RhythMick wrote: ↑Thu Dec 27, 2018 1:48 pm
One more thought, can someone help me eliminate this one from the clutter inside my head.
I'm playing direct from the CD player. No volume control. It's a low powered amp into inefficient speakers. Couldn't be that there's too much gain?
No there's loads of headroom on the 26. Bias is 10v. Plus it works fine single channel cross powered.
Steve, about that DC return path. Yeah made my head hurt a bit too. When cross powered, the right channel 26 power goes to the right channel interstage and is distributed to the left channel 26 valves. The b- from the left 26s (bottom of the tail) returns to the interstage where a solder post joins it to the PSU return. There's only one 0v ground reference which is from the left 26 tail.
I'll try the bias trimmer now.
I was thinking further over lunch. You have some DC over the choke as part of the bias so returning the grids to ground will give you way too much bias to the grids so you might have to do some calcs to work things out; maybe removing the bias pot altogether or at least returning the wiper to one side. All depends on bias point and choke resistance. But you designed it so you know all that.
Wish I was playing with my 813PP but I'm making stairs.
Yeah this is how the tail of each valve pair works. Every pair has the same structure, but different values of resistors and chokes.
2 X 200R resistors form VC for each valve
VC connect to either side of a 2W pot which allows me to vary the resistance to each cathode and thus the bias, so balance the current through each
Top resistor sets the minimum bias
Parallel paths from there.
- bulk of current goes through tail choke and bottom resistor to b-
- 100k trimmer to ground, wiper control Grid voltage and connects to interstage primary c/t
I'll remove the trimmer from circuit and insert another resistor to set the bias point
So from here I guess I do the same for each valve pair, change the top resistor, take the bias from there, take the 100k out of circuit. I lose the ability to control the bias point.
Is there an alternative? Cap from wiper to ground ?
RhythMick wrote: ↑Thu Dec 27, 2018 2:39 pm
So from here I guess I do the same for each valve pair, change the top resistor, take the bias from there, take the 100k out of circuit. I lose the ability to control the bias point.
Is there an alternative? Cap from wiper to ground ?
Well maybe. My pref would be choke in tail and grids to ground but you might not be able to do that, otherwise, yes, you might have to put a cap off the wiper if you want to keep the same adjustment/config.
Looking at your op points and circuit voltages, grids to ground may not be possible esp on the 26. Too high voltage for starters so maybe decoupled bias from cathode may be best option for now.
izzy wizzy wrote: ↑Thu Dec 27, 2018 5:04 pm
Looking at your op points and circuit voltages, grids to ground may not be possible esp on the 26. Too high voltage for starters so maybe decoupled bias from cathode may be best option for now.
Cheers,
Stephen
Yeah not designed for grounded Grid. I'll go simple for now
RhythMick wrote: ↑Thu Dec 27, 2018 9:30 pm
Well it was a good effort. All 6 bias trimmers are now out of circuit, top resistors changed to set bias. Interstages mounted back in amp.
Squeals like a stuck pig.
I recognise I'm pretty tired. Stopping now. Will check wiring tomorrow and make sure no mistakes.
Bloody knackering this kind of thing. Did you have the same bias arrangement on the other one with the wings?
Did you decouple the grid bias from the cathode; something like pic below?
RhythMick wrote: ↑Thu Dec 27, 2018 9:30 pm
Well it was a good effort. All 6 bias trimmers are now out of circuit, top resistors changed to set bias. Interstages mounted back in amp.
Squeals like a stuck pig.
I recognise I'm pretty tired. Stopping now. Will check wiring tomorrow and make sure no mistakes.
Bloody knackering this kind of thing. Did you have the same bias arrangement on the other one with the wings?
Did you decouple the grid bias from the cathode; something like pic below?
26bias.JPG
Cheers,
Stephen
Morning all
Excuse the rough sketches Steve.
This is the bias arrangement for Geoffs amp. The pot is a 2W Allen Bradley designed to give some flexibility over bias.
Screenshot_20181228_082358.jpg
This is mine as it stands now, with the parallel bias trimmer removed but the balancer in circuit.
