Problem with this fizzy sound

[Ross Grey]

We are on the home stretch now. I hope Silvio is watching this. I'm thinking now that the squashiness may just be listening fatigue.

Bumping the 33k grid stoppers to 47k had little effect on tone. Perhaps the gain dropped a little, but it is so slight that it is hard to tell. I'll just leave these in place -- again, I believe that these are key in removing fuzzy decay.

The real story is the stopper on the PI. I tried 500k, and the OD got harsh again. No frizzy decay, but harsh.

I went back and studied Merlin's guidance on the cathodyne. In particular, "100k to 1meg is usually necessary". He didn't say that this would be sufficient, and he is always very careful in his wording. The obvious next approach was to take the PI stopper to 1.5 meg.

All I can say is 'Wow'. This thing is over the top. I'm now really into 'seductively smooth'. If the sound got better with a 2meg stopper on the PI, I would flip out.

With the 1.5 meg stopper, there is plenty of clean tone on hand. Plenty of treble and gain on hand. Wonderfully smooth OD. I'm ready to buy a pair of these amps, update these then use them to bi-amp a 4x12 cab and use an A/B/A+B switch.

Ross

[kleuck]

Interesting too, there's not so many people who tried big stoppers on a cathodyne PI (i should have, but...) so any report is very valuable, thanks.
The only ones i know did not use anything above 1M.

[Ross Grey]

I also reinstalled R38, so now I am testing with both PI inputs.

For anyone tuning this amp, please be aware that the input to the PI driver from the first preamp stage is delivering a significant clean, and largely low-end signal into the mix.

This is a pretty nice feature for overdrive settings, but the signal mixing in the PI driver for clean settings isn't quite as clear. With R38 in place clean sounds also have more bottom end, which sounds better with smaller cabs.

[Ross Grey]

You must now decide.

With recent experiments, the design intent of the 'two input' PI is revealed I believe.

With small cathode bypass cap for the first two preamp stages, the original design will allow mid and high frequencies through the tone stack and overdrive path to (ultimately) the grid of the PI driver, where low frequencies are mixed with cathode current modulation from the first preamp stage to provide un-muddy bottom end to the PI.

This design was probably intended to deliver better overdrive/crunch tone by not allowing low frequencies contribute to tubby overdrive. Of course all bets are off in 'thick' mode when the tone stack is bypassed and the cathode input to the PI is run to ground through a couple of resistors.

Now I need to decide if I leave R38 in place and go to the original cap values for C7 and C21 or take R38 back out and voice the first two preamp stages like other more familiar designs with more bass content.

In either case, my testing shows that frizzy decay is addressed by grid stoppers as described earlier.

I'm hanging with 33k on V3B, 33k on V4B and 1.5 meg on V4A for the time being.

[kleuck]

Cool, i'm happy for the decay fix and to know your preferred value for the concertina grid-stopper, can be useful to me too As for the shaping of the low frequencies, it's up to you and your tastes, we can't choose for you.

[puke hat roy]

So, Kleuck and Ross, change R25 and R27 to 33k and R19 to 1 Meg or 1.5 Meg? I have the fizz decay also. Just to be sure. You are more advanced than I and after going back and forth between the Wizard, schematics and various forums I think I'm starting to understand. I reread the Dave Hunter book also. It has been an interesting quest. Thanks for all the work all the folks on this forum have performed.

[kleuck]

Looks like i gave a good response, but i did not teste myself (and my amp is not the same), i have no better advice than Ross's one : he finds that 1M5 is the best value, so try this one.

[kleuck]

Looks like i gave a good response, but i did not test myself (and my amp is not the same), i have no better advice than Ross's one : he finds that 1M5 is the best value, so try this one. The 33K resistors can be pushed to 47K, 100K if you want, but the sound can become too soft if you go too far, you must tune to your taste, but it seems that 33K is enough to get rid of the crappy decay (it should be slightly tube-dependent though).

[puke hat roy]

Yes Kleuck, you did, I just did not want to replace the wrong resistor. I just wanted to know if the ones I listed are the correct ones-R25, R27 and R19? I have not done this before. I want rid of the awful fizz.

[puke hat roy]

Yes Kleuck you did. What I was asking was-are the correct numbers R25, R27 and R19? I have not done this before and don't want to make a mistake.

[kleuck]

No, you don't have any resistor to replace : you must add them. Between the junction of c27/r36 you have the connection to the grid of a triode : so between this node and the grid (pin2) you must cut the path and add a 33K resistor. Same with c22/r27, this node to be connected through a resistor to the grid (pin7) Same with the c10/r19 node to the grid of V4B : 1,5 M here.

[puke hat roy]

Thanks Kleuck, I was way off on that. I have the parts coming tomorrow. I will let you know how it turns out. Merci beaucoup mon ami. Continuer le bon travail.

[kleuck]

Thanks Out of topic : "Continuez"

[ampdoc1]

I'm new on this site,..found it because my (on sale) VNT sounded crappy, with that horrible fade ripping sound. I've made all the changes to the unmodified amp per the discussions above, and was pleased with the results.

But it still wasn't right. Using a scope and a signal generator, I was seeing some nasty jittery distortion creeping in on the final stages. At first I though it was a bad OT, so I disconnected the spade lugs and checked everything. It was there that I noticed almost all of the spade lugs were very "loose" on their posts. Pulled them all off and tightened them up,...made a big difference in the stability of the signal. I am a little peeved at the poor quality of the foreign parts on these.

Then I attacked the last thing I didn't like,...the huge volume difference between the clean vs OD modes. As it turned out it was an easy mod, and improved everything. I installed a small 100k "long Legs", Peavey type pot across R36 (10M). This allows pre volume control of the OD mode, without affecting the clean mode. Now I get great even order distortion/sustain/feedback/ from the OD mode at a level proportional to the clean.

Thanks all, for your help. This woulda been a complete waste of money without some guidance.

[ampdoc1]

Lasr Fix For Me

I'm new on this site,..found it because my (on sale) VNT sounded crappy, with that horrible fade ripping sound. I've made all the changes to the unmodified amp per the discussions above, and was pleased with the results.

But it still wasn't right. Using a scope and a signal generator, I was seeing some nasty jittery distortion creeping in on the final stages. At first I though it was a bad OT, so I disconnected the spade lugs and checked everything. It was there that I noticed almost all of the spade lugs were very "loose" on their posts. Pulled them all off and tightened them up,...made a big difference in the stability of the signal. I am a little peeved at the poor quality of the foreign parts on these.

Then I attacked the last thing I didn't like,...the huge volume difference between the clean vs OD modes. As it turned out it was an easy mod, and improved everything. I installed a small 100k "long Legs", Peavey type pot across R36 (10M). This allows pre volume control of the OD mode, without affecting the clean mode. Now I get great even order distortion/sustain/feedback/ from the OD mode at a level proportional to the clean.

Thanks all, for your help. This woulda been a complete waste of money without some guidance.