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**AtomicMassUnit** · 09-17-2011, 04:50 AM

Does swapping the OT plate leads on the 6L6's make it stop?

**Prairie Dawg** · 09-17-2011, 05:50 AM

Maybe the artifact you're seeing is part of it oscillating at a higher frequency. Check your bias current to see what it's doing when the amp starts going crazy. While you're doing this if you take an AM transistor radio with an extending antenna and tune it between stations it may be helpful to locate the source of the oscillation if that's what it is. You'll hear white noise. Yeah, I know. My ten dollar Rat Shack AM shop radio is a shop tool that I discovered by accident.

One thing you might do is do a real careful evaluation of the output transformer, only because of the symptoms you described when testing. The other thing is to to check the board and see if there's voltage on it when you're running. Easy enough to do.

What I would try is set the amp up on the bench and turn it up until it is on the very edge of doing whatever it is determined it is doing, and then start probing with a dowel or chopstick. Here you're not looking for a yes/no binary answer but anything like a change in the parameters of what you're hearing. I resurrected an old Ampeg that came out of a barn where it had been for 25 years, and as I told the owner the overhaul was to get us back to why the amp ended up in the barn in the first place. It was oscillating like crazy. Using that method I found that the fix was moving a ground wire two inches. Did this recently on a clone of an old Champ too.

**pontiacpete** · 09-17-2011, 12:16 PM

forgot to mention, that I tried swapping OT leads and OT.

**pdf64** · 09-17-2011, 01:03 PM

How have you set the bias on the output tubes?
Don't just set a bias voltage according to a vintage schematic and assume all is good.
Modern 6L6GC generally need a higher bias voltage than USA made tubes, plus line voltage tends to be higher.
Far better to check plate dissipation.
That may be why your VB+ is low (tubes dissipating heavily and pulling the VB+ down).
Also some pre-amp tubes just act weird and can cause oscillation, so need to sub in known good ones, if a bias set up doesn't fix it.

**Enzo** · 09-17-2011, 01:49 PM

A couple thoughts...

ANy chance one or more of the filter caps are missing their ground connection? Verify with an ohm meter that the ground end of each one is indeed connected to chassis.

Does adjusting the volume/gain change the frequency of this noise?

Prairie Dawg has a good idea, check for RF. If the AM radio trick doesn;t do it for you, gogle up "rf probe" and find a ton of little articles on a million variations on the same simple little accessory for your meter. A diode and a cap, basically, and you have an rf probe, which will let you measure RF signal levels on a hand meter.

**km6xz** · 09-17-2011, 02:35 PM

Touching the plate lead and which starts an oscillation is probably unrelated to the motorboating problem. A parasitic oscillation like that will be in the RF range, at some frequencies there is a phase shift enough to provide positive feedback. Does that every happen when the probe is not on the plate lead? It might not be a problem at all if that is the case but a questionable layout.
You mention the volume has to be high for the MB to start so it is likely that there is a decoupling problem or filter problem.
With so many new solder connections and parts, the clues to the problem are getting buried under a slew of new variables. The problem should have been attacked directly instead of shotgunning so now it will be harder to track down...one of the main reasoned experienced techs do not shotgun.
What test instruments do you have available? If you have a scope, and there is a decoupling problem, where for example preamp tube B+ is modulated by power output section current draw, you can see that right away. Decoupling problems are usually poor filter grounds(on any of the B+ distribution chain) or bad capacitors. If you have an analog meter but no scope, you will be able to see low frequency variation in preamp B+ feed if the problem is in its filter or the filter up the chain closer to the raw B+ from the rectifier. Even with the power section drawing high current, fluctuation of the preamp B+ ought to be very low, invisible on a normally damped analog meter. A digital meter will not display this well even if the variation is high.
So, if the problem is only present with high signal levels:
1. What test gear is available to you
2. Do not Change anything unless you can prove it is defective
3. Check each of the distribution B+ taps in the supply, each with have a dropping resistor and capacitor as a low pass filter, for voltage fluctuation at the same rate as the motorboating. Easy to see on a scope and less on an analog meter and probably not visible with a DMM.
4. Bridge a capacitor, value is not important, across the later distribution chain filter caps where you see the variation with frequency of the MB. A weak cap there will raise the impedance of the B+ supply for that stage which will be reflected in signals at the output impacting the B+ of earlier stages before the plate load resistor.

If gain and signal volume setting impact the MB, but not the actual signal level...shorted input and gain up high, there may be a relationship between the probe induced parasitical and you are probably looking at a wiring placement problem or grounding problem.

**pontiacpete** · 09-17-2011, 03:37 PM

Thanks very much for all the replies and suggestions.
I'm moving a bit slow, had a hernia op. monday but hopefully I can get somewhere with this amp today.
Answer to some of the questions:

Voltage on the board? no voltage, less than a .05vdc

Bias of Tubes: I biased the tubes to 30mA which gives me 12 watts plate dissipation.(however this isn't accurate being that upon start up I am measuring 455vdc at the CT. As soon as i touch a note it starts to motorboat then plate voltage goes way down)

Ground connections: I double check all the grounds of the filter caps in the can under the chassis, even ran a wire to ground it on the chassis. This amp has the dog house with 4 holes on the side where the (-) poles of the filter caps come thru and are soldered to the can.)

Does turning of controls change the MB: yes, of course the volume knob kill the MB, but when turned back up to pass 8 or 9 it returns. The bass pot also has the same effect.

Test equipment: I have oscilloscopes, audio generators and function gen. DVMs and VTVMs. I don't have an RF probe(looking into making on now)
I have an AM radio, I can use my yaesu VX5R and I have 12' j-pole, I'm on the first floor in manhattan.

I tried clipping across filter caps w/ more new caps, with no results. I change out all electrolytics to start with being that they were 60 some odd years old and the person buying this amp is going to be play high level tours and needs reliability. I think the coupling caps need to come out but the guy selling this amp does want them to so I will probably put them back in once i solve the problem.

Thanks again!

**km6xz** · 09-17-2011, 04:50 PM

Do you see the low frequency modulation of the preamp B+source or is it clean? If seen before(towards the supply) the plate load that is how positive feed back is being introduced. Assuming it is clean, the problem gets a little more complicated in that some new route of in-phase return path to the input or other gain stage is present. I would forget RF, it is a redherring in a motor-boating case with actual positive feedback. A probe on a high z path...say, when a tube is near cutoff, can generate some really high RF voltages, even draw an arc.
Does the frequency of the oscillation change with any controls? Don't be surprised if it is some overlooked wiring mistake that looks right so has not been double checked to verify good or bad.
Have you ever seen this amp working correctly either before the parts replacements or during a prior visit? Are there signs of mods or "fixes" in the past.
Is the motorboating output level full saturation level or somewhere under clipping? If it is lower I would expect the amplitude would be variable with control and load changes.
In that regard, does the motorboating change when changing load Z? About what frequency is the oscillation, that can give a clue as to the reactive elements involved.
At lower gain settings do all controls behave exactly as you expect, and sound good?
Sorry for all the questions added to those others have asked but some key clues are not known yet.

**pontiacpete** · 09-17-2011, 08:19 PM

Some questions answered for Km6xz

"Does the frequency of the oscillation change with any controls?" -- yes the bass control and volume

"Have you ever seen this amp working correctly either before the parts replacements or during a prior visit? Are there signs of mods or "fixes" in the past. "
---No, it came here with the filter caps under chassis changed. I made the huge mistake of not listening to it. But I work a lot with the guy who brought it and he said it sounded dull and wanted me to take the bias pot that had been put in by a previous person and move it to the board and using a trim pot. He tells me it jumps all over the place from one extreme mAs to another. To me looked like it had been installed wrong,,, in place of the 56K that is paralleled the 8uf/150 bias cap to ground. I changed it to 6.8k in series w/ 50k pot between the 2 neg ends of the bias caps. So I went ahead and change the 60 yr old bias caps and the other to lytics on the board. Should have listen before I did that.,.,.

Also, there are some discrepancies betw the schematic and what actually on the board. The .02uf cap betw the treble and the bass was a .1 and the 56K resistor was a 100K. If you touch the lower leg closest to the 12ax7 it starts to motorboat.

"Is the motorboating output level full saturation level or somewhere under clipping? "--- yes, so when I lower the volume it stops, same with bass control.

"At lower gain settings do all controls behave exactly as you expect, and sound good?" yes

thanks again,

**MWJB** · 09-17-2011, 08:41 PM

Please list all dc voltages for power tube plates screens & negative grids, preamp tubes plates & cathodes. 455vdc at the OT centre tap seems low (bordrlineball park) at 30mA per tube, suggests a lot f current being draw somewhere. Confirm you have a GZ34/5AR4 installed.

Physically check the integrity of the ground connections, e.g. the "-" leads of caps were often used as ground wires and they can ping off the brass buss. Check all pots & jacks are tight, in fact drop them all out and remove any signs of corrosion between brass plate & chassis. There is often some voltage creep between the 100K V1 plate resistors & 68K input voltage dividers...so I'd remount these on the input jacks running grid wires straight down to the tube sockets, preferably with shielded cable.

The fact that you state that you changed out a selenim rectifier identifies this as a vintge bassman, it worked in the past because it left the factory...otherwise it would have ben dealt with at the test shop. Nevertheless, many people could have worked on it in the meantime, so please show some pics of pots, tube sockets & circuit board.

Your bias pot only allows your bias to be adjusted hotter than stock, it would make more sense to have a 39K or 47K resistor between the bias pot & ground. Currently your amp is biased hotter than stock.

You said you changed out the coupling caps, nowyou say the owner does not want them to come out? Are they in or out? It's not unusual to see afew changed caps in an amp of this age...all new caps suggests an issue that a tech couldn't pin down and I wouldn't take that amp on tour, even though there maybe thousands of amps of the same age that I would.

All that said, the guy who is selling it wants a sale, the amp isn't going to shift while it does not work, so he can't be too precious about the caps.

**MWJB** · 09-17-2011, 08:43 PM

.1uf bass cap and 100K treble slope resistors are actually correct & most common values.

**pontiacpete** · 09-20-2011, 02:51 PM

To wrap up this thread, I would like to thank everyone for their help. I found after going thru the schematic and layout an added 4.7K R on the presence pot, and the whole time staring me in face. I removed and start working like normal, sounds great. Can't believe I didn't notice it, oh well. (

**MWJB** · 09-20-2011, 03:35 PM

Hold on, does the amp have a 5K presence pot, or a 25K presence pot. If the 25K then the 4.7K is the NFB load resistor. On the earlier version there is just the 5K pot with no parallel resistor to ground.

**pontiacpete** · 09-20-2011, 03:53 PM

That pot measures 33K, so it probably is a 25K pot. But the 4.7K was soldered to ground.

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