My 36W Project

[TriodeLuvr]

Thought it would be fun to post a pic of the prototype I built four years ago. It's just been sitting in the garage, waiting for some kind person to give me a free combo cabinet. I'll try to add some pics of the current state of the project soon.



Jack

[TriodeLuvr]

Making some progress now...

[geoff 1965]

will you have any issues with all the transformers in close proximity?

[TriodeLuvr]

will you have any issues with all the transformers in close proximity?

I hope not. The power and output transformers are positioned at right angles to each other to minimize coupling. The small "transformer" under the chassis nearest the output is a 0.5H choke with very little AC current flowing through it. At any rate, if this arrangement does something I don't expect, I'll wrap a few turns of Ultraperm (mu metal) around the output trans. Gotta admit, I would like to have a little longer chassis for this project. Eight tubes plus all the transformers, chokes and other parts takes up more space than I realized. I think it will be OK though.

Jack

[TriodeLuvr]

More progress today... stripped the chassis down again (hopefully for the last time) and cut all the holes for the tube sockets. This floor plan is pretty crowded, so I've decided to mount all the large axial power supply caps between two rows of terminal strips on the top side. I'll need to fabricate a cover from .050" aluminum and screw it to the chassis. The caps will be opposite the turret board that's underneath (inside) the chassis, so this takes advantage of unused real estate. I'll post more pics when there's something to see.

Jack

[TriodeLuvr]

Making some progress. Still a lot to do, but was able to fire up the HV using a variac. Everything good so far. This layout was difficult due to chassis size and the number of tubes. One area where I had to compromise is the position of the turret board pins to drive the output tube grids. The wires will cross over other stages plus filament wiring, so shielded cable will probably be needed. No big deal, just one more part of the process.

Jack

[TriodeLuvr]

Decided to use low-capacitance miniature coax for the long runs back and forth. I'll need two lengths from the front side of the turret board to the EL34 grids, and another from the Effects Return jack on the back to the Master Volume pot on the front panel. I'm using Belden 9221 for this. It's terrific cable in terms of specs and ease of routing, but the center conductor is thin and fragile. Here's how I adapt it for hand-wired tube circuitry like this.

[TriodeLuvr]

Well, the phase inverter and outputs are done now. Fired it up with a sig gen connected to the master volume, and she's looking good. I even accidentally got the OPT phased correctly right off the bat! No oscillations or noticeable hum showing on the scope, the presence control is working, and the bias pots on the rear panel do what they should.

The only issue I found is excessive PS sag. Turns out the reservoir cap is a little too small (fooled my simulator because I forgot to account for the ESR). Anyway, they make the larger value it needs in a physical size that will fit in the same space. I'll finish up the preamp and reverb stages while I'm waiting for the replacement to arrive.

Overall, a big "Yay!"

[TriodeLuvr]

It's alive! Everything except the reverb stage is wired now. The PS sag was due to a missing connection. LOL, the first filter cap wasn't in the circuit. There's some low level humming and buzzing here and there, but I'll work it out. The guitar creates a slight buzz in this amp, goes away when I touch the pickups. Damn, this thing is loud! Really difficult to only turn it up a little. I even went in with my ohmmeter to be sure the gain pots are audio taper. Great tone, definitely has the Brit personality I wanted. Not harsh when it's crunched, and it sustains forever on the High input. I'll work more on the minor bugs after the reverb section is finished.

Jack

[Bieworm]

Good luck!! Looking good then?
Reverb... the final frontier!! Verbs are potentially noisy buggers though..

[crgfrench]

I even accidentally got the OPT phased correctly right off the bat!

I love it when that happens

[TriodeLuvr]

Man, this cold weather is killing my project. I need to fabricate one small insulated panel to finish the chassis, but the garage is just too cold to work or run the machinery. On a good note, I went through the entire amplifier stage by stage this evening checking gain and hum. The gain structure is just about perfect and agrees closely with SPICE simulations of each stage. That includes the modification to "linearize" the second stage when the Low input is in use.

The hum I noticed earlier has turned out to be a non-issue. One source was the 12AT7 reverb stage, which was humming because the recovery side isn't connected to the tank yet. The other source is the guitar itself. It's got a hum that goes away when I touch the bridge or turn down the volume to zero. The amp itself isn't the cause.

Bottom line, I think the amp is bug-free. If everything goes well, I should be able to finish the reverb hookup and get everything into the cabinet by the end of the week.

Jack

[JMPGuitars]

The hum I noticed earlier has turned out to be a non-issue. One source was the 12AT7 reverb stage, which was humming because the recovery side isn't connected to the tank yet. The other source is the guitar itself. It's got a hum that goes away when I touch the bridge or turn down the volume to zero. The amp itself isn't the cause.

Does the reverb tube hum go away with the reverb knob(s) turned all the way down?

[TriodeLuvr]

The hum I noticed earlier has turned out to be a non-issue. One source was the 12AT7 reverb stage, which was humming because the recovery side isn't connected to the tank yet. The other source is the guitar itself. It's got a hum that goes away when I touch the bridge or turn down the volume to zero. The amp itself isn't the cause.

Does the reverb tube hum go away with the reverb knob(s) turned all the way down?

Yes. It's just humming because the grid of that stage is hanging in the air with nothing attached. It has no clue what it should do.

I decided today to add a fixed attenuator at the output of the tone stack. The MV is just too touchy without it, and peak-peak voltage at the stack is way larger than it needs to be to completely crunch the output stage. The attenuator gives the MV a more useful range, and it also provides a more appropriate level for the effects loop.

In the course of all this work and testing, I managed to find an intermittent putt-putt oscillation with the MV at maximum. It was cured by breaking my rule about star grounds and running a second ground from the Effects Return jack where the MV connects to the power ground used for the reservoir cap. It's completely stable now, no matter how the controls are set. Go figure.

Jack

[TriodeLuvr]

Yay, it works! My 36W is now bug free and making lots of tone! I just need to tighten down all the hardware and perform some cleanup before installing it in the cabinet.

At this point, I have a question. I based this amp on the 2104 MV with Low and High inputs. The High input cascades an additional 12AX7 into the preamp. When that input is in use, is it normal for the Master Volume to be extremely touchy at low settings? What I mean is that the control is very difficult to set at lower volumes. It gets loud immediately when it's barely moved off the stop, and it's almost impossible to set for a reasonable level in a living room or bedroom. I confirmed the log taper of the pot with an ohmmeter and double checked the grounding. There's simply a huge signal at the output of the TMB. Is this behavior normal for this amplifier?

Jack

[Bieworm]

Jack, that's what you get with adding a gain stage. Did you attenuate the signal before cascading? You might wanna play with some values there with a/the voltage divider ...

[TriodeLuvr]

Jack, that's what you get with adding a gain stage. Did you attenuate the signal before cascading? You might wanna play with some values there with a/the voltage divider ...

I built the entire preamp section identical to the Marshall 2104. I did add a voltage divider just ahead of the PI, but it's only marginally helpful. The problem is that there's a huge difference in signal amplitude between the Low and High inputs (approximately 60X). If the PI is attenuated enough to make the High input work well at the MV pot, then the Low input doesn't have enough gain. On the other hand, High gain can't be reduced at the cascade stage without changing the tone of the amp. The additional gain is responsible for allowing the player to crunch the preamp.

I should add that I do have a solution for this, because there's an effects loop between the preamp and the PI that wasn't present in the original design. I can install a send-return patch cable, then use the Send Level pot to reduce MV sensitivity (that won't change the preamp tone). I just want to be sure this sensitive behavior of the MV pot doesn't point to a wiring error or bad component somewhere. That's why I asked whether it's normal for the MV series.

Thanks!
Jack

[TriodeLuvr]

No new pics yet, but there's progress. It's funny what you learn about amplifier construction when you do something like this for the first time. The amplifier chassis wouldn't fit into the Peavey combo cabinet all the way because the HV cage that I made hit the speaker. The chassis stopped about 1/4" back from where it needs to be. So, I scrapped the cage and made a new one that adds about 1/2" of clearance. Probably can't remove the speaker once the chassis is bolted in, but that might have been true of the original design, too.

Also, I found out yesterday that amplifier mounting straps are readily available in various lengths from Antique Electronic Supply and probably others. I had assumed this would be an OEM-only item, so I never looked. And of course, none of them will fit the 6-1/2" spacing I used for the cage nuts. Doh! So, I spent a few hours today milling two aluminum straps. Now I have to finish the metal and anodize them, much more work that I would have undertaken had I known.

I might have to build a few more amps and sell them just to get my educational investment back...

Jack

[TriodeLuvr]

Here are the final pics of the amplifier chassis. The chassis and Hammond speaker have both been mounted in the donated combo cabinet now. I'll post pics of that as soon as I finish the new nameplate. (It's not a Peavey anymore!)

[TriodeLuvr]

Finally! Here are pics of the amp in the free cabinet. Need to fix a few Tolex blemishes, but it's basically finished at this point. Kyoto Audio is the brand I use for my own creations.

Jack