Airline 8504 restoration now underway.

[1blueheron]

Hoping to fix the wiring issue with ch. 2 tonight and test.  If all is good I will attempt Sluckey's mod.

By performing the mod, will that make the volume control for Channel B like a gain control and the volume for channel A like a MV?  It appears I am just feeding Channel B output back into Channel A input correct?  I assume this will make channel A "normal" and channel B "High Gain" and when playing in normal mode Channel A,  the volume and tone should be turned down on Channel B.  Is this correct or doesn't it matter?

[sluckey]

According to the schematic, channel A connects to pin 7 and channel B connects to pin 2. Since the channels are identical it makes no difference what you call them as long as you know which channel you are talking about. I'm gonna use the channel designations shown on the schematic so I will probably stay confused.  (Wish you would use the schematic designations too.)

About the mod... The answer is yes to all your questions except it should not matter about turning Channel B vol/tone down when using Channel A. This is a simple/easy mod. If this mod proves to be useful I would replace the three input jacks for Channel B with a single switching jack wired so the jack grounds the B input when not used. Or, wire a simple Fender two jack (HI/LO) input. Probably a good idea for Channel A also.

[1blueheron]

Got it.  Thanks.  I will from hereafter refer to the channels as designated on the schematic and try to prevent further confusion. 

[1blueheron]

I dug into it last night for a bit.  Fixed the issue I was aware of but it did not fix the problem.  So that sent me troubleshooting.

Found I only had 90V at one of the plates.  The other side was reading 2V.  Oops.  Whats wrong.  Well it appears I connected R27 to pin 8 rather than pin 6.   That was dumb.  Gotta get better at checking myself against the schematic.

So once I found that, I set about to fix it, but then decided I would like to make the caps and resistors more accessible than what they currently are so each time I change one for experimenting/modifying, my wires don't keep getting shorter from the amp chassis. I don't have any length to spare on the heater wires.  So I started to redesign the board that they are mounted on.  I almost have a new board finished with some standoffs for all the wire terminations from the amp chassis, the caps, and the resistors.  Should have it finished up tonite with any luck and hopefully everything will work right this time and I can move forward.

I guess I am making some good troubleshooting practice for myself....  Learning a good bit in the process.

[sluckey]

I guess I am making some good troubleshooting practice for myself....  Learning a good bit in the process.

That's good. See how checking tube voltages can lead to a problem? It's really nice to have an identical circuit that's working so you can compare voltages. When you have the amp running properly consider measuring voltages on all tube pins and write them on the schematic for future reference. I even like to write down zero volts.

[1blueheron]

The amp is now working 100%.  Put the new board in this AM and both channels worked great.  Did the suggested cascade mod and wow.  Has some gnarly tones.  Still need to get everything labeled and I am going to check all voltages and document for future reference.

I think a new 10 or 12" speaker and a solid pine cabinet would really make this amp sound good.  we are getting a bit of non-harmonic distortion from bass notes when its cranked up a little. Might be the metal grill rattling or possibly the speaker has some issues to check into.  It is pretty bass heavy.  Hers some pics of the new board. 

[sluckey]

Nice job!

Did the suggested cascade mod and wow.  Has some gnarly tones.

So, will the mod be useful? I have some ideas to add a bit more gain or tame the gain a bit if you want to experiment. Your new board would make it easy.

[1blueheron]

Yes, The mod is very useful.  Thank you.  My son says it has some "early Marshall" tones.

I am open to further ideas.  I have only 2 input jacks now rather than 6.  Need to switch them out to shorting style.  Need to fill the unused holes to make it look decent.  Was considering making the cascade mod switchable by putting a selector switch in one of the jack holes.  Then I can run it as stock real clean, or switch to the mod for high gain.  You probably can see that I moved the 68K and 1M resistors from the jacks to the new board to shorten the length of signal exposed.  I am going to make the wires from the jack to the board shielded.  Once I get a self shorting jack, I am considering changing value on the 1M and adding a capacitor.

In the cascade mode, we are getting some nice breakup but have more than enough volume.  If I put in a DPDT 3 position selector switch as mentioned above and an attenuator resistor on the speaker output that engaged only in the high gain mode might tame some volume while still giving good breakup?  Or maybe lower the plate voltage on other half to encourage an earlier breakup?  I don't know whats your ideas?  Enjoying this amp as a test bed for ideas.

[sluckey]

I am considering changing value on the 1M and adding a capacitor.

Why change the 1M? That's the most common value used on the input jack. I'd let it be. What cap are you thinking about adding?

Switching will be very simple with a DPDT switch. No need to do anything with speaker output. You don't really want speaker signal up there in the preamp chassis.

You should be able to use CH B volume as a gain control and use CH A volume to set the volume level.

[1blueheron]

OK, scratch the change to the 1M.

Does this look like a reasonable set of mods?

Change 68K gridstoppers to 10K and compensate with 200pF caps.  Supposed to have lower hiss levels?

Install switchable (S1) cold cathode clipper using 10K resistor on cathode of V1B.  Supposed to achieve smoother/earlier overdrive?   Will this work or do I need to make additional changes if I do this?

Install switch (S2) to make amp switchable between stock and cascaded mode.

Thoughts?  Dumb idea?

[sluckey]

That will work, but... I don't think hiss will be a problem so I would drop the 200pF caps. I also don't think the 'cold clipper' is needed because this is not a real high gain mod. Cold clippers are usually seen when you have three (or more) gain stages.

Here's what I had in mind...

[1blueheron]

OK,

That looks simple enough. 

What should I expect the optional 2.2uf caps do in terms of tone?

Removing the .003 cap on the tone pot will render that tone pot disabled correct?  I am not connecting the pot wiper directly to grd?

[sluckey]

The optional cathode caps will give you a fatter tone and also a bit more gain.

Removing the .003 cap on the tone pot will render that tone pot disabled correct?  I am not connecting the pot wiper directly to grd?

Yes and yes. I would rather just disconnect one end of the jumper between the VOL and TONE pots. Easy to reconnect if you think you need it.

You can also try your hiss caps and cold clipper switch in my schematic.

If you try my circuit, don't overlook the diagonal jumper on the switch. It is required for proper operation.

[1blueheron]

What voltage rating is needed for those 2.2uf caps?

[sluckey]

16 or 25. They will actually only have a couple volts on them.

[sluckey]

I think I see a problem with your new board. Look at the attached pic. If that screw is used to mount the board to the chassis, there's a problem. Measure resistance from pin 2 to chassis. If it's only 68K then you indeed have a problem.

Easy solution would be to use a nylon screw.

[dude]

How did you clean that chassis, looks great?

[1blueheron]

I think I see a problem with your new board. Look at the attached pic. If that screw is used to mount the board to the chassis, there's a problem. Measure resistance from pin 2 to chassis. If it's only 68K then you indeed have a problem.

Easy solution would be to use a nylon screw.

Sluckey,

The screw you mention does not touch the chassis.  In fact the entire board floats on little rubber grommets.  I believe the purpose in this was vibration isolation to avoid microphonics?  Anyways, that is why I ran ground wires back to the chassis/jacks.  The standoffs and board is isolated.   Thanks for taking the time to look behind me though.  It is much appreciated as I am still making plenty of mistakes. 

I am going to check my parts bin tonight and see what I have in the way of low voltage 2.2uf caps.  I found a DPDT switch last night, but it is ON_OFF_ON.   Will this cause popping or will it work fine? Do I need to order a On-On?

[1blueheron]

How did you clean that chassis, looks great?

Dude,

Thanks,

First I get all the loose mess an dust off with a paint brush.  That got all the dog hair, crud, etc off. Then a soft cotton cloth dampened with some water and mild detergent. Then I came back and used some Mothers Aluminum polish to remove the oxidation.  That stuff is the bomb.  Brings back a mirror like finish and didn't seem to bother the stenciling on the amp.  I used a Dremel tool with wire brush to clean up the nuts/shafts on the jacks and pots as they were all showing some oxidation.

[sluckey]

The screw you mention does not touch the chassis.  In fact the entire board floats on little rubber grommets.  I believe the purpose in this was vibration isolation to avoid microphonics?  Anyways, that is why I ran ground wires back to the chassis/jacks.  The standoffs and board is isolated.   Thanks for taking the time to look behind me though.  It is much appreciated as I am still making plenty of mistakes.

I understand about the grommets acting as a shock absorber, but they look dry and hard to me. But... The screw head is in contact with the terminal strip lug and I bet the other end of the screw and nut is in contact with the chassis. Make the resistance check I asked about. I bet it is 68K.

I am going to check my parts bin tonight and see what I have in the way of low voltage 2.2uf caps.  I found a DPDT switch last night, but it is ON_OFF_ON.   Will this cause popping or will it work fine? Do I need to order a On-On?

2.2µF is not a magic value. You can try anything from .68µF up to 25µF. The bigger caps will give more bass, maybe even too boomy for some people. And you can use different values for each cathode too. That switch will work. Just remember that the center position will kill all sound.

[1blueheron]

Thanks for the answers.  Just ran the check on the screw/chassis ground.  It is isolated.  Meter registers nothing between Pin 2 and ground.  This was with the board in place.  If I check the nut to chassis, I've got 69k so it seems everything is correct.

Found some 2.2uf 50V caps. Will be giving that a try.  Will try some other values as well just for grins and giggles.

Also going to hook up a 12" speaker and see how that sounds in place of the 15". 

[sluckey]

Thanks for the answers.  Just ran the check on the screw/chassis ground.  It is isolated.  Meter registers nothing between Pin 2 and ground.  This was with the board in place.  If I check the nut to chassis, I've got 69k so it seems everything is correct.

I don't think we are communicating very well. Pin 2 to ground should read 1M. How can the nut possibly read 69K to chassis? Isn't the nut in contact with the chassis? Can you post a pic that shows the end of the screw and nut on the other side of the chassis?

[1blueheron]

I am sorry,  maybe these pictures will explain it better?  At no place does the screw or nut contact the chassis.  There is rubber between all the parts. Screw goes through grommet which has rubber inside chassis hole, nut screws against outside of grommet and does not touch the chassis.  Chassis ground connections for pre-amp are through the indicated black and green wires which connect to the ring terminal of the two input jacks.

[sluckey]

Screw goes through grommet which has rubber inside chassis hole, nut screws against outside of grommet and does not touch the chassis.

Ding! That was the missing link for me. I got it now. Sorry for my confusion.

I see a couple issues with your board but I'll do a separate post for those. So, have you tested the cascade mod with your new board? Any issues?

[1blueheron]

Screw goes through grommet which has rubber inside chassis hole, nut screws against outside of grommet and does not touch the chassis.

Ding! That was the missing link for me. I got it now. Sorry for my confusion.

I see a couple issues with your board but I'll do a separate post for those. So, have you tested the cascade mod with your new board? Any issues?

I took the preamp apart last night hoping to start on the new mod.  Got as far as trying to put my DPDT switch in but realized the switch shaft size is too large to fit in the empty 1/4" jack hole.  I really want to avoid doing anything irreversible to the chassis like boring larger holes so I am trying to find/buy a switch that will fit in stock hole. 

At that point I took some pics and called it a night. 

What has been tested so far is the output of channel b tied into the input of channel a.  That worked fine fed from separate input jacks. 

Have not lifted tone control, installed cathode caps,  relocated the 1M or installed the DPDT switch.

[sluckey]

You need a mini DPDT switch like Hoffman sells. The switch fits a 1/4" hole so you will need a bushing with 1/4" ID and 3/8" OD.  A piece of 1/4" ID copper tubing should work. Look in plumbers supplies. You could even use a piece of 1/4" plastic/rubber fuel line or air hose.

About that screw/nut... That screw is in contact with the terminal lug that will connect through a 68K to the grid of the first triode. If you touch the end of the screw/nut you will get a buzz just as if you had touched the tip of the plug on a guitar cord. That screw/nut is like an antenna and it "may" pick up radiated noise from the transformers, house lighting, radio stations, etc. I still recommend using a nylon screw.

I've put together a drawing that shows my mod ideas and also addresses a couple issues I see with your board. Should be easy to work into your new board. The cascade switch is wired the same as Hoffman's hot switch. If you have any questions, just ask. I'm following this project with interest.

And if you decide to use another plan that's OK too. Still take a look at the attached pdf. It may give you some ideas.

[1blueheron]

You need a mini DPDT switch like Hoffman sells. The switch fits a 1/4" hole so you will need a bushing with 1/4" ID and 3/8" OD.  A piece of 1/4" ID copper tubing should work. Look in plumbers supplies. You could even use a piece of 1/4" plastic/rubber fuel line or air hose.

About that screw/nut... That screw is in contact with the terminal lug that will connect through a 68K to the grid of the first triode. If you touch the end of the screw/nut you will get a buzz just as if you had touched the tip of the plug on a guitar cord. That screw/nut is like an antenna and it "may" pick up radiated noise from the transformers, house lighting, radio stations, etc. I still recommend using a nylon screw.

I've put together a drawing that shows my mod ideas and also addresses a couple issues I see with your board. Should be easy to work into your new board. The cascade switch is wired the same as Hoffman's hot switch. If you have any questions, just ask. I'm following this project with interest.

And if you decide to use another plan that's OK too. Still take a look at the attached pdf. It may give you some ideas.

Sluckey,

Your observations and recommendations are greatly appreciated and it all makes sense.  Moving the caps to avoid heater wires is a no brainer and I should have seen that.

So last nights progress was minimal.  I spent some time digging through my piles to find a mini-toggle.  Found 3 so I think I am in good shape.  Also found some multi position rotary selector switches, which I assume would work well for this application and fit in the stock 3/8" holes.  I looked breifly at what it would take to put one of these in the deleted tone control position but the splines on the knob doesn't match my stock knobs so I'm sticking with the toggle for now.  A peice of copper wire wrapped around the base of the switch makes a good bushing to fill the hole.

After rounding up the switch, I addressed the issue with my electrocution screw.   I'm not a big fan of plastic or nylon screws and don't have any on hand, so I took a slightly different approach that hopefully meets the same objective.  I am awash in standoffs of all shapes and sizes.  Found one that uses 2 mounts that straddle the chassis mounting screw.  drilled 2 new holes and bolted it to the board,  so now the standoff is truly isolated with no conductive parts hanging out the back of the chassis.  That's about as far as I got last night.

We have some guests coming for the weekend so my wife is demanding I clean up the house and put things in order   This will limit my time on amp projects but hope to get back on this Sunday afternoon and finish up the mods as you have drawn.  Looking forward to hearing how it sounds.

Thanks for taking so much time and interest in this project and mentoring me through this.  In case it isn't obvious, I'm having a ball with this and most of all learning a great deal. 

This Forum rocks!

[1blueheron]

So I finished up the mods Sluckey concocted for the Airline.  There are a few things I would like to clean up and tweek but overall I am happy with the result.

I relocated the standoff mounting screws. Installed the 2.2uf caps, deleted the tone controls, relocated the 1M, installed sheilded cable from input jack to board, installed "hot"/"normal" switch.

We tested it out last night.  Using the 15" speaker that was in it, it was still dark and bass heavy.  Tones ranged from very clean to extreme breakup.  My son says it gets "farty" when you max out the gain.

Next I hooked it up to a pair of 12" organ speakers.  On these it was extremely bright, almost harsh and peircing.  Of course these were not in a proper cab, just mounted on a baffle board so some bass cancellation was to be expected.

Then I hooked it up to the cab we built for the Webster/Allen amp. using 1-15" and 1-12"  Here is where it really came out with some nice balanced tone and breakup.

Last I used the 15" built in and added just 1 of the Baldwin 12"  It is livable but not as exceptional as I think it could be with the right speaker.   I suspect in these experiments I was playing a bit fast and loose with impedance match.  There are 2 taps on the transformer, not sure which one is 8 ohm and what the other tap is.  No markings or indication.  I am going on assumption they stayed on the 8ohm when they changed the original speaker out.
Need to figure this out before I go speaker shopping.

I'm thinking I may try 1uf caps in place of the 2.2uf, see if it lessens the "fartiness"

Also going to change out the current 12AX7's to some of my Baldwin/Raytheon black plate 12AX7A's as well.

This has become a great test/experiment project.

[sluckey]

Maybe keep the cap on pin 3 and remove the cap from pin 8? You don't need to insulate the lead on those 2.2K resistors since those leads connect to ground.

[1blueheron]

Spent a little time on the Airline last night trying to finish it up. 

First job was to replace the pressboard baffle.  It now has a plywood baffle board. 

I replaced the 15" speaker with a 12"speaker so I can get a back on the cab now and it will hopefully roll off some of the bottom end and add a little more sparkle which was lacking with the 15.

Put the screws back in the lower chassis and secured it to the cabinet again.

Last job was to replace the standard 1/4" jack with a 12A style jack and fill the empty jack holes.

Once that was all done, I buttoned it all up with great expectations....

Of course nothing goes as planned.  I screwed the pooch somewhere... I am now getting a severe 60hz hum as soon as any volume pot is turned up past 2.  The hum becomes louder if I touch the 12AX7 preamp tube glass, or the screws in the isolation mount for it.   Guessing I hosed up the grounding somewhere along the line.  It does have signal and it does amplify but it doesn't sound good with all the hum.  A couple times it started to squall a little for no apparent reason.  Sounded like feedback but no signal to cause feedback and it was self-sustaining.  My son said it sounded like I was trying to communicate with whales.  LOL.  I also noticed I have a faint blue glow coming from one of the 6V6 tubes.  Just one of them.   It will have to come back apart again.

[1blueheron]

Found the problem.  Made a mistake when I switched out the jack.  It is back to gether and working nicely now.  The 12" speaker sounds a lot better than the 15 did.  Considerably brighter and more balanced.  Not thrilled with the volume controls and tone control.  Might be a future project to work on these.  Thinking it might work to put a tone stack in using channels a and b tone controls as bass and treble rather than one as a cut and the other just diasabled...  I have plenty of gain to play with now.  It could actually use a little taming.  Will try to get a sound clip/video done and posted.  Besides the control issues I am pretty satisfied with it.  It needs a Tolex job.  I have never ventured into that territory so that will be a first.