Yellow Jacket opinions

[shortcircuit]

Anybody have any experience/opinions on these ?? I'm thinking of trying it in a Vibro Champ. Converts 6V6 to an EL84.
Thanks !!

[Chuck H]

The Yellow Jacket socket is supposed to limit screen current and correct the bias circumstances for the el84 tube. I'm not sure how "universal" this could be to any amp they may be used in since amps vary considerably. One concern I might have is that there's no indication that plate voltage is adjusted. And el84's have a max 300Vp. A typical Vibro Champ has a plate voltage of 420+!!! That said, the limitation of screen current may be sufficient to keep the el84 alive.?. I know that some guys have used these in their Marshall amps. And many Marshall amps have over 400Vp. So these would be my concerns. I'd want to know more about what's inside that converter socket so I could equate the changes to real world circuit analysis before putting my stamp on using them.

JM2C

[Helmholtz]

I guess it will safely work in a BF Vibro Champ (already operating in class A) having supply voltages like this:
https://www.thetubestore.com/lib/the...-Schematic.pdf
I assume that the adaptor contains an (additional) cathode resistor, which further reduces the effective plate to cathode voltage.

But it is simply not possible to get the same output power with the Yellow Jackets in an originally class AB PP amp, if the EL84s are actually operated in class A.

[Chuck H]

If only that schematic were accurate. I've never measured less than 390Vp in a Vibro Champ and later models are worse and typically around 420Vp. That schematic may or may not be accurate to particular year for that amp, but the very next schematic for that amp (ab764) shows 420Vdc measured at the first filter node for a 117Vac power transformer primary. I don't expect that it magically drops to 342Vp through the output transformer!?! All reports I've read from others put the measured plate voltage for these amps at over 400Vdc. Since both schematics indicate the same power transformer I think it's more likely that the aa764 schematic is in error. Which isn't unusual for Fender schematics.

[shortcircuit]

Going back in my notes , this VB with a Jan Philips 5y3 , across pins 3 & 8 on 6V6 386vdc , biased down.

[Chuck H]

Going back in my notes , this VB with a Jan Philips 5y3 , across pins 3 & 8 on 6V6 386vdc , biased down.

I've biased el84's pretty hot with up to 400Vp (but not over). They become noisy due to microphonics pretty fast like this, but they don't blow up and a shorter tube life isn't a deal breaker. I'd try it. But as Helmholtz said, you won't get the same power as with the 6V6. And since el84 tend to sound more strident and less squishy at higher voltages I don't think you'll notice any useful change in tone. Certainly not "chimey" or Vox like if that was the goal. But you may like it however it sounds. You can't know if you don't try I suppose.

[Helmholtz]

But as Helmholtz said, you won't get the same power as with the 6V6.

This might be a misunderstanding. My general remark above was about converting class AB designs to class A, which must decrease output power as losses significantly increase.

But the Vibro Champ already is a class A design. So things are different. The 6V6 in the VC has a cathode resistor of 470R. This is way too much for an EL84 and won't allow for center-biasing, resulting in reduced clean headroom.

From the product leaflet, I am not sure if the Yellow Jacket even is intended for class A amps, as they talk about converting to class A.

[Helmholtz]

Going back in my notes , this VB with a Jan Philips 5y3 , across pins 3 & 8 on 6V6 386vdc , biased down.

Sorry, but what is VB and what is biasing down?

[shortcircuit]

Sorry, but what is VB and what is biasing down?

VB is Vibro Champ, biased down, is I raised the cathode resistor(so now it's 825ohm) to lower my VDC from 402vdc to 386vdc , sorry for the confusion. And your post #7 is what I was wondering, it says converts from AB to A , just did a total re cap ,I don't want any thing to go poof. Thanks all for your input !!

[Chuck H]

VB is Vibro Champ, biased down, is I raised the cathode resistor(so now it's 825ohm) to lower my VDC from 402vdc to 386vdc , sorry for the confusion.

Actually...

It doesn't work like that. Increasing the cathode bias resistor would reduce current and RAISE voltage. Whatever your memory of how it went down I promise that you did not decrease plate voltage by putting in the higher value cathode resistor. You should reexamine what's going on with regard to this.

And your post #7 is what I was wondering, it says converts from AB to A , just did a total re cap ,I don't want any thing to go poof. Thanks all for your input !!

If I'm interpreting the literature properly I have a good guess at how the Yellow Jacket manages this. But it's only a good guess.

Notice that there's a black lead on the Yellow Jacket base. I'll bet this is intended to be grounded/earthed to the chassis. The Yellow Jacket lit says that it converts any amp to class A "self bias". From this I interpret that the base has a capacitor that blocks DC from the grid and eliminates the cathode connection from the amp, substituting the bases own cathode resistance. Which must be an arbitrarily chosen value that is "close enough" though not idealized for most amps. I can't imagine another way they could be doing this.

In which case it should work in a Vibro Champ with under 400Vp. But as I said above, I'd prefer to know what's in the Yellow Jacket base before endorsing it.

[Helmholtz]

VB is Vibro Champ

??? Why not VC?

it says converts from AB to A

I mentioned this aspect as the Yellow Jacket leaflet says "converts from class AB to class A operation", which does not hold for the VC.

I raised the cathode resistor(so now it's 825ohm) to lower my VDC from 402vdc to 386vdc

This makes the tube run very cold, significantly reducing clean output power. An EL84 wants something like 200R for optimum class A center biasing.

[shortcircuit]

Actually...

It doesn't work like that. Increasing the cathode bias resistor would reduce current and RAISE voltage. Whatever your memory of how it went down I promise that you did not decrease plate voltage by putting in the higher value cathode resistor.

Thank you, and I stand corrected . Went back and looked at my original post.. (did play with cath resistor , it took a 825ohm to get a 31.5 drop , 400v across pins 3 & 8 .38mA to get a 14v dissipation , and it did raise pin 5 to 8 from 26.62vdc to 31.50vdc) it was to lower dissipation, does that make a little more sense

[Chuck H]

Please see the edit in my post above

[Helmholtz]

Increasing the cathode bias resistor would reduce current and RAISE voltage.

Yes it will raise plate voltage. But the tube only cares about/sees plate voltage minus cathode voltage, so...

[shortcircuit]

??? Why not VC?



I mentioned this aspect as the Yellow Jacket leaflet says "converts from class AB to class A operation", which does not hold for the VC.



This makes the tube run very cold, significantly reducing clean output power. An EL84 wants something like 200R for optimum class A center biasing.

Sorry Hemholtz , VB was stupidity on my part, typing not thinking, one of those days.