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 NEW  Matt presents bias and operation data for the 6V6 tube in SE operation - 6V6 Single-Ended (SE) Ultra Linear (UL) Bias Optimization.

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PostPosted: 13 Jun 2009, 03:21 
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Joined: 27 Dec 2008, 04:48
Posts: 25
Location: QLD, Australia
If all else fails, bypass all the switches and just wire it up as per the original schematic on one channel, if that channel comes good, then work from there.

You double checked the pinouts on the valves I guess.

You could split the circuit between the input valves and output valves, ground the input signal to the grid of the output valves and see if you still get the oscillation.

Proly a very simple wiring error, easy to do, we have all done it at some time :)

Ian.


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PostPosted: 14 Jun 2009, 09:15 
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Joined: 17 May 2009, 23:48
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Hi Guys, I've done a a rebuild of my first channel, and replaced pretty much everything including PCBs.... However, I just plugged it in and my B+ is not looking healthy... With no tubes plugged in, the B+ (when on) starts at about 500v and slowly climbs (I saw it at 515v before I unplugged it) :confused: . Anyone have any suggestions as to what it could be? I have verified all the connections, and as far as I can tell everything looks ok. I realise the B+ would probably come down if I plug the tubes in, but I'm sure last time it was at 475v before plugging them in, so I fear a PCB/wiring error, but can't find one... Any help much appreciated!


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PostPosted: 15 Jun 2009, 00:08 
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Joined: 17 May 2009, 23:48
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Ignore me... I must have remembered incorrectly. Plugged everything in, and it measures nicely (422v/238v with load) :up: Just a little nervous I guess! Tried the Gold Lions this time, and do prefer them from the JJs. Only down side is they don't have super bright heaters like the JJs.


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PostPosted: 15 Jun 2009, 00:13 
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Joined: 12 Jun 2009, 14:03
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Location: Bombay, India
ok... i floated the outputs and there is one major change - although the high pitched oscillations still prevail, the audio does come through now.

also, the volume of the oscillations is still independent of volume knob settings.

so i am getting signal through now. any idea what else could be done to get the oscillations out?

i still haven't bypassed the pentode/triode/UL switch and impedance selector switch... will try that as well. i suppose the 100 ohm resistor on g2 will not make a difference?

thanks again! :up:


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PostPosted: 15 Jun 2009, 08:53 
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Good luck with amp, flood. I'm totally new to this, so can only wish you luck!

I've been breaking in/testing my rebuild and I think I'm quite happy with it. I have slightly more mains hum than the first time, even though it is now much tidier, but that doesn't bother me too much. The only real concern I have is over the heat of the PT. Can anyone who has run one of these confirm how hot the PT will get? It is the specced Edcor XPWR105 one.

The PT got to a temperature where I couldn;t touch it for more than a few seconds (80 degress c maybe?). Opened it up to look for problems and found nothing. Powered it back up and it is warm again and getting slowly warmer, but no signs of trouble. I'm running a 6 ohm speaker from the 4 ohm tap, so it should be an easy load I would think??? I never ran it for this long in its previous incarnation, so woudl appreciate knowing your experiences....

Possibly irrelevant but I am running this amp at 50Hz. Thanks,


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PostPosted: 15 Jun 2009, 19:27 
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Just looked on the Edcor site and I see they now have a 120/240 version of the XPRW105. I'm sure that wasn't there when I ordered mine! Anyway, the inefficiency of running at 50Hz seems to be pushing it too much, so I'm going to replace it with the 120/240 version and add a switch to convert it to 120v for future use.


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PostPosted: 17 Jun 2009, 00:37 
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Location: Bombay, India
i'm still extremely puzzled about the oscillations...

to start with, the oscillations are there even without an input. they begin all of a sudden after about 45 seconds of turning the amp on, which i suppose is the time needed for the tubes t get warmed up to the point of operation.

the heater supplies are all DC - a10A (or was it 15A?) bridge rectifier filtered with 5 4700uF and an additional 1000uF cap in parallel, giving us about 25000uF in total. Heater voltages are rock steady at 6V. this is referenced to a voltage divided B+2 (R1/R2=220k/100k, R2 bypassed with a 1uF 100V cap).

the voltages i'm getting on the EL84 plates are about range between 220V and 240V (i have a voltage selector switch for more power and headroom). all trannies are custom wound-to-specs toroidals, PT rated 200-180-0-180-200 at 300mA, 6.3V 4A. OTs are 8kCT, 40% taps and rated for 25W. overkill, i know, but i thought that this way would be the best way to extract the maximum low end from it... maybe i'll replace them with 15W toroids and use the 25 Watters for a guitar amp or something.

coming back to the problem... . i would suspect that the oscillations are arising from (i think chris pointed it out earlier) the EL84 grid itself, since they don't require an input signal and are independent of volume. i wonder if this is due to the fact that there is no grid stopper, but i would doubt it seeing that so many (including myself) have successfully implemented Bruce' schematic verbatim. will try grounding the grids nonetheless.

the coupling cap to the grid for both channels is a 470nF BC (now philips, i think) orange cap paralleled with a wima FKP1500 pF. could the latter contribute in any way, even if no signal was passing through it?

the chassis is wooden with a frontplate, a backplate and a top plate all made of aluminum. there is no base as yet, but i will probably be putting one in as soon as i get it working. all resistances to ground read 0.3 ohms or less... i was convinvced it was some sort of grounding error but i'm not so sure anymore.

just some ideas to what it could be... thanks again for reading. :up:


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PostPosted: 17 Jun 2009, 07:34 
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Joined: 12 Jun 2009, 15:43
Posts: 87
Location: College Station, Texas
Hi Flood,

It is very strange, since the circuit is so simple, and seems to have very few possibilities for problems!

Have you tried Ian's suggestion of bypassing your pentode/triode/UL switch yet? (Is the oscillation independent of which of these setting you use, by the way?) Is there any chance you might have a wiring error where the two screen-grid taps on the output transformer are connected to the wrong EL84 tubes? (I guess that that might cause positive feedback?) Might it even be a manufacturing error in the transformer, where the colour coding for the wires from the two screen-grid taps has been inadvertently reversed? Making resistance measurements between the various primary connections should establish that.

How about unplugging the driver tubes? Does that affect the oscillation?

Assuming the oscillation is unaffected by the removal of the driver tubes, there seem to be very few possibilities for what can be causing the problem. Bypassing your pentode/triode/UL switch, and temporarily hardwiring it in a mode where the screen-grid taps on the transformer are not used, would surely eliminate almost all possible causes.

Good luck!
Chris


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PostPosted: 17 Jun 2009, 18:57 
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Joined: 04 Jun 2008, 20:59
Posts: 4256
Location: Arizona, USA
Hi, This is most odd. However there is nothing that can occur that can not be fixed. Can you send photos of the top and bottom of the amp 1000X800 is usually enough so I can review them. Send them to gofar99@cox.net. Do not exceed 10meg for a single email as it will not go through. Clearly there is a phase or positive feed back situation here. The variations of the amp are extremely stable. Did you use the specified parts, especially the output transformers and driver tubes.

Good listening
Bruce

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PostPosted: 17 Jun 2009, 19:12 
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Location: Arizona, USA
Hi Another thought since I missed the fact you used different trannies, Increase the value of the grid stoppers to about 4.7K and see if it helps or at least changes anything. I suspect that the actual problems are related to the transformers, but without any way to verify it I can't be sure. The Edcors in that circuit will go flat to below 10Hz. That is considerably more than needed for audio. It can cause problems with older turntables that have low frequency wow and rumble. It gets amplified just fine and you can watch the speaker cone waste energy on it. It may be necessary to alter the feedback circuit a bit. With the Edcor trannies it is quite stable, but since all trannies have inductance and capacitance it is possible to create an resonant circuit. With the values shown in the project there is really no way the amp can oscillate.

Good listening
Bruce

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