WAD K5881 modded to take EL38s

[david]

Just thought some might be interested in the weird and wonderful WAD mods out there. This K5881 was modded to run EL38s, a TV line output tube. All I'm told about the mods is they changed the mains transformer and claim it sounded amazing........EL38.jpg

[bimyb10]

This is my amplifier, and my friend modified the amp to take EL38 tubes, which are essentially an EL37 with a top cap. The EL38 has ceramic insulators to withstand high voltages for sweep tube use. The EL38 was typically used to drive radar equipment CRTs.

The power transformer burnt out, so he had a new custom one wound in the same foot print but taller core -with uprated heater supplies and HT current. The cathode bias was modified for the EL38 which requires less bias, so the resistor values were reduced.

The next mod is a tube regulated PSU, uprated OPTs and dispensing with the EF86 ECC83 front end for better noise performance.

[Greg.]

I don’t see the relevance to the original WAD 5881. Everything is different from the original kit. Are you saying that the basic WAD circuit contributted to the final build? Nothing new there. That’s the same for many loads of valve amps and even solid state amps on the market. There’s nothing new under the sun. It means nothing and actually is just nothing more than a something else valve amp. Of course, it could knock whatever WAD amp it is targeted at into the weeds, but that is not really the point here, as we really don’t give a stuff about performance competition. If you have been clever, we’ll done. I’ll send you a Blue Peter badge! Sorry if I’ve upset you. No doubt you have been subjected to the misplaced influences of David Cant. What a bummer.....yet another one.....what a shame.

[bimyb10]

Same power supply, (replaced transformer though as it burnt out) same driver, same output transformers.

Seems very relevant to me.

Almost all the original kit remains. If you have a WAD5881 you could actually apply this mod by changing four resistors and adding an anode wire.

So a great mod, keeping most of the original in place.

The old adage remains, though best to say nothing if you can't say something nice, also best to say nothing if your analysis is incorrect. In your case both are applicable

[Greg.]

All your photo’s show monoblok amps that have no visual relevance to the original 5881 design. Going with the similarity you appear to be relying on and God knows what that is, how about getting into the detail and really explaining with text and on a schematic diagram the changes you have made and then go on and explain how what you have done has brought a sound benefit over the original kit construction? Absolutely nothing new or original here other than you shilling on behalf of David Cant. His reputation is quite rightfully not heralded here.

Many thanks for your spamming platitudes. Please go away and get a life elsewhere. You and David appear to feed on the ignorance and manipulation of less knowledgable others. Not needed here. Yours and David’s original post has nothing of value. Please stop the spamming/shilling and simply go away.

PS, one of your links leads to sloppy marriage pictures. Youthful but alien to most of us these days, I suspect.

Really, regardless of your personal attack, you are not very good at all. Please feel suitably uncomfortable accordingly! Ha ha!

[bob orbell]

Good Lord Greg, and I thought it was only me that got bloody mad, anyway, well said . BOB

[Richard]

Nice picture from David! No commercial links please folks without running them past Matthew first, thanks.

I did the mods.

I run the community pages which have all the details listed on what to do if you look closely.

Yes I will take the offer of the Blue Peter badge if the offer is still there.

I don't know who "David Cant" is, never heard of him before until someone referred me to the picture.

Over 95% of the 5881 amp is the same with the EL38 mods, so if you want something a little more exotic PM for details.

Why does it sound better, well, the EL38 is a high perveance tube meaning it has a large cathode surface area and much greater cathode current than a regular 5881. It also has higher gain than a 5881 in this circuit. So this means that the ECC83 has to swing less voltage for the same output (about half in this case) so the distortion is about 1/3 less at higher outputs. The higher gain means you can reduce the overall negative feedback somewhat. If you remove the cathode bypass capacitors in the cathode bias circuit you can reduce the gain, and get some local feedback as well which can be added or removed according to taste.

The EL38 also has a lower impedance drive than the 5881, the primary inductance limitation of the modest 5881 output transformers is mitigated almost entirely, this means that the bass response is improved quite markedly. The improved bass response means that the feedback circuit has less to correct at lower frequencies so low frequency signal distortion is vastly reduced.

The EL38 also has 20% more plate dissipation than a 5881, so you can bias hotter for more class A operation, which many people like to do, sometimes at the expense of reduced tube life - however running hotter draws more current at idle which introduces a little more hum, so to combat this we replaced the stock the electrolytics, which were a bit tired anyway.

Doing a load test on the amp with EL38s the power output peak was still with an 8ohm load meaning the 6.6K Raa primary was also a good match.

Finally passing the lot through an HP8903B audio analyser showed a 50% reduction in THD across the board, and greater resistance to blocking distortion, or the elimination of blocking distortion by removing the cathode bypass caps. The frequency response was no longer 3-6dB down at 20Hz being almost flat.

The EL38 has the same pinout as all "6L6-oids" except it is a "no-connection" where the anode pin would be.

The anode is on the top of the tube because it was designed to withstand the back-emf of driving CRT coils, if it were left on the base, in its usual configuration the pins would arc over. The tube also has gold grids and ceramic insulators as this tube is designed to run flat out day-in and day-out at maximum dissipation. Something the 5881 was not designed to do.

Hope this helps and hope it answers your questions. If you want to know more, PM me.

[Matthew Snell]

I have been asked to delete the forum membership of "peterfarrow" - requested from an apparently genuine email address. The message stated "I did not create this account". No reason to doubt it, so have done as asked. If anyone has a problem with this, would they email me directly please?

Cheers,

Matthew