Brighten tone of Ampeg B-15 "Big M" amp

[Paul Lebow]

The base-heavy was apparent to me whether I listened directly through the 15 inch speaker or headphones. I compare to other amps under the same listening conditions such as a brown face Deluxe and tweed Tremolux. The lower capacitance did make a noticeable improvement in suppressing the bass to not over power the higher harmonics. Seems in line with the 500pf caps used in other tone stacks. As some have mentioned, the old Ampegs were not designed to be Ferndery or to break up, but its interesting to see what tweaks can nudge it in that direction. Comments here were very helpful and appreciated.

[martin manning]

Don't know if there is a way to keep the vol and tone independent with a single tone control

You could try a 5E3 volume/tone control. The tone control is still effective at full volume as a treble cut. Just rewire with your 500k pots to see if it works for you.

[B Ingram]

Question - At full volume isn't C7 completely bypassed?

As per suggestion by B. Ingram, I changed the filter cap, C7 from 2000pf to around 600pf. Significant change in adding more sparkle but only with volume turned down half-way. Seems the volume control essentially shorts out that capacitor when at full volume as one can see in the schematic. ... Don't know if there is a way to keep the vol and tone independent with a single tone control.

Your observation is correct, but that's all you can manage with a single Tone control. That's just one of the drawbacks of that circuit-style.

The base-heavy was apparent to me whether I listened directly through the 15 inch speaker or headphones. I compare to other amps under the same listening conditions such as a brown face Deluxe and tweed Tremolux. ...

You could try a 5E3 volume/tone control. The tone control is still effective at full volume as a treble cut. Just rewire with your 500k pots to see if it works for you.

You don't have much to work with.
- You can shave cathode bypass caps, but imperfect tubes may hum.
- You can shrink coupling caps, but they're already about the right size, and there's only a couple to mess with.
- Your sonic impressions are very likely skewed by the digital cab models you're using, as you already experienced (why not tinker those instead?).

[Paul Lebow]

I think you are correct. The tone certainly changes with different OX UA cab models. The original 15" speaker also inhibits the highs. As you point out, some of the caps are there for a reason besides tone ( hum control). The Ampeg manual actually warns that the 15" speaker can easily be blown. Probably better just to appreciate them for what they are and still kicking after 65 years.

My guitar teacher and long time performer relies on his guitar tone control while keeping the amp settings fixed - no time to fiddle with amp knobs. His Strat bridge pickup is rewired to the middle pickup tone control to cut the glassy highs when needed. Neck and middle fixed at max treble. That 2 inches difference in position is important to him. He also points out that these amps were created at time when the options were mainly jazz or twangy country.

[martin manning]

Don't know if there is a way to keep the vol and tone independent with a single tone control

You could try a 5E3 volume/tone control. The tone control is still effective at full volume as a treble cut. Just rewire with your 500k pots to see if it works for you.

Another option is what people call a Big Muff tone control, which is parallel high- and low-pass RC. It is not affected by the volume setting. This would be easy to try, again keeping your 500k pots, it would look like this. It rolls around the corner frequency of the filters, which is 480 Hz with 1n5 and 220k. C1 is the existing coupling cap and R3 is approximating the existing load. I'd consider converting the accordion channel to this circuit, where you would change the 100n coupling cap to a 22n.

[Paul Lebow]

Thanks. Looks like a nice transition from base to treble. Do you think the -25 db is more attenuation than the original circuit?

[martin manning]

It has a bit more loss, ~10 dB. Here it is against the existing Instrument tone/volume, both at max volume. Note I had a 500k volume pot above, changed to 1M here.

[Paul Lebow]

For the original circuit I see how much the full volume compresses the curves, almost no variation - just what I hear. Out of curiosity, can you run the curves at half volume. Are both pots linear or is the volume an audio taper?
Just need to determine the layout for the mod. Lots of long wires crisscrossing and doubling back on themselves. Ch1 and Ch2 controls are spaced wide apart. Daps of varnish.

Thanks!

Paul

[martin manning]

Maybe you are seeing old solder flux as varnish? It tends to get brown and brittle with age. In the model I have a linear 500k tone pot and 15% taper 1M volume. Plots below are 100% and 50% rotation on the volume, and I changed the RC to 100k and 3n3 to get a little wider range and better balance. The existing tone has a lot of bass cut available, so it seems odd that you hear it as bass heavy.

Looking for a gut shot I found this blog post where the guy seems to like it as it is, but I think the parallel RC tone control would be an improvement.
https://leightonamplifiers.blogspot.com ... t-amp.html
I would try it out by tacking the new parts in, then if you like it figure out a solid layout.

[Paul Lebow]

Thanks. The tone is nicely isolated from the volume. Thanks for the link you sent on the M-15 rehab - good to hear someone else's experience. He had a blown PT. In my case it was the OT. Surprised me since the fragile speaker seems alive and well - would have thought that would go before or with the OT.

I might try a completely separate circuit on a breadboard. It's a little off topic but unlike my Fender amps I've serviced or my Ampeg V2 , the chassis is very cramped. Unused lugs on pots and jacks are used as convenient nodes that aren't related to the jack or pot. All wires are tightly wrapped on lugs - even a solder sucker or solder wick isn't sufficient. Connections are buried deep making desoldering risky in terms of touching other components. Wires crisscrossing all over. Surprised how quiet it is. Not to mention the clutch-head screws! Not the best amp to mod. Maybe someone has some thoughts on working in such a jungle.

[martin manning]

Kind of a Tweed Fender situation. Some nice photos here: https://www.creamcitymusic.com/vintage- ... combo-amp/

[Paul Lebow]

That's the baby. They don't show the pot and jack side of the chassis - I have to zoom in with my iPhone or use ohmmeter to trace the circuit. I made the mistake of removing a flickering pilot light (can be seen on right side of image) to Deoxit. Bulb faces down into the chasis. Had to be Houdini to get it back in - used an angled hemostat squeezing the glass and lots of cursing and prayer.

[maxkracht]

Not the best amp to mod. Maybe someone has some thoughts on working in such a jungle.

Not so bad once you get used to it. Just have to work carefully and move wires to avoid burning them. If you're really worried, you can tape wires out of the way or put a couple layers of aluminum foil around where you're working incase the iron slips, but I doubt any of that is really necessary.

If there is lacquer or something on there, you might want to get it off with a solvent. Keep in mind, harsher chemicals might remove things you don't want to remove, so start with IPA and work your way up and don't use more than necessary. I don't think i've seen this on a vintage ampeg, but some people do put lacquer/nail polish on their solder joints as a quality check. Easy to remove with cotton swab and acetone.

Sharp flush cutters and a dental pick should be able to get just about anything out once you remove enough solder. Just heat, wiggle, snip, repeat. The eyelets and tube socket lugs are pretty big, so if something is wrapped really tight and you don't think you can remove it, it's ok to leave a wrap of wire. You can use a stainless steel dental pick, or wood toothpick, to make a new hole in a solder joint, add the new wire, then reflow. I also find hemostats incredibly useful for work holding/temporary heatsync/pliers/ etc.

Edit: Just looked at that picture again and the last one of that era ampeg I worked on is coming back to me. I believe it is glue, not lacquer. I think it softened with IPA if i remember correctly. Double check the solder joints on the big resistors, they tend to fail.

[Paul Lebow]

Thanks for the tips. You've inspired me to learn a little LTspice. At the point where I should be able to replicate bode plots. There's a bug in the Mac version in selecting the resistance step curves - doesn't look like Analog Devices is too interested.

Yes, that is glue. All the long jumper wires have dabs of glue to the metal chassis and some caps are glued to the eyelet board. Guess it was built expecting some abuse. Shielded cable to preamp grids grounded on one end. Almost no hum. I like the toothpick idea to open the lug holes. I've had to snip the wrapped wire at the lug so I could at least pull off the wire when heated. Still thinking about a protoboard version before I venture in the weeds.

[Paul Lebow]

I notice that the circuit provided by Martin Manning might have an issue since the volume pot at minimum will set the output to ground. This output on the original circuit goes through a 270k resistor before going to the phase inverter stage as does the output from other channel tone stack. This isolates the two channels. Without that additional 270k resistor, setting the volume to minimum on Martin's circuit would also short the other channel to ground. I'll leave those resistors in. I finally came up with a fairly simple configuration to incorporate the new circuit soon.