Junkbox Trainwreck

[gingertube]

Guys,
This amp has had a problem, on the rare occassions when the volume is turned passed about 7 it breaks into oscillation. I tried grid stops and high frequency roll offs on anode loads etc. etc. - no change.

It was however stunning when behaving. I assumed it was a lead dress problem from the original quick and dirty conversion from a PA Amp so decided to rebuild it.

As usual, during the rebuild process I found a few things which could/should be improved.

Changes:
Individual Bias resistors for output tubes with increased bypass cap values.
22K Grid Stops on 2nd and 3rd Gain Stages
+50V standoff on heaters
300pF "Bright Tamer" across 3rd gain Stage Anode Load
AND
Additional Power Supply (B+) decoupling stage between the Phase Inverter and the final preamp gain stage (the 3rd gain stage).

I'm fairly sure it was that last thing which cured the original oscillation problem.

So updated schematics.

[gingertube]

And the Power Amp

[overtone]

Thanks for the update Ian. That is quite a hefty increase on the filtering. Has this changed the feel of the amp?

The timing of your update is perfect for me, because I stripped out the AWA PA872 two days ago and have begun planning the layout. Just pencil sketches and some CAD at the moment, but it is clear that the amp will remain tag-strip and point to point.

I have planned for individual cathode bias too, but adjustable for different output valve choices. That means giving up the free filament DC for V1 and V2, plus an additional filament transformer for the output valves, otherwise I will be over the mA rating with EL34s.

Best, tony

[gingertube]

Tony,
I think it now sounds a little tighter, certainly it is one hell of a lot quieter in terms of both hiss and hum. The Power Amp section is almost HiFi, I tested the rebuilt power amp section in isolation before moving on to rebuilding the preamp section, power out was 32Watts RMS into 4 Ohms just below clipping and frequency response was scary, 23Hz to well over 30kHz.
Cheers,
Ian

[gingertube]

Tony (and others),
I had the Junkbox Trainwreck back on the service bench over the Easter break, a re-occurance of that oscillation problem when the volume knob is turned passed about 1/2 volume. As the amp is very rarely used at these volume levels the problem wasn't noticed until just recently (The local guitar god used it on a tour around Oz and had to turn up the wick to match the other guitar player who was using twin 100W Amps, each with 4 x 12" speakers)

This was not the rail to rail oscillation you usually see but bursts of high frequency just after the turn over in the positive signal swing.

I've seen this behaviour before in an Ultralinear HiFi Amp where the screen resistor were too small so I first fitted 1K 5W resistors into each output tube screen - that did'nt help but was left in.

Next tried grid stops on the phase inverter, BINGO, that fixed the problem.
I used 47K in each grid, probably bigger than required. I plugged the in Telecaster to see if I'd changed the sound much, decided that I had changed the sound just subtly but for the better, maybe a little earlier breakup due to those new screen resistors on the output tubes, but a HUGE sound. Hard to believe its 32 Watts max.

Cheers,
Ian

[overtone]

Thanks Ian,

I have been out of building amps for a while due to construction site demands. The AWA Junkbox fired up perfectly last week. It is not pretty but gets complements from the neighbors. The schematic is fairly close to yours and I will update with the differences.

Went with as many old parts as possible and can dime it 100% with no issues so far. Only tested it with a couple of Tele's and am looking forward to trying P90s with it. There was a spiky-irregular 100 Hz buzz which turned out to be a florescent light nearby.

But I do have one daft issue. The pots are set so far back from the face plate that I cannot get the grub screws to bite. I am looking for collet knobs which have a flush, not recessed bottom. The trouble is, here in Europe the knobs are nearly all 6mm, not 1/4" 6.35mm. I don't really want to change out the pots for 6mm or longer shaft ones if i can help it.

Here is a picture inside the junkyard. Quite a bloody mess really.

All the best everyone,
tony

[Reeltarded]

Awesome! You guys rock!

[Phil_S]

That is one awesome build! I love junk box amps. Anyone (well, not really) can build in a new chassis. Recycling takes real imagination.

[overtone]

as promised - the as built schematic in a pdf file, plus a screen shot of the layout at an early stage.
This is not how it ended up.
It did run a while like this but it was not simple enough so I stripped out a lot of fat.

I was aiming for about 360V on the plates when using 6V6. Currently running 5881.

All the best, tony

EDIT:
Apr 5th 2013: Revision A uplaoded / correct values for the gain pot and NFB cap.
Aug. 26th 2013: Revision B uploaded / minor clarifications

[overtone]

Revision A:
the schematic has been updated to show the correct gain-pot value of 1M log,

all the best,
tony

[gingertube]

Tony,
A quick "nit pick". Your schematic shows one output tube cathode bypass cap at 20uF and the other at 50uF.

All,
Was looking at this again as I have "scored" 3 more old AWV PA Amps.
2 of them run a pair of 6CM5 (EL36) for the output which will make ideal Junkbox Expresses.

The other one of them runs 6 off 6CM5 for around 100 Watts, Plus an Optical compression circuit, that may have to become a bass amp.

6CM5 were "common as mud" here in Oz, used as Horizontal Deflection Tubes in Television.

I've also collected a lot of 6DQ6 which were also a TV Horizontal Output tube but a bit beefer. Must try those in a Git Amp.

Cheers,
Ian

[overtone]

Ian, thanks for the nit pick!
I am pretty sure I used 20uF, will check and update the schematic.

This TW circuit variation that you came up with really works well for me and the old AWA iron is great. I doubt I will ever run into another one here in Germany.
Sometimes I have seen those kind of odd tubes but I have mostly passed on them except for some EL38 6CN6 with the anode caps.
Who knows when I will find the time to try them,

cheers!
tony

[Tone Lover]

Ginger tube what was the impedance on your OT.6k6 , 3k3 or something in between. just curious.
Thanks Bill

[gingertube]

Bill,
The amp detailed here (2 x 6CA7 cathode biased) used an output transformer I bought here in Oz. A small time winder who had reverse engineered a JTM45 OT. It was 3K4 Raa.
Cheers,
Ian

[Tone Lover]

Thanks gingertube