Robrob Princeton delete acting weirdly - help!

[Markeebee]

Hi all

As the old cliche goes, long time lurker, first time poster.  This forum has been an invaluable source of wisdom over the last few years, I hope everyone is keeping healthy and happy in these strange times.

I've built six or seven amps before, mainly 18W derivatives.  All turned out surprisingly ok.  I thought I'd diversify a little and go for a fixed bias 6V6 amp rather than cathode biased EL34....it's all about progression, isn't it?  Robrob's Princeton Delete looked like a nice simple candidate.  Or so I thought. 

Here's the schemo and stuff from Rob's fantastic site: https://robrobinette.com/AA1164_Princeton_Reverb_With_Reverb_and_Tremolo_Deleted.htm

I came up with a layout that pretty much followed the schemo, but with a switchable bright cap, NFB on/off switch, SS rectifier, and selectable AC into the rectifier.

GULP!  Here's my build.....be kind......I'll give it a bit of a tidy up if & when I get it working:



You'll note that the blocking cap after gain stage 3 has been lifted on the board: I put in a pre-PI MV here and moved the cap on to the pot.

So, the problem.  It's oddly quiet.  Sounds pretty good, but nowhere near as loud as I would expect, more like a 3W amp.  So I measured the voltages, and that's when it got very odd.

I realise that I really need to use a transformer with a higher voltage, but I was hoping that 275V with a SS rectifier would get me close enough to where I want to be.  But look:



With the power valves inserted, the voltage drops falls away as the valves warm up.  The rectifier voltage falls from 357v to about 250v, and the plate voltages go all over the place.  The bias voltage looks very odd too....

There's probably all kinds of info that I've left out, but I'm hoping some kind soul will take some pity and open my eyes to what is likely to be a very obvious noob error.

Thanks!


EDIT: error in voltage table updated

[JB]

With the power valves out turn your bias pot right down and see if you can get -30 to -40V on Pin 5, not just a few mV.  If your readings are correct your power valves are massively underbiased and hence sucking big anode current which will be pulling the supply down. Did you notice if they were glowing red hot?  As and when you've got the bias in the right ballpark put them in and try again.

[sluckey]

Get a proper power transformer and go from there.

[Markeebee]

With the power valves out turn your bias pot right down and see if you can get -30 to -40V on Pin 5, not just a few mV.  If your readings are correct your power valves are massively underbiased and hence sucking big anode current which will be pulling the supply down. Did you notice if they were glowing red hot?  As and when you've got the bias in the right ballpark put them in and try again.

Oh crap.  I got the chart wrong.  With the power tubes in, I get -6v on V3 Pin 5, and -26V on V4 Pin 5.

[vampwizzard]

I can find a data sheet on the power transformer but what is that output transformer youre using?

Anyone else think this sounds like thermal runaway?

[Markeebee]

Power txf http://primarywindings.com/product/amp-maker-pwam01-275v-190v-160ma-6-3v3a/

Output txf https://docs.rs-online.com/f7be/0900766b81528910.pdf

[Markeebee]

Oooh, hang on.

Should I derive the bias from the rectifier -ve as the PT doesn't have a centre tap?

[vampwizzard]

That could be problem! https://el34world.com/Forum/index.php?topic=17898.0

Also, not sure if it matters to you, but youve got a 10W max at 40hz OT paired with tubes that want to put out 15W or so.

[Markeebee]

That could be problem! https://el34world.com/Forum/index.php?topic=17898.0

Also, not sure if it matters to you, but youve got a 10W max at 40hz OT paired with tubes that want to put out 15W or so.

Unfortunately the ever fragrant Mrs Bee has some perverse notion that it's my turn to make dinner this evening so I'd better get on that and put off rehashing the bias circuit until tomorrow.

I've used that OP txf on a couple of EL84 amps and quite liked the grungey compression.  I'm not aiming for massive volume or ultra headroom, this is primarily an amp for home use and rehearsals so I don't mind losing a bit of output.

Thanks for your help, it's much appreciated.

[nandrewjackson]

Your voltage reading at power tubes pins 5 really needs to be at about NEGATIVE 34V DC like the RobRob schematic says.  For you to be getting different readings at the 2 tubes' pin 5 locations may indicate an error in wiring. One negative VDC supply shouldn't give 2 different readings.