Tube-related questions

jp

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A couple of tube amp questions :?: :?:

1) Has anyone here ever tried THD yellow jackets? I'm kind of curious to see what they sound like and how well they work, especially if I can get Vox and Mash-all sounds out of my Mesa Boogie.

2) What kind of tube bias tools do you guys use? I'm looking around for a decent one, especially the kind where I can bias two valves at once. Some of the choices have drastic price differences, and I wonder if it's really worth it. I like the Groove Tubes kit, especially since it comes with a manual, but it only does one valve at a time. I could have sworn Ted Weber used to sell one, but it's not on his site. I'm also rather curious about one I've seen on eBay by a company called SRS. Anyone have experience with these?
 

dklsplace

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Heard a lot about the Yellow Jackets & even came close to buying a set when I still had my Hotrod Deville. Gotta wonder though, as I was just discussing with capnjaun the other day, I've never played an amp using EL 34's or EL 84's in the power section that I thought was worth a darn. Not bad mind you...just more sterile & lacking character than 6V6's. FWIW, if you're going to spend the money, I'd also look at the THD Hotplates.

What exactly are you after regarding biasing tools? I simply use a good multimeter.
 

matsickma

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I run a pair of Yellow Jackets through a Guild Superstar. I also replaced the standard speaker with a JBL K130.

The Yellow Jackets, with the EL84's, are run in class A mode. They break up with substantial warm distortion-kind of chimey. The sound closely resembles a VOC AC30. I normally perfer a cleaner sounding amp pushed into saturation. However the Yellow Jacket do provide the clasic British tone and can be fun for that type of playing. (Note: The Superstar is a pretty loud amp so getting a warm tone out of it requires pretty high volume. The EL84 Yellowjackets allow the Superstar to break up well before the volume is getting painful.)

M
 

jp

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dklsplace said:
What exactly are you after regarding biasing tools? I simply use a good multimeter.
I'm just looking for the socket adaptor that you put between the tube and the tube socket that you can use with a multimeter to get a bias reading.

matsickma said:
The EL84 Yellowjackets allow the Superstar to break up well before the volume is getting painful.
Cool, matsickma! That's what I want to hear. My DC-5 is 50 Boogie watts, which means really really loud. The THD site claims that the Yellow Jackets act as power atttenuators--like the hot plate--and I'd like to be able to play at less than big club volume. I really love my Boogie, especially how easily tweakable it is, and I know how to get the sounds I want out of it quickly. If I can swap out tubes to get that chimey Vox sound and a Marshall sound, I'd have a really versatile setup. Plus one doesn't have to bias a Mesa--just plug in tubes and go.
 

capnjuan

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jp said:
dklsplace said:
What exactly are you after regarding biasing tools? I simply use a good multimeter.
I'm just looking for the socket adaptor that you put between the tube and the tube socket that you can use with a multimeter to get a bias reading.

Hi JP: like this one? eBay link; Bias Probe This is the single tube version, they sell a dual. If you are interested, think it over a little; I have one of the single-shots - if you're struggling to get to a value, then it means pulling a hot tube ... do other tube ... no good? pull hot tube ...
 

jp

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capnjuan said:
Hi JP: like this one?
Yes that's the SRS product on eBay I was talking about. I'm thinking of the dual version. I'd like to be able to do two tubes at once to avoid cool-down/warm-up. Also like the switch to go back and forth. Looks like the leads just go straight to the meter. Will they fit a standard meter? I've got a decent Fluke.

dklsplace said:
I simply use a good multimeter.
Can one bias without a tube biasing adaptor, Don?
 

capnjuan

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jp said:
....Looks like the leads just go straight to the meter. Will they fit a standard meter? I've got a decent Fluke.
As far as I know ... I'd guess the meter jacks are pretty standard, no?
 

matsickma

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Hi jp,

I wouldn't say the Yellow Jackets act as a power attenuator or Hot-Plate. The attenuator approach is basically a beefy termination box that bleads off the output power thru an array of high power resistors. The Hot-Plate requires that you drive your amp hard into saturation and it converts a lot of that power into heat.

The Yellow Jackets operated by biasing the EL84's in a Class A mode. What that means is that the tubes are not driven over a +/- voltage range. Instead the tubes are biased from 0V to + Voltage or 0V to - Voltage. Audio guys love these amps because at low power levels they are capable of the best Hi Fi because there is no crossover distortion (i.e., crossing over from + Voltage to - Voltage to deal with.) For a guitar amp the reduced voltage range results in a lower power and therefore a lower power level before distortion occurs. There are some other attributes of the non-crossover voltage range that results in the harmonics of Class A distortion sounding different from the other approach (Class AB or B). This biasing approach combined with the particular tube (EL 84) is what gives you the AC30 sound.

The Vox AC30 uses 4-EL84's. That would push it to a power level similar + to a Guild Thunderbird-decent volume but not pain killer.

Another thing that is cool about the Yellow Jackets is that for big power amps like Twin Reverb or 100W Marshalls you can mix them up in pairs. For example you can have pair #1 of EL84 Yellow Jackets and pair #2 6L6's in a Twin Reverb. Similarly you can modify a 100W Marshall pair #1 of EL84 Yellow Jackets and pair #2 EL34's. I don't have any of those big amps but I sure would like to try that combination of tubes. Also you can always install 4 Yellow Jackets in either of those amps and simulate a AC30.

Good Luck,

M
 

jp

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Thanks for the info matsickma. I understand a little better. When I read about YJs the first time I just never thought much about how they function. It makes sense that something placed in between the tubes and sockets can't function the same way as an attenuator. These limit the current and block the grid voltage. I'm intrigued. I'll have to try them out on my Deluxe Reverb and Mesa Boogie. Pretty simple to run them in my Mesa which only has two 6L6s in the power section (six 12AX7s in the preamp).
 

Kap'n

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matsickma said:
The Yellow Jackets operated by biasing the EL84's in a Class A mode.

Actually, what Yellow Jackets do are:

Put a capacitor on the grid, to strip away any bias voltage supplied by the amp.

Put a big-donkey resistor on the cathode so that virtually any plate voltage won't burn up the tubes.

Virtually guarantees a load mismatch, to further reduce volume.




No "Class A" thing going at all - not that it matters.....of course.
 

dklsplace

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jp said:
Can one bias without a tube biasing adaptor, Don?

I suppose that would depend on the amp. I was able to with my HR Deville. It's been awhile since I've had it, but I believe I could check either tube individually, or together.

Now my disclaimer is that I just know enough about electronics to be dangerous. I can follow a schematic ok, but so far I can't really translate what I see into having a handle on how it will sound. A local friend IS what I would call a tube amp guru & he's walked me through more than one project. Having the Deville's schematic in hand, he was able to point me to a couple options regarding biasing the beast.
 

capnjuan

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Kap'n said:
Actually, what Yellow Jackets do are: Put a capacitor on the grid, to strip away any bias voltage supplied by the amp. Put a big-donkey resistor on the cathode so that virtually any plate voltage won't burn up the tubes.
Wouldn't this effectively convert the amp to a 'cathode bias' amp? cj
 

matsickma

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Hi Kap'n,

It sounds like you are describing how the Yellow Jackets impliment the bias of the EL84's. An amp that doesn't switch between +/- Voltages and only runs on on side of the voltage rails is typically referred to as a Class A amp. The push-pull amps are called Class B (typical high power guitar amp) and an amp that works in Class A at low power and changes over to Class B at higher power is Class AB (typical Hi Fi amp). Class A amps can be a single output tube on low power amps because they are biased on one side of the power supply voltage and don't require a pair of tubes and an inverter to amplify both +/- sides of the output waveform.

The Yellow Jacket operation and bias approach is described at the THD Electronics web site (http://www.thdelectronics.com/pdf/yj.pdf). Your description of biasing is similar to their description. They also reference that the resulting mode of amplification is class A. Ovrall it can be said that the Yellow Jackets are a pretty effective technique in changing the overall tone of a amplifier at a reasonable price.

M
 

capnjuan

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Interesting; 'converts' 6V6 / 6L6 / others amps to Class A operation using EL84s. Alternatively, can convert a 7591 amp (Hi T-Birdies !!!) into a Class A / lower power / EL84 amp because, like The Bruce says about jobs in My Home Town: "...these tubes are going and they ain't coming back...". This is also true for those Magnatones using the 7189A.

I guess you could say the yellowjackets are a workaround-of-last-resort for amps running extinct or soon-to-be-extinct tubes (an argument Boardistas to stick with 6V6 / 6L6 amps), and a way to get some tonal variety in 'convertible' amps however that variety is somewhat limited in that only the EL84 is in play.
 
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