thanks in advance for any advice on the below.

Back in the mid-80's as a young starving wannabe surf guitarist I purchased this cool reverb unit from a small shop in San Bernardino CA that dealt with used gear (don't remember the name but it was on E street a few blocks away from Lier's Music). I believe it to be a 1965 unit based on serial number. Although it had obviously seen some wear and tear (note missing Fender badge) it was fully functional although somewhat rough sounding, but drippy enough to get the job done. In all the time I played with it I just fired it up and went, didn't know to check the tubes or have an amp shop look it over. But paired with a blackface Super Reverb it served our little surf band well and paid for itself a few times over in free beer.

The output gradually became more harsh and grainy and eventually I stopped using it, lacking the funds to take it to a shop for any work. A few years later (mid-90's) a buddy of mine who was into building and modding amps offered to look it over. He replaced the power cord and some caps, and made mention about putting in a couple of caps with different values as advised by one of the books he had on Fender amps in an effort to squelch the piercing high tones it was sometimes producing. It sounded a bit better but still not quite right. I kept it due to sentimental value but rarely plugged it in as life in general took over.

2 or 3 decades later I am blowing the dust off and - now armed with accurate info - trying to look her over properly. Before doing any research here I took it down to the local boutique guitar shop and replaced the old tubes (some no-name 6V6 and a couple of 12ATX7's) with same-spec Electro-Harmonix tubes. Sounded worse! So after doing some reading here I ordered a proper 6K6GT and 12AT7 for it. Both are RCA (what Doug's Tubes had in stock). Just plugged them in yesterday and she already sounds warmer, softer, and drippier, but the grainy highs are still there a bit.

I am not an amp tech but can use a soldering iron well and have respect when it comes to dealing with electronics and voltages in gear from this era, so am looking for input on what to do with it next. I know to discharge all the caps and take precautions, and would like to do as much work as possible myself this time since I do all the work on my guitars etc. as well, and just don't want any more surprises. Apparently the filter caps were replaced by my buddy and in the pics you can see some of the other electrolytic caps were replaced as well - particularly the one near the end of the circuit near the 12AX7 (7025) output tube which now seems to have 2 x 100mfd caps (this must be the mod my buddy was talking about - unfortunately I can't reach him these days to ask).

So here is what I found on a little inspection tour... thanks for listening and comments certainly welcome:

Front side, please to excuse workbench clutter:

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Back:

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Under the cover:

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The original caps are long gone but I am not sure what to make of the new caps in place, I will double-check the values - I know 2 on the right are 100mfd instead of a single 250mfd but are these type even right for this application?

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Power side:

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Tube chart in good shape (and for the first time in 30 years those are now actually the tubes installed)...

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From what I understand the "death capacitor" should be removed when converting to a 3-prong, but I don't know where the original was, hopefully this all looks good...

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Yep I was very careful here and took precautions, one hand in pocket etc. and only had te cover off long enough for a quick peek. Original filter caps have been replaced, which actually my buddy didn't mention doing, so I have 3 big blue Sprague Atoms for nothing, unless they are much better than these?

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Careful inspection of the reverb pan - pulling the front cover seemed less intrusive than removing the chassis. I finally found out where all the yellow-orange dust was coming from, prior to this I was wondering if some little dog in years past must have been lifting it's leg on it.

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Seems to be a Gibbs pan (authentic for the era?) and no broken or sagging springs. All that orange dust got blown out, and it also seems less "crashy" now.

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All back together again with an RCA 6K6GT and RCA 12AT7 (still have a EH 12AX7 in the 7025 socket).

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Buttoned her up and plugged in, tone and sound improved with the proper tubes but still a little harsh/grainy. Instro surf is about the only style I am into or play very well... "it's a lifestyle, man" ... so any improvement is welcome. I've seen some of the other very helpful info here such as a 1-meg resistor to ground from pin 2 of the 12AX7 socket and - barring any common sense reason - would like to bring this unit fully up to snuff myself. Functionality is way more important over patina and originality, she's been through the mill anyway and is a working girl, just want to make her happy with good, correct internals so we can twang away together .... :)

Oh if it matters I'm plugging in a couple vintage Fenders ('66 JM, '63 Jag) and a DePinto Galaxy IV, into the clean channel of a Fender FM210R (yes sacrilege I know, but my blackface Super Reverb was lost years ago, and the clean channel isn't that bad).

Thanks for reading this far.


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Post subject: Re: Some TLC for my 1965 6G15 Reverb Unit
Posted: Tue Feb 28, 2017 4:55 pm
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You do a good job for somebody is not a electronic tech.
You may not know where to buy better capacitors for this real gem.

Yes 2 X100 mfd = 200

Tubesandmore and many other parts stores sell better caps

Sprague Atoms and F & T caps are a better choice for caps.

Here a good quality 250 mfd. 25 volts;

https://www.tubesandmore.com/products/c ... gue%20Atom


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Post subject: Re: Some TLC for my 1965 6G15 Reverb Unit
Posted: Tue Feb 28, 2017 6:23 pm
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Those dual cathode-follower 100mfd caps are polarized electrolytics and they are not specified as such on the schematic. They may be the source of your problem. Also, if you do not have a *real* 7025 for the V1 socket I'd recommend a 5751 in lieu of a 12AX7. The slight reduction in gain may prove sonically beneficial.

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: Some TLC for my 1965 6G15 Reverb Unit
Posted: Tue Feb 28, 2017 6:53 pm
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Way cool - thanks for the comments and feedback, gents.

Stratele - that's great to hear about the caps, I ordered those very ones from that very place - 3 x Sprague Atom 40uF / 500V and one 250uF / 25V, they're sitting on my dining room table.

Retroverbial - I'll take a look at those other replacement caps and order the correct values and type according to the schematic for this unit (I'm not an amp tech but technically oriented on basic electronical stuffs). I've seen some good clues on what caps to go for based on other threads here about these units. And Dougs Tubes has a JJ 5751 for a decent price that makes it worth a try. Lower gain sounds like exactly what the doctor ordered.

Gracias! ;)


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Post subject: Re: Some TLC for my 1965 6G15 Reverb Unit
Posted: Tue Feb 28, 2017 9:23 pm
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Stoopy, nice Reverb Unit! Couple of comments (or three or four) :)

1.) Nichicon make good electrolytic caps. However, it's been years since they made axial caps in the 450VDC range. The Unit must have been re-capped quite a long time ago.

If you have no hum and no "pimples" on the cardboard end of those Nichicon main filter caps, you are prolly okay. I'd replace them with F&T 40mfd/500VDC caps... if there is any hum or question as to how long the Nichi's have been in the unit.

2.) The parallel cathode tied resistors on the 6K6GT is odd. Orange-orange-orange and brown-gray-orange does not equal the OEM 1000-ohm, 2 watt resistor.

I get 33k ohm plus a 18k-ohm, in parallel = 11.65k-ohms. No way close to the 1k-ohm original. Replace the two with one 1k-ohm, 5 watt metal oxide or wire-wound resistor. This resistor (or resistors, in your case) set the idle bias of that 6K6GT tube. At 11.65k-ohms, the tube is idling way too cold (prolly closer to Class B). Big effect on the tube's ability to drive the tank.

Also, use a 25mfd/50VDC bypass cap. Sprague Atom axial cap will work fine.

3.) Keep as many of those Blue MOLDED caps as possible. If any leak DC, replace them.

4.) For the 250mfd/6VDC, I used a 250mfd/25VDC Atom. It'll fit fine and the extra voltage rating will increase life. No tonal changes. Replace 25/25 with 25/50, for other bypass cap (on 12AT7).

Good luck! Keep us informed of the tone, once set back to original specs AND after three-prong power wire conversion.

BTW... I have a BF Reverb Unit, too. Show you some photos... :)


Last edited by BMW2002Ti on Wed Mar 01, 2017 8:04 am, edited 8 times in total.

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Post subject: Re: Some TLC for my 1965 6G15 Reverb Unit
Posted: Tue Feb 28, 2017 9:39 pm
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http://ampwares.com/wp-content/uploads/ ... ematic.pdf

I replaced the stock 1k-ohm, 2-watt cathode tied resistor with a 820-ohm, 12-watt wire wound Mills resistor. A good wire-wound will last the longest, under heated conditions of this Class "A" 6K6GT tube. Use a wire-wound or a good metal oxide 5-watt resistor.

I put a 1meg-ohm, 1/2-watt resistor from grid-to-ground on the output 7025 (12AX7). This keeps this tube from being overloaded by the reverb unit/6K6GT and going unstable. You can see the 1 meg-ohm resistor (brown-black-green) going from that 12AX7 socket to grounding point in next photo.

I also have NOS black plate RCA and Sylvania 6K6GT tubes. I like the tone of the Sylvania tubes. Little more sparkle.

Image


Image

Image

Image


Last edited by BMW2002Ti on Wed Mar 01, 2017 8:46 pm, edited 1 time in total.

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Post subject: Re: Some TLC for my 1965 6G15 Reverb Unit
Posted: Wed Mar 01, 2017 4:26 am
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Retroverbial wrote:
Those dual cathode-follower 100mfd caps are polarized electrolytics and they are not specified as such on the schematic. They may be the source of your problem. Also, if you do not have a *real* 7025 for the V1 socket I'd recommend a 5751 in lieu of a 12AX7. The slight reduction in gain may prove sonically beneficial.

Arjay


These caps are always polarized and like the other 25MFD, wich are polarized, they are not specified on schematic.


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Post subject: Re: Some TLC for my 1965 6G15 Reverb Unit
Posted: Wed Mar 01, 2017 7:06 am
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stratele52 wrote:
These caps are always polarized and like the other 25MFD, wich are polarized, they are not specified on schematic.


You are correct, mon amí.

:oops:

Arjay

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Post subject: Re: Some TLC for my 1965 6G15 Reverb Unit
Posted: Wed Mar 01, 2017 11:40 am
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Retroverbial wrote:
stratele52 wrote:
These caps are always polarized and like the other 25MFD, wich are polarized, they are not specified on schematic.


You are correct, mon amí.

:oops:

Arjay




I'm sure this is something you know, just not enough coffee this morning :lol:


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Post subject: Re: Some TLC for my 1965 6G15 Reverb Unit
Posted: Wed Mar 01, 2017 7:07 pm
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Holy moly - absolutely great, helpful advice and pics - can't thank you enough!

BMW2002Ti - I was kinda hoping you'd weigh in as I'd seen your post about your reverb unit here before. I would love to emulate exactly what you did, although this will take me some time to digest and create a plan of attack - but first thing, I'll go after the really egregious stuff you've pointed out like the incorrect resistors and funky bypass caps first. I haven't dealt with tube circuits before so some of the terminology is new to me but fun to learn about.

Knowing that some of these caps were definitely changed back in about 1990 when my buddy cracked this open and put in the 3-prong power cord and "changed some stuff" I am pretty sure that all the various Nichicon caps were swapped in in one fell swoop. So getting things back to stock form will be the first order. Time to make the shopping list.

Despite the great description and pics I am not quite sure I follow the mod you did with the grid-to-ground on the 12AX7 socket. What exact pin on the socket is that tied to? I had thought it was pin 2 but the schematic I sourced shows a connection already going from there to the footswitch hot side, not ground.

After improvements are done as you've suggested, last thing is I'll probably change the filter caps as they looked OK (but I'll look closer) and I have the distinct feeling that these were changed by my bud and are OK. There hasn't been a lot of play time on this unit since then due to the way it sounded and that pesky reality-career-family thing stepping in between then and now. I kind of became an instant family man a short time later and as you can tell this has been gathering dust but I've always kept it with me waiting for this day.

Well anyway I will get started and if it's OK will pop up in here with progress and questions as I go. Can't tell you all how much I appreciate the advice and encouragement.


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Post subject: Re: Some TLC for my 1965 6G15 Reverb Unit
Posted: Wed Mar 01, 2017 7:23 pm
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Oh BTW if it's any contribution, here is a re-done schematic I found which is nicely done showing resistor colors etc:

http://www.mojotone.com/Amp%20Kit%20Sch ... f?gc=clear

Easier for a dunderhead like me to follow.... ;)


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Post subject: Re: Some TLC for my 1965 6G15 Reverb Unit
Posted: Wed Mar 01, 2017 8:21 pm
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Yes, the 1 meg-ohm resistor goes from pin 2 of the 7025 to ground. It looks like pin 2 goes to ground on the original layout. But, it really is going to one connection point on the reverb foot switch jack. This point is hot (not grounded). The footswitch is grounded and sends pin 2's signal to ground, when you turn the reverb off.

One of the footswitch's jack is not being used. This is the ground point. You can check which one this is using a DVM in resistance mode (checking the un-switched continuity to ground). This is where I soldered the other end of the 1 meg-ohm resistor to. Or you can make a grounding point using a small standoff connector board.

Here's the modded schematic:

Image


Last edited by BMW2002Ti on Wed Mar 01, 2017 8:38 pm, edited 2 times in total.

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Post subject: Re: Some TLC for my 1965 6G15 Reverb Unit
Posted: Wed Mar 01, 2017 8:31 pm
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Those Nichicon caps under the doghouse are prolly ok. Like I said, Nichi makes very good caps. If there is any hum injected when you turn on the reverb unit... change them out for F&T caps.

I'd change the Nichicon caps used in bypass positions. Only because radial style caps soldered where axial ones should be, looks funky (and vice-versa). But, I'm sure they are ok.

I tried various 6K6GT tubes. US made Tungsol mil spec, RCA blackplate, Raytheon, and Sylvania tall black plate sound the best in this unit. Very 3D, sparkling, deep reverb tone.

http://www.ebay.com/itm/Radio-Tubes-6K6 ... Sw~gRVnx-e

Mojotone layout clearly shows the ground point on the footswitch jack. Wonder why they decided to use 20mfd/500VDC caps, instead of 6G15 original 40mfd? With a three-diode rectifier (instead of full 4 diode bridge), you need more filtering to remove hum.

Good luck! Keep us posted.


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Post subject: Re: Some TLC for my 1965 6G15 Reverb Unit
Posted: Thu Mar 02, 2017 2:48 am
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Nichicon could be a "good" cap *, but I don't like to put Asian parts in vintage amp. And the look of these vertical caps do not fit with in-line original caps.

* Good caps like F & T or Sprague Atoms cost just few dollars more for a $700 ( and more ? ) Vintage Reverb


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Post subject: Re: Some TLC for my 1965 6G15 Reverb Unit
Posted: Thu Mar 02, 2017 7:04 am
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stratele52 wrote:
Good caps like F & T or Sprague Atoms cost just few dollars more for a $700 ( and more ? ) Vintage Reverb


Ha!

Double that $700 is a more realistic value for an original 6G15.

Arjay

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