AA764 Vibro Champ build (power supply and grounding)

[2L man]

For me "ground" has always been just "Zero Volt" and it is where operative current return to power supply when system uses positive voltage power supply. There are systems which also use negative operative voltage, for example fixed bias.

For signal the 0V is a "reference".

First role for Chassis is Safety Earth! Building mains fuse "burn" thru mains cable safety earth wire in case amp own fuse does not protect severe mains current leak to Chassis.

Another role for chassis is a Shield against electromagnetic noise both ways, what amp create and what other equipment and universe create. Thats why it is not good to use chassis for operative current!

Potentiometer case role is also a Shield so no operative current is allowed! When potentiometer is mounted to metal chassis without insulation it turns to a Shield. When signal to potentiometer is wired using Shielded wire the Shield can be soldered to the pot case but when only other end of shield is connected there does not flow any current to Chassis.

When Safety Earth is easily available it is also used to attenuate noise what inevitably form to the signal reference 0V and it is best to do at amplifier input where signal is lowest and amp is most sensitive so 0V is most effective to connected to Chassis at input. Also "ground loop breaker circuit" can be used to connect 0V to Chassis / Safety Earth and when stereo amp system is used obviously other amp should have it? There are good articles in i-net.

 

[andrewRneumann]

AA764 is single ended one tube 6V6. Sag should not be a problem.

Oh cool. I searched “AA764 schematic” and the first thing that popped up was a Princeton Reverb push-pull. I never noticed Fender had two different circuits with the same designation!

 

[King Fan]

Oh cool. I searched “AA764 schematic” and the first thing that popped up was a Princeton Reverb push-pull. I never noticed Fender had two different circuits with the same designation!

Good point, Andrew. Usually, specifying the model number decreases Fender ambiguity (5F2a instead of "Princeton"), but here Fender left an open trapdoor in the middle of the stage.

You know this, but for those who don't it's a crazy bit of Fender history (brief thread detour): There never actually was an AA764 Princeton Reverb schematic (or build). *BUT* Fender put out a grundle of BF AA1164s with a misprinted tube chart (below). So lots of folks believed (many still believe) there was a BF AA764 PR.

But it gets worse. Some years ago, a well-meaning hobbyist Photoshopped a PR schematic with an AA764 heading, apparently just for grins. But *then* Uncle Doug and many others, seeing it on the internet, fell into *that* open trapdoor, and gave confirmation to all the owners and sellers who'd been writing and talking about 'AA764' PRs.

Example tube chart (note they were still using up that bad print run as of Apr '67 here):



The 'annotated' Photoshop document (he added the text correction later, after the internet adopted this as a real schematic):

 

[King Fan]

@ King Fan,

The theories from Merlin and Aiken are familiar to me, as is for instance Rob Robinette's site.

To be fair I don't have much experience building tube amps besides fixing and adepting a flawed Blues junior clone with PCB layout issues. After performing all BillM mods I've learned that doing things because internet says so isn't always wise ;-).

So for the next build I really want to understand the theory behind what I propose to add to a venerable schematic like this (if any).

And yes I know that my proposed layout is uncommon, that's exactly why I asked the combined wisdom here.

The theory comes basically from www.valvewizard.co.uk/smoothing.html regarding calculating around a smooth and quiet powersupply. That's why I propose an additional filter stage with 470k dropping resistor (2 effects, one action)

Merlins grounding scheme is tough reading though. Some ideas on the isolated from chassis came from that part.

Good for you. I could tell you'd done a lot of research. And sorry. I suggested Merlin about ground planes, and ground theory, but I should point out here (I actually do, all the time): his principles are high-end, idealized, and intended for designing entirely novel, big, complex high-gain amps. It'd be fun to build a test-bed amp and try out different approaches. But it wouldn't be all that rewarding or useful in a small, simple, well-loved amp like a VC. We've seen several folks here build multiple 5F1 Champs, or even 5E3 Deluxes, using many increasing degrees of theoretic improvement, and most of the time I sense the differences are either inaudible or very small; very much smaller than a B+ shift, much less speaker change. And you're right, the billm BJ journey is a good example of good theories hunting for realization with um, variable success.

The principle many of us suggest for a first build is kind of the opposite of 'optimizing' or even modding. Build it stock -- even that will give you plenty of important choices that matter, like B+ and speaker, lead dress, etc. Then operate, listen, measure, and break it all in. Troubleshoot, if needed; much much less frustrating (still hard) in a stock standard build. And then mod. You can build to ease future mods -- a hole drilled here, tag strip there, extra eyelet there. But starting out with the mods in place, you're on a journey with an unknown point of departure, much less destination.

EDIT: More ideas. @2L man is a champion of a single ground scheme, and though it doesn't make much audible difference in these small amps, a Merlin-ideal ground scheme (his simple bus is equivalent to a star) might be easier if you fitted separate caps instead of a cap can. Filter experiments aren't that hard, and they do make a difference if big enough. It'd be possible to start with a 20uF reservoir and clip or solder in a parallel cap (or rewire the can) to try 40. I've never seen a 20, then a 40, but maybe you could wire that as well -- later. As for adding a whole extra node, if you can fit a tag strip and cap, maybe you could -- but again, I'd wire it up later, to have something to compare it too.

OTOH correct me if I'm wrong, but if SE ripple is the issue, don't we want to maximally filter the plate? I may be confused by words instead of pictures here.

 

[MichelB]

After this very interesting side step on AA1164 vs AA764 schematics I would like to get back to the interesting grounding questions.

After reading through Merlin's chapter on grounding I have to quote a very interesting part.

"There must be no connection between transformer, rectifier, reservoir and chassis, of course, but that is always true of a proper ground scheme."

This implies that the center tap of the PT is NOT tied to chassis and that we might have a semi floating ground for the main power supply. As far as I can see on all layouts, the center tap is bolted firmly to the chassis on the PT/output ground point. This confuses me a lot!

The next point from Merlin's book is giving me a very good idea (I quote him again)

"In stand-alone preamps and low power (usually single ended) amps there may be one or more additional stages of smoothing after the reservoir capacitor, before supplying the audio circuit. In such cases it is beneficial to split the usual dropping resistance into two parts (usually but not necessarily equal) and so deliberately create a real balanced filter, as illustrated in fig. 15.7. The added resistance in the negative side of the circuit helps isolate the noisy rectifier/reservoir circuit from the amplifier proper, and will keep ripple current out of the audio ground."


All my previous intentions are to create the most stable DC and quiet amp possible and I've already decided I need to lower the B+, so I'm going to use some dropping resistors. With Merlin's info I could place a single discrete reservoir cap on terminal strips with 2 x 235k dropping resistors, feeding the cap can. The same cap can common will the be the start point of the preamp bus ending at the chassis grounded input cap. The cap can will then power the actual amplifier. The center tap of the PT will not be tied directly to ground, but as it's tied to ground due to one of the 235 dropping resistors, we should have a firmly fixed ground at 0 Volts.

Will this work?

In this scheme the center tap is not directly connected to chassis (through a 235k dropping resistor and complete grounding bus). And the chassis is directly connected to earth via mains grounding. This part is sort of mind boggling for me.

Looking for your shared input again?

Michel

 

[MichelB]

Good for you. I could tell you'd done a lot of research. And sorry. I suggested Merlin about ground planes, and ground theory, but I should point out here (I actually do, all the time): his principles are high-end, idealized, and intended for designing entirely novel, big, complex high-gain amps. It'd be fun to build a test-bed amp and try out different approaches. But it wouldn't be all that rewarding or useful in a small, simple, well-loved amp like a VC. We've seen several folks here build multiple 5F1 Champs, or even 5E3 Deluxes, using many increasing degrees of theoretic improvement, and most of the time I sense the differences are either inaudible or very small; very much smaller than a B+ shift, much less speaker change. And you're right, the billm BJ journey is a good example of good theories hunting for realization with um, variable success.

The principle many of us suggest for a first build is kind of the opposite of 'optimizing' or even modding. Build it stock -- even that will give you plenty of important choices that matter, like B+ and speaker, lead dress, etc. Then operate, listen, measure, and break it all in. Troubleshoot, if needed; much much less frustrating (still hard) in a stock standard build. And then mod. You can build to ease future mods -- a hole drilled here, tag strip there, extra eyelet there. But starting out with the mods in place, you're on a journey with an unknown point of departure, much less destination.

@King Fan,

These are very wise words and there is good chance I'll just build everything completely stock first. Most of my proposed improvements will be reversible anyway with very limited amounts of tinkering. Most are related to the routing of leads around the cap can (with or without dropping resistors), even adding the extra 20uF cap is a minor revision to the actual inside of the amp.

However (and a this is a big however);
A: I think we all like to understand what the thing in our hands is doing.
B: We have to do something while waiting on the parts and waiting on time to actually handle the soldering iron.
C: Theorizing over our hobby is nice (why else is this forum so big ?)

Michel

 

[King Fan]

@King Fan,

These are very wise words and there is good chance I'll just build everything completely stock first. Most of my proposed improvements will be reversible anyway with very limited amounts of tinkering. Most are related to the routing of leads around the cap can (with or without dropping resistors), even adding the extra 20uF cap is a minor revision to the actual inside of the amp.

However (and a this is a big however);
A: I think we all like to understand what the thing in our hands is doing.
B: We have to do something while waiting on the parts and waiting on time to actually handle the soldering iron.
C: Theorizing over our hobby is nice (why else is this forum so big ?)

Michel

I like where you're going here. I always say planning is the only part of amp building that combines free, safe, and fun. Items I might research and plan on a VC, for example:

1. Layout -- this can be a journey all by itself. I start with Fender's, then figure out what needs safety or other modernization.
2. Chassis, faceplate, backplate, cab, cosmetics, etc.
3. Board -- do I need and can I make or order a custom board? If not, revisit item 1
4. PT specs and choices -- a really big deal, one we see go wrong surprisingly often
5. PT secondary tap -- a big deal for you specifically, as you say you seek lower B+. I asked before, did you say? Will you use the lower HT tap? Do you know how to calculate the resulting B+?
6. OT size, taps, winding specs (important and less obvious)
7. Cab and speaker choices. 8", 10", even 12" -- big big differences. And brand/type choices? Huge impact.
8. Power entry and safety wiring. In the EU you'll want IEC and DPST switching? Which, where, how?
9. Other US-to-EU issues, like fuse
10. Tube, cap, and resistor brands and types. Endless fun; some even have practical importance.
11. Resistor details. MF in input chain? 1/2W CC on plates for possible mojo? Or 1W MF for quiet?
12. Wire -- gauge, cloth v. plastic, etc. etc. Really makes a difference; shaping and shape-holding is big in little amps.
13. Soldering station and solder choices -- especially if you have to use lead-free
14. Fun details that don't matter much (so no wrong choices) -- simple split (Rob style) vs. single ground. Isolated input jacks? Isolated speaker jacks?
15. Cap can or fit separate caps?
16. If separate caps, HT CT to reservoir negative?
17. Does my PT need an artificial heater CT?
18. Either way, elevate 6.3V CT? try both ways?
19. Plan and prep for basic, obvious, audible but easy mods to try after build. (VC example: Replace 18k tone resistor on bass pot with 10k -- value used on DR ‘Nashville mod’ -- for slightly more scoop, slightly less hair. Or NFB mods, or a trem-slowing cap to clip in, etc.)

 

[MichelB]

Hi King Fan,

I went through basically all questions you wrote down allready;-).

FYI,

-Hammond 290AEX only has one HT at 650ct. Using the lower tap will not work.
-I'll be using cloth wire, on guitars pushback cloth works easier than normal wire. Plus it looks nice.
-I'll be using MF resistors or 2W MOx resistors. The latter are larger and look more vintage.
-The power switch was reason for some concern. My chassis has the slider switch cutout and the readily available fender switch is only rated 0.3A/120V and doesn't cut it in EU. I found a heavy duty 6A/250V DP slider which I think/hope fits.

 

[MichelB]

Plan and prep for basic, obvious, audible but easy mods to try after build. (VC example: Replace 18k tone resistor on bass pot with 10k -- value used on DR ‘Nashville mod’ -- for slightly more scoop, slightly less hair. Or NFB mods, or a trem-slowing cap to clip in, etc.)

Tone stack bypass is planned on push-pull volume pot, since its DPswitch I'm considering taking the bypass cap on V1b out of the circuit if tonestack is bypassed for more 5F1 circuitry.

NFB three way switch is planned. Normal, light, off.

That's about the amount of modding on the sound circuit that I'm planning.

 

[Lowerleftcoast]

Thanks for the read. It looks like he's only upping the first filter capacitance though, it does work

If you read a little further, the second cap is doubled without a significant reduction in noise.

As far as I can see on all layouts, the center tap is bolted firmly to the chassis on the PT/output ground point.

Let's challenge that thought. Below the layout does not have the HV CT bolted to ground.


Layout courtesy of Rob Robinette.

There is no need to use the chassis (or brass plate) as a current path for DC in your amp. If you want to follow Blencowe's ground advice, only one reference to ground will be made at or near the input jack. (Wire or a ground bus will be used for all return current.)

As pointed out, there may be no penalty for using the chassis for current flow. Using Blencowe's advice will theoretically be better but with other designs, the amp may not pick up noise. The noisy current may not be near the sensitive signal flow so the noise will not be amplified by the tubes.

The cap can with the single ground terminal will make it difficult to follow Blencowe's ground scheme to a T. Every ground node (cap negative terminal) is at the same location.

 

[MichelB]

@ Lowerleft coast, thanks for sharing, this does help!

 

[2L man]

All my previous intentions are to create the most stable DC and quiet amp possible and I've already decided I need to lower the B+, so I'm going to use some dropping resistors. With Merlin's info I could place a single discrete reservoir cap on terminal strips with 2 x 235k dropping resistors, feeding the cap can. The same cap can common will the be the start point of the preamp bus ending at the chassis grounded input cap. The cap can will then power the actual amplifier. The center tap of the PT will not be tied directly to ground, but as it's tied to ground due to one of the 235 dropping resistors, we should have a firmly fixed ground at 0 Volts.

Will this work?

In this scheme the center tap is not directly connected to chassis (through a 235k dropping resistor and complete grounding bus). And the chassis is directly connected to earth via mains grounding. This part is sort of mind boggling for me.

Looking for your shared input again?

Michel

235K dropping resistors drop voltage a lot!!!

There is no need to build complicated secondary "grounding" to current return! Actually it is much simpler than B+2, B+3, B+4 ...current delivery because there is no dropping resistors on current return (unless you use that Merlin method and then delivery and return are just the same before B+1). Then very late in building the ground wire is connect to chassis using input jack or ground lug close the input.

When building you can time to time check that ground show infinite to chassis until you connect it to chassis.

There are additional resistors on current return on four of my builds now and I can say it make amps hum mains frequency less!

But I have used four or five filter capacitors before B+1 so it must have effect as well. For example five RCR stage PS filter has been four 16uF electrolyts and six about 47 ohm resistors between them and then two 33 resistors and last filter is 4,7uF polycap. When bias current is 60mA there come 27V loss in this power supply filtering.

 

[Phrygian77]

I did 40uF-200ohm-20uF, using the extra section of the JJ can, in one of the Vibro Champs that I built.

 

[MichelB]

235K dropping resistors drop voltage a lot!!!

There is no need to build complicated secondary "grounding" to current return! Actually it is much simpler than B+2, B+3, B+4 ...current delivery because there is no dropping resistors on current return (unless you use that Merlin method and then delivery and return are just the same before B+1). Then very late in building the ground wire is connect to chassis using input jack or ground lug close the input.

When building you can time to time check that ground show infinite to chassis until you connect it to chassis.

There are additional resistors on current return on four of my builds now and I can say it make amps hum mains frequency less!

But I have used four or five filter capacitors before B+1 so it must have effect as well. For example five RCR stage PS filter has been four 16uF electrolyts and six about 47 ohm resistors between them and then two 33 resistors and last filter is 4,7uF polycap. When bias current is 60mA there come 27V loss in this power supply filtering.

Thanks for sharing

 

[MichelB]

I did 40uF-200ohm-20uF, using the extra section of the JJ can, in one of the Vibro Champs that I built.

Also thanks for sharing, was this a quiet build? Do I see both a screen and grid resistor on the 6V6?

 

[mountainhick]

While I like well implemented bus rails, I'm also a bit lazy and like the concept of easy soldering to the brass plate. By just using some shoulder washers between ground plate and pots, some isolation tape between plate and chassis and shoulder washers between chassis and input jacks, I think I've made a floating circuit ground that I can connect to the star wired cap can common ground. Theoretically better then Leo's design and really not much more work. The twister heaters with artificial center tap will be more work then these changes.

For the record, the pots will not be isolated from chassis, just from floating circuit ground.

Michel

Buss wire is simpler. Really.

Pot's cases grounded to chassis=faraday cage=good. ground pots' terminals to buss.

 

[Phrygian77]

Also thanks for sharing, was this a quiet build? Do I see both a screen and grid resistor on the 6V6?

Considering that it was built with CC resistors, yes. It wasn't noticeably noisier or quieter than any of my other builds.

It makes sense to add grid resistors for a number of reasons. I always add them.

 

[MichelB]

The mail man arrived.

These slider switches fit!

https://www.ebay.nl/itm/2pc-Heavy-D...=p2349624.m2548.l6249&mkrid=711-127632-2357-0

It's nice to have a switch rated for 240V mains, if you want to use it as power switch

 

[MichelB]

Just some pics to keep the build thread going.

Hopefully the bulk of the parts will be delivered soon, so I can start building.

Just on other build question, how important is it to place the screen resistors on the tube socket? I like to use the unused pins of the 6V6 socket, but with a grip stopper, a screen resistor and 2 100Ohm resitors to the cathode it will get pretty cramped there. I was considering placing the screen resistor on the circuit board.

 

[mjcyates]

I did 40uF-200ohm-20uF, using the extra section of the JJ can, in one of the Vibro Champs that I built.

I did something similar to this and my amp is dead quiet. Here is the layout

I originally used the 275v tap but with the added resistors it dropped the voltage too much so I re-wired using the 325v tap and it worked out just about perfect.