Dumble-ized pre-BF Fender circuit

[Fischerman]

Short version: Look at this schematic and tell me if you see any glaring problems. One issue that I'm completely in the dark about is whether I'll have enough reverb signal to mix with the dry...or too much...I just have no clue here.

Long version: Long time ago I built a tweeked-for-guitar version of the Fender Blonde Bassman (6G6-B) Bass channel. I thought it sounded better than the regular BF Normal circuit that shared the same chassis (BF Bassman). So I thought...why not Dumble-ize it and try to add reverb. I've never messed with amp reverb at all...ever. I've rarely ever USED amp reverb for that matter. I sort of blended BF Fender combo reverb with a Dumble Reverb schematic I saw here. My main concern is whether my Reverb signal will be the right level to mix with the dry signal. But I really have no clue about amp reverb so there may be all sorts of errors/potential problems.

The SSS thread got me thinking about trying to do my own (read: ripped off from different sources than others ripped their stuff from ) 'Clean Machine' and since I like this circuit I thought I'd start with it. The tone stack Bass and Treble controls are way more responsive than the BF or Skyliner (I've never tried a Mid control with this circuit yet)...especially the Bass control...very responsive throughout the sweep. But this stack has TONS of loss so it should be more responsive...I think I have gain to spare so I'm not really worried about the TS losses (besides...it sounds good to me).

Some other questions:
Is there supposed to be a bright cap across the Return Level pot? If so, what value?
Due to the output Z of the cathode follower...you use a 250K-A Volume pot and the tone stack values are scaled down too. So...what size resistors should I use for the Deep switch (in-place of the usual 270K and/or 10K tail)? I guessed at 100K for the 270K but might I have to use two different values (one value off the bass pot and another value off the volume pot)?
I didn't Skyliner-ize the Mid control...I'll probably experiment with that...not sure what effect that has on the Deep switch.

Here tis (Steel String...Rusty Nail...get it?):
[img:800:648]http://i108.photobucket.com/albums/n4/m ... ailPre.jpg[/img]

[novosibir]

The gain out of the two cascaded recocery stages will be plenty and you even don't need cathode bypass caps on any of the V3 cathodes. Also a bright cap IMO isn't necessary across the reverb level pot!

With the deep switch the 100K's are ok, but the tail then has to be diminished to about 3.3K...3.9K and the .01 cap increased to about .025

The bright caps on the volume are way too small for a 250K pot and I'd recommend 270...390p instead of your 68p and a .001 instead of your 120p

To come into the ballpark of a skyliner tonestack's behaviour (no HRM) w/ the scaled down values first the slope R has to be diminished from 220K to about 47K...68K, the bass cap increased to .22 and the treble series caps increased to .005 (1-st) and .001 (2-nd)

The mid control should be going separately to ground via a .022 cap and a 100K log. pot. The bass pot should be increased to a 100K log, wired as a variable resistor, bypassed w/ a .0022 cap and a 3.3K...3.9K instead of the 10K resistor to ground.

The 33K/68K treble resistors are concern of taste, there you can try also 39K/56K or 47K/47K - but their sum always should be close to 100K

Larry

[Fischerman]

Thanks Larry. Some of the things about this circuit are from another amp that doesn't have the switches. Like that 220k slope resistor...looks like a wierd value but it sounds great so I was hesitant to change it. All those weird values in the tone stack are not what I want/need to change because the 'base' circuit sounds great and I don't want to lose that.

I don't really want 'Skyliner behavior'...I used this base circuit because I like it better than Skyliner. But I did want to add some switches and the Deep switch is a challenge.

I originally used 82k/22k for that fixed treble part of the stack but I usually run the treble a bit high on that amp so I thought I'd bump it up to 68k/33k. I thought the bright caps were too small but I like a very subtle bright cap so I wanted to start small (the 47pF value is perfect for me for a regular BF circuit...for me 100pF is too big). One of the beauties of this circuit is that it has great sparkle without ANY bright cap...and that makes it a great platform for pedals too!

[Fischerman]

FWIW, over the holiday I finished rebuilding this circuit into an old homebrew that I did many, many moons ago and have gutted/rebuilt three times now. That amp is no-NFB, cathode-biased, 2xEL34/6L6 amp...only about 395vdc on the plates. Matchless-style cross-cut MV and Vox-style Cut control. I used a slightly different PI...something similar to a SF Fender PI: 12AT7, 47k plates, 470R cathode, 22k tail, 470k grid loads...very 'clean'. The amp is only like 21W clean and 30W cranked. Heyboer TW 5200ohm OT and some Heyboer PT I got when I built it (similar to a TW PT but less stout...IIRC only 200mA HT rating).

The Deep switch doesn't do much at all (almost inaudible difference) and the Treble pot is too big but the base circuit sounds awesome. I used 120pF and 220pF for the Bright switch but they're a little too small...but it doesn't really need a Bright switch imo. It's really clean/sparkley at low volume...then it gradually/smoothly goes into a Voxy-type of breakup and then when cranked it gets darker/warmer and has a really nice distortion/sustain (somewhere around an AC30 amount of distortion...maybe a little more). I was surprised at how good it sounds when cranked. It has quite a bit of compression (even when clean) which I like.

The Reverb doesn't work though...still sussing that out and have spent almost no time on it. Everything seems to be operating except I get nothing out of the pan.

Now that is one fugly amp! Labeling is from previous builds. Control panel from L-to-R is Input-Gain-Bright-Mid Boost-Deep-Bass-Mid-Treble-Rev Send-Rev Return-MV-Cut.
[img:800:600]http://i108.photobucket.com/albums/n4/m ... ottom1.jpg[/img]
[img:800:600]http://i108.photobucket.com/albums/n4/m ... /front.jpg[/img]

[novosibir]

I don't know, what you took from my recommendations above.

Can you post an updated schem at least of the preamp section including the reverb circuit? Then maybe I can be of some help.

Larry

[Fischerman]

The circuit is like the schemo above with the following changes:
Bright caps are 120p and 220p.
Deep switch 'tail' resistor is 3.3k.
Bypass cap on Reverb Send stage cathode is 25uF.
Bypass cap on first Reverb Return stage is 25uF.

That's it.

Pan is an Accutronics 4AB3C1B. I have 382vdc supply (rev tranny blue wire), 376vdc plates, 7.6vdc cathode. Black wire is to ground, green wire is to the Reverb Send jack.

When I input a 880Hz/100mV signal (true rms) to the amp and turn up the Volume to a reasonable level I have ~160mVac signal at the 12AT7 grid. Then I have ~3.1Vac at the plates but then only ~52mVac at the Reverb Send jack (I can barely even measure it). Then I get nothing out of the pan. EDIT: If I hook up a small 6 ohm nominal computer-type speaker to the Send jack I do get sound...low volume but I get sound. When I input signal to the Reverb Return jack it gets amplified just fine.

I measured the pan and got about 1 to 1.5 ohms for the input transducer and IIRC like 217 ohms for the output transducer. The input transducer is totally isolated from the chassis and everything else...the output transducer has it's ground common with the chassis.

[novosibir]

[quote="Fischerman"]When I input a 880Hz/100mV signal (true rms) to the amp and turn up the Volume to a reasonable level I have ~160mVac signal at the 12AT7 grid. /quote]
After the plate of V2b - what's just separated by a cap from V4's grid - with 100mV input level should occur somewhat about 20,000mV

You get 160mV
There's something fishy!
Check the level after the .047 from V2a's plate!
If there's more - check the Send pot!

Larry

[Fischerman]

Thanks Larry. I did think that was a small signal level there but it is of course dependant on the Volume control setting (this is at a low Volume setting with the Bass/Mid/Treble at halfway). The signal is taken right off of the Treble pot center lug...point-to-point with a 560pF cap to the Send pot. I measured both sides of the 560pF and they were like 175mV and 160mV...something like that. It might be just because the volume control is set so low...the amp is making sound but not really loud.

But it appears that messing with those little connectors inside the Reverb pan when I measured it made the difference. It now works! It's not super strong...no surf music but it is plenty for me. I actually like it best with the Send pot just below max...all the way up and it rings a little too long (for me). Return pot on 6 or so is about right. There is some hum when I increase the Return pot so now I'll have to work on that.

[novosibir]

There is some hum when I increase the Return pot so now I'll have to work on that.

The Reverb pan's output has to be far away from the PT, otherwise it might pick up some hum from the PT's stray field. In combo cases no prob, but in head shells the pan sometimes comes terrible close to the PT.

Sometimes flipping the pan left to right can help, but I've already had the case, that closing the pan's bottom with a metal shield was inevitable.

Larry

[heisthl]

A short tank helps but even with a long tank put the Pan's input end towards the power transformer with the whole tank as far away from the power transformer as possible. I don't know why this works but it does.

[Max]

There is some hum when I increase the Return pot so now I'll have to work on that.

The Reverb pan's output has to be far away from the PT, otherwise it might pick up some hum from the PT's stray field. In combo cases no prob, but in head shells the pan sometimes comes terrible close to the PT.

Sometimes flipping the pan left to right can help, but I've already had the case, that closing the pan's bottom with a metal shield was inevitable.

Larry

Hi,

Some info added:

I never found any audiable "hum pickup" in any of the original Dumble heads with reverb.

How HAD placed the tanks in his heads you can see in some of the pictures you find on Bill's or Rob's site of ODS 150 Watt, ODS Reverb, DL 300SL, and some 100 Watt SSS.

In all the original amps I met he always used the big tanks (some four springs, some six).

All the tanks of the originals were open to the wood they were mounted on. Never met some kind of additional metal shield in an original.

He always (in regard of the amps I met) used the kind of shielded wires to connect the tank to the amp that he used inside the same amp for the signal transport.

Cheers

Max

[novosibir]

All the tanks of the originals were open to the wood they were mounted on. Never met some kind of additional metal shield in an original.

From the mid to late 90' I've rebuilt several Fender SF Bassman 135 heads. The left side became Fender Blackface Clean, the right side Dumble OD - both channels run into Reverb & Loop, so that I've had altotal 7 preamp tubes and placed the Reverb tranny on top of the dog's house.

When you know the Bassman 135 head, a very flat head case with a very big PT inside, then you can imagine, that after putting a Reverb tank onto the bottom, that you even couldn't get with your thumb between the PT and the Reverb tank. In this case I only couldt get rid of the hum, after I'd closed the tank's bottom with a metal plate.

I don't know, what size (height) of the head case Fischerman is using with his build, how much space is between the tank and the PT, therefore my recommendation.

Larry

[Max]

All the tanks of the originals were open to the wood they were mounted on. Never met some kind of additional metal shield in an original.

From the mid to late 90' I've rebuilt several Fender SF Bassman 135 heads. The left side became Fender Blackface Clean, the right side Dumble OD - both channels run into Reverb & Loop, so that I've had altotal 7 preamp tubes and placed the Reverb tranny on top of the dog's house.

When you know the Bassman 135 head, a very flat head case with a very big PT inside, then you can imagine, that after putting a Reverb tank onto the bottom, that you even couldn't get with your thumb between the PT and the Reverb tank. In this case I only couldt get rid of the hum, after I'd closed the tank's bottom with a metal plate.

I don't know, what size (height) of the head case Fischerman is using with his build, how much space is between the tank and the PT, therefore my recommendation.

Larry

Hi Larry,

My post was not meant as a comment in regard of what you recommended. I'm no tech, so I don't have the knowledge to judge what you recommend.

I just would point to the way HAD does it in his original amps (after all this is a "Dumble Discussion" forum, isn't it?). I just thought that perhaps someone might find some fun in the info.

Cheers

Max

[novosibir]

No prob, Max!

I didn't read your post as a criticism or an affront.

Any helpful info is welcome anytime

Larry

[Fischerman]

Thanks for the responses. I don't think the hum is pan related...it hums the same with the pan disconnected (of course...then the Reverb Return input grid is just 'dangling' there at 220k above ground).

Right now it's not in a cabinet...it's just sitting on the bench with the pan at the preamp end. The cabinet will either be a Bandmaster Reverb head cabinet or an old leftover Kendrick Climax (aka Vibrolux Reverb combo) cab I bought many moons ago when Kendrick was clearing them out. That cab has 2 each Weber 10F150T and always sounded like shit but this amp actually sounds decent through it...but that cab eats power like nothing else and this amp doesn't have power to spare.