Keep your eyes open for a good sounding neo 15 inch speaker. That will cut the weight down on those steel amps.

What I was seeing when looking at the transfer function on the X-Y scope was the whole curve would move up about 1.6V depending on the amount of overdrive. The point where hard clipping occurs on the top of the trace would move to the right. You can see the waveform moving around in a normal Y-t display.

I built the preamp as described in the linked site. I even found a 2N4401 to use for Q1. Note that the Emitter of Q1 is about -0.5V so C2 needs it's + terminal to ground. The only thing I changed was an added diode from the input of U1 to Vee for protection.

It works pretty much as described. The one problem I found is there is a DC shift in operating point at the Collector of Q2 under heavy overdrive. It moves positive about 1.6V. The DC at the Emitter of Q2 also moves around but the Base Voltage is pretty stable. The Emitter of Q4 is also stable. This is not a problem when just a guitar is plugged in.

The DC at the Collector of Q2 is a little high to begin with causing premature clipping on the positive side of the output. An adjustment to R12 would probably fix this. THD was about 7% at the onset of clipping. At lower levels 2nd harmonic is dominate.

Have you checked the four 10W emitter resistors?

The Peavey Transtube does have the raising distortion characteristic, but THD only reaches about 3% before clipping. At low levels I didn't see the 2nd harmonic dominance, 3rd was always present at nearly the same level. It does sound kind of tube like, but not as warm as a real tube or my Bender preamp.

Link to Bender preamp: http://www.ssguitar.com/index.php?topic=3732.0

The Sunn Concert series is a very primitive early 70's design. There is no current limit other than the beta of the output transistors and the drive available through the drive transformer. The driver transformer has a primary and four secondaries. On some versions there is a trim pot in the driver stage, but no info anywhere I can find on how to tweek it. Probably a clipping symmetry adjustment.

There are two basic versions. The earliest is a "Flying Bridge" with a floating power supply of about 35V. Check that the four transistors and all the resistors are good. There should be very little DC on the output with no speaker connected, perhaps a Volt or two. If there is too much DC, a transistor is blown or a resistor has drifted or smoked. It is not uncommon for a short to develop in the driver transformer between the primary and one of the secondaries. This puts big DC on the output.

The second version is a conventional totem pole configuration with +/- 34V rails, but it still has the same driver transformer. This version has a 5 wire power transformer, the earlier version has an 8 wire power transformer.

If everything seems OK, a low beta output transistor is suspected. If you don't have a way to measure beta at high current, you'll need to replace all of them.

I'm not sure any 240V export versions of the Concert series were ever made, but some 120V ones were made in Mexico.

Edit: Schematic you posted is the early version.

Edit2: Note that the wire common to R305 and R307 connects to ground at point 33 on the PCB.

I like the simplicity of this design. It might make a nice beginner project. If not the whole amp, at least the power amp. It could run from a 24VAC wall wart with a couple of simple changes.

What are the transistor numbers?

The one thing rarely mentioned is that most of these supplies will operate from 100 to 250 VAC 50/60Hz input. You just need the right Mains cord or an adapter. I found a 48V 2A but haven't tried it yet.

Quote from: Charliegriffin on August 12, 2016, 04:56:57 AMAlso I forgot to mention the schematic calls for 2n5484 transistors but all I had on hand were 4401s that could be  my whole problem.

Do you mean 2N4401 ? That's an NPN bipolar transistor. The 2N5484 is an N-channel JFET.

MEF went down last night and was still down this morning. It's back up now.

Tone stack looks a little odd for a Marshall. Try C13:470pF, R24:33K, C14:.022uF, C15:.022uF.

With 25V rails I would expect an 8 Ohm total load for 30W. Speakers should be 2X 16 Ohm.

Bad software doesn't kill power supplys. Not even Microsoft.

It's working now. Seems a little slow to respond but that could be the new Firefox 47.0.1

Edit: This site seems to be working fine, maybe a little better than before!

I'm still getting a timeout. I even re-booted and thought it was working but it was just the last valid page from two days ago.

Three parallel "branches" of 8 Ohms results in a load of 2.66 Ohms. So connect three pairs of 4 Ohm speakers in series, then connect those series pairs in parallel.

Is the Silvertone 1485 head the one with four 6L6s and two output transformers? That won't work well with this cabinet because you need two loads of 4 or 2.66 Ohms each.