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

[bazookajoe]

thanks

well thanks, maybe because english is not my first language i somehow ignored that "capacitor coupled".
may try this some time, although i just found that the reverb tank i bought some time ago is a 8A... and i also have a fender reverb tranny here,
so i will use those and try the 1 tube 12ax7 reverb from the hoffmann forum first.
thanks for this thread.

[dchang0]

Ah yes, if you have the 8A tank and reverb transformer, it'd be much smarter to go with the single 12AX7 design.

Tubenit's single-12AX7 design works exceptionally well. I'm not sure if that's the same as the one you are using, but it is amongst the warmest I've built and heard. It's the one he used in the Little Wing with Reverb amp, which you should be able to Google and find easily.

[caps8419]

I know this is an old thread but is the final version of this build available from anyone? Thanks!

-Michael

[Jazz P Bass]

Here is an attachment as a test of the system.

[dchang0]

I can see your attachment just fine. Hmm. Yet I cannot see the one I just posted (and obviously caps8419 can't either). I wonder what the difference is... (Good choice of tube for the test, BTW--the 6BM8 has been one awesome little bottle.)

[dchang0]

Testing the attachment system--the 1.0l schematic should be visible inline here:

[Jazz P Bass]

Wait a minute.

The attachment that I submitted was from my harddrive.
I clicked on the fourth icon from the right, directly above.
"Insert Image".

If you went the "go advanced" route, you may have picked a file that the Forum 'lost'.

[caps8419]

thanks again!

[elipsey]

I would be interested to see the schematic, which doesn't seem to be visible now (sorry if you're already working on it). Thanks

[dchang0]

Repost #3, following tips from g1 and Jazz P Bass to modify the file (by adding a dot) and re-upload it from my hard drive.

This is the final version, 1.0l (as in Lima). The prototype amp still adheres to this schematic, although I replaced the 0.5W Reverb pot with a 1W version when the first one broke mechanically (one solder leg came loose from the carbon trace). This was just in case the current was the reason the carbon trace failed.



Looks like it worked. Thanks for the help!

[elipsey]

Awesome, most elegant solution I have seen so far!

[dchang0]

Just a reminder to anyone considering building this exact schematic--the reverb was designed to be a studio-style "'dry' room with hardwood floors" kind of reverb. It is very clean and clear and sounds good even when the amp is overdriven/dimed. For guitarists that like warm, fat, slightly-muddy reverb, it will be necessary to swap out the 500pF capacitor with something else, but this will require other mods to tame it.

From the experimentation described earlier in the thread, what I basically found out is that if you swap the 500pF with a 0.001uF cap, the pentode-strapped-as-triode reverb driver is too strong for the reverb tank resulting in obvious high-current overdrive/distortion. I suppose someday I might draw a new load line for the pentode-strapped-as-triode side OR redesign it as a pentode. I suppose I could also try lower-impedance reverb tanks as a band-aid, but the smart thing to do is to weaken the reverb driver side.

Just a heads-up for anyone thinking of trying this design out. If any of you do end up weakening the reverb driver and are happy with the results, please post your mods here.

Personally, the experimental changes I would make in order are:

1) Replace the physically huge 0.470uF/600V coupling cap with a 0.022uF/600V. Mostly to save space and money but the tonal change may be desirable.
2) Replace the 500uF cap with a 0.001uF (again) and then:
3a) Replace the 5K/5W resistor with a 6.2K/5W or 10K/5W resistor to select a lower (flatter) load line, keeping the existing tank. (I had already considered both of these alternatives back in the design phase but chose the 5K/5W resistor for the highest current.)
or
3b) Try a lower-input-impedance reverb tank like the 8DB2C1D (for Marshall amps). The 8D has 250 ohm input impedance versus the 8E's 600 ohms and the 8F's 1475 ohms. I suspect that the 8D may handle the high current without distortion.
5) Redesign the reverb driver as a pentode in pentode mode.

But I am very happy with my prototype already and am loathe to mod it further.

[dchang0]

There's another easy idea to try if going for a warmer reverb--I am inserting it at 3a). Also, there was a typo--it is 500pF not 500uF--so let's try the list again:

1) Replace the physically huge 0.470uF/600V coupling cap with a 0.022uF/600V. Mostly to save space and money but the tonal change may be desirable.
2) Replace the 500pF cap with a 0.001uF (again) and then:
3a) Replace the 470K resistor right after the 500pF cap with the original 220K.
or
3b) Replace the 5K/5W resistor with a 6.2K/5W or 10K/5W resistor to select a lower (flatter) load line, keeping the existing tank. (I had already considered both of these alternatives back in the design phase but chose the 5K/5W resistor for the highest current.)
or
3c) Try a lower-input-impedance reverb tank like the 8DB2C1D (for Marshall amps). The 8D has 250 ohm input impedance versus the 8E's 600 ohms and the 8F's 1475 ohms. I suspect that the 8D may handle the high current without distortion.
5) Redesign the reverb driver as a pentode in pentode mode.

[caps8419]

Dennis,

I noticed you are not attenuating any voltage to the input grid of the Pentode (just RC filtering w/ the 500pf/470k). Have you run into issues with the reverb distorting in the tank? Thanks!

-Michael

[dchang0]

Nope, not as currently designed in the 1.0l schematic. The reverb is nice and clear all the way up dimed on both pots. If you were to change out the 500pF with a 0.001uF, then yes, there's a ton of distortion on the driver side of the tank....