5F1 Champ w/ Capacitor Coupled One-Tube 6BM8 Reverb (Eric Barbour/Ampeg-style)

[Splash Zone]

Thanks to you both. This was my understanding on both accounts: likely a typo, and that the capacitor type could be interchanged, but that electrolytics have a design that requires polarization to make them work correctly.

Banana sized capacitor....this is funny.

[Splash Zone]

Great news, this circuit works in my Skylark! I’m posting prematurely because I’m excited, but there are a few tweaks to make and then I’ll share the schematic for others. Nice work dchang0!!

[Splash Zone]

Here’s the final schematic and some sound samples. In my opinion, the choke is a must with a tube rectifier (where you can’t increase the filtering of the power supply).

I have a 1 meg resistor following the reverb intensity pot and I don’t know why so high but it sounds quite good here like this. I think I might be running the reverb driver a little hot calculating 20 mA, when 10 to 15 mA are sufficient. Sounds just fine to me so not going to mess with it…



Voltages:



Sound clips to hear what the reverb can sound like:


Gibson GA-5TR

https://youtu.be/YUlXPeAiLNg

[Chuck H]

Not a lot of reverb.?. But enough I think. Great tone overall. (<period) And that's what matters.The 1M series resistor doesn't reduce the reverb much at all but it does help to keep the mixed dry signal stronger. It may reduce a little top end on the reverb signal and it might be worth experimenting with a capacitor parallel to that 1M resistor since reverb is mostly in the HF. That may reduce top end on the dry signal a little but it should add sparkle to the reverb. Values in the 47pf to 220pf range I think (based on instinct and not simulated or tested). But the tone you have now is really good so if there's too much detriment to the dry signal tone you can always undo any mods.

Kudos I actually wasn't a believer on this one but even a subtle reverb effect while keeping some amp grit sounds stellar. This is what that 1M resistor is doing for you. Happy accident. Or not. I'm sure you experimented as well. Anyway... Do try a capacitor parallel to that resistor and see what happens.

[Pedro Vecino]

I would try connecting the pentode in pentode mode to drive the tank more and get more intensity and liveliness. And with more gain I would experiment with other things like connecting a capacitor in parallel with the tank input (180K resistor) to create more emphasis on the highs if necessary, etc.
Finally I would look at the 1M resistor if necessary. On the 68 Deluxe reverb (new silverface) it significantly reduces the sensitivity of the effect and its presence. I usually use a series sum of 470K + 220K/330pF in parallel to replace it. Similar to what Chuck proposes.
These are just ideas with the circuit already assembled and working.

[Splash Zone]

Thanks to both of you for the suggestions. I have been off-line for a bit so hadn’t checked the forum but I appreciate the thoughts!

Not a lot of reverb.?. But enough I think. Great tone overall. (<period)

Thank you! I’m actually finding the reverb a bit tough to tame, and I have it mostly on a sliver above “none” on the dial to get it perfect to my tastes. The tank is a long decay and I have since tried a medium decay, but not sure if it is in improvement. I tried lowering the return 470k grid-leak with a range of values and didn’t notice much improvements. I need to reread the thread to see what the different tweaks were that the OP liked.

It may reduce a little top end on the reverb signal and it might be worth experimenting with a capacitor parallel to that 1M resistor since reverb is mostly in the HF. That may reduce top end on the dry signal a little but it should add sparkle to the reverb.

. I’ll give it a shot and report back!

Kudos I actually wasn't a believer on this one but even a subtle reverb effect while keeping some amp grit sounds stellar. This is what that 1M resistor is doing for you. Happy accident. Or not. I'm sure you experimented as well. Anyway... Do try a capacitor parallel to that resistor and see what happens.

A little bit of tinkering but mostly a happy accident, I’m a noob (mostly!). The biggest issue I had was when I powered it up initially, there was HORRIBLE blocking distortion. Being a noob (mostly!), I had an evening of panic and disappointment that ended with a few hours of reading on what to do, followed shortly by a morning of satisfaction and elation after installing the purple 470kR seen in the schematic before the V3 6BM8. I think this guy functions as a grid stopper and a voltage-divider with the two 220kR’s to dump about half-the signal to ground.

Honestly, the amplifier sounds PERFECT with my telecaster. Very nice with a little breakup on the neck pickup and the guitar volume rolled off. And then seriously gnarly on the bridge humbucker with everything up at 11. I’m playing it through a “JohnH” reactive attenuator which I built, and I can keep the volume in the 5-8 range with this and stay married.

If I build another one like this, I will put in a speed control on the trem and a dwell for the reverb and it will really be perfect.

I would try connecting the pentode in pentode mode to drive the tank more and get more intensity and liveliness.

do you mean unstrapping the the driver stage? The cathode has 11.8V across 560 ohms, which I calculated at around 20 mA of current here. Merlin Blencowe writes that 10-15 mA is good to drive this type of circuit so I was thinking I might be a little high. Thoughts?

And with more gain I would experiment with other things like connecting a capacitor in parallel with the tank input (180K resistor) to create more emphasis on the highs if necessary, etc.

Finally I would look at the 1M resistor if necessary. On the 68 Deluxe reverb (new silverface) it significantly reduces the sensitivity of the effect and its presence. I usually use a series sum of 470K + 220K/330pF in parallel to replace it. Similar to what Chuck proposes.

And just to be certain, you are referring to the same 1M resistor that follows the reverb pot that was just 27k in the original circuit?

Thank you both again!

[Splash Zone]

Answering my own questions after some tinkering…for this circuit, changing the reverb input grid-leak resistor to 270k tames the reverb nicely so the reverb pot is not so sensitive and it is now a lot of reverb at 10 and a perfect amount around 3-4 instead of the perfect amount at 0.3 or 0.4 on the dial.

I put a 250 pF cap across that 1M resistor and it does damp the dry signal a touch but it brightens the effect quite a bit in a good way, so it’s a good improvement.

Thanks again for the suggestions. I’m attempting to upload an updated schematic but I’m not having success at the moment. I’ll do so if conditions improve.

[Chuck H]

I put a 250 pF cap across that 1M resistor and it does damp the dry signal a touch but it brightens the effect quite a bit in a good way, so it’s a good improvement.

Easy to sort of teeter totter the effect by trying a smaller value capacitor in series with a resistor. Values ranging something like 180p to low as 22p for the cap and 220k up to 1.5M for the resistor. With 180p/220k being just a little less effect than you have now and 22p/1.5M being almost no effect. All sorts of options in between. Usually with the lower cap values matched with higher resistor values (and vis versa). But you can even change this method up too if you want to alter the frequency of the effect more dramatically. Like just trying a lower value cap by itself.

[Splash Zone]

Updated schematic: