18 watt fizz: please help me diagnose this?

[JdJ]

Which suggests in your amp it's not a grid current issue. Using something like 22k grid blocking resistors would also help confirm if that's the case. Maybe the problem in your amp is more at the OT end...

I'll try the 22k blockers just to be sure, but 18k2 didn't seem to do much. So if I were to try some snubbers or a conjunctive filter, what values would be a good start? Does more primary winding resistance = more HF roll-off? 5W on the plates adequate?

[VTAMPSUSA]

Might just want to try putting a couple 100R resistors between the primary and each plate and see if that damps it out before you go to the Conj Filter or Zobel.

IMO the Zobel is probably a better tone preserver because it is merely altering the reflected impedance and not messing around with the interactions of the primary and secondary.

I would use the values Randy (ontariomaximus) stated of 2u2 in series with 10R in parallel with the 8R tap (IIRC) - his EL84 amps sound good to my ear. Couldn't hurt.

[JdJ]

Might just want to try putting a couple 100R resistors between the primary and each plate and see if that damps it out before you go to the Conj Filter or Zobel.

Thanks for the suggestion. I have a couple of 150ohm 6.5 watt resistors here - is that rating okay, or should they be more robust? I am running at max dissipation right now...

[VTAMPSUSA]

That should be fine. I doubt an EL84 will draw over 200mA!

[katopan]

I can't help but wonder if the fizz issue is just a result of using a HiFi OT with the extended frequency response and EL84's....

Very interesting point. The direction of this thread is the first plausible reason that has surfaced. Maybe our beloved 18W is that close to this fizz being audible or not, from all the reasons mentioned, that it doesn't take much with tolerances to balance one way or the other. OT primary resistance, speaker selection and even EL84 plate resistance will have an effect on the dynamic characteristics of the reflected inductive load.

And with all the other tests you've done Josh, you've ruled out everything else we could think of.

So what are the differences between a conjunctive on the primary and the Zobel on the secondary? ..... Right now I have a selector wired in, so I'd imagine putting a conjunctive filter on the primary would be the simplest to execute in my situation

A filter on the primary is subject to the valves cutting off through the class B part of the signal swing. On the secondary conduction is all the time. I have no idea on what difference this would make to the sound. You can still fix the filter to the 8 ohm secondary even if you are switching the speaker between taps. The idea is just to reflect a load in parallel to the speaker at higher frequencies to 'cap' the max impedance, and this can be on a separate winding.

[kleuck]

Be aware that a Zobel is meant to flatten the peaks in the impedance curve seen by the amp's output (regardless of the technology of the amp, ss or tube -must be particularly useful with tubes) and therefore is supposed to be calculated from the louspeaker impedance and inductance point of view.
So this is not a "generic" solution theorically, but i did not check if a mismatched loudspeaker (for example) would negate the effect from the zobel network.
If someone here knows how to use a simulation software, i should be an intersting experiment..

[VintageCharlie]

Hello TomOlsen,

could you please tell me what exactly did you do to reduce or get rid of the fizziness in the decaying ntes in your 18w. My 18w clone has the exactly same problem. I brought it to a good tech, but, besides other improvements, he didn't findthe cause for the fizz, or a workaround for it. I'm a noob, so i would really appreciate it you could explain this in laymans terms. Then i could write it down and tel the tech what to try.

As it is now, i get the same fizz and the amp sounds a bit too quiet for 18w - i compared it to a tiny terror, which seemed to be quite a bit louder and and it justsounded better, which shouldn't be acording to the posts here on 18watt.

Thanks a lot in advance!

Best regards,

VintageCharlie

[TomOlsen1]

Hi VintageCharlie

I followed the Paul Ruby doc in the downloads section. Get to a tech with a frequency generator and a scope. The telltale area for the Paul Ruby buzz is the speaker impedance load test.

But, if you're amp is very soft then there is something else causing it. My amp was very loud, even with the buzz.

[TomOlsen1]

@Josh, man, I know that it sucks that we haven't been able to solve your problem yet, but I cannot help but be grateful that this thread made caused some serious analysis of the 18watt and the tendency to fizz.

Also, just to give an update on my amp, at Steve's recommendation I've changed the plate resistor on V1 from 100k to 68k to reduce the overall gain in my amp. With the hot HB's in my PRS, the amp would would EL84 fizzy when then gain was past 9 o clock. I do realise that now the gain level has just been reduced, but the tweak suites my ears and now my gain knob is usable from one to ten.

I think that I had a few problems:
1. Paul Ruby buzz
2. EL84 fizz
3. VVR needs to be turned up past halfway otherwise the amp sounds cold. As mentioned by ZP and Steve in other posts, this is due to the speakers not being driven hard enough and the tubes running in starved plate mode which can sound like a pedal.

Lastly, the 18 watt is a very toppy amp, and its been a bit of an adjustment to get used to it. Before this amp I never truly appreciated just how crucial speaker choice can be.

[JdJ]

You can still fix the filter to the 8 ohm secondary even if you are switching the speaker between taps. The idea is just to reflect a load in parallel to the speaker at higher frequencies to 'cap' the max impedance, and this can be on a separate winding.

That makes sense to me - I was thinking about how any tap should work before I read your post. Unfortunately it is made more complicated by the GDS Vintage OT I'm using. Hmmm - maybe I'll start with those plate snubbers.

Also, just to give an update on my amp, at Steve's recommendation I've changed the plate resistor on V1 from 100k to 68k to reduce the overall gain in my amp. With the hot HB's in my PRS, the amp would would EL84 fizzy when then gain was past 9 o clock.

Thanks Tom - I appreciate your sympathy as well as the update. When you say "gain was past 9 o'clock" are you referring to the volume on the amp at any given setting on your VVR? Is that with the guitar on 10? I'll try this, but I have to figure out what to change the plates to on V3 too. It has a 100k going to the trem side plate of v3 with a 220k between the plates.

I should have time tomorrow to test some of this stuff out - along with a new speaker.

Cheers,

J

[Merlinb]

increasing the size of the 8.2k grid blocking resistors?

+1
Reducing grid-leaks only treats the symptoms by limiting how hard you can actually hit the EL84s.
To actually treat the blocking itself you need big grid-stoppers- the bigger the better, especially with EL84s. Even 100k isn't unreasonable, and as I mentioned in another thread, if you use a VVR then there is even more need for big stoppers. For some reason, though, a lot of folks here seem to find the 8.2k value sacrosanct. I can't imagine why- they have less effect on tone than the grid leaks, and everyone seems happy to mess with those...
Also we should not be too quick to blame the EL84s- the preamp can suffer blocking too remember, and the PI can do some weird things also. Guess what fixes that... yep! More grid stoppers.

Reducing the size of the cathode bypass cap will also help, and removing it altogether is best, but this too will bother some people.
[/i]

[jhaas]

Thanks Merlin, I needed to hear that reinforced specific to an 18W. I've read your material on grid-stoppers it in your book and on your site, and even quoted the relevant site content recently in another forum, when my use of 56K on the grid of an SE 6V6 was challenged. Everyone's apparent reluctance to go over 10K with these 18W amps had me second guessing...

I'm currently dealing with this issue in a Superlite TMB, last night I upped the grids from 8K2 to 47K, and trailing ring/fizz was lesser but still there. I'll try 100K and see...

-John

[katopan]

Hmmm - maybe I'll start with those plate snubbers.

I meant to add in my last post that you should start with the small plate resistors first. Would be an easy fix rather than worrying about filter values.

Even 100k isn't unreasonable....

Thanks Merlin. I have wondered why we didn't see higher values here, and as you say how other resistors are tweaked all over the place but no-one seems to change the grid blockers by much at all.

[Merlinb]

Even 100k isn't unreasonable....

Thanks Merlin. I have wondered why we didn't see higher values here, and as you say how other resistors are tweaked all over the place but no-one seems to change the grid blockers by much at all.[/quote]
I think people get frightened that it will kill the high end, but pentodes have such low input capacitance than you can go pretty high without too much worry- and you can always compensate in the preamp. Obviously you need to be sure not to exceed the max total grid-leak resistance, but that shouldn't be a problem.
Grid stoppers are probably the number 1 most important resistors in an amp, tonally.

Incidentally, if you ever need ammunition (i.e., precedent) for using big stoppers on a belligerent internet forum, quote the Marshall Studio 15, which uses 100k.

[katopan]

I think people get frightened that it will kill the high end

I think people also worry about extra noise, but it's not exactly the sensitive end of the amp and we're not talking about Meg ohms.

[JdJ]

Okay - this is getting silly!

Today I tried:

- twisting the plate/ot primary leads + 150ohm resistors in series
- 100k PT grid blockers
- replacing the ground refs (470k), Coupling caps and PI plate resistors
- adding 2 470ohm screen resistors stemming from the existing 100ohm to isolate the screens
- removed all wires from the OT secondary and connected new wires directly to the speaker jack (bypassing the selector)
- reduced V1 plate resistor to 68k (was 100k)
- added a Zobel using a 10ohm and a 1uf Cap
- tried a new cab & 16 ohm speaker

What did I find? The Zobel helped tame the "blaaaat" fizz, but I can still hear it in the background. Nothing got rid of the leading edge spike on the scope when the volume goes above 7.

I'm thinking my OT is questionable, which I find hard to believe, but what else can be going on!?! Paging Greydon...

[ontariomaximus]

Add another 1uf or use a 2u2 in the Zobel.
The 10ohm + 2u2 would be average values to counteract the speakers VC inductance, but it can be tweaked to taste.

[kleuck]

Here, a Zobel calculator.

[rock_mumbles]

I don't have the fizz problem with my amp but, who knows what I may experience in the future. Because of messing with PA speakers and crossovers I am somewhat familiar with Zobel networks.

When I used the above linked Zobel calculator for a speaker that has a Re = 6.4 and Le = 0.8 (1kHz) which may or may not be typical for a guitar speaker in general, but are "close" values to a speaker I have:

The calculated results: Rz = 8 ohms Cz = 12.5uf

So OM's suggested 2.2uf is 18% of the calculated value... I have some 4.7uf 400V film caps I guess I should do an experiment today to see I can hear the effect of Zobel or no-Zobel ...

[kleuck]

0,8 is not really a typical inductance for a single loudspeaker i think
I have a lil'buddy, wich have a pretty high inductance, and it's only 0.46 mH
And the impedance to use is not -imo- the real speaker impedance at 1Khz, but the "normalized" one you want your output to see with the Zobel mounted.

So for me it would be 10 Ohms/4,7 µf