Mini-TOS OT wiring

[tubenit]

I am going to use a single ended OT for the mini-TOS with the 6BM8 pentode.

8W to 12W SE Guitar Amp Output XFMR
82Hz to 10KHz, 60mADC
7000 Ohm and 5000 Ohm inputs to 4/8/16Ohm Outputs

The 6BM8 tube data is:

Pentode Class A AmplifierPlate Voltage ................................. 200 VGrid No. 2 Voltage ............................ 200 VGrid No. 1 Voltage ............................ -16 VPeak Grid No. 1 Voltage........................ 6.6 VAmplification Factor .......................... 9.5Plate Resistance (approx) ..................... 20K ΩTransconductance .............................. 6400 µPlate Current ................................. 35 mAGrid No. 2 Current ............................ 7 mA

Load Resistance ............................... 5.6K Ω

Power Output (approx) ......................... 3.5 WTotal Harmonic Distortion ..................... 10 %


So, am I approaching this correctly to use the 5000 ohm primary to the 8 ohm speaker?

Thanks, Tubenit

[PRR]

What B+ are you expecting to have?

V^2/7W = trial value of SE pentode impedance.

[kagliostro]

Sorry ...

V^2/7W = trial value of SE pentode impedance.

do you mean (V x V) / 7W  -  (the square of the voltage divided by the wattage) ??

Thanks

Kagliostro

[tubenit]

What B+ are you expecting to have?

B+ will be 277 volts on the 6BM8 plates.   I apologize but I've had no training in electronics & I don't understand the formula you liste?

with respect, Tubenit

[sluckey]

That is just a basic power formula that has been shuffled around to solve for R (resistance)

Resistance is voltage squared divider by power....   R = E^2/P

Using your voltage and PRR's 7 watts yields 10.9K for plate resistance.

[tubenit]

I am thinking 6BM8 single ended will be 3.5 watts?

277 X 277 =  76729  divided by 3.5 wattage = 21.9k for plate resistance

Not sure how to hook this up now???    I'll be using a 8 ohm speaker.

What should I hook up for the primary?

What should I hook up for the secondary?

Help please!   With respect, Tubenit

[Geezer]

7000 Ohm and 5000 Ohm inputs to 4/8/16Ohm Outputs

The most you're going to get from the primary using your 8ohm speaker is 14k (7k primary using the 4ohm 2ndary tap)

I think you're going to need to temp tag the OT in and try different taps (starting with above mentioned one) & see what sounds "right" to your ears(?)

G

[DummyLoad]

277 X 277 =  76729  divided by 3.5 wattage = 21.9k for plate resistance

no, the 7W is the TOTAL dissipation of the tube, you will only be able to deliver 45% (about 3.5W) of that at best to the load.

formula is Va² x Pdiss (Total Plate Power Dissipated)

in your case, 8K would be best.

datasheests also say @ ~270V use 8K  - see pg. 4 of link below.

http://tubedata.milbert.com/sheets/010/e/ECL82.pdf

[Geezer]

in your case, 8K would be best.

So 7k/8ohm is going to be your closest bet.........

G

[PRR]

> B+ will be 277 volts on the 6BM8 plates

That is much higher than the 200V suggested condition.

The tube can only take so-much heat. 7 Watts.

> I am thinking 6BM8 single ended will be 3.5 watts?

Output, perhaps. But first we want to get the power INPUT near-but-not-over the 7 Watt rating.

If you INcrease voltage by 270/200 or 1.35 times, to avoid meltdown you must DEcrease current by 1.35 times.

Suggested 200V 35mA becomes 270V 26mA.

Check: 270V*0.026A= 7 Watts, OK/safe.

Now as a -rough- starting point, pentode load should be the ratio of V/I.

270V/0.026A= 10,385 ohms.

+/-30% variation will cause small change of Power and some change of flavor.

If you have a low-pay helper who will do tedious work well, try all variations and see which numbers you like best. ECL82 curves are fairly soft, the V/I guesstimate for load impedance doesn't give the "very best" numbers, but the difference must be slight.

Use the 7K tap.

[DummyLoad]

That is much higher than the 200V suggested condition.

datasheets do suggest a 270V condition.

--DL

[andrew_k]

I'm using 7.5K:8ohm OTs from Epiphone Valve Jnrs (series 1 and 2 shipped with these, series 3 onwards uses 5k:8ohm) for a 6BM8 stereo and they sound excellent. Very happy with that combination. B+ is just shy of 300V. hope that helps.

[tubenit]

I am seeing the maximum voltage ratings  (design center values) in the tube data being:

PentodePlate Voltage ................................. 600
VGrid No. 2 Voltage ............................ 300 V

http://www.nj7p.org/Tube4.php?tube=6bm8

I ran my Little Wing and my 56T amps with 6BM8 tubes at 305 volts with no issues at all.

I appreciate all the help!  Thanks,  Tubenit

[PRR]

> datasheets do suggest a 270V condition

Tubenit was attempting to use a 200V condition. If that is the only suggestion available, you can extrapolate: 1.35 times voltage means 1/1.35 times current and 1.8 times load impedance. 8.3K.

The 272V condition suggests 8K load, which is indeed what Tubenit should be looking at.

> I am seeing the maximum voltage ratings

The "Va0 = 600V" number is for _NO_ current. Inside a TV set, some strange things can happen during turn-on. The "Va = 300V" number is an operating limit.

But WHY should you run at MAX voltage? In SE it does not affect Power: output power is usually limited by Plate Dissipation. A wide range of B+ voltages give nearly the same power output. 200V: 3.3W, 272V: 3.5W. This assumes you scale the load impedance to the actual voltage and current you use.

For 270V in a 7 watt tube, get in the ballpark of 8K. 7K is plenty close enough.

[tubenit]

PRR & others,

Thanks for the help!

With respect, Tubenit