Jansen IA-33

[Jay]

Drew up a circuit diagram for this oldie and I am a bit puzzled with the connections around the power valve EL84.

Observations and questions to the collective wisdom:

1) The pre-amp circuitry is typical for that era, however R5 shows " red, brown, X and then silver. So could be 200 ohm or 2K. Multimeter shows 3.5K so it needs changing to something like 1K to 2K.

2) I am more puzzled about R3 which appears to have been added at some later stage. What does it do?

3) Also the tone circuit appears to be strange coming directly from the power valve's anode. I have seen this on a Poweray single ended amp, so maybe just normal?

4) B+ goes to the screen grid so the anode voltage is 'powered' via the OT. That means that the anode has a lower voltage than the screen grid. It does appear original but seems strange to me.

Any feedback welcome. Thanks.

Jansen IA-33 Draft Circuit Capture.JPG (126.29 KiB) Viewed 1488 times

[Optical]

My best guess is R5 will be 2k, it doesnt make much sense if it were 200R and fully bypassed by the 10u at the same time... The resistor value won't matter much for that reason either however.

The rest is has some oddities. Are you sure that's drawn accuratly?
Essentially R3 is feedback to the grid, maybe to stabilise the open loop gain. But also being DC coupled to the plate of stage 2... i havent got my head around that yet!

[Jay]

My best guess is R5 will be 2k, it doesnt make much sense if it were 200R and fully bypassed by the 10u at the same time... The resistor value won't matter much for that reason either however.

The rest is has some oddities. Are you sure that's drawn accuratly?
Essentially R3 is feedback to the grid, maybe to stabilise the open loop gain. But also being DC coupled to the plate of stage 2... i havent got my head around that yet!

I have checked it multiple times... I have marked up R3 on the photo with the pin connections of the respective valve....
Blow it up to see it in more detail.

Edit, also note the gang pot for tone. Why?

Jansen IA-33 Gut Shot JAY_1499.JPG (1.64 MiB) Viewed 1440 times

[sizzlingbadger]

Agree with Optical.... I would say R5 cathode resistor should be 2K, its sets the DC bias on the 2nd stage of the amp. R3 would provide negative feedback from the power tube back to the pre-amp, but its odd as the DC is not isolated, so its effectively DC coupled to the 2nd stage of the 12AX7, I'm not sure it should be there at all. The tone control is across the output stage which is also odd, something doesn't add up to me.

[sizzlingbadger]

It would make more sense like this... but then the tone control pot would need to have a much higher value. (It has been replaced though)

Screen Shot 2022-06-10 at 6.47.12 PM.png (503.29 KiB) Viewed 1417 times

[RectifiedAmps]

I think you should try removing R3 and see what the effect is. You say you suspect it was added, and it doesn't seem right as a feedback method, so pull it and find out what happens - it won't hurt anything. The tone control probably does belong on the anode of the output valve, which is why the value would be low (50k pot?). C4 is also there for some tone shaping - I've seen this on old WEM amps before. And having the screen and OT fed off the first node of the B+ is pretty common on old low-wattage amps. The screen will be a few volts above the anode, but for gnarly old EL84s it didn't seem to matter much.

[sizzlingbadger]

That tone pot value would certainly work better across the EL84 but would it handle the current ? I guess its only got to pass the high frequencies so maybe it will be ok. The circuit does make sense with R3 removed so maybe that's the simplest solution.

[Jay]

I think you should try removing R3 and see what the effect is. You say you suspect it was added, and it doesn't seem right as a feedback method, so pull it and find out what happens - it won't hurt anything. The tone control probably does belong on the anode of the output valve, which is why the value would be low (50k pot?). C4 is also there for some tone shaping - I've seen this on old WEM amps before. And having the screen and OT fed off the first node of the B+ is pretty common on old low-wattage amps. The screen will be a few volts above the anode, but for gnarly old EL84s it didn't seem to matter much.

Thanks guys for your valuable feedback. I shall remove R3 but leave the tonestack connected to the anode. I believe it is still the original EL84 but I don't know if it works as I haven't powered the unit up yet.

Would it be better if I moved the B+ to the anode side of the OT as it is done on more modern amps?

I am in the process of finding out its repair history so hopefully I get some answers to the R3 'adjustment'.

Next steps then:
Remove R3
Change R5 to 2.2K
Verify all resistor values and change if difference > 10%
Remove audio valves
Insert new EZ81
Measure all voltages and show on circuit and post updated circuit in this thread.

There's hoping the can capacitor is still okay

[sizzlingbadger]

Would it be better if I moved the B+ to the anode side of the OT as it is done on more modern amps?

No, it looks fine to me, just the way its drawn makes it look a bit different I think.

[RectifiedAmps]

Since the current for the OT has to be pulled through R20, you may get extra sag at higher volumes. This is in addition to the sag created by the EZ81 rectifier valve, too. If it’s too spongy you could move the OT connection to the other side of R20, but you might get more hum as well.

[Jay]

Would it be better if I moved the B+ to the anode side of the OT as it is done on more modern amps?

No, it looks fine to me, just the way its drawn makes it look a bit different I think.

Apart from my lack of drawing skills...
I am referring to the screen grid wired such that it has a lower voltage than the anode. See the first circuit in the valve wizard link: https://www.valvewizard.co.uk/se.html

My suggestion is that on this Jansen amp I should resolder it to reflect the wizard's circuit.

SE1.jpg (38.7 KiB) Viewed 1323 times

[sizzlingbadger]

Having the screen at a voltage higher than the anode is not a problem in practice, as long as the screen is not hundreds of volts higher than the anode.

The whole point of the screen is to be at a high voltage so that it accelerates electrons in its direction, but it has wide gaps so that those electrons pass right through the gaps to the plate.

When you are playing guitar a signal on the grid effects anode current which creates a voltage drop across the output transformer causing the anode voltage to fall. The screen pretty much stays at a fixed high voltage. So half of the time when you're playing, the anode voltage is well below screen voltage.

Screen voltage effects the peak anode current that the output stage can create; lower screen voltage means lower peak anode current, and lower output power. Having the screen stay at a high voltage during those half-cycles when anode voltage is falling is what keeps attracting electrons in the direction of the anode, think of it as a "Turbo" for electron flow. That's why triode mode operation (often used as a lower power mode on amps) generates less output power, the screen is tied to the anode, and screen voltage drops as anode voltage drops, disabling your "Turbo".

If the power dissipation is too high at idle then you could lower the screen voltage by adding a resistor between the wire and pin 9 screen grid. 1K 2W should do the trick, it will reduce the headroom of the amp further though.

Ultimately a lot of this will depend on the value of your B+.

I also noticed that tone pot is wired in parallel ? maybe to double its power rating which would make sense seeing where it placed in the circuit.

[Jay]

Thank you SB for such comprehensive response. I will keep the screen grid resistor in mind when measuring/calculating power dissipation

[Jay]

Since the current for the OT has to be pulled through R20, you may get extra sag at higher volumes. This is in addition to the sag created by the EZ81 rectifier valve, too. If it’s too spongy you could move the OT connection to the other side of R20, but you might get more hum as well.

Thanks, I had considered that but I fell it would be better to leave the old amp's character in place for nostalgic reasons

[sizzlingbadger]

Full disclosure... I have Covid induced brain fog this morning, but the more I look at R3 the more I reckon it would work as NFB without any any real ill effects despite no DC de-coupling. Maybe a bit more hum, and the pre-amp anode voltages may creep up a bit. I'd still remove it to start with and then maybe see what effect it has, someone must have liked it in the past