5b6 build

Mike Bagwell · From: Greenville, SC, USA

Matthew,

Do you have the jumper in place from pin 2 to 5 on the 6sc7 ?

Ken Fox · From: Nashville GA USA

I have a Gibson GA-75, a very similar amp and circuit (1-15" Jensen, much like the original Bassman amp you are cloning). I would expect no more than you are getting out of it. It is a low wattage amp and easily overdriven. The preamp would need to drive a different style inverter and the power tubes would have to be a fixed bias to get a cleaner sound. A good chance that is what the Little Walter amp has, as you can't get much more than 20-25 watts out of a cathode biased amp with 2 x 6l6GC tube.

The original amp was designed with 6L6 tubes, not 6L6GC, Those tubes are rated from 18.5 to 22 watts out in a push pull, cathode biased configuration. Putting in 6L6GC tubes will not yield more than about 1-2 watts more, if that.

A good sample of a GA-75, sounds exactly like mine, a superb guitar amp for some, but not a clean pedal steel amp.

http://www.youtube.com/watch?v=Ln1SJYFjp3E

Matthew Dawson · From: Portland Oregon, USA

Mike I do have that jumper in place. After rolling some preamp tubes I ended up with 200v on the 6sl7 and 142v on the 6sc7, which is much more in the ballpark of Tino Z's values. I'm going to adjust the dropping resistorthat feeds those and see if I can get the preamp voltages a bit higher to increase the headroom.

Then its time to deal with the ghost notes. I'm only using 3 sections of a 4 section 40/20/20/20 cap can so I have another 20uf of filtering I could bring into play. Also, Tino Z suggests adding a .1uf snubber cap across the output transformer as in the schematic below to quell oscillations. Any other ideas?

Dave Little · From: Atlanta

Question from an electronics newby.
I have become interested in this subject (amp building) and am just beginning to learn to read schematics, so the answer to this question is surely obvious to you vets. In Zottola's book, he describes this amp by saying, "The 6SC7 stage features a separate amplifier for each jack input." Here's the question: If you plug your guitar into one input jack only, are you only getting part (maybe half) of the possible output from this amp?

Dave Little

Stephen Cowell · From: Round Rock, Texas, USA

Tim Marcus · From: San Francisco, CA

I think what Tino means is that each input jack goes to its own half of the first preamp tube, whereas on modern fender designs both jacks share a tube. A tube stage is an "amplifier" because it takes your guitar level signal and amplifies it to a level that is useful for the rest of the circuit.
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Matthew Dawson · From: Portland Oregon, USA

Following a suggestion from Mark Fowler over at the Amp Garage, I used my decade resistance box to determine that 1k for the 1st dropping resistor will give me 390v on the screens with 396v on the plates. That also gave me 219v on the plates of the 6sl7 and 157v on the plates of the 6sc7, which seem to be in the ballpark. To my ears the amp sounds much better for steel guitar with the higher 6l6 screen and preamp voltages.

However, I still have ghost notes under what I'm playing. They are low 3rds, like a frog singing bass under my two and three note chords. The ghosts are very noticeable with the tone control turned down. As the tone control is turned up the ghost notes gradually disappear.

I added the snubber cap across the output transformer as suggested by Tino Z and it sucked the life out of the amp. I immediately snipped it out.

I also added the extra 20 uf to the 6l6 screen supply. I noticed a slight stiffening of the power supply but no help with the ghost notes. I also tried a number of different 6sl7s and 6sc7s. I noticed definite tonal differences between the different manufacturers of 6sl7s but no tubes we more or less susceptible to ghosting. I double checked all my solder joints and reflowed some of them. All the things that need to be grounded are solidly grounded. Looks like i'm going to have to use my oscilloscope for something other than testing for the outer foil of caps.....

Matthew Dawson · From: Portland Oregon, USA

Oscilloscope shots up on Amp Garage:

http://ampgarage.com/forum/viewtopic.php?t=20072&start=60

Stephen Cowell · From: Round Rock, Texas, USA

You're using the chassis as a ground buss... this can induce unwanted signals into your mix.

Try different places to put the tone shunt... it should be on one of the stages around it, try both driving and receiving cathode ground points. It looks the same on the schematic, but it makes a difference when you look at circulating currents. The chassis is not the best place to run your earth currents, you might consider a mix of star/buss techniques. Don't depend on the chassis alone... notice your input jacks, for instance. And keep the final ground separate from the preamp ground... they should string out, with the finals closest to the transformer, and the HV filter string paying out from there. It's not a simple task. You don't need to isolate the jacks, just buss them to the cathode ground of the input tube.

AFA what you should see on the scope... look for differences as you turn your tone control... use a sine-wave excitation. I'm betting hum is getting in there... with oscillation, it would be the opposite, your tone would sound better turned down than up... although, note that when you turn the tone down, you are injecting HF into the ground system! It ain't easy... you're at the last part, where the final 5% takes most of the work. Good luck!

PS I'm not seeing any pictures on AG, perhaps I have to have a login?
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Matthew Dawson · From: Portland Oregon, USA

Thanks Stephen. You do have to log in to amp garage, but its worth it. Nice bunch over there. I think playing around with the grounding scheme is the next thing for sure. I usually ground all the preamp stages in order on a buss ending at the input jack, ground the power stages at the cathode of the power tubes, and let the output jack ground at the chassis. Usually works out well but on this one I was really trying to see what rules I could break. I guess not as many as I was hoping! First I'll try moving the tone shunt ground. I need to make an artificial center tap for the heaters as well. I appreciate everyone who has taken the time to look over this build.

Matthew Dawson · From: Portland Oregon, USA

My signal at the plate of the 6sc7:

One plate of the phase inverter:

The other side of the PI:

One side of the output:

And the other side of the output. These are pictures of a 400 Hz sine wave with the tone control at 50%.

Stephen Cowell · From: Round Rock, Texas, USA

I'm seeing some LF on the output... and perhaps some parasitic too. The LF is pretty obvious... the HF parasitic is the top of the sine wave where it gets creamy. The output section should cancel out any LF from the filter section... but only if its driven in a balanced fashion.

The scope looks like a decent one... perhaps the trace could be focused more? Or it could be the camera.

There's definitely an amplitude difference in the PI... this normally results in even harmonics and some added sweetness, not necessarily a bad thing, unless one side clips... then expect real crap coming out.

Not sure where the tone control is in all this... are we in failure mode? Somewhere in between?
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Matthew Dawson · From: Portland Oregon, USA

Tone control is set at 50%. I used an iphone camera for this so the picture quality isn't great. I'm going to try experimenting with the grounding scheme tonight.

Matthew Dawson · From: Portland Oregon, USA

With the tone control centered there are noticeable but not extreme ghost notes.

Ken Fox · From: Nashville GA USA

Have you tried reversing the primaey windings on the plates? Possibly you may be getting incorrect feedback due to the phasing. I chased that early on in my tube amps repairs with a Hammond transformer. Their wire colors phased the transformer opposite as compared to a stock Fender transformer.

Matthew Dawson · From: Portland Oregon, USA

Thanks for the suggestion Ken. I swapped the leads and it seemed to make the amp less brilliant sounding. I still had the low frequency oscillation as well. I've been really hoping my problems are a result of a simple-to-fix error like that.

The amp wants to sound really good, and probably no one in my house would notice the oscillations very much except me, but they are there.

If my traces are nice and sharp until the output, what does that say about the preamp/pi section? I'm considering redoing the grounding system entirely. I'd move the filter cap and dropping resistor for the preamp/pi off the cap can and over to the other side of the amp and ground it on a buss with the stages it decouples. That bus would ground at one of the input jacks and everything on the 6sc7 and 6sl7 would ground to it in order, including the filter cap. I'll ground the cap can to where the cathodes of the power tubes ground.

Stephen Cowell · From: Round Rock, Texas, USA

Matthew Dawson · From: Portland Oregon, USA

Thanks Stephen. Rechecking everything.

Matthew Dawson · From: Portland Oregon, USA

Stephen Cowell · From: Round Rock, Texas, USA

OK... maybe it's amplitude is below the oscillating threshold. It should definitely work better one way over the other... try it with it disconnected, then use whichever way sounds best.

If it's not like this already, try grounding the 27ohm resistor at the speaker jack... if you ground it at the same place the cathode for PI V2 is grounded you could be getting FB there. Your problems are most likely related to layout, ground scheme, or shielding.
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Matthew Dawson · From: Portland Oregon, USA

Thanks for sticking with me though this one. Tonight I tried:

1) Grounding the 27r resistor to the speaker jack.
2) Grounding the cathode of the power tubes to the same place as the can cap
3) Moving the negative feed back from the 4 ohm to the 8 ohm OT tap.
4) Changing the routing of the long wire that connects pin 6 of the phase inverter to the output section.
5) Changing the OT leads around until I found which way sounded best (the original way).
6) Clipping out the negative feedback.
7) Substituting in 12 different 6sc7s.
8. Four different 5u4s
9) Subbing some old Tung Sol 5881's for the the new JJs I had as power tubes.
10) Different speakers
11) Different guitars

Throughout it all and in spite of everything, it still always sounds like someone is playing a jug in harmony with me whenever i play two notes or more at once.

J Fletcher · From: London,Ont,Canada

You might try isolating the high voltage supplies for V1 and V2. That is, put in another section of filtering to supply the V1 plates. Say a 10K resistor, and a 20uF cap. Jerry

Matthew Dawson · From: Portland Oregon, USA

Thanks Jerry. That is certainly a possibility if I run out of other options. Last ditch plan is to totally redo the grounding scheme with a bus and move the preamp filtering over to that side of the amp. If that doesn't work then I'd decouple that first stage and ground it on the bus as well.

Jerry Fleming · From: Roanoke, Virginia, USA

Hi Matt,

Great job on your new project build. Sounds like you are having fun. Based upon your description, the next thing I suggest is revising your grounding topology as well. Ground loops in a high gain stage can cause a lot of problems and behavior as you describe. Also make sure your scope is grounded to the speaker output with a short connection. I suggest using a star grounding technique to a common point like the negative side of the electrolytic capacitor. Keep in mind where the highest current flows and use heavy buss wire to form a ground bus and try to keep it short. I know it is difficult but avoid connecting ground to the aluminum chassis in several places.

As a test, I would isolate the preamp first and drive the PI with a sine wave and make sure the power amp is operating correctly into a load. Increase the PI input amplitude and make sure the power supply does not sag significantly with maximum power out.
Then enable the preamp and insert a sine wave into the input and follow the signal with your scope. Dress your wiring so the output transformer connections are as far away as possible from any of the preamp or other wiring.

These are just a few ideas and I hope they are helpful to you.

Kind regards, Jerry

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