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-   -   Anyone have experience with transformers for a 6p30b-r PP Amp? (http://www.diyaudio.com/forums/instruments-amps/220791-anyone-have-experience-transformers-6p30b-r-pp-amp.html)

usbekhistan 1st October 2012 04:50 PM

Anyone have experience with transformers for a 6p30b-r PP Amp?
 
Hi I have started to build an PPAmp with two 6p30b-r.I run it with 120 volt and use a transformer thats originally wound for 2 x EL84 with a primary of ca. 10kOhm . Anyone has experience with these tubes?

gingertube 4th October 2012 04:57 AM

usbekhistan,
Since no one has answered, here is what I think.

Datsheet here:
http://scottbecker.net/tube/sheets/113/6/6P30B.pdf

It is a low voltage tube, low power audio ouput pentode. Recommended anode voltage is 120V as you suggest, for single end use a cathode resistor of 330 Ohms gives an anode current of about 35mA. Anode dissipation is 5.5 Watts maximum.

While I have no experience with using these tubes, that limited amount of data suggests to me that the 10K Raa output transformers will not really be suitable and since those transformers were intended for 2 x EL84 (14 Watt tubes) the transformers will be much larger than you really need. You will get no more than about 7 to 8 Watts from your pair of 6p30b.

Lets look at some estimates. You are going to get half the power of 2 x EL84.
Normally that would mean 0.7 Voltage and 0.7 current into the output tranny primary.

However in this case the voltage swing is going to be about 0.3 so you will want about 1.5 times the current.

To get 1.5 times the current at 0.3 times the voltage you will want the Raa to be 10K/(1.5/0.3) = approx 2K.

So you want an output tranny with an Raa of 2K to 2K5 capable of handling about 8 watts maximum. Since it is for a guitar amp where you don't need bass response a 5 watt rated tranny would most likely be fine.

Hope this helps.

Cheers,
Ian

usbekhistan 4th October 2012 01:50 PM

Thank you...
 
3 Attachment(s)
indeed there is not much information to find for this tube.I found some schematics and they all used about 5-6 k for the transformer BUT every schematic i found used a much higher B+ (up to 300 volt!!!) than the 120 Volt in the datasheet of the tubes.While i trust in russian tubes to take some volts more i fear this is way to much...what do you think? The last of the 3 schematics is SE so its a different thing,it has a B+ of 150 Volts,so this may be ok

TheGimp 5th October 2012 03:47 PM

1 Attachment(s)
Looking at the datasheet Va max is listed as 250V, not 120V. 120V is a recommended operating point, and is not a restriction. Max voltage at cutoff is shown at 350V.

I would be inclined to operate at more like 175 to 200V, so with transformer drop and anode resistor drop, a B+ of up to 225V would be max. With 200V you would be operating the tube with about 175Va-k.

Unfortunately the curves don't go beyond 150V, so extrapolation is necessary.

Note that the G2 for the plate curve is at 120V, so your cathode resistor will need to be larger to compensate or you will draw more current than you expect to.

From the lower left curve, extrapolating for Vg2=200V and Ia =20mA, you need a Vg1 of -32V and a cathode resistor of 1600R (1W resistor). This is for the 4K curve.

Also to achieve this you need a B+ of 232V plus the drop in the transformer, so you are close to 250V B+.

Also, this analysis of for a single tube. For PP, double the impedance of the curves.

Using the purple curve, the 10K p-p transformer would work.

Use Ia=30mA, Ik=35mA, Vk=26V, Rk=750R 2W
B+ will need to be - 175 + 26 + Transformer drop (5V) = 206V

usbekhistan 7th October 2012 09:59 AM

Great
 
this will help me a lot! At the moment i have built it on my breadboard,but I prefer fixed bias at the moment.The G2s are connected va 1K resistors to the UL taps.B + is 130 Volt but I will increase it now that you told me Va max is 250.

TheGimp 10th October 2012 04:03 PM

Just be sure to watch your cathode current to stay below Pa-Max (5.5W).

usbekhistan 12th October 2012 06:49 AM

That for sure
 
meanwhile i tried it with 4 tubes and a transformer with 3k . I' ve wired it in UL so i think i will get about 12-15 watts with B+ of 200 volts


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