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

[pietro_moog]

hi. i'm playing the amp these days.
the reverb it's nice, it's good. it seems more like a space to methane fender one, like a room.
maybe i'll play with the voltage divider and the mixing resistor.

my big big problem is the high level of noise. that's not good. but if you say you don't have it i surely did something wrong.
i'll check for the reverb ground (it's probably that)
the tube gets very hot. do you have this too?

your reverb it's a nice thing. i see why you like it.
surely long delay time will make the sound richer.

[dchang0]

Yes! That's exactly what I was going for--a reverb that sounds as much as possible like a spacious room with hardwood floors!

We might be able to increase the dwell time easily by splicing in a pot or trimpot. Let me look into that. I happen to have spare pots laying around. We can simply replace the 220K grid leak on the driver side with a 1M or 500K pot. My guess is that a 470K resistor as well as the 470K resistor grid leak on the recovery side (which I already have in my amp) will raise the reverb to your satisfaction.

If you listen to any of the impromptu recordings I posted up, you'll hear how low the noise floor is on my amp--this can be built very quietly. I used two grounds: a star for the heavy current stuff and a buss for all the rest. The marked (B) and (C) grounds are at points along the buss.

My 6BM8 doesn't get very hot at all. However, I am using a metal tube shield, which may reduce some of the heat by acting as a heat sink.

I did have trouble in the beginning with one 6BM8 that was very microphonic--it kept having self-feedback problems and squealed like crazy. I sent it back to Tube Depot for a replacement and haven't had trouble since.

At about the same time, I was having a serious oscillation problem due to a poor layout; the 6BM8 did get very, very hot at that time. After re-laying out the board, the heat problems went away (though the tube was still microphonic until I replaced it). Perhaps that's what you're dealing with.

I'd say that it'd be worth your time to play with the mixing resistor--I put in several values before settling on 100K. But do this AFTER you solve the noise/heat problem.

[pietro_moog]

changed a few things, the noise is a tad lower but nothing mind-changing.
the noise is coming all from the reverb circuit.
what was wrong with you previous layout? i'm sorry for ll this questions.. you are basically building my amp..

thank you for your patience all your efforts

[dchang0]

Not a prob, man.

I had two problems with the previous layout.

1) Since the reverb board was a daughterboard mounted above the main 5F1 board, it ended up placing some components too close to the filter caps on the left side of the chassis (near the power transformer). I designed a new reverb daughterboard with all the components moved about 1" to the right, since all the circuitry is in effect "preamp" circuitry.

2) I segregated the signal-related components from the cathode-bias components and from the anode-side components as best I could. It wasn't anything mind-blowing--I just kept the signal components as close to the tube socket as possible, then put the cathode stuff together off to the side and the anode-side stuff together off to another side. The whole goal was to keep the signal path wires as short as possible and away from the other stuff.

Once I did that, the build got super-quiet, the tube temperature went down, and microphonics were lower (but still present until the 6BM8 was replaced).

OH, I should remind you that you should be using isolated jacks for all your reverb jacks (footswitch, reverb out, reverb in), or you might be creating an unintended ground loop through the chassis. That was a big source of noise for me when I built my Princeton Reverb clone--it was all in the reverb circuit. Also, make sure your reverb tank is only grounded on one end (return side). I use the Cliff S-H501 1/4" jacks and the S-H267W and S-H267R isolated RCA jacks from tubesandmore.com.

Some guys know how to build (and swear by) chassis-grounding, but I haven't built a single amp yet that worked right with that scheme.

[pietro_moog]

yeah yeah, ground chassis. that could be it, i'm having tons of problems with that. good catch.
i'll try and i'll let you know

[pietro_moog]

i officially had a case of bad ground. once i moved the reverb cable ground from the switch jack to the main ground, the noise disappeared.
now i just have to find the right mixing resistor. i'll try 220k, 330k and 470k.
i don't know why but i think 330k could be the right one.

[dchang0]

Awesome! Glad to hear it.

After you try a higher mixing resistor, try replacing the 220K grid leak on the driver side with a 470K. That'll beef up the reverb tone and add a lot more body (it's where a 1M dwell pot would normally go).

[pietro_moog]

mmm..
you mean the pentode side? did you try such an high value? i'm curious. what's the result?
i'm thinking i could put a 1M pot, since i have space on the chassis. (i had a tremolo installed before, so i have the holes)
do you think i have to raise the grid resistor on the recovery side too? something like 470k?
i think i'll stay with 220k as mixing resistor. the noise is increased a bit, so i'd like to stop.

i'm looking at the fender unit schematics. it calls for a dwell linear pot. do you think i should use a rev log one?

[dchang0]

Yup, pentode side. No, I haven't personally tried it on this particular build, but I did just do it on another single-triode reverb build about a month ago. It was a 1M linear taper trimpot in that case. We ended up cranking the (dwell) trimpot all the way up to 1M anyway. If it were me installing a dwell control, I'd do this: install one 220K resistor and one 250K audio taper pot in series with that (on the pentode side).

I'd expect that you'd get less noise if you raised the pentode (driver) side than if you raised the triode (recovery) side, since the reverb tank pickup is a large source of noise.

In my own build, I'd raise both grid leaks to simple 470K resistors. I just haven't had time to do it myself.

[pietro_moog]

i think i'll use a 2.2nf cap instead of the 500pf one on the reverb input and a dwell 1M log pot.
recovery side grid will go up to 470k

[dchang0]

Niiiice. That'll add a LOT of warmth and power. "Season to taste," LOL!

Post up short recordings of your amp after you've got it the way you like it--I might follow you on some of the changes, especially the 500pf -> 2.2nF mod.

EDIT:

By the way, I just tried swapping the 220K driver grid leak with a 1M. It's too much, IMO. The reverb starts kicking in at 2/10 on the reverb knob and goes all space-crazy by around 7/10. Very cool for those who play surf, but not very useful for people who just want a little atmosphere, since almost all the useful range of the knob is between 0/10 and 2/10.

I'm guessing 470K is the highest most people would go and still get a useful range on the reverb pot. My prediction is that "sweet spot" will be 470K on both driver and recovery grid leaks. Note that Eric Barbour originally used 330K as the grid leak on the pentode driver example. It might also work well with 330K for pentode grid leak and 1M for triode grid leak.

[dchang0]

Yo! I just tried this setup instead:

1M on the recovery (triode) grid leak
470K on the driver (pentode) grid leak

SOUNDS AWESOME. The reverb pot still has a decent range but goes space-crazy at around 8.5/10. The "sweet spot" is still at 5/10, just like with the 220K pentode grid leak and 470K triode grid leak. It starts dipping in volume at around 7.5/10, meaning I'll have to increase the 33K send leg resistor a bit (probably to 47K).

Last change that I'll make is to the 500pF--maybe to a 1nF. This I think is what you're really after--you want FATTER reverb instead of the "big spacious room with maple floors" sound I went for. That said, you'll probably get the biggest bang for your buck by changing the capacitor, since the resistor changes we talked about so far only mess with the wet/dry mix and not the tonal characteristics.

[pietro_moog]

i don't know what i'm after. i was (and intact i am not) a reverb addicted.
the point with the 500pf cap is that it forms a low pass filter with the grid resistor at 1.5khz! it's really cutting everything for me.

i was thinking to remove the reverb from the amp and make it as a standalone unit, but inside the amp cabinet (mine is a 5f2a, the cab is bigger)
i was thinking to take this circuit and insert it in the fender reverb unit schematics,from the dwell pot to the tone pot.
basically a 2 tubes unit reverb. i need to think about some stuff, find the right chassis and figure out a power filtering stage without a choke.

that could be seriously awesome.

what do you think about it?

[dchang0]

Sounds like a great idea! The last one-tube reverb build I worked on about a month ago was installed in the bottom of a combo and entirely contained within a single pedal-sized Hammond enclosure. It ended up sounding really awesome (we used a 0.001uF cap instead of a 500pF); it was very warm.

Yup, that 500pF cap is almost completely responsible for the overall tone of this reverb, but I happen to like the empty-room-"slapback" sound. The other two caps have much wider frequency windows. Tweak themall to your satisfaction!

[dchang0]

Okay, I replaced the 500pF with a 0.001uF cap, and NOW THIS THING'S INSANE!!!!!!!

Yeah, now the reverb is so strong that the "sweet spot" where it sounded like a nice empty room has moved from 5/10 on the reverb pot to around 2/10. The range between 8/10 and 10/10 is now more like a delay effect than reverb because the outer-space sound is so over the top (and the tank is on the verge of feedback, with the 6BM8 displaying slight microphonics). The wet signal is now about indistinguishable in tone from the original dry signal, versus it being a bit colder "slapback" echo with the 500pF in place. The result is a muddier overall sound, since the reverb signal "steps all over" the dry signal.

It's waaaaay too much, but it's still fun to play with, so I'm going to leave it this way for a while.

So, here's the setup right now: 470K grid leak on driver (pentode) side, 1M grid leak on recovery (triode) side, 0.001uF signal coupling cap. All other components exactly the same as in the schematic. The result: sweet spot range between 1/10 and 2/10. Everything above that pretty crazy.

With 500pF, 470K and 1M, it was very strong reverb but still had a sweet spot around 5/10 and outer-space above 8/10.

With 500pF, 220K (pentode) and 470K (triode), it was moderate reverb, with sweet spot around 5/10 and surf above 8/10 but no outer-space.

I think we've explored too far--my guess is that the 500pF with 470K (pentode) and 1M (triode) grid leaks will serve the greatest amount of people. One alternative, for those wanting warmer reverb, could be the 0.001uF signal cap with 220K (pentode) and 470K (triode) grid leaks.

(Main volume at 12/12 for all tests above.)