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SurfGuitar101 Forums » Gear »

Permalink 6g15 reissue rebuild questions!

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With film caps it really doesn't matter which way around they go.

There is some mojo that others get into about the polarity of film caps, but I'm blowed if I can perceive much (if any) difference. Try them either way and see what you think rings your bell or floats your boat.

Its a completely different story with electrolytic caps however - which have to be installed w.r.t. the correct polarity.

But if you want to pay attention to the mojo, the banded end of those film caps can go to the low impedance side of the circuit (in this case pointing towards the triode's plate(s), or away from the next triode's grid)

He who dies with the most tubes... wins

Last edited: Mar 09, 2012 15:35:23

Got it. Thanks. I figured it was the same as with electrolytics. I guess that is why they don't bother marking them. I'm sure I won't notice a difference.

The old blue molded caps have a black line at one end of the paper label so I figured it mattered.

image

Last edited: Mar 09, 2012 15:39:57

(ignore this post. I seem to have figured it out... see below)

Wired it all up, powered it all up...

DOESN'T WORK! Oh no.

So, let the trouble shooting begin.

The tubes glow, the signal gets through it to my amp, but no reverb.

Here is what the controls do:

DWELL: This know does nothing EXCEPT for when it is turned all the way counterclockwise to zero it suddenly delivers a sharp buzzzzzz. As you turn it clockwise the buzz stops once you start to turn it and nothing else happens as you continue the sweep of the pot.

MIXER: At zero the guitar plays with no reverb of course, a bit muffled. As you turn the knob a hummm builds up and a treble heavy transistor radio sort of tone emerges until that's all you have. Just a tinny, verbless guitar.

TONE: This just takes the hummy/tinny guitar tone and adds some more high frequency buzzing...

So. Where do I begin?!

I checked all my connections with a meter to make sure it's all hooked up with proper solder joints. Looks good.

The voltages all seem pretty close to the original schematic. 300-310DCV on the electrolytics, 280DCV just after the rectifier. Tubes are not red-plating. Pilot light is on.

?

Red "OUTPUT" from pan is plugged in between the AT7 and AX7.
White wire from "IN" on the pan is plugged into the RCA jack between the 6K6 and AT7.

I did replace the two old Aerovox caps with new mallory .1 uF caps before I did any testing.

Last edited: Mar 10, 2012 02:30:53

Voltage at the pins:

6K6:
1= 0
2= 25VDC
3= just under 300VDC
4= just over 300VDC
5= 0
6= 0
7= 25VDC
8= 25VDC


12AT7:
1= 145VDC
2=0
3= 2.2VDC
4= 25VDC
5= 25VDC
6= 155VDC
7= 0
8= 2.5VDC
9= 25VDC


12AX7:
1= 215VDC
2= 0
3= 1.2VDC
4= 25VDC
5= 25VDC
6= just under 300VDC
7= 130VDC
8= 155VDC
9= 25VDC

Okay. So. I was wrong. It kind of is working. I just need to work out some grounding issues.

I need to remove some isolation washers that I hadn't noticed on the RCA jacks. They are not grounded and are causing the major HUMMMMMM/BUZZZZZ

I played a bit while wriggling those cables and noticed the hum went away when I touched (grounded) the jacks.

So. I'm going to sleep now, but tomorrow I hope to have this in pretty good shape. Sounds pretty good so far. Hard to tell at 3 am with the volume on 0.5.

Plus the whole chassis is still out of the cab with the back open, so there is more noise that might be getting in that way.

Glad it's not a catastrophic error on my part.

It's alive! It's alive!

image

Smile

Old punkers never die... They just become surf rockers.

Ha ha! Yes. I do feel a pulse. Still some ways to go, but it's getting there.

Okay. So, I pulled out the RCA jacks and removed the plastic shields. That got rid of 90% of the hum. Still have to poke around with a chopstick to see if my wires are causing the issue. Also, I didn't get a brass plate for grounding. When I find a piece I'll install that and see it it helps.

The reverb sounds really nice, as I'd hoped and the hum issues are not surprising. I think most amp builds wind up with at least some hum to work on.

The one big question mark now is, when I turn the dwell knob all the way off, I get a strong, sudden "wrong answer" kind of buzz. I guess then you roll the dwell all the way off something is going 100% to ground and maybe that dwell pot isn't properly grounded? That's the biggest issue.

Many photos:

The layout pics you just posted look very tidy for a 1st build - well done!.

Re: your voltages - they look about ballpark and the amp is working (albeit with some glitches), so that's a good sign. (BTW when you check the voltages, you should also measure the VAC between each side of the PT windings, including the heater winding by measuring pin-to-opposite-pin. The DC-to-ground voltages you measured at the heater pins just reflect the way you have ground-referenced the heater to the 6K6 cathode). Just sayin', because the heaters must be working properly for the amp to be functioning as it is. Smile

Re: the dwell control, there shouldn't be a buzz with the wiper grounded: there may be a problem with the wiper contact failing at the ground lug end of the pot rotation (possibly due to the damage in installation?). Unsolder the pot lugs and test the pot rotation for continuity with your DC meter - you should still get DC continuity all the way to off (i.e. the wiper-to-ground resistance shouldn't go open either way - if it does, the best solution would be to chuck a new pot in). If its tests okay, then the hum could be from the way everything is grounded. (Edit - thinking about that way you have soldered the pot ground return without using a brass grounding plate, it could very well be)

Re: the hum, I suspect the hum is most probably grounding related, although it may be from several sources. Check the shielding of your pan send and return cables for DC-continuity with your r-meter. Also, the way you lay out the signal cables and AC wires is important in order to avoid unwanted signal-coupling. You could try shielding the signal leads from the input jack to the grid of V1a, and from the dwell pot wiper to the grid of V1b (such cable shields should be connected to ground at one end only ). With the mix control at the dry-signal-end of the pot rotaton, do you still get hum? - If you don't, then the source of a lot of the hum could be from unwanted coupling somewhere in the wet side. Is the pan oriented so the that output-transducer-end is furthest away from the transformer iron? - the output transducer is the most sensitive to unwanted EMF coupling in this regard.

Re: the grounding, as I said earlier - its quite a critical aspect in a SAR unit because of the ground loops between the unit and the main amp. You may need to keep working at it repeatedly to get it to behave. You may most likely need to totally re-do it. Whatever you do, don't lift or elevate the mains earth cable from the chassis - that could be fatal. It better to optimise the ground circuit within the SAR unit. Read those articles I linked in one of my earlier posts. If you do the R.G. Keen method, follow it to the letter. If you do what Merlin does, follow that to the letter.

Or use a brass grounding plate if you're going to follow the Fender layout. The way the ground wires appear to be soldered to the chassis in your pics looks far from ideal - the soldering joint(s) do not look very clean from what I can see. You need a really powerful iron to get solder to bond properly with a steel chassis - a lot of heat is needed for the reaction. But apart from that, steel has higher resistivity than brass, and that can wreak havoc with micro-voltages within the ground return.

Let us know how you progress.

Pete

He who dies with the most tubes... wins

Last edited: Mar 10, 2012 10:30:44

I'll try your wiper test. Makes sense. I never tested it before I installed it and was pretty gentle, but sounds like it could be the issue. Or maybe some solder got in the wiper? I don't know.

I was just poking around in the amp with the power on, plugged in. Moving the heater wiring between the pilot light and 6K6, and rotating the pilot light wires helped reduce the hum by a lot.

Yes, you're right about my joints at the chassis. They are the weakest link. Not easy to get it to stick. I'll try the brass plate first, or maybe just run the wires Keen style to the ground bolt I have coming in from the doghouse.

Seems like I'm very close. The hum I was expecting, the crazy dwell buzz is more of a concern. Overall It's been a good experience. You've been a real good resource Tubeswell.

I'll report back after more adjustments.

J

Here is a sound sample!

It's improvised so, all the usual apologies apply. That's my 3.5 month old son singing along live in studio.

Elliot's First Wave

jbennett wrote:

I'll try your wiper test. Makes sense. I never tested it before I installed it and was pretty gentle, but sounds like it could be the issue. Or maybe some solder got in the wiper? I don't know.

That's possible - in which case it'll be virtually impossible to recover the pot.

I was just poking around in the amp with the power on, plugged in. Moving the heater wiring between the pilot light and 6K6, and rotating the pilot light wires helped reduce the hum by a lot.

Re: the technique for that sort of troubleshooting, I know some people do 'chopstick' tests on live amps, but IMO that is verging on plain foolhardy. I never poke around in the chassis these days with the power on - the risks are too high. I have shocked myself previously from that sort of careless behaviour, and I have been extremely lucky to have survived relatively unscathed. But believe me, when you are lucky enough to survive being shocked by a tube amp's innards, you don't want to experience it a second time. If you have a young family, you owe it to them to be rigorous in your self-discipline.

Its better to look at the wiring first and work out where unwanted coupling is occurring, then with the amp switched off try moving wires around and then testing it.

As a general RoT, the twisted AC pairs should be reasonably closely twisted together and at least an inch away from signal wires/pins*, but if they have to be closer, then only have the twisted pair and the signal wires crossing at right angles. This minimises EMF coupling between the wires. Keeping all wires as close as possible to the chassis (where practicable) also help the chassis 'eat up' stray EMF around the wires. Otherwise keep each wires as short as practicable.

Re: those heaters, you could rewire the heater wires between the 6K6 to the 12AT7 to get the wires right into the bottom corner of the chassis and as far away as possible from any of the signal wires/pins. And lay the heater wires right against the bottom of the chassis where you have just a single wire.

*In your pics I notice the blue wire to the plate of the 6K6 is running along side the twisted pair for the heaters - that sort of thing can cause unwanted EMF coupling for instance. Also maybe separate a gap between where that sole heater wire is going to pin 7 on the 6K6 socket and it crosses the cathode wire for the 6K6, and keep that cathode wire clear above the heater wire at that point. JM2CW

He who dies with the most tubes... wins

Last edited: Mar 10, 2012 17:44:38

Put on the back cover. Dwell buzz is back. probably that dud ground connection. I'll work on that. Otherwise it sounds really great to my ear. An improvement over the reissue for sure. I always felt it was a little shrill, especially as it go really wet. I was happy with it on lower settings, but now it sounds great across the full range, from a light echo to a long crashing surf sound.

I'm very paranoid and cautions when it comes to a live amp opened up. I appreciate you words though. I am very careful to use one hand, rubber soles, gently move things around, unplug amp whenever I do anything more than a gentle prod or push.

I'll recheck those heaters, etc.

Re: the brass grounding plate, to save you making one yourself, you might be able to find one that is made to fit. Weber do chassis kits for their various amps which include brass grounding plates. The chassis drawing with dimensions for the Weber equivalent to the 6G15 is on this link (so you can check the chassis hole spacings and sizes against your unit). http://www.webervst.com/chassis/5g15.pdf

The order form for their pre-fab brass grounding plate for this kit is on this link FWIW https://taweber.powweb.com/store/chassis/chord2.html
Only US$12!

He who dies with the most tubes... wins

Last edited: Mar 10, 2012 22:52:42

Thanks. I might do that. I looked for Brass last week and they guy pointed to a a 50 dollar slab meant to finish off the bottom of a door.

I found a local jewelery makers supply store that sells brass of different thicknesses for cheap (3 bucks, 4 bucks) and I have the snips and bits that I got for this project so I think I'll continue down my DIY path and make the plate. Mostly because after shipping the 12 dollar plate becomes a 20 dollar item. If I'd thought ahead I'd have ordered one along with the rest of the pieces. I think I thought there was one in the reissue.

jbennett wrote:

I think I thought there was one in the reissue.

The RI gets there by traces on PC boards

He who dies with the most tubes... wins

Yeah. There was a green ground wire that came off the PCB and was screwed on to the chassis. It came from the narrow top pot PCB so that must have been it.

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