B+ below 300v - any good? (aka can valve radios make good amp donors?)

[Loomer]

Ok, gotta be honest with myself here and admit, I probably won't go through the trouble of making the chassis look quite as nice and reprimed as that...! A light buff to remove some patina and rust, yeah...

Main reason I liked the idea of reusing sockets is that the bloody things are riveted in!

Any thoughts about the PT? I'm a bit stuck. My instinct would be to simply keep it all as-is since power switch and transformers will be the same anyway, so while new wiring might be a good idea nothing should need changing. But understanding what's going on with that extra ground and the heater filaments would give me peace of mind..!

Basically waiting to make sure all that is clear in my head before I get gutting, just don't want to risk asking myself a question about what's what and not being able to reference the existing layout further down the line... 🤔

[sluckey]

I don't see a problem with the heater grounding. It was very common to ground one side of the heater winding. I bet if you examine the tube sockets, you'll likely find that one of the heater pins of each socket is also grounded.

[Loomer]

I don't see a problem with the heater grounding. It was very common to ground one side of the heater winding. I bet if you examine the tube sockets, you'll likely find that one of the heater pins of each socket is also grounded.

Thanks Steve, that clears it up! What puzzled me most though is the fact there are effectively two grounds on the primary side of the PT - one connects to the IEC via the on/off switch, the other is bridged to the heaters ground and therefore linked to the chassis. Or am I simply reading it wrong and it's not necessarily the primary? PT taps look like so:

Neutral (via on/off switch) ]                   [ 250
                                                           [ CT
                        200 - 225 ]                   [ 250
                        225 - 250 ]                   [6.3v
                Bridged to GND ] - - (link) - - [ Heaters GND (to chassis)

It's that extra tap (bottom left, as it were) that is bridged across to the heaters ground (and therefore the chassis) that puzzles me slightly. I'd imagine whatever its function it ultimately should be grounded at the power section's star ground so probably a simple case of leave as-is.

The on/off switch currently shorts Hot and Neutral from the IEC when switched on, so won't be switching the thing back on. Bit puzzled as it all worked as should and hardly moved anything in there since opening it up, but maybe a few solder joints or the switch itself were on the brink? My inclination would be to move the neutral straight to the PT and simply breaking the hot connection with the on/off switch rather than having both hot and neutral going through a 2-pole switch? Unless it's another case of "how things were done back then" which leaves me, as someone new to the party, stumped as it's nothing I'd seen before?!

[Loomer]

Couple more issues...

All three pots are wired backwards.

V1 pin 8 bypass cap should be 2µF, not 25µF.

More good spots Steve, thank you so much for your help!

About the bypass cap - the AA764 Champ has 2uF, the AA764 (and AB764) Vibro Champ have 10uF. My understanding is that a higher value should allow for more bass content and all the demos I've heard are of the Vibro version, so might stick to 10, but a potential place to experiment right there so might have both values at hand, though I have a feeling the difference might not be massive at least between 2 and 10; 2 and 25, as incorrectly previously listed, probably would be more noticeable but in my head I feel it could potentially make for a flabbier, fartier breakup with that lone 84/6v6.

Amended the layoutupturning the pots as ultimately that's how I will want them in the chassis for easier wiring, inspecting etc

[sluckey]

What puzzled me most though is the fact there are effectively two grounds on the primary side of the PT - one connects to the IEC via the on/off switch, the other is bridged to the heaters ground and therefore linked to the chassis.

No, there's only one ground on that side of the PT. NEUTRAL IS NOT A GROUND!

It's that extra tap (bottom left, as it were) that is bridged across to the heaters ground (and therefore the chassis) that puzzles me slightly. I'd imagine whatever its function it ultimately should be grounded at the power section's star ground so probably a simple case of leave as-is.

That bottom left lug is internally connected to a shield or just the frame of the PT. The schematic clearly shows three PT lugs connected to ground. Look at the attached pic.

 

The on/off switch currently shorts Hot and Neutral from the IEC when switched on, so won't be switching the thing back on.

I seriously doubt that. The primary winding is very low resistance, but not a dead short. With the switch on, your meter is reading that very low resistance between hot and neutral. Nothing wrong.

Amended the layout

But you are still showing a 47K connected to the bass pot. And the way that resistor is drawn makes it look like all three pots are connected together.

[Loomer]

But you are still showing a 47K connected to the bass pot. And the way that resistor is drawn makes it look like all three pots are connected together.

D'oh! Re-amended the layout... Hopefully for the last time.

As per all the other observations - it all makes sense so quickly once you break it down 🙃 Hugely grateful for your help. The schematic is indeed clear on the third ground and the fact it's the frame/shield, of course neutral is NOT ground and finally, internal R of the primary winding is a meagre 30ohm.

Might need to catch up on sleep tonight but will be looking at beginning the strip-down soon as well as putting an order together for the bits needed.

While I'll be sticking to 1w resistors basically throughout (aside from B+ and EL84 cathode bypass) because why not, pots won't actually need to be 1w and 0.25w will suffice right? Less choice when you need long shafts (these are roughly 55mm/2-and-a-touch inches), though I've found some rated for 0.4w that would fit the bill perfectly.

In other news, my ears perked up when, looking through the manuals, I saw the speaker mentioned as 8" - it's actually barely 8" on the very outer edge where the screw mount points are so doubt I would be able to swap it for a guitar-specific speaker even down the road. Again, this was always meant to be something I could use as a head or through a resistive load and into my DAW, so no big deal if that proved not possible.

[scstill]

I don't see a problem with the heater grounding. It was very common to ground one side of the heater winding. I bet if you examine the tube sockets, you'll likely find that one of the heater pins of each socket is also grounded.

I have a Leslie 25 booster amp that I added a preamp to for guitar. (discussed in this forum)
One side of heater is grounded. The hum is not too bad, but its present.
Was thinking to lift the heater adding artificial center tap. A lot of work.
Could there be improvement to warrant the effort?
Why have builders abandoned this one side grounded approach?

[sluckey]

I consider using a real or artificial heater center tap to have much better hum reduction than simply grounding one side of the filament string. I would do it if I had that amp. I would not bother if it were a converted car radio.     Heck, I even left my '57 Harvard amp filament string grounded on one side. Not gonna hear too much hum from a single 10" speaker.

All the builders are just following Leo's lead.

[PRR]

Grounded-one-side heater is "acceptable" in radios, "annoying" in guitar amps.

Radios run 4" and 6" speakers, and the station is ALWAYS playing something. (Also the oldest transmitters had hum.)

Guitar amps run larger speakers (more 60Hz response), and while you play a lot it may not be 99.9% of time. Sometimes you sit lost in thought (or pot), silent, except that darn hum. Yes, a lot comes in from the geetar picking up lighting circuits and ungrounded bodies in the room hum-field, but the heater-hum is the cherry on top. And, if objected-to before the build, is less than twice as much work to do balanced.

Also EVERYTHING hummed in the old days. Radios. Most telephones. Fans/blowers. Most fluorescent and some incandescent lamps. "The sound of progress!" Fashions change and now we object to even small hum in music. Nobody uses the old hummy radios any more, but the old hummy guitar amps confront new expectations.

[Loomer]

Thank you all, great food for thoughts there. The artificial centre tap seems like a very easy way to address this to some degree, so will be sure to include that.

For those stumbling upon the thread, Merlin Blencowe is of course providing the goods: https://www.valvewizard.co.uk/heater.html

The option of using a pot to balance it out also looks interesting, but might stick to resistors at least to begin with.

On the layout front, despite having basically worked out a way to build the Champ in a standard two-row format, I've just bitten the bullet and ordered a turret punch and a bunch of turrets. Since space allows I might simply end up adapting the Fender layout to an 18w-sized board which will also house the SS rectifier, heaters artificial CT, while keeping the grounding scheme to modern standards as per the layout I'd drawn.

[scstill]

yes lost in thought (I wish pot).
Interesting when running through a vintage  Jensen 15" ceramic the hum is not so bad
But when running a modern WGS 12" with likely better lower response the hum is quite annoying
I'm gonna lift the heater on the Leslie (Lizzily ??)

BTW - I just recently finished Phirty6 a 1936 Philco car radio into Guitar amp.
Gutted everything including the grounded heaters and with the grids on top caps
separated by a chassis panel from the AC and DC spaghetti it is really quiet
and sounds pretty good through a 6" eminence (biggest possible)

[PRR]

DUH! The audio amplifier in a radio is less sensitive, also typically set lower, than a guitar amp.

Most radios are like am AA Champ minus the first stage. Less gain.

For several reasons, radios spent 98% of their life with volume near "10 o clock" or "Fender 3", while thrilling guitar wants Volume well above 5 and often 8 to 10 (or 12).

So what is OK for a radio may not suit guitar.

OTOH, open-back box masks a lot of 60Hz. The Ampeg VT-40 had wicked hum from heater traces on PCB, and head-to-cone you could hear it all, but 2 feet away the front and back hums cancelled.

VT-40 had a hum balance and revealed the fallacy of that. Hum comes in different stages depending how the gain and bass knobs are set. For an ONE specific set of settings you could near-null the hum, but touch a knob and the null was spoiled, hum come in strong. Not sure what the factory wanted.

[Loomer]

So after reading the article about artificial heater CT I'm left with one question - I think I'll go for a pot because why not, if that allows for more precise dialling out of the hum it fells like it would be worth it.

In terms of elevation and/or reference though, am I better off connecting the ground straight to the star ground or would I be better off connecting it to the power valve's cathode? Which is meant to clean things up best?

[kagliostro]

If it is a cathode biased amp you can connect it to the power tube cathode

If you prefer you can use a resistor voltage divider (+ an e-cap) to connect the wiper of the Humdinger pot to a DC elevated voltage (in the order of 40 or 50 volts)

The voltage divider can be feed by one node of the amp, about the value of the resistors to be used on the divider adopt values that will result in a low current consumption, do a search on the web to see reference values you can use

Franco

[Loomer]

If it is a cathode biased amp you can connect it to the power tube cathode

If you prefer you can use a resistor voltage divider (+ an e-cap) to connect the wiper of the Humdinger pot to a DC elevated voltage (in the order of 40 or 50 volts)

The voltage divider can be feed by one node of the amp, about the value of the resistors to be used on the divider adopt values that will result in a low current consumption, do a search on the web to see reference values you can use

Franco

Yes, this will be cathode biased, so that would be an option too. Was curious to see if one way (humdinger pot wiper to ground) is better than the other (wiper to cathode), or if we're splitting hairs and either will simply do the job well enough.

Had to put this on the backburner a bit due to work, flu and a weekend away but trying to focus again. Which obviously means, a few more questions for the lovely people of the forum...! 

Is it worth adding a heaters fuse? I'm definitely thinking about adding a mains fuse (either with a simple clip holder or possibly swapping the hard wired power cable to a fused IEC lead socket at the back.

Secondly, I'd worked out the original layout on the 18 rows because of available tag/strip boards. However, I hadn't fully factored in how small these are! At 117mm wide they are basically as wide as the circumference of a CD, which is pretty small. Is that going to drive me crazy or should it work well enough? Ultimately the 1w resistors aren't a huge deal larger than the 1/4w I'm used to using in pedals, but the electro caps are my concern. Perhaps going radial instead of axial might help to distribute the components better (and save some pennies)?

I have a turret tool and a few dozen turrets coming in the post today, so the alternative is using a pre drilled board and turret it up as needed. This might be handy as it will allow for extra space for the rectifier diodes and pretty much any size caps. The standard 18w-sized board would fit inside the radio and is one of the smallest sizes I can find, so would probably be working on one of those, which of course is more than plenty big for a champ!

Another option is using a JMP power board, which has 18 rows but is 4cm wider, alongside a chassis mounted bridge rectifier like these.

[Esquirefreak]

Why would you not fuse the mains? It's a very affordable insurance for your amp.

/Max

[Loomer]

Why would you not fuse the mains? It's a very affordable insurance for your amp.

/Max

Yup, the plan is to fuse the mains, but wondering on the merits of fusing the heater filaments, some amps do, some don't. My guess would be that a fuse is always good insurance and might be worth installing anyway, especially given the sockets are less than £2.

The talk of mains fuses raises one more question - how do I decide on the value of the fuse itself?

[Esquirefreak]

I've never had any heater meltdowns in any of my amps. Although, a fuse can never hurt. Especially with a lesser known PT.

As for the mains fuse - I think the stock fuse value in the Champ AA764 schematic would be a good starting point.

/Max

[PRR]

A heater fuse seems to be a fad in a 21st-century electric code (OK, late 20th). Manufacturers "must" obey. Garage shops can do what they like.

[Loomer]

Thanks both for your input on the fuses - going for the standard Champ value makes sense and good to know heater fuses aren't anywhere near as important as a mains fuse - I might omit for simplicity.

Pretty set on using the JMP power board with turrets at this point - should hit a sweet spot between compactness and ease of use and the added flexibility from the turrets' location makes for a tidier layout in my opinion, which is attached below and might need reviewing/revising.

This latest layout is using the EL84 as the output tube of choice, as my build will be.

I've also attached is the schematic by Kley de Jong I'm taking the cue from, which only differs from the original AA764 Champ past the 22n coupling cap to the power tube.

I've always been temped to add a "tweed" switch to bypass the tone stack but the obvious place to have it would be right by the B+T pots and I'm not too keen to drill the front of the radio. Might look into options for having it round the back. Initially I wasn't keen as I didn't want too long a wire going back and forth, though I'm now thinking as long as I find a way to have the wire be effectively in use only when the tone stack is off then no signal should be going through it and therefore the length shouldn't matter too much? Or am I off the mark?

[Loomer]

Busy few weeks meant slow to no progress.

My boards ordered from Modulus have finally turned up and after double and triple checking the layout posted above I’m confident I’m on a good path and will be hammering in those turrets. Definitely feels like a drill press would be a smoother way to do it, but the old hammer seems to do an ok job if a bit louder and likely time consuming.

The only doubt I have is whether I should have the extra filter stage before getting to the OT to minimise the risk of audible hum rather than after it?


Lastly, any word of advice on the rectifier I posted above? Looks within spec to me but would love a more expert opinion just in case..!

[sluckey]

Rectifier is way overkill but will work just fine. Don't forget to ground the negative terminal.

[Loomer]

Rectifier is way overkill but will work just fine. Don't forget to ground the negative terminal.

Yeah I figured it's way over spec but that should not be an issue. I quite like that I can just bolt it to the chassis and not need extra tagboard space for the diodes.

Wanted to finish putting the turrets in but since the board is quite wide at 8 rows I want to wait and have the components to see if I should go a little shorter to avoid using the whole length of the legs/not have much wiggle room to wrap the legs around the turrets; alas since I'm soon off to the family for the festive break and have the new road bike to set up in time for that it means it'll be a 2024 project...! At least I should have plenty of time to put together a full BOM for both this amp and the AC15/Matchless/18w hybrid I'm planning to build next...

By the way, I saw there is a Hoffman Champ AA764 turret board in the shop, but was unable to find documentation about it anywhere - does it exist somewhere on the site? Would be good to compare notes :)

[sluckey]

By the way, I saw there is a Hoffman Champ AA764 turret board in the shop, but was unable to find documentation about it anywhere - does it exist somewhere on the site? Would be good to compare notes :)

That's all you get. You'll have to look at the original Fender layout. Hoffman's board is based on the original layout, but the board will be 3.125" wide. The original is narrower.

[AlNewman]

My suggestion to you would be, if you are going to build a guitar amp from a radio, gut everything but the power supply, and maybe the power section/phase inverter.  Replace all the electrolytics.  Install a 3 prong cord and a fuse.  The rats nest will disappear in a hurry.  Most of those old radios have schematics, and you will need to figure it out in order to make whatever you're doing work.

You have lots of voltage, depending on what you want to build.

[Loomer]

Reviving this thread as I'm now starting to make inroads.

I've done the bulk of the gutting this evening and most of the components are in (apart from the elctros as the supplier didn't have the right value axials).

The headache now is removing the tube sockets as the things are riveted in, I managed to break one off and it wasn't too hard a task but then the overthinker in me started wondering what those wafers are made of - should I be doing this outdoors and get rid of the bits ASAP? I know some plastics used for high temperatures applications and wafers in the past aren't particularly lung friendly...

Layout looks good and the turrets are going in, only thing I'm still wondering about is the "mini choke" in the OT - is the wiring as per the layout above correct? ie having a filter cap before and one after it as you would with a regular choke?

A

[scstill]

I drill out the old rivets I want gone
slightly larger bit
works really good for me.

[PRR]

....what those wafers are made of - should I be ....

Bakelite. The un-cured phenol, liquid and powder, is toxic, one reason you can't readily buy it today. The cured product is so inert, it isn't even fattening. Maybe don't make a meal of it because I am sure they bulked it up with factory sweepings and stuff. Russian missile heatshields were phenolic and asbestos fibers; generally Bakelite dust is bad for you. HOT Bakelite releases formaldehyde, which is bad, and also pretty noxious thus obvious. So bust it up, not too fine, and throw it in rubbish.

[WimWalther]

For some strange reason I can never throw any of the old guts away.
Thinking it might come in "handy" some day. In reality its not usually useful.

Considered getting your amateur radio license? If you're habitually saving all that old crap, you're already well on your way! 

But really.. the tuning caps from old AM sets are good for building passive radiator loop antennae, basically DIY versions of the famous "Select-A-Tenna" which still pull high prices on eBay.

[Loomer]

Thanks both for chiming in! I'd thought about drilling them out, but if the bakelite isn't too serious a threat I found pulling them out quick and easy enough so might stick to that..!

As per keeping the old guts... That rat's nest was so gunk'd up by time I had to use the hand washing routine I normally need after working on the bike or car - it was filthy in there! Definitely starting to look better now it's (almost) all gone..!