PP3521 idht

[Paul Barker]

Yes but I dont know which valves were in the good side. But There are 6 caps, and 2 of the 6 are new, one each side, they have the stickers on. So I could cut the new one on the bad channel. As the fault on the old amp was the opt smoked its primary which had nothing to do with cathode: so tomorrow ill bring it up slowly with the new cap on that side cut out first. Tomorrow then.

I think its on the cathode and might not have damaged the valves yet. No reason theres anything wrong with resistor, it passes all dead tests, so banking ona cap going into resistor. Theyre 160v and bias is much lower so must be a capacitor. Or a short i havent found. B+ arrives as a single ht so its not outside the signal module.


I think it has to be the new cap on that side. Heres hoping.

[Paul Barker]

I get youre point about eliminating valve issues by puting one in each channel. So I could put one new valve not yet used in each channel as you say. Then its down to the cathode path. As I bring it up on the variac having snipped the new capacitor on the bad side, both channels would bias closely as its brought up. Repeat with two different valves, and again with the final two to check for faulty valves at that moment. Fingers crossed. Meanwhile I’ll make sure I havent shorted a cap of the three on the bad side. Though I did visually check for that visually.

[simon]

Why do I feel like Simon?

Poor bugger!

[Nick]

Or first bring up with no output valves and check the grid voltage to make sure the coupling caps are ok.

[Paul Barker]

IT coupled hammond byfilar maybe that was breached when the opt primary on that side shorted to hround and went open circuit so it could act like a bad coupling cap I have a spare so I can put that in circuit instead. Seems more likely than a pio auto bias resistor cap. So starting to home in on it. Have a guy with no heating collecting me with my hand tools to get his boiler looked at first thing so resume later in day. My van is a right off stearing bent black ice heavy railway iron fence verses steering rack and front suspension geometry no contest. Ive had enough of the costly mot bills each year. Dont see me back full time in heating work Dianas future pretty shady but nobody knows when final blow will come until when im not free to drive round Yorkshire every day leving her home alone.

But getting this amp working will give me much happines again which is important to me. Im very positive there are good sound expectations. Pretty sure it will sound better than 6em7 which says a lot. The two valves that went cherry red might not have suffered too much lucky they werent mosfets.

[steve s]

If you can get them down to me they can all be tested Paul..

[Paul Barker]

Well, now there is a bigger problem than ever. I cut the bypass caps out on the bad side, and changed the Hammond interstage. On the bad side there is the same voltage on the grid as there is on the cathode which is the case for both parallel valves on that side. The other side the bias is 35v and the grid is neutral. So as it cant now be blamed on the bypass caps or the interstage that only leaves the valves or something else I havent yet found. Now, to cap it all off there is no audio out at all. I can only assume that is due to no output from any of the four valves because every single one has spent time in the bad side, since I didnt keep track of them.

I guess next thing is to check grid to cathode seperation of each valve removed.

This isnt going how I envisaged.

The boiler today went better, the return temperature sensor was out of range because the transit tub had cracked and dropped condensation on it. Both parts ready to collect in morning.

[Paul Barker]

Non of the valves removed from the bases is continuous between grid and cathode. Open circuit. The bases with no valves plugged in are 770 ohm on bad side (the side where the plates are red and the grid voltage is the same as the bias voltage so the valves are cooking their plates ) 740 ohm good side. To recap on that side all three cathode bypass caps are disconnected.

I cant figure where the 740/770 ohm is from, possibly the interstage dc resistance.

Ill send these four output valves to Steve for testing at least.

[Nick]

I would have expected it to be the interstage DC resistance + the cathode resistor. Maybe worth checking grid voltage with HT applied to make sure the interstage is not breaking down with applied voltage. What voltage is on the interstage primary? Have you checked the resitance between the primary and secondary of the interstage?

[Paul Barker]

Open between on all three interstages between primary and secondary. Youre right about dcr. Primary is 150v from vr150 plate which drops 300b+ via 25w chassis mounted 6k8 to share 14mA through ecc88 signal current and shunt regulate at 14mA.

One thought is change to lcl coupling with added coupling cap and grid chokes only use one side of interstage leave other side disconnected. Have valves tested by Steve first. My meter not able to test issues inside the valves. I’ll have a break!

[Paul Barker]

I might just build it again on a big piece of wood, rcl coupled from the 300v through the load resistor to 150v plate of ecc88. Make it single output valve through the 3k rcore opt’s. Cant be any oddities like these in that method. I think every one of these valves has had 160mA of cathode current through it. Better look after the other ones in the drawer.

Maybe it is at voltage the interstages are breaking down.

Nobody build this circuit please?

[Nick]

Whats the interstage?

[Paul Barker]

I might just build it again on a big piece of wood, rcl coupled from the 300v through the load resistor to 150v plate of ecc88. Make it single output valve through the 3k rcore opt’s. Cant be any oddities like these in that method. I think every one of these valves has had 160mA of cathode current through it. Better look after the other ones in the drawer.

Maybe it is at voltage the interstages are breaking down.

Nobody build this circuit please?

I think the interstage on one side and the spare one I substituted both break down and pass on high voltage to the grid on that side. Dead tests dont reveal anything but voltage measurments in circuit show the grids have gone positive which tells the another story.

[Nick]

Looking back to find the interstage info, the interstage spec state max voltage is 400v, so that should be ok, but it may be that the valve having a grid to anode short has passed excessive current through the secondary damaging the winding wire.

[Paul Barker]

Thanks Nick for applying youre mind to this. They are old valves and Im using them at max. Perhaps that was my mistake. I shouldn’t blame the Hammonds then. But its a hard lesson, I had too much trust in the components and it bit me.

Iam going to back track but keep this chassis in the mix. The hammonds are coming out. The bypass caps werent to blaim so theyre going in.

In my days with 6EM7’s those were cheap so I have only my historical cost in my head. The concept of the amp sounded ever so good, the output transformers werent involved in the failure mode. The b+ supply took a pounding with all the shorts they had to keep supply voltage to. But that huge shoke took the pounding as did the se output transformer on the bad side.

When I have less private life pressure I shall get my own transformer winding on track, and return to interstages, but I’ll section them not bifilar them. I’ll incorporate some amounts of Nikel, I have some appropriate EI laminations 50% Nickel and parafeed them. I’ll shed my assumptions about capacitors and deploy them.

Valves are my babies, and Im hurting enormously whats happening to my wife, but not helped by these old valves getting wasted. But my mental pressure is much worse for Diana.

So I’ll swap the bases for octals and get some 6EM7’s out of hiding. Order some more resistors, keep it simple, resistor load the driver, cap couple grid chokes. Might yet again change VA choice. Though the sound was great and for transformer loading it was a great choice. The bass was as good as the delicate top end the delicacy was lovely too. True triodes have an endeering quality with their lucious harmonics. I’ll get a win with this chassis with 6EM7’s. I know I shall. I’ll achieve my goal and enjoy the fruits at home.

First I’ll rethink for resistor loading the VA and that might bring different choices of valve. No longer depended on characteristics that suit interstage transformers. Maybe the old fashioned srpp or whose to say a common cathode resistor loaded direct coupled to a cf driving the 6em7, maybe best not use such old valves yet as the mesh plate 27 globe bottles though. Im still smarting from what Ive done to the Mazda’s. But was it my fault? Maybe my fault not puting the volt meter on the grid and my fault not keeping track of where each Mazda had been. My fault for trusting bifilar mechanical resiliance. But I’ll be on my guard next time for a current sucking short somewhere I may still not have found.

I’ll be back, but I’ll move the thread again when Ive desided a few things before just jumping in.

I wish ai hadnt lost the interstage phase splitting transformers with a feedback winding I brought that 1619 push pull amp to Owston with and then lost without trace. I’ll just have to wind my own recipe with Nickel when Im better. Im not done with transformers and Im not done with a push pull mix.