18 watt fizz: please help me diagnose this?

18watt-specific Tech Talk - Building, Fixing, Parts, Mods...

Moderators: CurtissRobin, colossal, zaphod_phil, Daviedawg, Graydon

Post Reply
katopan
Frequent poster
Frequent poster
Posts: 884
Joined: Mon 12/10/07 2:00 am
Location: Melb, Aust
Contact:

Post by katopan »

kleuck wrote:I think i understood the reason for a twin cunjuctive filter in the Watkins :
When both tubes are working together ("A class") they "see" the whole impedance of the primary of the OT, BUT, when only one is conducting, it "sees" only 1/4 of the primary impedance.
So with a cunjunctive filter for each half of the OT there should be a more even impedance on the primary all along the conduction cycle of the tubes.
Kinda what I was talking about - on the primary a filter across the whole winding will act differently as one valve cuts off. On the secondary there is conduction through the whole cycle.
0 x

kleuck
Occasional poster
Occasional poster
Posts: 253
Joined: Sun 11/18/07 2:00 am
Location: France

Post by kleuck »

katopan wrote:
JdJ wrote: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.
Indeed :oops:
0 x

User avatar
JdJ
Newbie
Newbie
Posts: 54
Joined: Tue 05/05/09 2:00 am
Location: Seacoast NH

Post by JdJ »

So I finally got around to taking a quick screen shot with the Zobel in place. This is with the 10ohm and 2uF across the secondary and 15v Zeners a la Ruby:

After (Volume on 10 400mV 333Hz Sine wave)
Image

Before with no Zeners and no Zobel (note the Vol on 7 - @10 the leading edge spike goes through the roof!)
Image

So the rising impedance is the culprit indeed! Thanks so much guys - you all have been a great help. I can't believe how much I have learned over the past month. If I can assist anyone by putting together more screen shots or any other testing, just let me know.

Next I'm going to try it without the Zeners again to see how I like the crossover distortion...

Special thanks to Katopan who talked me through the testing.

-Josh

PS I'm finally going to gig it this Friday!!
0 x

katopan
Frequent poster
Frequent poster
Posts: 884
Joined: Mon 12/10/07 2:00 am
Location: Melb, Aust
Contact:

Post by katopan »

Awesome news! Nice to see the CRO traces too - thanks for updating.

I'm interested to see what you think with the zeners removed, which will indicate if the problem noise was a combination of the two issues or just the one. Also did you put the grid blocker resistors back to some normal sort of value and did that return all of your treble, or does it still sound like the tone control is turned down a bit? I'm hoping that the tone is full range because on paper the zobel shouldn't be having an effect except right at and above the top end.
0 x

TomOlsen1
Occasional poster
Occasional poster
Posts: 159
Joined: Thu 09/18/08 2:00 am
Location: Jo´burg, South Africa

Post by TomOlsen1 »

Congratulations Josh!
0 x

User avatar
Gary
Newbie
Newbie
Posts: 93
Joined: Fri 08/18/06 2:00 am
Location: Leeds - as in "Live At ..."

fizz summary

Post by Gary »

Speaking as a novice, I think this deserves some kind of summary and a breakdown of the steps required to test and fix the problem. Could be useful for a lot of frustrated tweakers.
0 x

User avatar
JdJ
Newbie
Newbie
Posts: 54
Joined: Tue 05/05/09 2:00 am
Location: Seacoast NH

Post by JdJ »

Thanks for the congrats guys!
katopan wrote:I'm interested to see what you think with the zeners removed
I tried it tonight and didn't like it without them. You definitely can see increased crossover distortion on the scope (not to tune by eyes...), but it sounded phasey and too swirly. Markedly less definition. Back in they went. I didn't try removing them with the 100k blockers - that'd be an interesting experiment...
katopan wrote:Also did you put the grid blocker resistors back to some normal sort of value
I went to 22k grid blockers. The treble is much better. I also like the Wimas I used over some metal films I tried for the Zobel.

Strangely enough, I tried changing out my current screen resistor configuration (a shared 100R with individual 470Rs to the screens) going with split 1Ks and no shared resistor. It sounded horrible! Almost like the speaker jack was cutting in/out - so much so I tried a different cab. Anyway- I went back to the previous 100/470 setup and all is well. My screens are dissipating around 1.5watts and the voltage at pin 9 is 333VDC.
Gary wrote:I think this deserves some kind of summary and a breakdown of the steps required to test and fix the problem
I'm not quite sure how to go about doing this, but I agree it would be helpful. I think ultimately it would have to be a collection of sound clips illustrating the issues. I think that the idea of a clip bank of sorts has been brought up a few times. Aside from that, (and I don't have recorded clips but Tom posted some earlier of his Ruby Buzz-sick amp) all I can do is assemble CRO shots, but not everyone here has a scope...

Cheers,

Josh
0 x

User avatar
Gary
Newbie
Newbie
Posts: 93
Joined: Fri 08/18/06 2:00 am
Location: Leeds - as in "Live At ..."

zobel capacitor specification

Post by Gary »

what would be usable construction types and voltage ratings for Randy's application discussed earlier?

I notice the bit about not using electrolytics but I haven't seen any specifications mentioned.

Thanks
0 x

User avatar
JdJ
Newbie
Newbie
Posts: 54
Joined: Tue 05/05/09 2:00 am
Location: Seacoast NH

Post by JdJ »

gary wrote:what would be usable construction types and voltage ratings for Randy's application discussed earlier
The consensus seems to be 5w on the resistor (I'm using a 10w ceramic as I had it around) and I have 1000V Wimas for caps. I don't think the resistor is ever going to have to dissipate a whole bunch of current at this frequency, but AFAIK 5W is more than adequate.[/quote]
0 x

User avatar
Gary
Newbie
Newbie
Posts: 93
Joined: Fri 08/18/06 2:00 am
Location: Leeds - as in "Live At ..."

Post by Gary »

would like to experiment with the conjunctive filter and zobel suggestions, but I don't know what caps to buy, in terms of safety margins.

What voltage rating do I need? What material would you recommend?

Do these factors differ depending on whether its on the primary or secondary?


In a similar vein, what type and rating of cap would you use for a 'fizz cap' across the phase inverter ?

Thanks guys.
0 x

User avatar
zaphod_phil
Builder, Admin
Builder, Admin
Posts: 14991
Joined: Wed 03/19/03 2:00 am
Location: YYZ

Post by zaphod_phil »

PI fizz cap - 400V silver mica (preferred) or ceramic

Conjunctive filter - 1000V polywhatever

Zobel filter - depends on amplifier power output. V= sqrt(P*R). So for example, say if an 18W amp puts out 36W peak on distorted transients, and we're using a 16 ohm speaker load, then V = sqrt (36 X 16) = 24V So use a non-polarised electrolytic cap (of the kind sold for speaker crossovers) rated for 30V or above.
0 x
Nature abhors a clean tube amp

User avatar
Gary
Newbie
Newbie
Posts: 93
Joined: Fri 08/18/06 2:00 am
Location: Leeds - as in "Live At ..."

Post by Gary »

Thanks ZP.

Maplins had the grand total of two 2u2f non polarised caps in stock!
0 x

User avatar
zaphod_phil
Builder, Admin
Builder, Admin
Posts: 14991
Joined: Wed 03/19/03 2:00 am
Location: YYZ

Post by zaphod_phil »

Never mind, they have plenty of digital thermometers and radio-controlled cars. :roll: :x
0 x
Nature abhors a clean tube amp

User avatar
Snapcase
Occasional poster
Occasional poster
Posts: 176
Joined: Mon 08/23/04 2:00 am
Location: Spain

Post by Snapcase »

This is one of the best and more informative discussions I've read in a lot of time. Thanks a lot, guys. Certainly this is the very best EL84 related forum out there.
0 x

User avatar
zaphod_phil
Builder, Admin
Builder, Admin
Posts: 14991
Joined: Wed 03/19/03 2:00 am
Location: YYZ

Re: 18 watt fizz: please help me diagnose this?

Post by zaphod_phil »

Adding this into the thread. I haven't heard anyone talk about snubbers for quite some time, and I believe this idea has merit.
oh7hhi wrote:This worked really well in my amp: http://www.guitarplayer.com/article/smo ... -ride/5505
0 x
Nature abhors a clean tube amp

Post Reply