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Does anyone have a clean scanned schematic? All the ones that I have seen online are blurry...

From the Internet Archive, it's a beauty!

Well, I am sad to say that the tubes that I ordered never showed up, and I have asked eBay for a refund. Now I need to find and place another order, and it will take a few more weeks before they get here, oh well... :mad:

Nice find, an article by Prof. Terman! The drawing is just an example taken from Philips' application note... so just move the choke to the positive rail. Done!

Not much going on at the moment, I'm still waiting for the output tubes that I ordered to arrive... In the meantime, I am looking at making some changes to the power supply, in particular, regulating the screen grid voltage. Since I have some 0D3 on hand, I could connect two in series and have a...

Thanks for the reminder, I will check it out.

Yes, I do plan to change the bias scheme to partially cathode biased, and raise the idle current a bit for more Class A output, but not going for pure Class A, since that was never the idea. Because all the existing circuits will be removed anyway, so there isn’t that much extra work for the...

Wiring the output tubes as triodes or re-biasing them for Class A operation can be done easily, therefore not much of a challenge. For this engineering exercise, I chose to use the OPT for CFB, which is of course more difficult to do compared to the above options, but it also seems more...

These zero-bias tubes need lots of grid current, power! Check out the characteristic curves for the 811A. So a bunch of triode-strapped KT88s isn’t such far-fetched idea… :oops:

No, the Lundahls are off the shelf items.

Yes, because of the grid current needed for the 811 output tubes, which isn’t needed in this case. But that’s indeed a great design by Altec. Thanks for sharing it.

No, 200H is correct, it has to be quite high for good frequency response. He also described how it should be wound on his website. I plan to get the Lundahl LL1667 or LL1668.

Instead of using the small bench (tube) amp to test the Philips OPT, I took out an old SS amp from the storage shed and hooked it up to a spare OPT in the reversed direction as suggested. There’s no surprise… the Philips OPT worked as expected. So the next step is to design and breadboard the...

Thanks for the suggestions, I think I will try both and see how it goes. My little bench amp needs all the help it can get… :confused:

I'm not quite sure what you meant, could you please elaborate?

Thanks for the reminder on the wire gauges used in the various windings. I just did a quick test on the bench with the OPT removed from the EL6435, and everything checked out per Philips' specs, I used the Com and S2F (100V) terminals for CFB, with S2D (50V) grounded, which represented about...

Here is a better image of the OPT specs taken from the EL6436, which is a 19” rack mount version of the same amp, without the volume and tone controls.

Deleted.

Found a nice, vintage Philips poster.

Found a few more pictures of the EL6435 online, these are very clean examples! Unlike mine, sigh…

Yes, they could be good options if I can't find the Philips (see below). No, I have no experience with the G-807's at all, and I should clarify what I meant by "staying away", even though the G-807 is the most readily available tube type, and the cheapest by far, it is also the furthest away...

Thanks for the feedback, guys. I like to find some PE06/40's although it does not seem likely, but I have not given up yet. Next on the list is the QE06/50's & the NOS 807's. I'm trying to stay away from the Russian G-807's.

I have a question regarding the tube types - PE06/40, QE06/50 and 807, are they really interchangeable? From the data sheets, they appear to be different, but many vendors list them as being equivalent. For those with experience with these tubes, what do you think?

Something to look forward to…

Wiener & mustard, now you guys are making me hungry... :banana: BTW, the odd looking part in the lower photo is a part of the opto-compressor circuit, a pretty neat feature from Philips.