Building a modified Chieftain circuit, tweaked schematic available for comment

[Duncan]

Howdy everyone,

I have a gutted Traynor YBA-1 that I've started building a Chieftain circuit into. I've been going by the Chieftain Reverb schematic posted on here, but I've made some tweaks.

The main modifications I've made are removing the reverb circuit, running a diode stack for rectification, deleting the standby switch, and using a resistor in place of a choke.

I opted to do these modifications for a few reasons. Regarding the reverb, I would never use it, and I'm trying to get away with using on-hand parts (meaning jacks, reverb transformer, pan, etc would require buying more stuff). The diode stack vs tube rectifier is due to my stock power transformer (Hammond 291JX equivalent) doesn't have a 5v tap on it. And lastly, the resistor over choke is due to the schematic calling for a 20H/160mA choke that I just cannot find for the life of me (likely a custom-built unit for Matchless).

I passed on the reverb switch because the original schematic calls for the standby switch to lift the ground for the HV centre tap, and my PT doesn't have a centre tap on the HV.

I've attached the original schematic as well as my modified one. The changes in components are in red, and the entire reverb circuit has been removed. I do have the turret board mounted with the components to add the reverb back in (along with needing the driver, jacks, and to drill out another hole for an additional preamp tube socket), but for now, I'm not going to use it.

Can anyone see issues with my updates?

Thanks in advance!

[sluckey]

Your rectifier will not work. Needs to be a FWB configuration.

1K/25W is way overkill. A 1K/10W is more than sufficient.

[Duncan]

Your rectifier will not work. Needs to be a FWB configuration.

1K/25W is way overkill. A 1K/10W is more than sufficient.

Oh great. I'll run with the FWB config.

And the 1K 25W resistor is mostly out of convenience. When I built my Trainwreck Express circuit, the shop I bought my components from required a minimum of 10 of those resistors to be ordered. So I've got a bunch just sitting in my drawer.

[Duncan]

Your rectifier will not work. Needs to be a FWB configuration.

1K/25W is way overkill. A 1K/10W is more than sufficient.

Here's an updated schematic, reflecting the FWB rectifcation.

Forgot to have a ground reference. It's been updated.

[tubeswell]

Your rectifier will not work. Needs to be a FWB configuration.

1K/25W is way overkill. A 1K/10W is more than sufficient.

Oh great. I'll run with the FWB config.

And the 1K 25W resistor is mostly out of convenience. When I built my Trainwreck Express circuit, the shop I bought my components from required a minimum of 10 of those resistors to be ordered. So I've got a bunch just sitting in my drawer.

You don’t need a 2nd 1k resistor in series in the screen node filter - a single 1k is plenty (and you already have a 1k in there).

[Duncan]

Thanks for the insights, everyone!

I fired it up last night and I’ve got no sound, but I’m sure I’ve just wired something wrong.

I did notice that my voltage coming into the filter caps exceeds their rating, so I’ll either have to drop that voltage or get some 47UF/500v caps. Any advice on this?

[pdf64]

…
You don’t need a 2nd 1k resistor in series in the screen node filter - a single 1k is plenty (and you already have a 1k in there).

As I see it, the 1k choke substitute HT dropper between the anode and screen grid supply nodes is necessary, and the R35 1k is a key part of the circuit, as it acts to linearise things. Same as a JTM45.

…
I fired it up last night and I’ve got no sound, but I’m sure I’ve just wired something wrong.

I did notice that my voltage coming into the filter caps exceeds their rating, so I’ll either have to drop that voltage or get some 47UF/500v caps. Any advice on this?

After the initial power up surge, the HT voltage should come down once the valves warm up and conduct. Same as things did when it was a YB1A.

[Duncan]

I've finished wiring this thing up after fixing a pretty frustrating miss on my part (had pin 5 of the power tubes wired to the wrong spot).

There are a couple gremlins I'm chasing down (motorboating when I turn the treble all the way up, and an overall lack of expected gain), but it makes nice guitar sounds haha.

Thanks for your help, everyone!

[shooter]

overall lack of expected gain

if it's a tinker'n amp;
one at a time, any order;


Temp OUT the brilliance n MV stuff
make V1 2 gain stages
make V2 a DCCF

[Duncan]

overall lack of expected gain

if it's a tinker'n amp;
one at a time, any order;


Temp OUT the brilliance n MV stuff
make V1 2 gain stages
make V2 a DCCF

All great suggestions. Thank you!!

[Duncan]

I took a run at tweaking the schematic. Thoughts?

[sluckey]

I took a run at tweaking the schematic. Thoughts?

What are you trying to do? Everything in red is gonna smoke quickly.

[Duncan]

I took a run at tweaking the schematic. Thoughts?

What are you trying to do? Everything in red is gonna smoke quickly.

Bump up the gain some, ideally.

[shooter]

she'll be real loud for 13 micro-Seconds, then, well new tube time.
you need to decouple the VDC at the plate so ONLY VAC shows up at the grid.


I don't have my thinking cap on, but a shared Plate R is telling me you'll get the same "gain" since the developed signal for the "2nd" stage is locked to the 1st.

[tubeswell]

I took a run at tweaking the schematic. Thoughts?

It isn't clear from that schematic what you want to achieve. If you want to make the triode on pins 6, 7 and 8 of V2 into a DC coupled cathode follower, the grid (Pin 7) needs to be connected to the plate output (Pin 1) of the other triode, but Pin 6 (CF Plate) needs to be connected to the High Tension supply (and not to the plate load of the other triode). And if you have only 2k2 worth of resistance working as the load resistance, everything will smoke - You want a load resistance somewhere between 47k and 220k for a 12AX7 triode for the tube to sit in a happy place, AND if the CF is to be DC coupled, you want the load resistance to match the load resistance of the driving triode (in this case 100k) to get the CF to bias properly*. Also, the output of the CF (i.e. connection to the tone stack) needs to come from the cathode (Pin 8) via a coupling cap.


*Otherwise use another biasing method with AC-coupling (e.g. cathode bias or fixed bias) in the CF stage.

[Duncan]

Welp, I tracked down a couple issues (master and brilliance were wired wrong, a resistor lost ground contact) and it sounds exactly as it should. Though there’s a hum that I can’t seem to track down.

I suspect it’s interference from the power transformer, as it’s the same frequency as the natural hum of the PT.

Anyone have any guesses as to what it might be?

Here’s a video:

[Duncan]

Nevermind guys. It was a ground loop issue. All is well.