Ampeg B-12-XY....

[Jack_Hester]

Jack does your amp have this wire ????

Mine has no such wire.  I'm not sure if that line is intentional.  Mine has the footswitch, only, between the cathode resistor and ground. 

Not sure if I'll have time to make Steve's mod to mine, this evening.  I am getting really spoiled by these amps.  Unscrew the chassis, and flip it over on the transformers.  No need for a cradle.  I put down a padding to protect the finish.  If I can get a hour, I can do the mod and report back with the results. 

Jack

[Jack_Hester]

In the meantime, until I can get back to the amp, can someone explain Ampeg's reasoning for feeding the oscillator signal through the V5 and V6 stages?  Looks like the oscillator signal enters the second stage of V6, and again on the output of same stage, which feeds the first stage of V5.  Then, the oscillator signal enters at the second stage of V5, and again on the output of that stage.  This output then goes into the RC circuit of C13/R23, C14/R24, C15/R25, and C16/R26.  I don't understand that portion of the circuit, but the signal leaves there and feeds a last stage (one triode of a 6D10), before heading down to V3.  I'm impressed.  And, I sure would like to know what's going on with the above RC circuit.

Jack

[xm52]

The RC circuit is a high pass filter. It removes thumping which can sometimes be heard in tremolo outputs.

Check out the VOX AC-30 and AC-15 schematics and you'll see something very similar being done.

[Jack_Hester]

The RC circuit is a high pass filter. It removes thumping which can sometimes be heard in tremolo outputs.

Check out the VOX AC-30 and AC-15 schematics and you'll see something very similar being done.

Is this the very subtle thumping that I'm hearing in this amp, with volumes turned down and Vibrato turned on?  My early GA-40's do this, also.  One is rather noticeable.  I bought .1uf caps for C13,14,15, and 16, but have not put them in.  On this amp, you can't hardly hear it.  You can, for sure, in the 40's.  

Any thoughts on the extra stages that the oscillator feeds?  Looks like the final signal goes through a 1M resistor (R29) before going to V3.  I'm assuming the large value is there for attenuation.  Thanks for the info.  

Jack

[Jack_Hester]

xm52 -

I took a look at the two Vox schematics that you suggested.  Here are a couple links to the old Gibsons that I have:

'55 and later (50's, that is):

http://www.el34world.com/charts/Schematics/files/gibson/ga40.pdf

'52-'54:

http://www.el34world.com/charts/Schematics/files/gibson/ga40_2.pdf

The '55 has the high pass filter that you spoke of, along the lines of the Ampeg and Vox's.  The '54 and earlier have none that I can tell.  It has a very noticeable thumping, when turned on.  I don't care.  It is a wonderful sounding amp, and very quiet.  This was one of those slightly cannibalized/abandoned amps.  Decoupling caps bad, and all tubes missing.  Replaced and it is a keeper. 

The '55 needed some of the tubes and the chassis reinstalled.  For some reason, the threads were stripped, for the two mouning screws.  I made a quick fix on it by just putting nuts and new 10-32 x 1-1/4" screws in place of the missing ones.  I haven't had that one back for several months, to make the permanent repairs to the stripped holes. 

Off topic a bit, but is this common place to find that people just abandon amps, rather than have them fixed.  The '54 obviously had an active life at sometime.  Battle scars on the cab.  The '55 was well taken care of, but who knows why the screws would be stripped in the chassis?  Amp works great with a complete set of tubes.  Why would they be missing? 

Jack

[Jack_Hester]

Steve -

17:30 hrs. - Fired up the amp, just to verify the operation of the footswitches.  The Echo switching is quiet, grounding the tank output.  The Vibrato had a pop that was hardly noticeable, in either direction.  So, I now had a reference.  I don't depend on memory, from previous work.

Made the mods to the Vibrato Speed pot, ground, and footswitch, as you suggested.  Vibrato works fine, as before.  However, this configuration makes for a very loud pop when opening or closing the footswitch.  I checked and rechecked my wiring, and I had it wired as you asked.  

So, I rewired it back to the configuration, as per the original drawing.  Fired it up, and the pop went back to the low, hardly noticeable level that it was before.  So, I will leave it like this.  It was worth a try.  I have attached my rev 'C' drawing, which is the way it is, now.  

I think that I will begin chasing the hum in Channel 1.  I'll start first by connecting the Hum Control pot directly to the filaments of V7.  Before I do that, I will make some checks by grounding the grids all Channel 1 related gain stages, with the exception of V7b.  That one is already grounded with the footswitch, and doesn't change the hum when ungrounded.

Anyway, that will start tomorrow afternoon.  Have a good one.

Jack

[Jack_Hester]

Well, I couldn't wait.  The amp was warm, and I wanted to make the checks.  

A baseline note: With both channels turned completely down, there is some hum.  I turn Channel 1 up, and hum gets louder.  Back down and turn Channel 2 up full.  No real change in hum (maybe some), but a lot of hiss.  Slight hum coming from Echo speakers, regardless of the Dimension pot position.  I'll chase that one later.  

Also, another note: I was ready to call it quits on this amp, because it is such a good sounding, good working amp.  In case you haven't gathered from all this yet, these older model Ampegs are fun to troubleshoot.  I'm just nit picking.  It is not as quiet as the last amp.  But then, it's got a lot more going on in it, than the simplicity of the last.  

With the Channel 1 volume pot turned up full, and Channel 2 all the way down, a hum was there.  Not real loud, but you would not want to record it.  I grounded V3-pin7.  Got what I was expecting.  Total silence.  I moved to V8-pin9, with the ground.  Silence, again.  I walked my way down Caps C16, 15, 14, and the junction of C14 and 13.  Total silence, each time.  I even grounded the junction of C12 and R22.  No change in the hum.  Still there.  

That was a good stopping point.  Tomorrow, I'll start by grounding V7-pin9.  Just in case it's all coming from the open circuit J1.  Probably is.  I'll post my progress.  Have a good one.

Jack

[smackoj]

JH; I don't believe that it is common to find many abandoned amps anymore. Lots of would-be musicians bought amps during the 50s and 60s when the only thing available was tube type. Many of those found their way to the garage, basement or attic and lanquished there for yrs. Probably for two or three yrs now, the ebayers have figured out that those dusty attic queens are bringing five x what they thought, so most of them have found their way onto the auction stage. It seems to be mostly a huge waste of time looking on the bay nowadays for amps, chassis, tubes or even resistors. The people have pushed the 'baseline' price up so high that most of it just sits there without any takers. I bought a couple Bogan PA heads off the bay about 5 yrs ago for under a hundred dollars each. Now I see them for 350 BIN!

So, what is the answer? Well, seems to me like most guys have decided to build from scratch and to heck with the bungalow ebay fruitcakes. I mean, looking at the "Show us your favorite build" thread on this forum it is almost heart stopping the beauty of these amps.....total coolness and functionality. It gives sophomores like me inspiration to see what a motivated tone junkie can accomplish.

[Jack_Hester]

Yep.  Ebay has ruined being able to get anything at reasonable prices.  My wife and I enjoy an occasional antique fair.  We rarely ever buy anything.  And when we do, it's nothing more than what you can carry in one hands.  Old machinist tools are some of my favorites.  I missed a deal last year on several large boxes of tube related test equipment and load of tubes.  I do what I call a 'whirlwind' trip around and through it all, to see the highlights.  The tube stuff was one of those that I would come back to and haul to the truck.  It was all gone when I got back.  Someone else had a better idea.  I've revised my whole outlook on such finds, now. 

Jack

[smackoj]

I used to watch 'American Pickers' on the cable. I like to watch how other people get the good stuff for reasonable. The tall skinny guy is always saying, when you find a good item, make the buy right then, don't wait. I guess the law of averages has proven to wait is to more often lose.

keep on truckin'

JD

[Jack_Hester]

16:00 hrs. - Channel 2 Volume at minimum, Channel 1 Volume at maximum.  

Shorted J1 (open circuit jack).  No change in hum at full volume.  

Grounded V7-pin9.  Hum actually got louder.  Not considering that an issue at this point.  

Grounded the wiper of the Treble pot, R2b (Treble at full), and got rid of what sounded like a very high frequency hum.  Now, there's only a low frequency hum.  

Turned my overhead lights off and no real change.  Turned the lights on and picked a little bit of the higher frequency hum, though not much.  

Removed the ground from the Treble and got all the high freq hum back.  Lower freq hum unchanged, wiper grounded/ungrounded.  

Grounded V6-pin4.  Lower freq hum, same as with volume at minimum.

Plugged up a guitar and heard only a little drop in the high frequency hum.  Not much.  

Removed V7 and turned the standby back on, volume at full.  Nothing but the lower frequency hum.  

Put the other 6D10 (the one that won't oscillate) in and higher freq hum sounded the same.

Put a 6AC10 in and the higher freq hum sounded the same.  Put the good 6D10 back in.

Grounded the junction of C1 and C6.  All higher freq hum went away, leaving lower freq.  

So, looks like my problem high freq hum is upstream of C1.  I grounded V8-pin9 and lower freq hum went away.  Total silence.  That's what I want.

17:20 hrs. - I'll stop here, as the rain is so loud that I'm no longer able to hear the two hums like I could.  

More tomorrow.  Have a good one.

Jack

[xm52]

xm52 -

Here are a couple links to the old Gibsons that I have:

'55 and later (50's, that is):

The '55 has the high pass filter that you spoke of, along the lines of the Ampeg and Vox's.  The '54 and earlier have none that I can tell.  It has a very noticeable thumping, when turned on.  I don't care.  It is a wonderful sounding amp, and very quiet.  This was one of those slightly cannibalized/abandoned amps.  Decoupling caps bad, and all tubes missing.  Replaced and it is a keeper. 

The '55 needed some of the tubes and the chassis reinstalled.  For some reason, the threads were stripped, for the two mouning screws.  I made a quick fix on it by just putting nuts and new 10-32 x 1-1/4" screws in place of the missing ones.  I haven't had that one back for several months, to make the permanent repairs to the stripped holes. 

I like the tremolo used in the Fender Vibro-champ AA764. It doesn't have the four pole high pass filter. It sounds great as is. One of the nicest tremolos was made by DeArmond. It employs a shaking drum filled with liquid to make and break the signal from the instrument. It is so simple and yet it sounds fantastic. Very organic. There is some info on it here (http://danformosa.com/dearmond/). Building one is on my list.

I used to use an amp that added a characteristic thump. The added artifact was part of the sound. It had a very nice attack. I wouldn't want to change it because it sounded like nothing else.

[xm52]

Is this the very subtle thumping that I'm hearing in this amp, with volumes turned down and Vibrato turned on?  My early GA-40's do this, also.  One is rather noticeable.  I bought .1uf caps for C13,14,15, and 16, but have not put them in.  On this amp, you can't hardly hear it.  You can, for sure, in the 40's.  

Any thoughts on the extra stages that the oscillator feeds?

It could be.

The tremolo has a oscillator that is used to modulate the signal. On some of the Ampeg tremolos, they used an amp stage to amplify the oscillator pulse. Having a stronger tremolo oscillator can be used to provide more depth. Too much depth can lead to thumping so you have to balance things. Likewise you can experiment with the component values in the high pass filter to tune the filter frequency.

When you look at some of these designs, you wonder why they did some things the way that they did. Oscillators can be tempermental and sometimes need something to kick them into oscillation. Sometimes filters are added to the design to fix problems that creep in.

One thing that I like to do with tremolos is slow them way down. You can do this in the Fender Vibro-Champ that I mentioned above if you replace one of 0.01uF caps with a 0.02uF. I think that it sounds better that way.

[Jack_Hester]

Saturday will be my day to troubleshoot Channel 1 more.  Basically, there are two hums.  A recap of yesterday's snooping resulted in the following:

A higher frequency one that originates between the downstream leg of C1 and the input jacks.  Shorting J1 doesn't help.  Grounding V7-pin9 makes little change.  But, grounding the junction of C1/C6 eliminates the high freq hum.  Leaving a ground attached to this junction, a quick check showed that the lower freq hum originates before V8-pin9.  Ever so quiet.  This is where the noise from the heavy rain stopped my troubleshooting.

Channel 2 has neither of these hums, that I remember.  It does have a very high hiss when the volume is up.  Truth be known, there's probably some hum in there.  But, eliminating the hiss is what I will go after.  Then I'll know if a hum is there to address.  No troubleshooting has been done on this channel, and I won't do so, until Channel 1 is done.  Or, someone has a good idea of what to look for, as a starter. 

Jack

[Jack_Hester]

Well, I've been at this for awhile, today (Sat).  Not making much headway.

Troubleshooting Channel 1, Volume at full. 

With a ground on the junction of C2/pot R3c (Vol), total silence.  No hum/hiss of any kind. 

Move ground to the junction of C1/C6, very quiet with some background hum.  I would call this normal, as compared to some of the newer tube amps that I've begun listening to, with nothing plugged up.  I could really live with this. 

Still no change, when grounding V7-pin9. 

I'm wondering if this is something inherent to Compactrons.  Channel 2 has hiss in the upper half of the volume.  But, no hum like coming from Channel 1.  I've done some more tube swapping.  I already had a 6AC10 in V8.  I now have one in V7.  The channel is brighter than before.  I believe the oscillaor circuit works some better, though that may be just me. 

I did move the Hum Control pot over to V7.  Left the existing wires, but put heat shrink over the ends and pushed them back against the chassis.  They are still hooked to V1.  I twisted three conductors and routed them under everything else, against the chassis, and soldered them to V7 filament terminals and a nearby signal ground.  Anyway, it made a difference in the lower freq hum.  I can now null most of it out. 

I'm hearing hiss in the higher freq hum, that may have been there the last time.  It just got too noisy out here, then.  Grounding C1/C6 eliminates both the hiss and the higher freq hum, leaving the subtle lower hum. 

Anyway, time to take a mid-afternoon lunch break.

Jack

[Jack_Hester]

I didn't make any more progress, after the break.  So, I closed up for the rest of the weekend. 

I'm thinking my next move will be to lift everything from V7-pin9 and tie it directly to ground with maybe a 1M resistor.  The fact that grounding C1/C6 cleans it up pretty well, and the fact that I've replaced R6 and C1, leads me to think that something in the input circuitry of J1/J2 may be amiss. 

While I'm at it, I'll re-flow the solder on R5, if I haven't already done it.  I've done that quite a bit on this amp. 

Anyway, it's a place to start looking.  More to come.  Have a Blessed and Safe Christmas.

Jack

[Willabe]

Have a Blessed and Safe Christmas.

You too Jack! Take a break with your family and church, you deserve it.

Did I ever say I really love your bottom line personal signature?


              Brad     

[Jack_Hester]

You too Jack! Take a break with your family and church, you deserve it.

Did I ever say I really love your bottom line personal signature?

Thanks, Brad.  As to the quote, I read it as a kid.  And, it was attributed to George Orwell.  Others have claimed it, as well.  But, when I ran across it again several years ago, Orwell was given credit.  So, I continue to do so.  And, this coming from the man who gave us '1984'.  Had to read that in High School.  I've forgotten most of it, but maybe I can find it for my Kindle reader.  Have a good one. 

Jack

[PRR]

> the quote, I read it as a kid.  And, it was attributed to George Orwell

As a greybeard, it is unlikely you saw those words that way as a kid.

While it is something Orwell *might* have said, there is no record of that phrase before Richard Grenier used it in 1993.

http://quoteinvestigator.com/2011/11/07/rough-men/

I think it sounds more like Kipling, but he didn't say those words either.

[sluckey]

I thought Jack Nicholson said that.

[Willabe]

Well fellas to me truth is truth no matter who said it first.

Of course we can disagree on what is truth.


          Brad     

[Jack_Hester]

As a greybeard, it is unlikely you saw those words that way as a kid.

While it is something Orwell *might* have said, there is no record of that phrase before Richard Grenier used it in 1993.

That could very well be true.  I've given Orwell credit for so long that it's the only origin that I remember. 

[smackoj]

Hello leaders; Because I am a noobish oldster who makes mistakes building amps on a reg. basis, I would like just a few pointers re: this hum/hiss noise chase. I chase a lot of hum on my builds which are mostly re-works of 50's era tube radio/phono amps.

When you use a jumper and short a tube pin to ground, that's just grounded on the chassis? Are there specific pins you  DO NOT ground 'cuz they have HT volts? And, this then would isolate the noise culprit when a pin shorted makes the amp go quiet? Do you use this method on Rs and Cs also?

Sorry if this is so elementary but hey, "it ain't rocket science"  but it ain't simply opening a can of Libbys sweet corn either....know what i mean Vern?   

[sluckey]

When you use a jumper and short a tube pin to ground, that's just grounded on the chassis? Are there specific pins you  DO NOT ground 'cuz they have HT volts? And, this then would isolate the noise culprit when a pin shorted makes the amp go quiet? Do you use this method on Rs and Cs also?

Using a ground strap in the signal chain can be a very useful troubleshooting tool. But doing so requires an understanding of schematics and how circuits operate. Lacking that understanding and using a ground strap is an invitation to disaster.

[smackoj]

copy that SL.....not wanting 'disaster' anytime soon but would like to make these old tube amps as quiet as poss.

thanks for the reality check.

JD     

[Jack_Hester]

When you use a jumper and short a tube pin to ground, that's just grounded on the chassis? Are there specific pins you  DO NOT ground 'cuz they have HT volts? And, this then would isolate the noise culprit when a pin shorted makes the amp go quiet? Do you use this method on Rs and Cs also?

Practically all of the old amps that I have had my hands in use the chassis as the signal ground.  Some have followed the phylosophy that I tend towards, and have a dedicated signal ground, though I haven't found any that are completely above the chassis.  I troubleshoot from the component end that's supposed to be connected to signal ground.  On these last two Ampegs, they are built as if the chassis is out of the picture, and signal ground is kept for the most part on the eyelet board.  Only at the jacks (input, speaker, etc.) have they utilized the chassis for a signal grounding path.  On the last one, I made sure the jacks were all tied to the same signal ground as the eyelet board.  I will do so with this one, before I'm finished.  Strangely enough, this one has most of them tied together and connected to the eyelet board.  With only two or three depending on the chassis ground. 

I use a capacitor on the end of my jumper, when I'm testing for noise on the plate of a tube.  Mostly I don't have to resort to this, as I can usually find what I'm looking for by grounding control grids.  I have done so, with the first stage of Channel 1, and had the same results as grounding the junction of C1/R6.  At this point, I'm going to lift all from V7-pin9, connect it to signal ground with a 1M resistor, and see what it sounds like.  And then, short jumper this point to signal ground, as my long jumper picks up way too much noise. 

I'm hoping I'll be troubleshooting the input jacks and the resistors/wiring, because the grid grounding was successful in eliminating the hiss and maybe some hum.  Again, the hum is not really bad.  Just obvious at full volume with nothing attached.  But, it all goes away, back at the junction of C2 and the Volume pot.  This lets me know that the subtle hum may be something that can be made to go away. 

I'm hoping that whatever rids the hiss on Channel 1 will be the same solution to rid Channel 2 of it's big hiss (again, at full volume, no connections).  We'll see.  More to come.

Jack

[Jack_Hester]

I'm going to lift all from V7-pin9, connect it to signal ground with a 1M resistor, and see what it sounds like.  And then, short jumper this point to signal ground, as my long jumper picks up way too much noise. 

I'm hoping I'll be troubleshooting the input jacks and the resistors/wiring, because the grid grounding was successful in eliminating the hiss and maybe some hum.  Again, the hum is not really bad.  Just obvious at full volume with nothing attached.  But, it all goes away, back at the junction of C2 and the Volume pot.  This lets me know that the subtle hum may be something that can be made to go away. 

I'm hoping that whatever rids the hiss on Channel 1 will be the same solution to rid Channel 2 of it's big hiss (again, at full volume, no connections).

Well, I'm back.  Got one of the worst head colds that I can remember, right after Christmas Day.  By New Year's Day, I was recovered enough to begin my new work schedule at the plant, meaning I haven't touched the amp until yesterday. 

I lifted the input to V7-pin9 and connected it to ground with a 1M resistor.  Same hum and hiss as it had with the inputs connected.  Connected it to signal ground and got the same louder hum as I did by shorting J1, when the inputs were connected.  So, I wired it all back up and made the quick grounding checks that I did before.  Grounding the junctions of C2/Vol pot made it dead quiet.  Grounding C1/R6 cleared the hiss, but left the hum. 

I buttoned it all back up to make it movable, as I have so much other stuff to finish up outside the shop, before chasing this one again.  It is playable and the hum and hiss is not terrible.  I'm just bound and determined to clean it up to the quality of the B-12-N.  That one came home the weekend before New Year's.  What a fine sounding amp.

Let me comment on the sound of the B-12-XY.  The Vibrato has a really wonderful sound, but I made an interesting discovery.  I patched J1 to J4 and connected the guitar to J3.  With both the Vibrato and Echo on (just because I like the Echo with it), the Vibrato has a completely different response.  The oscillation is much greater sounding, as in swing.  I have to cut the Intensity back below half, so that it doesn't drop the volume to practically nothing.  And, it can be set from a really nice tremolo sound to something that I have no idea how it would be used.  Too much. 

So, I tried the opposite.  I patched J3 to J2, and connected the guitar to J1.  Now, the really nice sound is back that doesn't sound like the typical tremolo.  Both times I've had each Treble at max, and Bass at half.  And adjusting volumes only, I can change from a somewhat bright sound to a really warm sound.  Bright being with the Channel 1 volume at half or above, and Channel 2 volume being less than half.  Warm happens with the opposite on the volumes.  Channel 2 half or better, and Channel 1 less than half. 

I believe that this would be a really fun amp in the hands of a good musician.  I just can't do it justice.  But, I can enjoy the heck out of it.  No question that I'll be back on this one, to clean it up.  Life can get in the way of fun, at times.  Bad colds always get in the way.  New work schedule will curb my shop time, but I'll still work some in.  By Spring, I should have a routine worked out to spend more time on these projects.  Of course, mowing always gets in the way of fun.  I'll deal with that too.  More to come.  Have a good one. 

Jack