Help improve pp-18 tone

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

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asd
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Post by asd »

Sorry for the delay:

-unscaled: B+=330V, kV=9.85v

-scaled: B+=30V, kV=0.72V

The pot is the stock shipped by Dana Hall, it should be 1M linear pot (it has a circled "a" on it).

I installed a 12v zener Ruby mod, and the fizz seems to go way at max volume, while it's quite evident at medium VVR settings.
It seems that if I turn the VVR down enough (max attenuation) the fizz goes away again. Does this make sense?

And I found that if I strike a chord, kV goes up to 15-16 V. Does this mean that, since the bias shift to the cold side, I'm also getting less power? Could a bigger cathode cap give more output power?
I would be glad if the amp was a little bit louder : )

I also found true what Daviedawg said.
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Post by katopan »

Variable current source with a negative threshold voltage ranging from 12V down to around 1V, pot controlled. I'll work out the ratios and get back to you. Easy to try it out on another pot. Bit harder to match it to a dual gang pot installed on the VVR.
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asd
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Post by asd »

That's so kind of you, thanks a lot!!

And... NOS tubes in the mail!! I'm putting them in this evening, I was thinking to increase cathode resistor to dissipate about 10W and to add 100ohms screen resistor as suggested here, and maybe to decrese screen voltage a bit as suggested here: http://www.18watt.com/storage/18wattLite2b.gif

Is there anything else I should know to preserve NOS tubes life without sacrifing the tone too much?
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Post by zaphod_phil »

I was thinking to increase cathode resistor to dissipate about 10W and to add 100ohms screen resistor as suggested here, and maybe to decrese screen voltage a bit as suggested here: http://www.18watt.com/storage/18wattLite2b.gif
In that case increase the cathode cap size to 1000uF+, to help regulate the bias voltage.
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asd
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Post by asd »

I'll do!!
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katopan
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Post by katopan »

Almost there asd. Hope you haven't given up on this. I'll have something soon, but I've been busy finishing the testing of another amp I've been working on. Have worked out the basics but still thinking around what is needed to protect the NPN transistor. In its basic form the emitter to base max voltage will be exceeded when reverse biased (not conducting with the normal positive part of the signal). Anyway, should have something for you to try over the next few days.
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Post by rock_mumbles »

asd wrote:...
And... NOS tubes in the mail!! I'm putting them in this evening, I was thinking to increase cathode resistor to dissipate about 10W and to add 100ohms screen resistor as suggested here, and maybe to decrese screen voltage a bit as suggested here: http://www.18watt.com/storage/18wattLite2b.gif

Is there anything else I should know to preserve NOS tubes life without sacrifing the tone too much?
What did you get for NOS tubes?
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asd
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Post by asd »

Almost there asd. Hope you haven't given up on this.
I'm here Katopan, waiting faithfully : ) Thanks for your effort.
What did you get for NOS tubes?
Sgt. overdrive said that it's 2 saratovs 7189 and 2 sylvanias 12ax7.
The Saratov's should be from the 80's and and on the Silvania's there is written 59-2 so I suspect the are from late 50's...
But one 12ax7 is branded as westing-house, it's short grey plate and that should be the sylvania.
The other one has no brand, it's long grey plate and that's a 274 on it.
Confronting few pics on the net I suspect it's an RCA.
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Post by rock_mumbles »

... it's 2 saratovs 7189 and 2 sylvanias 12ax7.
The Saratov's should be from the 80's ...
As far as biasing the output tubes, the Saratov (Reflektor) 6P14P, 6P14P-V, and 6P14P-EV tubes are rated at 14 W plate (anode) dissipation.
So they can be biased hotter than a 'standard' EL-84
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asd
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Post by asd »

You're right thanks for the info. So I'm biasing way too cold at the moment, I'll rebias them soon
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Post by katopan »

OK, this is experimental and I haven't tried it out myself. The MJE350 is overkill but is readily available and should be bulletproof in this application. It is not even close to needing a heatsink. Likewise the three protection diodes feels like I'm going overboard, but they're 50c each and transistors really don't like much of a reverse bias voltage, especially emitter to base. So we need to protect it and also not provide any bypass paths around any of the resistance, so it stays around the required 470k in both directions. Lastly I couldn't think of any way to avoid using a dual gang pot, so this control has to be independent of the VVR control, and dialled in along side the VVR setting.

I'd just attach the file except attachments haven't worked here for a while now, so I've put it on my site - link below.

https://sites.google.com/site/chosenaud ... edirects=0
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TomOlsen1
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Post by TomOlsen1 »

Keep going guys, this is awesome!
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Post by katopan »

Just got to see if it works now! I know a variable zener mod is something that has been raised and dismissed a number of times now.
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asd
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Post by asd »

That's great!! Give me few day to find the time to build this, and I'll report asap.

To make it work with a single pot we'll need a triple ganged 1M+250k+250k pot...
And a question: how I can tell if it's working not just by hear?

Thanks a LOT Craig!!
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asd
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Post by asd »

I bought the parts but couldn't find any MJE340 nor any substitutes!!
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