Gutting a Peavey Classic 20

[Nthagroin]

Hi all. Brand new member.

I'm planning on taking out the boards of a Classic 20 and building something handwired in the chassis. I saw there was a pretty popular rendition of this type project for a Blues Jr. on the site and figured this would be a good place to get some information.

Planning to keep:
-single input
-2 12AX7
-2 EL84
-Output Transformer
-Volume
-Treble/Mid/Bass
-Master Volume
-Boost switch

Planning to add/change out:
-Fuse holder (Fender style)
-Progressive Power Switch (Off/Standby/On)
-Either replace power transformer or add 5V filament transformer from Weber for
-Tube rectifier (5Y3 or 5U4, open to suggestion)

My aim in posting is to feel out the members of the forum to see what circuit would be a good substitution, allowing me to retain most of the function of the original chassis. (AC15, Marshall 18w?)
I want to get a turret board from Hoffman soon, so when I settle on the circuit, I can design the layout and so on.
I realize this project sounds more trouble than it's worth, but it's probably the cheapest ground-up build I can do at the moment and have a terrible itch to get some real world application for recent knowledge acquisition.

Open to any and all suggestions.

Thanks so much.

-Andy

[Deric]

I'm not going to be a lot of help, but....


Assuming the amp functions as-is, I've always REALLY dug the Classic 20.  Back when they first came out a local store had one for a couple of years that they couldn't sell.  No one was into to small tube combos back then....  It was my favorite amp to plug into any time I went into the store.  I never bought it because....well...    Anyways....


Several months back a customer brought one in to have fixed.  Looked like it had been in barn/trunk/basement for 20 years.  Cleaned it up, put in new tubes (whatever you do some sort of tube guard is a must) and fired it up.


Every bit as cool as I remembered.  Actually, cooler than I remembered.  He was just fixing it so he could sell it.  Still kick myself for not offering him $50 for it - he probably would have taken it.


If you're going to do it anyways....  A Marshall 18 lite TMB might be a good option.  Stock voltage is in the ball-park IIRCC.  Keep in mind the stock PT has no CT and uses a FWB so not really practical for a tube recto.  You can do a SS diode/tube bridge.  May be other options too...


Anyways....good luck!!

[Deric]

Another cool - higher gain - candidate would be a Phaez Daisycutter.  One of my favorite low-watt (ish) high gainers.

[tubenit]

Something like this? 

With respect, Tubenit

[Nthagroin]

Thanks for the responses:

Tubenit, I was going to try to stay a little truer to the Classic 20 design (no FX loop, split load PI, no choke, Master Vol before PI), which I think the Daisycutter appears to be:

http://music-electronics-forum.com/attachments/24687d1376516627-18w_daisycutter_100422.jpg

Maybe just simplifying where possible and changing coupling cap values and other tonal adjustments.

A few issues I'm having studying the Classic 20 schematic, though:

http://www.blueguitar.org/new/schem/peavey/c20schem.gif

-There are a few caps and resistors in front of the first tube's screen grid that I am not sure of their function. Why all that?
-There are diodes between the EL84 plates and the OT primary that I'm not clear on either.
-I'm also a little iffy about the boost switch. Does it just put a resistor in the circuit before the Treble pot?

If I replaced the stock PT with something like a 290-0-290 with leads for filaments at 6.3V-CT/5A and 5V/3A, I could get away with heating a tube rectifier and eliminating the extra filter caps for the SS FWB in the schematic?

Could I otherwise stay fairly true to the original schematic? I'd put a single fuse in the chassis and also upgrade the switch to include a standby position.

Thanks again.

-Andy

[Willabe]

-There are a few caps and resistors in front of the first tube's screen grid that I am not sure of their function. Why all that?

R21/R22/C21?

That's the cathode (K) bias R/C's. The screen grid(s) are not connected to them. The suppressor grids are.

-There are diodes between the EL84 plates and the OT primary that I'm not clear on either.

Those diodes are not in series between the plates and OT, they go to ground and are for killing hi voltage spikes to protect the power tubes.

[Nthagroin]

No, I was looking at V1A. (Mainly C12 and C9)
I'm used to seeing an input/grid leak resistor for the pickups on the jack and a grid stopper between that and the 12AX7 grid, but not typically these caps.
Can they be eliminated?

[Willabe]

-There are a few caps and resistors in front of the first tube's screen grid........

The 1st tube/V1a is a triode, plate/grid (grid 1, G1, control grid, input grid)/cathode, no screen.

I'm not sure why they have the 2 caps there. Might be to bleed any RF to ground?

Can they be eliminated?

If your doing a gut and rebuild, then probably you can get rid of them. Just wire it up like a standard Fender/Marshall/Vox/............ input stage.

[PRR]

> a few caps and resistors in front of the first tube's screen grid that I am not sure of

"Screen Grid" is usually the *second* grid of a pentode.

You are pointing ta a triode's control gird (or just "grid").

You can bring signals direct from the big bad world to the 12AX7's grid. 90+% of the time this works.

If you come from a source with *no* DC conductivity, the 12AX7 will mis-bias and work bad.

If you play a truck-stop full of CB radios, or under an AM radio tower, you will have extra players shouting a totally different play-list.

If you come from a pedal with DC leaking from its output, the 12AX7 will mis-bias and work bad.

If you come from a loudspeaker output line (!!) or 120V AC wall-power ( ! ! ! ), you *could* melt the 12AX7's grid (ruin the tube).

The R8 C12 C9 R11 affair mitigates all these problems for less than a buck.

Even cheap Fender used about 50K (2*68K) and 1Meg to cut-down radio and set working bias. (He didn't have a lot of DC-leaky sources.)

To criticize a fine plan:

The 39pF is perhaps "umbrella and rain-hat" since the 12AX7 has about 100pFd inside it.

Some situations (piezo pickups) might favor a higher value than 470K.

C9 is the only part that "costs money". You may wonder how much to spend to defend against offensive sources (DC-leakers). However bad stuff happens, upsets the user who has no idea why the amp works bad. 40 cents (quantity) may be cheaper than answering irate calls from a few customers.

To critique the overall amp:

The "sound" may have a lot to do with the very low gain in the output stage, and high tonestack losses, forcing V2a to work at VERY high signal levels. They do that so you can clip the crap out of V2a, yet work the 18 Watt power stage down at 2 Watts for gnarly tone without ear-bleed in smaller venues. My heart says "play LOUDer!!" My decayed hearing says "keep it down!". Getting big-amp tone from a small or strangled amp is not easy (maybe not possible). The C-20 plan is a fine commercial compromise. If you are going to torch&gut, many fine Fender and Marshall designs can be taken entirely.

Boost switch extends the range of the Treble pot into middle frequencies. "Boost" may not be the best name, but it looks good in the showroom.

[Nthagroin]

Thanks for clearing up the "screen" grid, guess I understand the function of it better than the naming scheme.

I'll probably go with a more Fender-ish input -> V1A grid resistance (68k on the grid, 1M at the input jack)

Would I run into any trouble putting that tone stack behind V1B, using the same voltages from the power section at V2A for make-up gain and putting the master volume in between the V2A and V2B load splitter (where it currently lives)?

I'll work on drafting this up in hopes of getting some feedback.

Thanks again for the help.

[PRR]

> Would I run into any trouble putting that tone stack

IMHO: Don't get clever until you have a lot of disappointments behind you. There's hundreds of known-good designs, steal one verbatim.

[MFowler]

Keep it simple, how about a 18w Marshall Plexi with MV.

[Nthagroin]

Okay folks.

So, I drafted up this partial layout pieced together from a few different circuits.
Obviously missing are the PT, 5Y3GT, filter caps and resistors to step down plate voltages.
BUT, I was hoping I could get some feedback as to how this circuit might perform, if at all.
Would probably mount the filter caps right on the turret board between the EL84 cathode bias resistors and the feedback circuit.
Not sure if the direct coupling of triodes across different envelopes is frowned upon.
The 47k on the EL84 grids from the phase inverter seem higher than other circuits, but this looks like the Classic 20 value if I'm reading the schematic (linked in previous post) correctly.
Mainly, was hoping someone with more experience and knowledge might provide some insight.
Would be happy to answer any questions about my thinking on architecture or values etc.
Look forward to hearing back.
Been super helpful so far.

-Andy

[Nthagroin]

Also wide open to more traditional or cleaner arrangement suggestions.
Thank you thank you thank you.

[Nthagroin]

Hi again.

I drew a more complete layout diagram.
Was really wanting someone to take a look and see if it will blow up.
There are a couple of places where wires were drawn to the wrong spot, messily crossed out.
Please also be aware that the 1.5k resistor on pin 8 of V2 does not go through the 68k as it looks, that's a direct connection to the cathode.
The feedback loop was taken from the Classic 20 schematic, I'm not well versed in that and curious if it's overly complex for my purposes.
I'd truly appreciate any comments.
I know the recommendation is to copy a tried and true layout, but I wanted to retain the controls already in the Classic 20 chassis.
Not certain about the B+ resistors, either.  Have seen a pretty wide range across different circuits (even among different 18W plexi clones).

My thanks in advance.
Would love to get this thing off the ground.

-Andy

[Nthagroin]

"Cricket. Cricket."

Hmm.
Okay. Must be worse than I thought.
Guess I'll hit the books some more and then the drawing board.
 

[Willabe]

For me, it's hard to see the amp's circuit looking at a layout drawing.   

It might help if you drew up a schematic for the guys to see?

[sluckey]

It can be tough trying to understand a circuit just by looking at a layout. A layout is great for building something. A schematic is great for understanding a circuit. Post a schematic so we can see what your circuit looks like. Then we can compare to the layout to see if they match.

[MFowler]

Checkout Mark Huss's 18w schematic or view his website for complete details.

[Nthagroin]

Drew this up to clarify.
I calculate 328V at A. Rectifier is a 5Y3GT.
The layout drawing (above) would need a jumper on the left side of the split-load connecting the 68k - 1M - .047micro
The two should otherwise match up, whether or not it is sensible.
Please let me know.

[Nthagroin]

noticed after posting (of course) that I put an extra connection in the feedback circuit.
There should be no connection between the 2.2k 470k and the cap/resistor in series above it.
Sorry.

[shooter]

I was bored watching glue dry so I re-drew your schematic, check it for drawing errors.  Wasn't sure on the boost switch.  A suggestion, add a *D* node for V1, and use the *C* node for V2.  Don't know what power tubes you're using so the pin-outs are for EL84's

[Nthagroin]

Cool.
Thanks shooter.

Everything on there looks like what I drafted up with the exception of a second 220Ω/5W on the EL84 cathodes.
The boost switch is right, as I understand it.
Just curious about the reasoning behind another node? Not opposed at all, just an uneducated n00b.

[tubenit]

Please hear this respectfully because I mean this respectfully.  IF you build this amp with the original values you posted,  I will be surprised if you like the amp. 

Using EL84's,  those values will give you a very muddy distorted non-articulate tone like one might find in some metal/grundge amps.   It will lack transparency and articulation.  IF that is the tone you are going for use the values on your original schematic. 

IF you want something that is more touch sensitive and transparent then I would suggest trying values closer to these.  My guess is that your amp will need some smoothing caps to smooth out the buzzy tone of the EL84's?

With respect, Tubenit

[tubenit]

Here are a couple of tone stack options for you to consider.  The one on the left is just another way of doing what you are doing with a mid boost.   

The one on the right is what I've used on my D'Mars builds and on a recent HoSo56 build.  And I like how it sounds.   It allows the mids to be adjusted more independently of the bass/treble.  This also gives a nice tight bass tone, IMO.  The typical Marshall tone stack is great sounding but sounds more compressed to me.

Not necessarily better but another option that you might like?

With respect, Tubenit

[shooter]

the reasoning behind another node

The wise here can do better explaining, my understanding; If you "sting" to many pre-amp tubes on the same node you end up with problems like oscillations, positive feedback loops, overall bad things you don't wanna fix after the build.  It also give you a better regulation for the most sensitive V1 section.  you can control plate voltage easier by swapping 1 dropping R instead of changing 2 plate R's.

Tubnit is correct, the amp will probably lean to the "grungy" side. 

[Nthagroin]

Awesome.
Thanks for the feedback.
I'll certainly implement most of those changes in my next attempt.
Forum has performed as advertised.
Appreciate it.

Best,

-Andy

[John]

Forum has performed as advertised.

Thank God, we spent a fortune our ad campaign!






 

[Willabe]

 

[shooter]