NZGuitars.com

First observations:

• No fuses anywhere to be seen.
  Voltages all within acceptable range.
  The 8" Philips ADZ3800M speaker is underwhelming.
  Amp is remarkable quiet for such a 'loose' grounding scheme.
  R3 has a significant impact on the amp's volume and feedback

I fitted the new, old stock rectifier valve (bought from Clarry) in without the other two in and read normal voltages, no smoke or funny smells.
Then put the preamp and EL84 back in. Pots noisy as hell but some contact spray fixed most of that.

Playing on the 8" speaker with a hollow body guitar with R3 in the circuit: okay crunchy sounds but even on full volume the amp is quiet. No really much cleans.

Then to the Celestion 12" with hollow body guitar and R3 in circuit: Nice crunchy sounds with a reasonable volume but nothing even on full can't get the amp to howl. Cleans are there but only at low volume.

Then remove R3, everything else the same....

The amp turns into an untamed beast! Can't set the vol pot full open as it howls like a wolfpack or two... Amp becomes very loud but the volume pot is useful now. (To be fair, I stood very close to the speaker due to the space limitations in my workshop).

Need to try it with a solid body tomorrow but my take from this little test run today is that the amp needs a 12 incher.

I can see why R3 was put into the circuit many years ago- it allows for 'bedroom' playing. The question for the owner is - what is your taste these days?



For the amp gurus:
Suppose could put a 1MOhm resistor in series with R3 but then there would be DC on the pot... Or perhaps switch R3 in and out...

Jay wrote: ↑Fri Jun 24, 2022 4:52 pm The question for the owner is - what is your taste these days?

Is there a middle ground? One where the best of both is available or the dogs of war recall their puppyhood?

What’s the tone control like with R3 removed? And does it howl with no guitar plugged in but the volume cranked?

With R3 out, you could try removing C2 (or making it switchable) to tame the gain a bit.

RectifiedAmps wrote: ↑Fri Jun 24, 2022 9:27 pm What’s the tone control like with R3 removed? And does it howl with no guitar plugged in but the volume cranked?

With R3 out, you could try removing C2 (or making it switchable) to tame the gain a bit.

This could be good.
The owner agrees that a new speaker is in the near future. (I was thinking something like a Warehouse G10. Any other suggestions?)

As for the character, he would like some clean usable for home use, but his happy place is Neil Young chaos so neutering the amp won't be appreciated.

It is a home amp though; he has stage/gigging rigs as well.

A 10" might fit in the cabinet, a speaker out jack provides flexibility. It certainly is a good amp for NY chaos. Have to try Cinnemon Girl on a LP with it.

I have a 10” Eminence RF10C or a Celestion G10 Greenback that I could part with- both broken-in but otherwise like new. PM if you’re interested.

RectifiedAmps wrote: ↑Sat Jun 25, 2022 1:03 pm I have a 10” Eminence RF10C or a Celestion G10 Greenback that I could part with- both broken-in but otherwise like new. PM if you’re interested.

Will pass this on. It's a medium /short term project at the moment. Owner has covid, Jay hasn't finished with the amp etc.

RectifiedAmps wrote: ↑Fri Jun 24, 2022 9:27 pm What’s the tone control like with R3 removed? And does it howl with no guitar plugged in but the volume cranked?

With R3 out, you could try removing C2 (or making it switchable) to tame the gain a bit.

Good question. So without R3 the tone control still works fine. I did not unplug the guitar but with tone and vol on full there is some form of audible oscillation going on that disappears when you turn the tone a little down. Will need to try that without guitar.

Removing C2 makes the amp considerably quieter, with and without R3. Although my poor hearing tells me that without C2 there is some treble definition lost...

Now the volume pot has a switch that is not used and works fine (once the knob was removed). It is rated 250VAC / 2A.

So I have the option of making either C2 or R3 switchable. The voltage over R3 is about 100VDC so the switch should be able to handle that, yes?

My ears tell me, make R3 switchable which also means that there are at least some low volume cleans.

I think I will have to ask the owner to listen to the options and make a decision (as it requires some surgery on the volume knob to make the switch work).

This was a pig of an amp to work on... Without R3 the amp would squeal at higher volume which I had not noticed earlier as it seems to depend on some sort of interaction between tone and available gain.

I tried all sorts of things to get rid of the damned squeal... tried different valves, no change; put in grid leak resistors, no change; removed cathode cap on V1b, no change. I would have tried 10pF caps between grid and plate but didn't have any high voltage ones, so no go there. Fixing a noticeable earth loop also made no difference. Resoldering quite a few joints just to be sure also made no difference. Sigh!

So I resigned myself to retaining R3. But with V1b cathode cap removed I wanted to squeeze "as much gain out of R3" as possible. So I started an iterative process in which R3 would be increased but not to the extend that the amp would squeal again. I settled on 220K in series with a 470K resistor. Also changed the tone control to something more conventional.

Then after changing the arcing power switch, adding a mains fuse and fixing up the input circuit we now have a working amp again. It doesn't sound too bad on the original 8" speaker but sounds pretty decent on an GH12 Celestion.

Here the as built schematic:

You could put a cap in series with R3 to block the DC.

sizzlingbadger wrote: ↑Sat Jul 09, 2022 6:48 pm You could put a cap in series with R3 to block the DC.

Did that with a 0.1uF cap but it would still squeal. Somehow the DC component is 'important'. Pheck, I tried so many different things

Maybe it just has too much gain, dropping R27 and R4 to 100K would reduce some. A 250K tone pot would reduce it a bit more too.

sizzlingbadger wrote: ↑Sat Jul 09, 2022 7:17 pm Maybe it just has too much gain, dropping R27 and R4 to 100K would reduce some. A 250K tone pot would reduce it a bit more too.

Definitely too much gain. R4 drops to 130K due to R3 (220K) - pseudo parallel resistors. I dropped R27 to 110K initially but it needed to go back to 220k when I increased R3 to 690k.

Played with tone pot values too, would you believe...