But a 6V6 would feel right at home. mind you, I have about 20 6K6's so it would not bother me too much. On higher voltages they would run better in P-P circuits. For SE the bias is shifted over quite a bit even when you are running 100% dissipation. But is it doesn't sound too bad and you don't have red-plating or sparks flying then no harm done. I have had 6K6's in a 5E3 with 360V with no issues. Just a touch less loud than 6V6's and NOS 6K6GT's are still cheap.
6k6 3 watt build
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Lowerleftcoast
Poster Extraordinaire
A friend has a pair of 6K6 in a Trainwreck. It is still loud. hah. Sounds good though.
I was reading today that the *power pentode* has more 3rd order harmonics than a *beam power tetrode*. So does that mean an even betterer desirable distortion can be had with the 6K6G? The EL84 is a *power pentode* and I am not so sure I prefer the distortion of it compared to a 6V6. Horses for courses I guess. They all have a place. Roll them tubes.
EL84's need about half the signal to drive them as compared to a 6V6. A 6K6 needs more signal as compared to a 6V6. Might have something to do with it also.
Hi. I got this thread in a feed from TDPRI and read it with interest. I found a pertinent thread over at Hoffmans forum which has some useful info. One thing that can be done to lower the plate voltage for a 6K6 is to use a Hall VVR on the plate and screen. Here are some links with information to consider.
VVR and cathode biased amps
Hoffman Amplifiers Tube Amp Forum, VVR and cathode biased amps
el34world.com
NaptownGuitarman
TDPRI Member
The AC Voltage you measure is RMS, not peak. The peak voltage will be 1.414 x RMS so 316 x 1.414 = 446 Volts. The filter cap fills in the gaps (Filtering the rectified DC) less diode drop, yielding your measured 443 Volts.
-Bob
macpgh
TDPRI Member
all this is ....is a pirated Fender tweed Champ amp schematic. The 6K6 is in the same ffamily as a 6V6 and thus can be substituted. Easiest thing to do is get a JJ 6V6 tube and use it to fire it up and dial it in to see where the actual voltages fall. The original champ has a rectifier tube which will drop the plate voltage but the power rail filtering at the first node is limited by the rectifier tube. Generally rectifier tubes do not like to see more than 40uf of capacitance ......or they will burn up fast. Either they will fry when switched on or shortly there after. A solid state rectifier...hence silicon diodes can handle more power rail filtering and bigger capacitance since this first node acts as a reservoir of energy. I'm not ffamiliar with the 6K6 tube though it was used in the outboard reverb units...the original ones, as the modern reproductions used 6V6 tubes rather than the original 6K6. You should consult a TUBE DATA book to examine the maximum plate voltage recommended for the 6K6 running in CLASS "A". Without consulting the BOOK...I would guess its around 380 volts maybe slightly less, but usually under 400 volts. But at the end of the day, its all about the final bias of the power tube even though its CATHODE BIASED....that still need dialed in. I see a 680 ohm power resistor being used......it could be a higher value or a lower value depending on how the tube draws current as this will affect the end result for plate voltage. I notice that there is a 100uf electrolytic cap in parallel with the 680 ohm bias resistor. The 100uf is too high and will result in a very clean sounding amplifier.....the original value for a Champ is 20 to 25uf. The lower value will make the tube response more juicy rather than clean.....so its a judgement call by ears for what you seek as a response. Personally I do not care for the YOUTUBE channel and its host...he doesn't play guitar is the first tell tale sign in my book....which translates that he doesn't know how to TUNE a guitar amp for a guitar player....despite presenting a friend to play guitar through the amps he features. Its like an old school auto mechanic that can't tune a car motor......meaning he can unbolt stuff and bolt new parts in......but can't tune a carb or distributor....which means the ability to dial in a carb or to re-jet and re-meter a fuel system and make a car run tip top for performance. The auto industry sent products to specific locations which needed specific types of calibration....hence sea level & mountain areas....and then the rest of the country that didn't have the extremes. This was a general calibration which was meant to be adjust at the final destination and operating environment. Personally I wouldn't patronize a medical doctor that drinks alcohol, much less subscribe to anybody that engages in intoxication while working.....just saying. Don't be dazzled by the scenario of repurposing electronic devices....when essentially all you need is a power transformer putting out the right voltages and current to support a particular tube and string of tubes....and an output transformer that matches the plate load requirements for the power tube and the intended power wattage as a result. SO to be clear.....that guy can do repairs as its not rocket science......but working in the field after so many years experience.....you would acquire the ears and not really need test equipment or scoping everything......you just need a good multi-function volt meter....to determine a few parameters......and the unit either works or it doesn't. Parts have a service life.....so its best to do total overhauls rather than re-using aged parts that have exceeded their service life but appear to be still working properly...as you'll be chasing repairs as they fail in situation rather than having a piece that's totally reliable for years considering the service life of the NEW parts the whole way across the board. Repairmen make money by doing repairs.....>>>>>>as a side note, I personally wouldn't have used the rectifier system he used with the 2 silicon diodes......I would spend a few more pennies and have more diodes
ront
TDPRI Member
If I read what you wrote correctly you have the wrong xfmer. You should have the voltage across all windings. The center will be half. KA4WYO.
Perhaps someone has already noted this? The 330vac is across the total windings. This full wave not bridge is typical. Just put the right XFMR and she'll work. CT is ground half on each leg. DC will increase after rectification and filters smoothing. I pray you haven't applied this to the tube plates.I didn't look for circuit errors or death caps if this is an old schematic. DLAB is usually dead on accurate. Great Fenfer Tec. Sure glad I didn't make this error with my 4-1000 or 3-1000z's 5600 and 3500 vdc on the plates. rf amps not audio. Expensive mistake. Thanks hope this helps. You'll get it. Do I smell smoke?
Perhaps someone has already noted this? The 330vac is across the total windings. This full wave not bridge is typical. Just put the right XFMR and she'll work. CT is ground half on each leg. DC will increase after rectification and filters smoothing. I pray you haven't applied this to the tube plates.I didn't look for circuit errors or death caps if this is an old schematic. DLAB is usually dead on accurate. Great Fenfer Tec. Sure glad I didn't make this error with my 4-1000 or 3-1000z's 5600 and 3500 vdc on the plates. rf amps not audio. Expensive mistake. Thanks hope this helps. You'll get it. Do I smell smoke?
Yes Ront the xfmr issue was mentioned and realized not long after my posting. And fortunately, no fireworks. I have built several amps over the years and was hoping I could put this small package in an even smaller cabinet. The veritable 3# of sausage in a 1# skin. I have lots of old radios so had hoped I could find a adequate xfmr for this amp. No such luck though. Once I find a cabinet that I like, I'll probably use this to build up a bastardized champ which I've done in the past. And of course, use a proper xfmr. Thanks all for the input
