Can I make a Trainwreck out of this?

[StingRay85]

I'm not a firm believer of the Trainwreck layout having a large part to do with the sound (but surely I can be wrong), I think it's more the schematic/component (value) choice and making the amp stable and relatively noise free.

 

[playloud]

Probably the best $100 you have ever spent!

Dead heat between that and my first guitar!

 

[Pete Farrington]

The 1k HT dropper between OT CT and screen grid supply nodes has been shown to be significant in shaping the Express’ overdrive character.
That’s not to say it’s no good with a choke, just that it may be a bit different to a stock Express.
See katopan’s thread analysing the Express over on TAG.

 

[playloud]

The 1k HT dropper between OT CT and screen grid supply nodes has been shown to be significant in shaping the Express’ overdrive character.
That’s not to say it’s no good with a choke, just that it may be a bit different to a stock Express.
See katopan’s thread analysing the Express over on TAG.

Is it this one? https://ampgarage.com/forum/viewtopic.php?f=1&t=15694

A lot of interesting stuff here. The thing that jumps out initially is the difference on either side of the PI output. That probably explains why the oscillation after the PI presented the way it did (tube fed by 82k had bias run away, whereas the other remained stable), and why the solution worked.

Note my amp still has the 1k HT dropper (the choke replaces the initial dropping resistor, between rectifier and mains cap). Do you think it will still behave in a similar fashion?

I should still pick up my scope and signal generator in the next few weeks, so I'm looking forward to investigating...

 

[Pete Farrington]

Is it this one?

That’s it !

Note my amp still has the 1k HT dropper (the choke replaces the initial dropping resistor, between rectifier and mains cap)

I’m confused, can you show me the schematic you’re referring to?
A choke between the rectifier and reservoir the reservoir cap is a totally different arrangement to almost any valve guitar amp I can think of.
Some amps have a sag resistor between rectifier and reservoir, but not any standard Trainwreck.

 

[playloud]

That’s it !

I’m confused, can you show me the schematic you’re referring to?
A choke between the rectifier and reservoir the reservoir cap is a totally different arrangement to almost any valve guitar amp I can think of.
Some amps have a sag resistor between rectifier and reservoir, but not any standard Trainwreck.

Sorry I meant the bleeder (not dropping) resistors after the rectifier in a stock Express (2x100k):



That's where my choke went in the circuit, to create the choke input filter. I know these are highly unusual in guitar amps, but I spotted this in Merlin's book (discussion starts about p. 60) and thought it would be worth a shot - as it happened, it worked!

Here's the relevant formula (2.10, noting that R_l >> R_s, and my PT had 420V AC on the secondary). I'll photograph the other pages when I get home.

 

[Pete Farrington]

Haha, sorry, it’s all there in post #1, but in my defence it was a couple of months ago

 

[arthur.lowery]

Choke input supplies were always a lot kinder on the rectifier tube, as they smooth out the current demanded by the rectifier (chokes like to keep current through them constant). https://frank.pocnet.net/sheets/030/g/GZ34.pdf (page 9). They have a lower output voltage, as others have noted in this tread.

Anyhow looks good!

BTW: The anti-squeal cap is changing the phase around the feedback loop (the amp + feedback), to try and make it not 360 degrees total when the gain around the loop is >1. That would give oscillation. It's often quite difficult to get an appropriate value without doing detailed measurements with the feedback removed.