Will this circuit work??? Filmosound conversion to Fender Deluxe clone

[Lectroid]

Recently I started a Filmosound conversion as my first tube project ever.  My goal was to used the old transformers (which both check out) and clone a Fender Deluxe.

After reading several conversion stories I settled on a schematic that looks good to my untrained eye.  I don't think I'm completely off the rails, but I really don't have the design chops to evaluate it without actually plugging in the variac and bringing it up to speed.

Before I do that and possibly get what SpaceX refers to as a "rapid unscheduled disassembly," I'm hoping anyone interested could take a look at my schematic and give my any feedback you can.  I'm a long-time guitarist but new to tube amp design so I'll take any kind of advice or criticism you have to offer.

Here's the proposed schematic I used to convert the amp.  I have the B&H original schematic but so much has been changed/discarded that it wouldn't help--it contained a lot of extraneous oscillator/sound-reading circuitry.

Thanks in advance!

[PRR]

Gain of the pentode will be poor without a screen bypass cap.

Gain of the pentode will be excessive with a screen bypass cap.

It can be fiddled. But _I_ would spend the few bucks for an octal hi-gain triode. 6SF5 is one.

[2deaf]

V2a doesn't have a grid reference.   
Something looks funny with the secondary on the output transformer.
I don't like any of the cathode resistors.
I don't like the .047uF coupling capacitor coming off of V1.
I don't like the 500K Volume pot.
I don't like the 500K Tone pot.
The power supply isn't grounded.
The input doesn't have a shorting jack.

[DummyLoad]

one approach. should work well for you. attached expressSCH editable schema - fill in the volt chart when done.   


--pete

[bmccowan]

I have two of these on a shelf waiting for a rebuild. Every time I take one down and look at the circuit I put it back and choose another project. With this being your first tube project I hope you have patience and good dexterity. The chassis is small and crowded. As you said, there are many good articles on the Web regarding these amps - not so many good schematics of projects though. Did you check out this series: https://guitar.com/?s=filmosound+DIY

[Lectroid]

Everybody,

Thanks for all the help and suggestions and sorry for the delay getting back.  I posted this but never got notified you all had responded.  Have to check my settings...   

DummyLoad, thanks for the extra work in forwarding that modified schematic.  The good will is much appreciated.  I definitely will use your suggestions as I tweak this circuit.

2deaf, you're right, I left a few things off the schematic thinking they would be "understood."  I will be more complete in the future.  I have a p/s ground and a shorting input jack with a 1M resistor to ground.  As far as your other comments, why don't you like the .047uf cap coming off V1?  Did you mean it's too high?  Or the two 500K pots?  Would 1M be better?  The OT wiring is taken right from the original B&H schematic that had both 8 ohm and 16 ohm outputs.  I'd like to know your thinking on these.

PRR: You were right about the screen bypass cap.  After adding it, the pentode has a gain of around 16 or so and the output on the scope looks very clean.

bmccowan:  Yes, these are a pita to work on.  I built it point-to-point and would definitely use a board next time.  If you work on one, use your tiniest hands.

Currently--I have gotten it running (sort of!) but still have a couple of remaining problems.  Maybe they have the same cause, I don't know.

I've added a 0.22uf screen cap to the input pentode V1 and a 500K grid leak to V2a.  I was also advised that the bypass cap on V2b was unnecessary and it doesn't seem to affect the sound whether it's in or out.


1.  The sound is distorted, like my Marshall 30 watt on full gain.  All notes are clear enough, no drop-out on power chords and no P/S sag that I can detect.  The distortion is on the harsh side, not as warm as even numbered harmonics.  But I haven't seen clipping on the scope.  Since I'm also new to using a scope that might not mean much.

2.  The volume isn't all I'd hoped for out of 2 6V6's.  Doesn't sound like more than 5 - 7 watts.


Does this sound like anything you've come across?  Thanks again in advance.

[sluckey]

500K grid leak to V2a

Exactly how did you connect that?

[Lectroid]

sluckey:

From the grid of V2a down to just below the 2.7K cathode resistor, between the 2.7k and the 100k.

Today I changed the coupling cap from the pentode to V2b from .047 to .0047, and it's sounding much more stable and clear.  All notes ring and sustain nicely.  Still has strong distortion since I'm running it flat out.  Tomorrow I'll install the volume/tone network and that should really civilize this thing.

Rick

[DummyLoad]

what voltages are you seeing for B+ at nodes A-B-C-D? plates, screens & cathodes of V1,V3,V4? plates & cathodes of V2? 


--pete

[Lectroid]

DummyLoad,

I see
A - 322V
B - 308V
C - 266V
D - 252V

I'll measure the other voltages and post tomorrow.  Thanks

[2deaf]

As far as your other comments, why don't you like the .047uf cap coming off V1?  Did you mean it's too high?  Or the two 500K pots?  Would 1M be better?

.047uF cuts more bass than 0.1uF when the volume is turned down low.  The guy that originally designed this volume/tone control knew what he was doing.  The millions of attempts to tweak this volume/tone control were and continue to be futile.

 

[Lectroid]

.047uF cuts more bass than 0.1uF when the volume is turned down low.  The guy that originally designed this volume/tone control knew what he was doing.  The millions of attempts to tweak this volume/tone control were and continue to be futile.

Thanks for the insight.  I have a filter calculator for simple high and low pass filters and I can see the change as I raise the voltage.  But only on the low pass filter.  The formula for high-pass doesn't divide by the voltage squared, just the voltage itself.

Can I assume you would also stick with the 'original' 1M pots instead of the 500K's I'm using?

[Lectroid]

Here are the voltages I saw today:


Tube      Grid      Plate      Screen   Cathode
------------------------------------------------------------------------
6SJ7      0      53.7      41.4      2.9
6SL7b   0      286.3   -      0
6SL7a   47.8      227      -      67.7
6V6      0      324      303.2   16.8
6V6      0      332      352*      16.8


(*At some point today I measured this as 303.4, similar to the other 6V6, but 352 is what it reads at the moment.)

I replaced the three .047uf coupling caps with 2.2nf and that cleaned up the sound somewhat.  In running without a volume pot, just my guitar controls, the sound breaks up somewhere around 3 with Gibson humbuckers.  Below that volume, the notes are clear but very Bright--like Rickenbacker bright with a treble boost.

Again, thanks for any advice or suggestions!

[PRR]

In Reply #9 you reported A - 322V.

In Reply #12 you say
Tube      Grid      Plate      Screen   Cathode
6V6      0      332      352*  16.8

It seems unlikely that 30 Volts appear from thin air. More likely a bad joint or a bad tube sucks power some times and noth others. If one 6V6 is loafing you sure could have distortion.

[DummyLoad]

.0022uF - really? for the coupling caps to the 6V6 output tubes? if yes, those roll off at 800Hz and that's a really poor choice. you want to be around 50-60-70Hz. rolloff NOT 800Hz. .022uF puts you around 70Hz with the 100K grid leaks.

the V1a coupling cap feeding the tone & volume network needs to be at least .1uF for decent low frequency response at lower volumes. leo & the boys got that one right, so no need to tweak that one.

we have LARGE coupling caps where they're not needed and very SMALL-ish ones where they aren't. if that amp has the values that indicated in the last post, you have a nothing but treble passing. 

please see attached: just some suggestions.   

respectfully,

--pete   

[Lectroid]

Pete,

You are so right--while the amp is quite loud, the sound is terribly bright and distorted.  It also has some other odd quirks I won't detail here. Tomorrow I will change the caps as you suggest on your revised schematic and report back.  Maybe that will fix some of the other problems.  I'll research the stray voltage issues and see what I can find. I've tightened all the pins but there's got to be a bad connection somewhere.

Thanks also for the general lesson about looking for coupling caps creating high  or lowpass filters. A true 'duh' moment but fairly obvious once I thought about it.  I always learn when I come here.

Rich

[Lectroid]

UPDATE:

I got the amp working this morning and I'm very happy with it.  A really decent Deluxe-(ish) clone.  It's clean and loud up to ca. 5-6 then it begins to break up with a lot of sweet harmonics.

Thank to all who offered advice and encouragement for my first build.