Red hot coil in the centre of 6P14P-EV

[paggerman]

Hi all. Build #3 sorted (Sort of like a Gibson GA5 but largely modified to suit my box of bits. Replaced 250k anode resistors with 100k and it sounds MUCH better). I'll sit down one night and draw and publish the schematic.

I'll also draw and publish the schematic for build #4, which has a problem I haven't read about. I have built a pre-amp that closely follows the switchable parallel/series input triodes in Merlin's book (I have used 6N2P triodes instead of 12AX7), with a James tone stack (also following Merlin's book) and a cathodyne PI into a 6P14P push pull power amp, which follows exactly the Lamington power stage of my first build (other than using EL84s).

so, apart from a LOT of noise (earthing, lead dress etc.) it works. I haven't given it a workout yet because I noticed that one of the power tubes has a red hot coil down the centre shaft. I've read about red plating plenty, but not red grids/cathodes.

I know that there is probably a heap more information that anyone would need before being able to make a sensible conclusion as to the cause and remedy for this (I am reminded of the 'Lack of critical thinking' thread), but are there any suggestions as to where I should start investigating? I am reasonably confident that all the pieces have been put together according to the various sources.

Many thanks as always

[Steve Conner]

It's probably the screen grid. I've seen the screens on EL84s glow a dull red, but it shouldn't be any brighter than that. If it's orange or white you definitely have a problem. (Most likely the tube isn't getting plate voltage because of an open circuit somewhere)

[paggerman]

I'd say it was orange. basically it is a perfect colour match for the heater filament.

[Jazz P Bass]

Did you try swapping out that tube for a known good one.
If it is a bad tube, it is bad.
It just may be a "binner".

[paggerman]

Did you try swapping out that tube for a known good one.
If it is a bad tube, it is bad.
It just may be a "binner".

I'll try the EL84 pair from build #1 tonight. I'll also do a the schematics.
Thanks

[paggerman]

Build #4 - nearly there!

OK, so I swapped tubes - same problem. Voltages were reading around 307 at the anodes, so I popped a couple of 12V zeners on the PT CT and the coil is now a dull red colour which by Steve Connor's (who seems to be very trustworthy) statement is OK for an EL84. Next I had to deal with a LOT of noise, which from reading multiple posts on this forum I concluded was probably motorboating. A common remedy seems to swapping the OPT leads. So I checked the circuit diagram on which i based the power stage and saw that common terminal of the OPT was connected (indirectly) to the anode of the PI. Swapped the leads over, plugged in and what a difference. This is build number 4, it's the most complex build so far, my lead dress is probably less than perfect but the background noise is really low. And the sound from the parallel triodes is really sweet through my nice shiny new Fender Jaguar Special HH. The cascade mode is OK, but I haven't given it a good work out yet. I've attached the schematic here.

Just a couple of notes about construction. Chassis and PT are from a 1960's radiogram, valves are from ex-Soviet surplus (hence the Red15 name) @ $2 each and the OPT is an M1115 PA 100v line transformer @ $11.

There is a problem, though. When I flick the switch from closed (parallel) to open (cascade) I get the expected 'pop' but then absolute silence for around 45seconds. It then sounds like the amp is warming up and normal service is resumed. Flicking from cascade to parallel doesn't have the same effect. Can anyone shed any light on what might be happening to cause 45 seconds of silence (apart from John Cage)?

I have to give thanks here to those from whom I have taken ideas. Merlin Blencowe for the pre-amp, Grant Wills for the power amp, James for the tone stack, Steve Connor for the advice above, Enzo for his posts elsewhere that helped me resolve the motorboating and the zener trick and Gary, just in case I nicked an idea without knowing it. Thank you to everyone else on this board, your contributions have worked their way into my subconscious and reading this board is a daily ritual.