New PP Pentode Monoblocks

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Mike H
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#166 Re: New PP Pentode Monoblocks

Post by Mike H »

Oh yay-yas! :thumbright:

Last job is to add small Zeners so that they turn on harder when cathode Voltage goes over a certain threshold. The idea is to prevent the cathode caps charging up in class B and reducing the Va. That's the theory anyway. First I need to establish exactly what the nominal cathode Voltage is after a long run, at the mo I think it's 12 or 13V.
 
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#167 Re: New PP Pentode Monoblocks

Post by Mike H »

Yes looks like 13V.
 
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#168 Re: New PP Pentode Monoblocks

Post by Mike H »

Bit of panic this afternoon. Thought project was going to be scuppered. Did the above, and whilst doing so also found an unsoldered wire on the CCS board, that goes to one of the EL84 cathodes. Had only been touching in order to function. But not when Volts tested this time, hence how I found it.

Soldered it, connected up amp to play some toons, turned it on, after a few minutes to warm up right speaker is pumping in and out like a good 'un. Very low frequency oscillation, but inaudible, so I don't know how long it's been doing it! Do not normally watch speaker cones. Is it because of soldering the loose wire has changed something but was other channel! Or been doing it a long time I don't know.

Bollox. I think what it is the OPT's are too good. (130 Henries) In the end I had to reduce the open-loop gain to stop it. Behaving itself at the mo.
 
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#169 Re: New PP Pentode Monoblocks

Post by Cressy Snr »

Have you got some unwanted LF coupling from somewhere in the power supply too?
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#170 Re: New PP Pentode Monoblocks

Post by Mike H »

Possible as they both share the same HT rail.

HOWEVER the LTspice simulation was also LF unstable until I reduced V1 1st stage cathode bypass to 10uF. Conversely, the signal grid coupling to the o/p tubes had to have a lower LF roll off than the values I originally put in. So that's not really the answer, as the simulation is a single mono amp.

BUT I ignored LTspice and used 47uF instead for V1 bypass because I have done before OK. Was lowered to 22uF and still unstable, so have reduced V1 gain by adding 2.2k in series with this 22uF. I.e., V1 EF86 now 'sees' approx. a 1.5k cathode resistor. That's fixed it. So it's telling me too much gain, you'd think lots of gain with NFB is good, but not always!

Anyway been working for the last 3 hours OK and the gain 'loss' isn't noticeable in terms of 'performance'.
 
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#171 Re: New PP Pentode Monoblocks

Post by Mike H »

Image

Went into LTspice to modify the circuit to what I've actually built.

In sim it oscillates. Hmmm!

Seems the sim doesn't like each EL84 having its own cathode path CCS. They're identical. (As are the EL84's of course as they use the exact same model.)

If I link the cathodes together, it behaves itself. (Nothing else I do will stop it.)

This is how it was originally, single CCS for both cathodes as a long-tailed pair, then it went to having a bypass cap added, then it went to separate CCS's with caps. Reason being easiest way to equalizing the anode currents. Pretty much. Could be the caps' time constant is contributing to the oscillation.

In the real world discrete components are not identical, but by a fluke that one time I put the valves back in maybe the right channel pair WERE very closely matched so off it went. When I take the valves out to do whatever to the amp, then replace them, sometimes I keep them in the same order to go back in same sockets, sometimes I don't, especially if they were put back in their boxes. Maybe I should label the boxes?
 
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#172 Re: New PP Pentode Monoblocks

Post by Mike H »

Sir-kwit...

EL84_p-p_amp_005B.jpg
 
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#173 Re: New PP Pentode Monoblocks

Post by Mike H »

Appendix...

EL84_CCS_2.jpg
Edit: R1 is two resistors: 47R // 100R
 
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#174 Re: New PP Pentode Monoblocks

Post by Cressy Snr »

Look fwd to hearing this.
Another proper pentode amp.

Vive le revolution! :wink:
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#175 Re: New PP Pentode Monoblocks

Post by pre65 »

Far be it for me to advise you, but when I did a KT66 PP amp with a separate CCS in each cathode i did as the circuit shown.

Can't remember where I got the info though. :oops:

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#176 Re: New PP Pentode Monoblocks

Post by Mike H »

Yes I remember that actually, and I did consider it (honest guv), but LTspice showed the o/p stage can get noticeably more power out if each cathode was separately AC coupled to ground. It may be nit-picking actually and that the cross coupled version as in yours is adequate for most practical purposes.
 
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#177 Re: New PP Pentode Monoblocks

Post by Mike H »

Cressy Snr wrote:Look fwd to hearing this.
Another proper pentode amp.

Vive le revolution! :wink:
Oh yay-yas!

Cue the revolting peasants...

Image
Last edited by Mike H on Wed Jun 15, 2016 9:12 pm, edited 1 time in total.
 
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#178 Re: New PP Pentode Monoblocks

Post by pre65 »

Mike H wrote:Yes I remember that actually, and I did consider it (honest guv), but LTspice showed the o/p stage can get noticeably more power out if each cathode was separately AC coupled to ground. It may be nit-picking actually and that the cross coupled version as in yours is adequate for most practical purposes.
Fairy nuff. :)
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Mike H
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#179 Re: New PP Pentode Monoblocks

Post by Mike H »

Now is not the time so soon before the fest, while the amp is working, but I will try it later. :thumbleft:


Currently playing Fonogeri - Letters From Home. (again)
 
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#180 Re: New PP Pentode Monoblocks

Post by cressy »

How dare you! Im a peasant, dont think im that revolting :lol:
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