simple, cheap, 6L6 amp? help

[trespasser_guy]

jean-paul, I am assuming you are looking for schems for gabevee's amp, which are on the first page of this thread, one or to topics from top.

I am now going to look at some parts for various amps and compare... Any suggestions on resistor wattage ratings for gabevee's and Kaufman's amps? 1/2W unless noted otherwise? Will probably go with Vishay/Dale here.

Also, in a design like Kaufman's, where would a gridstopper go, will it help, and what value should I use?

Thanks

[trespasser_guy]

If going with the Gabevee amp, should I be looking at the bottom one? I am guessing it is most current and updated. This one is under the More Updates heading.

[G]

Sorry. Here's the link to the schematic:

http://members.tripod.com/~gabevee/setube.html

[fragman56]

Hey Tim,

Very nice se 6l6 amp design. A newbie question, please, what does the resistor and cap across the output transformer do?

Thanks in advance,
Rick

[Morse]

Tresspasser guy:

>>>where would a gridstopper go, will it help, and what value should I use?<<<

A gridstopper is used as a safety device to prevent parasitic oscillation. Usually values from 100 ohms to 1k Ohm are common. They ALWAYS go in series with the signal going in to the grid (if it's anywhere else it's something else...) and always are attached with the shortest possible lead (going in to the grid). Usually I butt the resistor body up against the solder tag of the socket so that there is almost no lead at all on that side.

Fragman:

>>>what does the resistor and cap across the output transformer do?<<<

If you're referring to the schematic labelled "final schematic' and you mean the 22k Ohm resistor in parallel with the .01uF cap that run from the secondary on the transformer, they are the feedback loop for the amp.

All the best,
Morse

[trespasser_guy]

Thanks, Morse. I will remember to add some extra resistors to the final parts list for gridstoppers when the time comes...

EDIT: Ok, I tried two ways to link to the pic of Gabe's amp, but his server won't allow outside linkes to images, so... it is the VERY bottom schem on the page. Sorry for the inconvenience of not being able to post it...

But, I wonder... on the missile amp, what would it take to 'adapt' it to use the 6L6 instead of the missile tube it uses? It seems a cheaper route, and much easier, which is good for a tube noob like myself. I am confident that I can read schems, and can build sold state, but when we are talking about 400V, with expensive tubes and caps, I get a little more cautious.

Thanks

[Sch3mat1c]

Originally posted by fragman56
Hey Tim,

Very nice se 6l6 amp design. A newbie question, please, what does the resistor and cap across the output transformer do?

Thanks in advance,
Rick

The 4.7k and .01uF stabilize it. For some reason, it likes to oscillate around 30kHz without it. (Currently, it uses 4.7k and 2200pF here. Also has .022uF (IIRC) across the 2.2k NFB resistor.)
Basically, it absorbs HF energy at the output. (The current values leave the audio spectrum untouched.) I think it could just as well be across the output, though the resistor would have to be much smaller (10 ohms or so) and the cap much larger.

Tim

[fragman56]

Thanks for the info, Morse and Tim

Aspiring DIYer,
Rick

[Morse]

Trespasser_Guy:

>>>But, I wonder... on the missile amp, what would it take to 'adapt' it to use the 6L6 instead of the missile tube it uses? It seems a cheaper route, and much easier, which is good for a tube noob like myself. I am confident that I can read schems, and can build sold state, but when we are talking about 400V, with expensive tubes and caps, I get a little more cautious.<<<

According to the prev posts, the 6AR5 is a 6L6 variant - but I'd be a little cautious about assuming that the 6L6 can handle as much abuse as a ruggedized military variant. Anyway, the "missile amp" is a fixed bias triode strapped design, operating in class A1. Quoting from the 1950 edition of the RCA Receiving Tubes Manual, a triode connected fixed bias 6L6 operated in class A1 has a max plate voltage of 275V with respect to the cathode. A suggested operating point is Vp =250V, -20V applied to G1 from the bias supply, a peak AF (Audio Frequency) voltage on G1 of 20V, zero signal plate current of 40mA, max signal plate current of 44mA, Rp (plate resistance) of 1700 ohms, an amplification factor of 8, transconductance of 4700umhos, and a suggested load resistance of 5000 ohms. This yields 1.4 watts of output at 5% THD (almost all 2nd order, so it's pretty innocuous).

Okay, let's go over the design. First off, the we'll calculate the voltage gain of the 6SL7's. Voltage gain = u * Load Resistance/(Load Resistance + Plate Resistance). Here u = 70, Rl = 51k ohm, and Rp = 44k ohm, thus voltage gain = 37.6 and if you have a 1 volt input you'll get a 37.6V output from that stage so if you change to the 1950 RCA specs you MAY need to change the voltage gain on the 6SL7 to prevent overdriving the 6L6 with too small an input signal. Back to the amp and our signal - I'm going to choose .373V (you'll see why later). It gets multiplied to 14.0 volts by the 6SL7's gain and travels via the .22uF coupling cap (which is there to isolate the two different DC levels of the 6L6's grid and the 6SL7's plate) to the grid of the 6L6. Using the specs for the 250V operating point I come up with a voltage gain of 5.97 for the 6L6. Multiplied by the voltage gain of the 6SL7 this yields a voltage swing of 83.7V at the plate of the 6L6. Now, this is stepped down in the OPT by a factor of the square root of 8/5000 (you can derive it by using V1^2/R1 = V2^2/R2) which gives us 3.35V out at the speaker terminals. Finally using Power = V^2/R we arrive at 1.4 watts output. See why I chose .373V input? I worked it backwards before I wrote this.... Hopefully my numbers all check out - it's easy enough to check them yourself (and before you build anything I STRONGLY advise that you acquaint yourself with the circuit this way and convince yourself of how it's supposed to work, as well as double checking my numbers).

Any input signal of higher amplitude than .373V is going to drive the output stage into clipping, since it will run out of current capacity - but the input stage is no where near being overdriven, as designed.

You MIGHT want to back off the B+ a tad to keep it under the old 6L6 specs HOWEVER, the 6L6WXT MIGHT be able to take 300V at the plate this way - in which case I'd try it as is. You should really ask around (I don't use 6L6's sorry - I can only speculate on the 'toughness' of it) for some experienced 6L6 user's input and decide on what to do based on that.

Sorry for the runon post!
All the best,
Morse

[Sch3mat1c]

Hell... ratings be damned.. I've ran 6W6s at 15W Pd, not even red plate... Popped a 6V6 into Revision 3 (6L6GC SE), only that loose tag on the plate glowed.. and that's like 20W (330V 60mA with a 6L6)...



Tim