Hoffman Stout Reverb Mod?

[Lectroid]

I am thinking about building an 18W amp with 6BQ5s and looking at the Hoffman Stout Reverb with a few changes. What I want is something with less power than my DR clone, and to substitute the 1-Tube reverb from tubenit's thread in Archives.

The Hoffman Stout Reverb splits the guitar input right before the preamp stage 1.  Half of the signal goes to another gain stage just before the reverb amplifier (V1-A) that can be switched in or out.  There is also a mod to switch out one of the gain stages which leads me to my questions, the idea being to see if it's possible to leave off the extra gain stage entirely?

https://el34world.com/Hoffman/files/Hoffman_StoutReverb.pdf

1.  Could the 'extra' gain stage and switching right before the reverb circuit (V1-A) be omitted? If not, why not?

2.  If so, could the reverb be brought back in after the second preamp stage, (pre-PI), similar to Fender BF circuits?  Would this compensate for the 'missing' gain stage right before the reverb amp?

3.  I think the 1-Tube Reverb would probably work in this circuit.  Any reasons it might not work, or be inadvisable?

Thanks for all help, opinions, and random comments.

[sluckey]

Since your goal is to gut Hoffman's reverb circuit and opt for tubenit's one-tube reverb, I suggest you use Hoffman's original Stout circuit. Insert the one-tube reverb between C5 and C6.

I would not be surprised to find that tubenit already has a drawing for this idea. If not, he could probably whip up a drawing before you read this.   

[AmberB]

After looking at the linked schematics, I'm curious, what is the purpose of C6?  It seems redundant to me unless it's there to restrict low frequencies, and then I would wonder why they didn't just use that value for C5?

[Lectroid]

I wondered about C6 as well.

sluckey, I see your point about building a modified Stout since I don't want the extra gain stage. I should have approached it from that angle.  Trust a noob to make something easy harder than it needs to be.   

[sluckey]

You don't need C5 or R12 unless you will be building the gain switch mod.

[tubenit]

A couple of thoughts for you to consider ................


An 18w EL84 amp will not be hardly any quieter than the 22w Deluxe Reverb clone. I doubt you'll notice any significant difference?  So if your goal is less volume/gain ............ I'm not sure your approach will work?


You could use 6BM8 tubes for a 7-9 watt amp. I play at open mic jams with a 6BM8 amp and don't even mic the amp cab and have enough volume with the amp to suffice quite well.        Just a thought to ponder for you ............


If you like the EL84 tubes and are sold on those ............... you could consider a cathode biased Plexi 18w with reverb approach?


With respect, Tubenit

[Lectroid]

tubenit,

Thank you very much for all that.  Yes, the 6BM8 version looks more like what I'm after.  I also found Da Geezer's 7W bassman that looks interesting too.

The MOSFET in your dwgs is the Marshall-inspired cathode follower, right?

What is the purpose of the 12V Zener diode? from the drain back to the gate?

In the Tung-Sol data sheet, I found a 6BM8 single-tube amp w/ transformer-less output that might turn out 2-3 watts.  Has anyone built anything like this?  I've searched here without joy.  Can anyone explain where the -E_CC1 connection should go?

[sluckey]

What is the purpose of the 12V Zener diode? from the drain back to the gate?

It's between the gate and the source! Protection for the MOS-FET. Prevents the gate voltage from ever rising more than +12V above the source voltage. Fewer meltdowns.

In the Tung-Sol data sheet... Can anyone explain where the -E_CC1 connection should go?

Connects to a negative bias source.

[PRR]

> 6BM8 single-tube amp w/ transformer-less output that might turn out 2-3 watts.

Only if you find a 5k-7k loudspeaker.

Direct to a 4-16 ohm speaker it is a few milliWatts. (Figure 30mA peak, or 21mA RMS, in 8 Ohms is 3.5mW.)

Few-milliWatt in a good speaker may seem loud at home after midnight, but may be disappointing otherwise.

[Lectroid]

sluckey,

fewer meltdowns is a good goal. I will learn to read data sheets more carefully.

PRR,

Thank you. Should have know that 1-tube amp was too good to be true.

[PRR]

> Thank you. Should have know that 1-tube amp was too good to be true.

Oh, with an output transformer the 6BM8 could be a lame but workable guitar amp. Eat your spinach and strum like Popeye the sailor, you'll get a large Watt.

[Lectroid]

@tubenit (and anyone else who wants to chime in),

In the 9W Plexi schematic tubenit posted above, https://el34world.com/Forum/index.php?action=dlattach;topic=30283.0;attach=105814, the first stage tube V1 isn't specified.  Can I assume it's as 12AX7?

Why does the first preamp tube have a such a high grid stopper value: 3M?

Why no negative feedback loop?

On the LTP, the 82K plate resistor is bypassed by a 220pF cap.  Why?

The 1-tube reverb circuit for the 9W, the reverb tube is a 12AX7.  On another thread, (https://el34world.com/Forum/index.php?topic=26365.msg288480#msg288480,) you posted a clip that showed the same circuit with a 12AT7.  I'm thinking a 12AT7 would be better.  Is that right?

One last thing.  DIYLC showed me the attached symbol for the MOSFET but it doesn't look like the symbol for the IRF820 tubenit's schematic shows.  Are they equivalent?

Thanks for everyone's help on this.  I hope to finish up my schematic for review Real Soon Now.

[tubenit]

In the 9W Plexi schematic tubenit posted above, https://el34world.com/Forum/index.php?action=dlattach;topic=30283.0;attach=105814, the first stage tube V1 isn't specified.  Can I assume it's as 12AX7?


Use whatever tube you want.  I usually use a 5751 or 12AY7 in V1 but that's only a preference thing.


Why does the first preamp tube have a such a high grid stopper value: 3M?


Look again.  It's not 3M .......... it's 33k


Why no negative feedback loop?


No particular reason.The PPIMV does begin to negate the effect of a NFB when the PPIMV is turned down.




On the LTP, the 82K plate resistor is bypassed by a 220pF cap.  Why?


Do a search on "enhance cap" on the forum.  I use the 220p cap on all my LTPI plate resistor in that position. It smooths out the treble tone, IMO.


The 1-tube reverb circuit for the 9W, the reverb tube is a 12AX7.  On another thread, (https://el34world.com/Forum/index.php?topic=26365.msg288480#msg288480,) you posted a clip that showed the same circuit with a 12AT7.  I'm thinking a 12AT7 would be better.  Is that right?


I've used a 12Ax7, 5751, 12AT7 and 12DW7 as one tube reverb tubes.   Pick whatever sounds best to you.  Currently, I like the 12DW7 the most.


One last thing.  DIYLC showed me the attached symbol for the MOSFET but it doesn't look like the symbol for the IRF820 tubenit's schematic shows.  Are they equivalent?


I use IRF820 mosfets.  I would suggest you doing the same.


With respect, Tubenit

[Lectroid]

Tubenit,

Thanks.  I appreciate the extra information, helps fill in the holes.  I was unaware of the 12DW7 before this but I will give it a try.  It looks pretty versatile.  I will use the IRF820 of course.  Replacing a triode CF this way seems like magic, (solid state infancy here,)    but it's an elegant solution. I like it.

[tubenit]

I own 3 amps.  Two of them use a mosfet cathode follower and have a psuedo pretend active FX loop in them.  The "Minimalist" in the SCH files is one of those two.


With respect, Tubenit