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Tesla88 28th March 2013 02:50 PM

Need a bit more power :)
I've built for a friend a tube amp made from stock parts . The output section is made from a pair of EL84 tubes in PP cathode bias with common resistor , Phase Inverter is a cathodyne using 5814 tube ...the amp sounds good but a bit underpower , putting out 8W rms with sine input .
I tried fixed bias but only minimal change , and the PI is not near clipping .
The problem seems related to the low output ht voltage of 250Vac , about 280Vdc with maximum load.

I wonder if there's a power tube with same socket of EL84 capable of a bit more power with this kind of supply voltage.

EL86 maybe ? I've a new pair of EL86 NOS from valvo

Midnightmayhem 28th March 2013 03:53 PM

Sounds like you need to redesign your power supply. If you are getting 280v ac that means after rectification you are only getting like 150v on the plates. I think you need a power transformer that puts out more voltage in order to drive more watts out of your 6bq5's they should have (guessing) 250 to 350v DC on the plates. What are the specs on your power tansformer? Voltage of secondaries and weather they are center-tapped?

Tesla88 28th March 2013 03:58 PM

No center tap . It's about 240-250Vac , so with a bridge SS rectifier i get 340V DC , under load drops around 290-300V DC (not 150V) depending on mains fluctuation...

Midnightmayhem 28th March 2013 04:17 PM

I may be out of line here but the fact that your power supply drops to below 90% of it's open circuit voltage under load, that suggests to me that you are drawing too much current from the trans. Do you possibly have a shorted tube somewhere? Do you have a bigger power tranny that you can swap out to see how that affects the issue?

An old timer once told me that to see the amp rating that a power trans is good for, load it down til the voltage is 10% below open circuit voltage and measure the current with that load. that is the maximum safe current to draw from an unknown transformer. Using this formula and the numbers you provided I am going to suggest a bigger (higher current) power supply. Do you have a schematic?

DF96 28th March 2013 05:40 PM

Before fixing the problem, first you have to find what the problem is. This might seem obvious, but it is often overlooked. Exactly where is the limiting happening?

An output stage can run out of current or voltage, for a start - depends to some extent on reflected load, so depends on OPT and external load.

A driver stage can run out of headroom, or can hit the grid current or grid cutoff limit of the output stage.

Find the problem, then think about a solution.

GoatGuy 28th March 2013 05:53 PM

Adding just a wee bit to DF96's advice ... also, make measurements.

Measure the cathode voltages of all the tubes. Measure the plate voltages. Summarize what values are in use for resistors, capacitors. Post a schematic here, to get more of us to think on it with you. Have an oscilloscope? Use your PC output jack to feed a 1V input to the amp at some frequency or another. Convenient signal generator, and it works. With oscilloscope, measure at various points in the signal chain. Are the A/C volts where you expect them? Are they enough?

The reason for all this, is this: using Ohm's law [E = I R], one can determine I from volts and resistance. Its very useful to know. It also will show operating points of the tubes, and their biases. Very useful in turn. There comes some predictability (or at least "analyzability by us Tube Clowns") of gain from the quiescent (no signal) measurements you make and the specs on the tubes (assuming they're behaving in normal ways).

So, i recommend, start with the schematic. Print it out on a big piece of paper (or two sheets, taped together after printing), so that you have a work-page to record values, draw squiggles, margins to make notes of questions and observations. Use pencil, not pen. Prepare to ERASE things.

Then, whatever "problem" you have should become basically trivial to ferret out.



DF96 28th March 2013 05:59 PM

Sorry, I should have said: if the output stage is running out of current (i.e. the valve cannot pull its anode down enough) then an EL86 might help, as that is designed to pull more current at lower voltage than EL84.

azazello 28th March 2013 06:14 PM

Schematic, please!

Tesla88 10th April 2013 10:59 AM

Sorry for not reply , i'm very busy from work , i'm an electrical engineer ... electronics is an hobby !


Some update . I stated a wrong voltage in previous post , the HT winding is at 200Vac , and i get 280V dc at idle , 260Vdc under load.

Schematic is quite simple , a Push Pull of EL84 with 200 ohm cathode bias , 1 resistor for each tube , 1K screen resistor from B+ , 5K grid stopper , driver is a cathodyne PI with 47K plate and cathode load.

I discovered the output transformer is for 4 Ohm and 16 Ohm load , not 8 ohm .

With EL84 seems it work really bad and the maximum power output is 8W on 8 ohm using the 4 ohm output , every other combination lead to less power .
4 ohm would run the tubes out of current , giving less power , same for a 16 ohm load on the 16 ohm tap.
Anyway the 8 ohm load on the 4 ohm tap lead to distortion due to primary saturation , at 8W i get 320Vac plate to plate , don't know TU ratings but i'm near TU saturation.


Tried EL86 , two NOS NIB from Valvo , 5,00 € a pair from a local repair man.
Swap is easy because they're pin to pin compatible , i've no time to change the cathode resistors so applied a negative voltage at grids , about -25V to get 45mA each tube.
Not really matched BTW

Now i get 14W rms continuos sine on resistive load @4 and 16 ohm, 16W rms with hotter bias , no red plating anyway. I also get 12W rms on 8 ohm using 16 ohm tap.
Also the 2 tubes are not matched (i read 8V and 10V on the cathode resistors) , so one tube start distorting earlier meaning a well matched pair would lead to a bit more power :D
EL86 are great !!

Next step is to change cathode resistor and find a more matched pair

Parafeed813 10th April 2013 12:14 PM

So you have 320Vac anode to anode with 8Vac on the 4 ohm tap, right? (8W into 8ohm).
That means a turns ratio of 40.
With an 8 ohm load on that tap, the output valves see an imedance of 12800 ohm.
For EL84's that is very high, resulting in low power output.
Putting 4ohms on that tap gives 6k4, which is too low.
A pair of EL86's are happy with 5k5, so 6k4 is not that far off. Check the voltage rating of the second grid. I think it has a limit of 200V.

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