Finished and working - 12 valve all DHT PP 26-71a-45

[izzy wizzy]

Put channel A probe on one phase and channel B probe on the other phase, Set trigger to channel A only. Other options might be B only, Alternate, Chopped. Then you can see the phase diff between them.

Cheers,
Stephen

[RhythMick]

Ok I now have

Xlrs still go to correct input tx

Left tx goes to right 26s

Right tx goes to left 26s

Left channel (right XLR and input tx) is ok

Right channel (left input) whistles.

Think that proves it's not the XLR or input tx

[izzy wizzy]

I hope you take my next comment the right way. You might as suggested by yourself have some feedback situation. From your pic near the beginning, you have a rather interesting layout with very long cables betwixt the transformers down the spine and the valves on the fold out wings. I'm guessing the cables fall where they may when closed up. Are you able to dress them in a way to avoid inputs and outputs being in close proximity? Maybe using card board props or the like to elevate and seperate for the purposes of fault finding

I might have missed something, but I assume you've done much swapping of valves between channels?

Cheers,
Stephen

[RhythMick]

I hope you take my next comment the right way. You might as suggested by yourself have some feedback situation. From your pic near the beginning, you have a rather interesting layout with very long cables betwixt the transformers down the spine and the valves on the fold out wings. I'm guessing the cables fall where they may when closed up. Are you able to dress them in a way to avoid inputs and outputs being in close proximity? Maybe using card board props or the like to elevate and seperate for the purposes of fault finding

I might have missed something, but I assume you've done much swapping of valves between channels?

Cheers,
Stephen

Thanks Steve. Perhaps my fault for muddying this thread. The pic you're referring to is Geoffs amp which is working fine. I'm now working on my build of a similar design, which is giving what I think is feedback on one channel.

By the way, I'll never be offended by attempts to help or constructive feedback (excuse the pun). All help very gratefully received!

[izzy wizzy]

I was assuming even tho that one was another amp, yours was of similar layout? Or is it completely different?
If different, can we see a pic?

Cheers,
Stephen

[RhythMick]

I was assuming even tho that one was another amp, yours was of similar layout? Or is it completely different?
If different, can we see a pic?

Cheers,
Stephen

Sure. I'll have to edit a few images down size and post later. It's a different layout. Mine is being mounted on rollers in a cupboard. Top level is a single piece of 30mm Cedar with valves and caps mounted plus mains inlet, speaker posts, umbilical in, XLR in and output transformer under covers.

Hanging down from there is a piece of 6mm ply, from which hang platforms for the toroidal, chokes and interstage transformers. Another piece of 6mm ply forms a base. On the back are 2 vertical sheets of copper with DC boards on the inside and heatsinks on the outside. On the front are a board per channel with all the components for the long tails on, which gives ready access to balancing pots, trimmers and resistors for measuring plate current.

[RhythMick]

I may be making progress pinning it down. Though it's making my head hurt.

The B+ for the 26s is applied to the centre tap of the primaries of the first interstage transformer, then to the anodes of the 26s, down through the cathodes and tail. I used clip leads to connect up the RIGHT 26 pair and tail to the left interstage power and return, and vice versa. So the right 26s anodes are inductively coupled to the left 71A and the left 26s are inductively coupled to the right 71A.

The left still plays music. That music is coming from the right hand XLR - I proved this by disconnecting the left XLR from the back of the CD player - music plays in the left channel. Disconnect the remaining right XLR and of course the music stops.

More interesting is doing the same to the opposite channel. I put the left XLR back in and removed the right. Music plays but very quietly in BOTH channels! The left XLR is going to the grids of the left 26 pair, which are powered from the right hand interstage primary.

There must somehow be a crossed connection somewhere around that interstage. Nothing for it but to whip the base off and check everything.

[Nick]

If there isn’t a crossed connection then maybe you are getting inductive coupling between the transformers?

[RhythMick]

If there isn’t a crossed connection then maybe you are getting inductive coupling between the transformers?

It's possible, or between wires. I'm just happy to have pinned it down a bit. I can stop scratching my head over the input tx and 26 stage. Both the left and right 26 pairs and long tail components have now produced music.

[RhythMick]

Found it. Within 2 minutes of taking the base off. Connection on interstage just as I thought it was.

Will confirm in about 30 minutes.

[IslandPink]

Happy Christmas !

[izzy wizzy]

Found it. Within 2 minutes of taking the base off. Connection on interstage just as I thought it was.

Will confirm in about 30 minutes.

I've waited my 30 mins at least. Hope that's a result. Fantastic if it is.

cheers,
Stephen

[RhythMick]

Found it. Within 2 minutes of taking the base off. Connection on interstage just as I thought it was.

Will confirm in about 30 minutes.

I've waited my 30 mins at least. Hope that's a result. Fantastic if it is.

cheers,
Stephen

I likes to keep my audience in suspenders. Ooh err missus & all that.

Well that was a fault, but the old problem of an upside down amp had me confused. That fault was in the left channel, which played (with a buzz). It now plays much better and without the buzz. Lol.

Still fault finding on the right.

[RhythMick]

Well the left channel input tx, 26 and 71A are fine. Problem pinned down to one of...

2nd interstage
45s
Opt
Speaker posts

Unfortunately getting at those means stripping back a large part of the build. Early start tomorrow. Really want this working for Xmas.

[izzy wizzy]

And take some pics. Wanna see

Cheers,
Stephen