Problem with this fizzy sound

[jmaf]

Left to right: V1, V2

[sickman82]

Left to right: V1, V2

V1 & V2 are the EL84's behind the two preamp valves, V3 & V4 are the ECC83's at the front. V3 being the first/second stage, V4 being the third/phase inverter.

Ok, Chuck and Diagrammatiks.. It sounds as though you guys agree on some modifications to try. Can you give details of some values and positions, i.e. R13 - 120k for example, so that I can try your suggestions out please?

Deltones, if we figure this out.. Do you know anybody capable of carrying out the modifications for you?

Pete, we seem to have lost you. It would be great to get your input on this.

[Deltones]

V1 & V2 are the EL84's behind the two preamp valves, V3 & V4 are the ECC83's at the front. V3 being the first/second stage, V4 being the third/phase inverter.

Deltones, if we figure this out.. Do you know anybody capable of carrying out the modifications for you?

Thanks for the clarification. As for somebody able to carry out the mods, I'll be able to manage with a local tech I can trust.

[pdf64]

I'm not certain that the cathodyne has been eliminated as being the cause; the pre cathodyne master volume experiment earlier seemed a bit shakey (no offence intended).
I'd like to rule that out first.
Once that's out of the way, all the suggestions of modding the pre-amp operating points to be more like regular guitar amp seem sensible.
In addition to that, I'm a fan of grid stoppers, as being basic good practice - 10k to 100k on all pre amp common cathode stage grids.

[diagrammatiks]

also other then the resistors I wouldn't take out any of the film snubbers across the electros in the power supply. vox did the filtering perfectly right.

[jmaf]

V1 & V2 are the EL84's behind the two preamp valves, V3 & V4 are the ECC83's at the front. V3 being the first/second stage, V4 being the third/phase inverter.

Right. I guess I've got nothing to help you guys out with - I'm out of ideas, besides calling the two EL84's Doris and Betty instead of V1 and V2.

[Chuck H]

Here's the mod I suggested before:

I might try replacing R13 with a 2.2k and replacing R9 and R26 with 150k.

I also agree that a grid stopper on V1b and/or V1b and V2a could help. But to keep it simple I would start with the three resistors mentioned above. diagrammatiks did do some more careful calculating for some mods but I'm not sure so much idealizing is needed. I wouldn't want to take the amp too far from it's stock sound/feel if possible.

I would start with the above three resistors (c'mon, it's only three resistors). If there is improvement but anyone wants more I would decrease the cathode resistor on V1b to 1k. If you still need more improvement I would add a grid stopper of 100k to V1b.

[sickman82]

I'm not certain that the cathodyne has been eliminated as being the cause; the pre cathodyne master volume experiment earlier seemed a bit shakey (no offence intended).
I'd like to rule that out first.

Ok, would you tell me exactly how to wire this up.. which resistor am I replacing with the 1M pot? If I take the signal up terminal 1, then out of the wiper.. do I leave terminal 3 disconnected or does it need to be grounded? Will any particular taper work better than another? I'm going to have to order in a pot, so it won't be until next week before I can try this out.

In addition to that, I'm a fan of grid stoppers, as being basic good practice - 10k to 100k on all pre amp common cathode stage grids.

This is something I can try right away, but I'm sceptical about cutting the traces on the pcb. I suppose for the sake of the project I will give it a go.. it would be for the greater good if we found out exactly how to solve this issue that many NT owners seem to have (do I sound like a politician?!). So looking at the schematic that Silvio gave us at the beginning of the thread (which seems to be 100% correct).. are you suggesting a grid stopper on each preamp stage (V3a, V3b, V4a & V4b), Chuck suggests only V3b and V4a? As far as cutting the trace goes, depending on which component I solder the grid stopper to, I am presuming I need to cut the trace that connected that component to the grid right? Effectively making the only connection to the grid through the new grid stopper. Any tips on cutting traces?

also other then the resistors I wouldn't take out any of the film snubbers across the electros in the power supply. vox did the filtering perfectly right.

I've tried that already. It made no difference whatsoever. But as somebody mentioned earlier, it's probably going to be various things that need to be looked at rather than one specific change. Do we all agree that the filtering here is ok, and should be left alone?

I would start with the above three resistors (c'mon, it's only three resistors). If there is improvement but anyone wants more I would decrease the cathode resistor on V1b to 1k. If you still need more improvement I would add a grid stopper of 100k to V1b.

Cool.. I will get on with that as soon as possible and see how it goes. I have the day off today but I've got a Les Paul to wire up this morning, but I might get the opportunity this afternoon to do some tinkering with the amp.. depending on what my other half is up to. I am going to assume you mean V3b and V4a when you mentioned the grid stoppers Chuck?

So, to recap here are the proposed mods so far in no particular order:

- Pre cathodyne master to rule out the cathodyne as the root cause
- Grid stoppers on V3b & V4a preamp stages (values between 10k-100k)
- Replace R13 with 2.2k, R9 & R26 with 150k - for more potential improvement replace R23 with 1k

If I have missed anything, please insert and repost.

Silvio, are you gonna try these things with me? Perhaps give the proper pre-cathodyne master another try with a 1M pot correctly wired?

And.. just so we're certain that we are all working from the same schematic: Vox_Night_Train pictures by Blinker_2009 - Photobucket

[Chuck H]

I am going to assume you mean V3b and V4a when you mentioned the grid stoppers Chuck?

Yes, you are correct. I just rechecked the schem. It's all cattywhompus compared to the usual 'first preamp tube in the signal chain is V1' scenario.

[Silvio55]

Silvio, are you gonna try these things with me? Perhaps give the proper pre-cathodyne master another try with a 1M pot correctly wired?

Sure Sickman! But I´m quite busy these days, and I have to get the componets yet, there´s no place to buy here, Some guys at TV repair shops have helped me with some resistors earlier but they don´t have 1 or 2 watts resistors, I will have to order them and I don´t think I´ll have those here untill late next week. But don´t worry, I´m still in the fight!

[pdf64]

'Ok, would you tell me exactly how to wire this up'
See



Needs a 470k pot, audio / log taper preferably, a 470k to 1M series resistor (it's fine to omit this for the purposes of the MV mod test), and a 0.01uF - 2.2nF coupling cap, any 400V rated film type will do.
It requires the track from C10 to R19 / V4A grid to be cut and the new parts to be fitted between the C10 side, ground, and R19/V4A.
Good luck!
Pete.

[sickman82]

Thanks for the description Pete, I need to get some bits ordered to try this out.. hopefully this week. I'm about to try the other suggested mods this morning, so I'll report back as soon as I have carried those out.

[sickman82]

R13 looks like a 1W resistor to me, I'm going to have to order in the correct wattage resistor to do these mods.. R9 and R26 are just 1/4W, which I have plenty of, but I'd rather swap out all at once.

EDIT: Just taken a reading of R13.. the schematic states it should be 9k, however I am getting 4k7?

[sickman82]

Just a quick couple of questions regarding the grid stoppers on V3b and V4a;

For the V3b, if you remember I have removed the circuitry at the input stage of the amp which has left only a 68k resistor in the position of R21, which means the input only passes through that 68k before hitting the grid. That has left R22 with no component in the PCB, only a bridge connection. I would assume that the grid stopper would normally be placed from what would have been R22 to pin 7, and then cutting the trace bewteen. Can I just drop in my grid stopper where R22 originally was?

For V4a, am I hooking the grid stopper up from R19 to pin 2 of V4 and cutting the trace between R19 and pin 4? Correct? I am just a little unsure of which is the correct pin.. but I think pin 2 is the grid of V4a.

[sickman82]

Slightly off topic, but relevant all the same. Due to me snipping the leads of the ceramic caps in the power section earlier in this quest, I am planning on replacing them. Would there be any merit to replacing the other film caps, mostly rated at 0.1uf 400v and a couple at 22nf 400v, with some higher end caps such as Sprague or TAD (they do a repro of the Mullard mustard caps)? I'd be interested to see if what people say about caps is true and whether it does improve tone (albeit subjective).. or whether it is, again, just a load of cork sniffers talking rubbish.

Just a thought, because while I am ordering in bits to do mods, and replace snipped caps.. I might as well order those parts too if there is any point to doing so. I figure as they are only £1-2 a piece, it would be worth it just to see. Thoughts? Maybe PM me to keep the thread on topic though. Cheers!