I've just noticed, you've got a 10K resistor between the first stage and the volume pot, that should be a 470K. Is that intentional? Not saying this is the cause of the hiss, just something I've noticed.
With regards to the hiss.........This is unlikely to be a wiring issue or interference, that would result in hum or buzz. It sounds to me like either a noisy valve or there's a lot of gain for some reason. Or a combination.
- Have you checked continuity on all connections? Confirmed the resistors are in spec?
- What did you have the gain set to in that video?
- And what does it sound like to play? Like a normal JCM800? Can you get much distortion plugged into the Low Input? From memory the Low Input should be pretty clean on a JCM800.
kuDo wrote: ↑Wed Jun 29, 2022 5:23 pm
Heater hum doesn’t seem to be present. It has been elevated and balanced via a humdinger pot.
But if all else fails, I can try.
you will be surprised what us coming through the heater line. on high gain this is important. easy test take smal 6VDC lead battery pack, disconnect the V1 heaters, and connect the heaters of the first valve on the battery. ground minus pole of battery on grpund ofcourse
Littlewyan wrote: ↑Wed Jun 29, 2022 6:43 pm
I've just noticed, you've got a 10K resistor between the first stage and the volume pot, that should be a 470K. Is that intentional? Not saying this is the cause of the hiss, just something I've noticed.
With regards to the hiss.........This is unlikely to be a wiring issue or interference, that would result in hum or buzz. It sounds to me like either a noisy valve or there's a lot of gain for some reason. Or a combination.
- Have you checked continuity on all connections? Confirmed the resistors are in spec?
- What did you have the gain set to in that video?
- And what does it sound like to play? Like a normal JCM800? Can you get much distortion plugged into the Low Input? From memory the Low Input should be pretty clean on a JCM800.
The 10k is not intentional. That should indeed be 470k. I will fix that. Thanks!
The noise exists with different tubes in v1. Could it be that the socket is not connecting well enough, could that cause noise?
The gain is set to the max.
All the resistors on the board are in spec, because I measured those before I put them in. The ones on the inputs and the ones ofboard I haven’t checked.
Tomorrow I could play a little and record it. But it sounds to me like it should.
The clean channel is completely clean with my guitar.
kuDo wrote: ↑Wed Jun 29, 2022 5:23 pm
Heater hum doesn’t seem to be present. It has been elevated and balanced via a humdinger pot.
But if all else fails, I can try.
you will be surprised what us coming through the heater line. on high gain this is important. easy test take smal 6VDC lead battery pack, disconnect the V1 heaters, and connect the heaters of the first valve on the battery. ground minus pole of battery on grpund ofcourse
kuDo wrote: ↑Wed Jun 29, 2022 8:12 pm
The 10k is not intentional. That should indeed be 470k. I will fix that. Thanks!
The noise exists with different tubes in v1. Could it be that the socket is not connecting well enough, could that cause noise?
The gain is set to the max.
All the resistors on the board are in spec, because I measured those before I put them in. The ones on the inputs and the ones ofboard I haven’t checked.
Tomorrow I could play a little and record it. But it sounds to me like it should.
The clean channel is completely clean with my guitar.
Have you checked the resistors are in the right places though? I would check through the circuit, at least around V1. i.e. Measure resistance from pin 1 on v1 to the power rail to ensure you read 100k etc.
Also, where is your circuit connected to the chassis ground?
Littlewyan wrote: ↑Wed Jun 29, 2022 8:38 pm
Have you checked the resistors are in the right places though? I would check through the circuit, at least around V1. i.e. Measure resistance from pin 1 on v1 to the power rail to ensure you read 100k etc.
Also, where is your circuit connected to the chassis ground?
I will check al of them, this weekend.
My circuit is attached to chassis ground via the input connector of the high input.
All other connectors are isolated via isolation washers.
That is the only connection to chassis ground.
Littlewyan wrote: ↑Wed Jun 29, 2022 9:39 pm
You really want a dedicated connection to ground through a ground lug to be safe.
When you tried replacing V1 with another valve did the level of hiss change at all?
Also, I would swap the master and presence controls around. It will shorten the leads for the master control a lot.
In my previous iteration of this amp, before I converted it to PCB.
I had a dedicated lug for this. But also back then, it was hissing. So my idea was to shorten this connection by attaching the input directly to chassis.
Changing tubes did not change anything. But maybe, I am the lucky owner of three noisy tubes
in hi gain amp all is in account. all signal /noise ratio is made in first valve V1. which need to have as much gain as possible, so 220k in anode gives less hiss than 100k and better signal/noise ratio if all is ok with the stage.
to prevent hiss need to be many condition satisfied.
1. lead dress, layout, grounding. hiss is also coming from the bad grounding too (people thinks that grounding causes only humm). grounding for hi gain is the best local star grounding. separate for V1a and separate for v1b.
V1a nad V1b to be supplied with separate capacitors otherwise ecc83 can oscillate on hi gain(hiss) . it is ok to put even 5k resistors between electrolitic caps for V1a V1b.
short path between input jack, cathode rwsistor and first tube. the best is input tube close to input jack and direct connection from input jack
to the input valve over grid stopper.
like mentioned before, 10-15k grid1 stopper , not more.
2. dc heaters for the V1
3. low noise resistors anode, g1, cathode
im doing all that and i dont have problem with hiss... i dont even try to start nw with ac heater for the first valve..hope that will help.
p.s. pcb construction for hi gain amp , all need to be predicted and then can work without problems
bepone, you're really generalising a lot of things in your post.
- A 220K anode resistor won't necessarily give you less hiss, it could actually make it worse. It all depends on the gain of the valve set by the bias, which is determined by both the anode and cathode resistors. You could actually achieve less noise by attenuating less between gain stages and lowering the gain of a stage.
- When you say separate capacitor for V1a and V1b I'm assuming you mean separate filter stages. But this isn't necessary. Plus we're not talking about a particularly high gain amp here. The general rule is two stages out of phase can share the same filter stage, which is what's happening in this case. So we should be fine. But I've seen a few Soldano amps and EVH amps share the same filter stage amongst 3/4 gain stages and be ok.
One of my amps has a 4 gain stage channel, it's in a fairly cramped chassis and it's pretty quiet. I didn't need DC heaters, it's AC but with DC elevated.
This is a JCM800, not a high gain amp by modern standards. We shouldn't have to go to extreme lengths to keep it quiet.
Last edited by Littlewyan on Thu Jun 30, 2022 8:32 am, edited 1 time in total.
kuDo wrote: ↑Thu Jun 30, 2022 5:12 am
In my previous iteration of this amp, before I converted it to PCB.
I had a dedicated lug for this. But also back then, it was hissing. So my idea was to shorten this connection by attaching the input directly to chassis.
Changing tubes did not change anything. But maybe, I am the lucky owner of three noisy tubes
I would go back to the dedicated lug for safety if I were you.
I've just noticed the order of your wires in your last few photos. Have you definitely got the 2.7K/.68uF on the first stage? The wires closest to the 2.7K/.68uF resistor/cap seem to go to the second gain stage.
If that's ok I would try swapping the valves in the amp around. See if the hiss level changes if you swap V1 with V2 etc.
Littlewyan wrote: ↑Thu Jun 30, 2022 8:13 am
bepone, you're really generalising a lot of things in your post.
- A 220K anode resistor won't necessarily give you less hiss, it could actually make it worse.
no i, it is increasing signal to noise level, increasing the gain on the first stage..ofcourse you need to adopt cathode rsistor too, but this is obvious no need to speak about very basic things.
