Re: push pull possible problem??

Patrick, thanks for the tips. I'll go over it one more time.
I worked on another pair of these amps about 8 months ago and they produced
59-60w RMS before clipping, so I figure these should produce the same.
I'm running them at 65ma as per the original Eico design.
I have checked the input to each power tube and they are the same level, so
the phase splitter stage seems to be ok.
My plate dissipation is about 490v X .65ma=31.85w.
I'll go and check the neg biases again for each output, but I think they are
all ok.
Thanks....
Dan

"Patrick Turner" <info@xxxxxxxxxxxxxxxxxx> wrote in message
news:43108109.CA65E13C@xxxxxxxxxxxxxxxxxxxxx
>
>
> Dan Nicotera wrote:
>
>> I've jsut rebuilt a pair of Eico HF60's and first off I'm only seeing
>> about
>> 50pp output across an 8 ohm
>> dummy load. I've injected in a 1 Khz sine wave and on the scope the lower
>> half of the sine wave seems to be distorting or not quite as linear as
>> the
>> top half.
>> As I incresse the input, the top half of the sine wave starts to clip,
>> but
>> not the bottom.
>> Is only part of the push pull not operating properly? I've tried another
>> EF86, 6sn7 and different EL34's and even 5AR4. The output transformer
>> checks
>> out ok also.
>> Both amps have the same problem.
>> All voltages check out, all components also check out ok.
>> Any ideas guys??
>> Thanks...Dan
>
> The 50V p-p is 17.7Vrms which is an output power of 39 watts.
>
> Isn't that about what you'd expect at the onset of clipping?
> What does the manual say about max power?
>
> Clipping in some tube amps isn't always "symetrical",
> ie, occuring at the same time on +ve and -ve peaks and there are a number
> of
> reasons why
> such as the output tubes being not matched, the driver amp topology
> gives more drive to one tube than the other when grid current begins to
> occur.
>
> Have you measured all the signal voltages at eacg anode and grid before
> any
> clipping
> begins to occur, say at ***32 watts?***
>
> I assume your schematic is as at
> http://www.triodeel.com/hf5060s.gif
>
> You should find that the drive voltages to each EL34 grid have the same
> amplitude.
> If they are not equal, then adjust the value of R12 ( which is shown as
> 33k )
> either larger or smaller until the drive voltages are equal.
>
> Each of the grid drive voltages should not be much distorted below
> clipping.
>
> The bias currents at idle can be balanced by adjusting R2, the 10k balance
> pot.
> Make sure that without any signal present that voltages across R18 & R19
> are equal, and approximately 0.40 volts which means
> about 40mA flows in each EL34 so that anode & screen dissipation is Ea x
> Ia
> = 490 x 0.04 = 19.6 watts.
> Check that when the bias is set that the applied -ve voltage to each gris
> is
> about the same for each
> tube.
> A difference of more than 10% grid bias may mean one tube is worn, ie,
> needs
> more
> grid bias than the other, so it'll be the other healthy one with less grid
> bias
> that clips first.
> Also measure the DC idle voltages across the EL34 grid bias resistors R16
> & R19
> of 180k. The voltage should be less than 0.2V, meaning that the grid
> voltage
> is not more than 0.2V *positive* with respect to the negative bias voltage
> at the ends of the 10k balance pot.
> Should you read a +ve voltage of say 1V across either 180k of both, you
> have
> either a problem with a leaky
> coupling cap ( C9 & C10 ), or you have a tube/s which are worn out and due
> for
> replacement.
> Measure this bias resistor voltage 3 minutes after turn on then agsain 15
> minutes later.
> There should be no slow increase of the DC voltage across either bias R.
>
> And if you don't like the sound you could try
>
> http://www.geocities.com/dmitrynizh/hf60-dht.htm
>
> But maybe KT88 or KT90 may work better.
>
> Biasing for KT88, KT90, KT66, 6L6, 5881 are all about the same,
> ie, for an equal applied grid voltage the bias current will be similar.
> EL34 is the odd one out, and bias should be adjusted for
> between 35 and 40 mA for EL34 or any of the above tubes if used.
>
> Also make sure that you have the 8ohm load connected to the 8 ohm outlet,
> not to the 16 ohm outlet.
>
>
>
>
> Patrick Turner.
>


.

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