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

[dchang0]

very nice. i have almost all the parts.
what about the tank? did you use the 800Ω impedance intentionally or it's spare part?
do you think i could use the 2500Ω one? i can find it at a good price, while the 800Ω one must come from oversea..

Actually, the circuit is really meant for the higher-impedance 8F tank (1925 ohm input), not the 8E (800 ohm input), but I couldn't find the 8F anywhere in the USA without custom-ordering one (not a big deal--I heard that the guys at MOD will build one for no extra charge).

So I expect that the 8F tank will actually sound much better (less springy/midrangey) than the 8E that I'm using. But I haven't actually tested an 8F, whereas I did test an 8E, so I put the 8E in the schematic because I can vouch for it.

If you find that the 8F works fine or better, please let us know (sound clips would be nice). I'd happily update the schematic with the 8F, since that's really what's supposed to be in the circuit.

[pietro_moog]

hi. i haven't finished yet the circuit.
i can only find 4,7k 5w resistors, is the ok or 5k is a must?
i image that reverb is off when the switch is connected to ground, am i right?

[dchang0]

4.7K is close enough. Stick with the 5W power rating no matter what resistor you choose--the calculated power requirement was high enough that 3W resistors are too close for comfort. We don't want your amp catching on fire, LOL.

Oh, BTW, the reverb switch is on the wrong side of the tank. Put it on the recovery side (mine is on the driver side in real life, which also works but doesn't shunt any noise picked up by the recovery side to ground). Yes, when it connects to ground, it shuts off the reverb completely.

I'll update the schematic for future builders; I'm just too lazy to switch my reverb switch around.

---

Also, I checked with MOD and the rumor was false. They DO NOT custom-build reverb tanks. The models they make are the most popular ones.

Accutronics/Belton still builds tanks to order, so I'll order one from them just to test the proper 8F 1925 ohm impedance input. At the very least, I'll order the long decay because that's what I've wanted all along (basically, I want the sound of the Princeton Reverb tank but in a short head-sized format).

[dchang0]

Post-final schematic version 1.0k. This is NOT tested but should work perfectly fine. The changes made are to match the ideal design; shematic 1.0j was a compromise made with available parts (the reverb tank, mainly).

[dchang0]

Just ordered one 8FB3C1B tank direct from Accutronics/Belton. Cost was $25 plus $20 shipping from Korea to the USA. I'll let you know whether the 8E or the 8F sounds better and post sound samples within a month or so. My expectation is that the 8F will sound much better with less spring sound.

[pietro_moog]

i'm slowly building mine. i'm still waiting a few pieces.
i hope i can finish it this week.
do you feel the champ to have a boost when reverb isn't engaged?
(is this phrase grammatically correct btw?)

[dchang0]

Awesome! Glad that you're almost done there.

There isn't any difference between reverb on or reverb off. This is most likely due to the low 100K value for the reverb mix resistor and its minimal influence on the dry signal. I have considered increasing this resistor to get even closer to the Fender Princeton Reverb, but I like that the reverb can be clicked on or off without any affect on the dry signal. I can strike a note really hard, click the reverb footswitch multiple times, and not notice any difference in volume or gain or tone.

Your grammar is fine. I suppose a smoother wording would be "Do you feel the Champ has a boost when reverb isn't engaged?" but it is also correct the way you wrote it.

[pietro_moog]

this is awesome.
you did a splendid job on this. i'm so grateful.
just to say, i have a jazzmaster coming from warmoth.
it will make love with my amp on my holidays.

(and you're a nice guy too for the reply to the question out of topic. yeah, initially i wrote that and then i changed. sometimes i feel insecure and write crazy things. the sopranos didn't help with my grammar)

[dchang0]

You're very welcome! Wish I could speak Italian--all I know is a teeny bit of French.

And thanks go out to everyone who helped me design this reverb circuit. I'm glad to have done it--it's got a very different character than the other one-tube reverbs out there. This one has more "muscle" than the others.

BTW, you can and should experiment with the coupling caps especially the 500pF cap at the 100K mixing resistor. If I could do this again, I'd probably double that to a 0.001uF cap (to warm it up) and reduce the 0.47uF cap to a 0.01uF cap (to tighten it up). You can also mess with the mix resistor and the grid leak resistor on the recovery side to try to get a different wet/dry mix.

[pietro_moog]

your infos are super-cool.
i noticed you removed the cap on the second stage cathode. why? too gainy? what kind of pots are you using? linear?

[dchang0]

Yes, the second stage was much too gainy with that cap there. It'd be nice to put a switch there to select it, though--the amp becomes a real beast with it inserted.

I'm using all audio taper pots.

[pietro_moog]

cool. i'm planning to keep the cap for now. mine is a princeton tweed with the one knob tonestack.
it stays cleanish untill half the turn.
champ-type amps are fun.

[dchang0]

cool. i'm planning to keep the cap for now. mine is a princeton tweed with the one knob tonestack.
it stays cleanish untill half the turn.
champ-type amps are fun.

Yeah, that's exactly where the version with the cap started getting dirty--around 5/10. I removed the cap and toned down the reverb recovery gain, and now the amp gets dirty around 9/10. I intended for this amp (version 1.0j) to be the perfect recording amp. Super low noise, clean most of the way up, sweet even-handed tone, big spring reverb sound, and easy to run pedals through. I can carry it in one hand, guitar in the other, into the studio for some late night recording sessions. It doubles as a fantastic apartment practice amp, especially with a Swart Night Light attenuator.

You'll love it!

[pietro_moog]

so, mine is almost done.
the champ part is ok, i haven't finished the reverb yet.

how do i have to wire the reverb pot?
it seems a normal turn-off pot, like the volume, but it's reversed in the schematics.

pin 6&7 are tied. am i right?

ground for (B) and (C) seems to be separated. how is that? don't you have only a big ground?
is the reverb tank grounded just on the return side?

[dchang0]

Cool--almost done!

The reverb pot is a normal pot without the on/off rotary switch. I used audio taper.

Correct, pins 6 and 7 on the 6BM8 are tied together to turn the pentode side into a triode-strapped-pentode.

The grounds are separated along a buss wire. I connected all the (B) grounds together at one point on the buss and all (C) grounds together at one point. You are welcome to use whatever ground scheme you wish.

Yes, the reverb tank is grounded only at the return side to prevent a ground loop from occurring on the send (driver) side.

Accu/Bell has shipped my new 8F tank; I'll get it within a few weeks.