Princeton 6G2 from Hammond L-100 (A-043) chassis

[sluckey]

Ah, ok. I guess that I wasn't sure if or how I should interpret the various part sizes. It would really be best to either mark them explicitly or create a  legend table of power vs. part size.

I thought the difference in size/shape of the 3W MO resistors and the 1/2W metal film was obvious. Both are scaled to the size of the real resistors sold by Hoffman. It's usually not an issue because my layouts are accompanied with a Visio schematic  and the resistor wattage is annotated on the schematic. Since I just used the original Fender schematic in your project, there was no way for you to really know what the wattage was. Anyhow, I've attached revision 3 just for you.

ETA: Using the extant gain pot in series with 8K25 puts us right where we want, with -15V near mid-range.  Resolution could be better, but it's what I'd call "acceptable" as-is. 1/2 of the pot track is doing only about 15mW, fine.

Sounds good, but more important is the range of adjustment. I would shoot for -10 to -20 for EL84s. Remember, you can change either/both of those resistors to achieve the desised range.

[WimWalther]

Sounds good, but more important is the range of adjustment. I would shoot for -10 to -20 for EL84s. Remember, you can change either/both of those resistors to achieve the desised range.

Right now it's at -8.3V to -19V.  Pretty close.

When I do the build I'll sub a 87K or 90K for the 100K, and that should put it very near the 10-20 range. Could also increase the 8K2 slightly if I have that value. Thing is, those pots are 20% parts, so..

[WimWalther]

A question has arisen..

On the original Hammond build there's an R-C network between the plates of the EL84 tubes**. This is shown as C314 & R335 in the layout - 0.001uF / 4K7. This gives a corner frequency of 33.8KHz

Do we know what this is doing? My guess is that it's possibly compensating some issue in the OT, like a tendency to ring at HF? It is a form of local feedback, correct?

So I'd removed these parts when I was stripping out the old circuit, but now I'm having second thoughts..

Any ideas?

(** or across the primary of the OT, to look at it another way.)

[sluckey]

You're building a 6G2 so forget about the Hammond circuit.

Don't worry. Be happy.

But if you really want to know search the forum for "conjunctive filter". Some more good info at the amp garage.

[WimWalther]

You're building a 6G2 so forget about the Hammond circuit.

While the Hammond circuit may be gone, the Hammond OT remains. My concern was that the R-C net is coping with some idiosyncracy of the OT.

[PRR]

A question has arisen.. ..... - 0.001uF / 4K7. This gives a corner frequency of 33.8KHz....

You (say) you don't know what it is. You speculate it may be specific to that OT. Proving that may make your head hurt (or your cat hate you). 34kHz is FAR FAR beyond the guitar band.

I'd leave it in for luck.

[WimWalther]

A question has arisen.. ..... - 0.001uF / 4K7. This gives a corner frequency of 33.8KHz....

You (say) you don't know what it is. You speculate it may be specific to that OT. Proving that may make your head hurt (or your cat hate you). 34kHz is FAR FAR beyond the guitar band.

I'd leave it in for luck.

Yes, I was concerned that it might be related to some issue with the OT, ringing at a frequency above the audio band or some other HF artifact. The fact that the corner frequency is 38Khz supports this idea.

Having read (very briefly) on conjunctive filters, it seems that this is not necessarily the case. At this point, I'll just go sans-filter and see what the oscope and my ears tell me once the amp is running.

But two things are for sure - the Hammond guys thought it was needed or it wouldn't be there, and it's tailored to act in a range almost a full octave above the commonly accepted audio band.

I just like to understand why I do what I do, that's all. Knowledge is power.

[WimWalther]

Not much here, just a footswitch (tremolo) made up from parts I already had.

[dude]

That OT is fine as it is, no unusual noises in guitar amps.  l have use that OT in many amps, even a Fender Blues Jr, it sounds great with 6v6’s and EL 84’s.
Build the amp, play, let us know what you think, or if the dogs bark.
Cheers, the dude 

[WimWalther]

That OT is fine as it is, no unusual noises in guitar amps.  l have use that OT in many amps, even a Fender Blues Jr, it sounds great with 6v6’s and EL 84’s.
Build the amp, play, let us know what you think, or if the dogs bark.
Cheers, the dude 

That's currently the plan, just waiting on parts and for my friend to do the metalwork so the new controls can be installed.

Funny you mentioned the Blues Jr. Years ago I needed an OT for my Zenith 10s669 radio (2x 6V6). A friend suggested that there should be plenty of stock BJ OTs out there, as many of them get replaced with better quality parts.

So I asked on some forum and just like that, I had one for about $12 shipped. Turned out that it bolted right up where the old Zenith part had been, couldn't have been easier. Still sounds just fine today.

[PRR]

> bolted right up where the old Zenith part had been

I often suspect the one or two 6V6 Fenders were pre-war "Parlor radios" at the core.

[WimWalther]

I think the boards are done..

Sprague 715P OD in the signal path, Illinois MPW/MWR everywhere else, except 500pf micas & orig. Hammond 0.047uF in eq network.

Mix of orig. & NOS Xicon carbon resistors in signal path & eq, metal films everywhere else.

25@25V Lytics are ???. Anyone recognize that "U" logo? No idea, but I had a bag full, so..

Reused orig. wire where possible.

[WimWalther]

Here's a thought..

Toying with the idea of adding a presence control to this 6G2-ish amp. As I understand, it's simply adjustable NFB?

So.. if the stock FB resistor is 56K, should I do something like 100K (lin) pot + 1K? 5K? backstop resistor? That easy?

[sluckey]

More to it that that. There's a cap involved also that causes the NFB to be frequency dependent and act similar to a tone control.

[WimWalther]

More to it that that. There's a cap involved also that causes the NFB to be frequency dependent and act similar to a tone control.

I'm seeing that. Looking at the 6G14 Showman circuit (chosen at random).. but its definitely not suitable as it uses a totally different phase splitter than the 6G2.

Which model, if any, would have an applicable pres control?

[66Strat]

Fender 5E7 Bandmaster implements a presence control to a cathodyne PI.

[sluckey]

Fender 5E7 Bandmaster implements a presence control to a cathodyne PI.

Perfect! Just what he needs. Very easy to just tie into the existing circuit.

While browsing the schematics library just now, I noticed that many of the 5Exx amps use that same presence control with a cathodyne.

[WimWalther]

Fender 5E7 Bandmaster implements a presence control to a cathodyne PI.

Yes, that looks like a very good possibility.. though one question arises.

Notice that the 1K5 Rk in the 5E7 Bandmaster is unbypassed? I don't think that I'd have to remove the bypass cap in the 6G2 circuit.. but I'm not sure. The 5E7 has an additional gain stage or two up front, so maybe they just didn't need the extra gain that the bypass would provide.

Guess it just needs to be tried. It's certainly easy enough.. just a pot + cap.

[sluckey]

The 6G2 is not bypassed either. The NFB loop would not work if it was bypassed.

[Toxophilite]

I think the boards are done..

Sprague 715P OD in the signal path, Illinois MPW/MWR everywhere else, except 500pf micas & orig. Hammond 0.047uF in eq network.

Mix of orig. & NOS Xicon carbon resistors in signal path & eq, metal films everywhere else.

25@25V Lytics are ???. Anyone recognize that "U" logo? No idea, but I had a bag full, so..

Reused orig. wire where possible.

I'd ditch all the noisy flammable carbon resistors and use nice quality flame proof modern ones. The whole old carbon resistors have a magical sonic quality thing is nonsense in my opinion. I've built several fender style amps using Hammond organ chassis, transformers and their turret boards , Love to reuse that stuff, but using old components?? There's a reason they have best before dates. Would you put NOS tires on your car?


My amps sound just as good as any from the factory fender amp. Remember Leo Fender wasn't a guitarist, he was a business man and he bought whatever parts he could get cheapest that would do the job, This goes for everything, speakers etc. etc.


THere is a Fender amp built around EL84s, run very hot. I think it was a 60s tremelux (could be wrong as to the name tremesomething I thing) Somebody was comparing it to a similar 6V6 amp and he said they both sounded like fender amps.  I built my stereo deluxe reverb using Russian 7189 Equivilents(uber El84) to save space and try it and It sounds no different than the deluxe I built using 6V6s.

[WimWalther]

The 6G2 is not bypassed either.

You're right, thanks for catching that. I was looking at the wrong cathode circuit..

The NFB loop would not work if it was bypassed.

That's what I was struggling to recall. But, yeah - with the cap present, it would shunt all of the NFB to ground.

[WimWalther]

I'd ditch all the noisy flammable carbon resistors and use nice quality flame proof modern ones.

Shrug, maybe. I just used what I had, that still measured in-spec. There aren't but a handful of old CCs in there, easy enough to change.. if I feel like it.

Thing is, if I'd have gone with the original plan (make a pair of mono audio amps), I'd have most likely left damn near everything in it.. and it would probably have been fine.

As for the sound of flameproofs, maybe I'll try them myself some day. Meanwhile, I'll let you enjoy them all to yourself. ;-)

[sluckey]

None of the resistors on that board will ever get even warm. No need for flame proof. The only place I would use metal oxide flame proof resistors in that amp are the places on the layout where I put those 3W resistors.

[Toxophilite]

I'd ditch all the noisy flammable carbon resistors and use nice quality flame proof modern ones.

Shrug, maybe. I just used what I had, that still measured in-spec. There aren't but a handful of old CCs in there, easy enough to change.. if I feel like it.

Thing is, if I'd have gone with the original plan (make a pair of mono audio amps), I'd have most likely left damn near everything in it.. and it would probably have been fine.

As for the sound of flameproofs, maybe I'll try them myself some day. Meanwhile, I'll let you enjoy them all to yourself. ;-)

ah my apologies, seems like you're building with what you have at hand, fair enough. I have to admit I don't believe resistors have a sound, some people do believe that though.

[WimWalther]

Metalwork is done on first chassis. Should be getting it later today. Unfortunately, it looks like I'm going to the hospital today.. and I expect to be admitted. Not very happy about it.

On the bright side, Steve does nice work, I hope you'll agree. Nothing fancy, but neat & workmanlike.

The lefthand socket had to rotate 90' so that the 6E2 bar display would be visible from the front of the amp. I'll need to figure out how to build the driver circuit for it at some point.

ETA: And.. total humiliation! The pot I gave him to size the holes has a 5/16" bushing, but all of my new pots have 3/8" bushings for some damn reason.  Do it again!

[PRR]

> it looks like I'm going to the hospital today.. and I expect to be admitted.

My housemate went to hospital this week, but escaped admission. Good luck to you.

[WimWalther]

Dodged the hospital, but I may end up there yet.. man do I hate this crap.

Anyway, on a far more interesting subject, I need a quick opinion..

I'm adding the very simple (1 cap + 1 pot) Presence control to my 6G2 build. Question is, where to put it? The stock control lineup is:

Vol, Tone, Speed, Int.

I'm thinking:

Vol, Tone, Pres, Speed, Int.

But the Bandmaster has it last in the row, so:

Vol, Tone, Speed, Int, Pres

Thoughts on which makes better sense?

[sluckey]

The BM presence pot is the last in the row because of where it connects to the circuit (PI cathode tail). The 6G2 Presence will connect to the cathode of V1, so Vol, Tone, Pres, Speed, Int may be a better lineup. However, if I was doing it, I'd put the presence pot on the back near the speaker jack. The Presence is a control that you will twist a lot the first week. Then you'll probably touch it very seldom after you find your sweet spot. So, put it outta sight.

[WimWalther]

I've already placed a hole for it on the front panel, so maybe we'll just go with Vol, Tone, Pres, Speed, Int.

[WimWalther]

@sluckey

I'm getting to wiring the board into the chassis, and it seems that you might want to revise your drawings.

You have the controls on the "back" of the amp, opposite the side with the tubes. Problem is, the board mounts to that side and there's really no way around it. The controls need to run along the same side as the tubes, as the board can't mount over top of the tube sockets or the control pots.

Or that's how it's shaped up.

[sluckey]

@sluckey

I'm getting to wiring the board into the chassis, and it seems that you might want to revise your drawings.

You have the controls on the "back" of the amp, opposite the side with the tubes. Problem is, the board mounts to that side and there's really no way around it. The controls need to run along the same side as the tubes, as the board can't mount over top of the tube sockets or the control pots.

Or that's how it's shaped up.

It was your choice to leave the board mounted on the side of the chassis. If you had followed my layout this would not be an issue.

[WimWalther]

@sluckey

So where did you anticipate the board should be mounted? It's not clear from your documents.

Underside of the chassis deck? When I looked at that option, it seemed prone to crowd either the controls or the sockets.

Then there's the aesthetic thing.. controls on the other side putz the tubes in the rear.. where you can't admire them! ;-)

ETA: Metalwork has only been done on the first amp. I can always try a different scheme for the second, though I might have to drop the eye candy tube.

[sluckey]

So where did you anticipate the board should be mounted? It's not clear from your documents.

Look at Dude's post #12. There's a pic of how the board would be mounted if you had followed my layout.

I thought my documents were crystal clear. Controls on the front, tubes lined up across the back, and board in between. Those three things work together to form a logical, well thought out layout. Change any one of the three and you could end up with a spaghetti dinner.

Really not gonna make much difference with this layout though. Since the input jack and volume control will be located very close to V1, just connect the 68K directly between the input jack and V1-2. Now take the .022 cap off the board and connect it directly between V1-1 and the high side of the VOL pot. Finally, connect the VOL pot wiper directly to V1-7 with a short unshielded jumper. Run the filament string flat against the chassis to keep away from all these changes. This will take care of all the sensitive stuff. There are only three more control wires to deal with and they are all non sensitive tremolo wires. Just run these three pot wires neatly and directly to the board.

All this should work out just fine. Get the first amp up and running ***BEFORE*** working on the second amp. You may just want to use these same changes again.

You're on your own with the magic eye tube. I don't think you have shared how you intend to interface that.

[dude]

That’s a narrow chassis, you can do what Sluckey mentioned or (if you used sluckey’s cone layout) just reverse the front and back, take the board off those two screws and use standoff and mount the bd as in my photo in reply #12.
You’ll maybe have to put metal plates on the front and, or back, drill new holes, yeah pain in the ass but a much neater build, you don’t want wires running like a rats nest. The side with the original Hammond bd attached is almost all sheet metal with no holes so no plate needed, hopefully you didn’t drill it, if so, you’ll have to use cover plates, maybe same will the other side. This is the reason l move the board off the side of the chassis. I explained in one of my posts that you have to be careful where you mount the bd on standoffs, or you won’t be able to get the pots out, look at my layout, open it in my post for a clear detailed photo. Also, if you take the bd off the side use 3/8” standoffs, those cones are long.
Plus, the 6G2 has a lot less inside than my 6v6 plexi l stuffed into that chassis.
 

[sluckey]

Hey Dude, your 6V6 Plexi board looks very familiar.   

Still got all those AO-43 chassis?

[dude]

Hey Dude, your 6V6 Plexi board looks very familiar.   

Still got all those AO-43 chassis?

Familiar, yeah I'm gloating again  .  But took a lot of time getting that chassis to look like it does, I wish I had a layout for that AO-43 chassis and board I cut down (from AES) to fit, hint, hint . Don't know the exact size of the board but you can count the lugs in the photo. I'm trying to get it together to run that program but the old man ain't as fast as he use to be, .
I have four complete untouched chassis's left, three more without just PT's (that PT is a nice fit for a lot of amps) and several OT's.

The Plexi sounds great, no hum, hiss all the way to 10. It's really a Fender Bassman with two volumes.., right. You have to crank it to get the Marshall crunch but a good pedal works well for lower volumes. I find that Stew Mac's Ghost Drive works very well with that amp, it's just like the original Klon.  Cheers and thanks again, from everyone on this forum for all you help, I wouldn't be where I'm at with your help.BTW, I'm in the process of building a 6G2 using your layout with the Hammond AO-43. All the work you did, I had to build it.

[sluckey]

Dude, what are the exact measurements (LxWxH) of the AO-43 chassis?

[dude]

Dude, what are the exact measurements (LxWxH) of the AO-43 chassis?

OK, I'll have to open the AO-43 Plexi, and give you accurate measurements of chassis also length, width of the board too, I'll do it later today and post, Thanks

[WimWalther]

You're on your own with the magic eye tube. I don't think you have shared how you intend to interface that.

Here's the circuit, it takes signal straight off of the OT sec. Most components can mount on the socket & adjust pot. Maybe one additional terminal might be required.

[dude]

Dude, what are the exact measurements (LxWxH) of the AO-43 chassis?

Hammond chassis AO-43: Length, 16", Width, 5", Height, 2"
AES stock bd, cut to: Length 8 5/8" Width 2 1/4" (8 3/4" L will fit)

Spacing of holes in board, 3/8"

Note: I put in a cathode/fixed bias switch in the fuse hole and internal fuse but never use it, skip it if you can see it. Also, I put in a 750 ohm screen R if I decided to try 6L6's, skip it, use 470 ohm. 5AR4, 6V6's, put two bleeding R, probably not needed.Would like to print it on a 8 1/2 X 11 paper.Thanks

[sluckey]

Here's the circuit, it takes signal straight off of the OT sec. Most components can mount on the socket & adjust pot. Maybe one additional terminal might be required.

That looks easy and straightforward. I'd like to see it in action.

Did you like my suggestions?

[WimWalther]

That looks easy and straightforward. I'd like to see it in action.

With a bit of luck on my part, you may just have that opportunity! =)

BTW, for the unaware, those display tubes can be bought NOS off AliEx for $2/ea. The chi-coms must have made a million of them, no idea what their application was.

Did you like my suggestions?

Honestly, I read over but didn't really process them. I'm already past that point in the build, so will go forward as planned. If I should have any issues, then I will give them full attention..

And when it's time to finish the second unit, everything that's been discussed / suggested so far will be reviewed.  All of your input, and of others, is very much appreciated.

[WimWalther]

I actually got to hear it make some noise last night. Still need to build a bias supply (using a bench supply atm), hook up the NFB, adjust the trem circuit and sort out some other general weirdness.

What range should the stock trem osc have? The lowest speed is now around 2Hz, I'd prefer it to be more like 0.5Hz..

Seems like there might be an issue with the PSU. If the input is overdriven, B+ crashes down to ~200V and the amp goes mute for several seconds until it recovers. Could be oscillating, but it acts like the supply impedance is way too high. Caps are original, but check out fine fwiw

[sluckey]

What range should the stock trem osc have? The lowest speed is now around 2Hz, I'd prefer it to be more like 0.5Hz..

3 to 8Hz is typical. Replace that .01 that's connected to the speed pot with a .02 to lower the speed. I've never been able to get down to 2Hz with this circuit and I've never seen any guitar tube amp phase shift LFO get down to 0.5Hz.

If the input is overdriven, B+ crashes down to ~200V and the amp goes mute for several seconds until it recovers.

What happens to the ACV on pins 4 and 6 of the rectifier tube? (Check both pins) If the AC remains constant then a fresh set of filter caps will likely fix that sag issue.

[WimWalther]

@sluckey

Years back I rebuilt an Elk amp head. They're a Japanese Fender copy, all-tube, though I'm not sure which specific model (if any) it replicated. My recollection is that it had a very slow trem setting, sub-1Hz.. but my memory could be off.

Agreed on the filter caps. I'll tack-in another 50uF on the 1st pole and see if it changes.

ETA: Just caught an error. Left out the 1K grid stops on the EL84s. That can't help things.

ETA2: Also left out the 1st stage 1M grid / input resistor.. that's why it kept going mute. The stage was going into grid cutoff when driven hard. Oops.

Amp is fairly quiet, despite not having installed any shielded cable. I'll probably do so anyway.

Trying to squeeze in a bias supply without adding any new terminals.

ETA3: Bias supply is in, EL84 grids are at -15V, B+ is 330V, Ia ~25.5mA. Tubes are doing about 8.5-9W/ea. Pa. Went with a fixed voltage setup, at least for now. Bias network values are 136K+15K, 22@160V cap across the 15K to gnd

Hey, if it was good enough for Leo Fender..

[sluckey]

A convenient place for the grid stoppers is on the socket between pins 2 and 8. Like this...

[WimWalther]

A convenient place for the grid stoppers is on the socket between pins 2 and 8. Like this...

I looked at that possibility, but none of the unused pins on the EL84 are really "free". Pin 8, like the others is marked "IC" (internal connection) as opposed to "NC" (no connection).

So in my case, I did the same thing Hammond did - ran my (1K5) grid stops direct from the board to Pins 2.

If you're confident that it's safe to borrow pin 8, I might use your technique on the second unit.

However, I did use two "NC" pins on the 5Y3 socket to build the bias supply. Those two pins plus a ground were sufficient to implement it.

[sluckey]

Grid stopper would be most effective if the resistor is mounted very close to the grid.

Hold your EL84s up to a light. Do you see anything internally connected to pin 8?

[WimWalther]

Grid stopper would be most effective if the resistor is mounted very close to the grid.

Yes, I'm aware.

Hold your EL84s up to a light. Do you see anything internally connected to pin 8?

I will at the next opportunity.  Is it common EL84 / 6BQ5 practice to borrow pin 8 for this purpose?

In case you didn't catch it, the muting issue was due to a missing grid (leak)  1M on the 1st triode section. It was going into grid cutoff when driven hard.

[sluckey]

I will at the next opportunity.  Is it common EL84 / 6BQ5 practice to borrow pin 8 for this purpose?

It is for me. The few layouts I've seen either have the grid stoppers mounted on the board or have them soldered to pin 2 with the dangling end soldered to a wire going to the board (I don't like that idea). I've never seen pin 8 internally connected. Hoffman puts the grid stoppers between pins 1 and 2. I don't like that because I have a pair or RCA 6BQ5s and pin 1 is internally connected to pin 2. So, if wired as Hoffman does, the grid stoppers would be useless, shorted out. Probably never even know in a low gain amp.

In case you didn't catch it, the muting issue was due to a missing grid (leak)  1M on the 1st triode section. It was going into grid cutoff when driven hard.

I saw that.

So, what are your impressions with the amp? Did the presence pot mod work as expected? Anything you don't like about the amp?