Hoffman 18 Watt Stout TMB in Hammond AO 43 Chassis--MV Questions??

[rlh5599]

I have been wanting to build an 18 watt for a while.  I found a Hammond AO43 chassis on eBay for a fair price, and began my search for the right circuit.  I knew I wanted a single channel with a full tone stack and thought about a Dr Z or Matchless style, and then Doug finished his Stout TMB and I knew what I wanted. 
I am just a hobbyist, a 60 year old retired/disabled guy that likes to work on and play guitars and amps.  I am not a pro musician or a fully qualified tech, but I know enough to safely mess with tube circuits.
When I got the chassis, it had been stripped and all that was left were the transformers, tube sockets and new jacks.  The holes for the controls and 2 input jacks had already been drilled.

I was kinda stuck with 2 inputs so I added the second input, and arranged the controls with the holes available.

Put it all together, and it squealed like a banshee, so I reversed the OT secondaries and VOILA, sound!!

With a vintage RCA 5Y3, the voltages in the power supply are about 10 volts higher across the board than called for on Doug's layout, but it works, sounds pretty good through my Cannabis Rex, and it really THUMPS.  It's a little noisy with my P-90 tele, but I haven't really be able to ring it out other than at my work table, sitting under a bunch of cfls, right next to the amp. 

Next??  figure what type of MV to use.  Figured it would be best to get the circuit working before I started changing stuff around...

All in all, I think it's a success.  Thanks Doug!!

[Willabe]

I don't know how many amps you've built/rebuilt but this build looks mighty fine to me!


                  Brad     

[rlh5599]

I don't know how many amps you've built/rebuilt but this build looks mighty fine to me!


                  Brad     

Thanks for the kind words!! 
This is my fourth build, a MOJO Champ, MOJO Deluxe and a little SE 12V6/12AX7 champ style preceded it. 
Good thing I didn't pick up an expensive hobby, eh??

[EL34]

Nice build and good job figuring out to reverse the OT wires.

My layout diagram is correct for the 18 watt OT I sell but it's a crap shoot with other OT's

Keep us posted on your master volume mod

[kagliostro]

Nice Job 

K

[rlh5599]

So I installed a simple pre phase inverter Master volume (Similar to the Matchless) in my newly finished AO-43 Stout, and I'm not sure I like it.  It seems to be all or nothing, Goes from Off to pretty much full by the time you get to 2-3 on the dial.  I used a 1m audio pot, as seen in the layout below.  Also included in the layout below is the voltages as compared to Doug's layout.

Any thoughts or recommendations would be greatly appreciated.  I would like to have a little more "sweep" or travel.  If that can't be accomplished like this, any ideas??

Thanks!
Ralph

[sluckey]

That is called a cross-line MV. I had it on my Matchless Lightning and thought it was worthless. Here's a much better MV. It requires a dual-ganged 500K audio taper. I would probably remove those 470K resistors on the board or replace them with 2.2meg resistors.

[rlh5599]

Thanks, I'll give it a try!!

[rlh5599]

Waiting on parts, over-thinking everything as usual.  Doug's Stout TMB has a single input, my chassis had holes for two inputs.  So I wired a second jack in like a Fender Hi/Lo set up, and just added a 33kohm grid resistor inline to pin 2 on v1.  I notice most all 2 input arrangements use 68kohm grid resistor.  Do I need to change my 33's now that I have 2 inputs?  what would be the advantage/disadvantage to changing?
Figure since I'll be in it this weekend, if I need to change anything I'll do it in one shot.
Thanks!

[sluckey]

I notice most all 2 input arrangements use 68kohm grid resistor.  Do I need to change my 33's now that I have 2 inputs?  what would be the advantage/disadvantage to changing?

I don't clearly understand what you are saying. Do you have the typical Fender hi/lo jacks with a 1m and two 68ks and then you put another 33k between the junction of the two 68ks and V1-2? If so, you have a functioning hi/lo jack arrangement, but you don't need the 33K.

If you mean something else, please explain in more detail.

Here's a good drawing of the hi/lo jacks. The junction of the 68ks goes directly to V1-2.
http://el34world.com/Hoffman/files/CommonHookups.pdf

[rlh5599]

Oops, sorry, no 68ks, just the 1meg, and a 33k from each input, in place of the 68ks, then to v1. 

[EL34]

To wire dual jacks, see the PDF file that Steve linked to in the post above yours

[jojokeo]

Depending on the amp I've found that I don't always like the ppimv and the standard volume control after the tone stack works beautifully. But then if you want preamp distortion there's got to be at least one Gain/Drive/Vol control in there somewhere between a stage or two. I've also had to install treble bleeders on a ppimv especially with the 500k value (unless it's an overly bright amp but then that gets adjusted and it's back to the treble bleeds). It can also be nice to get some phase inverter distortion too - again it just depends on the circuit. In a circuit similar to the Stout I found it useful to have a volume control before and after the phase inverter since EL84s give it up so easily. Lastly, for future reference it's always good to make sure to check the data sheet before deciding on the value of the ppimv pot being used or just use near the same value of the resistors that are being replaced in the circuit. You don't want the grid returns to be higher than the data sheets list depending if fixed or cathode biased.

ps - for your enjoyment here's a couple AO43s from 3 or 4 years ago. 5879>12AX7>6BM8s and then a Dumble-ish front end married to an 18watt back end.

[lego4040]

That is a great looking amp, Doug's stout was my lastest build from his site and I added the master volume mod. Thank God my wife doesn't complain about the bill when I order my parts

[rlh5599]

Thanks to sluckey I got my MV wired up and it's just what I was hoping for.  I still have a bit of noise to work out, but I thinks it lies in the lead dress somewhere, or maybe a ground. This is a real cool circuit, and as I said before, it THUMPS! 

A question for jojokeo or anyone else who has used these transformers; did you use an artificial center tap for the heaters? (2@100ohm to ground from the heaters)
In any case, this has quenched my thirst for an 18 watt... at least for now.  Although looking at jojokeo's ao43 projects has my mind a'running. 
Still thinking about a BF Deluxe Lite, no effects, just TMB, and maybe an MV.....   Or a Harvard.....

It never ends, does it??

[sluckey]

That PT has a center tap on the filament winding. I'd just use that.

The AO-43 is a great platform for a BF Deluxe. It'll do a Harvard too but the B+ will be higher than the original.

[jojokeo]

Thanks to sluckey I got my MV wired up and it's just what I was hoping for.  I still have a bit of noise to work out, but I thinks it lies in the lead dress somewhere, or maybe a ground. This is a real cool circuit, and as I said before, it THUMPS! 

A question for jojokeo or anyone else who has used these transformers; did you use an artificial center tap for the heaters? (2@100ohm to ground from the heaters)

The MV issue you had sounds like you used a linear pot instead of a logarithmic pot. Typical characteristic when using the wrong taper type. As for noise I see potential in several areas plus having a very high B+ can also contribute on it's own too. These are robust PTs and put out more voltage than what's needed for EL84s and their capacity could easily run 6L6/5881s and I think EL34's if wanted. The OT would have to change then of course. Does the floor noise level lessen as you turn down the MV a bit?

Your heater wiring could be improved but as it sits now I drew a couple things on one of your photos. Shorten and move the B+ CT to the ground bolt noted and move the heater's CT to the cathodes of the power tubes. This will put a small DC voltage on the heaters' center tap and give you "free DC" help here. It appears that you may have the OT secondary wiring backwards at the output jack but hard to see clearly? See pic of the small wiring changes.

[rlh5599]

Thanks for the tips.  I redid the wiring as you suggested and no real difference.  I am not sure about the speaker jack wiring, I set it up as the layout I found online, (Black to +, and Green to -).   I am beginning to think it's just the dreaded 60 cycle hum.  The hum does vary with the volume controls, and of course it's worse with single coil pups than with HB's.
At idle, with a guitar plugged in, I get a noise like rustling papers when I turn both the MV and the Volume up past around 8 or 9.  Chances are I'll never play it that loud, but I just want to make sure I'm not blowing something up...

And Doug, I changed my pics to attachments, sorry 'bout the big pics..

[jojokeo]

The heater CT wire will help with heater noise if it's an issue, the B+ will help with something else and is the way to hook that up so you want to get into good habits there. This is beginning grounding and lead dress 101. Your heater wiring under your board is not good. Those wires coming out from underneath get near it and you failed to continue with the winding of them in the preamp where it's most important. Anything amiss at your early preamp stage gets re-amplified more & more as it goes down the circuit and amplifying path. You made your circuit board larger than necessary so you have what you have and there's nothing you can do about it the way it sits presently. There's a lot more current which flows when you turn up the amp, therefore you get more of the bad things which can happen accordingly and what you report. The "rustling papers" noise sounds like either you have a poor solder connection with your load resistors or maybe just because you used carbon comp resistors for these too. That's par for the course when you use those types for load resistors. The more you read and build the more you'll learn about this stuff. Just trying to help...

[rlh5599]

I greatly appreciate all the helpful advice.  I'll redo the heater wires at the preamp and check the load resistors, and recheck all my joints and grounds.  I knew I took the lazy way out when I did the heaters, and it's only one wire....  
live and learn..

[jojokeo]

When it comes to noises there's usually several things to look at and you go one at a time methodically. I've also seen the OT wires cause it being too close to other wires. Troubleshooting over the net isn't as easy as having something in your hands.

[rlh5599]

You are right, but your knowledge helps get me (us) pointed in a direction that may provide some success.  Would you recommend shielded cable on the input wires going to and from V1, and to and from the MV??

[jojokeo]

It can never hurt. What may not be an issue at home could be at another location. The chassis itself helps provide some shielding and the grid resistor along with miller capacitance helps with radio signals but whenever wires are close - especially low level high impedance, these are the most sensitive and possible or likely to be affected.

[rlh5599]

Thanks for all your help!!

[rlh5599]

So I got the heater wires for the preamp redone, no change, 
Moved the input to V1 and V1 to the volume away from everything, no change,
but I have noticed 2 things:

1: when checking the voltages; when I touch either of the 100k plate resistors (or pin 1 or 6 of V1) with the probe, the hum goes away.

2: I ran the wires from the .022 coupling caps, under the resistors at the cathode end of the board, then to the MV pot.

I checked all my connections, and everything that should be grounded is grounded, still sounds like an open ground somewhere, and it's not REAL loud unless it's cranked, and the treble knob is up... it's just annoying

[Willabe]

The green wires from the TS coupling caps going to the TS pots, at least 1 of them looks to be laying right on a B+ wire? Once the signal goes through the cap it is now a grid wire and will pick up noise.

Try lifting up the TS wires OFF of the B+ wire(S).

Also the input jack tip wire might need to be changed to a shielded wire, if it's not already? 


             Brad   

[Willabe]

Look at the 3 blue arrows in this pic. Lift them all up and away from the B+ power supply wire (PSU). 2 are green and 1 is black.


             Brad 
   

[rlh5599]

Thanks for the tips!
a couple new pics to show the current state of things...
my worry now is this leads to the MV, green and yellow in the pics.

[Willabe]

Would you recommend shielded cable on the input wires going to and from V1, and to and from the MV??

It can never hurt. The chassis itself helps provide some shielding and the grid resistor along with miller capacitance helps with radio signals but whenever wires are close - especially low level high impedance, these are the most sensitive and possible or likely to be affected.

What Jojo said. (He's talking about grid wires.)

At this point, after you have moved the TS wires off the B+ wires and put shielded wires on the input jacks, if it's still noisy, try shielded wire to and from the MV and see if that gets it.


                Brad     

[rlh5599]

OK, time to dig back in.  Would high plate voltage on V1, cause problems, and could it be adjusted with different resistors?
Plate voltage is currently at about 240.  Doug's layout calls for around 219..

and would switched output jack leads cause noise, like a hum, not a squeal?

[Willabe]

Plate voltage is currently at about 240.  Doug's layout calls for around 219..

Not much difference in plate voltage, not causing the noise problem. Yes latter you can fiddle with the B+ PSU dropping  R's if you want to drop it a little.

and would switched output jack leads cause noise, like a hum, not a squeal?

I don't think that's it either. (Disconnect the FB wire temporarily and see if it goes away.)

I'd try the MV wires 1st. After the PI coupling caps the wires to the MV and then to the output tubes grid R's become grid wires. They can/will pick up noise.

Process of elimination, knock them down 1 at a time until you find it. Of course it can be a number of things that all add some noise, so you got to find them all.

After the MV wires you might have to mess with the preamp tube grid wires, shorten and/or move.


                Brad        

[rlh5599]

Thank You Brad, for all your help!!  I'll just start knocking stuff off a little at a time...

[sluckey]

OK, time to dig back in.  Would high plate voltage on V1, cause problems, and could it be adjusted with different resistors? Plate voltage is currently at about 240.  Doug's layout calls for around 219..

That's not a problem. Your voltages throughout the entire amp should be higher than Doug's voltages due to the power transformer you used. That's OK.

and would switched output jack leads cause noise, like a hum, not a squeal?

This would be a negative feedback (NFB) issue. Reversed phase can cause a whole bunch of symptoms that range from subtle to howling, depending on the amount of NFB used in a particular circuit. When building a new amp with an untried OT, there is always a 50/50 chance of getting the NFB phase correct. The only way to know for sure is to reverse it. You should always check this on a new build, especially one where you are using a donor OT such as yours. And bear in mind that it may have been correct to begin with.

In the few years that I've been playing with this stuff I've seen a wide variety of symptoms related to reversed phase of the NFB loop. I've even seen one case where the phase was wrong for several years and the guy just thought that was the way the amp should sound.

EDIT... I see from your first post that you have already verified that the NFB is correct.

[rlh5599]

Thanks Steve.  It's just gonna take some more detective work.  As I have said before, I really like the circuit, and my problems should in no way be a reflection of Doug's design.  I'll get it worked out...

[jojokeo]

Don't know if this will be a cure for you but here's what I mean. You have the OT's ground (black wire) going to the positive/tip lug of the jack and it's also where you're taking your NFB wire from. You then are using the green OT wire as the gound. Normally black is always ground/common on an OT and sometimes it's even soldered to the OT's frame. See attachment.

[rlh5599]

thanks jojokeo, I am going to try that tomorrow...

[rlh5599]

Don't know if this will be a cure for you but here's what I mean. You have the OT's ground (black wire) going to the positive/tip lug of the jack and it's also where you're taking your NFB wire from. You then are using the green OT wire as the gound. Normally black is always ground/common on an OT and sometimes it's even soldered to the OT's frame. See attachment.

So I tried switching those wires, and it squealed like a pig.  I originally wired the OT like the layout I found online, and it shows green to ground, black to tip.
I am going to order some metal resistors, and some wire from Doug, and just change one thing at a time till I fine the culprit....

in the meantime, I just playin it, noise and all

[TIMBO]

Stick with it , your doin a great job.

[rlh5599]

So I've been away for a while.  Last time I had this thing running, the volume went to nothing, and V4 started to glow nice and red, so I shut it off, and put it away for a while.  I decided to take it apart and clean up the lead dress, move the controls to line up with board a little better, and narrow the board down a bit so I could see the sockets, and generally just make sure I didn't screw something up.  Last night I got it all put back together, and it lit right up (with new power tubes), it sounded great, and most of the noise was gone.  Today when I started it up, the volume sated out fine, the volume went to nuttin, and V4 started glowing again.  So I shut it down.
I am lost, don't know what to look for or where to look.  a shot of the layout is below, provided by sluckey.  only changes to this layout is; put 2.2m resistors where the "x's" are between the .022s and the MV leads, a second input as in Doug's notes, and the center tap for the heaters goes to the cathodes.
Any ideas would be greatly appreciated.....
 

[Glennjeff]

Hi rlh5599,



Just thinking out loud here. I take it that you used Doug's circuit diagram as that is what I will be reading from.


OK only one of power tubes ( V4 ) is red plating, so, something may have failed in it's biasing circuitry.


With amp disconnected from mains power and the power tubes V3 and V4 removed.
Temporarily remove capacitors C8 and C9 - (the amp will no longer have any volume.)

Triple check: R21 R22 R23 R24 R25 R26 and R27 with ohm meter for correct value and check that all those components connections are good and are getting to where they are supposed to go and have not been compromised by something like a short circuit.

Also check the master volume dual gang pot for any dropped connections, bent lugs, solder dags etc.

Check that the valve sockets pins are not worn, old looking, dirty or loose. Inspect valve sockets for any sign of heat or arcing like signs. Clean if possible.

IN other words I would check that anything that is connected to the power tubes is still what it should be, goes where it is meant to go and has not been "interfered with" by a stray piece of wire or a dag of solder (soder), loose connection, etc.

Check the underside of the circuit board as well.


With power connected
(Without tubes in) check that voltage on pins 9 and 7 of V3 and V4 sockets is a B+ 350 or so.

Remove from mains power, put all valves in and switch on.  Is it still red plating?

If it is going again without red plating then check and write down what voltages are on V3 and V4 at pins 7, 3, 2 and  9, they should be close to what is on Doug's circuit specifications. (APPROX Pin 7 = 340, Pin 9 = 348, pin 3 = 11, Pin 2 = 0 with respect to chassis / ground)

Remove from mains power, replace C8 and C9, reconnect and switch on. Is it red plating ?

Another thought. I recently had a hum problem with NFB that was actually an oscillation instability. I resolved this (temporarily) by putting a 1nF 600V capacitor between the anodes of the power tubes (one end of capacitor to pin 7 of V3, the other end of the capacitor to pin 7 of V4 ), but worry about that after the red plating issue is solved.


That is how I would approach it if it were my problem.

All the best
Glenn.

[jojokeo]

RLH - Glen gives a good systematic approach but you have your ppimv wired incorrectly. Remove the 2m2 resistors from the circuit board and put them directly onto the dual MV pot as in the corrected layout attached. The dual pot is replacing those (the power tube's grid return resistors) which for cathode bias should never be more than 1meg total. See data sheet attachment.

Doug's "X" on the original 470k resistors means to remove these - not replace them. Lastly, even though the ppimv's pot goes to ground, I don't like it since this is the power amp section and it's returning to the preamp grounds. Instead look at the green wire and put this ground where it belongs as shown.

This will most likely cure/fix your red plating (hopefully if nothing else is amiss?)

Keo

[rlh5599]

Thanks Guys!!  I'll check this all out, and rewire as suggested, and report back tomorrow!!

[Glennjeff]

Do what jojokeo suggests first. See if that error correction fixes both red plating and hum.

[rlh5599]

Thank You Keo!! 
Rewiring the MV solved most of my problems,
however, SOMEtimes, when I first turn it on (from cold, with no input) V4 tries to redplate, then it calms down and plays nice. 
I re-checked all the connections, voltages, and all the values, and everything checks out, EXCEPT the heaters are about 7.25 volts.  My electric service is about 125 volts~.  Am I going to have to lower the heater voltage? or would a standby switch help?

I still have a bit of noise, but I think I'm just going to start changing all the carb comp resistors to metal, and see if that helps.

As a side note, when I first got it back up and running, it still had a nasty hum, with some ticking, at high volume.  after I shut my soldering station off, all that went away....  (It has an internal shut off that counts down 1 hour after it's last use....)

Thanks again Glenn and Keo, you helped a lot!!

[sluckey]

...the heaters are about 7.25 volts.  My electric service is about 125 volts~.

See page 2 of this pdf. You may need to build one or two of these...

     http://home.comcast.net/~seluckey/amps/misc/Amp_Scrapbook.pdf

[rlh5599]

I may do that, Steve. 
I did isolate my "intermittent" redplate problem, though;
I just fired it up, from cold, and it started to redplate, I took a piece of wood, and wiggled the tube around, got an audible "pop" and it calmed right down.  Figger it's a bad socket, cincher, solder connection, etc.

My question NOW is, which pin would cause these symptoms? it happened with several different tubes, so I know it's the socket or something attached to it.  if I have an idea what to look for, I may be able to save a little further frustration...

when this thing runs right, through my cannabis rex, it's a MONSTER.....

[sluckey]

My question NOW is, which pin would cause these symptoms?

most likely pin 2.

[rlh5599]

My question NOW is, which pin would cause these symptoms?

most likely pin 2.

THANKS!!

[Glennjeff]

Hi rlh,

It still seems a little bit suspicious to me.
How does it sound when you play guitar through it, good stable tone or squeaky / fizzy.

Anyway, hope it's all well for you.

(Oh I see there were several posts whilst I was typing, you only have one small problem to resolve by the sound of it.
In my experience, IF there are problems with a tube socket it is best just to put in a new socket.)

Glenn

[rlh5599]

I think I got my socket problem figured out...

I don't have a spare socket right now, and I'd have to just about disassemble the whole thing to change it, so I checked and sure enough, pin 2 was lose in the socket.  You could see the pincher was open wider than all the rest.  I tightened things up a bit, and lit her back up, and no crackles, no redplate. I can wiggle the tube about with no noise or changes.  I'm gonna play it for a while, and next time I make an order for parts, I'll get a complete new set of sockets.  The ones in it came with the stripped chassis.  They were new, but I never liked them, they seem to wiggle around too much in the chassis...

geez, I just noticed I do have a spare, the empty one at the end of the chassis...  If I have any more problems, I'll change it out, otherwise I'm just gonna wait and get all new sockets.  If I have to take it apart to change one, might as well change em all.  Likely the problem will continue with the other sockets at some point...

Thanks again for all your help!!
(Gratuitous faceplate pic below)