Gutting an old SS combo to put in a low power high gain monster

[Snicksound]

In exchange for some work on an amp, I was gifted an old Beckemer SS combo that doesn't work anymore. It didn't sound very good so no point in fixing it, instead: we gut!

Plan is to reuse the front controls as is, so that means a clean and a dirty channel with a shared EQ. So clean channel will be sort of like a Plexi, and then dirty channel will kick in 2 extra stages (one a cold clipper) and a Master Volume.
Speaker is a 10" (I believe of Eminence origin) that doesn't sound bad at all. Space in the cab is limited, and low budget is the name of the game, so I'm reusing transformers I already had and going for low power.
PT will be a Hammond 269GX which will give me just a tad over 300VDC. Power tube will be a single JJ ECC99 dual triode in PP good for 2-3W or something. For the OT, I have an unused 1750PA gathering dust so I'll use that. 8.4K is of course way too low so I'll be using the 4 ohm tap for a reflected impedance of 16.8k (8 ohm speaker). Still a bit low, but high enough.

I took some inspiration from Marshall's line of 1W amps. They used a cathodyne PI in a few of them. Dual triode power amp don't need a lot of signal swing and don't pull a lot of current, so this plus big 100k grid stoppers for the power amp means I can use a cathodyne easily without it sounding like a cathodyne amp (nothing bad about those, but not a good match for a high gain preamp). This allows me to use only 3 preamp tubes (3 triode gain stage, 2 triodes for the DC cathode follower driving the tone stack, and the 6th triode is the cathodyne). Should be fine without a recovery stage after the tone stack. Worst case... I'll add another tube... or a MOSFET.

The front panel also features a Reverb control (the reverb tank is trashed) and a headphone output. I will use the headphone output as a slave output with the Reverb control as a level for that. Coupled with a speaker output that shunts into a big resistor when unused, I will be able to use it Herzog style into a bigger amp. Or with the speaker connected, I can run the slave out into my HX Stomp and make it sound like I'm playing through a 412 for the FoH feed.

IMG_20200424_161606.jpg

I'm splitting the circuit over 2 boards, preamp and power amp. This way, if I really like it but want more power, I don't have to redo the preamp.

IMG_20200425_095647.jpg

Populating the preamp board. I don't like not having access to everything once installed, so power rail wires are on top of the board. Doesn't look as good, but it works. Running a parallel rail for the 1st stage (that one's job is gain and headroom, so I want the most voltage available), had to do the "under then over" thing to leave room for the 2nd cap.

IMG_20200425_111215.jpg

View of the donor amp and the almost finished boards. Rk is not final value, and it's missing a few caps. I'll play around with values before committing. Just want to get it running first. Proof of concept.

A 3PDT toggle switch will go in the "Channel" position. One pole switches between Level 1 and the output of the 3rd stage to feed the Cathode Follower stage. One pole grounds the circuit between the 2nd and 3rd stage in clean mode (for lower noise floor), and 3rd pole bypasses Level 2 (aka the MV) in clean mode. So clean is non-MV (like a Plexi).

Contour will be backwards Mid. Haven't decided what Bite will do yet.

94225283_671548050289399_7469092185242075136_n.jpg

Mocking around a layout.

[ChopSauce]

Of course, I do not cease being astonished by all these professionaly looking builds around but I do like this kind of sleepers ...
(especially when the builder does not attempt to redesign the outside of the amp, too)

The front panel also features a Reverb control (the reverb tank is trashed) and a headphone output. I will use the headphone output as a slave output with the Reverb control as a level for that. Coupled with a speaker output that shunts into a big resistor when unused, I will be able to use it Herzog style into a bigger amp. Or with the speaker connected, I can run the slave out into my HX Stomp and make it sound like I'm playing through a 412 for the FoH feed.

Not sure I understand this. I'll Google that as well ...

[thetragichero]

what if, instead of completely bypassing the output section when the slave out is used, you figure out the appropriate voltage divider and have the line out go through the tubes? I've used this to good effect, stealing the values from old acoustic brand schematics (the booster output to feed a power amp)

[Snicksound]

Of course, I do not cease being astonished by all these professionaly looking builds around but I do like this kind of sleepers ...
(especially when the builder does not attempt to redesign the outside of the amp, too)

Not sure I understand this. I'll Google that as well ...

Thanks! I've wanted to do a silly build like this for a long time but didn't want to spend too much money on the donor amp. Got this in exchange for a reduced rate on a new amp for a friend.

The slave out basically takes a tap from the output transformer and sends it out as a line level signal (just a resistor in series with a potentiometer). This allows you to get exactly what the speaker is getting, so you can run it through IRs and make it sound like it's going through whatever cab you want. So I can have this little guy close to me on stage for my own monitoring, but send a signal to the PA that sounds like a mic'd 412 if I want to.
Or you reamp this signal and this 50W amp can be a 1000W amp if you want.

And I'm wiring the speaker output so that when the speaker is disconnected, a dummy load is automatically engaged. This way I can have it completely silent and still get a "post transformer" signal. It's resistive load only though, but if you're reamping through a tube amp it can still sound pretty cool.

[Snicksound]

what if, instead of completely bypassing the output section when the slave out is used, you figure out the appropriate voltage divider and have the line out go through the tubes? I've used this to good effect, stealing the values from old acoustic brand schematics (the booster output to feed a power amp)

I'm not bypassing the output section, the slave out is taken from the speaker jack so it's getting the full "picture"

Only thing is, in silent mode (with speaker disengaged), it's only a resistive load which makes it sound like there's too much NFB. But it's better than nothing.

[thetragichero]

gotcha, that's what i was suggesting. guess i misinterpreted what you were saying. my b

[T Wilcox]

I just did this with a non working Fender Frontman 212R
Built a Matchless Lightning at first then after a week added 2 tube reverb as well.
Cool amp! Now I'll be on the lookout for other suitable donors

The chassis before I gutted it
https://imgur.com/fwaFyBC

The chassis after finishing the build ( before reverb, I havent taken any pics of the reverb ckt yet)
https://imgur.com/il36ak9

In the cab
https://imgur.com/pJgBbpt

https://imgur.com/hAKud6J

PS I dont think I know how to add images to posts correctly, please advise.

[Snicksound]

I'm gonna say, as I was wiring this little guy I started having some doubts. So much going on around the channel switch. But it turned out great! No hum, maybe a little more hiss than I'd like. No squealing, even though I forgot the usual small cap from plate to cathode on the 1st stage (common on high gain builds).

I'm still tweaking things. Initially had a 39k Rk on the cold clipper, à la SLO/Dual Rec, but I thought it sounded better with a 10k à la JCM800 so I switched that but now it has stupid amount of gain (but no squeal!). Circuit is basically a JCM800 with a boost built-in at this point. Except for the tone stack which favors a more scooped tone.

So I may work on lowering gain. Also, this thing is painfully bright unless I turn either mids or treble REALLY down. I don't really care if my controls look stupid, but I would like to get a bit more brightness out of the clean channel so I may change a few things around to better balance them.

Anyway, here's a little quick test so far. This is through the original 10" speaker that came with the amp. https://youtu.be/n-3L_8CMnAk

This is what the circuit looks like at the moment

Beckemer 2W v0.2.gif

[xtian]

Nice shredding!

Also, this thing is painfully bright

No kidding. I see a 470p/470k treble peaker AND a 1000p bright cap prior to the first stage, and another treble peaker prior to second stage. Yikes!

[Snicksound]

Nice shredding!

Also, this thing is painfully bright

No kidding. I see a 470p/470k treble peaker AND a 1000p bright cap prior to the first stage, and another treble peaker prior to second stage. Yikes!

Thanks!

Yeah I know, but that's pretty common for this type of circuit. Without this the overdrive is fuzzier and not as tight (I did try it). I just need to counterbalance it with some low passing later in the circuit. Running the Treble control low works, I just need a better balance between both channels.

[xtian]

My cheap advice: Adding more EQ to "too much" will make things worse. Why not lighten the harsh treble boost, instead of trying to compensate later?

[Snicksound]

My cheap advice: Adding more EQ to "too much" will make things worse. Why not lighten the harsh treble boost, instead of trying to compensate later?

Because said treble boost is actually more like a low cut (it's not adding treble, it's letting less bass through). Focusing on mids and highs early in the circuit gives the tight distortion I'm looking for. Without these the resulting sound is a lot muddier, with no definition when doing palm mutes. I could of course just use very small coupling caps, but I find it sounds a bit unnatural. The effect is too drastic.

I do agree about keeping things simple, but with this much gain, a little tailoring is required. At the end of the day, this pretty much has the same treble peaking as an original JCM800. Like I said, it's a JCM800 with the boost built in. Those Marshalls are also very bright.

Also have to remember that the classic Fender/Marshall tone stack emphasizes the highs whenever the Treble control is not on 0 (flat-ish is Bass 0, Mids 10, Treble 0). So there's nothing wrong about setting it very low.

EDIT: Should specify, the point is to reduce bass before where the distortion happens, else it takes over the distorted sound. EQ before and after distortion has different purposes.

[Snicksound]

So, after lots of tweaking, here I am:

https://1.bp.blogspot.com/-nrMNpXc0fyE/ ... 2Bv0.6.gif

Turns out, good old Marshall values for the tone stack make for great high gain tones. Presence was implemented as a Cut control due to the way the PI (stolen from the Marshall JTM1) works)

And here's how it sounds:
https://www.youtube.com/watch?v=N1KdwrFCNic

[drew]

PS I dont think I know how to add images to posts correctly, please advise.

Hit the “Attachments” tab, below the reply box, and follow the directions.

[ViperDoc]

That sounds awesome.